Metal-hydroxide surface precipitates formation

The mechanism for the formation of metal hydroxide surface precipitates is not clearly understood. It is clear that the type of metal ion determines whether metal hydroxide surface precipitates form, and the type of surface precipitate formed (i.e., metal hydroxide or mixed metal hydroxide) is dependent on the sorbent type. The precipitation could be explained by the combination of several processes (Yamaguchi et al., 2001). First, the electric field of the mineral surface attracts metal ions (e.g., Ni) through adsorption, leading to a local supersaturation... 

The formation of metal hydroxide surface precipitates and subsequent residence time effects on natural sorbents can greatly affect metal release and hysteresis. It has generally been thought that the kinetics of formation of surface precipitates was slow. However, recent studies have shown that metal hydroxide precipitates can form on time scales of minutes. In Figure 3.7 one can see that mixed Ni-Al hydroxide precipitates formed on pyrophyllite within 15 minutes, and they grew in intensity as time increased. Similar results have been observed with other soil components and with soils (Scheidegger et ak, 1998 Roberts et ak, 1999 Sparks, 2002, 2005). 

The formation and subsequent aging of the metal hydroxide surface precipitate can have a significant effect on metal release. In Figure 3.8 one sees that as residence time (aging) increased from 1 hour to 2 years, Ni release from pyrophyllite, as a percentage of total Ni sorption, decreased from 23 to 0% when HNO3 (at pH 6.0) was employed as a dissolution agent for 14 days. This... 

Fig. 6.10 shows idealized isotherms (at constant pH) for cation binding to an oxide surface. In the case of cation binding, onto a solid hydrous oxide, a metal hydroxide may precipitate and may form at the surface prior to their formation in bulk solution and thus contribute to the total apparent "sorption". The contribution of surface precipitation to the overall sorption increases as the sorbate/sorbent ratio is increased. At very high ratios, surface precipitation may become the dominant "apparent" sorption mechanism. Isotherms showing reversals as shown by e have been observed in studies of phosphate sorption by calcite (Freeman and Rowell, 1981). 

Mixed Co-Al and Zn-Al hydroxide surface precipitates can also form on aluminum-bearing metal oxides and phyllosilicates (Towle et al., 1997 Thompson et al., 1999a,b Ford and Sparks, 2000). This is not surprising, as Co " ", Zir+, and NP+ all have radii similar to AP+, enhancing substitution in the mineral structure and formation of a coprecipitate. However, surface precipitates have not been observed with Pb2+, as Pb-+ is too large to substitute for AP+ in mineral structures (Sparks, 2002, 2005). 

Ni Sorption on Clay Minerals A Case Study. Initial research with Co/clay mineral systems demonstrated the formation of nucleation products using XAFS spectroscopy, but the stmcture was not strictly identified and was referred to as a Co hydroxide-like stmcture (11,12). Thus, the exact mechanism for surface precipitate formation remained unknown. Recent research in our laboratory and elsewhere suggests that during sorption of Ni and Co metal ions, dissolution of the clay mineral or aluminum oxide surface can lead to precipitation of mixed Ni/Al and Co/Al hydroxide phases at the mineral/water interface (14,16,17,67,71). This process could act as a significant sink for metals in soils. The following discussion focuses on some of the recent research of our group on the formation kinetics of mixed cation hydroxide phases, using a combination of macroscopic and molecular approaches (14-17). 

In surface precipitation cations (or anions) which adsorb to the surface of a mineral may form at high surface coverage a precipitate of the cation (anion) with the constituent ions of the mineral. Fig. 6.9 shows schematically the surface precipitation of a cation M2+ to hydrous ferric oxide. This model, suggested by Farley et al. (1985), allows for a continuum between surface complex formation and bulk solution precipitation of the sorbing ion, i.e., as the cation is complexed at the surface, a new hydroxide surface is formed. In the model cations at the solid (oxide) water interface are treated as surface species, while those not in contact with the solution phase are treated as solid species forming a solid solution (see Appendix 6.2). The formation of a solid solution implies isomorphic substitution. At low sorbate cation concentrations, surface complexation is the dominant mechanism. As the sorbate concentration increases, the surface complex concentration and the mole fraction of the surface precipitate both increase until the surface sites become saturated. Surface precipitation then becomes the dominant "sorption" (= metal ion incorporation) mechanism. As bulk solution precipitation is approached, the mol fraction of the surface precipitate becomes large. 

It is well known that hydrolyzed polyvalent metal ions are more efficient than unhydrolyzed ions in the destabilization of colloidal dispersions. Monomeric hydrolysis species undergo condensation reactions under certain conditions, which lead to the formation of multi- or polynuclear hydroxo complexes. These reactions take place especially in solutions that are oversaturated with respect to the solubility limit of the metal hydroxide. The observed multimeric hydroxo complexes or isopolycations are assumed to be soluble kinetic intermediates in the transition that oversaturated solutions undergo in the course of precipitation of hydrous metal oxides. Previous work by Matijevic, Janauer, and Kerker (7) Fuerstenau, Somasundaran, and Fuerstenau (I) and O Melia and Stumm (12) has shown that isopolycations adsorb at interfaces. Furthermore, it has been observed that species, adsorbed at the surface, destabilize colloidal suspensions at much lower concentrations than ions that are not specifically adsorbed. Ottewill and Watanabe (13) and Somasundaran, Healy, and Fuerstenau (16) have shown that the theory of the diffuse double layer explains the destabilization of dispersions by small concentrations of surfactant ions that have a charge opposite to... 

In the presence of a high cation to sorbent ratio, and at high sorbate concentrations, surface sites become saturated and surface complexation may be replaced by surface precipitation, which involves the formation of a new sohd or gel metal hydroxide at the surface [12]. 

O Day et al., 1994a,b, 1996 Hayes Katz, 1996 Papelis Hayes, 1996 Schei-degger et al., 1997, 1998 Towle et al., 1997 Xia, 1997). The formation of these surface precipitates occurs below bulk solution saturation for precipitation of pure metal hydroxide phases and at submonolayer surface coverages in many of the cases cited above. 

The equilibrium equations that normally have to be considered in the EKR modeling of a soil contaminated by heavy metals can be classified into one of the following categories complex formation reactions, precipitation of the metal hydroxides or of other species, ion exchange reactions, surface complexation reactions, etc. Anyway, the autoionization of water always has to be considered and the precipitation of carbonates, together with the carbonate-bicarbonate equilibrium, should normally also be considered. However, the above equations have only considered the species in aqueous phase, so if a species precipitates, a new master species has to be included in this equilibrium system, whose concentration would be the amount of the precipitated species per unit volume of water. This additional degree of freedom is constrained by the solubility product constant of the precipitate (KO, because the new solid phase is in equilibrium with the aqueous phase. If there exists Np precipitated species, the pure-phase equilibria can be represented with the following equation ... 

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