| Humic substance (HS) is commonly present in source waters. On one hand, itreduces the treatment process efficiency; on the other hand, it may act actively duringwater treatment and may influence the removal of hazardous pollutants. As both acomplexing and reducing agent, it may affect the adsorption and oxidation system.Thus, it is fundamental and necessary to study the interaction mechanism among thehumic substances, the micro-pollutants, and the water treatment agents. This paperfirstly studied the aggregation property of humic substances as to provide insight intothe mechanism of pollutant adsorption and particle aggregation, on this basis, westudied the influence of HS on coagulation and PVDF microfiltration membranefouling. Then we studied the removal of reducing hazardous material As(III) and Tl(I)by oxidation, and analyzed the influencing factors on the removal performance ofthese pollutants. We also studied the effect of ozone pre-oxidation on the treatmenteffectiveness of natural organic matter polluted waters using a pilot scale treatmentsystem.The variation of particle size and Zeta potential changes detected by MalvenNano Zetasizer may indicate the extent of aggregation of particles. The three-dimensional fluorescence spectrum results showed that fulvic acid (FA) has morefunctional group than humic acid (HA), thus lead to different aggregation property.Complexation is the basis for HA and FA aggregation, and their complexation withCa2+was the main reason for the increase of particle size and Zeta potential. In thepH range of6.0~9.0, the complexation of Ca2+with HA is strong, and the extent ofparticle aggregation is remarkable; while in the case of FA, it had more functionalgroups, their inter repulsion suppressed the Ca2+complexation, which is unfavorablefor lowering the negative charge and thus causing aggregation. Ozonation increasedHA’s acidic functional group, and the oxidation products aggregated due to hydrogenbonding, and the particle size was increased. With the decreasing of pH from9.0to6.0, the average Zeta potential of the Fe-hydrolyzed species increased from-7to+24and the main particle average diameter decreased from1100nm to300nm. Ca2+cation may complex with the functional groups on the Fe-hydrolyzed species’ surface,and made the particle Zeta potential more plus and the particle size increase slightly.The sterical stabilization mechanism of hydrophobic NOM adsorbed on Fe(OH)3(s)was the main reason inhibiting the coagulation of low alkalinity, low turbidity andhigh NOM water, and “two-stage coagulation” could promote the formation of flocs effectively.Complexation was the key factor for the aggregation of ozone oxidationproducts of HA and FA. At low ozone dose, ozone could increase the acidicfunctional group and improve the complexation between ozone and Ca2+, and the sizeof the complexed particles was increased; but at high ozone dose, ozone may reducethe molecular weight of organics, and the size of the complexed particles wasdecreased. The chemical adsorption and aggregation of organic or inorganic particlesmight significantly reduce the PVDF membrane flux. Whilst the adsorbability of thepollutants with the membrane influenced the membrane’s cleaning effect and the fluxrecovery extent. The ozonation products of NOM had stronger complexing abilitywith Ca2+, which aggregated under the micro-filtration pressure, and led to theincrease of particle removal and the evident decrease of membrane flux. The Caelement content of the foulants on membrane surface was11.1%with the case ofozonated water, which was clearly higher than the case of unozonated water. Withthe sand filter backwash water, it contained aggregates that had weak attachmentwith the membrane, thus the membrane flux was easily recovered after flush.The experiment using simulated water showed that under natural pH condition,As(V) had strong complexing ability with Fe3+or the hydrolyzed species, As(V)could be easily removed by coagulation, and the impact of the competing Ca2+,humic acid, and HCO3-was weak. During the oxidation of As(III) by both O3andKMnO4, the oxidant competing ability by humic acid was very weak, and the impactof humic acid on As(III) removal by preoxidation-coagulation was not obvious.The complexing ability of Tl(I) was very week, while Tl(III) had goodcomplexing ability. Both O3and KMnO4could promote Tl(I)’ removal in FeCl3coagulation, but the mechanism was different. Ozone could oxidize Tl(I) into Tl(III),and the thallium removal effectiveness was related to the ozone dosage. When theCa2+concentration was high, it would compete the complexing site with Tl(III);humic acid had strong competing ability for O3, and a low dose of humic acid wouldprevent the oxidation of Tl(I) by O3. Under natural pH condition, KMnO4could notoxidize Tl(I) into Tl(III), the increased removal efficiency was due to the goodadsorption by in-situ formed MnO2·mH2O; a low dose of Ca2+significantlyinhibited Tl(I)’ adsorption on MnO2·mH2O; the presence of humic acid mightinfluence Tl(I)’ adsorption on MnO2·mH2O. Since both O3and KMnO4had strengthand weakness during the enhancement of Tl(I) removal, combined oxidation by O3and KMnO4was efficient for Tl(I) removal, and lower dose of these oxidants wasrequired, ozone could efficiently oxidize Tl(I) after KMnO4destroy the reducingfunctions of humic acid. The property of NOM had great influence on the enhanced coagulation ofparticles such as algae and inorganic particles. The coagulant dose was mainlydetermined by the hydrophobic fraction of NOM, and had better correlation withUV254than with TOC. When the NOM concentration was high, the ratio of larger particles(>5m) in the DAF effluent increased, and for larger particles, the major filtrationmechanism changed to physical straining and wedging, the filtration efficiencydecreased and the filter run time is reduced. Hydrophobic fraction of NOM wasactive component in ozonation, its difference in the content in water led to differentozonation effect. With the raw water containing high hydrophobic substance, the pre-ozonation on the enhanced coagulation was less effective; conversely, the pre-ozonation effectiveness was obvious. Preozonation was efficient for algal particlesreduction, and it was not effective with just increasing the coagulant dose. Pre-ozonation reduced the chloroform formation potential, but increased the chloroformformation potential. The GC-MS results showed that coagulation was efficient for theremoval of alcohols and hydrocarbons with unsaturated links, but not effective forthe removal of hydrocarbons and alcohols with low molecular weight, and pr-ozonation improved their removal. |
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