The role of humic substances in the transport and fixation of metals of economic interest (Au, Pt, Pd, U, V)

Abstract Humic substances are complicated, naturally occurring organic compounds of high molecular weight. Owing to the presence of functional groups containing oxygen, and to a lesser extent, nitrogen and sulfur, these substances are capable of interacting with economically interesting metals in solution in a number of ways. Humic substances may: (1) form soluble complexes with metal ions; (2) cause precipitation of metal ions via reduction; (3) cause precipitation of metal ions via adsorption, ion exchange or other sorption processes (when humic substances themselves are insoluble); (4) modify the sorption behavior of mineral surfaces with respect to metal ions; (5) modify the behavior of colloidal particles of metals; and (6) affect the rates of geological processes. The ability to develop quantitative geochemical models of these interactions is important in the study of the secondary dispersion of most ore metals in the surficial environment and of primary ore-forming processes for a number of ore metals (e.g., Au, PGE, U, V). Available data suggest that humic substances are drastically altered, especially with respect to their ability to complex metals, at temperatures exceeding about 100°C, so their role is probably limited to low temperature environments. In this review, attention is focussed on the binding and reducing effects of humic substances on Au, PGE, U and V. Thermodynamic modeling of the binding of metals by humic substances is complicated by their diversity and our incomplete understanding of their nature. However, a number of models have been employed. These models may be broken down into three main categories: type I or discrete models, type II or continuous models and type III or analogue-based models. Each category includes some examples which also take into account the polyelectrolyte properties of humic substances. In this paper, each class of model is briefly reviewed. So far, none of these models have been applied to Au or the PGE, and only relatively simple models have been applied to U and V. In spite of numerous experimental studies of the interaction of Au with humic substances, there is little agreement as to the predominant role these natural organics play in Au transport and deposition, and no thermodynamic data for these interactions have been derived. The weight of experimental evidence seems to be equally divided between that which supports the predominant formation of soluble Au-humate complexes and that which supports the predominant reduction and precipitation of Au by humic substances. The conflicting observations may be a result of numerous factors, including: variability in the source and method of preparation of humic substances employed, lack of control of redox conditions, sensitivity of the method employed to study Au-humate interactions, possibility of important roles for both complexation and reduction, etc. Gold is expected to bind most strongly at sites usually present in relatively low concentration, i.e., those containing S and N donor atoms. The lack of agreement as to the role of humic substances in experiments using relatively concentrated gold solutions may be a result of saturation of these sites, followed by reduction of the excess gold. Detectable formation of soluble Au-humate complexes may only occur when the amount of gold available does not exceed the amount of sites where Au can be preferentially bound. It may be necessary to conduct experiments at concentrations of gold approaching those likely in natural waters, i.e., in the ng/l range. Significant advances in the study of Au-humate interactions will probably require the derivation of thermodynamic data using one of the binding models described in this paper. Very few studies of the interaction of the PGE with humic substances have been conducted, and these have been limited to Pt and Pd. Solubility studies show that the presence of fulvic acid, and some simple organic analogues such as acetate, phthalate, and salicylate, can increase the concentrations of both Pt and Pd in aqueous solution over those found in distilled water at similar pH. Raman, IR, NMR and UV-visible spectroscopic studies provide unambiguous evidence for the formation of soluble complexes between Pd and acetate, oxalate, and phthalate, and by analogy, fulvic acid. Like Au, Pt and Pd should form even stronger complexes with S- and N-bearing sites than with O-bearing sites. There is little evidence that humic substances induce the reduction and precipitation of Pt2+ and Pd2+. Numerous experiments on the interaction of U(VI) with solid humic substances at room temperature demonstrate that, over laboratory time scales, U(VI) is strongly bound without reduction. On the other hand, V(V) is reduced to and bound as V(IV), without further reduction to V(III). Soluble U- and V-humate complexes also exist, and in the case of U, several independent studies yield conditional binding constants which are in generally good agreement when account is taken of differences in pH and ionic strength. These data suggest that U-humate complexes should predominate over inorganic carbonate complexes at mildly acidic pH. A number of other metals (e.g., Cu, Pb, Zn, Cd, REE) are known to form complexes with humic substances but the roles of such species in the concentration of these metals to economic levels and the secondary dispersion of these metals largely remain unexplored.