In situ Fe-sulfide coating for arsenic removal under reducing conditions

- An in situ Fe-sulfides coating for As removal has been developed.
- This approach achieved a much high As(III) removal efficiency from groundwater.
- Direct reaction between As(III), Fe(II) and S2− is the mechanism for As immobilization.

SummaryAn in situ Fe-sulfide coating approach has been developed for As-contaminated groundwater remediation. Alternate injection of Fe(II), O2-free water and S2− can realize Fe-sulfide coating onto quartz sands with minor changes in porosity. As(III) uptake experiment indicated that the retardation factor for As(III) was 37 and dynamic retention capacity was 44.94 mg As(III)/g Fe, which was much higher than the maximum adsorption capacity for As(III) by FeS and FeS2. This result indicated that adsorption cannot be the only mechanism for As(III) uptake by Fe-sulfide coating layer. The SEM image and FTIR spectra results suggested that interaction between As(III) and Fe-sulfides and formation of As-sulfide precipitates could significantly contribute to As(III) uptake by Fe-sulfide coating layer. Alternate injection of Fe(II) + As(III) and S2− was conducted to simulate in situ As immobilization from real groundwater. The SEM image showed that the quartz sands were mainly covered by crystalline framboidal pyrite after such amendment. The breakthrough of As(III) was not observed during this experiment and the removal capacity for As(III) was 109.7 mg As/g Fe. The As(III) immobilization mechanism during alternate injection of Fe(II) + As(III) and S2− was significantly different from that of As(III) uptake by Fe-sulfide coating. The direct interaction between As(III) and S2− produced As-sulfides contributed to the high As(III) removal capacity during alternate injection of Fe(II) + As(III) and S2−. This result indicated that alternate injection of Fe(II) and S2− approach has an attractive application for As-contaminated groundwater remediation under strongly reducing environment.