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Acid−base properties exert important influences on biochar's practical application as a soil amendment or contaminant sorbent. In this paper, a model with independent acidic/basic sites coexisting on a biochar surface is proposed to account for the acid−base behavior of biochar derived from two invasive species (Spartina alterniflora and water hyacinth). Modeling results show that a three-site model with two acidic sites and one basic site can reflect the acid−base behavior of biochar, increasing pyrolysis temperature results in the change in concentration (basic sites increase successively, while acidic sites decrease first and then increase) and strength (individual site dependent) of acidic and basic sites. Both the concentration and strength of the sites play important roles in the acid−base behavior of biochar. Theoretical simulations based on modeling results demonstrate that both surface net and local charge should be considered when electrostatic interaction is responsible for the biochar's environmental behavior. The site modeling procedure proposed in this study constructs a bridge between macroscopic pH and microscopic sites and is useful to describe the acid−base behavior of biomass and biomass-derived biochar.

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