Optimization of Ni(II), Pb(II), and Zn(II) Ion Adsorption Conditions on Pliocene Clays from Post-Mining Waste

Abstract

Lignite mining results in a significant amount of waste clayey materials—the so-called Pliocene clays, which are often considered useless mineral waste. One method of management is to use them as inexpensive adsorbents in water and wastewater treatment, or as permeable barriers in groundwater remediation. The study is an attempt to use the raw Pliocene clays to remove Pb(II), Ni(II), and Zn(II) ions from wastewaters. The mineralogical composition, physicochemical, granulometric, structural, and surface properties were examined. The value of the point of zero charge pH of the adsorbent was found to be 7.1. To determine the optimal adsorption conditions, the central composite design (CCD) method was used with the pH of the solution, metal ion concentration, and the adsorbent mass as independent variables, and the percentage removal as the response variable. The determined optimal values of the adsorbent mass and the solution pH turned out to be similar for all three metals, with a pH of 6–7 and an adsorbent dose of 18–24 g/dm3. In further tests, the pH was established slightly lower than it would appear from the CCD method in order to avoid the possible precipitation of metals in the form of hydroxides. The kinetics of sorption were investigated and it was found that the sorption process ran for several minutes. The effect of temperature showed that the adsorption was spontaneous and thermodynamically favored. The leachability of the metals indicated that Pb(II) and Zn(II) were strongly bound in the adsorbent, whereas Ni(II) manifested a larger mobility. It was found that the tested clays had quite good sorption properties in relation to the tested metal ions and could be used for water and wastewater treatment.