Quantitative Characterization of the Site Density and the Charged State of Functional Groups on Biochar

Charge density and the proton dissociation state of surface groups play a crucial role in understanding biochar's sustainable applications in the environment. Such knowledge is still limited due to the complexity of biochar. Biochar RWB and RPB and their base/acid-treated equivalents (TWB and TPB, respectively) are characterized to testify the graphitization degree, specific surface area, porosity, zeta potential, "Boehm" acidic functional group contents, and surface charge vs pH curves. A trimodal NICA model with strong and weak acid-type groups and a basic-type group, each with a continuous proton affinity distribution (PAD), is applied to describe the functional group protonations and charge densities of TWB and TPB. Acidic group densities are roughly matched with the Boehm titration results. The log K-H and m(j,H) values show a linear dependence with log c(KCI) caused by electrostatic interaction. The trimodal PAD, which is a material-specific "fingerprint" of biochar, and the charged state of functional groups were calculated. The model also describes successfully the charge curves of three series of biochar reported by Li, M.; et al. Method to characterize acid-base behavior of biochar: site modeling and theoretical simulation. ACS Sustain. Chem. Eng. 2014, 2, 2501-2509.Mia, S.; et al. Aging induced changes in biochar's functionality and adsorption behavior for phosphate and ammonium. Environ. Sci. Technol. 2017, 51, 15, 8359-8367. NICA modeling allows for a quantitative comparison between biochar and humic substances, although the latter has no basic-type group. Quantifying the relation between the surface groups and the charge density of biochar with the NICA model analysis provides crucial information for assessing and improving the targeted applications of biochar in nature.