Membrane Fouling During Ultrafiltration Treatment Of Wastewater Contained Heavy Metals Complexed By Palygorskite

Complexation-ultrafiltration technology is an efficient process for the treatment of heavy metals.The complexing agents commonly used in complex-ultrafiltration are mostly organic polymer complexing agents.But these complexing agents are expensive and are difficult to popularize.Palygorskite is a good material for the adsorption of heavy metals.It is cheap and rich in reserves which can be used as a substitute for traditional complexing agents.However,the use of palygorskite to adsorb heavy metals will cause high turbidity in wastewater.In the process of ultrafiltration,membrane fouling is serious.Membrane fouling has always been an important reason for limiting the development of membrane technology.In this paper,the membrane fouling and the membrane cleaning efficiency of complexation-ultrafiltration with palygorskite were studied.In this study,the palygorskite in Tianshui,Gansu Province was used as raw material.After crushing,sieving and purification,the palygorskite colloid was prepared by high-speed stirring.The palygorskite colloid was added to the artificially prepared heavy metal wastewater,and ultrafiltration was used to remove heavy metals.In the process of complexation ultrafiltration,flux declines in the treatment of three heavy metals under different membrane pore sizes and operating conditions were investigated.And the distribution of membrane resistance after the end of the run was examined.After the end of the ultrafiltration operation,the ultrafiltration membrane is cleaned using air-water flushing,ultrasonic+water washing,water backwashing,chemical cleaning to study the cleaning efficiency of different cleaning methods,cleaning temperature,cleaning time,cleaning cycle,and cleaning agent on membrane flux recovery rate.The results show:In the process of complexation-ultrafiltration,the decline in membrane flux during the treatment of heavy metals with four different molecular weight cut-off membranes is similar,which can be roughly divided into three stages.In the first stage,the flux declines slowly.In the second stage,the flux drops very quickly.And the decline in flux in the third phase tends to be gradual.After 150 minutes,the flux of the membrane after Cu²⁺and Zn²⁺treatment basically stabilized,but the flux after treatment of Cd²⁺continued to decline.When using 5 g/L palygorskite for complexation-ultrafiltration,the time for stable flux should be less than 300minutes.Comparing the flux reduction rates of the four membrane pore diameters,it can be seen that the higher the molecular weight of membrane retention is,the more severe the flux decline.The ultrafiltration membranes with MWCO of 6,000 and 10,000 Da have the highest proportion of resistance.For 50000,100000 Da MWCO ultrafiltration membrane,the cake layer resistance has the highest proportion of resistance.Operating pressure and temperature have similar effects on membrane flux.In the initial stage of operation,the higher the operating pressure and temperature is,the higher the membrane flux is.However,as the operating time increases,the operating pressure and temperature will have less effect on the membrane flux.The higher the pressure and temperature is,the higher the overall rate of decrease is in membrane flux.Under the same conditions,the membrane specific flux after treatment with Cd²⁺was significantly higher than the membrane specific flux after treatment with Cu²⁺and Zn²⁺.The proportion of the membrane's own resistance gradually decreases with the increase of operating pressure and temperature,and the proportion of cake layer resistance and concentration polarization increased.At different initial pH values,the decline in membrane flux has a slow decline followed by a rapid decline and finally tends to a stable trend.In the initial 20 min of operation,the rate of flux decline at pH 3 is higher than that at other pH.After 150 minutes of operation,the overall decrease in the flux of the ultrafiltration membrane increased with increasing pH.The ratio of the membrane's own resistance is the highest at all pHs.As the pH increases,the proportion of concentration-polarization resistance increases.The higher the amount of palygorskite is,the higher the turbidity of the solution is,and the greater the effect on the membrane flux is.When the amount of palygorskite was 1 g/L,the flux reduction rate is less than 12%.The higher the amount of palygorskite is,the higher the flux decline rate is.When the complex-ultrafiltration process used to treat the heavy metals Cu²⁺and Zn²⁺,the flux declines by more than 50%.While the complex-ultrafiltration process used to treat the heavy metals Cd²⁺,the flux declines by 36%.With the increase of the amount of palygorskite,the proportion of the membrane's own resistance decreases,and the total proportion of the cake layer resistance and concentration polarization resistance continues to increase.In gas-water cleaning,the recovery rate of membrane flux is the highest when the ratio of gas to liquid is 25:1.The recovery rate of flux is basically stable when the cleaning time is 20min.The recovery of membrane flux is better when the cleaning cycle is less than 60 min.Ultrasonic time and sodium hexametaphosphate can effectively improve the cleaning efficiency in the ultrasonic and water washing.The recovery rate of membrane flux is the highest when the ultrasound is five minutes and the concentration of sodium hexametaphosphate is 0.5%.The cleaning efficiency is gradually improved with the increase of repetition times of ultrasonic+water washing.After 3 times of repeated cleaning,the flux recovery rate is basically stable.And the recovery rate of membrane flux is over 97%when the cleaning cycle is less than 60minutes.When pure water is used for backwashing,the flux recovery rate basically stabilizes after 12 minutes of backwashing.After backwashing for 20 minutes,the backwash temperature and backwash flow can improve the recovery rate of membrane flux,but the increase in flux recovery rate is not obvious.When using four chemicals to enhance backwashing,the HCl solution works best and the flux recovery rate can reach 97%.Comparing the recovery rates of the different backwash cycles,the recovery rate of membrane flux when using 45°C pure water is best under the cleaning cycle of 30 and 60 minutes.The recovery rate of membrane flux when using hydrochloric acid is best under the cleaning cycles of 90 and 150 minutes.The chemical cleaning of the four cleaning agents can be found that the best is HCl,and the best chemical soaking time is 4h.Flux recovery rate increases with increasing HCl concentration.