The Membrane Fouling Mechanism Of Ca²⁺ And Natural Organic Matter Was Characterized By UV-vis Method

Although the direct health risks of natural organic matter?NOM?in drinking water or treated wastewater are not yet well known,these materials are usually recognized as precursors of harmful disinfection by-products?DBPs?.Thus,to minimize the adverse effects of NOM,a wide variety of treatment methods,such as coagulation/flocculation,activated carbon filtration,magnetic ion exchange resin processes,advanced oxidationprocesses and membrane filtration,have been developed and applied.Ultrafiltration?UF?is an effective strategy for removing NOM from waters because of the considerable ability of UF membranes to reject colloidal materials and suspended particles.However,the presence of NOM in water can result in substantial membrane fouling.Ultrafiltration?UF?can achieve excellent removal of natural organic matter?NOM?,but the main challenge for this process is the limited understanding of membrane fouling.As NOM is composed of numerous aliphatic and particularly aromatic hydrocarbon structures,various characterization methods and approaches have been applied to better understand the fouling behavior of NOM.For instance,size exclusion chromatography?SEC?and liquid chromatography with on-line organic carbon detection?LC-OCD?are promising for the analysis of the molecular weight distribution of a given NOM sample.Nuclear magnetic resonance?NMR?and Fourier transform infrared?FTIR?spectroscopy are capable of characterizing the functional and structural characteristics of NOM.Noticeably,it has been documented that high-resolution Orbitrap mass spectrometry and Fourier transform ion cyclotron resonance MS?FT-ICRMS?have shown great promise in characterizing NOM at molecular levels.Indeed,the use of these characterization methods can aid in understanding different aspects of NOM properties.However,the analysis of NOM by the above-mentioned techniques is time consuming or requires complicated sample processing.In comparison,spectrophotometric analysis,such as ultraviolet and visible?UV-vis?absorbance spectroscopy and fluorescence spectroscopy,is able to monitor NOM with less simple or no sample preparation.In addition,spectrophotometric analysis appears capable of achieving the in situ measurement of the amounts and fouling propensity of NOM.Limitations also exist for spectrophotometric analysis.Although fluorescence spectroscopy is of high sensitivity to NOM,less than 1%of molecules in the NOM bulk can emit fluorescent light.Importantly,fluorescence measurement is also largely dependent on water chemistry,such as pH.In contrast,UV-vis absorbance spectroscopy is limited by the featurelessness of the absorbance spectra of NOM,which makes it difficult to distinguish different NOM samples.Further spectral data processing,such as the calculation of spectral slopes and absorption ratios,is needed to extract more useful information from the UV-vis absorbance spectra.For instance,E₂:E₃,calculated using the absorbance values at 250 and 365 nm,has been proven to be a good proxy for characterizing the molecular size and aromaticity of NOM in aquatic waters.E₄:E₆?the ratio between the absorption values at 465-665 nm?was found to be less effective for characterizing aquatic humic solutes but is indicative of the molecular size and aromaticity of soil-derived NOM.More interestingly,previous studies have shown that the spectral slope from 275 to 295 nm(S_275-295)and the ratio of the two slopes(S_R=S_275-295/S_350-400)can semi-quantitatively measure the molecular size of NOM from various water sources,i.e.,a decreased S_275-295 or S_R corresponds to increased molecular size.Because a specific range of the spectral region is used rather than a single data point,the slopes or slope ratios(i.e.,S_275-295 or S_R)seem to be more robust than the absorption ratios?i.e.,E₂:E₃ and E₄:E₆?for describing NOM properties.Additionally,differential absorbance spectroscopy?DAS?can yield subtle changes in the UV-vis absorbance spectra of humic substances or NOM resulting from varying physicochemical conditions.Compared with the UV-vis absorbance spectra,the DAS can present the interpretable features of NOM more clearly,thus facilitating the capture of more information regarding changes in NOM properties.The objective of this study is to explore the potential of UV-vis spectroscopic analysis for the detection of membrane fouling caused by humic acids?HA?at different solution chemistries(i.e.,calcium ions(Ca²⁺)and pH).In the presence of Ca²⁺,several spectral parameters,including the DSlope_325-375?the slope of the log-transformed absorbance spectra over 325-375 nm?,S_275-295?the slope of the absorption coefficient over 257-295 nm?and S_R(the ratio of S_275-295 to S_350-400)of various HA solutions,were correlated with the molecule aggregation and the membrane fouling potential.Interestingly,increased DSlope_325-375 and decreased S_275-295 and S_R were observed for the HA-Ca²⁺interaction under alkaline conditions?i.e.,pH=8.25?relative to those in lower pH environments?i.e.,pH=6.50?,suggesting that spectral parameters were able to predict HA-Ca²⁺interactions under varying pH conditions.The strong correlations between the spectral parameters and the unified membrane fouling index?UMFI?obtained from UF experiments further corroborated that the spectral parameters were able to predict the membrane fouling potential.Moreover,the spectral parameters were also found to well reveal the fouling extent of the mixture of HA added with varying calcium concentrations,implying that the spectroscopic analysis was also available for the indication of practical NOM fouling.In addition,the measurement of S_275-295 and S_R of the permeate solution suggests an increasing proportion of small-molecule HA in the permeate during the UF process.This study not only expands our knowledge of NOM-Ca²⁺aggregates as well as their role in membrane fouling behavior but also provides an approach for the in situ characterization of membrane performance.