Removal of Pb²⁺ and Cu²⁺ from artificial geothermal brine by zeolite at various salinity and temperature conditions

Natural zeolite (predominantly clinoptilolite - Ca) was tested for application in geothermal facilities to remove copper (Cu²⁺) and lead (Pb²⁺) from formation fluids. Batch and dynamic flow-through (only for lead ions) experiments were performed at different salinities (I = 0.1 and 1 mol/L) in NaCl or CaCl₂ solutions at up to 115 °C (batch experiments) and up to 130 °C (flow-through experiments), respectively. The batch experiments resulted in an uptake of up to 24 % at an initial concentration of 0.3 g/L and up to 93 % at 0.03 g/L of Pb²⁺ and 115 °C. For Cu²⁺, the uptake reached 100 % from initial 0.004 g/L. The presence of CaCl₂ in the solution matrix reduced the uptake of the heavy metals as compared to the NaCl matrix. The Pb²⁺ uptake by zeolite granulates at dynamic flow-through conditions at 130 °C was 52 % of the initial value of 20.7 mg/L during the 180 min experimental time. Based on the results from batch experiments the data fitting indicated a site density of monovalent exchange sites of 3.0 ± 2.6 mmol/g solid and a cation exchange capacity (CEC) of 295 ± 26 meq/100 g zeolite. The Pb²⁺ uptake process by zeolite gave a K_D value of log K = 1.53. These calculations were not possible for Cu²⁺ uptake indicating that the uptake mechanisms of Cu²⁺ at high salinities are too complex to be simulated. To explain the uptake processes at given conditions various structural analysis were performed. Infrared spectroscopy indicated a cation exchange of lighter ions in the crystal structure of zeolite by the heavier ions Cu²⁺ and Pb²⁺ at 650 to 750 cm⁻¹. After the treatment with heavy metals, two new bands were detected at 2902 and 2982 cm⁻¹, which were not observed in the natural zeolite. With X-ray diffraction (XRD) new crystalline phases were detected in the treated zeolite samples that could be attributed in case of Pb²⁺ uptake to cottunite (PbCl₂) and laurionite (PbOHCl) and in the case of Cu²⁺ uptake to Cu₂Cl(OH)₃ and CuCl₂. The simulated G(r)s supported these observations regarding Pb²⁺ uptake. Using electron microscopy methods, both mechanisms (ion exchange and mineral precipitation) were confirmed: the formation of new Pb²⁺ and Cu²⁺ containing mineral phases on the surface of the zeolite granules and the uniform distribution of chloro-coordinated complexes of Cu²⁺ and Pb²⁺ on the pore surfaces inside the zeolite.