:LEAI
WATER POLLUTION CONTROL RESEARCH SERIES • 17010EFX 04/70
PHOSPHATE REMOVAL
FROM WASTEWATERS USING LANTHANUM
PRECIPITATION
U.S. DEPARTMENT OP THE INTERIOR • FEDERAL -WATER QUALITY ADMINISTRATION
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WATER POLLUTION CONTROL RESEARCH SERIES
The Water Pollution Control Research Reports describe the results
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For sale by the Superintendent of Documents, U.S. Government Printing Office
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PHOSPHATE REMOVAL FROM WASTEWATERS
USING LANTHANUM PRECIPITATION
by
Howard L. Recht
Masood Ghassemi
Atomics International
Division of North American Rockwell Corporation
Canoga Park, California 91304
for the
FEDERAL WATER QUALITY ADMINISTRATION
DEPARTMENT OF THE INTERIOR
Program #17010 EFX
Contract #14-12-183
FWQA Project Officer, Dr. S. A. Hannah
Advanced Waste Treatment Research Laboratory
Cincinnati, Ohio
April, 1970
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FWQA Review Notice
This report has been reviewed by the Federal
Water Quality Administration and approved for
publication. Approval does not signify that
the contents necessarily reflect the views
and policies of the Federal Water Quality
Administration, nor does mention of trade names
or commercial products constitute endorsement
or recommendation for use.
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ACKNOWLEDGEMENT
The authors wish to thank Dr. S. A. Hannah and Mr. J. M.
Cohen of the Cincinnati Water Research Laboratory for the
interest, expertise, and guidance provided during the course of
this work.
Also, thanks are due to the directors and staff of the Las
Virgenes Municipal Water District, Calabasas, California, for
their help in providing the authors with wastewater test samples.
11
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CONTENTS
Page
Acknowledgement ^
Contents iii
Abstract iv
Introduction 1
Experimental 4
Materials 4
Apparatus 5
Commercial Items 5
Kinetics Apparatus 5
Analytical Procedures 8
Test Procedures 11
Jar Test Experiments 11
Experiments with the Reaction Kinetics Apparatus 12
Thermogravimetric Experiments 12
Results and Discussion 13
Lanthanum Precipitation of Orthophosphate 13
Effect of Sulfate on Lanthanum Precipitation of Orthophosphate .... 19
Lanthanum Precipitation of Condensed Phosphates 21
Lanthanum Precipitation of Phosphate from Secondary Effluent .... 27
Comparison Between Lanthanum and Aluminum Salts for
Precipitating Phosphates 29
Jar Test Reaction Rate Studies with La(III) and Al(III) 36
Reaction Rate Studies Using the Reaction Kinetics Apparatus 37
Nature of La(HI) and Al(III) Orthophosphate Precipitates 38
Recommendations for Future Work 40
Summary 41
References 45
iii
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ABSTRACT
A parametric study was made of the use of lanthanum salts for the removal
of phosphates from pure aqueous solutions of orthophosphate, pyrophosphate,
and tripolyphosphate, and effluent from an activated sludge waste-water treat-
ment plant, and the results were compared with the results of similar tests using
Al(III).
The reactions of orthophosphate with both La(III) and Al(III), which result
in precipitate formation and phosphate removal, were found to be complete in
less than 1 sec.
Phosphate removal results indicated that La(III) is far superior to Al(IH)
for the precipitation of phosphates, especially condensed phosphates, in that it
has a broader effective pH range and yields a lower residual phosphate concen-
tration. For example, at a 2:1 cation-to-phosphate equivalence ratio, with
phosphates at about 0.001 Ni La(III) gave a residual phosphate concentration of
less than 0.1 mg/jf P for both ortho- and condensed phosphates over a pH range
of 6 to 9. With Al(IH) this same residual phosphate concentration was obtained
only with orthophosphate, and only at a pH of 6.
Within the pH range for optimal phosphate removal, the reaction of La (III)
with both ortho- and polyphosphates resulted in the formation of large, settle-
able floes. Immediately outside this pH range, fine turbidity developed which
did not settle very well. At higher pH levels outside this pH region, some residual
turbidity was observed in almost all cases. No turbidity or precipitate forma-
tion was observed at very low pH levels with either ortho- or polyphosphates.
In studies at constant pH, the removal of orthophosphate was found to be
directly proportional to the concentration of added lanthanum. Essentially com-
_3
plete removal of phosphate was observed at a La(HI)/PO4 molar ratio of
slightly less than 0.9. This direct stoichiometric relationship strongly suggests
that phosphate removal occurs through a purely chemical reaction between the
lanthanum and the orthophosphate and not through adsorption (physical or chemi-
cal) of phosphate on a precipitating metal hydroxide.
In tests with actual wastewater (secondary effluent), as with pure solutions,
a lower residual phosphate level was produced over a wider pH range with La(III)
Iv
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as compared with Al(III). At a 2:1 cation-to-phosphate equivalence ratio, a
residual phosphate concentration of less than 0.1 mg/I P was obtained with
La(IH) over a pH range of 4.5 to 8.5; Al(III) gave this residual concentration
only over a pH range of 5 to 6.5, and left 0.3 mg/jj P at a pH of 7. Thus lan-
thanum is effective for the removal of phosphates from domestic wastewater
(pH ~7 to 8) without pH adjustment. To reach the optimum precipitation pH of
6 with aluminum, large quantities of acid, or a considerable excess of aluminum
salt, must be added to the wastewater to overcome its natural buffer capacity.
Based on the above data, further studies of the use of lanthanum salts for
phosphate removal from wastewater, including pilot plant testing, appear
warranted.
This report was submitted in fulfillment of Program No. 17010 EFX,
Contract No. 14-12-183, between the Federal Water Quality Administration
and Atomics International, Division of North American Rockwell Corporation.
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INTRODUCTION
Phosphorus and nitrogen compounds, which are present in domestic waste-
water in appreciable amounts, are important nutrients. The discharge of large
quantities of these nutrients into natural waters promotes the growth of algae
and results in eutrophication of lakes and similar deterioration of water quality
in receiving streams. In recent years, the rapid population growth and the
astonishing economic and industrial expansion experienced in many areas of the
world have placed a growing burden on the available water resources. This has
generated an increasing need for abatement of water pollution, and reclamation
and reuse of wastewaters. Accordingly, considerable effort has been directed
toward the development of economical methods to achieve a high degree of
wastewater treatment and to effect near complete removal of undesirable nutrients.
Although both nitrogen and phosphorus compounds are essential to algae
growth, phosphorus is generally considered to be a more critical nutrient be-
cause, unlike nitrogen, it can enter a receiving body of water only by influx of
phosphorus containing compounds. In contrast, certain species of algae, particu-
larly the nuisance blue-greens, have been reported to be capable of satisfying
their nitrogen demand by direct utilization of the atmospheric nitrogen.* ' Thus,
control of eutrophication may best be achieved through control of phosphorus.
For this reason, the major portion of the research effort on the elimination of
nutrients from wastewaters has been directed toward the development of eco-
nomical methods for the removal of phosphates.
Phosphorus may be present in the wastewater in the form of organic phosphorus,
inorganic condensed phosphates, andorthophosphates. Most of the organically-
bound phosphorus compounds in the wastewater are present as particulate organic
matter and as bacterial cells. Very little Is known about the dissolved organic phos-
phorus compounds producedbybacterialmetabolismandcelllysis. Inorganic con-
densed phosphates such as tripolyphosphateandpyrophosphate originate mainly in
household detergents. Orthophosphate Ls an end-product of microbial degradation
of phosphorus-containing organic compounds; it is also excreted Ln the urine and is
the product of enzymatic hydrolysis of condensed phosphates. Orthophosphate is the
phosphorus form most readily available for biological utilization. The concentra-
tions of the various forms of phosphorus ina domestic wastewater are subject to wide
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hourly and daily fluctuations. Wastewaters received at or discharged from differ-
ent plants also contain varying concentrations of phosphates depending on the type of
community served and the nature of the biological treatment employed. Finstein and
Hunter* studied phosphate concentration in three activated sludge and three trick-
ling filter plants and found that in the influent to the biological treatment units inor-
ganic condensed phosphates constituted 15 to 75% of the total phosphorus, the value
varying on a daily cycle, andthatabout 50% of the condensed phosphates were hydro-
lyzed to orthophosphate on the ir pas sage through the treatment plants.
To date, the most common method of secondary wastewater treatment is
biological oxidation. The microorganisms present in the wastewater degrade
the complex organic molecules into simpler products, thereby acquiring energy
and material for their growth and the synthesis of new cells. As the supply of
the available food diminishes, the starving bacteria agglomerate into large
floes which are removed by settling. Although conventional biological treatment
processes can result in a substantial reduction of the carbonaceous organic
matter, due to the unique nutritional requirements of sewage bacteria, only
20 to 40% of the phosphorus compounds initially present in the raw wastewater
is converted into removable cell material. Accordingly, the effluents from
most conventional biological treatment units still contain substantial quantities
of phosphates (about 5 to 30 mg/i P* ') which have to be essentially completely
removed if serious deterioration of water quality in the receiving stream is to
be avoided. Phosphorus concentrations over 0.01 mg/I in natural waters have
been shown to lead to the development of massive algae growth/ '
Of the several methods available for the removal of phosphates, chemical
precipitation (often called coagulation) using aluminum, ferric, and calcium
salts has received the widest attention and is considered the most promising
IA\
of these/ ' Aluminum and ferric salts both hydrolyze in solution to an appreci-
able extent; their effectiveness in precipitating phosphate depends on pH. The
pH of maximum phosphate removal is close to 5 for aluminum and is near 4 for
ferric salts/ ' The pH of most wastewaters, however, lies in the range of
7 to 8. To reach the optimum precipitation pH with aluminum and ferric salts,
large quantities of acid must be added to the wastewater to overcome its natural
buffer capacity. In practice, the requirement for addition of acid is often partially
eliminated by the addition of excess quantities of aluminum and ferric salts. Even
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then, the extent of phosphate removal has been reported to be unsatisfactory.
For example, Lea et aP reported that in a pilot plant evaluation of phosphate
removal using alum, A12(SO. )_• 14H2O, phosphate removals, varying from
77 to 89.3%, were achieved with an alum dose of 200 mg/jg when the concentration
of soluble phosphate in the influent was in the range of 4 to 6 mg/j? P. An
appreciable amount of the precipitated phosphate did not settle out but was lost in
the effluent. The phosphate removal efficiency could be raised to 99% by filtra-
tion of the effluent. However, considering that such a large excess of alum was
used (aluminum to phosphate molar ratios in the range of 3:1 to 5:1), the effec-
tive molar capacity of aluminum to remove phosphate is very low.
The effectiveness of aluminum and ferric salts in precipitating condensed
inorganic phosphates has been the subject of some controversy. Sawyer, for
example, reported that both aluminum and ferric salts are highly effective in
/7\
removing all forms of phosphate. According to Stumm/ however, tripolyphos-
phates are not removed effectively by coagulation with either Al(HI) or Fe(III)
due to the formation of soluble complexes such as MP-O,Q" .
In the case of aluminum, disagreement has been expressed by some investi-
gators on the effect of contact time on the efficiency of phosphate precipitation.
Stumm/ ' for example, reported that under proper pH conditions and at low
cation-to-phosphate ratios, the cation-orthophosphate reaction occurring is
+3 - +
M + H2PO4 = MPO4 + 2H , provided that sufficient time is allowed for the
precipitation to occur. In the actual practice, however, even under optimum
pH conditions, far more than equivalent quantities of metal salts are required
for complete precipitation of phosphates. Also, Lea et aP ' reported that in the
treatment of sewage plant effluent with alum, mixing and flocculation time in
excess of 12 minutes resulted in a decrease in soluble phosphate removal.
Calcium salts are effective in precipitating phosphate at very high pH levels.
The calcium is generally added to the wastewater in the form of unslaked lime
(CaO) which serves the additional purpose of raising the pH. Because of the
large buffer capacity of most wastewaters, however, considerable amounts of
lime are required to raise the pH high enough to effect good phosphate removal.
Residual phosphate concentrations of less than 0.01 mg/jg P have been reported
with lime dosages exceeding 500 mg/jj CaO. Recarbonation of the wastewater
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following lime treatment is also necessary if the discharge of a highly alkaline
effluent is to be avoided.
Because of the special pH requirements and the low efficiencies of alumi-
num, ferric, and calcium salts in precipitating phosphates, there has been con-
siderable interest in developing better methods for phosphate removal. In some
preliminary work at Atomics International, the salts of lanthanum, the most
abundant of the trivalent rare earth cations, were shown to be effective in pre-
cipitating orthophosphate from solution. (It should be noted that there is an
abundant supply of the rare earths in nature so that application of lanthanum to
the treatment of domestic wastewater on a wide scale will not be limited by any
shortage.^)
Based on these very promising results, the present investigation was under-
taken. This investigation had as its principal objectives the carrying out of a
parametric study of phosphate precipitation with lanthanum and the comparison
of the lanthanum precipitation results with the data obtained using aluminum.
Specifically, an evaluation was made of the effects of pH and reagent concentra-
tion on phosphate removal, the rate of phosphate precipitation, and the nature
and characteristics of the precipitates. Although effluent from a biological
wastewater treatment plant was used in some of the experiments, most of the
precipitation studies were made on pure solutions of ortho-, pyro-, and
tripolyphosphates.
EXPERIMENTAL
This section describes the materials, apparatus, and procedures used
generally during the course of the investigation. Special methods and modifi-
cations are described with the results where appropriate.
Materials
Reagent grade sodium monohydrogen (ortho-) phosphate, tetrasodium pyro-
phosphate, sodium tripolyphosphate, and aluminum nitrate [A1(NO,)~- 9H2O],
and lanthanum nitrate [La(NO3)3« 6H2qJ of 99.9% purity, obtained from
Molycorp (Molybdenum Corp. of America), were used to prepare pure solutions.
All La(in) and Al(IH) solutions were prepared fresh daily. Adjustment of pH
was made using reagent grade HC1 or NaOH. All other reagents used were the
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purest grade available. Double distilled water was used to prepare reagent
solutions.
The orthophosphate solution used in most experiments contained 12 mg/4 P
Na2HPO4 (11.8 x 10" equivalents/S. PO4"3 or 3.86 x 10"4_M). This concentra-
tion was selected as representative of that to be encountered in a typical secon-
dary effluent. An 18 mg/I P solution of tetrasodium pyrophosphate and 21.6
mg/jj P solution of sodium tripolyphosphate, each containing the same number
of equivalents per liter of the anion as the orthophosphate solution, were used
in the experiments with condensed phosphates.
The wastewater (secondary effluent) used in these studies was obtained
from the Tapia Park Treatment Plant of the Las Virgenes Municipal Water
District in Calabasas, California. This treatment plant is a small ("2 mgd)
activated sludge plant which serves a primarily residential community. It
employs an aerobic sludge digestion process with the supernatant liquor re-
turned to the head of treatment plant. The small amount of large bacterial
floes which were present in the effluent was removed by prior filtration through
coarse filter paper.
The following is a partial analysis of the filtered effluent used in the phos-
phate precipitation experiments with Lia(III) and Al(III):
pH 7.8
Temperature (°C) 17.5
Conductivity (at 17.5°C)(mmhos/cm) 2.17
Turbidity (JTU) 0.73
Orthophosphate (mg/4 P) 7.75
Total phosphate (mg/i P) 7.75
Polyphosphate (by difference)(mg/i ) 0
Apparatus
Commercial Items
All "jar test" precipitation experiments were conducted using Phipps and
Bird six-place stirrers (Phipps and Bird, Inc., Richmond, Va. ). The metallic
paddles supplied with the instruments were replaced with clear plastic paddles
to avoid possible metallic contamination of test solutions. Radiometer PHM 26
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and PHM 28 pH meters (Radiometer Co., Copenhagen) and a Beckman Model
96 Zeromatic pH meter (Beckman Instrument Co. , Fullerton, Calif. ) were used
for pH measurements. All conductivity determinations were made with Radi-
ometer Model CDMZe conductivity meters (Radiometer Co. , Copenhagen). The
conductivity cell was modified by drilling numerous holes in the glass envelope
to permit more rapid passage of the solution into the space between the electrode
surfaces. A Radiometer automatic titration control unit, type TTT 11 (Radiometer
Co., Copenhagen) was used in conjunction with the Radiometer pH meters for the
constant pH precipitation experiments. A Moseley Model 7100 B two-pen strip
chart recorder (Hewlett-Packard Co., Pasadena, Calif. ) was used for recording
of pH and conductivity in the reaction rate studies. Turbidity measurements
were made with a Hach Laboratory Turbidimeter Model 2100 (Hach Chemical Co. ,
Ames, Iowa). Most of the colorimetric analyses were made with a Bausch and
Lomb Spectronic 20 Colorimeter/Spectrophotometer (Bausch and Lomb Co. ,
Rochester, N. Y.). Some colorimetric analyses were obtained using a Gary
Model 14 recording Spectrophotometer (Gary Instruments, Monrovia, Calif.)
with a 10 cm light path. A Beckman Model DU Spectrophotometer-Flame Pho-
tometer (Beckman Instrument Co., Fullerton, Calif.) was used for the flame-
photometric determination of lanthanum. All x-ray diffraction analyses were
made on a Norelco 50 kv Diffractometer (Norelco, New York, N. Y.). Weights
of precipitates in the study of weight loss on drying and heating were determined
using a Mettler M5 Micro Analytical Balance (Mettler Instrument Corp., Hight-
stown, N. J.) of ±0.001 mg stated precision.
Kinetics Apparatus
To obtain reliable data on the rates of the phosphate precipitation reactions,
a special reaction kinetics apparatus, shown schematically in Figure 1, was
designed and constructed. In this apparatus provision is made for monitoring
pH and residual phosphate and cation concentrations under steady-state condi-
tions. The metal salt and phosphate solutions flow by gravity from separate
reservoirs into a 50 ml reaction flask where they are rapidly mixed with a mag-
netic stirrer. The mixture then travels through a long 16mm OD tube assembled
from 4-ft Pyrex sections connected by short pieces of rubber hose, with open-
ings for sampling or for insertion of a pH probe electrode. Samples of the flow-
ing solution mixture are withdrawn under vacuum through 450 m/I membrane
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INITIAL LIQUID LEVEL
16 mm 00 PYREX TUBE (4-ft SECTIONS CONNECTED
BY SHORT PIECES OF RUBBER HOSE; HOLES IN HOSE
SERVE AS SAMPLING PORTS)
MILLIPORE SWINNEX-25
FILTER UNIT WITH 450 mft
MEMBRANE FILTER
5-GALLON
GLASS BOTTLE
X MAGNETIC
STIRRER
50ml
SAMPLING FLASK
wr
21 in.
— 1/2 in. TYGON TUBING
1/2 in. PLASTIC TEES
MIXING CHAMBER
(50mj0ERLENMEYER FLASK)
TO VACUUM
Figure 1. Schematic Diagram of the Reaction Kinetics Apparatus
(Not Drawn to Scale)
70-A15-032-1A
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filters installed in Millipore Swinnex-25 filter units (Millipore Corp., Bedford,
Mass.), modified and attached to small flow-through cells. These cells are
placed in the flow line so that the flowing liquid passes directly over the entire
filtration area (see Figures 2 and 3). The filtrates collected in this manner
represent the liquid phase prevailing at the sampling ports. The time of solution
travel to each port is thus the true reaction time prior to filtration. After col-
lection, the filtrates are analyzed for residual phosphate and metal ion content.
It was originally intended that the conductivity of the solution mixture also
be monitored at several points in the flow line. However, this did not prove to
be feasible because the available conductivity cell caused an excessive reduction
in flow rate.
The mixing apparatus was shown to be effective by reacting a dilute solution
of sodium hydroxide, with phenolphthalein indicator added, with a dilute HC1
solution. No color was observed in the fluid leaving the mixing flask. Indeed,
little, if any, color was observed beyond the point where the two streams en-
tered the flask.
In the initial reaction rate experiments an average total solution flow of
65 ml/sec was used. At this flow rate, the approximate total travel time from
the start of the mixing to the observation ports, A, B, C, D, and E (see Figure
l)are 1.3, 2.7, 4.3, 15.6, and 22.2 sec, respectively. In later experiments the
time of solution travel to the first port was reduced to less than 1 sec by further
elevating the solution reservoirs and by shortening the flow line. A photograph
of the apparatus in this arrangement is presented in Figure 3.
Analytical Procedures
The stannous chloride method described in Standard Methods was used
for all orthophosphate determinations. With this method, the 2 cm light path of
the Bausch and Lomb Spectronic 20 permitted phosphate concentrations as low
as 0.01 mg/iPto be detected. Using the 10 cm light path of the Gary Model 14,
phosphate concentrations as low as 0.002 mg/i P could be determined. The poly-
phosphates were hydrolyzed to orthophosphate by boiling with acid and deter-
mined as orthophosphate. The benzene-isobutanol extraction modification of the
stannous chloride method as recommended in the Standard Methods * for ob-
taining increased sensitivity and avoidance of certain interferences was used in
8
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FILTRATION AREA
TOP VIEW
—3.2 cm (ID) —
,16 mm PYREX TUBE
MEMBRANE FILTER (450
PYREX CELL
FRONT VIEW
FLOW OF SOLUTION OVER
THE MEMBRANE FILTER
16 mm (OD)
FILTER SUPPORT
MODIFIED MILLIPORE
SWINNEX-25
FILTER HOLDER
TO SAMPLE COLLECTION
FLASK AND VACUUM
Figure 2. Schematic Diagram of the Flow-Through Sampling Port
Used in the Reaction Kinetics Apparatus
2-10-70 UNCL 5128-4004
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Figure 3. Reaction Kinetics Apparatus, Modified for Rapid Flow
70 A1 03222
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all orthophosphate determinations involving wastewater. In the experiments
with pure phosphate solutions, the lanthanum analyses were made by an emission
spectrographic technique. The method, however, was found to lack the desired
sensitivity and precision and the values reported may be somewhat differentfrom
the actual concentrations. Of the several sensitive colorimetric and fluorometric
methods available for the analysis of lanthanum, none was found to be free from
interference from phosphate ion. In one phosphate precipitation experiment with
wastewater where the residual phosphate concentration was less than 0.01mg/6P
an extraction-flame photometric method was used to determine the residual
lanthanum concentration. The method was found to be sensitive to lanthanum
concentrations as low as 0.5 mg/i La. No interference was encountered from
other residual ionic and nonionic species in the wastewater since an identical
calibration curve was obtained when the wastewater (phosphate content less than
0.01 mg/£ P) was substituted for distilled water in the preparation of the lan-
thanum standards.
Test Procedures
Jar Test Experiments
Many of the studies were carried out in batch type ("jar test") experiments.
The usual procedure in these tests was as follows. Five hundred ml of the test
phosphate solution was placed in a 1 i. beaker. While stirring at 90 rpm, 5 ml
of a lanthanum or aluminum nitrate solution of appropriate concentration was
added to the beaker to establish cation-to-phosphate equivalence ratios of 2:1,
1:1, or 0.5:1. After 2 min of rapid mixing at 90 rpm, the stirring rate was
slowed down to 20 rpm and the mixing was continued at this rate for 10 min.
Following 20 min of quiescent settling the samples were filtered through What-
man No. 42 filter paper and the filtrates analyzed. Prior to filtration, a portion
of the sample supernatant was removed by decanting and its turbidity was deter-
mined. All precipitation experiments were conducted at ambient temperature
(25 ± 2°C).
In experiments with condensed phosphates and secondary sewage effluent,
and in some experiments with orthophosphate, pH adjustments were made by
prior addition of NaOH or HC 1 to the phosphate test solution. In most ortho-
phosphate precipitation experiments, a Radiometer automatic titrator was used
11
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to maintain desired reaction pH through automatic addition of NaOH while the
lanthanum or aluminum solution (1-10 ml) was added slowly from a separate
burette. In phosphate removal studies on secondary effluent where the desired
final pH was in excess of 8, the addition of the NaOH to the wastewater prior to
addition of lanthanum was found to result in precipitate formation (possibly cal-
cium carbonate and/or calcium phosphate). In performing these phosphate re-
moval experiment, therefore, the base and the lanthanum were added to the
sample concurrently.
Experiments with the Reaction Kinetics Apparatus
The phosphate solutions used in reaction rate studies in the kinetics appa-
ratus, described above, were prepared to contain 24 mg/i P Na2HPO^ (7.72 x
• ^L
10 M). Upon mixing with the metal salt solutions at nearly equal flow rate,
_4
a final phosphate concentration of about 12 mg/i P (3.86 x 10 M ) resulted.
The aluminum and lanthanum salt solutions were prepared fresh In concentrated
forms and were diluted to the necessary volume of 18 i immediately prior to
start of each experiment. All kinetics experiments were completed within 10
min of the preparation of the dilute metal solution. The final solution pH in each
experiment was maintained within the range for optimum phosphate precipitation
and good floe formation (6 to 8 for La (III) and 5 to 7 for Al(III)) by prior addition
of NaOH to the phosphate solution.
Samples of mixture leaving the kinetics apparatus were collected and floc-
culated by gentle mixing (20rpm), settled, and then the supernatants were filtered
through 450 m/4 membrane filters. These filtrates and those obtained at the
sampling ports were analyzed for residual cation and phosphate content.
Thermogravimetric Experiments
To investigate the nature of the precipitates formed in phosphate removal
using lanthanum and aluminum salts, orthophosphate was precipitated from solu-
_4
tions of 12 mgAP Na2HPO4(3.86 x 10 M) using a cation-to-orthophosphate
molar ratio of 1:1. The precipitation experiments were conducted at a constant
pH of 6.0 for aluminum and 7.0 for lanthanum using a Radiometer automatic
t it r a tor. Following flocculation and settling, the supernatant was decanted and
the precipitate slurry was centrifuged and washed into a preweighed Gooch cru-
cible containing a glass fiber filter paper (No. 934-AH, Hurlbut Paper Co.). In
12
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the experiment with aluminum, the precipitates from 3-2 liter samples were
combined and placed in one Gooch crucible; the lanthanum precipitation experi-
ments were run in triplicate. The fresh precipitates were dried in a desiccator
at room temperature and the weight loss with time was determined. The sam-
ples were then heated at 104°C for three 2-hr periods. Following each heating,
the crucibles were cooled in a desiccator for about 2 hr, weighed, stored in the
desiccator for an additional 24 hr and weighed again. After a portion of the
dried precipitate was removed for x-ray analysis, the precipitate was weighed
and ignited at 600°C for two 2-hr periods. X-ray diffraction analyses were made
on all fresh, dried, and ignited precipitates.
RESULTS AND DISCUSSION
The primary objective of the present program was the evaluation of the ef-
fectiveness of lanthanum salts for the removal of phosphates by precipitation.
As a supplementary objective, lanthanum results were compared with data from
similar tests using aluminum. An investigation of the rates of phosphate pre-
cipitation with lanthanum and aluminum salts, and studies aimed at the character-
ization of the precipitates formed were also done in the present study. The re-
sults obtained and discussion of them follow.
Lanthanum Precipitation of Orthophosphate
A major portion of this program was devoted to determining the effect of pH
and reactant concentration on the precipitation of orthophosphate with lanthanum
salts. These experiments were performed at ambient temperature (~25°C). The
data obtained are presented in Figures 4, 5, and 6.
Figure 4 shows that, at a 0.5:1 lanthanum-to-orthophosphate molar ratio,
the residual phosphate concentration was between 4.8 and 6.0 mg/£ P in the pH
range from 4 to 9, with the minimum value obtained at a pH of 5.0. The removal
of phosphate in this pH range was accompanied by near complete precipitation of
lanthanum. No appreciable removal of phosphate or lanthanum was observed at
pH 2, 10, and 11. Lanthanum was also quantitatively precipitated at a pH of 12,
accompanied by a 25% removal of phosphate.
As shown in Figure 5, at a 1:1 lanthanum-to-phosphate ratio, the phosphate
removal was greatest within the pH range from 5 to 9. The concentration of
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LIMIT FOR La ANALYSIS
SOLUTION pH
Figure 4. Precipitation of Orthophosphate with Lanthanum at a 0.5:1 Lanthanum-to-Orthophosphate
Molar Ratio (Initial Orthophosphate Concentration, 12 mg/j0P)
70-A15-032-43
14
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100
LIMIT FOR P ANALYSIS
12
SOLUTION pH
Figure 5. Precipitation of Orthophosphate with Lanthanum at a 1:1 Lanthanum-to-Orthophosphate
Molar Ratio (Initial Orthophosphate Concentration, 12 mg/jJP)
70-A15-032-42
15
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1000 r
LIMIT FOR P ANALYSIS
0.001
10
12
SOLUTION pH
Figures. Precipitation of Orthophosphate with Lanthanum at a 2:1 Lanthanum-to-Orthophosphate
Molar Ratio (Initial Orthophosphate Concentration, 12 mg/JJP)
70-A15-032-41
16
-------
residual phosphate within this region is given as 0.01 mg/jC P. This is the limit
of sensitivity and precision of the method and instrument used for phosphate
determination. Within the pH range of greatest phosphate removal and also at
lower pH levels, the removal of lanthanum closely followed that of phosphate
roughly on the basis of a 1:1 reaction stoichiometry. At a pH of 12, the precipi-
tation of lanthanum was accompanied by a greater reduction in phosphate con-
centration than that observed at a pH of 11.
At a 2:1 lanthanum-to-phosphate ratio (Figure 6) two pH regions of good
phosphate removal may be noted, one extending from a pH of about 4 to a pH of
about 9, and another in the pH range from 9.7 to 11.5. Near complete preci-
pitation of phosphate was obtained within both these regions. The removal of
phosphate at high pH levels, noted here and above at a 1:1 molar ratio, may be
due to adsorption of phosphate on precipitating lanthanum hydroxide and/or may
result from the formation of a lanthanum-hydroxy-phosphate precipitate.
The turbidity data obtained in these orthophosphate precipitation experi-
ments are also plotted in Figures 4 to 6. The close correlation between phos-
phate removal and the formation and settling of the precipitate is apparent. At
low pH values where no turbidity develops, no removal of phosphate is observed.
At higher pH levels in the pH range for optimum phosphate removal, the lantha-
num-phosphate reaction results in the formation of precipitates which agglomer-
ate into large readily settleable floes.
Figure 7 shows the results obtained when phosphate precipitation with lan-
thanum was carried out at a constant pH of 7.0 using La(III)/PO~ molar ratios
ranging from 0.1:1 to 1.0:1. Results of similar experiments with aluminum are
also shown; these are discussed below where La(III) and Al(III) results are com-
pared. The lanthanum data in this figure indicate that complete removal of phos-
phate can be achieved at La(III)/PO ~ molar ratios as low as 0.9. In the 0.1-
0.9 molar ratio range, the removal of phosphate is directly proportional to the
amount of added lanthanum, and is about 14% higher than that which would be ex-
pected from a 1:1 lanthanum-to-phosphate reaction stoichiometry. This suggests
that the mono- and dihydrogen orthophosphate ions in addition to the trivalent
orthophosphate ion are involved in precipitate formation. The linear relationship
between phosphate removal and amount of added lanthanum strongly suggests that
a chemical reaction occurs between these species, and physical processes such
as adsorption are little, if at all, involved in the removal.
17
-------
0.2
0.4
0.6 0.8 1.0 1.2 1.4 1.6
CATION-TO-ORTHOPHOSPHATE MOLAR RATIO
Figure 7. Comparison of the Effectiveness of Lanthanum and Aluminum for the Precipitation
of Orthophosphate; La (III) at pH = 7.0. Al (III) at pH = 6.0
(Initial Orthophosphate Concentration, 12 mg/j2 P)
70-A15-032-32
18
-------
In these studies of the lanthanum-orthophosphate reaction and In the poly-
phosphate precipitation experiments (to be described later) no quantitative as-
sessment was made of the nature and the extent of colloidal surface charges
and their role in agglomeration and dispersion of the precipitates. In qualita-
tive terms, it was found that the changes in pH can result in a dispersion of the
phosphate precipitate. The dispersion at higher pH levels can probably be ex-
plained in terms of a net negative surface charge on the colloid particles arising
from increased dissociation of the surface phosphate groups of the precipitates
and/or adsorption of phosphate and hydroxide ions from solution onto the particle
surfaces. The dispersion of the precipitates at low pH may be attributed to the
adsorption of excess cations onto the surface of the precipitates resulting in a
net positive surface charge.
Effect of Sulfate on Lanthanum Precipitation of Orthophosphate
The lanthanum-orthophosphate reaction data, presented in Figures 4-7, were
obtained using pure phosphate solutions. Before the effectiveness of lanthanum
for the removal of phosphate from actual wastewaters was evaluated, the effects
of sulfate and calcium, two common wastewater constituents, on the precipita-
tion of phosphate were investigated. These ions with their divalent charge might
be expected to affect the results significantly. The 12 mg/S. P Na7HPO . phos-
-2
phate solution was made 0.0025 M in sodium sulfate (240 mg/f. SO . ) and the
experiments were conducted at ambient temperature.
Figure 8 contains data on the precipitation of orthophosphate in the presence
of sulfate using a 1:1 lanthanum-to-phosphate molar ratio. For the purpose of
comparison, the phosphate precipitation data in the absence of sulfate (data on
Figure 5) are also reproduced in Figure 8. These data indicate that the overall
effect of sulfate on the phosphate precipitation curve was a slight narrowing of
the pH range of effective phosphate removal and a shift toward higher pH levels.
The pH shift was about 0.5 unit for pH levels above 9 and generally close to 1.0
for pH values less than 6.0. Figure 8 also shows data, obtained in the tests on
sulfate containing solutions, on the turbidity of the unfiltered samples following
20 min of quiescent settling. As was observed in other cases, a good correla-
tion exists between the development and settling of turbidity and the removal of
phosphate.
19
-------
TURBIDITY
-------
Another common divalent constituent of wastewater is calcium ion. Despite
the relative insolubility of calcium phosphate, wastewaters usually contain both
calcium and phosphate ions at concentrations above those which would be expected
from solubility considerations. No explanation is yet available for this apparent
anomaly. An evaluation of the effect of calcium ion on phosphate precipitation
with lanthanum could not be made, since the addition of calcium to a phosphate
solution resulted in the development of turbidity and formation of a precipitate,
probably calcium phosphate.
Lanthanum Precipitation of Condensed Phosphates
The effects of pH and lanthanum concentration on the precipitation of pyro-
and tripolyphosphates from pure solutions by lanthanum were investigated. The
results obtained are summarized in Figures 9 through 12. In these figures,
residual phosphate and turbidity of test samples following settling are plotted
as functions of solution pH for lanthanum-to-phosphate equivalence ratios of
1:1 and 2:1.
As with the precipitation of orthophosphate, the extent of polyphosphate re-
moval with lanthanum is a function of solution pH and reactant ratios. The opti-
mum precipitation pH range for both pyrophosphate and for tripolyphosphate is
5.5 to 6.0 at a 1:1 lanthanum-to-phosphate equivalence ratio, and 7 to 9 at a 2:1
ratio. Minimum pyrophosphate residuals of 0.06 and 0.002 mg/.£ P and mini-
mum tripolyphosphate residuals of 0.18 and 0.007 mg/^ P were observed for 1:1
and 2:1 lanthanum-to-phosphate ratios, respectively. It should be noted that the
phosphate data reported in Figures 10 and 12 were obtained using a Gary Model
14 recording spectrophotometer, whose 10 cm light path permitted phosphate
residual concentrations as low as about 0.002 mg/Jl P to be determined.
At a 0.5:1 lanthanum-to-tripolyphosphate ratio, no phosphate removal was
observed at any of the pH levels examined. Table 1 contains data on the turbidity
produced under these conditions. These data indicate that some reaction prob-
ably takes placed in the pH 4 to 6 region, but that particles produced are very
fine since no phosphate was removed by filtration through Whatman No. 42 filter
paper.
21
-------
100
0 Q.001 —
00
QC
0.01
12
SOLUTION pH
Figure 9. Precipitation of Pyrophosphate with Lanthanum at a 1:1 Lanthanum-to-Pyrophosphate
Equivalence Ratio (Initial Pyrophosphate Concentration, 18 mg/j2 P)
70-A15-032-33
22
-------
100
I V)
to
cc
10.0
1.0
0.1
0.01
0.001
LIMIT FOR P ANALYSIS —*
6
SOLUTION pH
10
12
Figure 10. Precipitation of Pyrophosphate with Lanthanum at a 2:1 Lanthanum-to-Pyrophosphate
Equivalence Ratio (Initial Pyrophosphate Concentration, 18 mg/j^P)
70-A15-032-39
23
-------
100
TURBIDITY
--OE
TRIPOLYPHOSPHATE
0.10
2;
0.01
0.001
8
10
12
SOLUTION pH
Figure 11. Precipitation of Tripolyphosphate with Lanthanum at a 1:1 Lanthanum-to-Tripolyphosphate
Equivalence Ratio (Initial Tripolyphosphate Concentration, 21.6 mg/jgP)
70-A15-O32-31
24
-------
100
UJ
Is
Hu.
II
UJ o
08
°i
OS
10
0.10
-JOC
< Ul
0.01
u.
O
0.001
TRIPOLYPHOSPHATE
6
10
12
SOLUTION pH
Figure 12. Precipitation of Tripolyphosphate with Lanthanum at a 2:1 Lanthanum-to-Tripolyphosphate
Equivalence Ratio (Initial Tripolyphosphate Concentration, 21.6 mg/j2p)
70-A15-032-30
25
-------
TABLE 1
LANTHANUM-TRIPOLYPHOSPHATE REACTION AT
0.5:1 EQUIVALENCE RATIO
(Phosphate Solution Used: 21.6 mg/4 P
pH
3.45
4.10
4.35
5.25
5.95
Turbidity
(JTU)
0.17
2.20
1.80
3.50
0.70
pH
6.70
7.50
9.05
10.00
Turbidity
(JTU)
0.13
0.20
0.18
0.15
Comparison of the lanthanum-polyphosphate reactions at 1:1 and at 2:1 lan-
thanum-to-phosphate ratios, reveals several interesting points. The two residual
phosphate curves for the 2:1 ratio (Figures 10 and 12) exhibit relatively constant
low values in the pH 5-6 range. This pH range also corresponds to the region of
maximum phosphate removal at the 1:1 ratio. At the 2:1 ratio, up to a pH of
about 6, essentially no additional removal of phosphate is effected by the pres-
ence of this excess lanthanum. At a 1:1 lanthanum-to-phosphate ratio, substan-
tial amounts of residual turbidity are observed in the pH 6.5-9 region (Figures 9
and 11). This pH region also corresponds to the region of maximum phosphate
precipitation when a 2:1 ratio of reactants is used.
The results of a similar study of the aluminum-tripolyphosphate reaction* '
has shown that a significant amount of residual phosphate remains in the pH re-
gions immediately outside the range of good floe formation. This phosphate is
presumably in the form of colloidal particles which cannot be effectively removed
by filtration through Whatman No. 42 filter paper; the range of good phosphate re-
moval is somewhat broadened when these solutions are filtered through fine mem-
branes (e.g., 100 mfj). It is possible, therefore, that the rise in residual phos-
phate observed in the pH 6.5 to 9.0 region at a 1:1 lanthanum-to-phosphate ratio
is due, at least in part, to the formation of a highly dispersed, fine precipitate
which is not removable by filtration through Whatman No. 42 filter paper. The
addition of excess lanthanum (use of a 2:1 La/phosphate ratio) would bring about
a destabilization of this dispersed colloidal turbidity and therefore result in a
26
-------
broadening of pH range for good phosphate removal. The possibility should be
considered that the lanthanum-polyphosphate reactions which take place in the
pH 6.5 to 9 range lead to different products than those occurring at lower pH
levels (pH 5 to 6), since the precipitates formed in the high pH region at the 2:1
ratio were noted to be more bulky and gelatinous than those observed at lower
pH levels.
Within the pH range of optimum phosphate removal, the lanthanum-polyphos-
phate reactions resulted in the formation of large settleable floes. Immediately
outside this pH region, colloidal suspensions were formed. At very low pH levels
where no turbidity was formed, the lanthanum-polyphosphate reactions either do
not occur or they result in the formation of soluble products.
Lanthanum Precipitation of Phosphate from Secondary Effluent
The effectiveness of lanthanum in precipitating phosphates from actual
wastewater, a secondary effluent, was evaluated in a series of batch type pre-
cipitation experiments. The experiments were conducted as a function of pH
using a 2:1 lanthanum-to-phosphate molar ratio. The residual concentrations of
phosphate following precipitation with lanthanum are plotted in Figure 13 as a
function of final solution pH. (Results obtained using aluminum salts on the same
effluent are also shown; these will be discussed later when results with La(III)
and Al(III) are compared.)
It may be seen, comparing the lanthanum results in Figure 13 with those
shown in Figure 6, that the phosphate removal from wastewater exhibits a de-
pendence on pH similar to the phosphate removal from pure solutions. With the
secondary effluent, a minimum residual phosphate concentration of 0.01 mg/f. P
or less was obtained in the pH 5.6 to 7.6 range. Also, the residual phosphate
concentration was less than 0.1 mg/j? P in the pH 4.4 to 8.8 range. The corre-
sponding range with a somewhat more concentrated pure orthophosphate solution
was from pH 4 to 9. As with pure phosphate solutions, the optimum pH for phos-
phate removal from wastewater with lanthanum is fairly broad and includes those
pH levels normally encountered in the effluents from biological wastewater treat-
ment plants. It is apparent, therefore, that in the treatment of wastewater with
lanthanum there would not be any need for costly pH adjustments, e.g., through
addition of acid.
27
-------
1000.0
100.0
I
r 10.0
o
s
UJ
u
1.0
o
oc
o
9
(A
UJ
OC
0.1
0.01
Al (III)
La (III)
LIMIT FOR P ANALYSIS
0.001
6
SOLUTION pH
8
10
12
Figure 13. Comparison of the Effectiveness of Lanthanum and Aluminum for the Removal
of Orthophosphate from Secondary Effluent at a 2:1 Cation-to-Orthophosphate Molar
Ratio (Initial Orthophosphate Concentration, 7.75 mg/j? P; Initial pH = 7.8)
70-A15-032-29
28
-------
In the experiments with lanthanum and secondary effluent, the filtrate from
the sample at pH 7.0 (phosphate residual less than 0.01 mg/jg P) was analyzed for
residual lanthanum using an extraction flame photometric technique. Despite
the use of excess lanthanum in the precipitation experiment (use of 2:1 La/PO.~
molar ratio), no lanthanum was detected in the pH 7.0 filtrate in contrast to the
high lanthanum residuals found in the comparable experiment with pure solutions.
The fact that this excess lanthanum, is precipitated is of significant importance
since in practice an excess amount of lanthanum could be used to ensure complete
removal of phosphate without encountering any significant loss of the lanthanum
in the effluent wastewater.
Comparison Between Lanthanum, and Aluminum Salts for Precipitating Phosphates
As stated earlier, the principal objective of the present study was to evaluate
the effectiveness of lanthanum for the removal of phosphates with a supplementary
objective to compare the lanthanum precipitation results with similar data ob-
tained using aluminum. The data on the lanthanum precipitation of phosphate from
pure solutions and from secondary effluent were presented and discussed in the
previous sections. In this section the lanthanum data are compared with the data
obtained using aluminum. The data on the precipitation of phosphate with alumi-
num will not be discussed in detail but only cited for comparison purposes. The
mechanism, rate and stoichiometry of phosphate precipitation with both aluminum
and ferric salts have been studied in detail in a separate investigation; the data
presented herein on the aluminum-phosphate reactions have been directly taken
from that studv. Since the behavior of aluminum-phosphate and ferric-phosphate
(5)
systems have been shown to be similar in many respects, most of the conclu-
sions drawn from the comparison of lanthanum with aluminum may also be ap-
plied to the comparison of lanthanum with ferric iron.
Studies of the aluminum-orthophosphate reaction* have indicated a slight
broadening of phosphate residual - pH curve on the basic side when the pH adjust-
ments are concurrent with the addition of the cation, rather than by prior addi-
tion of acid or base. However, the difference is not of sufficient magnitude to be
considered in the present discussion; for all practical purposes the data on alu-
minum-or thophosphate and lanthanum-orthophosphate reaction are directly com-
parable.
29
-------
The relative effectiveness of lanthanum and of aluminum in precipitating
ortho- and polyphosphates may be seen in the results shown in Figures 13 through
18. Lanthanum may be seen to give superior performance in the following ways.
The pH range for optimum removal of phosphate with lanthanum is considerably
broader than that for aluminum; this is especially pronounced in the precipitation
of condensed phosphates. For example, at a 2:1 aluminum-to-polyphosphate
equivalence ratio (Figures 17 and 18) practically no phosphate is removed at pH
levels ±1 unit from that for maximum phosphate removal; with lanthanum, on
the other hand, the corresponding polyphosphate residual is less than 0.01 mg/4 P
in the pH 7 to 9 range and is less than 1.0mg/£ Pfrom about pH 4 to approximately
pH 10.
On an equivalence basis, lanthanum is far more effective than aluminum for
the precipitation of phosphates. At a pH of 7.0 (Figure 7), complete removal of
orthophosphate was obtained with a lanthanum-to-orthophosphate molar ratio of
slightly less than 0.9:1.0 (i.e., at this ratio of the reactants, the phosphate re-
sidual was 0.01 mg/j? P or less). On the other hand, the minimum residual phos-
phate concentrations observed with aluminum at 1:1 and at 2:1 cation-to-ortho-
phosphate molar ratios were 3.5 mg/i P and 0.10 mg/Jt P, respectively (see Fig-
ures 15 and 16). Furthermore, as indicated in Figure 7, the efficiency of phos-
phate removal with aluminum (at the optimum pH of 6) decreases as the alumi-
num-to-phosphate molar ratio exceeds 1.0. With lanthanum, on the other hand,
up to the point of complete phosphate removal, the phosphate removal efficiency
remains independent of the cation-to-phosphate molar ratio used.
The reaction of aluminum with tripolyphosphate (Figure 18), using a 21.6
mg/i P solution of Na5P~O10, gave at a 2:1 ratio, a minimum residual phosphate
concentration of 3.8 mg/jE P at a pH of 5.3. At a 1:1 ratio, no removal of phos-
phate was observed at several pH levels examined within the pH 4.8 to 6.5 region.
The minimum residual tripolyphosphate concentrations obtained under the same
conditions with lanthanum-to-phosphate ratios of 1:1 and 2:1 were 0.2 mg/iPand
0.01 mg/i P, respectively. As indicated in Figure 17, lanthanum is similarly
far more effective than aluminum in the precipitation of pyrophosphate.
With aluminum and secondary effluent as shown in Figure 13, a minimum
residual phosphate concentration of 0.04mg/i P was observed at pH 6.0; however,
30
-------
z
o
DC
t-
U
o
8
UJ
85
O
a.
O
K
cc
O
UJ
oc
SOLUTION pH
10
12
Figure 14. Comparison of the Effectiveness of Lanthanum and Aluminum for the Precipitation
of Orthophosphate at a 0.5:1 Cation-to-Orthophosphate Molar Ratio
(Initial Orthophosphate Concentration, 12 mg/j2P)
70-A15-032-37
31
-------
100
10
I
UJ
u
UJ
I
fe
O
Q.
O
oc
O
O
35
UJ
CC
1.0
0.1
0.01
0.001
LIMIT FOR P ANALYSIS—*
6
SOLUTION pH
10
12
Figure 15. Comparison of the Effectiveness of Lanthanum and Aluminum for the Precipitation of Orthophosphate
at a 1:1 Cation-to-Orthophosphate Molar Ratio (Initial Orthophosphate Concentration. 12 mg/jZP)
70-A15-032-38
32
-------
1000
100
10
<
tr
o
o
o
UJ
<
X
Q.
o.
O
oc
O
13
g
55
UJ
oc
1.0
0.1
0.01
0.001
La (III)
LIMIT
FOR P ANALYSIS
At (III)
•-0 -
CD OQE
6
SOLUTION pH
8
10
12
Figure 16. Comparison of the Effectiveness of Lanthanum and Aluminum for the Precipitation of Orthophosphate
at a 2:1 Cation-to-Orthophosphate Molar Ratio (Initial Orthophosphate Concentration, 12
70-A1 5-032-36
33
-------
1000
100
in
E 10
<
-------
1000
100
10
at
<
CC
LU
O
O
o
UJ
I
a.
O
a.
ID
9 0.1
a>
ai
oc
0.01
0.001
XX) OO
La (III)
8
10
12
SOLUTION pH
Figure 18. Comparison of the Effectiveness of Lanthanum and Aluminum for the Precipitation of Tripolyphosphate
at a 2:1 Cation-to-Tripolyphosphate Equivalence Ratio (Initial Tripolyphosphate Concentration. 21.6 mg/JJP)
70-A15-032-34
35
-------
the residual phosphate concentration was above 0.3 mg/l P at all pH levels out-
side the pH 4.6 to 6.8 range. Thus, it is seen that the pH range for optimum
phosphate precipitation with aluminum lies below pH = 7. The effectiveness of
lanthanum for phosphate removal, on the other hand, extends well above pH 7
into the alkaline range and includes those pH levels normally encountered in
domestic wastewater (7.0 to 8.0). This is a very desirable feature for applica-
tion of lanthanum to wastewater treatment since it eliminates the necessity for
costly pH adjustment by addition of acid or excess aluminum salt. A loss of
buffer capacity will occur with pH adjustment. This may in itself be undesirable
since it necessitates careful control of chemical addition to avoid wide pH fluc-
tuations. Furthermore, it has been demonstrated in other studies that the
addition of excess aluminum can result in the formation of poorly settleable floes
which cannot be effectively removed by plain sedimentation or by ordinary filtra-
tion methods. Also, if the discharge of acidic effluents is to be avoided, the pH
of the wastewater must be readjusted with lime or soda ash following the treat-
ment with aluminum.
The pH dependence of phosphate removal efficiency for both lanthanum and
aluminum can be explained in terms of two competing reactions, hydrolysis and
precipitate solubility. Hydrolytic products of both Al(III) and La(III) with their
smaller ionic charge would be less effective in precipitating phosphates than the
trivalent cation species. Since the extent of cation hydrolysis increases with the
rise in the concentration of OH~ ions, the efficiency of the cations for phosphate
removal tends to decrease as the pH is raised. At lower pH levels where L/a(III)
and Al(ni) are less hydrolyzed, an increased solubility of the precipitates is
brought about by a decrease in the concentration of the orthophosphate ion. The
extent of the pH-dependence of the two reactions (cation hydrolysis and precipi-
tate solubility) determines the extent of the pH range of phosphate removal.
Since La(III) hydrolyzes to a lesser extent than Al(III), and has a lower phos-
phate solubility, it is more effective than Al(III) for phosphate removal.
Jar Test Reaction Rate Studies with La(III) and Al(III)
Preliminary tests were made using the "jar test" procedure to determine
the rate at which La (III) and Al(III) reacted with ortho- and pyrophosphates to
form a precipitate and thus remove the phosphate from the system. Changes in
36
-------
pH and conductivity were used to measure the course of the precipitation reac-
tions. In all cases examined, the recorded changes in pH and conductivity all
took place within 10 to 60 sec after the addition of the lanthanum or aluminum
salt solution. In one experiment with Al(III), aliquots of solution were removed
from the reaction beaker at selected intervals following addition of aluminum
nitrate, vacuum filtered, and the filtrates analyzed for residual phosphate. The
results indicated that the initial drop in pH and conductivity which took place
within 30 to 60 sec following additions of aluminum nitrate was accompanied by
a drop in phosphate concentration from 12 mg/j£ P to 0.10 mg/CP. No further re-
moval of phosphate was observed after this period despite the very noticeable
gradual growth and agglomeration of the floes.
An HCl-NaOH neutralization experiment, conducted under identical mixing
conditions, gave patterns of pH and conductivity change identical to those ob-
served in the phosphate precipitation experiments. Since this type of acid-base
reaction is known to be practically instantaneous, it was postulated that the
aluminum-phosphate and the lanthanum-phosphate reactions which result in
changes in pH and conductivity might also be instantaneous, and that the meas-
ured time delay in attaining constant pH and conductivity levels could in fact be
due to the time required for mixing the added reagents.
Reaction Rate Studies Using the Reaction Kinetics Apparatus
As was discussed in the section "Kinetics Apparatus," a reaction kinetics
apparatus was designed and built to permit monitoring of pH and residual phos-
phate concentration within a very short time following mixing of the phosphate
and the lanthanum or aluminum salt solutions. In the first tests with this equip-
ment, only the pH of the solution mixture leaving the reaction flask (see Figure
1) was monitored. The pH data collected at several points in the flow line showed
essentially constant values for both aluminum (6.4 to 6.6) and lanthanum (8.5 to
8.6). This constancy clearly indicated that within the period of observation the
reactions between orthophosphate and aluminum and lanthanum are completed
within 1.3 sec.
In experiments where phosphate removal data were collected, the reaction
kinetics apparatus was modified so that the time of solution travel to the first
sampling port was reduced from 1.3 sec to slightly less than 1 sec. In these
experiments, 0.5:1 lanthanum-to-orthophosphate and 1:1 aluminum-to-or tho-
phosphate molar ratios were used.
37
-------
The pH of the solution mixture was monitored at port B (see Figure 1) dur-
ing sample collection. A slight gradual increase in pH was observed in almost
all cases. It appears, however, that this rise in pH was not real but rather due
to instrumental factors (probably brought about by the gradual decrease in the
rate of solution flow around the pH probe electrode). Inmost cases a sample of
solution mixture was collected at the discharge point of the kinetics apparatus
and flocculated in the Phipps and Bird stirring apparatus. The analyses of fil-
trates from all the samples showed residual concentrations of 3.90 mg/j? P with
lanthanum and 4.12 mg/l P with aluminum. No changes occurred in residual
phosphate beyond that observed at the first sampling port.
These results indicate that the reactions between La(III) and Al(III) and
orthophosphate which results in precipitate formation and removal of phosphate
from solution are complete in less than 1 sec and are probably nearly instanta-
neous. Even when samples of the solution mixture discharged from the kinetics
apparatus were flocculated by gentle mixing, the growth and agglomeration of
the floes was not accompanied by any additional removal of the phosphate rela-
tive to that obtained by millipore filtration through the sample ports.
Although no phosphate removal data were collected on the reactions of
La(III) and Al(III) with polyphosphates, the constancy of pH observed Ln the batch
pyrophosphate precipitation experiments indicates that, at or near the pH of opti-
mum phosphate removal, the cation-pyrophosphate reactions are also very rapid.
From the results of these kinetic studies, the requirement of addition of a
molar excess of the aluminum or other metal salts to obtain essentially complete
phosphate removal cannot be attributed to inadequate time allowed for precipitate
formation. Instead, as discussed before, this can be satisfactorily explained by
the occurrence of competing reactions and the dispersion of the phosphate pre-
cipitates into extremely fine, nonsettleable particles.
Nature of La(III) and Al(III) Orthophosphate Precipitates
In order to learn more about the precipitates being formed in the reactions
of orthophosphate ion with La (III) and Al(IH), thermogravimetric measurements
and x-ray diffraction studies were carried out. These were done as described
earlier in the section, "Thermogravimetric Experiments."
38
-------
The aluminum precipitates did not reach a constant weight even after a 9-
day storage period in the desiccator. However, the extent of weight loss after
the 7th day was very small. Accordingly, the weight of this precipitate after
the 9th day was used in calculating the total water loss at room temperature,
and the percentage weight loss which resulted on subsequent heating at 104°C.
The percentage of total weight loss resulting from room temperature drying is
substantially lower for the lanthanum precipitates (0.73%) than for the aluminum
precipitate (9.72%). If these values of total weight loss, based on the weight
change of the precipitate in the desiccator after one day of drying, represent
approximately the initial unbound water contents of the precipitates, the lantha-
num phosphate precipitate is seen to have a lower unbound water content. This
constitutes a desirable feature for large-scale application of lanthanum salts to
the removal of phosphates from wastewater.
The precipitates, on heating at 104°C, were found to be somewhat hygro-
scopic and slight increases in weight were observed following the first weighing
after each heating. Accordingly, the smallest weights observed were used in
calculating the weight losses resulting from heating at 104°C. Essentially no
further change in weight was observed following the first 2 hr of ignition at600°C.
To summarize these thermogravimetric results, lanthanum and aluminum-
orthophosphate precipitates, obtained under the particular precipitation condi-
tions used, gradually lose weight when heated at 104°C. The total percent weight
loss resulting from 6 hr of heating at 104°C, based on the dry weight at room
temperature, was 9.9% for the aluminum precipitate and 2.3, 2.4, and 2.5% for
the three lanthanum precipitates, one of which had undergone only 4 hr of heating.
Correcting for the small portion of the precipitate removed for x-ray analysis
from one lanthanum sample, the total weight losses on heating at 104°C and at
600°C were 17.5% for the aluminum and 9.8%, 10.3%, and 10.3% for the lanthanum
precipitates. If the assumption is made that the total loss in weight on heating
represents water bound in LaPC«4.nH2O, the value of n is approximately 1.5.
Fresh, desiccator-dried and ignited precipitates of aluminum and the fresh
precipitate of lanthanum were found to be amorphous on examination by x-ray
diffraction. The lanthanum precipitate dried at 104°C and the residue resulting
from ignition at 600°C were found to be crystalline with the latter exhibiting a
much sharper diffraction pattern. The material was identified as LaPO4 on the
basis of its x-ray diffraction pattern.
39
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Recommendations for Future Work
The basic objectives of the study described in this report have been to
assess the potential use of lanthanum salts for the removal of phosphates from
aqueous systems and to compare the results with the phosphate precipitation
data obtained using aluminum salts. A pure lanthanum salt was used in this pre-
liminary study to represent the lanthanide family. Except for a few precipita-
tion experiments in which actual wastewater was used as the phosphate-containing
solution, most of the precipitation studies were conducted on pure phosphate solu-
tions. Lanthanum has been found to be so effective in precipitating phosphates
and so superior to aluminum that further experimental work is recommended in
order that the lanthanum process be developed for large-scale application to the
treatment of wastewater for the removal of phosphates.
The following areas of research and development are recommended as logi-
cal extensions of the present study:
1) Laboratory evaluation of the effectiveness of lanthanum and mixed
lanthanides for removing phosphates from secondary effluent obtained
from several different sources and comparison of the results with
similar data obtained using Al(III) or Fe(III) salts.
2) Characterization of the lanthanum (and mixed lanthanide) precipitates
obtained in the treatment of secondary effluent for phosphate removal.
Parameters to be evaluated should include: settleability, filterability,
and strength (resistance to shear) of the floes; compaction and dewater-
ability of the sludge; and means of improving and effecting the solid-
liquid separation. For the purpose of comparison, some limited data
should also be obtained using Al(in) or Fe(III) salts.
3) Evaluation of lanthanide recovery methods using both pure phosphate
solutions and secondary effluent. Criteria to be defined should include
optimum regeneration conditions, extent of chemical losses and the
effectiveness of the recovered lanthanide s for precipitating phosphates.
4) Laboratory-scale pilot plant evaluation of the lanthanide treatment
method for the removal of phosphates from secondary effluent.
40
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5) BLoassay studies of the possible effects of lanthanides on living
organisms.
6) Application of the lanthanide treatment process to a small scale
wastewater treatment operation and collection of field data on the
basis of which the economics of the treatment process could be
assessed with reasonable accuracy.
SUMMARY
Atomics International has conducted a parametric investigation of phosphate
removal from aqueous systems using lanthanum precipitation in which the effect
of pH and reagent concentration on the rate and the efficiency of phosphate re-
moval were specifically evaluated. The phosphate precipitation data obtained
with lanthanum are compared with similar data obtained using aluminum.
Pure solutions of orthophosphate at concentrations representative of those
in wastewater, and of pyrophosphate and tripolyphosphates as well as effluent
from an activated sludge wastewater treatment plant were used in this investi-
gation. Reaction rate studies were conducted using both batch precipitation and
steady-state reagent flow in a specially-designed reaction kinetics apparatus
which permitted rapid mixing of the reactant solutions and subsequent monitor-
ing of pH and residual reactant concentrations of the mixed stream. The effects
of pH and reactant concentrations on the efficiency of phosphate removal were
evaluated in batch precipitation experiments. The precipitates obtained in the
reactions of orthophosphate with lanthanum and aluminum were examined by x-
ray diffraction and were characterized by weight loss in thermogravimetric ex-
periments.
Reaction rate studies showed that the precipitation of orthophosphate with
both aluminum and lanthanum, which results in the removal of phosphate from
solution, is very rapid and is completed in less than 1 sec. No further removal
of phosphate is effected following the initial drop in the concentration of soluble
phosphate.
Orthophosphate removal with lanthanum was found to depend on both pH and
the concentration of added lanthanum. The pH range for optimum phosphate re-
moval with lanthanum was found to be quite broad. For an initial orthophosphate
41
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concentration of 12 mg/l P and a lanthanum-to-phosphate molar ratio of 0.5:1,
the residual phosphate concentration was between 4.8 and 5.6 mg/l P in the pH
4 to 9 region, with the minimum at pH 5.0. At 1:1 and at 2:1 lanthanum-to-
orthophosphate molar ratios, the residual phosphate concentration was less
than 0.01 mg/l P in the pH 5 to 9 range.
When the pH was kept constant, the removal of orthophosphate was found
to be directly proportional to the concentration of added lanthanum. Nearly
complete removal of phosphate was observed at a La(III)/PO4 molar ratio of
slightly less than 0.9. The existence of such a constant stoichiometric rela-
tionship strongly suggests that a chemical reaction is occurring between the
lanthanum and the orthophosphate and that the phosphate removal is not occur-
ring by a mechanism involving adsorption (physical or chemical) on a precipitat-
ing metal hydroxide.
Precipitates obtained in the reactions of orthophosphate with lanthanum and
aluminum were examined by x-ray diffraction and were characterized by weight
loss on air drying and heating at 104° and at 600°C. The aluminum precipitate
was found to be amorphous under x-ray examination. The lanthanum precipitate
heated at 104°C and the residue resulting from its ignition at 600°C were both
found to be crystalline with diffraction patterns corresponding to those of lan-
thanum phosphate (LaPO A
Lanthanum was also found to be effective in precipitating condensed inor-
ganic phosphates. For an initial pyrophosphate concentration of 18 mg/l P,
minimum phosphate residuals of 0.05 mg/l P(at pH 5.6) and 0.002 mg/l P (avpH
8-9) were obtained with lanthanum-to-pyrophosphate equivalence ratios of 1:1
and 2:1, respectively. For an initial tripolyphosphate concentration of 21.6 mg/l P,
minimum residual tripolyphosphate concentrations were 0.18 mg/l P (at pH 6)
and 0.007 mg/l P (at pH ~9) at these same ratios of the reactants, respectively.
Within the range of optimum phosphate removal, the reaction of lanthanum
with both ortho- and polyphosphates resulted in the formation of large, settleable
floes. Immediately outside this pH range fine turbidity developed which did not
settle out very well. At higher pH levels, some .residual turbidity was observed
in almost all cases. No turbidity or precipitate formation was observed at very
low pH levels with either ortho- or polyphosphate.
42
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The effectiveness of lanthanum in precipitating phosphates from actual
wastewater (secondary effluent) was evaluated as a function of solution pH using
a 2:1 lanthanum-to-orthophosphate molar ratio. The effluent used in the precipi-
tation experiments had a pH of 7.8 and contained 7.75 mg/j? P orthophosphate and
no condensed phosphates. A minimum residual phosphate concentration of less
than 0.01 mg/i P was observed in the pH 5.6 to 7.5 range. Also, the phosphate
residual concentration was less than 0.1 mg/j? P in the pH 4.4 to 8.8 region. The
filtrate from the precipitation experiment at pH = 7.0 was analyzed for residual
lanthanum content using an extraction - flame photometric technique which was
shown to be capable of detecting lanthanum concentrations as low as 0.5 mg/4 P.
_3
Despite the addition of excess lanthanum (use of 2:1 La(III)/PO4 molar ratio),
no lanthanum was detected in the sample filtrate.
Comparison of the lanthanum phosphate removal data with the similar data
obtained using aluminum indicates that lanthanum is far superior for the precipi-
tation of phosphates. For the 12 mg/j? P orthophosphate solution, the minimum
residual phosphate concentrations with aluminum were 6.8, 3.5, and 0.09 mg/£P
for the conditions of 0.5:1, 1:1, and 2:1 aluminum-to-orthophosphate molar ratios,
respectively. The corresponding values of minimum residual orthophosphate
concentrations obtained with lanthanum were 4.8, <0.01, and <0.01 mg/£ P, re-
spectively. The superiority of lanthanum to aluminum is especially pronounced
in the precipitation of condensed phosphate. For example, at a 1:1 cation-to-
phosphate equivalence ratio, no removal of tripolyphosphate was obtained with
aluminum at several pH levels examined. Lanthanum, on the other hand, showed
good removal of tripolyphosphate over a broad pH range with a minimum residual
phosphate concentration of 0.18 mg/i P observed at pH 6. At a 2:1 cation-to-tri-
polyphosphate ratio, the minimum residual phosphate concentrations with alumi-
num and lanthanum were 3.8 and 0.007 rag/* P, respectively.
Lanthanum exhibits a considerably broader effective pH range for phosphate
removal than observed with aluminum; this is particularly true in the case of
polyphosphates. For example, at 2:1 equivalence ratios, practically no removal
of pyro- and tripolyphosphates was observed with aluminum at pH levels ±1 unit
from that for optimum phosphate removal (pH ~5.5). In the case of tripolyphos-
phate precipitation with lanthanum, the residual phosphate concentration was less
than 0.1 mg/t P between pH's of 6 and 10 and was less than 1.0 mg/S. P in the 4 to
10.5 pH range. One very attractive feature of the use of lanthanum for the removal
43
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of phosphates from wastewater is that the pH of almost all domestic wastewaters
lies within the pH range where lanthanum is most effective. To reach the opti-
mum precipitation pH of about 6 with aluminum, large quantities of acid, or a
considerable excess of aluminum, must be added to the wastewater to overcome
its natural buffer capacity.
Based on the results of this investigation, it is evident that lanthanum salts
show great potential for removal of phosphates from wastewater, and further
studies including pilot plant testing appear warranted.
44
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REFERENCES
1. C. N. Sawyer, "Some New Aspects of Phosphate in Relation to Lake Ferti-
lization," Sew, and Ind. Wastes. 24, 768 (1952)
2. M. S. FinsteLn and J. V. Hunter, "Hydrolysis of Condensed Phosphates
During Biological Sewage Treatment," Water Res., J_, 247 (1967)
3. C. N. Sawyer, "Fertilization of Lakes by Agricultural and Urban Drainage,"
Jour. N. E. Water Works Assn., ^J., 1925 (1944)
4. J. B. Nesbit, "Removal of Phosphorus from Municipal Sewage Plant Efflu-
ents," Engineering Research Bulletin B-93, The Pennsylvania State Univer-
sity, February 1966
5. "Parametric Study of the Kinetics of Precipitation and Nature of the Precipi-
tate Obtained in Phosphate Removal Using Iron and Aluminum Salts," Con-
tract No. 14-12-158, between the FWPCA and the Atomics International Div.,
North American Rockwell Corporation
6. W. L. Lea, G. A. Rohlich, and W. J. Katz, "Removal of Phosphates from
Treated Sewage," Sew, and Ind. Wastes. 26, 261 (1954)
7. W. Stumm, Discussion in "Advances in Water Pollution Control Research,"
Proc. 1st Intl. Conf. Water Poll. Res., Pergamon Press Ltd., London,
England, Vol 2, 216 (1964)
8. R. Owen, "Removal of Phosphorus from Sewage Plant Effluent with Lime,"
Sew. Works. 25, 548 (1953)
9. E. V. Kleber and B. Love, The Technology of Scandium, Yttrium, and the
Rare Earth Metals, Pergamon Press (1963)
10. Standard Methods for the Examination of Water and Wastewater. APHA,
AWWA and FWPCA, 12th Ed. (1965)
11. D. Menis and T. C. Rains, "Extraction and Flame Spectrophotometric De-
termination of Lanthanum." Anal. Chem.. 31. 187 (1959)
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Accession Number
Subject Field St Group
SELECTED WATER RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
Organization
Title
J Q Authors)
16
Project Designation
21 Note
22
Citation
23
Descriptors (Starred First;
Identifiers (Starred First)
27
Abstract
A parametric study was made of the removal of ortho- and polyphosphates from pure
solutions and secondary effluent with La(III), and the results compared with those from
gfm-ny tests with Al(IU). The reactions of orthophosphate with both La(III) and Al(III),
resulting in precipitate formation and phosphate removal, were complete in less than 1 sec.
La(III) showed a broader effective pH range and lower residual phosphate concentration and
thus proved to be far superior to Al(III) for phosphate, especially polyphosphate, precipi-
tation. Within the pH range for optimal phosphate removal, the La(III)-phosphate reactions
produced large, settleable floes. Immediately outside this range, poorly settling fine
turbidity developed. At pH levels above this region, some residual turbidity was generally
observed. No turbidity or precipitation was observed at very low pH levels. At constant
pH, orthophosphate removal was directly proportional to the La(IU)/PO. ~3 molar ratio, with
complete removal at a ratio of ~ 0.9- This strongly suggests that orthophosphate removal
with La(UI) occurs solely through a chemical reaction and not through an adsorption pro-
cess. W.th secondary effluent, as with pure solutions, lower residual phosphate resulted
over a wider pH range with La(III) than with Al(III) ; no pH adjustment is needed for effec-
tive removal with La(III), while considerable amounts of acid or excess Al(III) are re-
quired to achieve the optimum pH of 6 for phosphate removal.
Abstractor
Institution
WN:10S (REV. JULY >•«•)
wwtic
SEND TO: WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON. D. C. 20240
* U.S. GOVERNMENT PRINTING OFFICE : 1870 O - 409-149
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