Interactions Between Natural Organic Matter (NOM) And Metal Lons/Nanoparticles And Their Effects In Membrane Fouling Process

Natural organic matter (NOM), ubiquitous existing in various environment media on the surface of the earth, is the predominant source of organic matter in aqueous and soil ecosystems. NOM is an important complexant and absorbent for environmental pollutants. By affecting the chemical bound forms, solubility and mobility, NOM subsequently alters the fate and toxicity of pollutants. Thus the in-depth interaction mechanisms between NOM and pollutants as well as the establishments of effective approaches for NOM removal have long been an important hotspot in the field of wastewater pollution control. Low-pressure membrane (LPM) technique is a newly-emerging technique in producing drinking water in the past decades. LPM can entrap most of the particles, colloids and partially remove NOM, remarkably reducing the amount of disinfect chemicals and the formation of disinfect by-products (DBPs). NOM was among the most important foulants in LPM systems, which potentially interact with various pollutants and affect the mobility and removal efficiency of NOM by membranes. In addition, membrane fouling remained one of the most important barriers for the wide applications of LPM systems. The application of convenient pretreatment approaches with high-efficiency was the most efficient way to promote the quality of eluent and mitigate membrane fouling. In this work, advanced analytical approaches were developed to characterize the interactions between NOM and typical metal ions/nanoparticles. The roles of solution chemistry on NOM-induced membrane fouling and the interaction mechanisms were explored. Microgranular adsorptive filtration (μGAF) system was used to enhance the quality of effluent and mitigate fouling developments. Multiple spectral analytical tools were performed to explore the fouling mechanism in μGAF systems and to probe the key foulants in such processes. These works will provide theoretical basis and technical guidance on enhancing the removal of foulants in pretreatment approaches. The main contents and results are listed below:1. Dynamic light scattering was performed to explore the coagulation kinetics of typical NOM, i.e., humic acids (HA) aggregates in the presence of mono-and di-valent cations. The results show that the coagulation kinetics of HA was remarkably affected by the valent of cations and solution pH. At high concentration of NaCl, the hydrodymanic radius (<Rh>) of HA colloids could be scaled to the time t as <Rh>∞ta, suggesting a diffusion-limited colloid aggregation process. At pH 7.1 the coagulation value of NaCl was between 61.3 and 84.8 mM. Di-valent Mg2+ significantly promoted the coagulation of HA, reflected by a lower coagulation value (1.0-1.7 mM) and faster coagulation rates. The highest coagulation rate (d<Rh>/dt) and coagulation value at different pHs followed the order of:acidic> neutral> alkaline, and alkaline> neutral> acidic, respectively. TEM analysis confirmed that the HA colloids shifted from spherical aggregates with rough morphology in acidic solution to smooth aggregates at alkaline condition. These results will help understand the dynamic interactions between HA and cations.2. The aggregation, aging and dissolution of copper nanoparticles (CuNPs) induced by typical NOMs were explored. The results show that the presence of NOM will disperse the CuNPs clusters. The release of copper from CuNPs was attributed to the complexation between CuNPs surfaces and the surface functional groups in NOM. The copper releasing capacity of tested NOM was in the order of:Sigma HA> extracted HA> SRFA> BSA> alginate. Humic substances have higher copper-releasing ability among five tested NOMs, mainly attributed to higher functional group density and lower molecular weight. These characteristics promoted the contact and complexations between NOM and CuNPs. Chlorine treatment destroyed the functional groups on NOM. As a result, treated NOM was less absorbed onto CuNPs and copper release was highly reduced. The interaction mechanisms between NOM and CuNPs will help understand the mobility and fate of CuNPs in natural aqueous systems.3. The roles of typical divalent cations, i.e., Ca2+ and Mg2+ on the HA-induced ultrafiltration membrane fouling were explored by combined use of attenuated total reflectance (ATR)-FTIR spectroscopy, quartz crystal microbalance (QCM) and isothermal titration calorimetry (ITC). The results show that Ca2+ was more effective in enhancing HA-related fouling. HA showed higher adsorption onto hydrophobic PES membrane than hydrophilic cellulose membrane. ATR-FTIR results indicated that the interaction between HA and PES membrane was mainly attributed to polysaccharide C-O, aromatic C=C and carboxylic C=O groups. QCM data provided quantitative evidence that Ca2+ was more effective in accumulating HA onto membrane surfaces. ITC results showed that Ca2+ exhibited higher heat release and higher binding capacity with HA compared to Mg2+. As a result, more Ca2+ could bind with HA through hydrophobic interactions with more HA-Ca2+ complexes formed. These results are of importance in explaining the roles of Ca2+ and Mg2+ on the HA-induced ultrafiltration membrane fouling mechanisms.4. The pretreatment of natural water in μGAF systems was investigated and its effects in mitigating membrane fouling were tested. The results show that hydrophobic fraction caused highest fouling among individual fractions with different hydrophobicity. The key foulant was probably the small hydrophobic portion accumulated on membrane surface. At the same dissolved organic carbon (DOC) concentration, mixed solutions composed of more than one individual fraction resulted in a higher fouling potential, indicating that the interactions among fractions are important factors which can accelerate membrane fouling. μGAF system embedded with HAOPs can effectively remove humic-like substances, fulvic-like substances and high-apparent molecular weight (AMW) fractions from raw water. However, HAOPs layer showed low efficiency in removing aromatic proteins. Membrane fouling was much relieved after alkaline washing. The fractions accumulated on membrane surface contained more aromatic proteins but less humic and fulvic substances than raw water. These results show that the major foulant are, but not limited to, some hydrophobic fraction accumulated on membrane and some non-fluorescent moieties.5. Based on the results of natural water pretreatments in μGAF systems, multiple spectral analytical tools were performed to elucidate the effects of pretreatment modes of HAOPs in affecting the characteristics of NOM and membrane fouling. The results show that the two typical pretreatment modes, i.e., pre-adsorption and pre-deposition both removed a substantial fraction of the DOC, selectively removing UV254-absorbing molecules, humic and fulvic substances, and high-AMW fractions of the NOM. The absorbance ratios derived from multi-wavelength high performance size exclusion chromatography (HPSEC) data confirmed the adsorptive selectivity for aromatic over aliphatic groups within a wide AMW range by the deposited HAOPs layer. The total trihalomethane (TTHM) formation potential analysis confirmed that pre-deposition mode could not only mitigate fouling potential than pre-adsorption mode, but could remove more DBP precusors from feed water as well. The results support the view that the key foulants comprise only a small portion of the NOM in the source water and that they can be substantially retained by a HAOPs layer, perhaps due to the formation of a condensed NOM phase (e.g, a gel) that efficiently collects molecules responsible for membrane fouling. The different ineraction mechanisms between NOM and HAOPs in two pretreatments modes were well clarified.