Thermodynamics ofCu2+Adsorption on soilHumin

The adsorption thermodynamic characteristic of Cu2+ from aqueous solution onto humin and also humic acid from a typical black soil in northeast China were examined at three different temperatures (298, 318 and 338 K) by batch isotherm experiments. Results showed that humin was structurally different from humic acid. The increase of temperature had a positive effect on the adsorption process. Freundlich equation described the equilibrium data better with respect to Langmuir equation. Thermodynamic parameters, i.e. standard free energy changes (ΔGº), standard enthalpy change (ΔHº) and standard entropy change (ΔSº), revealed that Cu2+ adsorption by humin and humic acid were a feasible, spontaneous and endothermic process with an increase in disorder. The values of isosteric heat of adsorption (ΔHx) increased with increasing surface Cu2+ loading, and the increase in ΔHx values was smaller for humin than for humic acid. The results indicated that humin and humic acid possessed energetically heterogeneous surfaces. Humic acid had greater surface heterogeneity than humin.