Effect of the type of ion-exchange resin on Mn²⁺ adsorption in the presence of competing cations

Abstract

Present in soils, ground and surface waters, manganese is among the most common metals in Earth crust. It is also an essential trace element to the functioning of several enzymes in the human body. However, exposure to high manganese concentrations can also be harmful to humans with psychiatric and motor effects and therefore, manganese concentrations in drinking water and also industrial effluents are regulated. In the current work, the adsorption of Ca<sup>2+</sup>, Mg<sup>2+</sup> and Mn<sup>2+</sup> on three different ion-exchange resins: (i) aminophosphonic acid - chelating (Purolite S950), (ii) polyacrylic weak acid cation (Purolite C104E) and (iii) polystyrene strong acid cation (Purolite C100) was investigated. The results revealed that Purolite S950 had the highest Mn2+ uptake (37.9 mg/mL-resin or 0.69 mmol/mL-resin) as compared to Ca<sup>2+</sup> (3.2 mg/mL-resin or 0.08 mmol/mL-resin) and Mg<sup>2+</sup> (~0 mg/mL-resin) and was selected for further kinetics and equilibrium studies. The results indicated Purolite S950 as particularly suited to be applied in the treatment of neutral mine waters with high Mg/Mn ratios. Additionally, Purolite S950 showed a small affinity for Ca<sup>2+</sup> and therefore an efficient Mn<sup>2+</sup> removal will depend on the Ca/Mn ratio of the mine water under treatment. According to the kinetic analysis, manganese sorption on Purolite S950 was described by the pseudo-second order model (r<sup>2</sup> > 0.98) with an activation energy of 10.40 kJ/mol and thus pore-difussion was the rate controlling step of the process. In terms of equilibrium studies, manganese sorption on Purolite S950 followed the Langmuir model with maximum loadings of up to 41.5 mg/mL-resin. The thermodynamic modelling indicated an exothermic process (-85.0 kJ/mol, as standard enthalpy) with a standard entropy of -274 J/mol.K, which was ascribed to the release of two adsorbed H<sup>+</sup> ions for each Mn<sup>2+</sup> ion taken up from solution