Environ. Sci. Technol. 2007, 41, 4613-4619
Removal Mechanism of As(III) by a Novel Fe-Mn Binary Oxide Adsorbent: Oxidation and Sorption G A O - S H E N G Z H A N G , †,‡ J I U - H U I Q U , * ,† HUI-JUAN LIU,† RUI-PING LIU,† AND G U O - T I N G L I †,‡ State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China 100085, and Graduate School, Chinese Academy of Sciences, Beijing, China 100039
A novel Fe-Mn binary oxide adsorbent was developed for effective As(III) removal, which is more difficult to remove from drinking water and much more toxic to humans than As(V). The synthetic adsorbent showed a significantly higher As(III) uptake than As(V). The mechanism study is therefore necessary for interpreting such result and understanding the As(III) removal process. A control experiment was conducted to investigate the effect of Na2SO3treatment on arsenic removal, which can provide useful information on As(III) removal mechanism. The adsorbent was first treated by Na2SO3, which can lower its oxidizing capacity by reductive dissolution of the Mn oxide and then reacted with As(V) or As(III). The results showed that the As(V) uptake was enhanced while the As(III) removal was inhibited after the pretreatment, indicating the important role of manganese dioxide during the As(III) removal. FTIR along with XPS was used to analyze the surface change of the original Fe-Mn adsorbent and the pretreated adsorbent before and after reaction with As(V) or As(III). Change in characteristic surface hydroxyl groups (Fe-OH, 1130, 1048, and 973 cm-1) was observed by the FTIR. The determination of arsenic oxidation state on the solid surface after reaction with As(III) revealed that the manganese dioxide instead of the iron oxide oxidized As(III) to As(V). The iron oxide was dominant for adsorbing the formed As(V). An oxidation and sorption mechanism for As(III) removal was developed. The relatively higher As(III) uptake may be attributed to the formation of fresh adsorption sites at the solid surface during As(III) oxidation.
Introduction Arsenic, a relatively scarce but ubiquitous element, is of serious concern due to its toxicity and carcinogenicity even at low concentrations (1). In natural water, arsenic is primarily present in inorganic forms and exists in two predominant species: arsenate [As(V)] and arsenite [As(III)]. As(III) is much more toxic (2), soluble, and mobile (3) than As(V). Therefore, developing an economical, effective, and reliable water treatment technique that is capable of removing both As(V) and As(III) from contaminated drinking water and understanding arsenic behavior in the treatment processes are gaining considerable attentions in recent years. * Corresponding author phone: +86 10 62849151; fax: +86 10 6292355; e-mail:
[email protected]. † State Key Laboratory of Environmental Aquatic Chemistry. ‡ Chinese Academy of Sciences. 10.1021/es063010u CCC: $37.00 Published on Web 05/26/2007
2007 American Chemical Society
Adsorption is considered to be one of the most promising technologies because it can be simple in operation and costeffective (4). Iron hydroxides and oxides are the most used in the adsorption due to the higher affinity of iron oxide toward inorganic arsenic species. Numerous studies (5-7) have documented the adsorption of As(V) and As(III) on iron(III) (hydr)oxides. At the concentrations normally found in natural water, the As(III) adsorption is normally less effective than the As(V) adsorption by these adsorbents. To achieve higher arsenic removal, a pretreatment for As(III) oxidation is usually adopted. The adsorption mechanism has also been investigated by a number of researchers using EXAFS and IR spectroscopic techniques and they show that arsenic forms bi-nuclear bidentate as well as monodentae complexes on the iron oxide surface (8-10). Manganese oxides have been extensively investigated as oxidizing agents for arsenite (11-14), and some researchers observed the overall oxidation reaction (shown in reaction 1). Recent works by Moore et al. (15) and Nesbitt et al. (13) demonstrated that the oxidation of As(III) by the synthetic birnessite surface proceeds by a two step pathway (given in reactions 2 and 3), involving the reduction of Mn(IV) to Mn(III) and then Mn(III) to Mn(II).
MnO2 + H3AsO3 +2H+ ) Mn2+ + H3AsO4 + H2O
(1)
2MnO2 + H3AsO3 ) 2MnOOH* + H3AsO4
(2)
2MnOOH* + H3AsO3 + 4H+) 2Mn2+ + H3AsO4 + 3H2O (3) where MnOOH* represents Mn(III) intermediate reaction product. An electron-transfer mechanism and a substitution mechanism were established and could well interpret the redox reaction process (13). In our study, a novel Fe-Mn binary oxide adsorbent developed by a chemical coprecipitation method was found to have a much higher adsorption capacity toward As(III) than that of As(V). The similar phenomenon was observed by Deschamps et al. (16) who used a natural Fe and Mn enriched material to remove arsenic. They ascribed the higher As(III) uptake to the oxidation ability of manganese oxides content. However, the detailed mechanism of As(III) removal by the Fe-Mn material was yet to be elucidated. Infrared vibrational spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) are powerful techniques for the study of As(III) removal mechanisms by iron oxides and manganese oxides and were used by many researchers (9, 10, 13). Their results provide very useful information on As(III) removal mechanism by single iron oxides or manganese oxides. Nevertheless, these results cannot be easily applied to predict/explain the sorption behavior of As(III) on binary oxides. Thus, a control experiment was designed to investigate the effect of the MnO2 content in Fe-Mn binary oxide on arsenic removal, in which Na2SO3 was used as reducing agent to lower its oxidizing capacity by reductive dissolution of the Mn oxide. A detailed study on the oxidation and adsorption of As(III) by Fe-Mn binary oxide has been described using FTIR and XPS techniques in the present paper. The objectives of this study were (i) to investigate the respective roles of iron oxide and manganese oxide content on As(III) removal; (ii) to compare the FTIR and XPS spectra of the adsorbent before and after reaction with As(V) or As(III); and finally (iii) to explain the removal mechanism of As(III) by Fe-Mn binary oxide. VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4613
Materials and Methods Materials. All chemicals were analytical grade and were purchased from Beijing Chemical Co. (Beijing, China). The As(III) and As(V) stock solutions were prepared with deionized water using NaAsO2 and NaHAsO4‚7H2O, respectively. Arsenic working solutions were freshly prepared by diluting arsenic solutions with deionized water. Adsorbent Preparation. The Fe-Mn binary oxide adsorbent was prepared according to the method described in detail in our previous publication (17). Fe oxide and MnO2 were synthesized by similar methods. XRD pattern of FeMn binary adsorbent shows that both the Fe oxide and Mn oxide exist mainly in amorphous form. Batch Adsorption Tests. Experiments to determine isotherms were performed by adding 10 mg of the dry adsorbent to a 150 mL glass vessel containing 50 mL of arsenic solution. Initial arsenic concentrations were varied from 6.67 × 10-3 to 6.67 × 10-1 mM. As(III) and As(V) adsorption isotherms were determined at pH of 6.9 and 4.8, respectively. pH 6.9 was controlled by adding 0.1 M NaOH or HCl while pH 4.8 was maintained using a buffer of 5 mM sodium acetate. Ionic strength was adjusted to 0.01 M with 1 M NaNO3 solution. The supernatant was filtered through a 0.45 µm membrane after the solutions were mixed for 24 h. To determine the respective role of manganese oxide and iron oxide content on arsenic removal and elucidate the mechanism of As(III) removal, a control experiment was carried out with an adsorbent content of 200 mg L-1 and pH 4.8 (5 mM sodium acetate buffer). Our primary study indicated that Mn oxide was reduced by sodium sulfite while Fe oxide could not under the employed conditions. Sodium sulfite was therefore used to react with Fe-Mn binary oxide to lower its Mn oxide content and oxidizing capacity. The Fe-Mn binary oxide was first reduced by reacting with different amounts (from 0 to 10 mM) of sodium sulfite (Na2SO3) for 24 h, and then As(V) or As(III) (initial concentration 0.20 mmol L-1) was introduced into this system and mixed for another 24 h. The supernatant was filtered through a 0.45 µm membrane and total arsenic in the filtered solution was determined using an ICP-OES. All batch experiments were carried out at 25 ( 1 °C and all the suspensions were shaken on an orbit shaker at 140 rpm. The quantity of adsorbed arsenic was calculated by the difference of the initial and residual amounts of arsenic in solution divided by the weight of the adsorbent. The control experiments were repeated three times. Analytical Methods. Total arsenic (As(III) + As(V)) concentrations were determined using an inductively coupled plasma atomic emission spectroscopy (ICP-OES) (SCIEX Perkin-Elmer Elan mode 5000). Prior to the analysis, the aqueous samples were acidified with concentrated HCl, and stored in acid-washed glass vessels. Selective As(III) analysis was performed using hydride generation-atomic fluorescence spectroscopy (HG-AFS) with an AF-610A instrument (Beijing Ruili Analytical Instrument Co., Ltd. China) (18). A 5 mL portion of filtered supernatant was transferred into a 10 mL test tube and 2 mL of 0.5 mol L-1 citric acid solution was added. The solution was diluted to 10 mL with deionized water (solution pH 3.1) and then was analyzed. Citric acid (0.1 mol L-1) was used as carrying fluid. All samples were analyzed within 24 h of collection. Characterization. To determine the arsenic species adsorbed on the surface of the adsorbent after reaction with As(V) or As(III), some selected samples were freeze-dried for further analysis using FTIR and XPS. The change of oxidation state of surface elements was simultaneously investigated. To exclude the interference of sodium acetate in the IR and XPS spectra, 0.1 M NaOH and HCl were used to control the suspensions pH value of 4.8, instead of 5 mM sodium acetate 4614
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 13, 2007
buffer in the control experiment. In addition, the initial arsenic concentration changed from 0.20 to 0.133 mmol L-1. FTIR spectra were collected on a Nicolet 5700 spectrometer using transmission mode. Samples for FTIR determination were ground with spectral grade KBr in an agate mortar. To analyze Fe-OH quantitatively, a fixed amount of sample (1% w:w) in KBr was used to prepare the pellet. All IR measurements were carried out at room temperature. X-ray photoelectron spectroscopy (XPS) data were collected on an ESCA-Lab-220i-XL spectrometer with monochromatic Al KR radiation (1486.6 eV). C1s peaks were used as an inner standard calibration peak at 284.7 eV. XPS data process and peak fitting was performed using a nonlinear least-squares fitting program (XPSpeak software 4.1, Raymund W. M. Kwork).
Results and Discussion The determination of residual arsenic species in solution is essential to investigate the oxidation of As(III) by the adsorbent. This is also helpful for understanding the arsenic species adsorbed on the adsorbent, which can be further confirmed by FTIR and XPS analysis. Supernatant analysis showed that arsenic species remaining in solution was As(V) whether the initial arsenic was As(V) or As(III). When the adsorbent was pretreated with 10 mM Na2SO3 solution and then reacted with As(V) and As(III), however, residual arsenic in the solution was As(V) and As(III), respectively. These results showed that the Fe-Mn binary oxide could convert As(III) to As(V) and Na2SO3-treatment significantly lowered its oxidizing capacity. Arsenic Adsorption Isotherms. The adsorption capacities of the adsorbent for As(V) and As(III) were assessed using the isotherms (Supporting Information (SI) Figure 2). The adsorbent had high adsorption capacities toward both As(V) and As(III) at two selected pH values. The maximum adsorptive amount of As(V) at pH 4.8 was 0.96 mmol g-1. This amount decreased to 0.72 mmol g-1 with pH increasing to 6.9. This typical sorption behavior of As(V) is well documented in the literature. The same trend was observed with the initially added As(III) species. The maximum adsorptive amount was 1.77 mmol g-1 at pH 4.8 and reduced to 1.34 mmol g-1 at pH 6.9 and much higher than the results obtained with As(V) at two selected pH values. However, for pure iron systems, As(III) typically shows greater adsorption with increased pH up to a maximum value at about pH 7.0. So the conversion of initial As(III) to As(V) must occur during the adsorption process. Previous study (18) also shows that the adsorbent can effectively oxidize As(III) to As(V). The higher maximal As(III) adsorption capacity than that of As(V) indicated that something else must take place on the removal of As(III), besides only oxidation reaction. Production of fresh adsorption sites at the solid surface during As(III) oxidation may be responsible for this. Effect of Na2SO3-Treatment on Arsenic Removal and Mn2+ Release. No significant soluble iron was observed in solution after the adsorbent reacted with As(III). The same result was obtained when pure iron oxide reacted with As(III). The conversion of As(III) to As(V) by the adsorbent in the solution can therefore be attributed to the presence of manganese dioxide. If the oxidizing capacity of adsorbent was lowered with pretreatment by Na2SO3 which can reductively dissolve the Mn oxide, the arsenic uptake and Mn2+ release would change. The redox reaction between the adsorbent and Na2SO3 is similar to that of As(III). The overall reaction can be represented by eq 4.
MnO2* + SO32- + 2H+ ) Mn2+ + SO42- + H2O (4) where MnO2* represents the manganese dioxide in the adsorbent.
FIGURE 1. FTIR spectra of (A) Fe oxide, Fe oxide + As(V), and Fe oxide + As(III); (B) Mn oxide, Mn oxide + As(V), and Mn oxide + As(III); (C) Fe-Mn oxide, Fe-Mn oxide + As(V), and Fe-Mn oxide + As(III); (D) pretreated Fe-Mn oxide, pretreated Fe-Mn oxide + As(V), and pretreated Fe-Mn oxide + As(III). Initial arsenic concentration was 0.133 mM and a solid concentration of 200 mg L-1 was used for all samples. Solution pH value was 4.8 (maintained by intermittent addition of dilute HCl or NaOH). The effects of Na2SO3-treatment on arsenic removal and Mn2+ release are shown in SI Figure 3. Removal of As(V) increased with an increase in reduction extent of the adsorbent, whereas the removal of As(III) decreased as the Na2SO3-treatment increased. The increase in As(V) uptake would be expected given that more active adsorption sites were produced, due to the reductive dissolution of MnO2 in the adsorbent. The decrease in As(III) removal could be also explained. The Na2SO3-treatment lowered the oxidizing capacity of the adsorbent and consequently significantly inhibited the As(III) oxidization. As(III) removal is not as effective as As(V) removal for iron oxides. As a result, the As(III) removal decreased, despite that more fresh reaction sites were created during the reductive dissolution of MnO2. Without the Na2SO3-pretreatment, the concentration of Mn2+ in the solution was as low as about 1.0 mg L-1 (SI Figure 3b) after the adsorbent reacted with As(V). The release of soluble manganese might be ascribed to the reductive dissolution with the possibility of ligand-assisted dissolution of MnO2 since 5.0 mM sodium acetate was used as buffer. However, the concentration of soluble manganese in the solution was as high as 7.2 mg L-1 after the adsorbent reacted with As(III) alone, mainly due to the reductive dissolution of MnO2. Without readsorption, the released Mn2+ concentration in the solution should be 11 mg L-1. This indicated that some Mn2+ was re-adsorbed onto the solid surface. With the increase in Na2SO3-treatment, the Mn2+ concentration in the solution increased. For the As(V) system, the increase in As(V) removal corresponded to that of Mn2+ concentration in the solution, indicating the formation of new adsorptive sites with the Mn2+ release. However, for the As(III) system, the As(III) removal declined as the Mn2+ concentration increased, corresponding to the increase in Na2SO3-treat-
ment. The results of both arsenic removal and Mn2+ release reveal that MnO2 in the adsorbent plays a key role in enhancing As(III) removal. Analysis of FTIR Spectra. FTIR spectra of original adsorbent before and after reaction with arsenic are shown in Figure 1C. Figure 1-D presents the FTIR spectra for the pretreated adsorbent and the pretreated then reacted with arsenic samples. The FTIR spectra of pure Fe(III) oxide and pure Mn oxide before and after reaction with arsenic were also collected and are demonstrated in Figure 1A and B, respectively. The band at 1625 cm-1 was assigned to the deformation of water molecules and indicated the presence of physisorbed water on the oxides. The peak at 1384 cm-1 was ascribed to the vibration of NO3- because sodium nitrate was used to adjust the solution ion strength. For the Fe oxide spectra, three peaks at 1125, 1050, and 976 cm-1 correspond to the bending vibration of the hydroxyl group (Fe-OH) (19). After reaction with As(V), the peaks at 1125 and 976 cm-1 disappeared and the peak at 1050 cm-1 weakened greatly, while a new band, corresponding to As-O stretching vibration, appeared at 820 cm-1. This indicates that the As(V) is bound as a surface complex and not as a precipitated solid phase. After reaction with As(III), the peaks at 1125 and 976 cm-1 also disappeared and the peak at 1050 cm-1 slightly weakened. A new peak that appeared at 585 cm-1 instead of 820 cm-1 may be attributed to the As-O vibration in As(III) species. For the Mn oxide spectra, the weak absorbance at 1048 cm-1 may be assigned to vibration of the hydroxyl group. No obvious change was observed for this peak after the adsorption with As(V), while a new weak peak appeared at around 820 cm-1. The weak peak shows that As(V) binds weakly to Mn oxide. The spectrum of Mn oxide after reaction with VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4615
TABLE 1. Composition of Unreacted and Reacted Fe-Mn Binary Oxide Surface sample
Fe(t) at.%
Mn(t) at.%
O(t) at.%
As(t) at.%
Fe-Mn oxide Fe-Mn oxide after reaction with As(V) Fe-Mn oxide after reaction with As(III) Fe-Mn oxide after pretreatment with Na2SO3 pretreated Fe-Mn oxide after reaction with As(V) pretreated Fe-Mn oxide after reaction with As(III)
18.2 17.2 17.4 19.8 18.6 19.1
6.4 6.3 5.4 3.4 3.3 3.1
75.4 74.1 73.7 76.8 74.6 75.8
0.0 2.4 3.5 0.0 3.5 2.0
FIGURE 2. Fe2p core level photoelectron spectra of (a) Fe-Mn binary oxide, (b) Fe-Mn binary oxide after reaction with As(V) (Ci ) 0.133 mM); (c) Fe-Mn binary oxide after reaction with As(III) (Ci ) 0.133 mM); (d) Fe-Mn binary oxide after pretreatment with Na2SO3 (Ci ) 10 mM); (e) Fe-Mn binary oxide after pretreatment as (d) and then reaction with As(V) (Ci ) 0.133 mM); (f) Fe-Mn binary oxide after pretreatment as (d) and then reaction with As(III) (Ci ) 0.133 mM). As(III) was quite similar to that of Mn oxide after reaction with As(V), indicating that initial As(III) was oxidized to As(V). For original adsorbent spectra, three peaks at 1130, 1048, and 973 cm-1 are due primarily to Fe-OH vibration. The occurrence of a peak at 1538 cm-1 may be attributed to the interaction between Mn oxide and Fe oxide in the adsorbent because this peak appeared in neither the spectra of Fe oxide or the spectra of Mn oxide. After reaction with arsenate, the peak at 1538 cm-1 did not change, while those three peaks disappeared completely. Simultaneously, a new band, corresponding to As-O stretching vibration, appeared at 820 cm-1. After reaction with arsenite, the peak at 1538 cm-1 disappeared. This may be due to a surface alteration for the occurrence of redox reaction between As(III) and Fe-Mn binary oxide. The other changes were the same as that of As(V), which suggested indirectly that the arsenic species adsorbed onto the surface of the original adsorbent was As(V). Compared to the original absorbent, FTIR spectra for pretreated adsorbent did not significantly change expect that the peak at 1538 cm-1 almost completely disappeared. However, the spectra of pretreated adsorbent then reacted with arsenic were greatly different from those of the original after reaction with arsenic. The peaks at 1130, 1048, and 973 cm-1 did not disappear completely for samples reacted with both As(V) and As(III). And the intensity of hydroxyl group vibration for the sample reacted with As(III) was higher than that of sample reacted with As(V). This suggested that more hydroxyl groups were consumed after reaction with As(V) 4616
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 13, 2007
than that of As(III). Furthermore, the As-O stretching vibration peak shifted from 820 cm-1 to 811 cm-1 for the sample reacted with As(III). This would indicate that the arsenic adsorbed on the adsorbent contained As(III) species since the sorptive band corresponding to As(III)-O vibration located at 794 cm-1 (10). Furthermore, the intensity of As-O stretching vibration at 820 cm-1 for sample reacted with As(V) increased compared with that of the original one and contrary result was obtained with the sample reacted with As(III). These results also indicated that the conversion of As(III) to As(V) was responsible for the high As(III) removal. Surface Analysis of the Fe-Mn Binary Oxides. The surface compositions of the selected samples were determined by XPS and results were presented in Table 1. Mn atom content did not change after reaction with As(V), whereas it decreased a little after reaction with As(III), resulting from the reductive dissolution of MnO2. After Na2SO3-treatment, the Mn atom content decreased obviously. No significant change of the Mn atom content was found after reaction with As(V) or As(III), indicating available MnO2 for As(III) oxidation had been depleted after pretreatment. The XPS spectra of Fe2p are illustrated in Figure 2. The binding energy of 711.0 eV and the peak shape indicate that the oxidation state of Fe in the adsorbent was +III. It did not change after reaction with arsenic or the Na2SO3-treatment. The content of Fe atom on the surface did not change greatly as demonstrated in Table 1. However, a decrease of Fe2p spectra intensity was observed after reaction with As(V) or As(III), indicating the occurrence of strong interactions between As(V) and Fe atoms (19). A similar phenomenon
FIGURE 3. Mn2p core level photoelectron spectra of (a) Fe-Mn binary oxide, (b) Fe-Mn binary oxide after reaction with As(V) (Ci ) 0.133 mM); (c) Fe-Mn binary oxide after reaction with As(III) (Ci ) 0.133 mM); (d) Fe-Mn binary oxide after pretreatment with Na2SO3(Ci ) 10 mM); (e) Fe-Mn binary oxide after pretreatment as (d) and then reaction with As(V) (Ci ) 0.133 mM); (f) Fe-Mn binary oxide after pretreatment as (d) and then reaction with As(III) (Ci ) 0.133 mM).
FIGURE 4. As3d core level of the Fe-Mn adsorbent after reaction with As(V) or As(III). (a) After reaction with As(V) (Ci ) 0.133 mM); (b) after reaction with As(III) (Ci ) 0.133 mM); (c) same as (a) but with pretreatment using Na2SO3; (d) same as (b) but with pretreatment using Na2SO3. was also observed after the Na2SO3-treatment. No obvious change was observed for the Fe2p spectra intensity of pretreated adsorbent after reaction with arsenic. The Mn2p spectra of the adsorbent before and after reaction with As(V) or As(III) are shown in Figure 3a-c,
respectively. The fit data and peak shape demonstrated that no great change occurred in Mn2p spectrum after reaction with As(V), unlike that of Fe2p. This indicated that no strong interactions occurred between As(V) and Mn atoms. Nevertheless, a little decrease in intensity (by 18%) and binding VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4617
energy (from 642.6 to 642.5 eV) of Mn2p spectrum after reaction with As(III) was observed. This could be explained by increasing the proportion of reduced Mn species relative to that of Mn(IV), which resulted only from reaction with As(III). Nesbitt et al. analyzed thoroughly the Mn2p spectra of synthetic birnessite before and after reaction with As(III). They found that binding energies of Mn(IV), Mn(III), and Mn(II) species were very close to each other and the binding energy of Mn with low oxidization state was located on the low-energy side (13). After pretreatment, the intensity of Mn2p spectrum (Figure 3d) was sharply decreased (by 55%) and the binding energy shifted to the low region (from 642.6 to 642.3 eV). This indicated that part of MnO2 was reductively dissolved by Na2SO3 and the near surface was enriched in reduced forms of Mn relative to untreated Fe-Mn oxide. The Mn2p spectra of the samples pretreated then reacted with As(V) or As(III) are shown in Figure 3e and f, respectively. The relative intensity and shape of Mn2p spectra did not change significantly after reaction with As(V), similar to that of the untreated one after reaction with As(V). The binding energy shifted from 642.6 eV down to 642.1 eV, compared with that of the untreated one after reaction with As(III). This also suggested more reduced Mn species adsorbed on the surface of the adsorbent. It can be concluded that manganese dioxide mainly responses for oxidizing As(III) during As(III) removal. Figure 4 exhibits the As3d core level of the untreated adsorbent after reaction with As(V) and As(III) and the pretreated adsorbent after reaction with As(V) and As(III). There was no obvious difference between the As3d spectrum of the adsorbent after reaction with As(V) (Figure 4a) and that after reaction with As(III) (Figure 4b). This indicated that the arsenic species adsorbed after reaction with As(III) was As(V) and further confirmed that the adsorbent was effective for As(III) oxidation. After Na2SO3-treatment, the As3d spectrum of the adsorbent after reaction with As(V) (Figure 4c) was almost the same as that without pretreatment (Figure 4a) except for higher intensity, indicating more As(V) was adsorbed on the pretreated adsorbent. However, the As3d spectrum of the Fe-Mn oxide after reaction with As(III) (Figure 4d) was quite different from that of the untreated one. Fit results (SI Table 1) show that A3d line can be fitted with two components having binding energies at 44.34 and 45.55 eV, respectively. This indicates that both As(III) and As(V) species are on the surface of the pretreated then reacted with As(III) sample. Commonly, binding energy of As3d core level for As(III) and As(V) in arsenic oxides are 44.3-44.5 and 45.2-45.6 eV, respectively (13, 20, 21). They can be slowly shifted up to 44.6 ( 0.13 eV for As(III) and 46.0 ( 0.17 eV for As(V) when arsenic anions or molecules are adsorbed onto iron oxide (22). From the above discussion, it may be concluded that As(III) removal mechanism by Fe-Mn binary oxide is an oxidation coupled with sorption approach. The whole process can be briefly represented by reaction eqs 5-7.
As(III)(aq) + (-SFe-Mn) f As(III)-SFe-Mn
(5)
As(III)-SFe-Mn MnO2 + 2H + f As(V)(aq) + Mn2+ + H2O (6) As(V)(aq) + As(III)-SFe-Mn f As(V)-SFe-Mn + As(III) (aq) (7) where (-SFe-Mn) represents an adsorption site on the FeMn adsorbent surface. As(III)-SFe-Mn represents the As(III) surface species and As(V)-SFe-Mn represents the As(V) surface species. This oxidation and sorption mechanism could well 4618
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 13, 2007
explain the higher As(III) uptake and other experimental results.
Acknowledgments This work was supported by the Funds for Creative Research Groups of China (Grant 50621804) and by National Natural Science Foundation of China (Grant 20577063, 20577063).
Supporting Information Available Additional figures are provided. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Hughes, M. F. Arsenic toxicity and potential mechanisms of action. Toxicol. Lett. 2002, 133, 1-16. (2) Ferguson, J. F.; Gavis, J. A review of the arsenic cycle in nature waters. Water Res. 1972, 6, 1259-1274. (3) Duel, L. E.; Swoboda, A. R. Arsenic toxicity to cotton and soybeans. J. Environ. Qual. 1972, 1, 317-320. (4) Jang, M.; Min, S. H.; Kim, T. H.; Park, J. K. Removal of arsenite and arsenate using hydrous ferric oxide incorporated into naturally occurring porous diatomite. Environ. Sci. Technol. 2006, 40, 1636-1643. (5) Pierce, M. L.; Moore, C. B. Adsorption of arsenite and arsenate on amorphous iron hydroxide. Water Res. 1982, 16, 1247-1253. (6) Raven, K. P.; Jain, A.; Loeppert, R. H. Arsenite and arsenate adsorption on ferrihydrite: kinetics, equilibrium, and adsorption envelopes. Environ. Sci. Technol. 1998, 32, 344-349. (7) Dixit, S.; Hering, J. G. Comparison of arsenic(V) and arsenic(III) sorption onto iron oxide minerals: implications for arsenic mobility. Environ. Sci. Technol. 2003, 37, 4182-4189. (8) Waychunas, G. A.; Rea, B. A.; Fuller, C. C.; Davis, J. A. Surface chemistry of ferrihydrite. 1. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate. Geochim. Cosmochim. Acta 1993, 57, 2251-2269. (9) Sun, X. H.; Doner, H. E. An investigation of arsenite and arsenite bonding structures on goethite by FTIR. Soil. Sci. 1996, 161 (12), 865-872. (10) Goldberg, S.; Johnston, C. T. Mechanisms of Arsenic Adsorption on Amorphous Oxides Evaluated Using Macroscopic Measurements, Vibrational Spectroscopy, and Surface Complexation Modeling. J. Colloid Interface Sci. 2001, 234 (1), 204-216. (11) Oscarson, D. W.; Huang, P. M.; Defosse, C.; Herbillon, A. Oxidative power of Mn(IV) and Fe(III) oxides with respect to As(III) in terrestrial and aquatic environments. Nature 1981, 291, 50-51. (12) Scott, M. J.; Morgan, J. J. Reactions at oxide surfaces. 1. Oxidation of As(III) by synthetic birnessite. Environ. Sci. Technol. 1995, 29, 1898-1905. (13) Nesbitt, H. W.; Canning, G. W.; Bancroft, G. M. XPS study of reductive dissolution of 7Å-birnessite by H3AsO3 with constraints on reaction mechanism. Geochim. Cosmochim. Acta. 1998, 62 (12), 2097-2110. (14) Manning, B. A.; Fendorf, S. E.; Bostick, B.; Suarez, D. L. Arsenic(III) oxidation and arsenic(V) adsorption reactions on synthetic birnessite. Environ. Sci. Technol. 2002, 36, 976-981. (15) Moore, J. N.; Walker, J. R.; Hayes, T. H. Reaction scheme for the oxidation of As(III) to As(V) by birnessite. Clays Clay Miner. 1990, 38 (5), 549-555. (16) Deschamps, E.; Ciminelli, V. S. T.; Ho¨ll, W. H. Removal of As(III) and As(V) from water using a natural Fe and Mn enriched sample. Water Res. 2005, 39 (20), 5212-5220. (17) Zhang, G. S; Qu, J. H.; Liu, H. J.; Liu, R. P.; Wu, R. C. Preparation and evaluation of a novel Fe-Mn binary oxide adsorbent for effective arsenite removal. Water Res. 2007, 41 (9), 1921-1928. (18) Shi, J. B.; Tang, Z. Y.; Jin, Z. X.; Chi, Q.; He, B.; Jiang, G. B. Determination of As(III) and As(V) in soils using sequential extraction combined with flow injection hydride generation atomic fluorescence detection. Anal. Chim. Acta. 2003, 477, 139-147. (19) Zhang, Y.; Yang, M.; Dou, X.-M.; He, H.; Wang, D.-S. Arsenate Adsorption on an Fe-Ce Bimetal Oxide Adsorbent: Role of Surface Properties. Environ. Sci. Technol. 2005, 39 (18), 72467253. (20) Fullston, D.; Fornasiero, D.; Ralston, J. Oxidation of synthetic and natural samples of enargite and tennantite. 2. X-ray photoelectron spectroscopic study. Langmuir 1999, 15, 45304536.
(21) Ouvrard, S.; Donato, P.; Simonnot, M.O.; Begin, S.; Ghanbaja, J.; Alnot, M.; Duval, Y. B.; Lhote, F.; Barres, O.; Sardin, M. Natural manganese oxide: Combined analytical approach for solid characterization and arsenic retention, Geochim. Cosmochim. Acta. 2005, 69, 2715-2724. (22) Ding, M.; De Jong, B. H. W. S.; Roosendall, S. J.; Vredenberg, A. XPS studies on the electronic structure of bonding between solid and solutes. Adsorption of arsenate, chromate, phosphate,
Pb2+ and Zn2+ ions on amorphous black ferric oxyhydroxide. Geochim. Cosmochim. Acta. 2000, 64, 1209-1219.
Received for review December 19, 2006. Revised manuscript received April 13, 2007. Accepted April 19, 2007. ES063010U
VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4619
