Cadmium removal by FeOOH nanoparticles accommodated in biochar: Effect of the negatively charged functional groups in...

Metallic oxide nanoparticles (NPs) anchored in biochar provide a promising measure forward into the scaled-up application of these NPs in water treatment, and reducing the size of the dwelled NPs is expected to boost the adsorption performance of biochar-based composites because of the size and surface effect. Nevertheless, it is still of great challenge to regulate the size of the impregnated NPs due to their intrinsic self-agglomeration caused by high surface energy. In this study, we fabricated the charged biochar (C-BC) bearing high-density negatively charged groups (i.e., carboxyl and hydroxyl groups) via HNO₃ oxidization to load the model metal oxide FeOOH NPs. The average sizes of anchored FeOOH NPs were ultrasmall, ranging from 19.9 ± 1.5 to 3.1 ± 0.5 nm, and decreased with the increased amount of carboxyl and hydroxyl groups in C-BC. Whether in batch adsorption or fixed-bed column setting, adsorption of Cd(II) onto the as-made composites was greatly enhanced by carboxyl and hydroxyl groups in carrier. The normalized adsorption capacities of Cd(II) by ferric mass of the loaded FeOOH were 499.9-724.9 mg/g-Fe, approximately 18.6-27.1 and 2.51-3.64 folds over the bulky FeOOH and FeOOH-impregnated biochar. Our study results should provide a significant reference on how to acquire highly efficient biochar-based composites for water decontamination.