Effectiveness And Mechanism Of Photothermal Activated Peroxodisulfate And Photocatalytic Pre-oxidation To Control Ultrafiltration Membrane Fouling

Ultrafiltration(UF)has been widely used in the field of water and wastewater treatment.However,membrane fouling has always been one of the main obstacles hindering its wide spread applications.In addition,the poor removals of dissolved organic matters and micro-pollutants in natural waters also limit the application of UF membranes in some extend.In order to solve the problems above,an advanced oxidation process(AOP)based on light and heat was proposed as a pretreatment for UF.The oxidation process could effectively degrade natural organic matters(NOMs)and micro-pollutants while alleviating UF membrane fouling.This research used heat and solar energy-activated peroxodisulfate oxidation process as the pretreatments for UF,and systematically investigated the effects of AOP based on light and heat as a pretreatment for UF on membrane fouling and removal efficiencies.The mechanisms were explored through various methods.Firstly,it is proposed to use the heat to activate peroxodisulfate to degrade NOMs in the water and at the same time alleviate UF membrane fouling.To study the effects of heat-activated persulfate pre-oxidation on the membrane fouling and water purification performance caused by different types of dissolved organic matters in natural surface water,and the Songhua River water representing natural surface water was used as the target raw water.The effects of temperatures and dosages of oxidant on the removal efficiencies of DOC,UV₂₅₄ and fluorescent organics as well as membrane fouling were explored.It was found that heat-activated peroxodisulfate pre-oxidation showed an excellent degradation on the NOMs in the Songhua River water.The removals of DOC and UV₂₅₄ were 71% and 52%,respectively,at 80? in a reaction time of 60 min when the PDS dose of was 0.8mmol/L.In addition,the heat-activated peroxodisulfate(PDS)pre-oxidation showed a similar removal trend for humic-like substances(C1 and C2),while the removal of microbial-derived protein substances(C3)increased rapidly in the first 10 min and then remained stable.The heat-activated PDS pre-oxidation could effectively reduce membrane fouling.For example,the flux increased by 4%,40%,92%,152% and 260%,respectively compared with the raw water when the temperature was at 50,60,70,80,and 90?,respectively.The heat-activated PDS pre-oxidation could affect the surface morphology and hydrophobicity of the membrane.The surface roughness and uniformity of the membrane contaminated by the pre-oxidized water was lower than the membrane contaminated by raw water.Contaminants made the membrane more hydrophilic and the contact angle of the membrane contaminated by raw water was smaller than that of membrane contaminated by pretreated water.The mechanisms of mitigating membrane fouling included reducing the concentrations of NOMs,degrading large molecular weight humus into small molecular weight substances,and the destruction of the humus structure hindered the possibility of binding to high-valent cations.It was precisely because of the above comprehensive effects that heat-activated PDS pre-oxidation could better control membrane fouling.Secondly,on the basis of the studying the effects of heat-activated PDS pre-oxidation on the membrane fouling and water purification performance caused by different types of NOMs in natural surface water,and the application of green and clean energy-the solar energy was further proposed.As a pre-treatment the UF,the solar photothermal chemical pre-oxidation and the UF were combined to study the effect of solar photothermal chemical pre-oxidation on membrane fouling and water purification performance caused by different types of NOMs in natural surface water.In this process,solar energy was used to activate persulfate to degrade NOMs and micro-pollutants in surface water,which not only effectively removed NOMs and micro-pollutants in the raw water,but also effectively alleviated membrane fouling.The solar light effect dominated the activation of PDS when the temperature was below 50?.The solar light and thermal effect both played an important role in the activation of PDS when the temperature was above 50?.In addition,the thermal effect had the dual effects of accelerating the reaction speed(30?70?)and generating free radicals(50?70?).The solar-activated pre-oxidation could effectively reduce membrane fouling caused by the Songhua River water even at room temperature(30?)and the effect of mitigating membrane fouling increased significantly with the increase of temperatures.Solar and heat-activated PDS had the same effect on reducing membrane fouling at 70?.This was because a large amount of NOMs in the influent water was degraded resulting in very few pollutants in the water sample prior to UF under these two conditions.The relationship between various water quality indicators and membrane fouling indexes was effectively evaluated to predict membrane pollution through the establishment of a mathematical model.For the direct UF,HP30/UF,SP30/UF,HP50/UF and SP50/UF systems,membrane fouling was mainly caused by middle blockage and cake layer in the initial filtration stage;the membrane fouling was mainly caused by the cake layer in the middle and late stages of filtration.For the HP70/UF and SP70/UF systems,the four mathematical models could not describe the first three filtration cycles due to the fact that there were fewer pollutants in the water samples after the pre-oxidation.However,the cake layer formed on the membrane surface eventually led to membrane fouling due to the gradual accumulation of pollutants in the later stage of filtration.In addition,the irreversible membrane fouling was mainly caused by tryptophan-like proteins(T2)and aromatic proteins(T3)rather than humic substances due to the stronger hydrophilicity of protein than humic substances.In addition,the terrestrially derived humic-like matters(C2)and the protein-like substances derived biological production(C3)were the main causes of total membrane fouling and irreversible membrane fouling,respectively.The UV₂₅₄ of the influent prior to UF was more effective than DOC on the irreversible membrane fouling when treating Songhua River water,which could be used as an indicator of membrane fouling control strategy in treating the Songhua River water.Finally,based on the above research,an attempt was made to coat the Fe₂O₃-Fe₃O₄ composite material that absorbs visible light on the membrane surface to catalyze and oxidize pollutants in the water.This heterogeneous catalytic oxidation could directly use solar energy without artificially adding chemicals during the catalytic oxidation process and would not cause secondary pollution to water.The prepared Fe₂O₃-Fe₃O₄ composite material and modified membrane were characterized by various methods.The effects of membrane catalytic oxidation on water purification efficiency and membrane fouling were systematically investigated.The results showed that the modified membrane had a very obvious effect on alleviating membrane fouling.Compared with the control group,the total relative transmembrane pressure(TMP)of the modified membrane was reduced by about 56.5%.In addition,it had a good degradation on UV₂₅₄ in Songhua River water,of which was about 22.0%.However,the ability to degrade DOC was poor.In addition,it also had a good degradation for both the terrestrially derived humic-like matters(C2)and the protein-like substances derived biological production(C3),and their degradation rates were 19.38%and16.89%,respectively.