Humic substances surface soils

Metals constitute an important source of pollution in soils. As discussed in Chapters 5 and 6, they can bind to soils through humic substances, surface complex-ation, or ion exchange. In some cases more than one type of interaction can occur, as in clay minerals (e.g., montmorillonite and vermiculite) that bind metals through ion exchange as well as surface com-... 

Samson JAR, Ederer DL (eds) Academic Press, San Diego, p 225-261 Aiken GR, McKnight DM, Wershaw RL, MacCarthy P (1985) Humic Substances in Soil Sediment and Water Geochemistry, isolation, and characterization. Wiley, New York Amemiya K, Kitajima Y, Yonamoto Y, Terada S, Tsukabayashi H, Yokoyama T, Ohta T (1999) Oxygen -edge X-ray-absorption fine-structure study of surface methoxy species on Cu(lll) and Ni(lll). Phys Rev 59 2307-2312... 

Heavy metals compete for sorption sites on soil components. Reports on competition in the adsorption of two or more heavy metals on variable-charge minerals and humic substances in soils are rare. Benjamin and Leckie (1981) found a nearly complete lack of competition among Cd, Cu, Zn and Pb added to a poorly crystalline Fe- oxyhydroxide. Probably, the lack of competition among the heavy metals could be attributed to low surface coverage effect. Saha et al. 

Aiken GR, McKnight DM, Wershaw RL, MacCarthy P (eds) (1985) Humic substances in soil, sediment, and water. Wiley-Interscience, New York, 692 pp Amrhein C, Suarez DL (1988) The use of a surface complexation model to describe the kinetics of ligand-promoted dissolution of anorthite. Geochim Cosmochim Acta 52 2785-2793... 

Hydrodechlorination is a common reaction of chlorinated pesticides such as atrazine (eq. 15), alachlor, and metolachlor (2) (eq. 16). These reactions are catalyzed primarily by transition metals or by soil surfaces (clays or humic substances). 

Dissolved humic substances (DHS) are the main constituents of the dissolved organic carbon (DOC) pool in surface waters (freshwaters and marine waters), groundwaters, and soil porewaters and commonly impart a yellowish-brown color to the water system. Despite the different origins responsible for the main structural characteristics of DHS, they all constitute refractory products of chemical and biological degradation and condensation reactions from plant or animal residues and play a crucial role in many biogeochemical processes. 

Humic substances. Analogous to the reactions described above, humic substances (the polymeric pigments from soil (humus) and marine sediments) can be formed by both enzymatic and non-enzymatic browning. High concentrations of free calcium and phosphate ions and supersaturation with respect to hydroxyapatite can sustain in soil, because adsorption of humic acids to mineral surfaces inhibits crystal growth (Inskeep and Silvertooth, 1988). A similar adsorption to tooth mineral in a caries lesion can be anticipated for polycarboxylic polymers from either the Maillard reaction or enzymatic browning. 

The behavior of phenolic compounds derived from decaying plant residues, or released from degrading humic substances, is dictated by the physico-chemical processes of adsorption and desorption. Equilibria between these processes determine the concentration of phenolic compounds in the soil solution and consequently the bioactivity, movement, and persistence of these substances in the soil. Surface interactions between phenolic compounds and colloidal matrices may promote their polymerization (25, 26) or protect them from microbial degradation and mineralization. 

Water table should be more than several feet from the land surface. Humic substances in the soil can interfere with the process. 

Standard materials were also investigated for comparison. These included two humic acids, one from Alfa Aesar (CAS 1415-93-6) obtained from an unknown source, and one from the International Humic Substances Society (IHSS) obtained from standard Elliot soil. IHSS fulvic acid from Elliot soil was also used for comparison. Humic acids are ubiquitous geologic contaminants present in archaeological materials that have been exposed to soil or groundwater, and are important comparative materials for surfaces that are to be (or in this case, have been) radiocarbon dated by the PCO-AMS technique. 

The hydroquinone-derived polymers formed in the presence of the tephroite system (Shindo and Huang, 1985a) have similar IR absorption bands to those of soil humic substances (Schnitzer, 1978). The surface features of these polymers... 

Environmental organic matter is a composite of humic and nonhumic substances, which is formed through operation and interactions of various biotic and abiotic processes. Humic substances are formed through both selected preservation (residue) and catalytic synthesis mechanisms. Both enzymatic and mineral catalyses contribute to the formation of humic substances in the environment. The relative importance of these catalytic reactions would depend on vegetation, microbial population and activity, enzymatic activity, mineralogical composition and surface chemistry of environmental particles, management practices, and environmental conditions. Selective preservation pathways would play a more important role in humification processes in poorly drained soils and lake sediments, compared with more aerated environmental conditions. 

Thermal analysis of humic-mineral complexes has shown there is an overall reduction in the decomposition temperatures of humic acid that has been com-plexed to a mineral surface. Changes in the exothermic peak temperatures of humic substances in the free and complexed state are well-documented for synthetic mineral complexes with humic and fulvic acid (Schnitzer and Kodama, 1972 Tan, 1977 Schnitzer and Ghosh, 1982) as well as for authentic soil complexes. 

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