Recorded molecular weights (MWs) for humic substances (HS) range from a few hundred to millions of daltons. For purposes of defining HS as a specific class of chemical compounds, it is of particular importance to ascertain if this broad range of MWs can be attributed to actual variability in molecular properties or is simply an artifact of the analytical techniques used to characterize HS. The main objectives of this study were (1)to establish if a preferential range of MWs exists for HS and (2) to determine any consistent MW properties of HS. To reach the goal, we have undertaken an approach to measure under standardized conditions the MW characteristics of a large set of HS from different natural environments. Seventy-seven humic materials were isolated from freshwater, soil, peat, and coal, such that each possessed a different fractional composition: humic acid (HA), fulvic acid (FA), and a nonfractionated mixture of HA and FA (HF). Size exclusion chromatography (SEC) was used as the analytical technique to determine molecular weight characteristics. The MW distributions were characterized by number (Mn) and weight (Mw) average MW, and by polydispersity. The complete range of Mw values varied within 4.7-30.4 kDa. The maximum Mw values were observed for peat HF and soil HA, whereas the smallest weights were measured for river water HF. Maximum values of polydispersity (3.5-4.4) were seen for peat HF and soil HA, while much lower values (1.6-3.1) were found for all preparations isolated with XAD-resins. Statistical evaluation showed consistent Mw and Mn variations with the HS source, while polydispersity was mostly a function of the isolation procedure used. A conclusion was made that HS have a preferential range of MW values that could characterize them as a specific class of chemical compounds.

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The effectiveness of water treatment processes in removing natural organic matter varies with the nature of the natural organic matter (NOM), its molecular size, polarity and charge density, and with properties of the raw water such as turbidity and hardness. In some cases conventional alum treatment is inefficient. We have compared NOM removals achieved by conventional and polymer-based processes in bench-scale treatment of reconstituted ground and surface waters of varying colour, made from NOM isolated from the same waters. NOM isolates were fractionated by adsorption on non-functionalised resins and an anion exchanger, and characterised by size exclusion chromatography. Jar tests with the isolated NOM compared coagulation with polyelectrolytes, alum, clays and metal oxides, with each of the inorganics being in conjunction with a polyelectrolyte. Jar tests on reconstituted waters with alum and/or cationic polyelectrolyte show synergistic benefits from combinations of the two. The more hydrophobic NOM fractions were the most easily removed by polymer. The performance of cationic polymers improved significantly with increasing charge density and molecular weight. An alum/polymer combination is the most attractive treatment option.

of specific interactions between metal oxide surface hydroxyl groups or adsorbed water and the oxygen of Eleven well characterized humic substances (HSs) were adsorbed adsorbed carboxyl or hydroxyl groups of organic acids from aqueous solution onto a Na-kaolin clay. The adsorption affinity (K L), maximum adsorption capacity (b), a coefficient of desorption including HA (Parfitt, 1977; Yost et al., 1990; Biber and hysteresis (H), and the concentration of irreversibly adsorbed HS Stumm, 1994). Ligand exchange is highly affected by (IHS) were derived from adsorption-desorption isotherms. These pathe pH value of the adjacent solution. As a rule, adsorprameters were correlated with structural features of the HS. The tion of HS onto metal oxide surfaces increases with adsorption affinity was shown to correlate directly with the aromaticity decreasing pH value, passing a maximum at pH ϭ 4.3 of the HS and inversely with their polarity, expressed as the O/C to 4.7, corresponding to pK a values of most abundant atomic ratio. A dependency between polarity and maximum adsorpcarboxylic acids (Davis, 1981; Perdue, 1985; Murphy, tion capacity was not confirmed. The parameters b, H, and IHS expose 1990). The pH value determines the protonation state close correlation with the molecular weight (MW) and the partial negative charge of HS (Z) at the operating pH value. The following of the sorbate as well as of the surface hydroxyl groups. quantitative relationship was obtained: b ϭ 715 Ϫ 0.06 ϫ MW Ϫ As a result, the surface complexation via ligand ex-529 ϫ Z (r ϭ 0.92). It allows a selection of HS with respect to the change becomes less favorable as soon as the pH value largest content of organic matter in HS-kaolin clay complexes. Among exceeds the point of zero net surface charge (pH zpc) the HS studied the high molecular weight materials enriched with Csimply because of increasing electrostatic repulsion beand H-substituted aromatics, such as coal and peat humic acids (HAs), tween the surface and the anionic humic ligands. Noneare shown to be the most preferential materials for preparing stable theless, significant HS adsorption can be still observed HS-clay complexes. at these high pH values, for example, about 30% of the maximum adsorption in a hematite system at pH ϭ 9 (Vermeer et al., 1998a), and about 37% in a kaolin clay

Natural organic matter from drinking water sources was fractionated, and the fractions were characterized by NMR and SEC with the aim of relating NOM structure to treatability. Organic matter was isolated from two Australian surface waters (Horsham, Moorabool) by reverse osmosis and from a groundwater (Wanneroo) by anion exchange. The isolates were fractionated according to polarity and charge by resin adsorption. 13C NMR spectra of the freeze-dried fractions showed the most hydrophobic fraction to be high in aliphatic and aromatic carbon while slightly hydrophobic organics have more carbonyl and alkoxyl carbon. The Horsham and Wanneroo hydrophilic fractions show strong alkoxyl signals attributed to carbohydrate. Moorabool hydrophilics contain aromatic (phenolic) carbon; the apparent absence of carbohydrate appears to be an artifact. Size-exclusion chromatograms were recorded on the original and fractionated organics with both UV and dissolved organic carbon detection. The Horsham and Moorabool organics have similar molecular size distributions while Wanneroo is dominated by strongly absorbing species having large hydrodynamic radii. The hydrophobic and charged hydrophilic fractions also have high apparent MW, while the neutral fraction is higher in low-MW compounds of relatively low specific absorbance, suggestive of carbohydrates.