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Abstract Foulant hydrophobicity has been regarded as a key factor in membrane fouling. While hydrophobic/philic fractionation using DAX (or XAD) resin column has been extensively utilized in fouling studies, the critical retention factor (k ' cr , the ratio of water volume to column volume) has been long neglected which can alter the definition and relative degree of "hydrophobicity" of the obtained fractions and affect fouling results. To this end, this study compared fouling behavior of a membrane bioreactor (MBR) supernatant fractionated at varied k ' cr , and found that not the distinctively strong "hydrophobic" (k ' cr > 50) or "hydrophilic" (k ' cr < 5) fractions, but rather the nominally moderate "hydrophobic/philic" fractions had the greatest contribution to overall fouling. The joint influence of foulant "hydrophobicity" and coexisting factors (molecular weight, carboxylic group, UV/fluorescence properties and polysaccharide/protein/humic contents) was elucidated via multivariate statistical analysis. Therefore, special attention should be placed on the nominally moderate "hydrophobic/philic" fractions for fouling control. Graphical abstract fx1 Highlights • The critical retention factor (k ' cr) determines the degree of "hydrophobicity". • Fouling propensity of hydrophobic/philic fractions was re-evaluated at varied k ' cr. • Moderately "hydrophobic" fractions (k ' cr = 5–50) had great contribution to fouling. • MW and hydrophobic segments were important underlying factors in fouling. • Fouling control strategies need to be oriented to moderately "hydrophobic" fractions. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) with complex functional groups affects the trace of binding and transformation of hexavalent chromium (Cr(VI)) in the environment. However, the mechanisms and quantitative indicators are still ambiguous. In this study, the perspectives of spectroscopy with synchronous and excitation-emission matrix (EEM) fluorescence, and ultraviolet-visible (UV–vis) were employed to probe the interaction of DOM and Cr(VI). The spectroscopic parameters with Dslope 240–280 (the slope of the log-transformed absorbance in the range of 240–280 nm), DSlope 325–375 (the slope of the log-transformed absorbance in the range of 325–375 nm), S R (the ratio of the slope of the log-transformed absorption coefficient with 275–295 nm to 350–400 nm), SFI (specific fluorescence intensities), AvgFI/AvgUVA (the ratio of average fluorescence to average UV), and SS (Stokes shift) of UV–vis and fluorescence were extracted to characterize the DOM properties. The UV–vis results showed carboxylic-like and total phenolic/carboxylic-like groups were contributed to the complexation of Cr, which resulted in forming macromolecular aggregates, however, the degree of participation of different functional groups was uneven and the binding sequence depended on the types of fractions especially the ratio of protein. Meanwhile, the EEM results proved interaction between DOM and Cr also affected the hydrophobicity and quantum yield of solutions. In addition, according to the change orders of substances and functional groups by Two-dimensional correlation spectroscopy (2DCOS) analysis, aromatic amino acids substances were much more sensitive binding onto Cr than fulvic-like fractions and humic-like fractions, and the mechanisms for Cr(VI) retention were developed. Statistical analysis results showed that S R and SFI were significantly correlated with the concentration of Cr(VI), and multiple linear regression models were developed to predict concentrations of Cr(VI). Moreover, it was found the goodness of fit of SFI was better than that of S R at low Cr concentrations, but opposite at high concentrations. [Display omitted] • Aromatic amino acids were much more sensitive binding onto Cr than fulvic and humic-like. • The binding sequence of functional groups depended on the proportion of protein. • Models for predict concentrations of Cr(VI) with SFI and S R were established. • SFI fitting was more suitable than S R at low concentration, but opposite at high. [ABSTRACT FROM AUTHOR]

Model suspensions are important substances for characterizing fouling processes to study the mechanism of membrane fouling, thus, choosing a suitable standard suspension is critical for researches on membrane fouling processes. The pretreatment methods of model suspensions have long been neglected, which can alter the physical and chemical properties and affect fouling results. This study compared four different pretreatment methods (centrifuging and drying, magnetic stirring, ultrasonic processing, and dissolving in buffer solution) and found effects on size distribution, dispersion stability, pH variation over time, and fouling properties. Among the characterization of model suspensions, different pretreatment methods led to various changes in physical and chemical properties in this study. Membrane filtration experiments showed that these four pretreatments changed the rate of membrane fouling. The results of the analysis of filtration flux for suspensions indicated that for ultrasonic processing of all suspensions and magnetic stirring of kaolin suspensions, pore blocking was the main mechanism, while cake formation was dominant for the others. Therefore, special attention should be placed on different pretreatment methods and selecting appropriate model foulants. Finally, some guidance on selecting appropriate model foulants was given in the study. [Display omitted] • The effects of four different pretreatment methods were compared. • Properties that model suspensions need to be equipped were put forward. • The fouling mechanism of suspension after ultrasonic was pore blocking. • Zeta potential and turbidity were used to characterize sedimentation. [ABSTRACT FROM AUTHOR]

The polymer coagulant aid can effectively enhance the coagulation-ultrafiltration (C-UF) process for the purification of drinking water. However, when coagulant aid entered the filtration, it may also cause serious membrane fouling as polymer. In-situ Raman spectroscopy and electrochemical impedance spectroscopy(EIS) were applied to monitor the effects of coagulant aids on the membrane. The causes of fouling were assisted discussed through stage cleaning of the membrane. The equivalent circuit fitting was performed on the EIS data and the Raman spectral data were statistically analyzed after peak fitting. EIS and the cluster analysis of Raman spectroscopy provided an earlier feedback on membrane fouling layers compared to flux. The cause of membrane fouling was explained via variation of characteristic functional groups obtained by Raman spectroscopy. When the molecular weight of the coagulant aid was 160 times,80 times and 16 times larger than the MWCO of the UF membrane, the equivalent circuit obtained by fitting the EIS of the UF system satisfied Rs + c(QpRp), Rs(QcRc)(QpRp) and Rs(Qt(Rc(QpRp))) respectively. Partial correlation analysis showed that the corresponding factors causing irreversible fouling of membrane were humic acid(HA), HA and coagulant aids, coagulant aids. Combined with the mean roughness (Ra) of membrane, the coagulant aid performed differently in the cleaning of contaminated membrane and also affected the cleaning of HA. Image 1 • On-line monitoring of fouling in C-UF by simultaneous Raman spectroscopy and EIS. • The accumulation of foulants was explained by compound functional groups. • Cluster analysis divided the membrane fouling layer into three stages. • Membrane property is the key factor to role of coagulant aid in irreversible fouling. [ABSTRACT FROM AUTHOR]

Excitation-emission matrix (EEM) spectra were used to analyze the fluorescence properties of water-soluble organic compounds (WSOC) in PM 2.5 during winter and summer seasons in Lanzhou city, northwest China. Protein-like substances, humic-like substances and microbial by-products were found to be the main fluorophoric organic matter. Humification index (HIX), biological index (BIX) and fluorescence index (FI) were 1.2 ± 0.1, 1.4 ± 0.1 and 1.7 ± 0.2 in winter and 2.0 ± 0.3, 1.3 ± 0.1 and 1.2 ± 0.4 in summer, respectively, indicating higher aromaticity of WSOC in summer and terrestrial biogenic sources of WSOC in both seasons. Strong correlations were found between regional average fluorescence intensities (RAFI) at the fluorescence regions of II-V and water-soluble inorganic ions (K + , Cl − , NO 3 − and SO 4 2− ) in winter with the Spearman correlation coefficients ρ being larger than 0.7 and mostly around 0.9, suggesting significant contributions of bioaerosols, coal combustion and vehicle exhaust to PM 2.5 . Moderate correlations were found between RAFI and K + , Cl − , and NO 3 − in summer, indicating the more important roles of biogenic and vehicle exhaust than coal combustion sources in summer. High temperature in winter was conducive to and in summer prohibitive to polycondensation of WSOC, indicating the existence of an optimum temperature for such a process to occur. High relative humidity was unfavorable to the formation of fluorescent WSOC. [ABSTRACT FROM AUTHOR]