Performance and mechanisms of NaOH and ball-milling co-modified biochar for enhanced the removal of Cd2+ in synthetic water: A combined experimental and DFT study

A multifunction magnetic carbonaceous adsorbent (MBM/OH/BC) was synthesized by facile NaOH and ball-milling co-modified biochar, and applied for removing Cd²⁺ in water. The results showed MBM/OH/BC exhibited excellent adsorption capacity (183.59 mg/g for Cd²⁺), short equilibrium time (60 min) and good reusability (the declining efficiency <20% after four adsorption and desorption cycles) compared with pristine biochar. Meanwhile, the modification mechanisms of NaOH and ball-milling to biochar were explored by a series of characterizations (XRD, SEM-EDS, BET, XPS and FTIR, et al). The results indicated the remarkable adsorption performance of MBM/OH/BC was mainly attributed to the co-modification significantly increased the specific surface area, mineral content and cation exchange capacity of biochar, thereby further improving the precipitation, cation exchange and complexation with Cd²⁺. In addition, the results of adsorption mechanisms showed that the joint contribution proportion to the total adsorption capacity of precipitation, cation exchange and complexation was up to 90%, indicating the three mechanisms were the primary adsorption mechanism. To sum up, NaOH and ball-milling co-modification was an effective strategy to increase the adsorption capacity of the pristine biochar for heavy metals from water.