The pH-dependent adsorption of Ni²⁺ ions to sepiolite considering Ni precipitation

One of the challenges in measuring the adsorption of metal cations is that they may form metal hydroxide compounds at certain pH ranges. This becomes problematic when adsorption is quantified in terms of measuring a decrease in metal ions in solution, because metal hydroxy complexes are removed from solution through precipitation, leading to erroneously high determinations of adsorption. Within this context, the present study aimed to analyze the pH-dependent adsorption behavior of Ni²⁺ ions onto sepiolite, a naturally occurring magnesium silicate clay mineral, while simultaneously accounting for the precipitation of nickel compounds during the process. For this purpose, zeta potential, ion exchange capacity, and the adsorption behavior of 2.5×10^–3 mol L^–1 nickel (II) sulfate hexahydrate onto relatively high-quality sepiolite were investigated. Kinetic studies and thermodynamic assessments were delineated to enhance the understanding of adsorption isotherms, both for future research and practical applications in environmental remediation. The results showed that the adsorption of Ni²⁺ ions onto sepiolite before the onset of precipitation is governed by an ion exchange process involving the release of Mg²⁺ ions from the sepiolite matrix and the uptake of Ni²⁺ ions from the solution. Above the threshold of pH ~8 where Ni-hydroxide complexes begin to form, ~97% of nickel ions were present in the forms of NiOH⁺, Ni(OH)₂, and Ni(OH)^3–. Isotherms for total Ni²⁺ removal were constructed to distinguish true adsorption from precipitation phenomena. The calculated and experimental values for true adsorption were found to be in good agreement.