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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Environmental Pollution 179 (2013) 160e166 Contents lists available at SciVerse ScienceDirect Environmental Pollution journal homepage: Arsenic K-edge X-ray absorption near-edge spectroscopy to determine oxidation states of arsenic of a coastal aquifereaquitard system Ya Wang, Jiu Jimmy Jiao *, Sanyuan Zhu, Yiliang Li Department of Earth Sciences, The University of Hong Kong, Hong Kong, China article info abstract Article history: Received 31 January 2013 Received in revised form 25 March 2013 Accepted 3 April 2013 Keywords: Oxidation states Arsenic XANES analysis Aquifereaquitard system The Pearl River Delta Determination of oxidation states of solid-phase arsenic in bulk sediments is a valuable step in the evaluation of its bioavailability and environmental fate in deposits, but is difficult when the sediments have low arsenic contents and heterogeneous distribution of arsenic species. As K-edge X-ray absorption near-edge spectroscopy (XANES) was used to determine quantitatively the oxidation states of arsenic in sediments collected from different depths of boreholes in the Pearl River Delta, China, where the highest aquatic arsenic concentration is mg/l, but the highest solid arsenic content only 39.6 mg/kg. The results demonstrated that XANES is efficient in determining arsenic oxidation states of the sediments with low arsenic contents and multiple arsenic species. The study on the high-resolution vertical variations of arsenic oxidation states also indicated that these states are influenced strongly by groundwater activities. With the help of geochemical data, solid arsenic speciation, toxicity and availability were further discussed. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction In sediments of Quaternary age, enriched arsenic (>10 mg/l) in groundwater due to various geochemical processes has been identified to be serious environmental problems in many parts of the world (Nickson et al., 1998; Ahmed et al., 2004; McArthur et al., 2004; Agusa et al., 2006; Berg et al., 2007; Guo et al., 2008; Chakraborti et al., 2010; Thi et al., 2010; Chetia et al., 2011). Determination of oxidation states of solid-phase arsenic in bulk sediments is a valuable first step in the evaluation of its bioavailability and environmental fate in Quaternary deposits, including speciation, mobility and solubility. However, there have been very limited studies to quantitatively determine the contents of solid arsenic phases with different oxidation states in bulk sediments. This may be because even in areas with enriched aquatic arsenic in groundwater, such as aquifer systems in Bangladesh, the Pearl River Delta (PRD), Red River floodplain and a coastal plain in USA, solid arsenic in bulk sediments is generally < 50 mg/kg (Ahmed et al., 2004; Postma et al., 2007; Barringer et al., 2011; Wang et al., 2012). Moreover, arsenic phases in bulk sediments could be heterogeneous due to complex hydrogeological and geochemical conditions, and it is not easy to separate solid arsenic with different oxidation states. Species of solid arsenic are traditionally identified using timeconsuming indirect methods that determine the identity of the * Corresponding author. addresses: wangya@graduate.hku.hk (Y. Wang), jjiao@hku.hk (J.J. Jiao). compound through chemical methods, however, pre-treatment of the samples may alter the chemical forms of arsenic (Cui and Liu, 1988; Matanitobua et al., 2007). X-ray absorption fine structure spectroscopy (XAFS) is capable of providing detailed chemical and structural information about a specific absorbing element in situ with minor or no pre-treatments (Foster et al., 1997). X-ray absorption near-edge spectroscopy (XANES), one of the primary techniques of XAFS, is sensitive to oxidation state and local environment, which makes it a valuable technique in identifying oxidation states of various elements in earth materials (Wilke et al., 2001; Peak and Sparks, 2002; Roberts et al., 2002; Manceau et al., 2008). This technique has been widely used to study various As-bearing systems (Foster et al., 1998a, 1998b, 2003; Marcus et al., 2004; Lowers et al., 2007; Majzlan et al., 2007; Manceau et al., 2007; Sahai et al., 2007; Voegelin et al., 2007; Endo et al., 2008; Paktunc et al., 2008). Arsenic speciation identification studies have been carried out in many As-enriched minerals and mine tailings with arsenic contents ranging from several hundred to several thousand mg/kg (Foster et al., 1998b; Marcus et al., 2004; Lowers et al., 2007; Endo et al., 2008), which are much greater than those in the Quaternary sediments where elevated arsenic concentrations have been identified in groundwater (Ahmed et al., 2004; Postma et al., 2007; Barringer et al., 2011; Wang et al., 2012). This technique has also been successfully performed on Xylem sap at concentrations as low as 30 ng/ml (Meirer et al., 2007). However, to the best knowledge of the authors there are few studies about oxidation states of arsenic in bulk sediments because of low arsenic contents. In fact, the relatively low contents /$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.

3 Y. Wang et al. / Environmental Pollution 179 (2013) 160e of heavy metals in bulk sediments and nearly no overlap between arsenic fluorescence and scattered X-rays results lead a very good background during the whole XANES measurements, making it possible to collect well-resolved data and quantitatively identify the oxidation state of arsenic even for bulk sediments with arsenic content as low as several mg/kg. This study applied the XANES technique for quantitative determination of oxidation states of arsenic in bulk sediments of Quaternary age, in which concentrations of solid arsenic range from 11.1 to 39.6 mg/kg. The PRD was chosen as the study area, as enriched aquatic arsenic (up to mg/l) was found in the Quaternary coastal basal aquifer (Wang et al., 2012). Above the sand basal aquifer which is 36.4 m and 34.4 m below ground surface at the sites of SD1 and MZ4, respectively, there are thick layers of marine clay and silt. X-ray powder diffraction observation on sediment samples from both boreholes suggested that sediments are similar and dominantly composed of quartz, illite, kaolinite, feldspar and mica (Wang et al., 2012). Studies on heavy minerals in the PRD sediments indicate that they consist chiefly of oxides, silicates and hydroxides, and make up about 2.8e16 wt% of the analyzed samples (Huang et al., 1982; Lan, 1991; Long, 1997). It is important to understand the environmental fate of arsenic in such a Quaternary coastal aquifer, because the aquifer is key source of water supply and there are vital links between terrestrial and marine biogeochemical cycles (Moore, 1999; Charette and Sholkovitz, 2002; Slomp and Van Cappellen, 2004). 2. Experimental section 2.1. Site description The PRD is located in the coastal areas of south China (Fig. 1A). Sediments of the plain area were formed as a result of the interactions between rivers and the South China Sea during the late Quaternary. The stratigraphic units of Quaternary sediments can generally be divided into four sequences: terrestrial units of T1 and T2 as well as marine units of M1 and M2 (Zong et al., 2009) (Fig. 1B). The sand and gravel alluvial sediments (T2), widely distributed in paleo-valleys, were deposited prior to the last transgression in the late Pleistocene. The old silt and clay marine unit (M2), overlaying on T2, was formed during the interglacial period started from about 130 ka before present (Zong et al., 2009; Yim, 1994; Yim et al., 2002). During the last glacial period, the upper part of M2 was largely weathered due to the low sea level between 100 and 120 m below the present sea level (Wang and Sun, 1994), and formed a layer of weathered clay in most of the plain areas. Meanwhile, a layer of sand alluvial sediments was laid down along paleo-river channels. Both the weathered clay and alluvial sediments constitute T1. Rapid rise of sea level after 8.2 calibrated ka before present (cal. ka BP) resulted in a large-scale transgression in Holocene, and formed a layer of younger marine sediments (M1) of 5e20 m thick (Zong et al., 2009). The calibrated radiocarbon ages of the sediments collected from various studies of the PRD (Huang et al., 1982; Li et al., 1991; Zong et al., 2009) are shown in Fig. 1B. The silty and clay layers of M1 and M2 compose the aquitards, and the sand and gravel terrestrial unit of alluvial sediments T2 is the confined basal aquifer of this coastal groundwater system Sample preparation and XAFS analysis Core samples from two boreholes SD1 and MZ4 were selected for detailed arsenic study. Borehole MZ4 is located very close to the coastline, and about 48 km away from SD1 (Fig. 1A). Bulk sediment samples were taken from the centre part of the cores to avoid contamination. Wet sediments of 1e2 g were collected from the centre part of each bulk sediment sample, frozen dried, and ground into powder by using an agate pestle and mortar. As K-edge XANES was recorded in 14W1 beamline at Shanghai synchrotron radiation facilities, which was operated at 3.5 GeV with maximum beam current of 300 ma. The estimated photon flux on the sample at 10 kev was phs/s/100 ma. The X-rays were monochromatized with a pair of Si(311) crystals, and the instrumental energy resolution was about 0.6 ev at As K-edge position. The beam size at the sample position was about 1 mm 1 mm, and its intensity (I0) monitored with an ionization chamber of 30 cm. The natural samples were pressed to small thin pills for obtaining smooth plain, and then placed at 45 from the incident beam, and the fluorescent X-rays measured with a 7 element Si(Li) solid-state detector. The total count rate was about , and the total counts (10 s) of As K-alpha was about 200e2000, depending on contents of As. The dead time was controlled to be less than 20% by adding aluminium attenuation sheet. Dead time correction was done by the method of Nomura (1998). Energy was calibrated by assigning the energy of the first peak of As(III) reference compounds at ev. XANES spectra was recorded with 10 s counting time per step, 5 ev step from 150 ev to 20 ev, 0.3 ev from 20 ev to 100 ev and 1.0 ev from 100 ev to 150 ev. The model compounds chosen for reference were laid on 3M tapes and collected with transmission mode. The jump was optimized to about 1 by adjusting the thickness of samples. All spectra were normalized to unit step using Athena software of IFEFFIT package (Ravel and Newville, 2005). 3. Results 3.1. Oxidation states of bulk sediment sample The basic feature of As K-edge XANES spectra is the position of absorption edge that occurs over approximately a 10 ev interval. The absorption edge increases in height, and shifts to higher energy with increasing As oxidation state: the sharp increase in absorption beginning at about , , and ev for As(-1), As(III), and for As(V) compounds, respectively. Fig. 2 demonstrated unsmoothed As K-edge XANES spectra collected from the chosen model compounds for this study, including arsenopyrite, As 2 O 3 and H 3 AsO 4. The model compounds, including both reduced arsenic in arsenopyrite (As(-1)) (Jones and Nesbitt, 2002) and oxidized forms of arsenic in As 2 O 3 and H 3 AsO 4 (As(III) and As(V)), served as reference spectra for known oxidation states and chemical species of arsenic. Each prepared sediment sample was exposed to the air and the beamline for about one hour for XANES measurement. For a reliable quantitative determination of arsenic species of the bulk sediments, it is important that the oxidation states of arsenic in the samples remain unchanged while scanning. Fig. 3 demonstrated that sample 1-2 was repetitively scanned three times and no alterations of the arsenic K-edge XANES spectra were observed. Therefore, even after about 3e4 h exposure to the air and the beamline while XANES scanning, oxidation and radiation damage of arsenic species were negligible. The position of absorption edge lies on the oxidation states of arsenic, and two peaks for the bulk sediment sample 1-4 from SD site in Fig. 4 indicated the co-existence of both reduced and oxidized forms of arsenic. To quantitatively estimate the amount of each arsenic species, we applied least squares fitting method, of which the accuracy was approximately 10% (Voegelin et al., 2007). The least squares fitting of bulk XANES spectra of sample 1-4 was shown as the top dotted line in Fig. 4. As K-edge XANES spectra of model compounds were the primary reference spectra for the fitting, and carefully selected standard compounds, covering a range of oxidation states of arsenic of the samples, were all important for a

4 162 Y. Wang et al. / Environmental Pollution 179 (2013) 160e166 Fig. 1. A. Map of the study area. The piezometer sites MZ4 and SD1 are marked with red triangles. The boreholes along the cross-section are marked in green dots. B. Simplified cross-section of the PRD Quaternary sediments. Calibrated carbon-14 ages were shown on the cross-section for reference. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) reliable quantitative analysis. Dashed lines at the bottom of Fig. 4 denoted fractional contribution of each principal component making up the fitted spectra, including 46.5% As(V), 23.8% As(III), and 29.7% reduced arsenic as arsenopyrite for sample Oxidation states of solid arsenic in the PRD coastal Quaternary aquifereaquitard system In some parts of the PRD, groundwater in the confined basal aquifer was found to be enriched in arsenic concentrations (>10 mg/ L). The aquatic arsenic concentrations in the basal aquifer at sites SD1 and MZ4 were identified to be and 38.2 mg/l, respectively (Wang et al., 2012). The sedimentary cores of the two sites were representative for those locations of the PRD with enriched aquatic arsenic. Seven sediment samples taken from cores of SD1 at the depth ranging from 3 to 36 m, and nine samples from cores of MZ4 at the depth ranging from 4 to 36 m were selected for As K-edge XANES spectrum analysis (Fig. 5 and Table 1). These samples included both aquitard and aquifer materials. The lithofacies of these samples varied from silt and clay to sands from top to the bottom of the sediment profiles. The position and shape of the absorption edge of the XANES spectra of all the selected samples

5 Y. Wang et al. / Environmental Pollution 179 (2013) 160e Arsenopyrite Fitting Normalized Intensity As 2 O 3 H 3 AsO 4 Normalized Intensity As 2 O 3 H 3 AsO 4 Sample spectra Fig. 2. Normalized As K-edge XANES spectra of selected model compounds: arsenopyrite (blue line), As 2 O 3 (red line) and H 3 AsO 4 (purple line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) were provided in Fig. 5. The spectra of arsenopyrite (blue solid line), As 2 O 3 (red solid line) and H 3 AsO 4 (purple solid line) at the bottom of Fig. 5, demonstrating that all the oxidation states of arsenic of bulk sediment samples were covered by the selected model compounds provided for reference. One high energy absorption edge peak was observable for each of the shallowest samples, 1-1 and 4-1, from sites SD1 and MZ4, respectively (Fig. 5), these absorption edges were attributable to As(V). Conversely, for the deeper samples, a higher energy peak for As(V) and lower energy peak for As(III) and As(-1) were observable. Although SD1 and MZ4 are located 48 km apart from each other, the absorption edge in the XANES spectra of the two sites had very similar patterns. The nominal arsenic oxidation values of SD1 and MZ4 bulk sediments ranged from 1.8 to 4.3 with the standard deviation of Normalized Intensity Time 1 Time 2 Time 3 Fig. 3. Normalized As K-edge XANES spectra of sediment sample 1-2 from the depth of 9.5 m at the site SD1. Repetitive determinations on the same sample show the same spectra. Arsenopyrite , indicating that both reduced and oxidized states of arsenic were present in the sediment samples. Linear combination leastsquares fits of model compounds spectra representing arsenic of various species to XANES spectra for bulk sediments were used to ascertain the identity and quantitatively analyze the abundance of different arsenic species of all the selected bulk sediment samples. The results were provided in Table 1. Studies of Rowland et al. (2005) indicated that arsenic in sediments was easily oxidized to arsenate; however, a significant amounts of As(III) and reduced arsenic in both boreholes (Fig. 5 and Table 1) suggested that oxidation of arsenic during storage was minimal. Most of the c 2c values in the 10 th column of Table 1, ranging from 0.16 to 0.41, indicated good least-squares fitting results for the sediment samples. For the shallow depth samples 1-1 and 4-1, it can be seen that the fitting curve was not in good agreement with the experimental data (Fig. 5). The relatively high c 2c values (1.21 and 0.56 for 1-1 and 4-1, respectively) confirmed these deviations. It can be seen that both the XANES spectra of 4-1 and 4-2 presented a strong peak, which was located at ev and typically attributed to As(V); conversely, the features of the low energy peaks were different (Fig. 6). For sample 4-1, this peak shifted to high energy and became a shoulder peak at ev, while for sample 4-2, the peak was at ev and arose from not well resolved arsenic as arsenopyrite and As(III) (Fig. 6). Therefore, the relatively higher deviation of fitting for sample 4-1 than for sample 4-2 was caused by their XANES spectra difference at low energy. It was found that arsenic XANES spectra at low energy for sample 4-1 could not fit well with the spectra of the chosen standards. Because the edge position of 4-1 at lower energy was very close to that of As(V) (Fig. 6), it can be deduced that the XANES spectra at lower energy for 4-1 could also be generated by the oxidized form of arsenic (As(V)). The slight difference for edge positions of As(V) in the shallowest samples could suggest that oxidized forms of arsenic were incorporated in significantly different species of the compounds. 4. Discussion Fig. 4. Normalized experimental and least squares fits for the As K-edge XANES spectra of bulk sediment sample 1-4 from 18.5 m depth at site SD1. The three dashed lines at the bottom denote fractional contribution of each principal component making up the fitted spectra, including 46.5% As(V), 23.8% As(III), and 29.7% As(-1). The co-existence of different oxidation states of arsenic in the bulk sediments can be attributed to the complex geochemical and hydrogeological environment within the Quaternary sediments.

6 164 Y. Wang et al. / Environmental Pollution 179 (2013) 160e166 A 1-1 Data Fitting B 3 m Data Fitting Normalized intensity 4.3 m 9.5 m 14.5 m 18.5 m 25.5 m 31.5 m 36.5 m Arsenopyrite 7.3 m 11.4 m 17.4 m 22.4 m 27.4 m 29.1 m 35 m 36 m Arsenopyrite As2O3 As2O3 H3AsO4 H3AsO Fig. 5. As K-edge XANES spectra of selected sediment samples (green solid lines) and linear combination least-squares fitting lines (red dotted lines) at different depth of sites (A) SD1, and (B) MZ4. The spectra of arsenopyrite (blue solid line), As 2 O 3 (red solid line) and H 3 AsO 4 (purple solid line) were provided for reference. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Although the sites SD1 and MZ4 are about 48 km apart from each other (Fig. 1A), the distribution of oxidation states of solid arsenic in the sediments was very similar, as shown in Fig. 5. This suggested that the two sites had similar geochemical environment responsible for the arsenic oxidation states. This observation was consistent with previous studies which showed that these two places had similar sedimentary sequences, mineral assemblage, geochemical composition and groundwater activities (Huang et al., 1982; Lan, 1991; Long, 1997; Zong et al., 2009; Wang et al., 2012). Most As was associated with amorphous ferric oxyhydroxide and clay minerals (Wang et al., 2012), and amounts of As(V) took significant parts of total solid arsenic (Table 1), ranging from 3.7 to 26.6 mg/kg in sediment samples. Examination of As(V) sorbed on Fe- and Al-oxyhydroxides suggested that As 5þ O 4 tetrahedra primarily formed strong, bidentate complexes on amorphous ferric oxyhydroxide and clay minerals (Foster et al., 1998b). Since ph values indicated that groundwater in SD1 and MZ4 was about neutral to slightly basic (Wang and Jiao, 2012), the availability of As coprecipitated with ferrihydrite-like material should remain very low. Arsenic of a reduced form (nominally As(-1)) was identified in most aquitard sediments (Table 1), with the highest concentration of 8.7 mg/kg. Most of this As phase was likely to be incorporated in arsenical pyrite, which was derived from sulphate reduction and was found to be abundant (Huang et al., 1982; Wang et al., 2012). In such a reducing aquifereaquitard system, this phase generally retained As in a state of low mobility, toxicity and bioavailability. Dissolved arsenite, which is more toxic than most other forms of arsenic, was identified to be dominantly present in the Table 1 Abundance of arsenic with different oxidation states indicated by analyses of XANES spectra of bulk samples. Sample Depth Lithofacies As (mg/kg) Edge position (ev) Fit range (ev) Percent of model compound (normalized) c 2c Arsenopyrite As 2 O 3 H 3 AsO 4 SD Silt SD Silty clay SD Silty clay SD Silty clay SD Silty clay SD Silty clay SD Fine sand MZ4-1 3 Silt MZ Silt MZ Silt MZ Silt MZ Silt MZ Silt MZ Silt MZ Coarse sand MZ Coarse sand Note: the edge position was defined as the maximum peak in the derivative spectrum. Contents of solid arsenic at different depth of the profiles were obtained from the studies of Wang et al. (2012).

7 Y. Wang et al. / Environmental Pollution 179 (2013) 160e Fig. 6. Comparison of the As K-edge XANES between the sample 4-1 and 4-2. Normalized XANES spectra of sample 4-1 and 4-2 were provided on the top. Derivative spectra were shown at the bottom for distinguishing the oxidation states of arsenic. groundwater in contact with these authigenic pyrite-rich sediments at SD1 and MZ4 (EhepH figure was not shown). This result was consistent with the previous finding that under reducing conditions at ph less than 9.2, the uncharged arsenite species H 3 AsO 3 0 predominated (Brookins, 1988; Yan et al., 2000). Under this circumstance, it was possible that the arsenian pyrite was coated with arsenopyrite-like surface precipitates, but the quality of arsenopyrite-like surface precipitate was of minor abundance. In this study, groundwater activities also seemed to play an important role in affecting oxidation states of solid arsenic in such an aquifereaquitard groundwater system. Oxidation states of arsenic of sediments at the depth of 3 m and 4.3 m of sites SD1 and MZ4 were mainly As(V) (Table 1 and Fig. 5). These two samples were in the shallow unconfined aquifer zone with active groundwater flow. Groundwater activities here were mainly influenced by local infiltrated precipitation and surface water, which brought oxidants, such as dissolved oxygen or Fe 3þ, from the oxidation zones to cause the oxidation of solid arsenic. However, at the deeper part of the aquitard, results of XANES analysis on bulk sediments demonstrated two peaks of XANES spectra (Fig. 5) which indicated the presence of reduced form of arsenic (Table 1). At those locations, groundwater activities were restricted by the low permeability of Quaternary marine sediments, and oxidants could hardly reach those depths due to extremely slow groundwater flow within the aquitard. Active groundwater movement and limited groundwater movement at the shallow and deeper part of the aquitard were confirmed by the long-term groundwater level variation data obtained by micro-divers installed in piezometers in the study area (Wang, 2011). Arsenic K-edge XANES analysis on the sand samples from the confined basal aquifer also showed two peaks of XANES spectra (Fig. 5). This was also consistent with the limited groundwater movement in the confined basal aquifer, as groundwater flow in the confined basal aquifer was identified to be rather stagnant (Wang and Jiao, 2012). 5. Conclusions This study demonstrated that XANES was a very efficient tool to quantitatively determine the oxidation state of arsenic in bulk sediments. This method required minor pre-treatment on sediment samples and would not affect arsenic speciation during analyzing. Reduced form of arsenic (nominally As(-1)), arsenite and arsenate were identified to be the major forms of arsenic in bulk sediments, and they were clearly quantitatively distinguished. The availability of As coprecipitated with ferrihydrite-like material remained very low as ph values were about neutral to slightly basic. Most of As of a reduced form (nominally As(-1)) was likely to be incorporated in arsenical pyrite and this phase generally retained As in a state of low mobility, toxicity and bioavailability. It was possible that the arsenian pyrite was coated with arsenopyritelike surface precipitates, but the quality of arsenopyrite-like surface precipitate was of minor abundance. Vertical variations of arsenic oxidation state were consistent with the groundwater activities. Oxidized As(V) in the shallowest part of the aquitard was attributed to active groundwater movement that can bring oxidants. 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