Effective removal of Hg²⁺ and Cd²⁺ in aqueous systems by Fe-Mn oxide modified biochar: A combined experimental and DFT calculation

The Fe-Mn oxide modified biochar (FM-BC) was synthesized using straw biochar, and the adsorption mechanism of Hg2+ and Cd2+ was explained in-depth through adsorption experience data, characterization, and density functional theory (DFT) calculation. Adsorption kinetics and isotherm fitting suggested that FM-BC contained more adsorption sites than pristine biochar, and chemisorption was the rate-limiting step in Hg2+ and Cd2+ adsorption. The maximum adsorption capacities of Hg2+ and Cd2+ reached 86.82 and 131.03 mg center dot g(-1), respectively. Combining the characterization results (SEM-EDS, XRD, FTIR and XPS), it can be found that chemical complexation reaction (monodentate or multidentate inner-sphere complexes) was the main adsorption mechanisms for Hg2+ and Cd2+ adsorption. Moreover, the coprecipitation reaction was another significant mechanism of removing Cd2+ by FM-BC. The DFT calculation results illustrated that oxygen-containing functional groups and metal-pi interaction were crucial for the excellent adsorption performance of Hg2+ and Cd2+ onto FM-BC. The carbon structures doped with Fe/Mn atoms have significant adsorption energies for Hg2+ and Cd2+, and Fe/Mn atoms provided more electrons to promote the electron transfer. These innovative findings provide theoretical basis for the application of FM-BC as a high-performance adsorbent in the purification of Hg and Cd contaminated wastewater.