Isothermal, Kinetic, and Thermodynamic Study of Copper Ion Adsorption Using Raw Ranufe Bentonite and Its Sodium and Pillared Forms

Abstract

Heavy metals in industrial waste cause environmental damage because they metals are toxic, nonbiodegradable, and are bioaccumulated. As such, treatments are needed to reduce heavy metals to legally acceptable levels before disposal. As such, the aim of this study was to assess copper ion adsorption in aqueous solution using Ranufe bentonite (raw, sodium and pillared), which is abundant in Northeastern Brazil. Sodium bentonite was obtained using ionic exchange with sodium acetate and pillared bentonite was formulated with a pillarizing agent (aluminum hydroxide and sodium). Treatment efficiency was assessed using Brunauer–Emmett–Teller (BET), X-ray fluorescence (XRF), and X-ray diffraction (XRD) analyses. The surface area obtained by BET showed behavior characteristics of a mesoporous material. XRD exhibited basal spacing consistent with the pattern of clays. XRF analyses confirmed the changes on the raw bentonite surface, proving its transformation in the sodium and pillared forms. Adsorption capacity was assessed as a function of pH, contact time, adsorbent mass, initial metal concentration, ligand and co-ion effects on copper removal efficiency, reaching a maximum rate of 85.76%. Adsorption isotherms were fit to the Langmuir model, assuming adsorption on homogeneous surfaces in the form of monolayers, with maximum adsorption capacity of 12.92 mg/g−1 (raw bentonite) and 24.51 mg/g−1 (sodium bentonite). The kinetic study demonstrated that bentonite (raw and sodium) fit the pseudo-second-order kinetic model, indicating a process governed by ionic exchange between copper and the ions present in the bentonite layers. The thermodynamic adsorption parameters demonstrated that adsorption using raw and sodium bentonite is endothermic (favored by the rise in temperature) and nonspontaneous (energy is needed to promote adsorption). In a study on desorption, four adsorption and desorption cycles were sufficient to assess the regeneration capacity of the material, showing a decrease in adsorption capacity from the first to last cycle from 69.47% to 41.33% (raw bentonite) and from 48.36% to 22.46% (sodium bentonite). A decline in adsorption capacity is related to chemical degradation caused by acid solutions and mechanical agitation during the adsorption cycles.