Respective Role of Fe and Mn Oxide Contents for Arsenic Sorption in Iron and Manganese Binary Oxide: An X-ray Absorption Spectroscopy Investigation

Abstract

ABSTRACT:In our previous studies, a synthesized Fe−Mn binary oxide was found to be very effective for both As(V) and As(III) removal in aqueous phase, because As(III) could be easily oxidized to As(V). As(III) oxidation and As(V) sorption by the Fe−Mn binary oxide may also play an important role in the natural cycling of As, because of its common occurrence in the environment. In the present study, the respective role of Fe and Mn contents present in the Fe−Mn binary oxide on As(III) removal was investigated via a direct in situ determination of arsenic speciation using X-ray absorption spectroscopy. X-ray absorption near edge structure results indicate that Mn atoms exist in a mixed valence state of +3 and +4 and further confirm that MnO x (1.5 < x < 2) content is mainly responsible for oxidizing As(III) to As(V) through a two-step pathway [reduction of Mn(IV) to Mn(III) and subsequent Mn(III) to Mn(II)] and FeOOH content is dominant for adsorbing the formed As(V). No significant As(III) oxidation by pure FeOOH had been observed during its sorption, when the system was exposed to air. The extended X-ray absorption fine structure results reveal that the As surface complex on both the As(V)-and As(III)-treated sample surfaces is an inner-sphere bidentate binuclear corner-sharing complex with an As−M (M = Fe or Mn) interatomic distance of 3.22−3.24 Å. In addition, the MnO x and FeOOH contents exist only as a mixture, and no solid solution is formed. Because of its high effectiveness, low cost, and environmental friendliness, the Fe−Mn binary oxide would play a beneficial role as both an efficient oxidant of As(III) and a sorbent for As(V) in drinking water treatment and environmental remediation. ■ INTRODUCTIONArsenic, a ubiquitous trace element in the aquatic environment, is introduced from both natural processes and anthropogenic sources. 1 In natural water, arsenic is mostly found in two inorganic forms, namely, arsenate and arsenite. In welloxygenated water, As(V) is the major arsenic species, while As(III) is dominant in reduced groundwater. As(III) is more toxic, soluble, and mobile than As(V). 2,3 A high level of arsenic in groundwater and surface waters is a major concern worldwide, because of its high toxicity and carcinogenicity. To minimize the possible health risks, the World Health Organization (WHO) has set a stringent limit of 10 μg L −1 As for drinking water. 4 The adsorption reactions between arsenic and minerals (particularly metal oxides) play a very important role in its mobility and potential bioavailability because arsenic can be strongly adsorbed onto the surface of metal oxides, such as iron (hydr)oxides, alumina, and manganese oxides, which commonly occur in the aquatic environment. 5,6 Adsorption is also regarded as a promising method for the removal of arsenic from water, owing to its simple operation, high efficiency, and cost effectiveness. Numerous investigations have focused on arsenic immobilization by natural and synthetic Fe (hydr)oxides and Fe-containing materials. 7−10 Recently,...

In situ arsenic oxidation and sorption by a Fe-Mn binary oxide waste in soil

“…Based on the literature to date, the nearest neighbour peaks suggest that As is sorbed in a bidentate corner-sharing configuration for both the binary oxide and MnO2 ( Figure S6). This configuration is in agreement with previous studies [16]. Specifically, on the binary oxide waste we conclude that As is sorbed by mononuclear bidentate corner-sharing, with 2 Fe at c. 3.27 Ǻ (Table S1, Figure S7).…”

“…Specifically, on the binary oxide waste we conclude that As is sorbed by mononuclear bidentate corner-sharing, with 2 Fe at c. 3.27 Ǻ (Table S1, Figure S7). This concurs with previous spectroscopic studies that have shown that the structure of the As(V) surface complex on Fe-Mn binary oxides is a bidentate bridging complex on singly coordinated surface sites similar to that on Fe(III) hydroxides [16].…”

“…Sorption capacities of both As (III) and As (V) were significantly higher in the waste than in the synthetic -MnO2 ( Figure S2) which is in agreement with previous reports [16]. Specifically, the binary oxide waste sorption capacities were 95 % for the As oxyanions while synthetic -MnO2 showed sorption capacities of 7 % for both As(III) and (V).…”

“…In the environment Mn and Fe occur together and are chemically similar. Recent laboratory experiments have showed that the combination of these metal oxides in the form of synthetic Fe-Mn binary oxides show significantly higher As immobilization than the pure phases alone [16]. A binary oxide produces a synergistic two-step oxidation and adsorption reaction whereby As(III) is oxidized to As(V) by the Mn oxide portion, and As(V) is consequently strongly sorbed onto the Fe oxide via the following reaction series [17]:…”

Adsorption of antimony(V) onto Mn(II)-enriched surfaces of manganese-oxide and Fe Mn binary oxide

“…In previous studies we successfully developed a FeAMn binary oxide (FMBO) with more significant removal capability to As(III) and Sb(III) than iron oxide (Fe-oxide) and manganese oxide (Mn-oxide), and an oxidation and adsorption mechanism was proposed (Xu et al, 2011;Zhang et al, 2007). A recent X-ray absorption near edge structure (XANES) study indicated that Mn-oxide (1.5 < x < 2) within FMBO was responsible for As(III) oxidation, whereas Fe-oxide dominated in the adsorption of the formed As(V) (Zhang et al, 2014). At pH 3.0, FMBO achieved a maximum Sb(III) removal capacity as high as 1.76 mmol g À1 , whereas that of Fe-oxide and Mn-oxide was 0.83 and 0.81 mmol g À1 , respectively (Xu et al, 2011).…”