United States
Environmental Protection
Agency
EPA/540/F-92/008
September 1992
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Emerging Technology Bulletin
Metals Treatment at Superfund Sites by Adsorptive Filtration
University of Washington
Technology Description: This project evaluated an innova-
tive approach for removing inorganic contaminants from the liquid
phase at Superfund sites. In the process, called adsorptive filtra-
tion, metals are collected by attachment to a thin layer of
ferrihydrite (iron oxide) that has been immobilized on the surface
of sand grains. Some relevant features of the plain sand and the
two batches of coated sand used in the project are presented in
Table 1. The modification of the sand surface allows the grains to
simultaneously adsorb soluble heavy metals, and remove particu-
late metals by filtration in a column packed with the media.
Table 1. Characteristics of Plain Ottawa Sand and Iron Oxide Coated
Sand Used in Adsorptive Filtration Study. All Sands Were 20-
30 Mesh Size.
Plain Sand Media I Media II
Fe salt used for coating
% iron by weight
surface area by BET, m2/g
pHofthePZC
none
0
0.04
O.T
Fe(NOa)3
2.1
2.4
9.2
FeCI3
3.2
9.1
9.8
Model influent solutions contained 0.5 or 5.0 mg/L of three metals
(Cu, Cd, and Pb), sometimes individually and sometimes in
combination. The pH of the test solutions ranged from 7.0 to 9.5,
with most tests conducted at pH 9.0. Test solutions also con-
tained 0.01 M NaNOS. Also, several tests were conducted in
which an additional substance was added to the influent solution,
in order to assess its effect on metal behavior in the column. The
substances tested in this way included ammonia (as a complexing
agent), EDTA (as a chelating agent), sodium dodecyl sulfonate (a
surfactant), motor oil, and antifreeze. In addition, some tests
were conducted using a column containing biogrowth. Finally, a
few tests were run with a synthetic influent containing As and Se,
and with a solution collected from a Superfund site where con-
ventional treatment is currently being applied.
Waste Applicability: The process was shown to be applicable
for removing Zn from the effluent of a conventional precipitation/
coagulation process at a Superfund site. This test gave an indica-
tion of the incremental metal removal that could be obtained by
using adsorptive filtration as a polishing step after a conventional
treatment process. The total and soluble Zn concentrations in the
samples collected were in the ranges 0.6 to 4.0 and 0.3 to 0.6
mg/L, respectively. The corresponding Zn concentrations in the
effluent were around 0.2 and <0.1 mg/L, respectively.
The solutions investigated contained one to three heavy metals,
at total heavy metal concentrations ranging from 0.5 to 15 mg/L.
These solutions were treated successfully at empty bed contact
times (EBCTs) of 1 to 4 min, with the only significant effect of
EBCT being that headloss develops more rapidly at the higher
throughput rate. Additionally, a reasonable data base has been
developed that identifies acceptable regeneration conditions.
Though the optimum treatment and regeneration conditions may
depend on the specific water being treated, the results presented
here provide a good baseline from which to start such an evalua-
tion. ;
Most types of organic contaminants that are likely to co-exist with
metals at Superfund sites do not interfere significantly with the
adsorptive filtration process. Mild complexing agents such as
ammonia do not prevent sorption of the metals, and this is a
significant advantage of adsorptive filtration over conventional
precipitation. Strong complexing agents such as EDTA do pre-
vent adsorption. The process might be affected slightly, but
appears not to be affected dramatically, by the presence of
surfactants, oily substances, and nutrients that allow a biofilm to
grow on the media.
The long-term stability of the adsorbent appears adequate, al-
though additional tests that last even longer than those described
here would be useful to quantify the effective lifetime of the
media. • •
Test Results: Runs were conducted using a packed bed con-
taining 250 mL (bulk volume) of the coated sand. Once a pre-
determined criterion was met (related to either the duration of the
run or the headloss), the bed was cleaned by backwashing and/
or acid ^regeneration, and the 'cleaning' solutions were analyzed.
Regeneration was generally accomplished using a solution ad-
justed to and maintained at pH near 2.0.
In general, adsorptive filtration proved to be an efficient and
effective treatment process. Soluble and paniculate forms of all
the metals tested could be removed from the water stream at
both concentrations tested. The contact time required for treat-
ment was minimal (<5 min), and treatment was successful at
moderate pH values (near 9). Removal efficiencies ranged from
about 70 to >99%, depending on treatment conditions. For in-
stance, i when influent solutions contained 0.5 mg/L each of Cu,
Cd, and Pb, the effluent concentrations gradually increased from
near zero to about 0.1 mg/L each of Cu and Cd and 0.2 mg/L Pb
during treatment of 7000 and 13000 bed volumes of influent.
Printed on Recycled Paper
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l« the runs with 5 mg/L of each metal in the influent, most of the
metal load was participate;; soluble influent concentrations were
typically around 1,5 mg Cd/L, 0,8 mg Pb/L, and 0,2 mg Cu/L
Under these conditions, for the process to be successful, filtration
must be at least as significant a mechanism of metal removal in
the columns as adsorption.
The total concentrations of all the metals in the effluent were well
babw 0,1 mg/L until a few hundred bed volumes had been
{mated (a 6- to 12-hour period), at which point paniculate metals
began breaking through the column, Backwashtng of the media
allowed additional influent to be treated effectively. Removal of
sofuW© metal was substantial throughout these runs. Typical
removal efficiencies for soluble metals in the influent were 80%
for Cuf 90% for Pbr and 98% for Cd, and typical overall removal
eflicfencfas (comparing total effluent and total influent) were 99%
or greater for all three metals.
Regeneration was also fairly rapid and efficient. The metal con-
carttraSbn in the recirculation fluid increased rapidly at first and
than only sfowfy thereafter. A recirculatrng period as short as 10
minuses released a large fraction of the available metal. Metal
concentrations In the first and second regenerant solutions wera
as high as 3000 and 500 mg/L after the 5 mg/L runs. The overall
recovery efficiencies (backwash plus regeneration) were almost
always greater than 80% and were often 100% ± 10%. During the
course of these runs, the media was backwashed over 20 times
arsd regenerated about 10 times overa period of a few months,
with no apparent deterioration in performance.
In tests with ammonia-complaxed metal in the influent, about
1500 mg of each metal sorbed per liter of media before the
effluent concentration exceeded a few tenths of a mg/L, and
about 4000 mg of each metai sorbed per liter of bed before the
effluent concentration reached 4 mg/L. Regeneration of this col-
umn at pH 2,0 recovered 93% of the sorbed Cd and 100% of the
Cu, Similarly, the presence of 0, 15, or 30 mg/L of the surfactant
sodium lauryl sulfonate had no noticeable effect on mstal sorp-
tion. When the metals ware eomplexed with EDTA, however,
they broke through the column almost immediately.
The presence of a biofilm on the media reduced the capacity of
media for the metals by about 50%, It is expected that this
interference could be partially reversed by exposing the column
to a high pH solution, which would probably soiubilize a substan-
tial amount of the biofilm.
In runs evaluating the removal of As and Se by the coated sand,
the influent was adjusted to pH 3,5, but conditions were other-
wise simitar to those for removal of cationic metals. Significant
amounts of Se or As began appearing in the effluent after about
200 to 300 bed volumes of solution had been treated. The
removal pattern was remarkably consistent, regardless of the
metai (As or Se) being treated and its oxidation state (4-3 or+5 for
As; +4 or +6 for Se). The latter result was particularly surprising,
since selenate (SeO42-)is generally much more difficult to remove
from solution than is selenite (SeQ32)
For Further Information:
EPA Project Manager
Norma M. Lewis
U.S. EPA Risk Reduction Engineering Laboratory
26 West Martin Luther King Drive
Cincinnati, OH 45268
513/569-7665
Mark Benjamin
University of Washington
Dept. of Civil Engineering
Seattle, WA
206/543-7645
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Panaiiy for Private Use
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EPA
PERMIT No, G-35
EPAAB40/F-92/008
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