Removal Of Cadmium In Raw Water By Enhanced Water-soluble Polymer Ultrafiltration Process

Water-soluble polymer enhanced ultrafiltration process?PEUF?that combining polymer capture technology and ultrafiltration membrane separation technology,is a new and efficient way to treat heavy metal in raw water.In this paper,PEUF process was used to treat cadmium-containing simulated polluted raw water.Sodium polyacrylate?PAANA?,chitosan?CTS?and polyvinyl alcohol?PVA?were used as complexing agents,then compare the treatment effect of Cd²⁺in simulated polluted raw water and the pollution degree of ultrafiltration membrane under various control factors,and selecte the best polymer.On the basis of revealing the mechanism of the complexation reaction between Cd²⁺and polymer,the optimal complex was used to optimize the experimental parameters,the effects of pH,loading ratio P/M,co-existence cation,competitive complexing agent,Cd²⁺initial concentration and other control factors on its retention rate and membrane fouling were studied to determine the optimal technological conditions,the cleaning method suitable for this process was also investigated.Finally,the acid decomposition recovery of the polymer was carried out and its recycling efficiency was evaluated.,the experimental results are as follows:1.Selection of water-soluble polymers.The processing capacity of Cd²⁺was ranked from strong to weak as PAANA>CTS>PVA;PAANA has a high retention rate for Cd²⁺at low P/M;None of the three complexants is suitable for acid environment;Ion strength had minimal effect on PAANA-Cd²⁺system;The optimal water-soluble polymer is PAANA.2.The influence of all control factors on the retention rate and reaction mechanism of PEUF process for treatment of Cd²⁺.The complexation reaction rule of PAANA and Cd²⁺can be well described by the quasi-first-order rate equation,The complexation capacity of Cd²⁺is 0.033 g Cd/g PAANA;With the increase of P/M and pH,as well as the decrease of HA concentration and co-existed cation,the retention rate of Cd²⁺increases;When P/M is high,the retention rate is less affected by pH drop;The degree of influence of co-existence cation on retention rate decreased from strong to weak was Ca²⁺>Mg²⁺>Na⁺;The presence of HA reduces the retention rate of Cd²⁺;The increase in the initial concentration of Cd²⁺will result in a decrease in its retention rate;Under neutral conditions,the suitable loading ratio of Cd²⁺treated by PAANA is P/M=10,and the optimal pH is 7.0.3.The affects of each control factors on membrane fouling and cleaning of PEUF process.With the increase of P/M,the degree of membrane fouling increases,mainly reversible pollution;Membrane fouling is serious and the proportion of irreversible pollution is large under acidic conditions;The degree of membrane pollution caused by co-existence cation was ranked from strong to weak as Ca²⁺>Mg²⁺>Na⁺,and the majority of membrane pollution was reversible pollution;The presence of HA,a competitive complexing agent,led to the increase of membrane fouling,and the proportion of irreversible pollution increased;2%NaOH alkaline washing and 3ppm NaClO washing are good membrane cleaning solutions,and acid washing is not suitable for the membrane cleaning of this process.4.Study on polymer recovery and polymer reuse effect.Under pH=2.5,the first-order rate model can effectively describe the dissociation kinetics of Cd²⁺and PAANA;With the increase of volume dilution factor?VDF?,the recovery rate of polymer decreases,and the recovery rate of Cd²⁺increases;VDF=2 is the best value,and the actual recovery rate of PAANA reaches 98.5%;The recycling efficiency of the regenerated polymer through acid decomposition is good,and the retention rate of Cd²⁺under optimal conditions is only 1.2%lower than that of fresh polymer.