Page 4: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Colour
Although much research has been performed on the nature of organic colour in water during the past 60 years,Footnote 17 important chemical information on these "colour bodies" is still lacking. The dissolved organic colouring material in water consists almost totally of humic substances,Footnote 1,Footnote 18-20 and the gross characteristics of these substances are very similar in the surface waters of many different parts of the world.(Footnote 1,Footnote 6,Footnote 18,Footnote 21-23 The genesis of humic substances is still subject to conjecture.Footnote 24 Most of the humate content of water is probably derived from soil, but some of it is also produced by aquatic micro-organisms.Footnote 25,Footnote 26 Dissolved humic substances isolated from water are predominantly fulvic acidsFootnote 1,Footnote 5,Footnote 19,Footnote 22; humic acid is an important constituent of sedimentsFootnote 27 and of the particulate matter in surface water.
Humic substances isolated from soil can strongly adsorb a variety of organic substances.Footnote 28,Footnote 29 Part of this adsorption capacity is probably due to the presence of pore spaces, or voids, in these polymeric moleculesFootnote 30; part may be due to the long lipophilic aliphatic side chain "building blocks" in humic substances; part is probably due to an affinity of certain adsorbates for the functional groups present in humic substances (ester, ether, carboxyl, Colour ring, etc.); and, for polar and charged adsorbates, part is probably due to the polyelectrolytic nature of the humate polymers.
The adsorption by humic substances of organic compounds -- many of which are of concern because of their toxic properties -- in amounts that can exceed their aqueous solubilitiesFootnote 31,Footnote 32 is a matter of potential importance and merits further investigation. In addition, because of their polyanionic electrolytic properties, humic substances play an essential role in the dissolution, transport, and deposition of positively charged inorganic ions, the most important of which, from a public health viewpoint, are those of the heavy metals. Most metals will complex to a degree with humic substances in water. Complex formation can dramatically increase the solubility of the metal; for example, naturally occurring humic substances in water may render iron up to a billion times more soluble.Footnote 33 The detection of iron in highly coloured waters in amounts greatly in excess of its ionic solubility is probably due more to iron-organic complexes than to colloidally dispersed ferric hydroxide, as was commonly supposed. Mixed complexes with common anions are also known and include fulvic acid-iron phosphate complexesFootnote 34 and fulvic acid-copper citrate, phosphate and salicylate complexes.Footnote 35
Some metals also form insoluble complexes with humic substances under certain circumstances. This phenomenon serves to precipitate them from natural water systemsFootnote 36 and is exploited in the production of potable water from coloured surface waters.Footnote 14 In general, the order of precipitation of insoluble metal complexes decreases with the charge of the metal; thus, trivalent metals are 700 to 1000 times more efficient as coagulating agents than monovalent metals, and doubly charged metal ions are 30 to 60 times more efficient than monovalent metal ions.Footnote 14,Footnote 28,Footnote 29 Iron and aluminum, which are employed in drinking water purification, appear to be capable of forming insoluble bridged complexes between the metal and two or more molecules of fulvic acid; polymeric hydroxy-aquo forms of iron and aluminum may also be involved in the precipitation of humic substances from solution.Footnote 37
The fate of complexed toxic metals during drinking water treatment is a particularly relevant question. The most prevalent view has been that, although toxic metals associated with the suspended solids content of water may be at least partially removed, dissolved trace metals are probably removed to a negligible extent during conventional treatment.Footnote 38 On the other hand, if certain toxic metals in solution are strongly complexed to coloured organic molecules, which are themselves removed during treatment, it might be the case that colour removal by precipitation could also affect the removal of toxic metals. Although a small amount of information is available that suggests that 30 to 65% removal of trace metals can be achieved by conventional water treatment plants,Footnote 39 the degree to which complexes are involved in this removal remains unknown.
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