Redox Transformations of As and Se at the Surfaces of Natural and Synthetic Ferric Nontronites: Role of Structural and Adsorbed Fe(II)

Abstract

Adsorption and redox transformations on clay mineral surfaces are prevalent in surface environments. We examined the redox reactivity of iron Fe(II)/Fe(III) associated with natural and synthetic ferric nontronites. Specifically, we assessed how Fe(II) residing in the octahedral sheets, or Fe(II) adsorbed at the edge sites alters redox activity of nontronites. To probe the redox activity we used arsenic (As) and selenium (Se). Activation of both synthetic and natural ferric nontronites was observed following the introduction of Fe(II) into predominantly-Fe(III) octahedral sheets or through the adsorption of Fe(II) onto the mineral surface. The oxidation of As(III) to As(V) was observed via catalytic (oxic conditions) and, to a lesser degree, via direct (anoxic conditions) pathways. We provide experimental evidence for electron transfer from As(III) to Fe(III) at the natural and synthetic nontronite surfaces, and illustrate that only a fraction of structural Fe(III) is accessible for redox transformations. We show that As adsorbed onto natural and synthetic nontronites forms identical adsorption complexes, namely inner-sphere binuclear bidentate. We show that the formation of an inner-sphere adsorption complex may be a necessary step for the redox transformation via catalytic or direct oxidation pathways.

Direct Visualization of Arsenic Binding on Green Rust Sulfate

“…Although STEM and PDF analyses gave us highly quantitative information about the structure and adsorbed species of the As­(III)- and As­(V)-reacted GR, we combined these results with As K-edge XAS data (i.e., XANES and EXAFS) to determine if any changes in the As oxidation state occurred during the 5 day reaction with GR (Figure a) and confirm the As adsorption geometry using shell-by-shell fits (Figure c). Our XANES observations (Figure a) support previous studies that have shown that GR cannot oxidize As­(III) or reduce As­(V). − , It is noteworthy, however, that As­(III) oxidation to As­(V) under anoxic conditions has been reported for other iron-bearing minerals such as siderite, Fe­(II)-bearing nontronite, , lepidocrocite, and goethite . Furthermore, As­(V) reduction has been observed during Fe 2+ -catalyzed transformations of Fe-bearing minerals, wherein GR has been found as an intermediate phase. , …”

“…Our XANES observations (Figure 4a) support previous studies that have shown that GR cannot oxidize As(III) or reduce As(V). [27][28][29]31 It is noteworthy, however, that As(III) oxidation to As(V) under anoxic conditions has been reported for other iron-bearing minerals such as siderite, 56 Fe(II)-bearing nontronite, 58,59 lepidocrocite, 60 and goethite. 61 Furthermore, As(V) reduction has been observed during Fe 2+ -catalyzed transformations of Fe-bearing minerals, wherein GR has been found as an intermediate phase.…”

The Role of Defects in Fe(II)–Goethite Electron Transfer

“…Annealing of goethite by oxidative sorption of Fe­(II) followed by inhibition of electron transfer may also explain the recent report of decreased goethite recrystallization rates over time . Our hypothesis that oxidative sorption of Fe­(II) anneals surface defects is in agreement with results that show addition of Fe­(II) inhibits rates of microbial Fe­(III) reduction. , How defects will impact heterogeneous redox process such as contaminant reduction rates, ,,,− and the observed paradoxical oxidation of As­(III) at the Fe­(II)–Fe oxide interface, however, remains open to experimental investigation. ,, …”

“…73,74 How defects will impact heterogeneous redox process such as contaminant reduction rates, 3,33,34,[75][76][77][78] and the observed paradoxical oxidation of As(III) at the Fe(II)-Fe oxide interface, however, remains open to experimental investigation. 8,79,80 Our work also shows that electron transfer between Fe(II) and goethite is sensitive to diagenetic temperature and can be altered by relatively small changes in the structure. We note that we were only able to observe these changes with surface-sensitive techniques (i.e., XMCD and oxygen XAS).…”

Enhanced Immobilization of Arsenic from Acid Mine Drainage by Detrital Clay Minerals

“…The use of phyllosilicate clay minerals (hereafter “clays”) as substrates for As removal from aqueous solutions has also been reported. − Clays can immobilize As via the formation of surface-bound As complexes and exhibit higher affinity for As­(V) than As­(III) under most environmental conditions common in soils and sediments. ,− On the surfaces of kaolinite and nontronite, for example, the adsorbed As­(V) has been shown to coordinate with Al octahedral sites via inner-sphere bidentate binuclear binding, , with the maximum As­(V) adsorption occurring at low pH followed by a sharp decrease between pH 3 to 7. , Other natural clays (i.e., kaolin KGa-1 and illite IMt-2) have been shown to oxidize adsorbed As­(III) to As­(V), through multiple pathways depending on the clay type and the presence of particular inorganic complexes, ,, thus favoring As immobilization through As­(V) adsorption as inner-sphere complexes on the clay surfaces. Yet, under low-pH conditions, clays are prone to extensive dissolution and thus are not commonly used for dearsenication of low-pH aqueous systems.…”