Abstract: A batch experiment combined with the zeta potential, FTIR and XRD methods, was carried out to investigate adsorption of Cr(Ⅵ) by Fe(Ⅲ)-modified crop-straw biochar and its mechanism. Results show that in modification, Fe(Ⅲ) complexed with functional groups and formed a physical coverage layer of iron hydroxide precipitation on the surface of the biochar, thus lowering negative charge and increasing positive ones on the surface, and hence promoting Cr(Ⅵ) adsorption.  The effect of Fe(Ⅲ) modification was more significant on peanut straw biochar than on rice straw biochar. At pH 5.0, the modification increased the maximum Cr(Ⅵ) adsorption of peanut straw-biochar and rice straw biochar by 79% and 29%, respectively. When pH of the suspension varied in the range of 4.0-6.5, Cr(Ⅵ) adsorption of the original biochar and the Fe(Ⅲ)-modified biochars decreased with rising pH, and a similar trend was found with the effect of Fe(Ⅲ) modification on Cr(Ⅵ) adsorption of the biochars. The increase in negative charge on the surface of biochars was the main reason for the decrease in Cr(Ⅵ) adsorption with the rising pH. Electrostatic adsorption and specific adsorption of Cr(Ⅵ) occurred simultaneously on the surface of Fe(Ⅲ)-modified biochars. Meanwhile, specific adsorption of Cr(Ⅵ) also decreased with rising pH. Therefore, as Fe(Ⅲ)-modified crop straw biochars are quite high in Cr(Ⅵ) adsorption capacity under acidic conditions, they can be used as absorbents to remove Cr(Ⅵ) from acidic wastewater.

Key words: Cr(Ⅵ) adsorption, biochar, Fe(Ⅲ) modification, Zeta potential