Elaborating the mechanism of lead adsorption by biochar: Considering the impacts of water-washing and freeze-drying in preparing biochar.

[Display omitted] • The impacts of water-washing and freeze-drying pretreatment on biochar are studied. • Biochar with combined pretreatments (W-FD-PB) has a large surface area (212.69 m2/g). • Q m of Pb2+ on W-FD-PB is 406.99 mg/g, 1.53-fold of W-PB and 2.19-fold of PB. • Precipitation and ion exchange were enhanced during Pb2+ adsorption by W-FD-PB. • The addition of biochar significantly reduced DTPA-Pb concentrations in soil. This paper examined the impacts of different pretreatments on the characteristics of biochar and its adsorption behavior for Pb2+. Biochar with combined pretreatment of water-washing and freeze-drying (W-FD-PB) performed a maximum adsorption capacity for Pb2+ of 406.99 mg/g, higher than that of 266.02 mg/g on water-washing pretreated biochar (W-PB) and 188.21 mg/g on directly pyrolyzed biochar (PB). This is because the water-washing process partially removed the K and Na, resulting in the relatively enriched Ca and Mg on W-FD-PB. And the freeze-drying pretreatment broke the fiber structure of pomelo peel, favoring the development of a fluffy surface and large specific surface area during pyrolysis. Quantitative mechanism analysis implied that cation ion exchange and precipitation were the driving forces in Pb2+ adsorption on biochar, and both mechanisms were enhanced during Pb2+ adsorption on W-FD-PB. Furthermore, adding W-FD-PB to Pb-contaminated soil increased the soil pH and significantly reduced the availability of Pb. [ABSTRACT FROM AUTHOR]