Arsenate and arsenite removal by zerovalent iron: Kinetics, redox transformation, and implications for in situ groundwater remediation

Batch tests were performed utilizing four zerovalent iron (Fe-0) filings (Fisher, Peerless, Master Builders, and Aldrich) to remove As(V) and As(III) from water. One gram of metal was reacted headspace-free at 23 degreesC for up to 5 days in the dark with 41.5 mL of 2 mg L-1 As(V), or As(III) or As(V) + As(III) (1:1) in 0.01 M NaCl. Arsenic removal on a mass basis followed the order: Fisher > Peerless approximate to Master Builders > Aldrich; whereas, on a surface area basis the order became: Fisher > Aldrich > Peerless approximate to Master Builders. Arsenic concentration decreased exponentially with time, and was below 0.01 mg L-1 in 4 days with the exception of Aldrich Fe-0. More As(III) was sorbed than As(V) by Peerless Fe-0 in the initial As concentration range between 2 and 100 mg L-1. No As(III) was detected by X-ray photoelectron spectroscopy (XPS) on Peerless Fe-0 at 5 days when As(V) was the initial arsenic species in the solution. As(III) was detected by XPS at 30 and 60 days present on Peerless Fe-0 when As(V) was the initial arsenic species in the solution. Likewise, As(V) was found on Peerless Fe-0 when As(III) was added to the solution. A steady distribution of As(V) (73-76%) and As(III) (22-25%) was achieved at 30 and 60 days on the Peerless Fe-0 when either As(V) or As(III) was the initial added species. The presence of both reducing species (Fe-0 and Fe2+) and an oxidizing species (MnO2) in Peerless Fe-0 is probably responsible for the coexistence of both As(V) and As(III) on Fe-0 surfaces. The desorption of As(V) and As(III) by phosphate extraction decreased as the residence time of interaction between the sorbents and arsenic increased from 1 to 60 days. The results suggest that both As(V) and As(III) formed stronger surface complexes or migrated further inside the interior of the sorbent with increasing time.