Arsenate and Arsenite Removal by Zerovalent Iron: Kinetics, Redox Transformation, and Implications for in Situ Groundwater Remediation

Abstract

Batch tests were performed utilizing four zerovalent iron (Fe⁰) 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 °C 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 ≈ Master Builders > Aldrich; whereas, on a surface area basis the order became: Fisher > Aldrich > Peerless ≈ Master Builders. Arsenic concentration decreased exponentially with time, and was below 0.01 mg L^-1 in 4 days with the exception of Aldrich Fe⁰. More As(III) was sorbed than As(V) by Peerless Fe⁰ 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 Fe⁰ 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 Fe⁰, when As(V) was the initial arsenic species in the solution. Likewise, As(V) was found on Peerless Fe⁰ 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 Fe⁰ when either As(V) or As(III) was the initial added species. The presence of both reducing species (Fe⁰ and Fe²⁺) and an oxidizing species (MnO₂) in Peerless Fe⁰ is probably responsible for the coexistence of both As(V) and As(III) on Fe⁰ 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.