Study On Pellet Co-precipitation Micro-filtration Process For Removal Of Strontium From Liquid Waste

The chemical precipitation method for radioactive wastewater treatment hasadvantages such as simplicity and cost-effectiveness. However, the difficulties insolid-liquid separation and sludge concentration are bottlenecks restricting thedevelopment of the method. In this paper, a pellet co-precipitation microfiltrationprocess was developed to treat the wastewater containing strontium.In the tests of pellet co-precipitation, the seed was prepared by CaCO3powdersand Na2CO3was used as the precipitating agent. The results on influence factorsshowed that30cycles was adequate for seed preparation, the optimum strringintensity was99.9s-1and the calcium in the raw water was helpful for strontiumremoval. During the continuous operation, the average concentration of strontium inthe raw water was4.622mg/L and the strontium decontamination factor was48aftersettlement. Membrane filtration and FeCl3coagulation could enhance the effect ofstrontium removal and the decontamination factors were97and109, respectively.With the operation time, the precipitate particle size gradually increased. When thetest was over, the precipitate formed had good settling characteristics and theconcentration factor reached10,000.Based on the tests of pellet co-precipitation, the pellet co-precipitationmicrofiltration process was studied to treat the strontium-containing wastewater at alab scale. FeCl3was added in the membrane separator. The average concentration ofstrontium in the raw water was12.05mg/L and the effluent strontium concentrationgradually declined to a stable level, with an average decontamination factor of577.The precipitate particles formed in the pellet reactor gradually grew larger with goodsedimentation properties. When the experiment was over, the precipitate in the systemwas separated easily from the liquid phase and directly discharged. The concentrationfactor was1958. The effluent was colorless and the turbidity maintained below0.1NTU. The average value of pH in the effluent was10.39, which needed to be adjustedwith acid before discharge. The removal of calcium and magnesium were96.33%and33.04%, respectively. The dosed iron could be completely removed by formingFe(OH)3precipitate.The strontium removal mechanism in the pellet co-precipitation microfiltration process was studied based on the lab-scale experiments. Chemical precipitation wasthe main reaction mechanism and the strontium concentration in the raw water playedan important role on the strontium removal. By fitting with the standard hole blockingmodel, it was found that membrane pore blockage and cake layer formation furtherintercepted strontium crystallites. In the membrane separator, the pH of the solutionwas above10and thus the negative charged flocs formed by FeCl3hydrolysis had anadsorption capacity for strontium in a certain degree.The effect of the ferric coagulant on the process was investigated by changingthe dosage of FeCl3. The experimental results showed that the addition of10mg/LFeCl3was helpful to improve the strontium removal. When compared with chemicalprecipitation, the contribution of coagulation on strontium removal was small.However, the ferric coagulation had a good function on reducing membrane fouling,which could reduce the membrane filtration resistance by78%. When the dosage ofFeCl3was increased to20mg/L, the membrane fouling was serious in the lateoperation period. Thus, the ferric dosage of10mg/L was appropriate for maintainingcontinuous operation of the process.