Effect of inherent alkali and alkaline earth metals in biochar on adsorption of Pb2+ in aqueous solution: Different roles of Na/Mg/K/Ca

It is significant to investigate the effect of inherent alkali and alkaline earth metals (AAEMs) content on the adsorption characteristics for the selection and modification of biochar in the application of Pb2+ removal. The AAEMs content in biomass is modified by acid washing and impregnation. The different roles of Na, Mg, K, Ca are determined through quantitatively analyses of the adsorption mechanisms. The results reveal that the adsorption performance of biochar is improved with the increase of AAEMs content by enhancing the precipitation and ion exchange capacities. The maximum adsorption capacities of 0.5MNa-BC, 0.5MMg-BC, 0.5MK-BC, and 0.5MCa-BC are 195.8, 342.7, 235.5, and 317.7 mg g(-1) respectively, whereas the AAEMs content in biochar is around 2.2 mmol g(-1). Na and K in biochar mainly promote the precipitation capacity (Na > K), while Mg and Ca mainly enhance the ion exchange capacity (Ca > Mg). Moreover, the rate of precipitation reaction is higher than that of ion exchange. Therefore, Na and K enriched biochar is recommended for short contact time application scenarios, while Mg and Ca enriched biochar is recommended for high adsorption capacity requirements.