Tracking the Dynamic Evolution of NF Membrane Fouling Through Clustering Analysis Based on ATR-FTIR Spectra
LI Meng-chen1, XIAO Kang2, HUANG Xia1*
1. State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China
2. College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China
Abstract：Sewage reclamation is one of the effective countermeasures for solving the water shortage problem. Nanofiltration (NF) process is an effective method for reclaiming secondary effluent since it can provide high-qualitied water. However, during the nanofiltration process, complicated and dynamic membrane fouling occurred, which can cause the decrease in flux and effluent quality. Tracking the dynamic evolution of NF membrane fouling in water treatment is important for controlling the membrane fouling depending on different fouling stages. Organic matters are important indicative contaminant for the dynamic evolution of fouling layer. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) is one of the most significant methods to characterize the change of functional groups in fouling layer. However, the peaks of ATR-FTIR are complex and the variations between different samples are tiny, especially when the fouled membrane samples are similar in time. It is difficult to directly discriminate the variation and trend of different ATR-FTIR spectra and cannot be the convictive evidence for fouling stages recognition. To investigate the dynamic evolution of membrane fouling, this study categorized the NF membranes samples obtained at different fouling time through combining the ATR-FTIR spectra and clustering analysis. Considering the influence of distance measurement method between categories, normalization of ATR-FTIR peak absorbance, correlation between peaks, and interaction between peaks and samples, this study utilized the correspondence analysis as the pretreatment of the ATR-FTIR spectra to obtain the scores of different membrane samples along main dimensions and then clustered the samples based on normalized Euclidian distance. During the 1-month NF experiment using sewage secondary effluent, because of the deposition of foulants, membrane fouling occurred and 13 fouled membranes were obtained at different time. Based on the hierarchical clustering of ATR-FTIR spectra, the fouling process can be clearly divided into the stages of: virgin membrane, stage Ⅰ(3 h～8 d), stage Ⅱ (10～15 d), and stage Ⅲ (20～30 d). The results of the clustering analysis was further interpreted by X-ray photoelectron spectroscopy (XPS) and adenosine triphosphate (ATP) content on the membrane surface. It was shown that with the evolution of membrane fouling, the organic composition and coexisting microorganism amount on the membrane surface changed concordantly. The characteristics of different stages may be interpreted as: in stage Ⅰ, the membrane was initially covered by organic foulants, and microorganism began to gather; in stage Ⅱ, the proportion of polysaccharide-like substance increased and the gathering of microorganism became stable; in stage Ⅲ, the membrane fouling became mature and the hydrogen bond characteristics of organic foulants became more evident. In this study, the variation of different ATR-FTIR spectra was detected sensitively through clustering analysis. The study provides an objective, automatic and measurable auxiliary method for recognizing and characterizing membrane fouling stages. Besides, it is meaningful for investigating the ATR-FTIR spectra of a series of samples in not only membrane fouling research but also other fields such as materials science and adsorption research.
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