Effect of cations on the enhanced adsorption of cationic dye in Fe3O4-loaded biochar and mechanism

Fe3O4-loaded biochar (MBC) was synthesized through oxygen-limited pyrolysis of FeCl3-impregnated sorghum straw biomass, and its performance and mechanism for the adsorption of cationic methylene blue (MB) were investigated. The FeCl3 concentration of the modified solution used for MBC synthesis was optimized, and the physicochemical properties of the prepared adsorbents were evaluated. The results showed that iron oxide particles were successfully introduced on the biochar, resulting in a higher adsorption capacity for MB. The adsorption kinetics of MB onto raw biochar (BC) and MBC were better fitted to the pseudo-second-order model. The Freundlich isotherm model best described the experimental isotherm for MB adsorption onto BC, while the Langmuir model was best for MB on MBC. The presence of cations with different valences (Na+/Cu2+/Zn2+/Cr3+) induced a positive effect on the adsorption capacities of MBC for MB, which increased from 133.13 mg/g to 166.67, 160.21, 162.95, and 155.72 mg/g, respectively. Importantly, there was synergistic adsorption between MB and Cu2+/Zn2+/Cr3+, and the mechanistic study indicated that the MB molecules and Cu2+/Zn2+/Cr3+ acted as a bridge between the adsorbed and unadsorbed pollutants, which significantly improved the synergistic adsorption between dyes and heavy metal cations.