An X-ray absorption spectroscopy study of the coprecipitation of Tc and Re with mackinawite (FeS)

Metal ion binding to iron oxides

The uptake of Cu and Cd in aqueous solution by interaction with the surfaces of carefully synthesized finely sized particles of goethite, lepidocrocite, mackinawite, and pyrite was measured as a function of initial metal concentration in solution. The results show how reactions between metal ions in solution and mineral surfaces depend in a subtle way on the nature of the surfaces and, in certain cases, on the initial concentration of metal in solution. The uptake curves fall into two groups; type I in which the efficiency of uptake decreases with increasing concentration (Cu and Cd on goethite and lepidocrocite, Cd on pyrite), and type II in which it remains constant (Cu on mackinawite and pyrite, Cd on mackinawite). The total uptake is an order of magnitude greater for the latter group. X-ray absorption spectroscopies (XANES and EXAFS) were used to define the local environments of the metals taken up at the mineral surfaces. All examples showing the type I behavior yield information on local environments consistent with their being bound to the surfaces by an inner sphere complex formation mechanism. Thus, Cu on goethite appeared to form a Jahn-Teller distorted octahedral complex (four O atoms at 1.94 Aa; two O atoms at 2.41 Aa) but with evidence for interaction with two further Fe (or Cu) at 2.92 Aa. On lepidocrocite, the first coordination sphere was essentially identical to that on goethite but with a second Cu or Fe shell at 3.04 Aa and, for the highest Cu loadings, a third shell (2 Cu or Fe) at 3.67 Aa. The Cd on goethite showed best fits for sixfold coordination to O (at 2.26 Aa) but with evidence for a second shell of Fe atoms at 3.75 Aa. On lepidocrocite, the first shell was essentially the same as for goethite, but a second shell of Fe atoms appeared to occur at the shorter distance of 3.31 Aa. For Cd-loaded pyrite, best fits were given by a single shell of six O atoms at 2.27 to 2.28 Aa and with no evidence for a second shell of metal atoms. The systems exhibiting the type II behavior yielded spectra consistent with the formation of new phases on the surfaces, either by precipitation or replacement reactions. In the case of Cu interaction with mackinawite, a chalcopyrite phase appeared to form, whereas interaction with pyrite seemed to produce binary Cu sulfides--covellite at lower loadings and chalcocite at higher loadings. Cd interaction with mackinawite seemed to produce a CdS phase.

/pdf/reactions-of-copper-and-cadmium-ions-in-aqueous-solution-4u3buxdfj7.pdf

Reactions of copper and cadmium ions in aqueous solution with goethite, lepidocrocite, mackinawite, and pyrite

The adsorption of copper, chromate, and arsenate (CCA)onto iron-oxide-coated sand (IOCS) was examined insingle-metal and mixed-metal systems. Copper andarsenate were strongly adsorbed or formed inner-spheresurface complexes with the IOCS surface while chromatewas weakly adsorbed or formed an outer-sphere surfacecomplex with the IOCS surface. Copper adsorption byIOCS slightly increased in the presence of arsenatebut was not affected by the presence of chromate. Arsenate adsorption was not affected by the presenceof copper and/or chromate. Chromate adsorptionincreased in the presence of copper by the combinationof electrostatic effects and possiblesurface-copper-chromate ternary complex formation. The presence of arsenate significantly decreasedchromate adsorption due to competition for adsorptionsites and electrostatic effects. Using inner-spheresurface complexation constants for copper and arsenateand outer-sphere surface complexation constants forchromate, the triple-layer model (TLM) was successfulin describing adsorption of copper, chromate, andarsenate in single-solute systems. The modelindicated that each metal uses a different number ofadsorption sites. For CCA metals-IOCS systems, theequilibrium constants determined from single-solutesystems were not able to predict adsorption frommulti-solute systems. The TLM does not currentlyaccount for the heterogeneity of oxide surface sitesand the formation of ternary complexes and/or solidphases that do not exist in single-solute systems.

Copper, Chromium, and Arsenic Adsorption and Equilibrium Modeling in An Iron-Oxide-Coated Sand, Background Electrolyte System

The adsorption of lead(II) and zinc(II) onto goethite was studied as a function of pH, total dissolved metal concentration, surface area of goethite, and ionic strength. The results for zinc and lead were compared with those for copper reported earlier. The adsorption edge of lead ranges from pH 4 to 7, similar to that of copper, but the adsorption edge of zinc is displaced by 1.5 pH units toward higher pH. A fourfold increase in goethite surface area had a significant effect on the adsorption edge of lead, but a tenfold increase in the ionic strength of the medium did not affect the adsorption edge of lead and zinc. At neutral pH, 50 percent of the zinc was still available for transport and reactions in aqueous solution, whereas almost 100 percent of the lead and copper were bound to the goethite surface. The distribution coefficients increase sharply with the increase in pH and ranged from 60 to 30,000 ml/g in 2.5 pH units for lead and from 60 to 3000 ml/g in 1.5 pH units for zinc, depending on the goethite surface area and metal concentration. Distribution coefficients were used to calculate the number of protons released per mole of metal adsorbed during the adsorption process, with the average number of protons released per mole of lead and zinc adsorbed estimated to be 0.97±0.07 and 1.32±0.06, respectively. Proton coefficients of copper, lead, and zinc were correlated to their ionic radii and apparent equilibrium binding constant. Although the adsorption behavior of copper and lead were similar and both have the same charge, the drop in pH per mole of metal adsorbed is more in copper than in lead.

Comparative study of adsorption behavior of copper, lead, and zinc onto goethite in aqueous systems

The adsorption of copper(II) onto goethite was studied as a function of pH, total dissolved copper concentration, surface area of goethite, and ionic strength. The adsorption of copper was similar to that of other hydrolyzable metals. A tenfold increase in goethite surface area had a significant effect on the adsorption edge, but a tenfold increase in the ionic strength of the medium did not effect the adsorption edge. The distribution coefficients increase sharply with increase in pH and ranged from 10 to 60,000 ml/g over a range of two and half pH units, depending on the goethite surface area and copper concentration. A tenfold decrease in ionic strength as well as a tenfold increase in surface area of goethite did not have any effect on the magnitude of distribution coefficients. Distribution coefficients were used to calculate the number of protons released per mole of copper adsorbed during the adsorption process. The average number of protons released per mole of copper adsorbed was estimated to be 1.40 ± 0.10.

Zorawar S. Kooner

Papers

Adsorption of copper onto goethite in aqueous systems