Solubility and Aggregation of Natural Organics

The solubility of natural organics is an important issue in membrane processes where concentration polarisation is a common effect. Concentration polarisation may lead to gel layer formation if the solubility is exceeded. Solubilities of mixtures are difficult to determine. Many parameters influence solubility FA is, by definition, more soluble than HA at low pH. The complexation with metal ions (see section 2.8) also influences solubility. Metal complexes of FA are more soluble than those of HA due to the lower MW and the higher charge of FA. The solubility of each compound depends on the saturation of the complex with metal ions (Stevenson (1985)). Tipping et al. (1988) established a direct relationship between solubility and charge of HA. 

Trivalent and some bivalent cations can precipitate HSs under the right pH conditions from dilute solutions. Precipitation or aggregation of organics with cations is facilitated at low temperatures, most likely due to the copredpitation effect with carbonates (Steinberg and Muenster (1985)). However carbonates ate more soluble at low temperatures. 

An ionic strength increase in water treatment occurs, for example, in the membrane boundarjf layer or along RO modules. In natural systems this occurs when freshwaters mix with seawater. 

Natural organics also readily adsorb onto mineral matter which transforms the organic into particulate form. Depending on the size of the mineral colloid, it will remain in suspension or settle. This issue will be addressed separately (refer to section 2.9). 

10 Micelle Formation of Natural Organics and Surfactant Characteristes 

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