Study on the Pb(II) adsorption mechanism on acid‐modified ceramsite made from sewage sludge and industrial solid wastes.

Ceramsite was made from sludge, phosphogypsum, and red mud and then modified with different concentrations of hydrochloric, nitric, or citric acid. The resulting composite ceramsite was subsequently utilized for the adsorption of Pb(II). The maximum adsorption capacities of hydrochloric acid– and nitric acid–modified ceramsites were 90.38 and 88.57 mg/g, respectively, which were much higher than those of unmodified ceramsite (66.86 mg/g) and citric acid–modified ceramsite (57.85 mg/g). The variation of isoelectric point, acidic and basic groups, and Fourier transform infrared spectra of ceramsite were compared before and after adsorption. A decrease of isoelectric point implied weakening of electrostatic repulsion between Pb(II) and the modified ceramsite. Functional groups, such as O─H, C═O (or C═C), and Si─O─Si (or Al─O─Al), were involved in the adsorption. In addition, phenol hydroxyl, which exhibited a significant increase in modified ceramsite and a noticeable decrease after adsorption, was the most important group. Besides, the modified ceramsite had larger specific surface areas and pores (30.66 g/m2,.11256 cm3/g), more functional groups, and lower isoelectric points (8.57), which were favorable for adsorption. The adsorption processes described by pseudo‐second‐order kinetics were controlled by chemical adsorptions. Isothermal studies indicated that monolayer adsorption occurred on hydrochloric acid–modified ceramsite and multiple processes occurred on nitric acid– and citric acid–modified ceramsites. Thermodynamic analysis confirmed Pb(II) adsorption on modified ceramsite was spontaneous and endothermic. [ABSTRACT FROM AUTHOR]