Synthesis of MnFe₂O₄-biochar with surficial grafting hydroxyl for the removal of Cd(II)-Pb(II)-Cu(II) pollutants: Competitive adsorption, application prospects and binding orders of functional groups

A novel biochar material with magnetic modification by MnFe₂O₄ and surficial hydroxyl grafting (h-MFO-BC) was synthesized for capturing HMs (Cd, Pb and Cu) and their competition in composite systems was investigated. The modification of hydroxyl considerably improved the adsorption capacity of HMs. Chemisorption and monolayer and homogeneous reaction dominated adsorption processes. Moreover, a pronounced competitive adsorption effect between HMs was observed in composite systems. The order of selectivity by h-MFO-BC was Pb > Cu ≫ Cd. The distinction in the adsorption of HMs was related to different adsorption pathways and binding sequences of functional groups. Two-dimensional correlation spectroscopy revealed that Pb and Cu preferred to bind to the active sites (Mn/Fe-OH) on h-MFO-BC surface. Moreover, they could generate hydroxide precipitation more easily, which prevented further adsorption of Cd due to the occupation or coverage of binding sites and electrostatic repulsion. Furthermore, h-MFO-BC could be effectively regenerated and recycled and possessed fascinating performance in HMs removal from real water, indicating its potential for widespread applicability. This work provided a novel composite material for the treatment of HMs in wastewater or selective recovery of Pb and Cu and gave a new perspective on understanding the competition mechanisms between HMs on adsorbents.