Water-soluble natural products general methods

Humic substances, which are biopolymers widely and abundantly present in natural waters and soils, also have a high complexing ability with various heavy metal ions. These compounds are formed by the random condensation of breakdown products of terrestrial and aquatic plants and extracellular metabolites of phytoplankton. Concentrations of metals in marine and fresh waters are often higher than predicted from the solubility products of corresponding hydroxide and carbonate compounds. The complexation of metal ions with dissolved humic substances is responsible for the apparent supersaturation of metals in natural waters [9-21], Water-soluble humic substances are usually divided into two fractions, humic acid (HA) and Mvic acid (FA). HA is defined in operational terms as the fraction of humic substance soluble in alkaline solutions and insoluble in acidic solutions, while FA is the fraction soluble in both alkaline and acidic solutions. A general method for the fractionation of humic substances is illustrated in Fig. 1. HA is easily obtained as a precipitate in acidic solution (pH < 1.5). Although HA appears to be an attractive adsorbent for the recovery of heavy metal ions, there is little information on its practical application as adsorbent. It is difficult to use humic acid as the adsorbent because of its high solubility in water. 

Sol id Sol utions. The aqueous concentrations of trace elements in natural waters are frequently much lower than would be expected on the basis of equilibrium solubility calculations or of supply to the water from various sources. It is often assumed that adsorption of the element on mineral surfaces is the cause for the depleted aqueous concentration of the trace element (97). However, Sposito (Chapter 11) shows that the methods commonly used to distinguish between solubility or adsorption controls are conceptually flawed. One of the important problems illustrated in Chapter 11 is the evaluation of the state of saturation of natural waters with respect to solid phases. Generally, the conclusion that a trace element is undersaturated is based on a comparison of ion activity products with known pure solid phases that contain the trace element. If a solid phase is pure, then its activity is equal to one by thermodynamic convention. However, when a trace cation is coprecipitated with another cation, the activity of the solid phase end member containing the trace cation in the coprecipitate wil 1 be less than one. If the aqueous phase is at equil ibrium with the coprecipitate, then the ion activity product wi 1 1 be 1 ess than the sol ubi 1 ity constant of the pure sol id phase containing the trace element. This condition could then lead to the conclusion that a natural water was undersaturated with respect to the pure solid phase and that the aqueous concentration of the trace cation was controlled by adsorption on mineral surfaces. While this might be true, Sposito points out that the ion activity product comparison with the solubility product does not provide any conclusive evidence as to whether an adsorption or coprecipitation process controls the aqueous concentration. 

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