Adsorption of sulfate ion from water by zirconium oxide-modified biochar derived from pomelo peel

Zirconium oxide-modified pomelo peel biochar (ZrBC) was synthesized for the adsorption of sulfate ion from aqueous solution. Zirconyl chloride octahydrate (ZCO) was used to modify pomelo peel biochar into ZrBC. The optimal dose of ZCO for modification is 0.5 mol/L, at which ZrBC shows the highest adsorption of sulfate ion. The adsorbents were characterized by the field emission scanning electron microscopy, X-ray photoelectron spectroscopy and surface area measurement. The results confirm that the presence of zirconium oxides and hydroxide groups on the ZrBC surface, and ZrBC has a porous structure and a higher specific surface area in comparison with pomelo peel biochar. ZrBC shows good affinity for sulfate ion with a maximum sulfate adsorption capacity of 35.21 mg/g, which is much higher than that of pomelo peel biochar (1.02 mg/g). The adsorption of sulfate on ZrBC is pH dependent, and acidic conditions favor the adsorption. The adsorption can reach near-equilibrium in approximately 120 min. The adsorption kinetics and isotherm follow the pseudo second-order equation and Langmuir adsorption model, respectively. Furthermore, nitrate and fluoride anions exhibit little influence on the adsorption of sulfate by ZrBC, whereas phosphate inhibits the adsorption under the same concentration conditions. ZrBC has the potential to be used for removal of sulfate from aqueous solution.