Characterization of natural organic matters using flow field-flow fractionation and its implication to membrane fouling

摘要

Transport and deposition characteristics of natural organic matter (NOM) are systematically investigated using flow field-flow fractionation (Fl-FFF) at various chemical and physical conditions. Humic acid (HA) was chosen as model organic foulants. Prior to Fl-FFF analysis, HA was fractionated by membranes with different molecular weight cut-offs. To elucidate physicochemical factors affecting the deposition and transport characteristics of organic foulants, various concentrations of NaCl (i. e. up to seawater level) and CaCl_2 were employed as carrier solutions in Fl-FFF. Each fractionated NOM showed different transport and deposition characteristics with respect to the chemical and physical conditions employed during Fl-FFF analysis. When the total dissolved solids (TDS) concentration increased, there was more significant variation in the retention time for large NOM fractions compared with small NOM fractions. This means that the transport and deposition tendency of the larger NOM fractions varied more significantly with the alteration of ionic strength in Fl-FFF channel than the smaller ones. However, the smaller NOM fractions showed more considerable variation in retention time with increasing cross-flow intensity (i. e. flow perpendicular to channel flow in Fl-FFF) in Fl-FFF channel. This also means that the variation of physical factor could affect the transport and deposition tendency of the smaller ones more influentially. Results also elucidated that the retention time and area of elution peak of fractionated NOM were directly related to the amount of organic foulants attached to the membrane in Fl-FFF channel. It has been demonstrated that the deposition tendency of organic foulants increased at the higher TDS concentration, calcium concentration, and cross-flow intensity. This has been quantitatively determined using fouling index, Q_f, derived from the data obtained from Fl-FFF. Based on this study, it is implied that Fl-FFF can be a useful tool to characterize the transport and deposition behavior of organic foulants in the solid-water interface and optimize pretreatment options for reducing membrane fouling.