Sorption of ionizable organic chemicals to carbonaceous adsorbents: Solution pH change and contributions of different species

Abstract

The adsorptive performance of carbonaceous materials (CMs) for the removal and sequestration of ionizable organic contaminants (IOCs) is heavily dependent on the pH-controlled speciation of the objective chemicals. To date, there is no effective method for measuring the sorption of IOC species at a solution pH near the IOC pKa, which is usually of environmental relevance and essential for validating sorption models. In this study, a unique “pH dynamics method” was developed. The method relates the uptake of the IOC species with the solution pH changes during sorption on the basis of the proton mass balance in the adsorptive process. The effectiveness of the method to quantify the sorption of IOC species at a pH near the pKa was validated by determining the extents and mechanisms of the pH change in a series of sorption systems employing graphite, biochar, carbon nanotube and activated charcoal as representative CMs, and 4-methylbenzoic acid, 2,4,6-trichlorophenol, 4-methylaniline and quinoline as model IOCs. Moreover, with the derived species uptake using the pH dynamics method, the effectiveness of already-reported speciation models for predicting species sorption at a pH around the pKa was verified for the first time. It was shown that the speciation models, including the linear speciation model and nonlinear speciation model, were convincing for estimating the sorption of individual species at the pH around the chemical pKa when the collected uptake of individual species could be reliably predicted. This research advances the application of pH dynamics for quantifying the uptake of individual species of IOCs and enhances the understanding of the speciation models and carbonaceous materials in managing risks of IOCs.