Colloidal And Membrane Fouling Behavior Of Natural Organic Matter In Aquatic Systems: Mechanism And Prediction

Natural organic matter（NOM）is a complex mixture of decomposition products of plant and animal residues as well as substances synthesized biologically and chemically from these products.During the environmental transport process,the colloidal behavior of NOM occurs on the interface,which has an important influence on the environmental transport and fate of both NOM and adsorbed pollutants.On the other hand,NOM is a key pollutant that causes membrane fouling in filtration treatment of surface water and sewage.The colloidal behavior of NOM has an important impact on its membrane fouling.Therefore,explore the mechanism of colloidal behavior of NOM and establish a membrane fouling prediction model is of great significance to study the transport and transformation of NOM in the environment,assess the environmental fates and risks of pollutants,as well as promote the control and prediction strategy of membrane fouling caused by NOM.The structure parameters and hydrochemical conditions have important influence on the colloidal behavior of NOM in natural water according to the literatures.In order to evaluate the existence state and transport capacity of both NOM and adsorbed pollutants in natural water,it is necessary to explore the influence of the structure effect and ionic effect on the colloidal behavior of NOM,and to reveal its regulation mechanisms and controlling factors.In addition,the prediction of membrane fouling behavior of NOM has practical significance for the environmental application of natural water.In order to improve control strategy of membrane fouling of NOM,it is necessary to explore the mechanism of membrane fouling behavior and establish an effective and reliable prediction model of membrane fouling.The mechanism of colloidal behavior of NOM and the prediction model of membrane fouling caused by NOM are still unclear.In this dissertation,the structure effect and ionic effect of colloidal behavior as well as membrane fouling prediction model of NOM（dissolved black carbon（DBC）,natural organic matter standards and natural water samples）were studied under relevant environmental conditions.Dissolved black carbon（DBC）,as a special component in NOM,is of great significance in the environmental geochemistry research because of its high aromaticity and abundant oxygen-containing functional groups.Natural organic matter standards,which are widely used in related studies of NOM,provide comparable and repeatable experimental results,and can help to integrate the results into the existing research system.The application of natural water samples can verify the feasibility and practical significance of our research.On the one hand,the structure effect of colloidal behavior and its regulation mechanism of NOM were investigated.The ion effect of colloidal behavior and its influence mechanism of NOM were revealed.On the other hand,based on the importance of NOM hydrophobicity,the practical prediction model for membrane fouling caused by natural water samples was established in this study,and the effect of p H conditions and oxidation pretreatment on the applicability of the prediction model were also illustrated.The main findings achieved are summarized as follows:（1）We studied the main controlling force of aggregation behavior of DBC in the aqueous environment and its fractionation in the estuarine system.Our results demonstrated that Na⁺and Mg²⁺with high abundance in the aqueous environment can estuaries and Marine systems can readily induce the aggregation of DBC,with the critical coagulation concentration（CCC）values of 75 m M and 60 m M,respectively.It is clarified that hydration force is the main controlling factor of the colloidal behavior of DBC,and hydrophobicity is the main controlling mechanism of the colloidal behavior of DBC.We reported that the simulated sunlight exposure can increase the oxygen-containing groups and hydrophilicity of DBC,thus resulted in increase of its CCC value and colloidal stability.The transport process of natural organic matter standards（SRHA）over the estuary salinity gradient is conservative（i.e.,without addition or loss）,whereas DBC is non-conservative due to its aggregation and sedimentation.Our results suggested that the aggregation behavior of DBC in river and estuary systems may lead to the fractionation of DBC from the DOM pool and further enriched in the sediment,thus weakening its Land-Sea transport flux.（2）We studied the ion effect on the aggregate kinetic and thermodynamic of the aggregation behavior of natural organic matter standards（SRNOM）in the presence of monovalent and divalent ions.We found that there has strong ionic effect on the Li⁺.The monovalent ions（Cs⁺、Rb⁺、K⁺、Na⁺、Li⁺）and Mg²⁺can induce the structural can readily induce the aggregation of NOM.Extended classical colloidal stability theory（XDLVO）analysis revealed that hydration force is the main controlling factor for the colloidal stability of NOM by calculating NOM particle interaction energy and the ratio of each force.We found that there has a good correlation between the thermodynamic properties of the cation-NOM interactions(ΔS_hydration/ΔH_interaction)and its colloidal behavior（CCC values）.Our results indicated that the ability of cations to induce NOM aggregation was determined by cation-NOM interactions and their ability to modulate surface tension.（3）We studied the filtration kinetics of natural water samples in the ultrafiltration membrane treatment.Based on the results of the colloidal force analysis,the decisive role of NOM hydrophobicity is revealed.The hydrophobicity of NOM in natural water samples was quantitatively characterized by K_ATPS,and membrane fouling index（FI）was used to quantify the membrane fouling behavior of NOM.We found that the membrane fouling behavior of natural water samples was linearly correlated with the hydrophobicity index of NOM,K_ATPS,but not with the spectral indexes,SUVA₂₅₄ and F_max,which commonly used in membrane fouling prediction.The data of natural water samples were divided into two parts,including the training group and the validation group.Based on the K_ATPS of NOM,the membrane fouling prediction model of natural water samples was established by using the training group data,and the validity and reliability of this model were verified in the validation group data.We found that the good predictive ability of this model for FI of natural water samples under different p H conditions.Our results indicate that the prediction model will help assess the performance of pretreatment and develop suitable mitigation strategies.（4）In order to alleviate NOM membrane fouling,we studied the effect of two oxidation pretreatments（O₃ and Cl O₂）on the structure of NOM,and the applicability of the prediction model of NOM membrane fouling after two oxidation pretreatments.Both O₃ and Cl O₂ pretreatment increased hydrophilicity of NOM,but the increased hydrophilicity of NOM induced by O₃ pretreatment was more significant than Cl O₂pretreatment.The good linear correlation between NOM hydrophobicity index(K_ATPS)and membrane fouling index（FI）after O₃ pretreatment,indicated that hydrophobicity is the main control mechanism of membrane fouling behavior of NOM after O₃pretreatment.However,there is no correlation between those two indexes after Cl O₂pretreatment,indicating that the control mechanism of membrane fouling behavior of NOM has changed,which may be related to the molecular size and structural composition of products after Cl O₂ pretreatment.We further confirmed that the prediction model of membrane fouling in natural water samples is still applicable after O₃ pretreatment,but cannot be used to predict membrane fouling after Cl O₂pretreatment.Our results suggested that O₃ pretreatment can alleviate membrane fouling of NOM by reducing hydrophobicity of NOM.Cl O₂ pretreatment with low concentration can alleviate NOM membrane fouling,but Cl O₂ pretreatment with high concentration will generate products similar to the size of membrane pore,thus aggravating membrane fouling.