Metal-citrate complexation formation

Brookhaven National Laboratory s (BNL s) biochemical recovery of radionuclides and heavy metals is a patented biochemical recovery process for the removal of metals and radionuclides from contaminated minerals, soil, and waste sites. In this process, citric acid, a naturally occurring organic complexing agent, is used to extract metals and radionuclides from solid wastes by the formation of water-soluble, metal-citrate complexes. The complex-rich extract is then subjected to microbiological biodegradation that removes most of the extracted heavy metals. 

The possible combinations and stoichiometries of AP" " or Fc +, cit -, H+, and OH that may occur in a metal-citrate species are numerous. In general, the formation of soluble metal—citrate complexes can be described by... 

Interactions of Metal Salts with the Formation. Interactions of metal salts with the formation and distribution of the retained aluminum in a porous medium may significantly affect the location and strength of gels. This interaction was demonstrated with polyacrylamide-aluminum citrate gels [1514]. Solutions were displaced in silica sand. The major findings of this study are that as the aluminum-to-citrate ratio increases, the aluminum retention increases. Furthermore, the amount of aluminum retained by silica sand increases as the displacing rate decreases. The process is reversible, but the aluminum release rate is considerably slower than the retention rate. The amount of aluminum released is influenced by the type and the pH level of the flowing solution. The citrate ions are retained by silica sand primarily as a part of the aluminum citrate complex. Iron, cations, and some divalent cations cannot be used in the brine environment. 

Anthocyanins can form complexes with metal ions such as tin, iron and aluminium. The formation of a complex, as expected, alters the colour, usually from red to blue. Complex formation can be minimised by adding a chelating agent such as citrate ions. Another problem with anthocyanins is the formation of complexes with proteins. This can lead to precipitation in extreme cases. This problem is normally minimised by careful selection of the anthocyanin. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

Figure 6.10. Complex formation of Fe(III) and of Cu(II) by various ligands The Lability cannot be predicted alone from complex stability constants but competitive effects of (with metal ions) and of OH (with ligands) need to be considered. Multidentate complex formers form more stable complexes, especially at high dilutions, than monodentate ligands (e.g., F, NH3). To solutions of TOTFe(III) = 10 M and TOTCu(II) = 10 M, respectively, complex formers (at the concentrations indicated in the figures) were added (points are calculated). (Cit = citrate, gly = gly

Effects of Added Ligands. Investigation of the effects of ligands added to seawater samples was imdertaken to determine whether the presence of these compounds might interfere with the formation and extraction of metal-PCD complexes. Glycine, thiamine, citrate, cysteine, EDTA, salicylate, pyrogallol, and phosphate were the hgands studied. 

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