Using UV–vis absorbance spectral parameters to characterize the fouling propensity of humic substances during ultrafiltration

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. 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 (Ca(2+)) and pH). In the presence of Ca(2+), 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-Ca(2+) interaction under alkaline conditions (i.e., pH = 9) relative to those in lower pH environments (i.e., pH = 7 or 6), suggesting that spectral parameters were able to predict HA-Ca(2+) 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 and Suwannee River NOM (SRNOM) or the pure SRNOM 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-Ca(2+) aggregates as well as their role in membrane fouling behavior but also provides an approach for the in situ characterization of membrane performance.