Sulfur-Modified Biochar Efficiently Removes Cr(VI) from Water by Sorption and Reduction

Sulfur-modified biochar was prepared and used to efficiently remove Chromium (Cr) from water. It was found that the sulfur-modified biochar prepared from lower primary pyrolysis temperature (<400?), higher secondary pyrolysis temperature (>400?), appropriate secondary pyrolysis time, and S/C ratio was favorable for Cr removal. According to the response surface methodology analysis, the biochar prepared under the primary pyrolysis temperature of 350?, secondary pyrolysis temperature of 450?, secondary pyrolysis time of 60 min, and S/C ratio of 2:1 could achieve the maximum Cr removal (92%). The S-modified biochar prepared under the optimum condition exhibited greater capacity of adsorbing Cr(VI) or reducing Cr(VI) into Cr(III) compared with the biochar without the S modification. The enhanced Cr(VI) adsorption and reduction were likely because some of the sulfur-containing groups formed on the modified biochar could bind and interact with the Cr(VI). Besides, the enhanced surface area by the sulfur modification also played a role in prompting the removal and reduction of Cr(VI).