Effects of fulvic fractions on the pH-dependent sorption of Cu(II) to kaolinite

Sorption of copper on kaolinite in the absence and presence of four fulvic acid (FA) fractions fractionated using XAD-8 resin, including F4.8, F7.0, F11.0 and Feth fractions (eluted by pH4.8 buffer, pH7.0 buffer, pH11.0 buffer, and ethanol (95%), respectively, was investigated by batch experiments. Results showed that the binding of Cu(II) by pure kaolinite increased with an increase in pH values. The presence of each FA fraction significantly affects the sorption of Cu(II) to kaolinite. Below pH 6.3, Cu(II) sorption was pronouncedly promoted after adding FA fraction to binary systems, compared to that in pure kaolinite suspensions. Magnitude in enhancement of Cu(II) sorption to solid phase, which was caused by FA fractions, followed an order of F4.8>F7.0>11.0>Feth. Above pH 6.3, nearly all the Cu(II) were removed from solution in pure koalinte system, while a portion of Cu(II) left in aqueous phase via formation of dissolved Cu–FA complexes, was observed in systems in the presence of FA fractions. The largest Cu(II) distribution coefficients (Kd) between solid phase and aqueous solution at pH 3.15–5.7, are obtained from the ternary system with F4.8 fraction. The smallest Kd are from system in the presence of Feth fraction. Conversely, over a pH range from 5.7 to 7.0, the highest Kd values are from system with Feth fraction. Overall, F4.8 fraction exhibited the greatest effect on Cu(II) pH-dependent sorption to kaolinite, and Feth fraction had the least. Functional groups such as carboxyl, phenolic moieties of FA played the vital role in Cu(II) sorption to kaolinite at lower pH conditions, and solubility under neutral conditions.