Factors affecting the adsorption capacity of dissolved organic matter onto activated carbon: modified isotherm analysis

Abstract

Effects of aqueous phase Ca(II) concentrations and pH levels on the adsorption capacity of dissolved organic matters (DOMs) as well as the capacity dependency on the sources and types of DOMs and activated carbons (ACs) were examined. The Ca(II) effect was studied for three coal-based ACs having different pore size distributions (PSDs) and for three DOMs contained in three water samples. For each water sample, four working solutions prepared by adding different dosages of Ca(II) (0–1.5 mM) were used. For the water sample whose adsorption capacity was least affected by Ca(II), the effect of pH was subsequently examined for four water pH levels (pH=5.5–10) and these three ACs. Isotherm data were analyzed in terms of a modified isotherm model selected from three models developed for normalizing isotherms of heterogeneous organic mixtures. The Ca(II) effect depended obviously on the DOMs used. For all three ACs, increasing Ca(II) greatly enhanced the adsorption capacity of a commercial humic acid. However, the capacity of the DOM in a naturally colored surface water source was much less affected and that of the DOM remaining after precoagulation was not affected. The DOM–Ca(II) interactions, rather than the AC–Ca(II) interactions, seemed to be the predominant mechanism that controlled the extent of the Ca(II)’s impact. Water pH greatly affected the adsorption of the surface water DOM remaining after precoagulation. This effect seemed to be attributed to pH’s capability in changing ACs’ charge characteristics reflected by zeta-potentials. The adsorption capacity increased as the water pH decreased and was well correlated as a function of pH and the volume of AC pores in sizes of 30–100 Å. ACs having more pores in this size region generally exhibited larger adsorption capacities.