Humic substance metal adsorption

Benedetti, M.F., Milne, C.J., Kinniburgh, D.G., van Riemsdijk, W.H. and Koopal, L.K. (1995) Metal ion binding to humic substances application of the non-ideal competitive adsorption model. Environ. Sci. Technol, 29, 446-457. 

An actual contribution of humic substances to metal oxide reduction in natural systems has not been demonstrated, and there are processes such as adsorption or decomposition that could limit their effectiveness. Kostka et al. (2002a) observed that AQDS additions elicited a larger increase in Fe(III) reduction by S. oneidensis growing on ferrihydrite than smectite clay minerals. This suggests that the influence of humic substances may depend on soil or sediment mineralogy. Nevertheless, there is ample evidence to suggest that a portion of the anaerobic metabolism that was previously attributed to direct enzymatic Fe(III) and Mn(IV) reduction was actually none-nzymatic reduction by microbially reduced humic substances. 

The preceding discussion relies on an analogy between complexation in solution and complexation at the mineral surface, a fundamental tenet of the surface complexation model (27). Strong complexation of metals in solution by humic substances is well-documented (16, 42-44). Thus surface complex formation is a likely mechanism for the adsorption of humic substances on oxide surfaces. 

Humic substances, although a major fraction of soil DOM, do not facili- ate oxide dissolution in laboratory studies. The mobilization of metals in soils mav, however, be indirectly affected by humic substances. Solubilization of -metals may be inhibited by competitive adsorption of humic substances and LMW organic ligands. The opposite effect—an enhancement of metal morn ility—might result from the stabilization of dissolved metals as humate -omplexes. 

A number of processes affect the solubility characteristic of metal-hu-mate and metal-fulvate complexes in soils, as well as in natural waters. A major factor is the extent to which the complex is saturated with metal ions. Other factors affecting solubility include pH, adsorption of the complex to mineral matter (e.g., clay), and biodegradation. Under proper pH conditions, trivalent cations, and to some extent divalent cations, are effective in precipitating humic substances from very dilute solutions monovalent cations are generally effective only at relatively high particle concentrations. 

The chemical and biochemical behaviors of humic substances can also be changed by GPC. Frimmel and Sattler (1982) studied the complexation/ adsorption of trace metals by dissolved humic substances and discovered that the affinity of humic substances for metals markedly increased following GPC. Similarly, Stewart and Wetzel (1982) observed that all Sephadex G-lOO fractions of dissolved humic material obtained from the aquatic macrophyte Typha were more stimulatory to C assimilation by algae than were the same humic substances that had not been fractionated. The observations indicated that the gel, eluent, or processing procedure (e.g., lyophilization, reconstitution, cleavage during separation) either reduced the toxicity of the humic substances or enhanced its stimulatory nature or affinity toward trace substances. 

Complexation of metals and trace anions (e.g., phosphate) by humic substances leads to a decrease of toxicity of certain metals toward microorganisms and increases the availability of some metals, but decreases phosphorus resources. At this time, the role of humic substances in reducing the toxicity of trace metals is more clearly understood than the other roles. Adsorption of micropollutants by aquatic humic substances may enhance their toxicity toward microorganisms in many cases. The specific modes of action, however, are mostly unknown. On the other hand, dissolved humic substances may assist in the degradation of organic pollutants. Under environmental conditions, the predominating process is not obvious. 

The majority of adsorption studies deal with the interactions between metal ions and either real or model sediment phases. Amorphous FeCOH), hydrous MnO.. clays (bentonite, illite, kaolinite), SiO., Al Oj and organic matter (often humic substances) are the solid phases most frequently studied. 

In our work we concentrate on the interactions between di- and tri-valent metal radionuclides which enter estuarine and seawater in ionic forms and humic and fulvic acids of different origins. However, as humic substances react with metals, not only in dissolved but also in undissolved states, we report some data on the adsorption of trace metal radionuclides on suspended humic acids. 

When present in undlssolved states, humic substances act as adsorbents for trace metals. The presence of suspended humic substances increases the adsorption ability of suspended matter and sediment. 

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