Sorption of As(V) by Aluminum-Modified Crop Straw-Derived Biochars

Abstract

Biochars derived from the straws of rice, soybean, and peanut were prepared and modified with aluminum [Al(III)]. These modifications shifted zeta potential–pH curves of the biochars in a positive-value direction and changed surface charge of biochars from negative to positive under acidic conditions. The isoelectric points for 0.6 M Al(III)-modified rice, soybean, and peanut straw biochars were 8.0, 7.8, and 7.5, respectively. Electrostatic attraction of the positively charged surfaces on Al(III)-modified biochars to arsenate [As(V)] enhanced its sorption. The sorption of As(V) by these Al(III)-modified biochars was investigated in batch experiments. Al(III)-modified biochars had greater sorption capacity under acidic conditions compared with corresponding unmodified biochars. While unmodified biochars sorbed negligible amounts of As(V), their Al(III)-modified forms sorbed 445–667 mmol kg⁻¹ at pH 5.0, which were predicted by the Langmuir equation. Modifications with 0.3 M Al³⁺ improved sorption capacity of As(V) on soybean straw biochar to 445 mmol kg⁻¹, which was further increased by 50 % after modification with 0.6 M Al³⁺. These As(V) sorption capacities of biochars modified with 0.6 M Al³⁺ were larger than those of Fe/Al oxides determined at the same pH, which were < 500 mmol kg⁻¹. Thus, biochars modified with 0.6 M Al³⁺ could substitute Fe/Al oxides used for water purification. However, the sorption of As(V) by the Al(III)-modified biochars increased with decreasing suspension pH. Thus, As(V) removal by Al(III)-modified biochars is suggested to be conducted under acidic conditions, but at pH > 4.0.