Adsorption of pentachlorophenol to a humin-like substance–bentonite complex prepared by polycondensation reactions of humic precursors

Humic substances have natural surfactant characteristics and have a relatively high affinity for pentachlorophenol (PCP). However, humic substances cannot be loaded into bentonite (Bent) via intercalation because of their larger size. In the present study, a humin-like substance (HuLS) was prepared by polycondensation reactions of humic precursors, such as glycine and catechol, and the product was loaded into Bent. XRD patterns showed that the basal spacing, calculated from the montmorillonite peak, for the HuLS–Bent complex was larger than that for Bent itself, indicating that the intercalation of HuLS was successful. The adsorption capabilities of PCP were evaluated from adsorption isotherms at pH 4.0, 5.5 and 6.5. At all pH values, the linear adsorption coefficients of PCP, Kd (L kg− 1), for the HuLS–Bent complex (37 at pH 6.5, 247 at pH 5.5 and 804 at pH 4.0) were significantly larger than those for Bent itself (not determined at pH 6.5, 40 at pH 5.5 and 94 at pH 4.0). These results show that the adsorption capabilities of PCP can be enhanced by loading HuLS onto Bent. In addition, the Kd values for Bent and the HuLS–Bent complex were found to be pH-dependent, in that the Kd value decreased with increasing pH. Because of the pKa for PCP (4.75), more than 90% of the PCP was present as the anionic form at pH 5.5 and 6.5. In addition, the values for the Zeta potentials for Bent and the HuLS–Bent complex remained negative at pH 2–12 and decreased with increasing pH. It thus appears that electrostatic repulsion between the anionic form of PCP and the adsorbents can affect the adsorption capabilities.