Enhanced removal of metal-cyanide complexes from wastewater by Fe-impregnated biochar: Adsorption performance and removal mechanism.

Metal-cyanide complexes are common contaminants in industrial wastewater. Removal of these refractory contaminants is essential before their discharge into the environment. This study investigated a biochar (BC)-based sorbent material that could be applied for the efficient removal of metal-cyanide complexes from wastewater. In consideration of the strong electrostatic repulsion of the pristine BC toward anions, iron-modified BC (Fe-BC) composites were fabricated by a one-step co-pyrolysis of corn straw and FeCl3 at 600-800 °C. The adsorption performance and corresponding sorption mechanisms of representative metal-cyanide complexes (ferricyanide [Fe(CN)6]3- and tetracyanonickelate [Ni(CN)4]2-) onto the Fe-BC composites were investigated. The results indicated that the Fe-BC composites had significantly high affinity toward the metal-cyanide complexes, reaching a maximum sorption capacity of 580.96 mg/g for [Fe(CN)6]3- and 588.86 mg/g for [Ni (CN)4]2-. A mechanistic study revealed that Fe-impregnation during BC fabrication could effectively alter the negatively charged BC surface, forming more functional groups that could interact with the metal-cyanide complexes. Moreover, the transformation of carbon structure promoted the carbothermal reduction process, leading to the formation of various reductive-Fe minerals in the resulting Fe-BC composites. These modification-induced alterations to the surface and structural characteristics of BC were expected to facilitate the adsorption/precipitation of target contaminants. Different sorption mechanisms were proposed for the two metal-cyanide complexes that were the focus of this study. For [Fe(CN)6]3-, precipitation by Fe-bearing species in the Fe-BC composites was the major factor controlling [Fe(CN)6]3- removal, while for [Ni(CN)4]2- hydrogen bonding interactions between surface functional groups (especially hydroxyl (-OH) and carboxyl (-COOH)) and [Ni(CN)4]2- were the main factors controlling removal.