Batch and column studies on zinc removal from industrial wastewater using natural clinoptilolite: factors affecting sorption capacities and kinetics

Abstract

The ability of natural zeolite (clinoptilolite) to take up zinc (Zn) ions from industrial wastewater was studied in batch reactors and fixed bed columns. The sorption capacities and kinetic behavior of clinoptilolite for Zn ions was studied as a function of adsorbent dosage, particle size, pH, temperature, and conditioning conditions. Fixed bed experiments were conducted in order to examine Zn uptake by clinoptilolite (0.5–0.85 mm) using three different volumetric flow rates of 2–4–6 BV h−1 under a initial concentration of 18.4 mg L−1 and initial pH value equal to 4 and ambient temperature (23°C). The sorption capacities and removal efficients was strongly dependent on pH, and to a lesser extent on other parameters examined. The highest sorption capacities were obtained at pH 4, leading to 0.34 and 2.38 mg g−1 of clinoptilolite for the batch and column studies, respectively. The XRD pattern of the as-received, and treated clinoptilolite (with both 10% HNO3 and 0.73 M NaCl at pH 10) showed significant change in the position of the most intense peaks, leading to some destructions on the crystal structure of clinoptilolite. The analysis of sorption kinetics were examined using pseudo-second-order and intra-particle diffusion models. The sorption of Zn on clinoptilolite at different ranges of experimental parameters followed only pseudo-second-order kinetic model with high correlation coefficient, suggesting that the sorption mechanism might be a chemisorption process. The highest rate coefficient (k 2) was obtained at pH 2, which was 7.2 g mg−1 min−1. The intra-particle diffusion was not rate limiting step since the low correlation coeefficients were obtained and deviation from the origin was observed. In column studies, the decrease of the flow rate resulted in increase in both breakthrough and ion exchange capacities, optimal results were obtained at flow rates lower than 6 bed volume (BV h−1). The column, which was saturated at flow rate of 3 mL min−1, was regererated with 0.73 M NaCl at 0.5 mL min−1. Desorption efficiency was 83%. Clinoptilolite samples from Turkey are effective for removing heavy metals from industrial wastewater.