Cold Experimental Study On The Treatment Of Wastewater Containing Fission Product Nuclides By Combined Microfiltration Process

Microfiltration process combined with traditional process for the treatment of liquid radioactive waste can obtain higher decontamination factor and concentration factor. An adsorption-microfiltration process and a coagulating coprecipitation-microfiltration process were developed to treat the wastewater containing cesium and strontium respectively and the operating parameters of the coagulation-microfiltration process for the treatment of actinide elements wastewater were optimized as well in this paper.The adsorption-microfiltration process was researched to treat the low level cesium wastewater with the potassium zinc hexacyanoferrate as the adsorbent. The molecular formula of the adsorbent was K₂Zn₃[Fe(CN)₆]₂, and its structure was compacted with particle size of 0.275¹7.4μm. Its adsorption ability for cesium was good and the adsorption behavior was in accordance with the Freundlich type sorption isotherm and the pseudo-second order kinetics model. The results of the jar test showed that the decontamination factor was 75.8 and the removal reached 98.7% when the dose of the adsorbent was 0.330 g/L, whenas, the results of the lab-scale test with the same parameters showed that the effect of cesium removal was better than that of the jar test, which resulted from the continuous renewal of the adsorbent surface by which the adsorption capacity of the old adsorbent was improved. The more the adsorbent dose, the largerer the decontamination factor was while the smaller the concentration factor was. When the dose of the adsorbent was 0.330 g/L, 0.165 g/L, 0.083 g/L respectively and the cesium concentration of the raw water was about 100μg/L, the decontamination factor was 208.7, 128.8, 77.2 and concentration factor was 539, 1218, 2306 respectively. The effluent was colorless and transparent, the lowest turbidity was 0.05 NTU and the highest was 0.10 NTU. The calcium and the magnesium were not rejected while iron was rejected all and the concentration of CN- in the effluent, whose toxicity was serious, was lower than 0.002 mg/L and met the Integrated Wastewater Discharge Standard (GB8978-1996).The adsorption-microfiltration process with the montmorillonite as the adsorbent was not feasible because of the adsorbent does being too high to satisfy the engineering requirement.The coagulating coprecipitation-microfiltration process was feasible to treat wastewater containing strontium with Na₂CO₃ as the precipitant and FeCl₃ as the coagulant. In the batch operation mode, the decontamination factor reached 458 when the Na₂CO₃ dose was 2000 mg/L, the FeCl₃ dose was 10 mg/L and the strontium concentration in the raw water was about 10 mg/L, but the membrane fouling was serious which limited the application of the process. In the continuous operation mode, the mean decontamination factor was 183.6 and the concentration factor was 249 when the Na₂CO₃ dose was 2000 mg/L, the FeCl₃ dose was 20 mg/L and the strontium concentration in the raw water was 5.3 mg/L. The effluent quality was good with the turbidity being smaller than 0.10 NTU. The iron was removed all, the removal of the calcium was higher than 88% and that of the magnesium was lower, the softening of the water was favorable to the strontium removal.The operation effect of the full-scale cold test of the coagulation-microfiltration process for the treatment of actinide elements wastewater was good and the turbidity of the effluent was normal no matter the main or the auxiliary membrane worked. When the concentration factor was 565, the fluidity of the sludge was good and discharging from the membrane reactor was feasible.With reference to the results of the treatment of fission produce nuclides wastewater and actinide elements wastewater, a process for the treatment of mixed wastewater containing strontium and cesium simultaneously was designed preliminarily by which the strontium was removed first by coagulating coprecipitation-microfiltration with Na₂CO₃ as the precipitant and FeCl₃ as the coagulant, and then the cesium was removed by adsorption-microfiltration with the potassium zinc hexacyanoferrate as the adsorbent.