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Although biochar (BC) has been widely used to adsorb pollutants in environment due to its natural and green characteristics, the structural defects of BC limit the ability to remove various environmental pollutants in aqueous solution. In this study, oxidized biochar (OBC) and sulfhydryl biochar (SBC) derived from pomelo peel (PP) were prepared through an oxidation and esterification reaction. BC and modified BC were used for the removal of methylene blue (MB), Cd 2+ , and phenanthrene (PHE) in aqueous solution. The adsorption behavior and efficiency toward different types of pollutants were studied by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), Raman, X-ray photoelectron spectroscopy (XPS), kinetics, and isotherm model fitting. The results showed that the change of pH had great effect on MB and Cd 2+ adsorption, but not on PHE. SBC not only possessed the newly formed sp 2 -hybridized domains with easy access to aromatic pollutants but also had multiple functional groups (-COOH, -OH, -SH, -NH 2 ) that provided adsorption sites for positively charged pollutants. SBC was more flexible and efficient in purifying pollutants compared to BC and OBC, with the saturated adsorption capacities of MB (209.16 mg/g), Cd 2+ (786.19 mg/g), and PHE (521.58 mg/g). Moreover, the adsorption kinetic and isotherms fitting showed that the adsorption mechanisms were closely related to the structure of biochar and the properties of pollutants, including π-π interaction, surface charge, electrostatic interaction, surface functional groups, and Van der Waals force. In addition, the analysis of structure-function relationship demonstrated the enhanced hydrophilicity and the easy exposure of the binding sites on OBC and SBC. Hence, it was significantly effective to regulate microstructure and interfacial properties to promote its adsorption behaviors of biochar.

Biochar (BC) has been widely used to remove heavy metals from wastewater. However, due to the hydrophobicity of BC and the lack of its surface functional groups, the effect of metal ions adsorption onto BC is limited. In order to improve the adsorption efficiency, L-cysteine was used to modify biochar derived from pomelo peel (PP) to regulate surface structure. The characteristics of BC and cysteine/biochar composite (cys/BC) were analyzed by various characterization methods. Results showed that the hydrophilicity of biochar was enhanced, and the number of surface functional groups was increased, resulting to strong adsorption ability of Ag(I) (618.9 mg/g), Pb(II) (274.5 mg/g), and As(V) (34.7 mg/g) for cys/BC, which increased approximately by 15%, 35%, and 29% compared with that of BC, respectively. The adsorption process of Pb(II) onto cys/BC was fitted better by the Freundlich isotherm model and for Ag(I) and As(V) by the Langmuir isotherm model. Moreover, the adsorption kinetics followed pseudo-second-order equation and the adsorption process was controlled by the intraparticle diffusion for Ag(I), Pb(II), and As(V) adsorption onto cys/BC. In addition, the adsorption capacities of cys/BC for Ag(I), Pb(II), and As(V) decreased slightly after five adsorption/desorption cycles. Finally, the multiple adsorption mechanisms including functional groups, pore adsorption, surface complexation, and cations-π were analyzed. The paper demonstrated that the cys/BC composite could be reused as effective adsorbents for removing contaminants in the environment.

For soil and environmental remediation, biochar/struvite composites are prepared by the crystallization-adsorption method. The recovery rates of N, P, and Mg in the solution increase to 99.02%, 97.23%, and 95.22%, respectively, by forming 10% biochar/struvite composite. X-ray diffraction (XRD) patterns acquired from the 10% biochar/struvite composite show a crystalline structure of MgNH 4 PO 4 ·6H 2 O (PDF no. 15-0762) and release of the main nutrient elements (N, P, Mg) from the 10% biochar/struvite composite increases significantly compared to struvite. The solubility of the biochar/struvite composite is the highest in 0.5 mol/L HCl, second in 20 g/L citric acid, and lowest in water. The power function equation describes more precisely the cumulative release of N, P, and Mg from the biochar/struvite composite in distilled water, whereas it follows the simple Elovich equation in 20 g/L critic acid and first-order kinetics equation in 0.5 mol/L HCl. Leaching experiments are performed on the biochar/struvite composite in soil, and the results indicate that the biochar/struvite composite has a longer cycle of release of nutrients than traditional chemical fertilizers and has large potential as a slow-release fertilizer.