Organic complexes with clay minerals

Lead enters surface water from atmospheric fallout, run-off, or wastewater. Little lead is transferred from natural minerals or leached from soil. Pb ", the stable ionic species of lead, forms complexes of low solubility with major anions in the natural environment such as the hydroxide, carbonate, sulfide, and sulfate ions, which limit solubility. Organolead complexes are formed with humic materials, which maintain lead in a bound form even at low pH. Lead is effectively removed from the water column to the sediment by adsorption to organic matter and clay minerals, precipitation as insoluble salt (the carbonate, sulfate, or sulfide) and reaction with hydrous iron, aluminum, and manganese oxides. Lead does not appear to bioconcentrate significantly in fish but does in some shellfish such as mussels. When released to the atmosphere, lead will generally occur as particulate matter and will be subject to gravitational settling. Transformation to oxides and carbonates may also occur. 

Humic acids are soluble in weak alkaline solutions and are essentially insoluble in water and mineral acids. They may be precipitated from solution by the action of mineral acids and bivalent or trivalent cations, however, they are fairly resistant to the acid hydrolysis. They are dark spherocol-loids with a cross-linked structure which plays a part in their high sorption capacity. They exhibit different degrees of a tendency to aggregation and very different degrees of dispersion. In comparison with other types of natural organic substances, the humic acids are characterized by their extraordinary stability in the soil. This stability is due to their ability to form organomineraJ complexes, particularly with clay minerals and with aluminium and iron hydroxides. 

The inositol hexa- and pentakisphos-phates are prevalent in soils compared to lower-order esters, probably because stability in the soil is linked to the number of phosphate groups. The most widespread stereoisomer is myo-inositol (Dalai, 1977). Inositol phosphates are more resistant to mineralization than the other fractions of the soil organic phosphorus and, therefore, are probably poorly available to plants (Williams and Anderson, 1968). They are present in soils in highly complex forms associated with clay minerals, fulvic and humic acids (Anderson and Arlidge, 1962), proteins and some metallic ions (Rojo et al., 1990). The various forms of inositol phosphates are often imprecisely referred to as phytic acid, which is reserved exclusively for the free acid form of myo-inositol hexakisphosphate. Salt forms of myo-inositol hexakisphosphate, also known as phytates, are very stable and consequently accumu-... 

In the case of neutral NACs two types of adsorption mechanisms have been found to play a predominant role in most of the subsurface sediments (i) partitioning into the organic fraction of the sediment (43, 59, 70) and (ii) adsorption due to complex formation with clay mineral surfaces that bear exchangeable NHJ or K cations. This type of interaction has been postulated as an electron donor-acceptor (EDA) complex where oxygen atoms at the siloxane surfaces of clay minerals act as electron donors and NACs act as electron... 

Figure 2. Distribution of DNOC (open symbols), and TNT (filled symbols) between aqueous and solid phases of typical aquifer and soil materials. Shown is the fraction of sorbed species as a function of pH and K" -saturation of the clay minerals. The relative importance of the two dominating sorption mechanisms are compared hydrophobic partitioning into sediment organic matter (O, ) and specific EDA-complex formation with clay mineral surfaces (O, ) triangles (A, A) represent the overall speciation for DNOC and TNT respectively. Sediment parameters were chosen as follows fraction of organic carbon 4. (soil) = 0.05 f (aquifer) = 0.001 fraction of clay minerals f igy (soil) = 0.35 fciay (aquifer) = 0.05 porosity e(soil) = 0.4 (aquifer) = 0.3 bulk density p(soil) = p(aquifer) = 2.5 kg L water saturation 0(soil) = 0.5 0(aquifer) = 1 fraction of K -saturation of clay minerals = 0.001 (when pH was used as system variable) pH = 7.0 (when f(, +, was used as system variable). Linear adsorption isotherms of TNT and... 

Clay minerals or phyllosilicates are lamellar natural and synthetic materials with high surface area, cation exchange and swelling properties, exfoliation ability, variable surface charge density and hydrophobic/hydrophilic character [85], They are good host structures for intercalation or adsorption of organic molecules and macromolecules, particularly proteins. On the basis of the natural adsorption of proteins by clay minerals and various clay complexes that occurs in soils, many authors have investigated the use of clay and clay-derived materials as matrices for the immobilization of enzymes, either for environmental chemistry purpose or in the chemical and material industries. 

The distribution of the major elements (Ca, Mg, Na, K,. ..) in soils is well known to be governed by ion-exchange processes (1). The behaviour of transition elements such as Co, Ni, Cd, Cu, etc. in natural systems (soils, sediments) often results from a combination of different effects such as precipitation, sorption in oxides, exchange in clay minerals and complexation with organic... 

In addition to stabilizing organic products by reaction with metal-exchanged clays, as indicated above, aluminosilicate minerals may enable the preparation of metal organic complexes that cannot be formed in solution. Thus a complex of Cu(II) with rubeanic acid (dithiooxamide) could be prepared by soaking Cu montmorillonite in an acetone solution of rubeanic acid (93). The intercalated complex was monomeric, aligned with Its molecular plane parallel to the interlamellar surfaces, and had a metal ligand ratio of 1 2 despite the tetradentate nature of the rubeanic acid. 

Mordand MM (1970) Clay-organic complexes and interactions. Adv Agron 22 75-117 Mordamd MM (1986) Mechanism of adsorption of nonhumic organic species by clays. In Huang PM, Schnitzer M (eds) Interaction of soil minerals with natural organics and microbes. Soil Sci Soc Amer, Madison, Wisconsin, pp 59-76... 

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