Sorption behavior of Cu(II) ions on insolubilized humic acid under acidic conditions: An application of Scatchard plot analysis in evaluating the pH dependence of specific and nonspecific bindings

Sorption behavior of Cu(II) ions on a humic acid-based solid phase, insolubilized humic acid (IHA), were investigated under continuous column conditions. The quantitative data were derived on the basis of breakthrough curves obtained at different experimental conditions. For the studied experimental conditions, the highest and lowest observable capacities of IHA toward Cu(II) were found as 48.34 and 0.84 μmol g−1, respectively. In order to evaluate the pH dependence of Cu(II) binding on IHA, the effect of influent concentration was investigated at three different pH values (1.0, 2.0 and 3.0), and adsorption isotherms were derived for each acidic condition. The characteristics of Cu(II) bindings were evaluated by using Scatchard plot analysis, and it was observed that under strongly acidic condition, pH 1.0, nonspecific and/or multi-type interactions between Cu(II) and IHA were the prevailing effects causing the Cu(II) sorption. On the other hand, at pH 2.0 and 3.0, the role of specific interactions was more clearly observed, and so at pH(s) 2.0 and 3.0, the binding of Cu(II) on IHA was mainly attributed to specific interactions rather than nonspecific ones. For pH 2.0 and 3.0, the binding types of Cu(II) on IHA were sub-classified according to different affinities observed between Cu(II) and IHA, and thus two main types of specific bindings were proposed for the studied influent concentration range (1.0 × 10−3 to 1.0 × 10−1 mol l−1): Type-1, high-affinity/low capacity and Type-2, low-affinity/high capacity bindings. Finally, probable benefit of each specific binding type in separation of Cu(II) from acidic aqueous media has been discussed in detail.