Interactions of Arsenate and Arsenite with Nanoparticles of Zerovalent Iron Under Anaerobic Conditions.

Arsenic is a known carcinogen for humans. Even low levels of exposure for an extended period of time can cause cancer and sores internally and externally. Arsenic is found in wells and groundwater all over the world, and it is a huge health risk in many countries today. Previous studies have shown that nanoscale zerovalent iron (NZVI) is a promising sorbent for removing arsenic from water. However, the removal kinetics and mechanisms are not well understood. In this study, batch tests were conducted using 25-nm size nanoiron to remove the two predominant forms of arsenic: arsenate [As(V)] and arsenite [As(III)] under anaerobic conditions as a function of time and pH with or without phosphate and silicate. Both As(V) and As(III) removal increased with increasing time to approach a steady state after 4-5 days. There is generally more removal of As(III) than As(V). Complete or near complete removal of As(V) and As(III) was achieved at pH levels less than 10. Competition of phosphate and silicate against As(V) and As(III) was observed at alkaline pHs. New solid phases formed such as parasymplesite in the As(V) system and vivianite in the phosphate system. This study demonstrated that As(V) removal involves both solid precipitation and adsorption; whereas, As(III) removal involves surface adsorption.