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Effective and economical removal of selenium (Se) in agricultural drainage water is very important in Se bioremediation. Zero-valent iron (ZVI) and a redox mediator [anthraquinone-2,6-disulfonate (AQDS)] were assessed for their ability to enhance the removal of Se(VI) or Se(IV) (500 microg/L) in synthetic drainage water by Enterobacter taylorae. The results showed that E. taylorae was capable of using inexpensive sucrose to remove Se from the drainage water. During a 7-day experiment, Se(VI) was almost entirely reduced to Se(0) and transformed to organic Se in the drainage water with sucrose levels of 500 to 1000 mg/L. Addition of ZVI to the drainage water increased the removal of total soluble Se to 94.5-96.5% and limited the production of organic Se. Addition of AQDS to the drainage water with or without ZVI decreased Se(VI) removal, but enhanced the removal of Se(IV), suggesting that E. taylorae only can use anthrahydroquinone-2,6-disulfonate (AHQDS, a reduced form of AQDS) to respire Se(IV), and not Se(VI). These results show that ZVI has promising application potential in the bioremediation of Se in Se-contaminated water.

Biological reduction of perchlorate by autotrophic microorganisms attached to zerovalent iron (ZVI) was studied in flow-through columns. The effects of pH, flow rate, and influent perchlorate and nitrate concentrations on perchlorate reduction were investigated. Excellent perchlorate removal performance (> or = 99%) was achieved at empty bed residence times (EBRTs) ranging from 0.3 to 63 h and an influent perchlorate concentration of 40-600 microg L(-1). At the longest liquid residence times, when the influent pH was above 7.5, a significant increase of the effluent pH was observed (pH > 10.0), which led to a decrease of perchlorate removal. Experiments at short residence times revealed that the ZVI column inoculated with local soil (Colton, CA) containing a mixed culture of denitrifiers exhibited much better performance than the columns inoculated with Dechloromonas sp. HZ for reduction of both perchlorate and nitrate. As the flow rate was varied between 2 and 50 mL min(-1), corresponding to empty bed contact times of 0.15-3.8 h, a maximum perchlorate elimination capacity of 3.0 +/- 0.7 g m(-3) h(-1) was obtained in a soil-inoculated column. At an EBRT of 0.3 h and an influent perchlorate concentration of 30 microg L(-1), breakthrough (> 6 ppb) of perchlorate in the effluent did not occur until the nitrate concentration in the influent was 1500 times (molar) greater than that of perchlorate. The mass of microorganisms attached on the solid ZVI/sand was found to be 3 orders of magnitude greater than that in the pore liquid, indicating that perchlorate was primarily reduced by bacteria attached to ZVI. Overall, the process appears to be a promising alternative for perchlorate remediation.

A series of batch experiments were performed to study the combination of zero-valent iron (ZVI) with perchlorate-reducing microorganisms (PRMs) to remove perchlorate from groundwater. In this method, H2 produced during the process of iron corrosion by water is used by PRMs as an electron donor to reduce perchlorate to chloride. Perchlorate degradation rates followed Monod kinetics, with a normalized maximum utilization rate (rmax) of 9200 microg g(-1) (dry wt) h(-1) and a half-velocity constant (Ks) of 8900 microg L(-1). The overall rate of perchlorate reduction was affected by the biomass density within the system. An increase in the OD600 from 0.025 to 0.08 led to a corresponding 4-fold increase of perchlorate reduction rate. PRM adaptation to the local environment and initiation of perchlorate reduction was rapid under neutral pH conditions. At the initial OD600 of 0.015, perchlorate reduction followed pseudo-first-order reaction rates with constants of 0.059 and 0.033 h(-1) at initial pH 7 and 8, respectively. Once perchlorate reduction was established, the bioreductive process was insensitive to the increases of pH from near neutral to 9.0. In the presence of nitrate, perchlorate reduction rate was reduced, but not inhibited completely.

Zero-valent iron (ZVI) is an inexpensive agent that can remove many common environmental contaminants. We investigated the effects of arsenate [As(V)] and molybdate [Mo(VI)] on the removal of selenate [Se(VI)] (1000 microg/L) in a 5 mM Cl- and SO4(2-) solution by ZVI. Analysis of selenium (Se) species in a control experiment revealed that there was no detectable selenite [Se(IV)] in the solution and Se(IV) was the only dominant form of Se in the PO(4)3- extract, revealing that the removal of Se(VI) by ZVI appeared to be partly attributed to the reduction of Se(VI) to Se(IV) by Fe(II) oxidized from ZVI, followed by rapid adsorption of Se(IV) to Fe oxyhydroxes (Fe(OH)). Addition of As(V) in a range of 5 to 25 mg/L in the solution did not affect Se(VI) removal. As(V) was removed at much faster rates than Se(VI). It took 16 h for ZVI to completely remove As(V) from the solutions, whereas it needed 31 h to remove 99% of the added Se(VI). Addition of Mo(VI) to the solution significantly affects the removal of Se. After 16 h, 99%, 98%, and 73% of the added Mo(VI) was removed from the solutions with 10, 50, and 200 mg/L Mo(VI) added, respectively. Whereas about 18%, 38%, and 78% of the added Se(VI) still remained in the solutions, respectively. This study suggests that a sequence for rapid element removal by ZVI is As(V)>Mo(VI)>Se(VI) and that ZVI can be used to effectively remove a high concentration level of As(V), Mo(VI), and Se(VI) from water.

Zerovalent iron (ZVI) has been widely used in the removal of environmental contaminants from water. In this study, ZVI was used to remove selenate [Se(VI)] at a level of 1000 microg L(-1) in the presence of varying concentrations of Cl-, SO(2-)4, NO(-)3, HCO(-)3, and PO(3-)4. Results showed that Se(VI) was rapidly removed during the corrosion of ZVI to iron oxyhydroxides (Fe(OH)). During the 16 h of the experiments, 100 and 56% of the added Se(VI) was removed in 10 mM Cl- and SO(2-)4 solutions under a closed contained system, respectively. Under an open condition, 100 and 93% of the added Se(VI) were removed in the Cl- and SO(2-)4 solutions, respectively. Analysis of Se species in ZVI-Fe(OH) revealed that selenite [Se(IV)] and nonextractable Se increased during the first 2 to 4 h of reaction, with a decrease of Se(VI) in the Cl- experiment and no detection of Se(VI) in the SO(2-)4 experiment. Two mechanisms can be attributed to the rapid removal of Se(VI) from the solutions. One is the reduction of Se(VI) to Se(IV), followed by rapid adsorption of Se(IV) to Fe(OH). The other is the adsorption of Se(VI) directly to Fe(OH), followed by its reduction to Se(IV). The results also show that there was little effect on Se(VI) removal in the presence of Cl- (5, 50, and 100 mM), NO(-)3 (1, 5, and 10 mM), SO(2-)4 (5 mM), HCO(-)3 (1 and 5 mM), or PO(3-)4 (1 mM) and only a slight effect in the presence of SO(2-)4 (50 and 100 mM), HCO(-)3 (10 mM), and PO(3-)4 (5 mM) during a 2-d experiment, whereas 10 mM PO(3-)4 significantly inhibited Se(VI) removal. This work suggests that ZVI may be an effective agent to remove Se from Se-contaminated agricultural drainage water.

Zerovalent iron (Fe0) has tremendous potential as a remediation material for removal of arsenic from groundwater and drinking water. This study investigates the speciation of arsenate (As(V)) and arsenite (As(III)) after reaction with two Fe0 materials, their iron oxide corrosion products, and several model iron oxides. A variety of analytical techniques were used to study the reaction products including HPLC-hydride generation atomic absorption spectrometry, X-ray diffraction, scanning electron microscopy-energy-dispersive X-ray analysis, and X-ray absorption spectroscopy. The products of corrosion of Fe0 include lepidocrocite (gamma-FeOOH), magnetite (Fe3O4), and/or maghemite (gamma-Fe2O3), all of which indicate Fe(II) oxidation as an intermediate step in the Fe0 corrosion process. The in-situ Fe0 corrosion reaction caused a high As(III) and As(V) uptake with both Fe0 materials studied. Under aerobic conditions, the Fe0 corrosion reaction did not cause As(V) reduction to As(III) but did cause As(III) oxidation to As(V). Oxidation of As(III) was also caused by maghemite and hematite minerals indicating that the formation of certain iron oxides during Fe0 corrosion favors the As(V) species. Water reduction and the release of OH- to solution on the surface of corroding Fe0 may also promote As(III) oxidation. Analysis of As(III) and As(V) adsorption complexes in the Fe0 corrosion products and synthetic iron oxides by extended X-ray absorption fine structure spectroscopy (EXAFS) gave predominant As-Fe interatomic distances of 3.30-3.36 A. This was attributed to inner-sphere, bidentate As(III) and As(V) complexes. The results of this study suggest that Fe0 can be used as a versatile and economical sorbent for in-situ treatment of groundwater containing As(III) and As(V).