Treatment Of Cadmium Containing Wastewater By Enhanced Ultrafiltration And Foam Separation Below Critical Micelle Concentration

Micellar enhanced ultrafiltration?MEUF?is a new wastewater treatment technology which combines the adsorption characteristics of surfactant micelles and the interception characteristics of ultrafiltration membranes.It can efficiently remove metal ions from wastewaters.However,the traditional MEUF has the problem of large consumption of surfactant and secondary pollution of residual substances in permeate solution,which limits the application of MEUF technology in the treatment of heavy metal wastewater.In this paper,the enhanced ultrafiltration and foam separation with surfactant concentration below the critical micelle concentration?CMC?was used to treat the wastewater containing cadmium.Under the condition that the surfactant dosage was lower than CMC,low pressure membrane filtration and high efficient removal of heavy metal ions were achieved.SDS and Tritonx-100/SDS were used as surfactant systems to treat cadmium containing wastewater with enhanced ultrafiltration technology lower than CMC.The influence of various control factors on the separation efficiency of ultrafiltration was systematically studied.The adsorption and aggregation rules of SDS in solution,fouling layer and concentration polarization layer were studied,which provided theoretical basis for the realization of Cd²⁺removal by enhanced ultrafiltration below CMC.Then,the foam fractionation?FF?process was carried out by simple aeration of the permeate solution from ultrafiltration to achieve advanced treatment,and the control conditions of the foam separation process were optimized.Finally,the separation efficiency of the cadmium containing wastewater and the control of the minimum SDS dosage were analyzed by the MEUF-FF combined process.In this study,the adsorption of surfactants on the surface of solid membrane materials and concentration polarization under transmembrane pressure make it possible to enhance ultrafiltration performance with surfactant concentration lower than CMC.At the same time,by simple aeration after ultrafiltration,the utilization rate of surfactant can be effectively improved,and the dosage of surfactant can be significantly reduced under the same effluent requirements.In the first part of this paper,the characteristics of SDS solution lower than CMC and its adsorption on membrane surface were studied.The effects of other substances in the feed solution on the aggregation of SDS in the solution and the static adsorption on the membrane surface were investigated.The results showed that the surface tension of solution decreased significantly when Cd²⁺and Tritonx-100 were added into SDS solution.The existence of Cd²⁺and Tritonx-100 promoted the formation of SDS micelles in the solution and increased the zeta potential of SDS micelles.In addition,Cd²⁺and Tritonx-100 can also promote the static adsorption of SDS on the membrane surface.And under the static condition,SDS mainly adsorbs on the membrane surface,and the adsorption degree at the membrane pore is relatively low.In the second part of the paper,the efficiency and mechanism of enhanced ultrafiltration treatment of cadmium containing wastewater which is lower than CMC are studied.Under the condition of lower than CMC?SDS concentration range 0.05mmol/L-8 mmol/L?,the effect of operation conditions on the separation efficiency of ultrafiltration was investigated.The aggregation behavior of SDS in the fouling layer and the mechanism of Cd²⁺retention were analyzed.The results showed that when SDS concentration is merely 4 mmol/L,the retention rate of Cd²⁺can reach more than 90%.The complex system formed by Tritonx-100 and SDS further improved the retention of Cd²⁺.The concentration of Cd²⁺,SDS,the molecular weight of the membrane and the molar ratio of Tritonx-100/SDS all have significant effects on the separation of MEUF.The fitting results of the general adsorption model and Hermia's blocking model showed that the adsorption of SDS on the membrane surface occurred at low concentration,and different types of blocking occured at high concentration.The existence of SDS aggregates in the fouling layer was an important reason for the retention of Cd²⁺in the process of enhanced ultrafiltration lower than CMC.In the third part of the paper,the concentration polarization phenomenon and its effect in the process of cross flow ultrafiltration of cadmium containing wastewater were studied.The influence of operating pressure and cross flow filtration velocity on the separation efficiency of cross flow ultrafiltration and the characteristic parameters of concentration polarization were investigated.The results showed that the operating pressure and cross flow filtration velocity have influence on the concentration polarization and the retention of Cd²⁺.Concentration polarization resistance accounts for a large proportion of the total filtration resistance,and the concentration of SDS in the concentration polarization layer is far greater than that in the main phase solution,and greater than its standard CMC value.Therefore,SDS micelles with different structures will accumulate in the concentration polarization layer.The existence of SDS micelles in this region is also an important reason for the effective retention of Cd²⁺in the enhanced ultrafiltration process which is lower than CMC.In the fourth part,the foam frantionation was used for advanced treatment of the permeate from ultrafiltration.The foam properties of Tritonx-100/SDS composite system and the degree of removal and enrichment of Cd²⁺and surfactant under different conditions were investigated.The mechanism of Tritonx-100 in foam separation process was also discussed.The results showed that the molar ratio of Tritonx-100/SDS,Cd²⁺concentration and pH value all affect the performance of Tritonx-100/SDS foam.When the molar ratio of Tritonx-100/SDS is 0.1,Cd²⁺concentration is 10 mg/L,without adjusting the pH of the solution,the removal degree and enrichment degree of Cd²⁺and surfactant reached the highest value.There are two reasons for the effect of Tritonx-100 on foam separation efficiency.The foaming ability and foam stability of Tritonx-100/SDS composite system are better than those of pure SDS system.On the other hand,excessive Tritonx-100 promotes the formation of SDS micelle,therefore,Cd²⁺and SDS micelles may stay in the water phase,resulting in the reduction of separation efficiencyIn the fifth part of the paper,the effect of enhanced ultrafiltration foam separation on the treatment of cadmium containing wastewater below CMC and the minimum control of surfactant dosage were studied.The effect of single MEUF and MEUF-FF combined process on the treatment of cadmium containing wastewater with different surfactant systems was investigated.The results showed that MEUF-FF combined process had higher removal rate of Cd²⁺and lower residual rate of surfactant in both systems.In addition,under the same total concentration of surfactant,the separation effect of pure SDS system was good,The separation effect of Tritonx-100/SDS composite system is slightly lower than that of pure SDS system,but the amount of SDS is lower than that of pure SDS system.Under the condition of lower than CMC,the application of MEUF-FF combined process and Tritonx-100/SDS composite system can realize the minimum dosage control of SDS in the treatment process of wastewater containing cadmium.