Reverse Osmosis-membrane Distillation-adsorption Combined Process for Advanced Treatment of High-salt and Low-level Radioactive Wastewater

A combined process of reverse osmosis (RO), membrane distillation (MD) and adsorption was used to treat high-salt and low-radioactive wastewater. The use of molybdenum-technetium generators in the nuclear medicine industry generates some wastewater containing 0.9% NaCl and multiple radionuclides, such as ⁹⁹Tc, ¹³⁷Cs and ¹⁰⁶Ru. Long-term exposure to ⁹⁹Tc and ¹³⁷Cs will cause a great damage to human health and even lead to cancer. Hence, in-depth research on the purification treatment of this wastewater is necessary. Perrhenate ions and stable isotope Ru and Cs were used to prepare a mixed feed solution consisting of \mathrmR\mathrme\mathrmO_4^- , Cs⁺, Ru³⁺ and 0.9% NaCl. The effects of operating conditions (such as pressure, temperature, and flow rate) on the flux as well as the effect of feed solution pH and nuclide ion concentration on the retention performance of RO and MD processes were investigated. The RO flux is linearly related to the inlet water pressure, and the MD membrane fluxes increase with the increase of feed liquid temperature (50-75 ℃) and flow rate (30-90 L/h). Under the conditions of a hot side flow rate of 90 L/h and a hot side temperature of 75 ℃, MD membrane flux reaches a maximum of 27.07 kg/(m²·h). The rejection rates of Re, Cs, and Ru are dependent upon the feed solution pH, and the minimum rejection rates for all nuclides are observed at the pH_IEP of the membrane (about 4). Due to the Donnan effect, the rejection rates of Cs and Ru increase when the feed solution pH is above 4, which also results in a high rejection rate of Re because of the electrostatic repulsions between the membrane and \mathrmR\mathrme\mathrmO_4^- . In comparison, the rejection rates of nuclides during MD process slightly change (<0.1%) over the range of feed pH values (3-9), remaining a high level of 99.9% or above. When the ion concentrations of Re, Cs and Ru exceed 0.75 mg/L, the rejection rates of RO for nuclides show an obvious decreasing trend, which is ascribed to the effect of concentration polarization. On the contrary, the rejection rates of membrane distillation for each nuclide increase with the increase of the feed concentration. When the concentrations of Re, Cs and Ru reach 1 g/L, the corresponding decontamination factors exceed 10⁴. The results of dynamic adsorption experiments show that the rejection rates of CL-N235 extraction resin for Re and nickel ferrocyanide potassium for Cs are more than 99.0% in the lower mass concentration range (<5 mg/L). Under the optimal operation conditions, the total decontamination factors of Re, Cs and Ru by the reverse osmosis-membrane distillation-adsorption combined process all exceed 10⁴, and the volume concentration factor reaches 40. Based on the high decontamination factor and volume reduction factor, the reverse osmosis-membrane distillation-adsorption combined process is an effective method for the treatment of wastewater generated by the use of molybdenum-technetium generator.