Enhanced removal capacities and mechanisms of Mn/Fe-loaded biochar composites functionalized with chitosan Schiff-base or hydroxyl toward Pb(II) and Cd(II) from aqueous solutions

The efficient capture of heavy metal ions from actual wastewater is a global challenge. To enhance Pb(II) and Cd(II) removal, novel biochar composites (MF@CBC, MF-S@CBC and MF-OH@CBC) were successfully fabricated by loading magnetic Mn-Fe binary oxides and functionalizing chitosan Schiff-base or hydroxyl on biochar in the study. The maximum adsorption capacities of MF-OH@CBC and MF-S@CBC toward Pb(II)/Cd(II) were 196.69/187.29 mg/g and 161.61/153.69 mg/g (pH = 5.0, adsorbent dosage = 0.5 g/L), which were 2.09/3.05 and 1.54/2.33 times higher than corn cob biochar (CBC), respectively. The adsorption processes were conformed to Langmuir isothermal model and pseudo second order model. The numerous functional groups presented on MF-OH@CBC resulting from loading Mn-Fe nanoparticle and hydroxylation might be the main reason for dominating adsorption of Pb(II)/Cd(II) by complex interaction (contribution rate (CR) = 45.66%/43.12%). And also, the negative charge on MF-OH@CBC might be another reason for enhancing adsorption of Pb(II)/Cd(II) by electrostatic interaction (CR = 23.11%/22.45%). Besides, ion exchange (CR = 17.95%/19.33%) and physical adsorption (CR = 13.28%/15.10%) could also contributed to Pb(II)/Cd(II) removal. In addition, MF-OH@CBC displayed favorable recycling ability and its adsorption capacity retained over 80% after five cycles. It also exhibited anti-interfering ability, the adsorption capacity for Pb(II)/Cd(II) in actual sewage was 85.27%/80.47%, respectively. The results indicated that MF-OH@CBC possessed good potential for eliminating Pb(II) and Cd(II) from the complex aqueous environment.