Effects and mechanisms of anionic and nonionic surfactants on biochar removal of chromium

This work found that the removal of chromium by a straw-derived biochar was significantly promoted or inhibited by various surfactants. For example, the anionic surfactant sodium dodecyl benzene sulfonate (SDBS) inhibited the removal of Cr(VI) by the biochar but significantly promoted the removal of Cr(III) by the biochar. The nonionic surfactant Triton X-100 (TX-100) promoted the removal of Cr(VI) at low concentrations (< 100 mg L^-1) but inhibited the removal at high concentrations. Different mechanisms were found for the two surfactants. As an anionic surfactant, surface-sorbed SDBS changed the surface functional groups of the biochar, making the biochar negative charged and changing the sorption ability of the biochar. For the nonionic TX-100, monomers and micelles in the aqueous phase had a major influence on the sorption of chromium due to the impact on the interfacial tension between the biochar and the solution phase as well as the solution pH. The results suggest that when biochar is used to treat heavy metal wastewater containing coexisting surfactants, the type and concentration of surfactants must be considered as important factors. Under certain surfactant conditions, biochar will enable the simultaneous and efficient removal of heavy metals and surfactants.