Kinetic and isothermal study of lead ion adsorption onto natural bentonites with different cation exchange capacity (CEC) from Milos Island, Greece

A laboratory batch study has been performed to investigate the adsorption characteristics of lead (Pb²⁺) metal ions onto natural bentonite samples (B1, B2, B3) with different cation exchange capacity (CEC) values. Bentonite samples come from Milos island, Greece and were supplied by S&B Industrial Minerals S.A. Bentonites consist mainly of Ca-montmorillonite (>85%) with minor and different amounts of kaolinite, calcite and quartz. The CEC values of B1, B2 and B3 were 86.5 meq/100g, 95.7 meq/100g and 67 meq/100g, respectively. The specific surface area of B1, B2 and B3 was measured as 87.3 m²/g, 66.6 m²/g, and 80.1 m²/g, respectively. Equilibrium and kinetic experiments were performed. The effect of various physicochemical factors that influence adsorption, such as solution pH (2-6), amount of adsorbent (1-10 g/L), initial metal ion concentration (5-150mg/L), and contact time (20-360 min) were studied. The measured adsorption capacity was appreciably high for most experimental conditions. It has been found that the amount of adsorption of lead metal ion increases with initial metal ion concentration, contact time, solution pH but decreases with the amount of adsorbent. The adsorption process was strongly dependent on the pH of the medium with enhanced adsorption as the pH turns from acidic to alkaline side till precipitation sets in. The amount of Pb²⁺ adsorbed per unit mass (qe) of the adsorbent decreased with an increase in the amount of the clay adsorbent. This may be attributed to two reasons: (i) a large adsorbent amount effectively reduces the unsaturation of the adsorption sites and correspondingly, the number of such sites per unit mass are reduced resulting in comparatively less adsorption at higher adsorbent amount, and (ii) higher adsorbent amount creates particle aggregation, resulting in a decrease in the total surface area and an increase in diffusional path length both of which contribute to decrease in amount adsorbed per unit mass. The removal rate of bentonite increased with an increase in the initial metal ion concentration. Both Langmuir and Freundlich isotherm models fit well (R2>0.93) the adsorption process. By using the Langmuir isotherm, the maximum adsorption capacities for B1, B2 and B3 were found as 85.47 mg/g, 73.42 mg/g and 48.66 mg/g, respectively. In order to investigate the mechanism of adsorption, particularly potential rate-controlling step, the Lagergren pseudo-first-order kinetic model, the pseudo-second-order kinetic model and the intra-particle diffusion model were used to test the dynamic experimental data. Kinetic analyses not only allow the estimation of sorption rates, but also lead to suitable rate expressions characteristic of possible reaction mechanisms. The calculated kinetic parameters are of a great practical value for technological applications, since kinetic modeling successfully replaces time- and material-consuming experiments, necessary for process equipment design. Kinetic experiments clearly indicated that adsorption of lead metal ion (Pb²⁺) on bentonite was a two steps process: a very rapid adsorption of lead metal ion to the external surface was followed by possible slow decreasing intraparticle diffusion in the interior of the adsorbent which has also been confirmed by intraparticle diffusion model. Overall the kinetic studies showed that the lead adsorption process followed pseudo-secondorder kinetics.