Iron Adsorption on Clays Inferred from Atomistic Simulations and X-ray Absorption Spectroscopy.

The atomistic level understanding of iron speciation and the probable oxidative behavior of iron (Feaq2+ → Fesurf3+) in clay minerals are fundamental for environmental geochemistry of redox reactions. Thermodynamic analyses of wet chemistry data suggest that iron adsorbs on the edge surfaces of clay minerals at distinct structural sites commonly referred as strong and weak sites (with high and low affinity, respectively). In this study, we applied ab initio molecular dynamics simulation to investigate the structure and the stability of the edge surfaces of trans- and cis-vacant montmorillonites. These structures were further used to evaluate the surface complexation energy and to calculate reference ab initio X-ray absorption spectra (XAS) for distinct inner-sphere complexes of iron. The combination of ab initio simulations and XAS allowed us to reveal the Fe-complexation mechanism and to quantify the Fe partitioning between the high and low affinity sites as a function of the oxidation state and loadings. Although iron is mostly present in the Fe3+ form, Fe2+ increasingly co-adsorbs at increasing loadings. Ab initio structure relaxations of several different clay structures with substituted Fe2+/Fe3+ in the bulk or at the surface site showed that the oxidative sorption of ferrous iron is an energetically favored process at several edge surfaces of the Fe-bearing montmorillonite.

Thallium Adsorption onto Illite.

We investigated the adsorption of Tl+ onto purified Illite du Puy (IdP). Distribution coefficients (Kd) for trace Tl adsorption indicated a moderate pH-dependence from pH 2.5 to 11. Adsorption isotherms measured at Tl+ concentrations from 10-9 to 10-2 M at near-neutral pH on illite saturated with Na+ (100 mM), K+ (1 and 10 mM), NH4+ (10 mM) or Ca2+ (5 mM) revealed a high adsorption affinity of Tl+ in Na+- and Ca2+-electrolytes and strong competition with K+ and NH4+. Cation exchange selectivity coefficients for Tl+ with respect to Na+, K+, NH4+, and Ca2+ were derived using a 3-site sorption model. They confirmed the strong adsorption of Tl+ at the frayed edges of illite, with Tl selectivity coefficients between those reported for Rb+ and Cs+. X-ray absorption spectra of Tl adsorbed onto Na-exchanged IdP indicated a shift from adsorption of (dehydrated) Tl+ at the frayed edges at low loadings to adsorption of (hydrated) Tl+ on planar sites at the highest loadings. Our results suggest that illite is an important adsorbent for Tl in soils and sediments, considering its often high abundance and its stability relative to other potential adsorbents and the selective nature of Tl+ uptake by illite.

Fe(II) uptake on natural montmorillonites. II. Surface complexation modeling.

Fe(II) sorption edges and isotherms have been measured on low structural Fe-content montmorillonite (STx) and high structural Fe-content montmorillonite (SWy) under anoxic (O2 < 0.1 ppm) and strongly reducing conditions (Eh = -0.64 V). Under anoxic conditions Fe(II) sorption on SWy was significantly higher than on STx, whereas the sorption under reducing conditions was essentially the same. The uptake behavior of Fe(II) on STx under all redox conditions (Eh = +0.28 to -0.64 V) and SWy under reducing conditions (Eh = -0.64 V) was consistent with previous measurements made on other divalent transition metals. All of the sorption data could be modeled with the two-site protolysis nonelectrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model including an additional surface complexation reaction for Fe(II) which involved the surface oxidation of ferrous iron surface complexes (≡S(S,W)OFe(+)) to ferric iron surface complexes (≡S(S,W)OFe(2+)) on both the strong and weak sites. The electron acceptor sites on the montmorillonite are postulated to be the structural Fe(III).

Fe(II) uptake on natural montmorillonites. I. Macroscopic and spectroscopic characterization.

Iron is an important redox-active element that is ubiquitous in both engineered and natural environments. In this study, the retention mechanism of Fe(II) on clay minerals was investigated using macroscopic sorption experiments combined with Mössbauer and extended X-ray absorption fine structure (EXAFS) spectroscopy. Sorption edges and isotherms were measured under anoxic conditions on natural Fe-bearing montmorillonites (STx, SWy, and SWa) having different structural Fe contents ranging from 0.5 to 15.4 wt % and different initial Fe redox states. Batch experiments indicated that, in the case of low Fe-bearing (STx) and dithionite-reduced clays, the Fe(II) uptake follows the sorption behavior of other divalent transition metals, whereas Fe(II) sorption increased by up to 2 orders of magnitude on the unreduced, Fe(III)-rich montmorillonites (SWy and SWa). Mössbauer spectroscopy analysis revealed that nearly all the sorbed Fe(II) was oxidized to surface-bound Fe(III) and secondary Fe(III) precipitates were formed on the Fe(III)-rich montmorillonite, while sorbed Fe is predominantly present as Fe(II) on Fe-low and dithionite-reduced clays. The results provide compelling evidence that Fe(II) uptake characteristics on clay minerals are strongly correlated to the redox properties of the structural Fe(III). The improved understanding of the interfacial redox interactions between sorbed Fe(II) and clay minerals gained in this study is essential for future studies developing Fe(II) sorption models on natural montmorillonites.

Competitive Fe(II)-Zn(II) uptake on a synthetic montmorillonite.

The interaction of Fe(II) with clay minerals is of particular relevance in global geochemical processes controlling metal and nutrient cycles and the fate of contaminants. In this context, the influence of competitive sorption effects between Fe(II) and other relevant transition metals on their uptake characteristics and mobility remains an important issue. Macroscopic sorption experiments combined with surface complexation modeling and extended X-ray absorption fine structure (EXAFS) spectroscopy were applied to elucidate competitive sorption processes between divalent Fe and Zn at the clay mineral-water interface. Sorption isotherms were measured on a synthetic iron-free montmorillonite (IFM) under anoxic conditions (O2 <0.1 ppm) for the combinations of Zn(II)/Fe(II) and Fe(II)/Zn(II), where the former metal in each pair represents the trace metal (<10(-7) M) and the latter the competing metal at higher concentrations (10(-7) to 10(-3) M). Results of the batch sorption and EXAFS measurements indicated that Fe(II) is competing with trace Zn(II) for the same type of strong sites if Fe(II) is present in excess, whereas no competition between trace Fe(II) and Zn(II) was observed if Zn(II) is present at high concentrations. The noncompetitive behavior suggests the existence of sorption sites which have a higher affinity for Fe(III), where surface-induced oxidation of the sorbed Fe(II) to Fe(III) occurred, and which are not accessible for Zn(II). The understanding of this competitive uptake mechanism between Fe(II) and Zn(II) is of great importance to assess the bioavailability and mobility of transition metals in the natural environment.

Redox properties of structural Fe in clay minerals. 1. Electrochemical quantification of electron-donating and -accepting capacities of smectites.

Clay minerals often contain redox-active structural iron that participates in electron transfer reactions with environmental pollutants, bacteria, and biological nutrients. Measuring the redox properties of structural Fe in clay minerals using electrochemical approaches, however, has proven to be difficult due to a lack of reactivity between clay minerals and electrodes. Here, we overcome this limitation by using one-electron-transfer mediating compounds to facilitate electron transfer between structural Fe in clay minerals and a vitreous carbon working electrode in an electrochemical cell. Using this approach, the electron-accepting and -donating capacities (Q(EAC) and Q(EDC)) were quantified at applied potentials (E(H)) of -0.60 V and +0.61 V (vs SHE), respectively, for four natural Fe-bearing smectites (i.e., SWa-1, SWy-2, NAu-1, and NAu-2) having different total Fe contents (Fe(total) = 2.3 to 21.2 wt % Fe) and varied initial Fe(2+)/Fe(total) states. For every SWa-1 and SWy-2 sample, all the structural Fe was redox-active over the tested E(H) range, demonstrating reliable quantification of Fe content and redox state. Yet for NAu-1 and NAu-2, a significant fraction of the structural Fe was redox-inactive, which was attributed to Fe-rich smectites requiring more extreme E(H)-values to achieve complete Fe reduction and/or oxidation. The Q(EAC) and Q(EDC) values provided here can be used as benchmarks in future studies examining the extent of reduction and oxidation of Fe-bearing smectites.

Angiotensin-(1-7) reverts the stimulatory effect of angiotensin II on the proximal tubule Na(+)-ATPase activity via a A779-sensitive receptor.

Recently, we demonstrated that the stimulatory effect of Ang II on the Na(+)-ATPase activity in proximal tubules is reversed, in a dose-dependent manner, by Ang-(1-7) [Biochim. Biophys. Acta 1467 (2000) 189]. In the present paper, we characterized the receptor involved in this phenomenon. The preincubation of the Na(+)-ATPase with 10(-8) M Ang II increases the enzyme activity from 7.50+/-0.02 (control) to 12.40+/-1.50 nmol Pi mg(-1) min(-1) (p<0.05). Addition of 10(-9) M Ang-(1-7) completely reverts this effect returning the ATPase activity to the control level. This effect seems to be specific to Ang-(1-7) since Ang III (10(-12)-10(-8) M) does not modify the stimulation of the renal proximal tubule Na(+)-ATPase activity by Ang II. Saralasin abolishes the Ang-(1-7) effect in a dose-dependent manner being the maximal effect obtained at 10(-11) M. The increase in A779 concentration (from 10(-12) to 10(-7) M), a specific Ang-(1-7) antagonist, also abolishes the Ang-(1-7) effect. On the other hand, PD123319 (10(-8)-10(-6) M), an AT(2) antagonist receptor, and losartan (10(-12)-10(-7) M), an AT(1) antagonist receptor, does not modify the effect of Ang-(1-7). Taken together, these data indicate that Ang-(1-7) reverts the stimulatory effect of Ang II on the Na(+)-ATPase activity in proximal tubule through a A779-sensitive receptor.