Insights into the removal of Cr(VI) by a biochar–iron composite from aqueous solution: Reactivity, kinetics and mechanism

Recently, the contamination of heavy metals in aquatic environment is greatly received attention all over the world. In this work, a new type of biochar–iron composite (FBC) was prepared with water hyacinth waste straw and ferric chloride through a simple and operable process. Results demonstrate that the maximum adsorption efficiency and amount of Cr(VI) over FBC reached 96% and 18.78 mg g⁻¹, respectively. Cr(VI) adsorption by FBC process was better followed by the pseudo second-order and Langmuir isotherm models, and a monolayer adsorption was predominant in the Cr(VI) removal. Elovich model suggested that Cr(VI) adsorption onto FBC was a heterogeneous or multi-mechanism process, and the intra-particle diffusion model confirmed that this adsorption process was dominated by surface or intra-particle diffusion reaction. The thermodynamic analysis confirmed that Cr(VI) adsorption by FBC was a spontaneous, endothermic and chemical adsorption reaction process. Citric acid (CA) itself can donate electrons for Cr(VI) and this process was a slow-reaction. Most of Cr(VI) species were transformed into Cr(III) species such as Cr₂O₃ and Cr(OH)₃, and the reaction mechanism for Cr(VI) adsorption involving in a combination of surface precipitation, complexation and reduction reactions over FBC was proposed. Cr(VI) and malachite green (MG) were efficiently adsorbed by FBC in mixed systems. This finding of this study suggests that biochar–iron composite was an effective adsorbent for remediating Cr(VI) and MG contaminated wastewater.