Removal of Cr(VI) from aqueous solutions by Na2SO3/FeSO4 combined with peanut straw biochar

• Cr(VI) was reducted to Cr(III) by Na2SO3 and FeSO4.
• Removal of Cr(III) from aqueous solutions by biochar.
• Biochar neutralized solution acidity and increased solution pH.
• Adsorption and precipitation were main mechanisms for Cr removal.

Discharge of Cr(VI)-containing industrial effluents leads to the pollution of surface waters and ground waters. In this study, Cr(VI) was first reduced by Na2SO3 or FeSO4 and then biochar generated from peanut straw at 500 °C was used to remove the Cr(III). Results indicated that the reduction of Cr(VI) by Na2SO3 must be conducted under strongly acidic conditions within a narrow pH range of 2.0–2.4, whereas the reduction of Cr(VI) by FeSO4 can be conducted under acidic, neutral and weak alkaline conditions because protons are generated from the hydrolysis of Fe3+ via Fe2+ oxidation. When the initial concentration of Cr(VI) was no more than 1.5 mM, and after Cr(VI) had been reduced by Na2SO3 at pH 2.0 or FeSO4 at pH 7.6, 4 g L−1, peanut straw biochar was able to neutralize solution acidity and remove Cr from the aqueous solution. The optimal reaction time for biochar in the Cr-containing solutions was 6 h. The precipitation of Cr(OH)3 and the formation of Cr3+ surface complexes with the functional groups on the biochar were the main mechanisms for Cr(III) removal by biochar. These results suggested that the combination of reductants (Na2SO3/FeSO4) and biochar generated from peanut straw can be used to efficiently remove Cr(VI) from aqueous solutions.