United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-91/063 Mar. 1992
EPA Project Summary
Radium Removal from Water by
Manganese Dioxide
Adsorption and Diatomaceous
Earth Filtration
Rahul Patel and Dennis Clifford
This study reveals that radium ad-
sorption onto precipitated MnO2 fol-
lowed by diatomaceous earth (DE)
filtration is a very effective treatment
process for radium-contaminated wa-
ter. Radium removals in the range of
80% to 97% were observed for pre-
formed MnO2 feed concentrations of 0.63
and 1.26 mg/L as Mn in groundwaters
with hardness in the range of 100 to 245
mg/L as CaCO3. Radium removal in-
creased slightly with increasing pH
whereas it decreased slightly with in-
creasing hardness and iron (II) concen-
trations.
Pilot studies were performed in
Lemont, IL, using DE filtration and mul-
timedia filtration on a groundwater con-
taining 12 pCi/L 226Ra and 6 pCi/L "'Ra.
Radium removals for both pilot plants
ranged from 90% to 97% at a MnO2 feed
concentration of 1.26 mg/L as Mn, a
total hardness of 245 mg/L as CaCO3,
and a pH of 6.5. The costs of water
treatment by MnO2 adsorption and DE
filtration were estimated at $0.71 per
1000 gal for 280,000 gpd plants and
$0.47 for 1 Mgd plants. These costs are
competitive with ion exchange soften-
ing for similar plant capacities.
This Project Summary was devel-
oped by EPA's Risk Reduction Engi-
neering Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Radium is an alkaline-earth metal with
chemical and physical properties similar to
the other elements in this group—calcium,
magnesium, barium, and strontium. Ra-
dium, found primarily in groundwater sup-
plies, has three naturally occurring isotopes,
^Ra, ^Ra, and ^Ra. ^Ra, a member of
thorium series, is a weak beta emitter with
a half life of 5.7 yr. ^Ra, a member of
uranium series, is a long-lived alpha emit-
ter with a half life of 1620 yr. ^Ra, the
second generation progeny of ^Ra, is a
short-lived alpha emitter with a half life of
3.64 days. All radium isotopes are bone
seekers; ^Ra is considered to be the most
detrimental radium isotope because it is so
long-lived, and it has five alpha-emitting
progeny including its immediate daughter
^Rn, the inert gas radon.
Based upon the potential health haz-
ards of radium, the maximum contaminant
level (MCL) of total radium (^Ra + ^Ra)
is currently 5 pCi/L. Analysis of ^Ra is,
however, only required if activity of ^Ra
exceeds 3 pCi/L Using the compliance
data from 50,000 public water systems,
the U.S. Environmental Protection Agency
(EPA) estimated that 500 systems exceed
the radium MCL of 5 pCi/L. The highest
^Ra concentration reported was approxi-
mately 200 pCi/L. Few supplies exceed 50
pCi/L, however, and about two-thirds of
the supplies exceeding 5 pCi/L are below
10pCi/L
Several radium removal technologies,
including reverse osmosis, sodium ion-ex-
change softening, and lime-soda soften-
ing, have been applied for small community
Printed on Recycled Paper
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water supply treatment but not widely used
because of cost and complexity. This has
led to the search for simpler methods of
radium removal. Some new technologies
now in the experimental stage involve ad-
sorption onto one of the following
adsorbents: a BaSO4-impregnated cation
resin called the DOW radium selective
complexer (RSC); microbial biomass;
BaSO4-impregnated and plain activated alu-
mina; manganese-impregnated acrylic fi-
bers; filter sand; and iron and manganese
precipitates. This report on removing ra-
dium deals with adsorbing radium onto
MnO2 and then removing the manganese
precipitate by filtration.
Manganese dioxide (MnO2 or MnO2(s)),
often called hydrous manganese oxide
(HMO), was chosen as the radium adsor-
bent because it had been used in several
previous studies to abstract radium from
both seawater and groundwater. Dtl was
chosen as the filter media for most of the
experiments because it is effective in re-
moving fine particulates like MnO2 from
water.
The major objective of this study was to
determine the feasibility of MnOS! adsorp-
tion followed by DE filtration for removing
radium from water. Additional objectives
were to:
(a) determine the effects of pH, MnO2
concentration, total hardness, iron
and manganese concentration, and
the method of iron and manganese
oxide formation on radium removal,
(b) determine radium loadings on the
MnO2 surface as a function of radium
concentration in the water,
(c) determine the magnitude of MnO2
leakage into the filter effluent,
(d) compare multimedia depth filtration
to DE filtration for MnO2 and radium
removal on a pilot scale, and finally,
(e) estimate the cost of the MnO2 ad-
sorption/DE filtration process for small
community water supply treatment.
Materials and Methods
The bench- and pilot-scale experimen-
tal work was done in several stages. Pre-
liminary screening experiments were
performed by precipitating MnO2 in ^Ra-
spiked Houston groundwater and then fil-
tering the suspension with a paper filter in
a Buchner funnel or a DE-coated filter to
determine the feasibility of MnO2 adsorp-
tion followed by DE filtration as a treatment
process. In these bench-scale tests, the
pH, hardness, radium, and MnO2 concen-
trations were varied to determine their ef-
fect on radium removal. Then, a 0.3 gpm
bench-scale DE filtration system was tested
with ^Ra-spiked Houston tap water to
determine radium removal efficiency in a
continuous process. Finally, two pilot plants
(DE filtration and multimedia filtration) were
run in Lemont, IL, to establish the perfor-
mance of each process with actual radium-
contaminated groundwater.
Preparation of Manganese Dioxide
Stock Solution
MnO2 stock solution was prepared us-
ing MnSO4-H2O and KMnO4. Reagent
grade chemicals were used in bench- and
pilot-scale experiments performed at the
University of Houston. Food-grade MnSO4
and water treatment grade KMnO4 were
used in the Lemont pilot studies. The pre-
cipitation of MnO2 is an oxidation/reduction
reaction in which MnSO4 (Mn(ll)) is oxi-
dized and KMnO4 (Mn(VII)) is reduced to
Mn(IV). The precipitation reaction is:
3MnSO4-H2O + 2KMnO4
> 5MnO2(s) + K2SO4
+ 2H2SO4 + H2O
First, the predetermined amount of
MnSO4-H2O was added to water and mixed
thoroughly before adjusting the pH into the
8 to 9 range by adding NaOH. KMnO4 was
then added gradually while NaOH was
added periodically to maintain the pH be-
tween 8 and 9. A 5% stoichiometric excess
of KMnO4 was added to achieve complete
oxidation of manganous ion and to oxidize
iron in the water. To make a stock solution
of 1000 mg/L MnO2, 1166 mg MnSO4'H2O
and 738 mg KMnO4 were added to a liter of
water. This preformed MnO2 slurry was
kept in suspension by continuous mixing
and was normally used within 24 hr. This
suspension was fed to the raw water to
achieve the desired MnO2 concentration in
the feedwater.
Bench-Scale Experiments with
Houston Groundwater
^Ra-spiked Houston groundwater was
adjusted to a pH of 7.5 (except in pH
studies), and then placed into a 2-L polypro-
pylene beaker where preformed MnO2 was
added. For the MnO2to adsorb the radium,
this water was then magnetically stirred for
1 hr to equilibrate radium with MnO2. To
prepare a DE filter, a slurry containing 3.11
g of DE was distributed onto a 90-mm filter
placed into a Buchner funnel where a 0.1
Ib/ft2 DE coating on the filter paper was
formed by vacuum filtration. The feedwater,
containing radium equilibrated with MnO2,
was then vacuum filtered through this DE-
coated filter. In the screening experiments,
the filtrate was collected and passed
through the same DE filter a second time
to ensure equilibration and good filtration.
The filtrate was then analyzed for radium
and manganese concentrations. All the
experiments were carried out in duplicate,
and a new filter was used for each experi-
ment. Controls were also run in which
MnO2-free feedwater was filtered through
a DE-coated filter.
Radium activity and MnO2 concentra-
tion were varied to determine their effects
on radium removal. In these experiments,
Houston groundwater was spiked with ei-
ther 13 or 25 pCi/L of ^Ra, and the pre-
formed MnO2 concentrations were
maintained at 0.05, 0.15, 0.35, 0.63, or
1.26 mg/L as Mn.
To determine the effect of pH on ra-
dium removal, the raw water pH was ad-
justed to 6.5,7.5, 8.5, or 9.5 with the use of
1N NaOH or concentrated HCI. Total hard-
ness values of 60,120, 250, and 500 mg/L
as CaCO3were used to establish the effect
of hardness on radium removal. Because
Houston groundwater had a total hardness
of 120 mg/L as CaCO3, the 60-mg/L total
hardness sample was prepared by adding
equal amounts of deionized (Dl) water and
Houston groundwater. The 250 and 500
mg/L total hardness samples were pre-
pared by adding CaCI2«2H2O and
MgCI2'6H2O salts in the 3:1 Ca:Mg ratio
existing in the groundwater.
Experiments were also done to com-
pare the addition of preformed MnO2 with
the simultaneous, in-situ oxidation of iron
and manganese to Fe(OH)3 and MnO2,
respectively. The in-situ preparation of
MnO2 was carried out by adding MnSO4 to
the feedwater. Then, KMnO4 was gradu-
ally added to oxidize MnSO4 to MnO2. In
another experiment, manganese and iron
were simultaneously oxidized in the
feedwater to establish the influence of iron
on radium removal by MnO2 adsorption.
Experiments were also done to compare
the addition of preformed Fe(OH)3 with in-
situ oxidation/hydrolysis of ferrous ion to
Fe(OH)3 with the use of CI2.
Continuous Bench-Scale
Experiments with Houston Tap
Water
A laboratory apparatus was assembled
for the purpose of running continuous ra-
dium removal experiments with MnO2 ad-
sorption followed by DE filtration. Two
55-gal tanks were used for radium-spiked
feedwater. 226Ra-spiked Houston tap water
(pH a 7.5 and radium concentration =. 25
pCi/L) was pumped at a rate of 0.3 gpm
(0.1 gpm/ft2 of surface area) and preformed
MnO2 was blended in-line to achieve a
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MnO2 concentration in the feedwater at
0.63 mg/L as Mn. The water was then
passed through an Everpure* point-of-use
DE filter and subsequent radium removals
were measured. The DE filter surface area
was about 3 ft2 with 100 g of DE filter
media impregnated on it. The water/MnO2
contact time ahead of the filter was only 10
sec, so another experiment was clone to
determine the effect of mixing and MnO2
contact time on radium removal. In this
"greater mixing" case, the water was re-
cycled, after MnO2 addition, at three times
the flow through the filter, i.e., a quarter of
the flow was passed through the filter while
three quarters was recycled. A long tubing
of small diameter was added in-line to
increase the turbulence and mixing of MnO2
with the feedwater. With recycle, the con-
tact time was increased to about 60 sec. A
control run was also done to determine
radium removal by the DE filter when no
MnO2 was added to the feed.
Pilot-Plant Studies with Lemont
Groundwater
We found results of the bench-scale
experiments sufficiently encouraging to fur-
ther pursue radium removal studies in
Lemont. There, two pilot plants were run in
parallel using MnO2 as an adsorbent. One
pilot plant was a DE filtration system with a
1 -ft2 filter, and the other was a multimedia
filter consisting of coal, sand, and ilmenite
(10-in. height of each) packed in an 8-in.
diameter column. A flow schematic of the
pilot plants is depicted in Figure 1.
Diatomaceous Earth Pilot
Plants
Two DE pilot plants were obtained from
Manville Company. One was a 3,.14-in.2-
filter pilot plant, and the other was a 1-ft2-
fitter pilot plant. The smaller pilot plant was
used to optimize process parameters for
DE filtration. The 1 -ft2 pilot plant was then
used to perform the actual runs.
Optimization of Process
Parameters for DE Filtration
To optimize process parameters, efflu-
ent manganese concentrations were de-
termined for three commercially available
grades of DE: Celite 560, Celite 545, and
Hyflo Super-Gel. Hyflo Super-Cel was the
finest of the three, whereas Celite 560 was
the coarsest. Penetration of black colloidal
particles of MnO2 into the filter cake was
* Mention of trade names or commercial products does
not constitute endorsement or recommendation for
use.
also checked. To determine the optimum
body feed ratio, differential pressure and
run time were recorded and plotted for
body feed ratios of 2:1,5:1,10:1, and 15:1.
One experiment was also run with no body
feed, i.e., with precoat only. The optimum
body-feed ratio was determined from the
differential pressure versus run time curves
by calculating total water throughput per
gram of DE.
The DE pilot runs were made with the
1-ft2-filter pilot plant. The previously opti-
mized body feed ratio of 10:1 (20 mg/L DE
body feed) was used for all the runs. MnO2
stock solutions were made in a 30-gal
(114-L) tank using raw water. Stock solu-
tion was prepared every 12 hr and continu-
ously mixed. Another 30-gal tank, also
equipped with a propeller mixer, was used
as a feed tank in which MnOj/radium equi-
librium was achieved. Preformed MnO2 was
pumped into this tank to achieve a MnO2
concentration of 1.26 mg/L as Mn, and a
conservative residence time of 10 to 25
min was allowed for radium to adsorb onto
MnO2 surface. DE body-feed solution was
prepared in a 15-gal (57-L) tank, and this
solution was pumped in-line with feedwater
to achieve a feedwater DE concentration
of 20 mg/L. Both the precoating and filtra-
tion rates were 1 gpm/ft2. The effluents
were sampled every 4 hr, and the run
ended after about 20 hr, at which time the
pressure differential had reached 30 psi.
In one DE pilot run, there were two
deliberate stagnation periods of 8 and 16
hr. During each stagnation period, the flow
through the filter was stopped. Extreme
caution was used when closing and open-
ing the valves to the filter so as to minimize
cake dislodging. Nevertheless, some cake
was dislodged when restarting the run af-
ter the stagnation period. Hence, the efflu-
ent was sampled only after the water had
cleared up (about 10 to 15 min after the
start of the run). Another run was done to
determine the radium adsorption capacity
or desorption potential of radium-loaded
MnO2 already on the filter. In this run, the
feedwater containing MnO2, radium, and
DE body feed was passed for 20 hr, and
then only the radium-contaminated raw
water (MnO2-free) was passed through the
fitter cake for an additional 24 hr.
Multimedia Filter Pilot Plant
The multimedia and DE pilot plant
feedwaters were taken from the same tank
to compare the two means of post-filtra-
tion. The feedwater containing MnO2
passed at the rate of 1.75 gpm (5 gpm/ft2)
to the multimedia filter. The filter continu-
ously operated for 84 hr before a deliber-
ate 8-hr stagnation period was observed.
The filter was then restarted without
backwash ing and operated for 12 more
hours before another 16-hr stagnation pe-
riod. Finally, the filter was restarted and
operated for 12 more hours before termi-
nating the run. The effluents were sampled
every 12 hr for the normal run. Following
the stagnation periods, the effluent was
sampled immediately on the restart and
every 6 hr thereafter.
Analysis Methods
The Quality Assurance/Quality Control
(QA/QC) procedures incorporated into the
daily operation of the research included
good laboratory practices regarding sam-
pling, sample labeling, preservation, and
glassware washing. To guarantee confi-
dence in the experimental outcomes, all
important experiments were duplicated. All
the data and observations were promptly
written in log books with carbon copies
sent to the principal investigator. The qual-
ity assurance and radium analysis proce-
dures passed the scrutiny of an EPA
radioanalytical chemist and an EPA quality
assurance contract auditor.
Radium Analyses
Radium analysis procedures were cali-
brated with ^Ra standards obtained from
EPA's Environmental Monitoring and Sup-
port Laboratory (EMSL) in Las Vegas, NV.
All ^Ra activity measurements in this study
followed a modified EPA Method 900.1—
the gross radium alpha screening proce-
dure for drinking water with a carefully
controlled aging time to minimize inter-
ference from ^Ra. This procedure with
modified holding time had been used to
analyze more than 2,000 radium-contami-
nated water samples in Lemont before the
MnOg/DE pilot study.
During MnOj/DE research, approxi-
mately 200 water samples were analyzed
for ^Ra with the use of EPA Method 900.1.
In preparation for the research, the labora-
tory researcher performed and interpreted
approximately 100 analyses of standard
solutions to develop the calibration curve
and to achieve a high degree of analytical
competence.
Other Analyses
All the cation analyses in this study
were done with a Perkin-Elmer ICP 5500,
inductively coupled plasma spectrometer.
Standards run at the beginning and at the
end of every set of analyses determined
deviation of the standard curve during the
course of the analyses.
The miscellaneous analyses included
pH, temperature, and flow rates. The pH
meter was calibrated regularly using stan-
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Multimedia
Filter
Figure 1. Flow schematic of parallel-operated diatomaceous earth and multimedia pilot plants.
dard buffers. The flow rates were mea-
sured using a 1 -L graduated cylinder and
stop watch. Proper care was observed
during all the measurements to ensure
reliability.
Results and Discussion
Bench-Scale Experiments with
Houston Groundwater
Effect of MnO2 Concentration and
Feed Radium Activity
Figure 2 shows the effect of MnO2 con-
centration on radium removal when filter-
ing out the MnO2 with a DE-coated filter in
a Buchner funnel. The removal increased
from 30% to about 90% when MnO2 con-
centration was increased from 0.05 to 1.26
mg/L as Mn. The effluent radium activity
ranged from about 2 to 17 pCi/L for a
radium feed activity of 25 pCi/L. Thus, for a
radium feed activity of 25 pCi/L, a MnO2
concentration of 0.63 mg/L as Mn or greater
would be adequate to reduce the effluent
radium activity to less than 5 pCi/L. No
difference in percent removal was observed
for the two different radium feed activities
used—13 and 25 pCi/L Thus, it was con-
cluded that (a) percent removals can be
estimated from Figure 2 as a function of
MnO2 concentration irrespective of radium
feed activity and (b) an equilibrium iso-
therm plot of solid-phase radium loading
versus liquid-phase concentration would
be linear. A control run to determine ra-
dium removal by DE showed that, as ex-
pected, no significant radium removal was
observed with DE alone.
Effect of pH
pH showed little influence on the ra-
dium removal, which only increased from
80% to about 95% as the pH increased
from 6.5 to 9.5 at a MnO2 concentration of
1.26 mg/L as Mn. Some improvement was
expected because increasing the pH in-
creases the negative charge on the MnO2
particle, thus increasing its capacity to ad-
sorb radium—a divalent, hydrophobic cat-
ion. The magnitude of the increased ra-
dium removal was not great, however,
because the point of zero charge (ZPC) for
MnO2 occurs at tow pH, 2.8 to 4.5, which is
far from the 6.5 to 9.5 range tested. Al-
though slightly better radium removals were
achieved with increasing pH, the higher
phis may not be practical for full-scale
treatment.
Effect of Total Hardness
Figure 3 shows the effect of total hard-
ness on radium removal. The removal de-
creased from 95% to about 70% as the
total hardness increased from 60 to 500
mg/L as CaCO3 at an MnO2 concentration
of 1.26 mg/L as Mn. This trend was ex-
pected because increasing the total hard-
ness level increases the divalent cations,
viz, calcium and magnesium, which in-
creases competition for adsorption sites
on MnO2. The decrease in percent re-
moval in this study was not linear, i.e., at
low hardness, the decrease in percent ra-
dium removal was more dramatic than at
higher hardness. Beyond a certain hard-
ness, about 250 to 300 mg/L as CaCO3 in
this case, the increase in hardness caused
little further decrease in percent radium
removal. We caution, however, that the
hardness effect shown in Figure 3 cannot
be extrapolated to brine concentrations
such as 0.5 M calcium concentration and
beyond.
Effect of Iron and Manganese
Experiments were done to determine
the effect of iron and manganese on ra-
dium removal and to compare the addition
of preformed MnO2 and ferric ion with the
simultaneous oxidation of manganous ion
to MnO2, and oxidation of ferrous ion to
ferric ion. The results are shown in Figure
4. About 90% was removed when pre-
formed MnO2 was added at 1.26 mg/L as
Mn; slightly less (85%) was removed when
2.5 mg/L Fe was added as a ferric ion
along with preformed MnO2. Although a
5% decrease in removal was observed,
the results are statistically inconclusive
because the relative standard deviation for
radium analyses was also 5%. Only 70%
was removed, however, when these same
concentrations of Fe and Mn were simulta-
neously oxidized by KMnO4. The reduction
in radium sorption in the latter case is
thought to be due to the influence of posi-
tively charged oligomers of hydrous iron
(III) oxide that adsorb to the negatively-
charged surface of preformed MnO2. These
oligomers are short-lived and can only in-
fluence the MnO2 surface when Fe(OH)3 is
precipitated in-situ. Opposite behavior was
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100
90
80
70
60
50
5 40
30
20
10
Ra-226 Spiked Houston Groundwater
Total Hardness = 120 mg/L as CaCOg
pH = 7.5
B 25 pCi/L Radium Feed
• 13 pCi/L Radium Feed
0.0
0.2
0.4
1.2
0.6 0.8 1.0
Concentration (mg/L as Mn)
Figure 2. Effect of MnO2 concentration and teed radium activity on radium removal during
screening rests with the use of DE and a Buchner funnel.
1.4
100
90
80
70
§ 60
o
50
40
30
20
10
0
Ra-226 Spiked Houston Groundwater
Ra-226 Feed: 25 pCi/L
MnO2 Dose: 1.26 mg/L as Mn
pH = 7.5
Figure 3.
50 100 150 200 250 300 350 400 450 500 550 600
Total Hardness (mg/L as CaCQJ
Effect of total hardness on radium removal during MnOJDE screening tests.
5
observed between preformed Fe(OH)3 and
oxidation of ferrous ion to Fe(OH)3 by CI2 in
the absence of MnO2. In this experiment,
oxidation to Fe(OH)3 resulted in more re-
moval as compared with preformed
Fe(OH)3. This agrees with the typical be-
havior of Fe(OH)3 during conventional co-
agulation where in-situ precipitation is best.
Radium removal by Fe alone was observed
in the range of 5% to 30%, which agrees
with the published literature values for iron-
removal plants.
Continuous Bench-Scale
Experiments with Houston Tap
Water
The pilot-scale results from experiments
done to evaluate continuous radium re-
moval by MnO2 adsorption followed by DE
filtration were very encouraging. As a con-
trol, ^Ra-spiked Houston tap water was
passed through a DE-coated filter without
any MnO2 being added or present on the
filter. This resulted in 15% radium removal,
which was comparable with the removal of
radium by pure DE filtration during the
batch bench-scale experiments.
When MnO2 was fed at 0.63 mg/L as
Mn to the bench-scale continuous filter,
about 80% of the radium was removed,
which was similar to that observed in batch
experiments. The radium loading on MnO2
was about 20 pCi/mg MnO2 when the
equilibrium effluent radium activity was
4.6 pCi/L, i.e., a radium loading similar to
that observed in batch experiments. The
radium/MnO2 contact time in the system
was about 10 sec. This contact occurred
following in-line mixing at 0.3 gpm in 3/8-
in. ID tubing after adding MnO2 to the
feedwater. The value of velocity gradient,
G, for this system was estimated to be
1150 sec-1 using Camps' equation. Thus,
the Gt value for this system was approxi-
mated at 11,500, which falls in the 10,000
to 100,000 range recommended for con-
ventional mixing in water treatment.
After 6 hr (108 gal throughput), in-line
MnO2 feed to the filter was stopped and
only ^Ra spiked Houston tap water (MnO2-
free) was passed through the filter. At this
point, the filter had about 400 mg of MnO2
accumulated on it. An immediate increase
in effluent radium activity was observed as
soon as the MnO2 feed was stopped. Re-
sults suggest that MnO2, when fed along
with radium-contaminated raw water,
quickly comes to equilibrium with the efflu-
ent radium activity. When the MnO2 feed
was stopped, radium removal gradually
decreased as the DE filter came to equilib-
rium with the feed radium activity in about
72 hr.
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100
90
80
1
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c\i 60
4
^ 50
40
30
20
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Ra-226 Spiked
Houston Groundwater
Total Hardness = 120
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pH- 75
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2.5 mg/L
Fe & 1.26 mg/L Mnfyas Mn
(Inline Oxidation)
2.5 mg/L
Fe& 1.26 mg/L MnO^as Mn
(Preformed)
1.26 mg/L MriO^as Mn
(Preformed)
Figure 4. Effect of iron and manganese on radium removal during MnO^DE screening tests.
During a second experiment with the
bench-scale filter, the contact time was
increased to about 60 sec by adding a loop
in the line and recycling part of the flow.
The turbulence was increased by the loop,
which was a 1/8-in. ID tubing with diameter
contraction and expansion where it was
connected to the 3/8-in. OD feedv/ater tub-
ing. The Gt value for this system was
77,400. No significant difference in radium
activity of the effluent was observed as a
result of increasing Gt from 11,500 to
77,400. The radium removals for both were
near 80%. Thus, initial mixing (Gt == 11,500)
and 10-sec contact time were sufficient for
near-equilibrium radium adsorption onto
MnO2 with this continuous 0.3 gpm DE
filter unit.
Throughout the experimental program,
contact times from 10 sec to 2 hr were
used to equilibrate MnO2 with radium in
bench- and pilot-scale experiments. The
similar removals in all these experiments
indicate the lack of kinetic effects within
this contact-time range for radium adsorp-
tion onto MnO2. It was thus concluded that
a very short contact time, e.g., 10 sec, is
sufficient for good radium removal.
Pilot Plant Studies with Lemont
Groundwater
Further pilot-scale radium removal stud-
ies were carried out in Lemont, where
radium removal by resins, membranes, and
adsorbents was being studied in the UH/
EPA Mobile Drinking Water Treatment Re-
search Facility. To determine the best DE
grade to use, the small pilot-plant filter
cake was visually inspected after the run
was terminated. Hyflo Super-Gel, with the
least MnO2 penetration into the DE cake,
was chosen as the optimum DE grade. It
was then used to optimize the body feed
rate, which was subsequently used for all
the runs with the 1-ft2 pilot plant. An opti-
mum filtered product water yield of 29 L
throughput/g DE occurred at a body feed
ratio of 10:1 (20 mg DE/L).
The radium removals observed during
three runs with the 1-ft2 DE pilot plant
operated at a MnO2 feed of 1.26 mg/L as
Mn were excellent—90% to 97%. Again,
these removals were comparable to the
removals observed in the bench-scale ex-
periments with Houston groundwater. At
an average influent activity of 11.4 pCi/L,
the effluent 226Ra activity ranged from about
0.5 to 1.2 pCi/L during the 20-hr run. The
^Ra loading on the MnO2 surface was
about 6.7 pCi/mg MnO2. Table 1 summa-
rizes the radium loadings observed in this
study. Theoretically, they represent indi-
vidual points on a linear equilibrium iso-
therm in which radium on the solid MnO2
(pCi/mg) is in equilibrium with the effluent
radium activity (pCi/L).
The Lemont pilot-scale run was ended
after 20 hr when the differential pressure
across the filter reached 30 psi. The dry
weight of the filter cake was 168 g, and its
radium activity was 565 pCi/g. In actual
practice, the filter septum could be cleaned
by spray washing the filter with an esti-
mated 20 ga! (75.7 L) of water, which
would result in a radium activity in the filter
spray wastewater of about 1250 pCi/L in
the form of radium on particulate MnO2.
After shutting down the filter without
spray washing and then restarting, radium
removal decreased for a brief period. De-
creases of 3% and 8%, respectively, were
observed after 8- and 16-hr stagnation
periods. When resuming DE filtration im-
mediately after a stagnation period, the
product water was cloudy and had to be
directed to waste for about 15 min until it
became clear. The higher radium activity
after the stagnation surely resulted from
cake dislodging, which, in turn, leaked ra-
dium-contaminated MnO2 into the effluent.
Because cake dislodging will always be a
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Table 1.
Summary of Radium Loadings on MnO, Surface
Experiment
Batch bench-scale with Houston groundwater
Batch bench-scale with Houston groundwater
Continuous bench-scale with Houston tap water
Pilot plant with Lemont groundwater
MnO2
Dose
(mg/L)
1
2
1
2
Radium
Loading
(pCi/mg MnOJ
20
12
20
10.4
Equilibrium
Radium Activity
(pCi/L)
5.2
2.7
4.6
1
problem after stagnation, a run should not
be interrupted once it begins. On the posi-
tive side, in spite of cake dislodging, the
^Ra activity in the effluent after the stag-
nation was never greater than 2 pCi/L,
which is well below the 5 pCi/L MCL
A pilot-scale radium breakthrough curve
(Figure 5) very similar to the continuous
bench-scale results was observed in
Lemont when the MnO2 feed was stopped
after 20 hr of effective radium removal. As
can be seen in Figure 5, it took only 24 hr
for the effluent radium activity to level off at
about 15% below the influent radium activ-
ity. This radium breakthrough is much faster
than that occurring in deep-bed sand filters
being fed MnO2. In this latter case, in-
depth filtration occurs, the sand grains be-
come coated with MnO2, and the bed
behaves like a packed-bed adsorber with
a radium concentration gradient from inlet
to outlet. Also, the sand itself adsorbs and
desorbs radium in response to radium con-
centration variations in the feed. The result
of these phenomena is a much slower
sand-filter response to changes in MnO2
addition such as Valentine observed in his
MnO2 radium removal studies in Forest
City and in West Liberty, IA. In his studies,
radium removal continued for as long as
30 days after stopping MnO2 addition. Simi-
larly, effective radium removal did not be-
gin until many days after starting MnO2
addition to a sand bed already in equilib-
rium with radium contaminated waters.
Results from the multimedia-filter pilot
plant operated to compare its performance
with that of the DE pilot plant were also
encouraging. Excellent radium removals
(> 96%) were observed throughout its 108
hr of operation, including the periods im-
mediately following 8- and 16-hr flow inter-
ruptions. The effluent ^Ra activity ranged
from 0.2 to 0.5 pCi/L for an average feed
radium activity of 14 pCi/L and a feed
MnO2 concentration of 1.26 mg/L as Mn.
The multimedia pilot plant performed slightly
better than did the DE pilot plant during
both continuous and interrupted flow tests.
Manganese Leakage
Fortunately, no significant manganese
leakage was observed during this entire
study, in which manganese effluent con-
centrations ranged from 0.01 to 0.03 mg/L.
The only exception to this immediately
followed the stagnation period for DE filtra-
tion when higher manganese concen-
trations were observed due to cake
dislodging. Multimedia filtration also showed
no manganese leakage into the effluent. It
should be noted that the filter had a new
filter media (coal, sand, and ilmen'rte). The
performance of the multimedia filter after
backwashing is yet to be studied, how-
ever.
Conclusions
1. Adsorbing radium onto MnO2 followed
by DE filtration is very effective for
treating radium-contaminated water.
Radium removals in the range of 80%
to 97% were achieved at MnO2 con-
centrations of 0.63 and 1.26 mg/L as
Mn in waters with hardness in the
range of 100 to 245 mg/L as CaCO3.
2. Radium adsorption onto MnO2 ap-
peared to follow a linear isotherm
because percent radium removal was
not dependent on feed radium activ-
ity in the range of 13 to 25 pCi/L.
3. Radium removal by MnO2 was not
very sensitive to pH in the 6.5 to 9.5
range.
4. Radium removal decreased asymp-
totically from 95% to about 70% as
the total hardness increased from 60
to 500 mg/L as CaCO3. In-srtu oxida-
tion of iron (II) reduced the percent
removal of radium by preformed
MnO2.
5. The radium loading on the MnO2 sur-
face was in the range of 10 to 20 pCi/
mg MnO2with equilibrium radium ac-
tivities in the range of 1 to 5 pCi/L.
16
14
12
10
•
Average Feed
Effluent (MnO2 Feed = 1.26 mg/L as Mn)
. ....„_... Effluent (Raw Water Passing through Cake)
1 gpm/ft2 DE Filtration Rate
0.15 Ib/ft 2 DE Precoat
10:1 Body Feed Ratio
MnO2 Feed Stopped
12
16
20
24
28
32
36
40
44
Run Time (hr)
Figure 5.
Effect of MnOgfeed on radium removal by MnO2 adsorption and DE filtration during
pilot-plant studies at Lemont, IL
•&U.S. COVEKNMENT PRINTING OFFICE: 1992 - 648-0*0/40211
-------
7.
Manganese concentrations in the DE
filter effluent ranged from < 0.01 to
0.03 mg/L for the entire study, which
is below its secondary MCL of 0.05
mg/L.
The Lemont pilot-scale studies
showed that following MnO2 addition
multimedia filtration was as effective
for radium removal as was DE filtra-
tion.
8. The costs of water treatment by MnO2
adsorption and DE filtration were es-
timated at $0.71 and $ 0.47 per 1000
gal for 280,000 gpd and 1 mgd plants,
respectively. These costs are com-
petitive with ion exchange for similar
plant capacities and much less than
MnO2 adsorption and multimedia fil-
tration for smaller plant capacity.
The full report was submitted in fulfill-
ment of Cooperative Agreement No. CR-
813148 by the University of Houston under
the sponsorship of the U.S. Environmental
Protection Agency.
Rahul Patel and Dennis Clifford are with th& University of Houston, Houston, TX 77204-
4791.
Thomas J. Sorg is the EPA Project Officer (see below).
The complete report, entitled "Radium Removal from Water by Manganese Dioxide
Adsorption and Diatomaceous Earth Filtration,'' (Order No. PB92-115260/AS; Cost:
$19.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA PERMIT NO. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-91/063
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