Sorption properties of Pb²⁺ ions from water by alkali activated slag/biochar composites

In this study, alkali activated slag and biochar composites were investigated as a new sorbent for the removal of Pb²⁺ from aqueous solutions. The combination of biochar and alkali activated slag matrix resulted in successful removal of Pb²⁺ from aqueous solutions. The sorption properties are closely related to the mineral composition and microstructure of the sorbents. The maximum sorption capacity of Pb²⁺ was found to be 395.0 mg/g for the sample S2 (0.100 g) made from alkali activated slag with 15.4 wt% of biochar at an initial concentration of 1000 mg/L. Ion exchange and chemisorption dominated the mechanism of Pb²⁺ sorption using the alkali activated slag and biochar composites, and precipitation can occur because of complex formation during adsorption. XRD and FTIR analysis confirmed the precipitation of new chemical compounds: lead carbon hydrogen oxide (2PbCO₃∙Pb(OH)₂) and susannite (Pb₄(CO₃)₂(SO)₄(OH)). The incorporation of lead cations into the structure of calcium silicate hydrate (CSH) was confirmed by FTIR analysis. The elemental composition (according to EDX) of the sorbent materials showed that Pb²⁺ appeared after the sorption process, and sodium cations were not detected, confirming the ion exchange process during sorption. Therefore, all three composite sorbents showed satisfactory sorption capacities and can be considered efficient sorbents for the removal of Pb²⁺ cations from aqueous solutions. The conversion of low-cost composites of alkali activated slag and biochar as a sorbent benefits both waste management and environmental protection.