Pellet Coprecipitation Microfiltration For The Treatment Of Wastewater Containing Strontium, Iron And Nickel

Aiming at representative fission product90Sr and neutron activation products55,59Fe,63Ni in low level radioactive wastewater, application on radioactivewastewater treatment by a pellet coprecipitation microfiltration (PCM) process wasstudied. The influencing factors such as temperature, ionic strength and the dose offlocculant on the removal of strontium, iron and nickel were investigated.In our study, when Na2CO3and CaCO3were used as the precipitating agent andcrystal seed, the PCM process that produced dense and large size particles waseffective to remove strontium, iron and nickel ions from the wastewater. Thesuspended solid concentration of supernatant was declined and the load of thefollowed microfiltration process was alleviated. Simutaneously10mg/L of FeCl3usedas the flocculant was added into the membrane reactor to accelerate the settlement ofthe particles.The purpose of the crystal seed prilling was to increase the particle size, decresethe the suspended solid concentration of supernatant effluent and increase the quantityof aragonite. Strontium carbonate and aragonite has the same crystal structure withoctahedral form. So aragonite has stronger capture capability for Sr2+. The test resultshowed as follows: that after30times granulation, the turbidity and the hardness ofthe supernatant were low and stable. The particles were dense and the mean particlesize was48.90μm. The seed crystals possessed good fluidity and moderatedispersivity. XRD spectra indicated the presence of strontianite as mixed crystalprecipitated, evidenced by sharp peaks in seed samples.In the lab-scale tests, the strontium concentration in the effluent decreased withthe operation time. The effluent iron levels kept stable in the whole period of tests. Athigher temperature, the PCM process had better treatment effect on removal of thestrontium and iron. When the temperature was approximately16.5°C, the meanconcentrations of the strontium and iron in the effluent were23.55μg/L and1.00μg/L.The corresponding decontamination factor (DF) values were544and6590,respectively.As ionic strength declined and flocculant was added, strontium removalefficiency was improved. The PCM had also a higher removal capacity for nickel ionwith the higher temperature and lower ionic strength. As data indicated, the average concentration and DF of strontium could reach18.49μg/L and559, while those ofnickel were2.08μg/L and2519.Ferric chloride coagulation played an important role in relieving membranefouling. Membrane pore blocking and the cake layer deposited on it had great effecton reducing effluent strontium, iron and nickel concentrations. In the pellet reactor,Sr2+incorporated into CaCO3lattice to form strontianite. Fe(OH)3precipitated fromsolution solely. Compound xNiCO3·yNi(OH)2·zH2O adsorbed on the precipitatesurface and coprecipitated from the solution. The crystals were dense and had goodsettling performance to alleviate the burden of the membrane separator. Aftersedimentation for24h, the sludge volume could be minimized largely. In the PCMprocess the concentration factor (CF) value was more than1×103.