Simultaneous Sequestration of Humic Acid-Complexed Pb(II), Zn(II), Cd(II), and As(V) by Sulfidated Zero-Valent Iron: Performance and Stability of Sequestration Products

Heavy metal(loid)s (HMs) such as Pb(II), Zn(II), Cd(II), and As(V) are ubiquitously present in co-contaminated soil and shallow groundwater, where the humic acid (HA)-rich environments can significantly influence their sequestration. In this study, sulfidated zero-valent iron (S-ZVI) was found to be able to simultaneously sequestrate these HA-complexed HMs. Specially, the HA-complexed Pb(II), Zn(II), Cd(II), and As(V) could be completely removed by S-ZVI within 60 min, while only 35-50% of them could be sequestrated within 72 h by unsulfidated ZVI. Interestingly, different from the S-ZVI corrosion behavior, the kinetics of HM sequestration by S-ZVI consisted of an initial slow reaction stage (or a lag phase) and then a fairly rapid reaction process. Characterization results indicated that forming metal sulfides controlled the HM sequestration at the first stage, whereas the enhanced ZVI corrosion and thus-improved adsorption and/or coprecipitation by iron hydroxides governed the second stage. Both metal-oxygen and metal-sulfur bonds in the solid phase could be confirmed by X-ray photoelectron spectroscopy and extended X-ray absorption fine structure analysis. Moreover, the transformation of S species from SO₄^2-, SO₃^2-, and S₂^2- to S^2- under reducing conditions could allow the sequestrated HMs to remain stable over a long period.