Effect of adsorption nonlinearity on the pH-adsorption profile of ionizable organic compounds

Solution pH is an important factor in the adsorptive behavior of ionizable organic compounds (IOCs) in many industrial, commercial, and environmental contexts. A linear speciation model (LSM) that assumes the concentration-independent adsorption of charged and neutral species is often employed to model the pH-adsorption profile (edge). Deviations from that model--including the shift of the adsorption edge from its expected inflection point at pH = pK(a) and the appearance of an adsorption maximum ("hump") near the pK(a)--are sometimes used to infer the mechanism. We investigated the adsorption of six organic acids and bases on the nonfunctionalized, extremely low variable-charge surface of graphite. Isotherms at constant pH of both charged and neutral species were usually highly nonlinear, and the adsorption edges typically showed a shift, hump, or both. We postulate that this behavior is due to the gradual extinction of the dissolved neutral or charged species as the pH approaches and then crosses the pK(a). This leads to an increase in the affinity of that species for the solid resulting from the inherent nonlinearity of its isotherm. The extinction of the more strongly adsorbing species mainly causes the shift, whereas the extinction of the less strongly adsorbing species gives rise to the hump. A nonlinear speciation model (NSM) based on Freundlich or Langmuir equations was employed to fit the adsorption edge. The NSM captured both the shift and the hump and was superior to the LSM. Increasing adsorption nonlinearity of the neutral species shifts the adsorption edge in the acidic direction (organic bases) or alkaline direction (organic acids), whereas increasing nonlinearity of the charged species increases the hump size. Both the shift and hump size increase as the difference in adsorption affinity between neutral and charged species decreases. The results show that the concentration dependence alone can strongly affect the shape of pH-adsorption curve and should be taken into account in future modeling.