Comparative study on characteristics and mechanism of phosphate adsorption on Mg/Al modified biochar

Biochar-based adsorption process, for its environmental and economic benefits, has recently become a new method of pollution control. Nevertheless, biochar generally has a negatively charged surface that limits its binding affinity toward phosphate anions. In order to address this problem, Mg/Al-modified biochars derived from corn-cob (based on the Mg (Al): biomass = 5, 10, 15 and 20 wt%) are synthesized to remove phosphate from water. The physicochemical properties of biochars such as elemental composition, surface area, morphology, crystalline structure and surface functional groups are characterized. Batch adsorption experiments are performed to determine the adsorption performance and optimal adsorption conditions of phosphate on biochar. It is found that the structural properties and surface morphology of biochar modified by Mg/Al are greatly improved, and its phosphate adsorption capacity also significantly increases, so the optimal Mg/Al content is determined to be 20%. The theoretical maximum phosphate adsorption capacities of 20%Mg-BC and 20%Al-BC are 56.12 mg/g and 44.79 mg/g, respectively. Compared with 20%Al-BC, 20%Mg-BC has more stable removal efficiency as well as a wider pH range for phosphate removal. Spectroscopic solid-phase analyses demonstrate that the adsorption mechanisms mainly include electrostatic attraction, the formation of complexes, cationic bridging and chemical co-precipitation. The adsorption mechanisms, however, are different between Mg-BC and Al-BC. The results indicate that Mg/Al-modified biochar is an environmental friendly material for the recovery of eutrophic water.