Mn-embedded biochar for enhanced removal of Cd(II), Pb(II) and Ni(II): Physicochemical properties, high-efficiency adsorption capacities and quantitative mechanism analysis

Heavy metals (HMs) in wastewater, including Cd, Pb and Ni, were toxic and needed urgent treatment. Using biochar as an adsorbent to purify HM-containing wastewater is effective. However, there are few studies on screening of high-efficiency modifier and comparing their adsorption capacity. Herein, three different modification methods, including heteroatomic doping (urea), alkali hydroxide (NaOH) and chemical oxidation (KMnO4), were used to improve removal ability of HMs by biochar. The difference in impregnation solution resulted in completely differences in microscopic morphology, physicochemical properties and adsorption efficiency of modified biochar. Biochar modified by KMnO4 (Mn-BC) had significantly better adsorption capacity for Cd, Pb and Ni, reaching 46.40 mg/g, 80.20 mg/g and 18.65 mg/g, respectively, compared with raw biochar and other modification biochar, which might be due to the highest surface area and pore volume, lower zeta potential and loaded MnOx onto Mn-BC surface. Chemical and monolayer adsorption was dominant during adsorption of HMs. After adsorption, HMs were detected on the Mn-BC surface in the form of carbonate, hydroxide or oxide. Mineral precipitation and cation exchange were the dominant mechanisms during adsorption, accounting for 17.4%-41.8% and 43.0%-77.2%, respectively. Moreover, according to the correlation analysis, the increase in biochar pH, pore volume and degrees of carbonization were beneficial for HM adsorption. Mn-BC could adapt to a wider pH range (pH=4-6), and its adsorption capacity would not be interfered by coexisting ions (50 mg/L). This research proved advantages of biochar with KMnO4 modification and highlighted its application in treatment of wastewater contaminated with multiple HMs.