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IN-SITU REMEDIATION OF ARSENIC IN SIMULATED GROUNDWATER USING ZEROVALENT IRON: LABORATORY COLUMN TESTS ON COMBINED EFFECTS OF PHOSPHATE AND SILICATE
Citation:
Su, C. AND R W. Puls*. IN-SITU REMEDIATION OF ARSENIC IN SIMULATED GROUNDWATER USING ZEROVALENT IRON: LABORATORY COLUMN TESTS ON COMBINED EFFECTS OF PHOSPHATE AND SILICATE. ENVIRONMENTAL SCIENCE & TECHNOLOGY 37(11):2582-2587, (2003).
Description:
We performed three column tests to study the behavior of permeable reactive barrier (PRB) materials to remove arsenic under dynamic flow conditions in the absence, as well as in the presence, of added phosphate and silicate. The column consisted of a 10.3-cm depth of 50 : 50 (w : w, Peerless iron : Oil Creek sand) in the middle and a 10.3-cm depth of a sediment from Elizabeth City, SC in both upper and lower portions of the 31-cm long glass column with three side sampling ports. The flow rate was maintained at 4.3 m d-1 during the three- to four- month experiments. As expected, dissolved As concentrations in different positions of the column followed the order: column influent > bottom port effluent > middle port effluent > top port effluent > column effluent. The steady state As removal in the middle Peerless iron and sand mixture zone might be attributed to the continuous supply of corroded iron that served as the sorbents for both As(V) and As(III). Consistent with previous batch study findings, dissolved phosphate (0.5 or 1 mg P L-1) and silicate (10 or 20 mg Si L-1) showed strong inhibition for As(V) and As(III) (1 mg As(V) L-1 + 1 mg As(III) L-1 in 7 mM NaCl + 0.86 mM CaSO4) removal by Peerless iron in the column tests. The presence of added phosphate and silicate resulted in earlier breakthrough ? = 0.5 C0) and earlier complete breakthrough of dissolved arsenic relative to absence of added phosphate and silicate in the bottom port effluent. Phosphate and silicate were also removed by the PRB materials, especially by the Peerless iron. Competition between As(V)/As(III) and phosphate/silicate for the sorption sites on the Peerless iron seems to be the cause of the observations. This effect is especially important in the case of silicate for designing a PRB of zerovalent iron for field use because silicate is ubiquitous in terrestrial waters.