Adsorption of humic substances onto kaolin clay related to their structural features

Eleven well characterized humic substances (HSs) were adsorbed from aqueous solution onto a Na-kaolin clay. The adsorption affinity (K-L), maximum adsorption capacity (b), a coefficient of desorption hysteresis (H), and the concentration of irreversibly adsorbed HS (IHS) were derived from adsorption-desorption isotherms. These parameters were correlated with structural features of the HS. The adsorption affinity was shown to correlate directly with the aromaticity of the HS and inversely with their polarity, expressed as the O/C atomic ratio. A dependency between polarity and maximum adsorption capacity was not confirmed. The parameters b, H, and IHS expose close correlation with the molecular weight (MW) and the partial negative charge of HS (Z) at the operating pH value. The following quantitative relationship was obtained: b = 715 - 0.06 x MW - 529 x Z (r = 0.92). It allows a selection of HS with respect to the largest content of organic matter in HS-kaolin clay complexes. Among the HS studied the high molecular weight materials enriched with C- and H-substituted aromatics, such as coal and peat humic acids (HAs), are shown to be the most preferential materials for preparing stable HS-clay complexes.