Observations and modelling of the reactions of 10 metals with goethite: adsorption and diffusion processes

Abstract

A sample of goethite was mixed for periods which ranged from 2 hours to 8 weeks with solutions of dilute nitrate salts of Pb, Hg, Cd, Zn, Cu, Ni, Co, Mn, Cr and Al. The amount of sorption after each period was measured for an appropriate pH range for each metal. The sorption behaviour was characterized both by using characteristics of the sorption curves such as the pH at which half of the added metal was sorbed (pH 50 ) and by fitting a model in which sorption was mainly characterized by an affinity constant and by a diffusion constant. Initial sorption, whether characterized by the pH 50 or by the affinity constant, was closely correlated with the appropriate dissociation constants of the metals. The greater the affinity of the metals for hydroxide ions, the greater their affinity for the goethite surface. The metals differed in the rate at which they continued to react with the goethite. Lead had the slowest continuing reaction, cobalt the fastest. The continuing reaction was due to diffusion into the particles. It was characterized by the fitted diffusion constant and by the change with time in the pH 50 . For seven of the eight divalent metals, these were correlated with the ionic radius of the metals: the larger the radius, the slower the reaction. For Al and Cr, rates were slower than would be expected from the ionic radii and we suggest this shows that these ions react as the larger M(OH) 2þ ions. The behaviour of Ni was consistent with oxidation of the surface species and diffusion of Ni(OH) 2þ ions. The continuing reaction was similar to that observed when metal ions react with soils and suggests that their reaction with iron oxides is important in soils. The results also show that studies in which sorption is measured at only one period of reaction are incomplete and the application of equilibrium models to such results is misleading.

Kinetics of neptunium(V) sorption and desorption on goethite: An experimental and modeling study

“…However, modeling results and their implications for the relevance of aging, hysteresis and irreversible sorption behavior may strongly depend on the metals and mineral phases under investigation. For instance, different elements may form various types of surface complexes (Garnier et al, 2006), or may be characterized by different pore diffusion rates depending on their ionic radius (Fischer et al, 2007).…”

“…Furthermore, metal diffusion into goethite pores cannot be ruled out, as this has been shown to be a relevant process for other metal contaminants (Mustafa et al, 2006;Fischer et al, 2007). Previous studies focusing on Np(V) sorption onto Fe-oxide minerals suggest that the following surface chemical processes may be relevant: (1) the transformation of neptunium outer-into inner-sphere complexes, as indicated by X-ray absorption spectroscopy (Jerden and Kropf, 2007), and (2) the surface-mediated reduction of Np(V) to Np(IV), which has been reported for goethite (Jerden and Kropf, 2007) and magnetite (Nakata et al, 2002;Nakata et al, 2004).…”

Geochemistry and spatial distribution of heavy metals in Oxisols in a mineralized region of the Brazilian Central Plateau

“…Heavy metal adsorption on Fe and Mn oxides has been studied by several researchers and in general the studies have suggested the following order of adsorption of heavy metal ions on these oxides: Pb N CuN Mn N Co N Zn N Ni for the Mn oxides (McKenzie, 1989); CuN Pb N ZnN Co N Ni for goethite (Fischer et al, 2007), Pb N Cu N Zn N Co N Ni and PbN CuN Zn N Ni N Co for hematite and amorphous Fe, respectively (Ross, 1994). For soils, specially the highly weathered ones, the order has been, generally, Cr N Pb N Cu N CdN Zn N Ni (Matos et al, 1996;Fontes et al, 2000;Gomes et al, 2001;Fontes and Gomes, 2003).…”

Zinc adsorption by perlite: Effects of pH, ionic strength, temperature, and pre-use as growth substrate

“…Zinc adsorption by soil constituents is commonly considered to be specific, i.e., its affinity to charged surfaces is considerably higher than that of other cations coexisting in the solution phase, and is characterized by higher adsorption than expected from cation-exchange reactions with soil constant-charge surfaces (Alloway, 1995). It is common for equilibrium not to be attained throughout weeks of reaction, and Zn adsorption progresses as a result of metal-ion diffusion into inner sites (Fischer et al, 2007;Li et al, 2010;Nibou et al, 2010). Tiller et al (1984a) suggested that the kinetics of specific adsorption is slower than that of nonspecific adsorption; it is more pH-dependent than, and dominates over non-specific adsorption, at low initial concentrations.…”

“…Zinc adsorption onto metal oxides and clay minerals was successfully simulated as a function of solution concentration, pH, and I by employing models based on two or three adsorption layers (Barrow et al, 1981;Fischer et al, 2007). Adsorption by soil as a function of solution concentration and pH (but not I) was simulated by a single-layer model (Gu and Evans, 2007;Saeki and Kunito, 2009), or by a phenomenological, modified Langmuir competitive-adsorption model (Bar-Yosef, 1979;Bar-Yosef et al, 1988).…”