Water Purification Efficacy And Mechanism Analysis Of Ultra-Low Pressure Driven Catalytic Ceramic Membrane

In view of the two major challenges of water shortage and water pollution,it is necessary to develop advanced water treatment technologies for water reuse.Membrane separation technology has been widely applied in the field of water reuse.Comparing with traditional ultrafiltration,the emerging ultra-low pressure driven membrane filtration technology has the advantages of low energy consumption,easy operation and low capital cost,and thus has been studied in a variety fields of water treatment,including:gray water treatment,rainwater reuse,printing and dye wastewater treatment,and disaster-relief application.The hydraulic driving pressure of ultra-low pressure driven membrane filtration is usually 0.1 bar and below.Such hydraulic driving pressure is generally provided by the gravity flow to reduce the use of pumps and motors so as to further reduce costs.Compared with traditional ultrafiltration,ultra-low pressure driven membrane filtration often possesses better retention performance on pollutants.However,its retention mechanism other than the size exclusion mechanism has not been reported in detail,and the related membrane fouling mechanism is not well-rounded.Albeit the superior retention performance than traditional ultrafiltration,ultra-low pressure driven membrane filtration technology mostly uses mesoporous and macroporous membranes.Therefore,its removal efficiency for dissolved organic matter in water is still not ideal,and its removal of the emerging micropollutants is minimal.In response to the above challenges,this paper examines the retention and membrane fouling mechanisms of ultra-low pressure driven membrane filtration technology,and integrates in-situ chemical catalysis and in-situ electro-catalysis with membrane filtration to further improve the efficiency of ultra-low pressure driven membrane filtration.Ceramic membranes are used as substrates for the preparation of catalytic membranes due to their strong chemical and thermal stability.This paper first investigated the filtration behavior and retention mechanism of the ultra-low pressure driven membrane filtration.Aiming at three typical organic pollutants with different physicochemical properties and sizes far smaller than the membrane pore size:humic acid（HA）,bovine serum albumin（BSA）,and lysozyme（LYS）,ceramic membrane with a molecular weight cut-off of 300 k Da is applied in the gravity-driven membrane filtration for the pollutant removal.The retention and membrane fouling performances are studied under different hydraulic driving pressures（10 cm-110 cm gravity-driven height）.Combining the performance of membrane filtration,XDLVO theory（Extended Derjaguin–Landau–Verwey–Overbeek theory）and permeation drag force are used to analyze the interaction between membrane and pollutants.It is found that the electrostatic（EL）force is crucial for the retention performance of the gravity-driven ceramic membrane（GDCM）.The hydraulic driving pressure is positively correlated with the permeation drag（PD）force.The change of PD force mainly affects the EL performance,while its influence on van der Waals force and acid-base force is weak.When GDCM filters the like-charged pollutants（i.e.,HA,BSA）,the lower operation height reduces the PD force,resulting in the formation of a larger and stronger EL repelling zone,rendering the pollutant particles moving away from the membrane surface,and thus causing the higher retention rate by GDCM.Although the rejected pollutants are more than those under the situation of higher operating height,the cake layer is weakly attached to the membrane surface,causing the filter cake layer to be looser and thus easier to be removed by hydraulic cleaning.In addition,according to the"trade-off"relationship between pore blocking and cake layer formation caused by the reduction of hydraulic operation height,changing the electrical properties of the membrane is expected to improve both the retention and anti-fouling performance of GDCM.This research provides theoretical guidance for the selection/preparation/modification of ultra-low pressure driven ceramic membranes and the optimization of GDCM filtration performance.To enhance the performance of the ultra-low pressure driven membrane filtration regarding the removal of aquatic dissolved organic matter,the copper ferrite ceramic membrane（Cu Fe CM）is prepared by the sol-gel method.The peroxymonosulfate（PMS）/Cu Fe CM in-situ catalytic filtration is constructed to improve the water purification performance.Aiming at the typical organic pollutant humic acid（HA）,the PMS/Cu Fe CM in-situ catalytic filtration has achieved a total organic carbon（TOC）removal rate of up to 76.2%for HA,which is much higher than the TOC removal rate of traditional ceramic membrane filtration（30.1%）.Compared with traditional ceramic membrane filtration,the PMS/Cu Fe CM in-situ catalytic filtration reduces irreversible fouling(0.13×10^-12 m^-1)and significantly improves the hydraulic cleaning efficiency（flux recovery of 25.9%）of the membrane after HA fouling.Such performance improvement is attributed to the synergistic effect of catalytic oxidation and membrane filtration.According to the various characterizations of HA before and after the catalytic reaction,it is found that the phenolic compounds and aromatic rings in HA are degraded to a certain extent after PMS/Cu Fe CM catalytic oxidation,and the two characteristic fluorescence groups of HA are significantly degraded.HA is thus transformed into more hydrophobic,low molecular-weight and unsaturated structure.The unsaturated bond promotes the aggregation of HA particles and thus renders the effective steric retention by Cu Fe CM.In addition,the total attractive energy between Cu Fe CM and oxidized HA is lower than that between the ceramic membrane substrate and the original HA before oxidation.This assists effective detachment of HA from the membrane surface during hydraulic cleaning after PMS/Cu Fe CM catalytic filtration.The PMS/Cu Fe CM in-situ catalytic filtration has potential application value in the practical cross-flow operation treating highly polluted wastewater.Finally,in response to the challenge of the minimal removal rate by the ultra-low pressure driven membrane filtration treating emerging micropollutants,a Janus reactive electro-catalytic membrane is constructed.Using the ceramic membrane as the substrate,the cathode and anode regions are constructed by sputtering palladium and platinum metal catalysts on each side of the membrane by magnetron co-sputtering.Compared with the current electro-catalytic membranes using single-active surfaces and only combining the half-cell reaction with the membrane carrier,the Janus reactive electro-catalytic membrane makes use of the full electrochemical redox reaction and the spatial confinement effect of the membrane,improving the electro-catalytic reaction efficiency and expanding the function of the electro-catalytic membrane.Under the low voltage of 1.6 V between the two electrodes,the Janus reactive membrane（Pd-Pt-CM）uses the dissolved oxygen in the feed water as the precursor during the ultra-low pressure driving filtration process.By sequential electro-reduction and electro-oxidation reaction in different membrane regions during the electro-filtration,the hydrogen peroxide and superoxide radicals are generated in-situ as intermediates,leading to effective generation of singlet oxygen within the membrane structure.Sulfamethoxazole（SMX）,a typical toxic micropollutant,is added into the feed water to explore the water purification performance of Pd-Pt-CM.The results show that 83%of SMX can be removed after only 23 s of the retention time within Pd-Pt-CM.Compared with other electric filtration modes,the performance and energy consumption of the Pd-Pt-CM“flow-through”electro-filtration mode are 3 to 7 times higher than those of the electro-filtration modes using only single-active surface of the Pd-Pt-CM.The performance and energy consumption of the Pd-Pt-CM“flow-through”electro-filtration modes are also an order of magnitude higher than those of the conventional“flow-by”electrochemical modes.The detection of reactive oxygen species（ROS）in different regions of the membrane and electrochemical voltammetric characterization showed that the ROS generated in-situ in the Pd-Pt-CM electro-filtration system played a major role in the efficient removal of micropollutants.This research provides a theoretical guidance for the future development of ROS-based electro-catalytic membrane technology for water purification,and also provides a new idea for the design of efficient electro-active membrane geometry with broader application prospects.