Iron-modified biochar-based bilayer permeable reactive barrier for Cr (VI) removal

Iron (Fe)-modified biochar (FeBC) has been developed to remove hexavalent chromium (Cr(VI)) from groundwater and is suitable for use in permeable reactive barriers (PRBs). However, Cr(VI) removal behavior and chemical processes in FeBC-based PRBs are not fully understood, and the potential for Fe release has not been addressed. In this study, three FeBC-based PRBs were assessed in column experiments for 563 days with respect to their ability to remove Cr(VI). Bilayer column filled with FeBC+limestone and BC+limestone in two separate layers (FeBC_Ca_BC) showed the best performance in terms of Cr(VI) removal with a low treatment cost. The corrosion of FeBC was mainly related to pH and Cr(VI) concentration rather than flow rate. Leached Fe was attenuated by BC and limestone and reutilized in FeBC_Ca_BC. Cr(VI) was reduced to Cr(III) and then adsorbed or precipitated on the biochars. Cr and Fe formed inner-sphere complexes and then transformed from double corner sharing to edge sharing. During the reaction, Cr penetrated from the surface to the interior of the biochars and became a more stable species. This study provides evidence of the effectiveness of a new combination of biochars for Cr(VI) removal and insights into the reaction mechanisms.

Automated summary

This study evaluates the capability of iron-modified biochar (FeBC) in removing hexavalent chromium (Cr(VI)) and explores the reaction mechanisms. The results show that a bilayer column with FeBC and limestone layers performs the best in Cr(VI) removal with low cost. The corrosion of FeBC is related to pH and Cr(VI) concentration, and the leached Fe is attenuated and reutilized in the FeBC column. Cr(VI) is reduced to Cr(III) and adsorbed or precipitated on the biochar.