Insight into mechanism of aged biochar for adsorption of PAEs: Reciprocal effects of ageing and coexisting Cd2+.

Abstract Biomass derived biochar is a stable carbon-rich product with potential for soil amendment. Introduced into the natural environment, biochar will naturally experience 'ageing' processes that are liable to change its physicochemical properties and the mobility of sorbed pollutants over the longer term. To elucidate the reciprocal effects of biochar ageing and heavy metal adsorption on the affinity of biochar for organic pollutants, we systematically assessed the adsorption of diethyl phthalate (DEP), representative of phthalic acid esters (PAEs), to fresh and aged biochars with and without coexistence of Cd²⁺. Long-term oxidative ageing was simulated using 5% H 2 O 2 and applied to biochar samples made from corn cob, maize straw and wheat straw made by pyrolysis at both 450 °C and 650 °C. Our results showed that biochar made at lower temperature (450 °C) and from straw exhibited the higher adsorption capacity, owing to their greater polarity and abundance of O-containing functional groups. The adsorption of DEP onto fresh biochars was found to be driven by van der Waals force and H-bonding. Biochar made at the higher temperature (650 °C) displayed higher carbon stability than that produced at lower pyrolysis temperature. Oxidized biochar showed lower adsorption capacity than fresh biochar owing to the formation of three-dimensional water clusters on biochar surface, which blocked accessible sites and decreased the H-bonding effect between DEP and biochars. The coexistence of Cd²⁺ suppressed the sorption of DEP, via competition for the same electron-rich sites. This indicates that cation/π-π EDA interactions are the primary mechanism for PAE and Cd²⁺ stabilization on biochar. Our study sheds light on the mechanism of organic pollutant sorption by biochar, as well as the potential susceptibilities of this sorption to ageing effects in the natural environment. Graphical abstract Image 1 Highlights • The oxidative ageing and coexisting Cd²⁺ decreased adsorption capacity of DEP on biochars. • Lower temperature straw biochar has high adsorption capacity driven by van der Waals force and H-bonding. • Formation of three dimensional water clusters on oxidized biochar surface suppressed H-bonding adsorption of DEP. • Coexisting Cd²⁺ suppressed the adsorption of DEP via competition for the same electron-rich sites on biochar surfaces. • Cation/π -π EDA interactions between Cd²⁺, PAEs and biochars were the primary mechanism of DEP stabilization. Our results show that ageing increases the affinity of biochar for PAEs but decreases the differences between them. Sorption of PAEs will always be diminished by coexistence and sorption of heavy metals. [ABSTRACT FROM AUTHOR]