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Effects of long-term paddy rice cultivation on soil arsenic speciation
Journal of Environmental Management 254 (2020) 109768
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Research article
Effects of long-term paddy rice cultivation on soil arsenic speciation Puu-Tai Yang a, Yohey Hashimoto b, Wen-Jing Wu c, Jang-Hung Huang c, Po-Neng Chiang d, Shan-Li Wang a, * a
Department of Agricultural Chemistry, National Taiwan University, Taipei, 10617, Taiwan Department of Bioapplications and Systems Engineering, Tokyo University of Agriculture and Technology, Tokyo 184-8588, Japan c Department of Soil and Environmental Sciences, National Chung Hsing University, Taichung, 40227, Taiwan d Experimental Forest, National Taiwan University, Nantou, 55750, Taiwan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Paddy soil Arsenic speciation Wet-dry cycles Redox transformation
Geochemical behavior of arsenic (As) in rice paddy soils determines the availability and mobility of As in the soils, but little is known about the long-term effects of paddy rice cultivation on As speciation in the soils. In this study, surface soil samples were collected from a rice paddy land and its adjacent dry land with similar soil properties and known cultivation histories. The soils of the paddy land and dry land contained 378 and 423 mg As kg 1, respectively. The predominant As species in the soils were investigated using As K-edge X-ray absorption spectroscopy (XAS) in combination with two sequential chemical fractionation methods. The XAS results showed that the predominant As species in the soils were As(III)- and As(V)-ferrihydrite, As(V)-goethite and scorodite. In comparison to the dry land soil, the paddy land soil contained a higher proportion of As(V)-ferrihydrite and a lower proportion of scorodite. The results of chemical fractionation revealed that As in the paddy land soil was more labile than that in the dry land soil. It is therefore suggested that long-term rice cultivation enhances the mobility and availability of As in paddy soils.
1. Introduction
change in microbial activities from aerobic to anaerobic respiration €gel-Knabner et al., 2010). Consequently, the reductive dissolution of (Ko As-associated Fe hydrous oxides results in the release of sorbed or occluded As into soil solution and the reduction of As(V) to As(III) (Rinklebe et al., 2016; Takahashi et al., 2004; Yamaguchi et al., 2011). Meanwhile, microbial methylation can result in the transformation of inorganic As to organic As species, such as monomethylarsonic and dimethylarsinic acids (Cullen and Reimer, 1989; Kumarathilaka et al., 2018). The kinetics of these reactions determine the temporal dynamics of As species in soils and, subsequently, the relative amounts of inor ganic and organic As taken up by rice roots and accumulated in rice grains (Kumarathilaka et al., 2018; Suriyagoda et al., 2018). Under strongly reduced conditions, the reduction of sulfate to sulfide can result in the precipitation of As sulfide, leading to decreasing As availability (Burton et al., 2014; Hashimoto and Kanke, 2018). In the post-harvest period, rice fields are drained, and soils gradually return to oxic con ditions. While sulfide and Fe(II) are oxidized, the oxidation of As(III) to As(V) and re-precipitation of As(V) may occur and the availability of soil As may be decreased (Hashimoto and Kanke, 2018; Kumarathilaka et al., 2018).
Arsenic (As) is one of the ubiquitous contaminants in rice paddy lands in some areas of the world, and this exposes millions of people to the risk of As toxicity through the ingestion of As-contaminated rice (Davis et al., 2017; Kumarathilaka et al., 2018). For example, in Bangladesh, irrigation of paddy lands with groundwater containing geogenic As has led to elevated levels of As in paddy soils and, conse quently, in cultivated rice (Meharg and Rahman, 2003; Rhman et al., 2018). The availability of soil As to rice roots is dependent on As speciation in paddy soils, which is dynamic with time (Kumarathilaka et al., 2018). Understanding the key processes and factors controlling As speciation in paddy soils is essential for developing strategies to decrease As uptake by rice plants and consequently lower the environ mental risk of As in contaminated paddy soils (Kumarathilaka et al., 2018; Punshon et al., 2017; Suriyagoda et al., 2018). The characteristics of paddy rice cultivation are the wet-dry cycles of paddy fields, i.e., alternative submerging rice cultivation period and dry inter-cultivation period. Submerged conditions during rice cultivation period lead to a progressive reduction in rice paddy soil with time due to
* Corresponding author. E-mail address: [email protected] (S.-L. Wang). https://doi.org/10.1016/j.jenvman.2019.109768 Received 6 June 2019; Received in revised form 15 October 2019; Accepted 21 October 2019 Available online 4 November 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.
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Journal of Environmental Management 254 (2020) 109768
The current understanding of temporal dynamics of As speciation in paddy soils is gained from the study of a single wet-dry cycle, whereas the information is not consistent about the evolution of As speciation in paddy soils along continuous wet-dry cycles. For example, Parsons et al. (2013) found that repeated redox cycles of As-spiked floodplain soils decreased As mobility in the soils during the reducing period, which was attributed to decreased iron reduction due to the depletion of labile organic matter, decreased As sorption by Fe hydrous oxides, increased coprecipitation of As with Fe oxyhydroxide, and increased precipitation of noncrystalline ferric arsenate (Parsons et al., 2013). On the contrary, Jiang et al. (2017) found that a paddy soil had a lower arsenate sorption capacity than non-paddy soil, which implied a higher As mobility in the paddy soil (Jiang et al., 2017). Couture et al. (2015) found that, although As was mobile under the anoxic cycle, the change in the As mobility was insignificant over three redox cycles (Couture et al., 2015). The inconsistency among the observations of different studies may be attributed to the limited sensitivity of analytical methods and the methods to prepare As-spiked soils. Arsenic is predominantly associated with Fe in soils through sorption on Fe hydrous oxides or precipitation with Fe (Kumarathilaka et al., 2018). In particular, Fe hydrous oxides, such as ferrihydrite, goethite, and hematite, serve as good sorbents of As in soils (Campbell and Nordstrom, 2014; Majzlan et al., 2014). Because the As sorption of Fe hydrous oxides varies in terms of both sorption capacity and rate, the types and amounts of these Fe hydrous minerals will affect the overall sorption of As by soils. In paddy soils, continuous redox cycles drive the repeated reductive dissolution and precipitation of Fe hydrous oxides, which can lead to the transformation of Fe hydrous oxides and conse quently affect the mobility and availability of As (Campbell and Nord strom, 2014; Moreno-Jim� enez et al., 2012; Pedersen et al., 2006). However, the mineralogical evolution of Fe hydrous oxides after repeated redox cycles has been a matter of debate (Coby et al., 2011; Li et al., 2012; Thompson et al., 2011; Tomaszewski et al., 2016; Vogelsang et al., 2016). The key question about how long-term rice cultivation affects As availability in paddy soils remains to be studied. Elevated levels of As (≦500 mg kg 1) in surface soils were detected in the Guandu area in Taipei, Taiwan (Chang et al., 2010). The develop ment history of this area can be dated back to 1875. The occurrence of As in these soils was attributed to the irrigation of As-containing water 50–100 years ago and the source of As in the irrigation water was from nearby hot springs (Chiang et al., 2010). Owing to remediation of the irrigation system, the soils were no longer irrigated with this hot-spring affected water after the 1970s (Chang et al., 2010; Chiang et al., 2010). In a recent field survey, we found a rice paddy land and its adjacent dry land with similar levels of soil As. The high As contents in these two soils may provide an advantage to differentiate As species in the soils in terms of analytical resolution and sensitivity. The paddy land is flooded twice each year for rice cultivation (March–July and August–November) and is left fallowed between cultivations. On the dry land, the vegetation is bamboo (Dendrocalamopsis oldhami) all year round. The current condi tions of these two lands can be traced back to the 1970s (personal communication). Comparison of the As speciation in these two soils may provide an opportunity to investigate the long-term effects of rice cultivation on the evolution of soil As speciation. In this study, As speciation in the soils of a paddy land and its adjacent dry land was characterized using X-ray absorption spectros copy (XAS) in conjunction with two sequential extraction methods. Synchrotron-based XAS is a powerful tool to determine the speciation of a target element at low concentrations in natural materials with complex chemical compositions (such as soils and sediments), because it is an element-specific and non-destructive technique with high sensitivity (Foster and Kim, 2014; Grafe et al., 2014). Unlike X-ray diffraction, XAS can be used to characterize surface-adsorbed and poorly-ordered phases, in addition to crystalline phases. Distinguishing between adsorbed phases and precipitates is critical in assessing As availability in soils. Nonetheless, the solubility of a soil mineral generally varies due to the
continuous variation of its crystallinity, which cannot be distinguished using XAS. A relative measure of As solubility in the soils of the paddy land and dry land was therefore given by two sequential chemical fractionation methods, including the Wenzel and Hall methods. The Wenzel method has been widely used to fractionate soil As, because this method is targeted the general association of As with Al and Fe in soils (Wenzel et al., 2001). In considering the possibility that sulfide may form under the submerged condition of paddy soils, the Hall method was also used to fractionate soil As phases, including sulfide- and organic-bound As (Hall et al., 1996). Although As fractionation is operationally defined by the extraction schemes of the methods and provides almost no information about the chemical identity of As species in soils, it can still provide useful information to assess As solubility in soils. Through comparing As speciation in the soils of the paddy land and its adjacent dry land, this work clarified the effects of long-term rice cultivation on As availability in soils, which is useful for developing strategies to lower the environmental risk of As contamination in rice paddy lands. 2. Materials and methods 2.1. Collection, preparation and characterization of soil samples Surface soil samples (0–30 cm in depth) were collected from a rice paddy land (25� 070 4000 N, 121� 290 4700 E) and its adjacent dry land (25� 070 4100 N, 121� 290 4600 E) (Fig. S1 in Appendix). These two studied sites are located at the southeastern foot of Tatun Volcano Group, near the confluence of Keelung and Tanshui Rivers (Fig. S1). The annual precipitation is approximately 2120 mm and the monthly mean tem perature ranges from 18 to 30 � C. The paddy land and dryland soils (hereafter referred to as PLS and DLS, respectively) were air-dried and ground to pass through a 2-mm sieve. Soil pH was measured in a 1:1 soildeionized water suspension (Thomas, 1996). Organic matter content in the soils was determined using the Walkley-Black method (Nelson and Sommers, 1996). Cation exchange capacity was measured by saturating the soil with 1 M ammonium acetate at pH 7.0 (Summer and Miller, 1996). Soil texture was analyzed using the pipette method (Gee and Bauder, 1986). The pseudo-total As content in the soils was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES; Spectro genesis) after the soils samples of 0.2 g were ground to a particle size < 100 μm and then digested in 12 mL aqua regia at room temperature for 16 h and at 180 � C for 2 h. The free Al and Fe oxides (Ald and Fed) in the soils were extracted with the mixture of sodium dithionite, sodium bicarbonate and sodium citrate (DCB) (Jackson et al., 1986). Noncrystalline Al and Fe oxides (Alo and Feo) were extracted with 50 mL of 0.2 M ammonium oxalate-oxalic acid (AOD) solution at pH 3.0 for 4 h in the dark (Jackson et al., 1986). After each extraction, the samples were centrifuged at 3000 g for 10 min on a Hitachi CR21GII centrifuge and the supernatants were filtered using Whatman No. 42 filter papers to collect the filtrates. The Al and Fe concentrations in the extracts of both methods were then analyzed using ICP-AES. 2.2. X-ray absorption spectroscopic analysis X-ray absorption spectroscopic (XAS) data of As in soil samples and reference compounds were collected at Beamlines 07 A in National Synchrotron Radiation Research Center (NSRRC), Taiwan. The mono chromator was calibrated to the absorption edge of As metal at 11867 eV. Spectra were then collected from 200 eV below and approx imately 800 eV above the absorption edge in fluorescence mode using a Lytle detector with a 6-μm germanium filter. The spectrum of an As foil was also collected simultaneously with those of the samples for energy calibration. At least three scans were obtained and averaged for each sample. Spectral processing and analysis were conducted using the Athena 2
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program (Ravel and Newville, 2005). After baseline correction and normalization, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were isolated from each XAS spectrum. Linear combination fitting (LCF) was conducted to determine the components of As in the samples using the spectra of reference materials, including As(III)- and As(V)-sorbed on goethite and ferrihydrite, Al arsenate (AlAsO4⋅2H2O), scorodite (FeAsO4⋅2H2O), beudantite (PbFe3(OH)6SO4AsO4), and arsenopyrite (FeAsS). The fitting range was k ¼ 1.6–11.2 (Å 1) for EXAFS. The sorbed As(III) and As(V) phases were prepared by the adsorption of 1000 mg L 1 sodium arsenite and arsenate by goethite and ferrihydrite at pH 4. The synthetic methods for Al arsenate, beudantite, and scorodite, followed those described in Hess and Blanchar (1976), Alcobe et al. (2001) and Harvey et al. (2006), respectively. Arsenopyrite was a natural mineral specimen obtained from the National Museum of Natural Science in Taiwan.
3. Results and discussion 3.1. Basic properties of soils The major properties of PLS and DLS are listed in Table 2. The pseudo-total As contents of PLS and DLS were determined to be 378 and 423 mg kg 1, respectively. The origin of As in the soils was believed to be geothermal springs located in Tatun Volcano Group, which comprises 20 volcanoes and volcanic domes (Fig. S1) (Chang et al., 2010; Chiang et al.; Liu et al., 2016). The lower As content in the PLS may be attributed to the leaching effect of irrigation water after the 1970s, when water contaminated by As-containing hot-spring water was no longer used for irrigation. These two soils both have an acidic pH and a silty clay texture. The pH of PLS was 5.9, which was slightly higher than that of DLS (pH ¼ 5.5). The organic matter content and particle size distribution of the soils are slightly different. Since organic matter and clay of a soil predominantly contribute to its CEC, these two soils exhibited similar CEC values. Therefore, the difference in the crop cultivation history on the paddy and dry lands has not significantly altered these properties of the soils. The contents of noncrystalline and free Fe and Al in the soils, oper ationally extracted using the AOD and DCB methods, respectively, are listed in Table 2. These two methods are traditionally applied to differentiate pedogenic Al and Fe species, even though their applica bility for extracting the target materials has been questioned (Rennert, 2019). The contents of AOD-extractable Al (Alo) and DCB-extractable Al (Ald) of PLS were 1.6 and 2.5 g kg 1, respectively, and their counterparts of DLS were 2.5 and 4.1 g kg 1, respectively (Table 2). The large Alo/Ald ratios of both soils may be attributed to the contribution of volcanic materials from the nearby Tatun Volcano Group (Fig. S1 in Appendix) (Rennert, 2019; Wada, 1989). The Feo and Fed contents of PLS and DLS were 21.3 and 26.1, and 21.2 and 38.3, respectively (Table 2). Accordingly, the noncrystalline Fe percentage (i.e., Feo/Fed x 100%) in PLS and DLS were calculated to be 81.6% and 55.3%. Despite the reliability of the methods for differenti ating noncrystalline and crystalline phases (Rennert, 2019), the results indicated that the Fe in PLS is more labile in comparison with the DLS counterpart. This is similar to previous findings of Vogelsang et al. (2016), which investigated soil Fe minerals along a chronosequence of 100-, 700- and 2000-year-old paddy soils and found that prolonged paddy cultivation resulted in the loss of silicate- and oxide-bound Fe and the organic-rich weakly crystalline phases became predominant. Paddy soils are subject to intermittent submerged and drained periods. During submerged periods, the reductive dissolution of Fe hydrous oxides and the leaching loss of dissolved Fe(II) may result in the lower content of Fe in PLS (Thompson et al., 2011; Vogelsang et al., 2016; Winkler et al., 2018). After the soils are drained during the inter-cultivation period, oxidative re-precipitation of remaining Fe(II) may lead to the formation of poorly crystalline Fe hydrous oxides and consequently the relative enrichment of poorly crystalline Fe hydrous oxides (Thompson et al.,
2.3. Sequential extraction of As in soils Arsenic fractionation of the soils was conducted using two different sequential extraction procedures modified from those described in Hall et al. (1996) and Wenzel et al. (2001). The extraction solutions and parameters of each method are summarized in Table 1. In the Hall method, soil As was fractionated into the exchangeable (H1), bound to noncrystalline Fe oxyhydroxides (H2), bound to crystalline Fe oxy hydroxides (H3), bound to sulfides and organic matter (H4) and bound to silicate (H5) phases. Comparatively, in the Wenzel method, soil As was partitioned into five fractions, including non-specifically adsorbed As (W1), specifically adsorbed As (W2), As bound to noncrystalline Al and Fe hydrous oxides (W3), As bound to crystalline Al and Fe hydrous oxides (W4) and residual As phase (W5). The extraction solution and parameter for each As phase generally followed those in Hall et al. (1996) and Wenzel et al. (2001), except for the last phase (i.e., H5 or W5), which was digested using aqua regia solution, instead of HF/HClO4 (for H5) or HNO3/H2O2 (for W5). Accordingly, the H5 and W5 phases both referred to the residual As phase. Each sequential extraction procedure was conducted for both soil samples of 1.0 g in 50 mL polypropylene centrifuge tubes. After each extraction step, the samples were centrifuged at 3000 g for 10 min and the supernatants were filtered using Whatman No. 42 filter papers to collect the filtrates. The residue solids in the tubes were then washed with 10 mL de-ionized water at room temperature and the washings were combined with the corresponding extraction solutions. The sub sequent extraction was preceded for the residual solids. All analyses were performed in duplicate. The As concentrations in different frac tions were determined using ICP-AES.
Table 1 The procedures and targeted As phases in the sequential extraction methods modified from those in Hall et al. (1996) and Wenzel et al. (2001). Hall method
Wenzel method
Fraction
Extraction conditiona
As phase
Fraction
Extraction conditiona
As phase
H1 H2
1 M NaOAc at pH 5 for 6 h, twice 0.25 M NH2OH⋅HCl in 0.25 M HCl at 60 � C for 2 h and 0.5 h 1 M NH2OH⋅HCl in 25% HOAc at 90 � C for 3 h and 1.5 h 30% H2O2/0.02 M HNO3 at 85 � C for 2 h; 3.2 M NaOAc at 85 � C for 3 h
Sorbed/carbonate Bound to noncrystalline Fe hydrous oxides Bound to crystalline Fe hydrous oxides Sulfides and organics
W1 W2
0.05 M (NH4)2SO4 for 4 h 0.05 M (NH4)H2PO4 for 16 h
Non-specifically sorbed Specifically sorbed
W3
Bound to noncrystalline and poorly crystalline Fe and Al hydrous oxides Bound to crystalline Fe and Al hydrous oxides
Aqua regia/microwave digestionb
Residual
W5
0.2 M NH4-oxalate buffer (pH 3.25) in the dark for 4 h 0.2 M NH4-oxalate buffer þ 0.1 M ascorbic acid (pH 3.25) in the light and at 96 � C for 20 min Aqua regia/microwave digestionb
H3 H4 H5 a b
W4
Extraction temperature was 25 � C unless otherwise specified. Microwave digestion: the temperature was increased to 200 � C in 20 min and then maintained constant for 40 min. 3
Residual
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Table 2 Basic properties and As content of the paddy-land and dry-land soils in Guandu, Taipei, Taiwan. Soil sample
As content (mg kg 1)
pH
CEC (mmolc kg 1)
OM (%)
Sand (%)
Silt (%)
Clay (%)
Texture
Feo (g kg 1)
Fed (g kg 1)
Alo (g kg 1)
Ald (g kg 1)
Paddy land Dry land
378
5.9
320
5.0
5
47
48
21.3
26.1
1.6
2.5
423
5.1
302
4.5
6
49
45
Silty clay Silty clay
21.2
38.3
2.5
4.1
OM: organic matter content; CEC: cation exchange capacity; Feo and Alo: oxalate extractable; Fed and Ald: Dithionite-bicarbonate-citrate extractable.
2011; Vogelsang et al., 2016; Winkler et al., 2018). According to Ost wald ripening, poorly crystalline Fe hydrous oxides are expected to gradually transform to crystalline Fe hydrous oxides over time (Cornell and Schwertmann, 2003). However, the duration of the dry inter-cultivation period is approximately 50–60 days, which may not allow a significant increase in Fe crystallinity before the subsequent submergence period. Meanwhile, the enrichment of organic matter in topsoils may also favor the formation of poorly crystalline materials, because the presence of organic matter retards mineral ripening (Thompson et al., 2011; Vogelsang et al., 2016).
target element (i.e., As in this study), which could yield promising fit results when the XANES spectra are similar to each other. The LCF re sults are listed in Table 3. The best fits were in excellent agreement with the corresponding measured spectra (Fig. 2). For both soils, the best fits were obtained with four reference materials, including As(III)- and As (V)-sorbed ferrihydrite, As(V)-sorbed goethite and scorodite. The sec ond best fits were both obtained with the same reference materials but no As(V)-goethite. Thus, As(V)-sorbed goethite may be present in the soils, but its content was determined with uncertainty due to the re striction from the sensitivity and detection limit of XAS. The LCF results of As K-edge XAS spectra revealed a strong associa tion of As with Fe in the soils. Although the association of As with Al and other metals cannot be completely ruled out, the inclusion of any other reference materials did not further improve the LCF results, indicating their insignificant contributions to soil As. Those reference materials were therefore excluded from LCF to avoid overfitting. Especially, when considering the presence of Al hydrous oxides in the soils, As(III)- or As (V)-adsorbed gibbsite and Al-arsenate were ever tested with the abovementioned best fits, but no improvement in the LCF results was obtained. Because the contents of Fe hydrous oxides in both soils were one order of magnitude higher than the corresponding Al contents, the amounts of As bound to Fe hydrous oxides were expected to be much higher than those bound to Al hydrous oxides in these soils. Thus, it is plausible that As associated with Al has a relatively insignificant contribution to the As speciation in the soils, compared to that by As associated with Fe. Meanwhile, the presence of beudantite in the soils was also tested, because it was detected in nearby river sediment and was suggested to be present in the soil in this area (Chiang et al., 2013). However, no beudantite was detected in both samples.
3.2. As speciation in soils by X-ray absorption spectroscopy Arsenic K-edge XANES data were collected for the surface soil sam ples to provide a direct characterization of As speciation in the soil samples (Fig. 1). The rising edge in XANES spectra is correlated to the oxidation state of the target element (Fig. 1a). As shown in the corre sponding first-derivative spectra (Fig. 1b), the edges of the sample spectra are close to that of As(V). Therefore, the predominant oxidation states of As in the soils are As(V), which is consistent with the factor that As(V) is more stable in aerobic environments, such as the surface soils under investigation. Meanwhile, the first derivative spectra also revealed the presence of As(III) as the minor components of As in the samples, where the peaks raised slightly at the edge of As(III) when comparing to As(V) standards. LCF was performed on the As K-edge EXAFS spectra of two surface soil samples using a wide range of reference compounds to represent various soil As species. Compared to XANES spectra, EXAFS spectra are more sensitive to minor changes in the bonding configurations of the
Fig. 1. As K-edge XANES spectra of the paddy-land and dry-land soils and reference materials. 4
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Table 3 As speciation in the soils based on the LCF of As K-edge EXAFS. Soil Paddy land Dry land
Best fit Second best fit Best fit Second best fit
As(III)-ferrihydrite
As(V)-ferrihydrite
As(V)-goethite
Scorodite
R factor
0.349 (23) 0.346 (23) 0.341 (21) 0.335 (21)
0.410 (64) 0.489 (35) 0.277 (58) 0.413 (33)
0.088 (58) – 0.150 (53) –
0.153 0.165 0.232 0.252
0.01885 0.01940 0.01625 0.01728
(35) (34) (32) (32)
The number in parentheses is uncertainty estimated by the Athena software.
the reduced condition during rice cultivation, scorodite can also un dergo reductive dissolution along with Fe hydrous oxides (Drahota and Filippi, 2009). The dissolution of scorodite results in the release of As, which may be re-adsorbed on Fe hydrous oxides or other soil minerals. Consequently, the content of scorodite is expected to decrease after long-term rice cultivation. This may explain why the relative content of scorodite in PLS was lower than that in DLS. Sorption of As on soil minerals, especially Fe and Al hydrous oxides, has been considered to play a vital role in immobilizing As in soils (Kumarathilaka et al., 2018). Both As(III) and As(V) are strongly sorbed onto Fe and Al oxides by forming surface complexes or precipitates (Campbell and Nordstrom, 2014). Precipitation of As with Fe and other metals has also been considered to be one of the key reactions in immobilizing As (Kumarathilaka et al., 2018). However, As-associated with Fe may be turned into a source of As during reductive dissolution of Fe(III)-containing minerals, such as ferrihydrite, goethite and scor odite detected in the soils under investigation. During rice cultivation, anaerobic respiration of microorganisms induces reductive dissolution of Fe hydrous oxides, which results in the releases of Fe(II) and associ ated As (Hashimoto and Kanke, 2018; Rinklebe et al., 2016; Takahashi et al., 2004; Yamaguchi et al., 2011). Due to kinetic limitation, Fe hy drous oxides cannot be completely dissolved during rice cultivation. The released As may be re-adsorbed by residual Fe hydrous oxides or other soil minerals. The adsorption of As may preferentially occur on poorly crystalline materials due to the relatively high surface area and surface reactivity of poorly crystalline materials. In the dry inter-cultivation period, Fe(II) is rapidly oxidized to Fe(III), which precipitates and forms Fe hydrous oxides (Winkler et al., 2018). At the same time, As may be associated with Fe through adsorption or co-precipitation with Fe hydrous oxides (Jiang et al., 2017; Kumarathilaka et al., 2018). As suggested above, the relative content of poorly-crystalline Fe hydrous oxides increased over long-term rice cultivation. Thus, a higher portion of As is associated with poorly crystalline Fe hydrous oxides, such as ferrihydrite. Accordingly, the long-term rice cultivation resulted in a decrease in the stability of As phases in the paddy-land soil, as compared to the adjacent dry-land soil.
Fig. 2. As K-edge EXAFS spectra of (A) paddy-land and (B) dry-land soils (open circles) and the linear combination fits (solid lines) using the reference mate rials (C) As(III)- and (D) As(V)-sorbed on ferrihydrite, (E) As(V)-sorbed on goethite and (F) scorodite.
The best fits revealed that the predominant As phases in both soils were As(III)- and As(V)-sorbed on ferrihydrite, As(V)-sorbed on goethite, and scorodite. The results were consistent with previous ob servations that As is predominantly associated with Fe in soils through precipitation, coprecipitation, or surface complexation (Campbell and €fe et al., 2008; Kocar et al., 2006). In particular, Fe Nordstrom, 2014; Gra hydrous oxides, such as ferrihydrite, goethite, and hematite, exhibited strong sorptivity toward As(III) and As(V) through the formation of surface complexation (Couture et al., 2013; Gimenez et al., 2007). Therefore, the sorbed As(V) and As(III) phases composed significant portions of As in both soils, i.e., 84.7% and 76.8% in PLS and DLS, respectively. The relative contents of As(III)-sorbed on ferrihydrite were 34.9% and 34.1% in PLS and DLS, respectively, which were similar. The As(V)-sorbed on ferrihydrite and goethite were 41.0% and 8.8%, respectively, in PLS and 27.7% and 15.0%, respectively, in DLS. Compared with DLS, PLS contained less scorodite and As(V)-sorbed on goethite but more As(V)-sorbed on ferrihydrite. Scorodite has low solubility (Ksp ¼ 10 25.68); therefore, it has been suggested to act as a sink of As in the environment (Kocourkova et al., 2011). The formation of scorodite may be attributed to the continuous input of Fe and As through irrigation water contaminated by As-containing hot-spring water (Chiang et al., 2010). However, scor odite is metastable at higher pHs, such as those of the soils; thus, scor odite can undergo incongruent or congruent dissolution in soils, which results in the formation of Fe hydrous oxides such as ferrihydrite and goethite (Bluteau and Demorpoulos, 2007; Harvey et al., 2006; Krause and Ettel, 1989; Langmuir et al., 2006; Nordstrom et al., 2014). Under
3.3. As fractionation in soils by sequential extraction methods Fig. 3 shows the As fractions in PLS and DLS determined by the Hall and Wenzel methods (Hall et al., 1996; Wenzel et al., 2001). With the Hall method, the fractionation profiles of As showed the order of H3 > H2 > H5 ≫ H4 ≫ H1 in PLS and H3 > H5 > H2 ≫ H4 ≫ H1 in DLS. The H1 and H4 fractions (i.e., sorbed/carbonate and sulfide/organic As phases, respectively) were insignificant in both soils. Accordingly, the predominant fractions in both soils were As bound to noncrystalline (H2) and crystalline (H3) Fe hydrous oxides and residual phases (H5), which accounted for 33.2%, 46.6% and 17.4%, respectively, in PLS and 18.6%, 59.0% and 20.3%, respectively, in DLS. The PLS contained a higher H2 fraction but lower W3 and W5 fractions than the DLS. With the Wenzel method, the fractionation profiles of As were W3 > W4 > W5 > W2 > W1 in both PLS and DLS. The W1 phase, i.e., non-specifically adsorbed As, was trivial in both soils. The relative contents of the W2 and W3 phases (i.e., As specifically sorbed and bound to noncrystalline and poorly crystalline Fe and Al hydrous oxides) in PLS were 8.0% and 44.0%, which were higher than the counterparts (i.e., 4.0% and 37.3%) 5
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Fig. 3. As fractions in paddy-land and dry-land soils determined by the sequential extraction methods of Hall et al. (1996) and Wenzel et al. (2001).
goethite and scorodite. Large portions of As were associated with Fe hydrous oxides in these soils, and As sorbed to Fe hydrous oxides was less stable in PLS than in DLS. During rice cultivation, the reductive dissolution and oxidative precipitation of Fe occur in the wet and dry periods, respectively. The consecutive wet-dry cycles lead to the for mation of more poorly crystalline Fe hydrous oxides and, consequently, a higher portion of As associated with them. Accordingly, it is suggested that As becomes more labile and, thus, poses a higher risk after longterm rice cultivation. Further studies will be conducted to investigate agronomic strategies to reduce the availability of soil As to rice plants under the influences of different soil properties and climate conditions.
in DLS. On the contrary, the relative contents of the W4 and W5 phases (i.e., bound to crystalline Fe and Al hydrous oxides and residual phases) were 28.8% and 19.0% in PLS, which were lower than their counterparts (i.e., 36.8% and 21.8%) in DLS. The solubilities of the different As fractions follow the order of the extraction sequence, i.e., the latter the fraction, the lower the solubility (Larios et al., 2013; Mihaljevi�c et al., 2003). Despite the differences in the extraction schemes and defined As phases of the Hall and Wenzel methods (Table 1), similar trends were observed in both soils. The H5 and W5 fractions (i.e., 17.4% and 19.0%) in PLS were less than their counterparts (i.e., 20.3% and 21.8%) in DLS. It is also worth noting that the residual fractions were close to the corresponding contents of scor odite, i.e., 15.3% and 23.2% in PLS and DLS, respectively, determined by As K-edge EXAFS. Thus, scorodite may constitute the most recalci trant As phase, whose relative content in PLS was lower than in DLS. Meanwhile, the sum of H2 and H3 fractions in each soil was close to the corresponding sum of W2, W3 and W4. Thus, the As fractions of these two methods consistently indicated that As associated with noncrystal line and crystalline Fe hydrous oxides was predominant in both soils. The relative content of As bound to noncrystalline Fe hydrous oxide in PLS was higher than that in DLS, while those of As bound to crystalline Fe hydrous oxides and residual phases were lower in PLS than in DLS. The relatively high availability of As in PLS may be attributed to the increased amount of poorly crystalline Fe hydrous oxides, resulting from the consecutive reductive dissolution and precipitation of Fe along continuous wet-dry cycles, as discussed in previous sections (Thompson et al., 2011; Vogelsang et al., 2016; Winkler et al., 2018). Poorly crys talline Fe hydrous oxides exhibit higher affinity toward As than their crystalline counterparts because of their high surface area and surface reactivity (Campbell and Nordstrom, 2014). Combining the results of As fractionation with those of As speciation by EXAFS, it is concluded that As was more labile in PLS than in DLS.
Acknowledgment This work was financially supported by the Ministry of Science and Technology, Taiwan (Grant No. MOST 103-2313-B-002-024-MY3), Environmental Protection Administration, Taiwan (Grant No. EPA-1051603-02-01) and National Taiwan University (Gant No. 103R7748). The authors are grateful to NSRRC for providing beamtime to conduct As XAS measurements and to Dr. Shih-Chang Weng for the assistance in the XAS measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jenvman.2019.109768. References Alcobe, X., Bassas, J., Tarruella, I., Roca, A., Vinals, J., 2001. Structural characterization of synthetic beudantite-type bhases by Rietveld refinement. Mater. Sci. Forum 378–381, 671–676. Bluteau, M.-C., Demorpoulos, G.P., 2007. The incongruent dissolution of scorodite solubility, kinetics and mechanism. Hydrometallurgy 87, 163–177. Burton, E.D., Johnston, S.G., Kocar, B.D., 2014. Arsenic mobility during flooding of contaminated soil: the effect of microbial sulfate reduction. Environ. Sci. Technol. 48, 13660–13667. Campbell, K.M., Nordstrom, D.K., 2014. Arsenic speciation and sorption in natural environments. Rev. Mineral. Geochem. 79, 185–216. Chang, T.K., Shen, C.C., Chen, S.K., Lo, Y.C., Cheng, B.Y., Shyu, G.S., Lin, S.C., 2010. Identifying pollution source of paddy soils by using lead isotope ratio measurements at Guandu, Taipei. J. Taiwan Agric. Eng. 56, 1–10.
4. Conclusions This study characterized As speciation in a paddy land and its adja cent dry land to provide the required knowledge to predict and control the environmental risk of As in paddy soils. The predominant As species in these soils were As(III)- and As(V)-sorbed ferrihydrite, As(V)-sorbed 6
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Research article
Effects of long-term paddy rice cultivation on soil arsenic speciation Puu-Tai Yang a, Yohey Hashimoto b, Wen-Jing Wu c, Jang-Hung Huang c, Po-Neng Chiang d, Shan-Li Wang a, * a
Department of Agricultural Chemistry, National Taiwan University, Taipei, 10617, Taiwan Department of Bioapplications and Systems Engineering, Tokyo University of Agriculture and Technology, Tokyo 184-8588, Japan c Department of Soil and Environmental Sciences, National Chung Hsing University, Taichung, 40227, Taiwan d Experimental Forest, National Taiwan University, Nantou, 55750, Taiwan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Paddy soil Arsenic speciation Wet-dry cycles Redox transformation
Geochemical behavior of arsenic (As) in rice paddy soils determines the availability and mobility of As in the soils, but little is known about the long-term effects of paddy rice cultivation on As speciation in the soils. In this study, surface soil samples were collected from a rice paddy land and its adjacent dry land with similar soil properties and known cultivation histories. The soils of the paddy land and dry land contained 378 and 423 mg As kg 1, respectively. The predominant As species in the soils were investigated using As K-edge X-ray absorption spectroscopy (XAS) in combination with two sequential chemical fractionation methods. The XAS results showed that the predominant As species in the soils were As(III)- and As(V)-ferrihydrite, As(V)-goethite and scorodite. In comparison to the dry land soil, the paddy land soil contained a higher proportion of As(V)-ferrihydrite and a lower proportion of scorodite. The results of chemical fractionation revealed that As in the paddy land soil was more labile than that in the dry land soil. It is therefore suggested that long-term rice cultivation enhances the mobility and availability of As in paddy soils.
1. Introduction
change in microbial activities from aerobic to anaerobic respiration €gel-Knabner et al., 2010). Consequently, the reductive dissolution of (Ko As-associated Fe hydrous oxides results in the release of sorbed or occluded As into soil solution and the reduction of As(V) to As(III) (Rinklebe et al., 2016; Takahashi et al., 2004; Yamaguchi et al., 2011). Meanwhile, microbial methylation can result in the transformation of inorganic As to organic As species, such as monomethylarsonic and dimethylarsinic acids (Cullen and Reimer, 1989; Kumarathilaka et al., 2018). The kinetics of these reactions determine the temporal dynamics of As species in soils and, subsequently, the relative amounts of inor ganic and organic As taken up by rice roots and accumulated in rice grains (Kumarathilaka et al., 2018; Suriyagoda et al., 2018). Under strongly reduced conditions, the reduction of sulfate to sulfide can result in the precipitation of As sulfide, leading to decreasing As availability (Burton et al., 2014; Hashimoto and Kanke, 2018). In the post-harvest period, rice fields are drained, and soils gradually return to oxic con ditions. While sulfide and Fe(II) are oxidized, the oxidation of As(III) to As(V) and re-precipitation of As(V) may occur and the availability of soil As may be decreased (Hashimoto and Kanke, 2018; Kumarathilaka et al., 2018).
Arsenic (As) is one of the ubiquitous contaminants in rice paddy lands in some areas of the world, and this exposes millions of people to the risk of As toxicity through the ingestion of As-contaminated rice (Davis et al., 2017; Kumarathilaka et al., 2018). For example, in Bangladesh, irrigation of paddy lands with groundwater containing geogenic As has led to elevated levels of As in paddy soils and, conse quently, in cultivated rice (Meharg and Rahman, 2003; Rhman et al., 2018). The availability of soil As to rice roots is dependent on As speciation in paddy soils, which is dynamic with time (Kumarathilaka et al., 2018). Understanding the key processes and factors controlling As speciation in paddy soils is essential for developing strategies to decrease As uptake by rice plants and consequently lower the environ mental risk of As in contaminated paddy soils (Kumarathilaka et al., 2018; Punshon et al., 2017; Suriyagoda et al., 2018). The characteristics of paddy rice cultivation are the wet-dry cycles of paddy fields, i.e., alternative submerging rice cultivation period and dry inter-cultivation period. Submerged conditions during rice cultivation period lead to a progressive reduction in rice paddy soil with time due to
* Corresponding author. E-mail address: [email protected] (S.-L. Wang). https://doi.org/10.1016/j.jenvman.2019.109768 Received 6 June 2019; Received in revised form 15 October 2019; Accepted 21 October 2019 Available online 4 November 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.
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The current understanding of temporal dynamics of As speciation in paddy soils is gained from the study of a single wet-dry cycle, whereas the information is not consistent about the evolution of As speciation in paddy soils along continuous wet-dry cycles. For example, Parsons et al. (2013) found that repeated redox cycles of As-spiked floodplain soils decreased As mobility in the soils during the reducing period, which was attributed to decreased iron reduction due to the depletion of labile organic matter, decreased As sorption by Fe hydrous oxides, increased coprecipitation of As with Fe oxyhydroxide, and increased precipitation of noncrystalline ferric arsenate (Parsons et al., 2013). On the contrary, Jiang et al. (2017) found that a paddy soil had a lower arsenate sorption capacity than non-paddy soil, which implied a higher As mobility in the paddy soil (Jiang et al., 2017). Couture et al. (2015) found that, although As was mobile under the anoxic cycle, the change in the As mobility was insignificant over three redox cycles (Couture et al., 2015). The inconsistency among the observations of different studies may be attributed to the limited sensitivity of analytical methods and the methods to prepare As-spiked soils. Arsenic is predominantly associated with Fe in soils through sorption on Fe hydrous oxides or precipitation with Fe (Kumarathilaka et al., 2018). In particular, Fe hydrous oxides, such as ferrihydrite, goethite, and hematite, serve as good sorbents of As in soils (Campbell and Nordstrom, 2014; Majzlan et al., 2014). Because the As sorption of Fe hydrous oxides varies in terms of both sorption capacity and rate, the types and amounts of these Fe hydrous minerals will affect the overall sorption of As by soils. In paddy soils, continuous redox cycles drive the repeated reductive dissolution and precipitation of Fe hydrous oxides, which can lead to the transformation of Fe hydrous oxides and conse quently affect the mobility and availability of As (Campbell and Nord strom, 2014; Moreno-Jim� enez et al., 2012; Pedersen et al., 2006). However, the mineralogical evolution of Fe hydrous oxides after repeated redox cycles has been a matter of debate (Coby et al., 2011; Li et al., 2012; Thompson et al., 2011; Tomaszewski et al., 2016; Vogelsang et al., 2016). The key question about how long-term rice cultivation affects As availability in paddy soils remains to be studied. Elevated levels of As (≦500 mg kg 1) in surface soils were detected in the Guandu area in Taipei, Taiwan (Chang et al., 2010). The develop ment history of this area can be dated back to 1875. The occurrence of As in these soils was attributed to the irrigation of As-containing water 50–100 years ago and the source of As in the irrigation water was from nearby hot springs (Chiang et al., 2010). Owing to remediation of the irrigation system, the soils were no longer irrigated with this hot-spring affected water after the 1970s (Chang et al., 2010; Chiang et al., 2010). In a recent field survey, we found a rice paddy land and its adjacent dry land with similar levels of soil As. The high As contents in these two soils may provide an advantage to differentiate As species in the soils in terms of analytical resolution and sensitivity. The paddy land is flooded twice each year for rice cultivation (March–July and August–November) and is left fallowed between cultivations. On the dry land, the vegetation is bamboo (Dendrocalamopsis oldhami) all year round. The current condi tions of these two lands can be traced back to the 1970s (personal communication). Comparison of the As speciation in these two soils may provide an opportunity to investigate the long-term effects of rice cultivation on the evolution of soil As speciation. In this study, As speciation in the soils of a paddy land and its adjacent dry land was characterized using X-ray absorption spectros copy (XAS) in conjunction with two sequential extraction methods. Synchrotron-based XAS is a powerful tool to determine the speciation of a target element at low concentrations in natural materials with complex chemical compositions (such as soils and sediments), because it is an element-specific and non-destructive technique with high sensitivity (Foster and Kim, 2014; Grafe et al., 2014). Unlike X-ray diffraction, XAS can be used to characterize surface-adsorbed and poorly-ordered phases, in addition to crystalline phases. Distinguishing between adsorbed phases and precipitates is critical in assessing As availability in soils. Nonetheless, the solubility of a soil mineral generally varies due to the
continuous variation of its crystallinity, which cannot be distinguished using XAS. A relative measure of As solubility in the soils of the paddy land and dry land was therefore given by two sequential chemical fractionation methods, including the Wenzel and Hall methods. The Wenzel method has been widely used to fractionate soil As, because this method is targeted the general association of As with Al and Fe in soils (Wenzel et al., 2001). In considering the possibility that sulfide may form under the submerged condition of paddy soils, the Hall method was also used to fractionate soil As phases, including sulfide- and organic-bound As (Hall et al., 1996). Although As fractionation is operationally defined by the extraction schemes of the methods and provides almost no information about the chemical identity of As species in soils, it can still provide useful information to assess As solubility in soils. Through comparing As speciation in the soils of the paddy land and its adjacent dry land, this work clarified the effects of long-term rice cultivation on As availability in soils, which is useful for developing strategies to lower the environmental risk of As contamination in rice paddy lands. 2. Materials and methods 2.1. Collection, preparation and characterization of soil samples Surface soil samples (0–30 cm in depth) were collected from a rice paddy land (25� 070 4000 N, 121� 290 4700 E) and its adjacent dry land (25� 070 4100 N, 121� 290 4600 E) (Fig. S1 in Appendix). These two studied sites are located at the southeastern foot of Tatun Volcano Group, near the confluence of Keelung and Tanshui Rivers (Fig. S1). The annual precipitation is approximately 2120 mm and the monthly mean tem perature ranges from 18 to 30 � C. The paddy land and dryland soils (hereafter referred to as PLS and DLS, respectively) were air-dried and ground to pass through a 2-mm sieve. Soil pH was measured in a 1:1 soildeionized water suspension (Thomas, 1996). Organic matter content in the soils was determined using the Walkley-Black method (Nelson and Sommers, 1996). Cation exchange capacity was measured by saturating the soil with 1 M ammonium acetate at pH 7.0 (Summer and Miller, 1996). Soil texture was analyzed using the pipette method (Gee and Bauder, 1986). The pseudo-total As content in the soils was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES; Spectro genesis) after the soils samples of 0.2 g were ground to a particle size < 100 μm and then digested in 12 mL aqua regia at room temperature for 16 h and at 180 � C for 2 h. The free Al and Fe oxides (Ald and Fed) in the soils were extracted with the mixture of sodium dithionite, sodium bicarbonate and sodium citrate (DCB) (Jackson et al., 1986). Noncrystalline Al and Fe oxides (Alo and Feo) were extracted with 50 mL of 0.2 M ammonium oxalate-oxalic acid (AOD) solution at pH 3.0 for 4 h in the dark (Jackson et al., 1986). After each extraction, the samples were centrifuged at 3000 g for 10 min on a Hitachi CR21GII centrifuge and the supernatants were filtered using Whatman No. 42 filter papers to collect the filtrates. The Al and Fe concentrations in the extracts of both methods were then analyzed using ICP-AES. 2.2. X-ray absorption spectroscopic analysis X-ray absorption spectroscopic (XAS) data of As in soil samples and reference compounds were collected at Beamlines 07 A in National Synchrotron Radiation Research Center (NSRRC), Taiwan. The mono chromator was calibrated to the absorption edge of As metal at 11867 eV. Spectra were then collected from 200 eV below and approx imately 800 eV above the absorption edge in fluorescence mode using a Lytle detector with a 6-μm germanium filter. The spectrum of an As foil was also collected simultaneously with those of the samples for energy calibration. At least three scans were obtained and averaged for each sample. Spectral processing and analysis were conducted using the Athena 2
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program (Ravel and Newville, 2005). After baseline correction and normalization, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were isolated from each XAS spectrum. Linear combination fitting (LCF) was conducted to determine the components of As in the samples using the spectra of reference materials, including As(III)- and As(V)-sorbed on goethite and ferrihydrite, Al arsenate (AlAsO4⋅2H2O), scorodite (FeAsO4⋅2H2O), beudantite (PbFe3(OH)6SO4AsO4), and arsenopyrite (FeAsS). The fitting range was k ¼ 1.6–11.2 (Å 1) for EXAFS. The sorbed As(III) and As(V) phases were prepared by the adsorption of 1000 mg L 1 sodium arsenite and arsenate by goethite and ferrihydrite at pH 4. The synthetic methods for Al arsenate, beudantite, and scorodite, followed those described in Hess and Blanchar (1976), Alcobe et al. (2001) and Harvey et al. (2006), respectively. Arsenopyrite was a natural mineral specimen obtained from the National Museum of Natural Science in Taiwan.
3. Results and discussion 3.1. Basic properties of soils The major properties of PLS and DLS are listed in Table 2. The pseudo-total As contents of PLS and DLS were determined to be 378 and 423 mg kg 1, respectively. The origin of As in the soils was believed to be geothermal springs located in Tatun Volcano Group, which comprises 20 volcanoes and volcanic domes (Fig. S1) (Chang et al., 2010; Chiang et al.; Liu et al., 2016). The lower As content in the PLS may be attributed to the leaching effect of irrigation water after the 1970s, when water contaminated by As-containing hot-spring water was no longer used for irrigation. These two soils both have an acidic pH and a silty clay texture. The pH of PLS was 5.9, which was slightly higher than that of DLS (pH ¼ 5.5). The organic matter content and particle size distribution of the soils are slightly different. Since organic matter and clay of a soil predominantly contribute to its CEC, these two soils exhibited similar CEC values. Therefore, the difference in the crop cultivation history on the paddy and dry lands has not significantly altered these properties of the soils. The contents of noncrystalline and free Fe and Al in the soils, oper ationally extracted using the AOD and DCB methods, respectively, are listed in Table 2. These two methods are traditionally applied to differentiate pedogenic Al and Fe species, even though their applica bility for extracting the target materials has been questioned (Rennert, 2019). The contents of AOD-extractable Al (Alo) and DCB-extractable Al (Ald) of PLS were 1.6 and 2.5 g kg 1, respectively, and their counterparts of DLS were 2.5 and 4.1 g kg 1, respectively (Table 2). The large Alo/Ald ratios of both soils may be attributed to the contribution of volcanic materials from the nearby Tatun Volcano Group (Fig. S1 in Appendix) (Rennert, 2019; Wada, 1989). The Feo and Fed contents of PLS and DLS were 21.3 and 26.1, and 21.2 and 38.3, respectively (Table 2). Accordingly, the noncrystalline Fe percentage (i.e., Feo/Fed x 100%) in PLS and DLS were calculated to be 81.6% and 55.3%. Despite the reliability of the methods for differenti ating noncrystalline and crystalline phases (Rennert, 2019), the results indicated that the Fe in PLS is more labile in comparison with the DLS counterpart. This is similar to previous findings of Vogelsang et al. (2016), which investigated soil Fe minerals along a chronosequence of 100-, 700- and 2000-year-old paddy soils and found that prolonged paddy cultivation resulted in the loss of silicate- and oxide-bound Fe and the organic-rich weakly crystalline phases became predominant. Paddy soils are subject to intermittent submerged and drained periods. During submerged periods, the reductive dissolution of Fe hydrous oxides and the leaching loss of dissolved Fe(II) may result in the lower content of Fe in PLS (Thompson et al., 2011; Vogelsang et al., 2016; Winkler et al., 2018). After the soils are drained during the inter-cultivation period, oxidative re-precipitation of remaining Fe(II) may lead to the formation of poorly crystalline Fe hydrous oxides and consequently the relative enrichment of poorly crystalline Fe hydrous oxides (Thompson et al.,
2.3. Sequential extraction of As in soils Arsenic fractionation of the soils was conducted using two different sequential extraction procedures modified from those described in Hall et al. (1996) and Wenzel et al. (2001). The extraction solutions and parameters of each method are summarized in Table 1. In the Hall method, soil As was fractionated into the exchangeable (H1), bound to noncrystalline Fe oxyhydroxides (H2), bound to crystalline Fe oxy hydroxides (H3), bound to sulfides and organic matter (H4) and bound to silicate (H5) phases. Comparatively, in the Wenzel method, soil As was partitioned into five fractions, including non-specifically adsorbed As (W1), specifically adsorbed As (W2), As bound to noncrystalline Al and Fe hydrous oxides (W3), As bound to crystalline Al and Fe hydrous oxides (W4) and residual As phase (W5). The extraction solution and parameter for each As phase generally followed those in Hall et al. (1996) and Wenzel et al. (2001), except for the last phase (i.e., H5 or W5), which was digested using aqua regia solution, instead of HF/HClO4 (for H5) or HNO3/H2O2 (for W5). Accordingly, the H5 and W5 phases both referred to the residual As phase. Each sequential extraction procedure was conducted for both soil samples of 1.0 g in 50 mL polypropylene centrifuge tubes. After each extraction step, the samples were centrifuged at 3000 g for 10 min and the supernatants were filtered using Whatman No. 42 filter papers to collect the filtrates. The residue solids in the tubes were then washed with 10 mL de-ionized water at room temperature and the washings were combined with the corresponding extraction solutions. The sub sequent extraction was preceded for the residual solids. All analyses were performed in duplicate. The As concentrations in different frac tions were determined using ICP-AES.
Table 1 The procedures and targeted As phases in the sequential extraction methods modified from those in Hall et al. (1996) and Wenzel et al. (2001). Hall method
Wenzel method
Fraction
Extraction conditiona
As phase
Fraction
Extraction conditiona
As phase
H1 H2
1 M NaOAc at pH 5 for 6 h, twice 0.25 M NH2OH⋅HCl in 0.25 M HCl at 60 � C for 2 h and 0.5 h 1 M NH2OH⋅HCl in 25% HOAc at 90 � C for 3 h and 1.5 h 30% H2O2/0.02 M HNO3 at 85 � C for 2 h; 3.2 M NaOAc at 85 � C for 3 h
Sorbed/carbonate Bound to noncrystalline Fe hydrous oxides Bound to crystalline Fe hydrous oxides Sulfides and organics
W1 W2
0.05 M (NH4)2SO4 for 4 h 0.05 M (NH4)H2PO4 for 16 h
Non-specifically sorbed Specifically sorbed
W3
Bound to noncrystalline and poorly crystalline Fe and Al hydrous oxides Bound to crystalline Fe and Al hydrous oxides
Aqua regia/microwave digestionb
Residual
W5
0.2 M NH4-oxalate buffer (pH 3.25) in the dark for 4 h 0.2 M NH4-oxalate buffer þ 0.1 M ascorbic acid (pH 3.25) in the light and at 96 � C for 20 min Aqua regia/microwave digestionb
H3 H4 H5 a b
W4
Extraction temperature was 25 � C unless otherwise specified. Microwave digestion: the temperature was increased to 200 � C in 20 min and then maintained constant for 40 min. 3
Residual
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Table 2 Basic properties and As content of the paddy-land and dry-land soils in Guandu, Taipei, Taiwan. Soil sample
As content (mg kg 1)
pH
CEC (mmolc kg 1)
OM (%)
Sand (%)
Silt (%)
Clay (%)
Texture
Feo (g kg 1)
Fed (g kg 1)
Alo (g kg 1)
Ald (g kg 1)
Paddy land Dry land
378
5.9
320
5.0
5
47
48
21.3
26.1
1.6
2.5
423
5.1
302
4.5
6
49
45
Silty clay Silty clay
21.2
38.3
2.5
4.1
OM: organic matter content; CEC: cation exchange capacity; Feo and Alo: oxalate extractable; Fed and Ald: Dithionite-bicarbonate-citrate extractable.
2011; Vogelsang et al., 2016; Winkler et al., 2018). According to Ost wald ripening, poorly crystalline Fe hydrous oxides are expected to gradually transform to crystalline Fe hydrous oxides over time (Cornell and Schwertmann, 2003). However, the duration of the dry inter-cultivation period is approximately 50–60 days, which may not allow a significant increase in Fe crystallinity before the subsequent submergence period. Meanwhile, the enrichment of organic matter in topsoils may also favor the formation of poorly crystalline materials, because the presence of organic matter retards mineral ripening (Thompson et al., 2011; Vogelsang et al., 2016).
target element (i.e., As in this study), which could yield promising fit results when the XANES spectra are similar to each other. The LCF re sults are listed in Table 3. The best fits were in excellent agreement with the corresponding measured spectra (Fig. 2). For both soils, the best fits were obtained with four reference materials, including As(III)- and As (V)-sorbed ferrihydrite, As(V)-sorbed goethite and scorodite. The sec ond best fits were both obtained with the same reference materials but no As(V)-goethite. Thus, As(V)-sorbed goethite may be present in the soils, but its content was determined with uncertainty due to the re striction from the sensitivity and detection limit of XAS. The LCF results of As K-edge XAS spectra revealed a strong associa tion of As with Fe in the soils. Although the association of As with Al and other metals cannot be completely ruled out, the inclusion of any other reference materials did not further improve the LCF results, indicating their insignificant contributions to soil As. Those reference materials were therefore excluded from LCF to avoid overfitting. Especially, when considering the presence of Al hydrous oxides in the soils, As(III)- or As (V)-adsorbed gibbsite and Al-arsenate were ever tested with the abovementioned best fits, but no improvement in the LCF results was obtained. Because the contents of Fe hydrous oxides in both soils were one order of magnitude higher than the corresponding Al contents, the amounts of As bound to Fe hydrous oxides were expected to be much higher than those bound to Al hydrous oxides in these soils. Thus, it is plausible that As associated with Al has a relatively insignificant contribution to the As speciation in the soils, compared to that by As associated with Fe. Meanwhile, the presence of beudantite in the soils was also tested, because it was detected in nearby river sediment and was suggested to be present in the soil in this area (Chiang et al., 2013). However, no beudantite was detected in both samples.
3.2. As speciation in soils by X-ray absorption spectroscopy Arsenic K-edge XANES data were collected for the surface soil sam ples to provide a direct characterization of As speciation in the soil samples (Fig. 1). The rising edge in XANES spectra is correlated to the oxidation state of the target element (Fig. 1a). As shown in the corre sponding first-derivative spectra (Fig. 1b), the edges of the sample spectra are close to that of As(V). Therefore, the predominant oxidation states of As in the soils are As(V), which is consistent with the factor that As(V) is more stable in aerobic environments, such as the surface soils under investigation. Meanwhile, the first derivative spectra also revealed the presence of As(III) as the minor components of As in the samples, where the peaks raised slightly at the edge of As(III) when comparing to As(V) standards. LCF was performed on the As K-edge EXAFS spectra of two surface soil samples using a wide range of reference compounds to represent various soil As species. Compared to XANES spectra, EXAFS spectra are more sensitive to minor changes in the bonding configurations of the
Fig. 1. As K-edge XANES spectra of the paddy-land and dry-land soils and reference materials. 4
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Table 3 As speciation in the soils based on the LCF of As K-edge EXAFS. Soil Paddy land Dry land
Best fit Second best fit Best fit Second best fit
As(III)-ferrihydrite
As(V)-ferrihydrite
As(V)-goethite
Scorodite
R factor
0.349 (23) 0.346 (23) 0.341 (21) 0.335 (21)
0.410 (64) 0.489 (35) 0.277 (58) 0.413 (33)
0.088 (58) – 0.150 (53) –
0.153 0.165 0.232 0.252
0.01885 0.01940 0.01625 0.01728
(35) (34) (32) (32)
The number in parentheses is uncertainty estimated by the Athena software.
the reduced condition during rice cultivation, scorodite can also un dergo reductive dissolution along with Fe hydrous oxides (Drahota and Filippi, 2009). The dissolution of scorodite results in the release of As, which may be re-adsorbed on Fe hydrous oxides or other soil minerals. Consequently, the content of scorodite is expected to decrease after long-term rice cultivation. This may explain why the relative content of scorodite in PLS was lower than that in DLS. Sorption of As on soil minerals, especially Fe and Al hydrous oxides, has been considered to play a vital role in immobilizing As in soils (Kumarathilaka et al., 2018). Both As(III) and As(V) are strongly sorbed onto Fe and Al oxides by forming surface complexes or precipitates (Campbell and Nordstrom, 2014). Precipitation of As with Fe and other metals has also been considered to be one of the key reactions in immobilizing As (Kumarathilaka et al., 2018). However, As-associated with Fe may be turned into a source of As during reductive dissolution of Fe(III)-containing minerals, such as ferrihydrite, goethite and scor odite detected in the soils under investigation. During rice cultivation, anaerobic respiration of microorganisms induces reductive dissolution of Fe hydrous oxides, which results in the releases of Fe(II) and associ ated As (Hashimoto and Kanke, 2018; Rinklebe et al., 2016; Takahashi et al., 2004; Yamaguchi et al., 2011). Due to kinetic limitation, Fe hy drous oxides cannot be completely dissolved during rice cultivation. The released As may be re-adsorbed by residual Fe hydrous oxides or other soil minerals. The adsorption of As may preferentially occur on poorly crystalline materials due to the relatively high surface area and surface reactivity of poorly crystalline materials. In the dry inter-cultivation period, Fe(II) is rapidly oxidized to Fe(III), which precipitates and forms Fe hydrous oxides (Winkler et al., 2018). At the same time, As may be associated with Fe through adsorption or co-precipitation with Fe hydrous oxides (Jiang et al., 2017; Kumarathilaka et al., 2018). As suggested above, the relative content of poorly-crystalline Fe hydrous oxides increased over long-term rice cultivation. Thus, a higher portion of As is associated with poorly crystalline Fe hydrous oxides, such as ferrihydrite. Accordingly, the long-term rice cultivation resulted in a decrease in the stability of As phases in the paddy-land soil, as compared to the adjacent dry-land soil.
Fig. 2. As K-edge EXAFS spectra of (A) paddy-land and (B) dry-land soils (open circles) and the linear combination fits (solid lines) using the reference mate rials (C) As(III)- and (D) As(V)-sorbed on ferrihydrite, (E) As(V)-sorbed on goethite and (F) scorodite.
The best fits revealed that the predominant As phases in both soils were As(III)- and As(V)-sorbed on ferrihydrite, As(V)-sorbed on goethite, and scorodite. The results were consistent with previous ob servations that As is predominantly associated with Fe in soils through precipitation, coprecipitation, or surface complexation (Campbell and €fe et al., 2008; Kocar et al., 2006). In particular, Fe Nordstrom, 2014; Gra hydrous oxides, such as ferrihydrite, goethite, and hematite, exhibited strong sorptivity toward As(III) and As(V) through the formation of surface complexation (Couture et al., 2013; Gimenez et al., 2007). Therefore, the sorbed As(V) and As(III) phases composed significant portions of As in both soils, i.e., 84.7% and 76.8% in PLS and DLS, respectively. The relative contents of As(III)-sorbed on ferrihydrite were 34.9% and 34.1% in PLS and DLS, respectively, which were similar. The As(V)-sorbed on ferrihydrite and goethite were 41.0% and 8.8%, respectively, in PLS and 27.7% and 15.0%, respectively, in DLS. Compared with DLS, PLS contained less scorodite and As(V)-sorbed on goethite but more As(V)-sorbed on ferrihydrite. Scorodite has low solubility (Ksp ¼ 10 25.68); therefore, it has been suggested to act as a sink of As in the environment (Kocourkova et al., 2011). The formation of scorodite may be attributed to the continuous input of Fe and As through irrigation water contaminated by As-containing hot-spring water (Chiang et al., 2010). However, scor odite is metastable at higher pHs, such as those of the soils; thus, scor odite can undergo incongruent or congruent dissolution in soils, which results in the formation of Fe hydrous oxides such as ferrihydrite and goethite (Bluteau and Demorpoulos, 2007; Harvey et al., 2006; Krause and Ettel, 1989; Langmuir et al., 2006; Nordstrom et al., 2014). Under
3.3. As fractionation in soils by sequential extraction methods Fig. 3 shows the As fractions in PLS and DLS determined by the Hall and Wenzel methods (Hall et al., 1996; Wenzel et al., 2001). With the Hall method, the fractionation profiles of As showed the order of H3 > H2 > H5 ≫ H4 ≫ H1 in PLS and H3 > H5 > H2 ≫ H4 ≫ H1 in DLS. The H1 and H4 fractions (i.e., sorbed/carbonate and sulfide/organic As phases, respectively) were insignificant in both soils. Accordingly, the predominant fractions in both soils were As bound to noncrystalline (H2) and crystalline (H3) Fe hydrous oxides and residual phases (H5), which accounted for 33.2%, 46.6% and 17.4%, respectively, in PLS and 18.6%, 59.0% and 20.3%, respectively, in DLS. The PLS contained a higher H2 fraction but lower W3 and W5 fractions than the DLS. With the Wenzel method, the fractionation profiles of As were W3 > W4 > W5 > W2 > W1 in both PLS and DLS. The W1 phase, i.e., non-specifically adsorbed As, was trivial in both soils. The relative contents of the W2 and W3 phases (i.e., As specifically sorbed and bound to noncrystalline and poorly crystalline Fe and Al hydrous oxides) in PLS were 8.0% and 44.0%, which were higher than the counterparts (i.e., 4.0% and 37.3%) 5
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Fig. 3. As fractions in paddy-land and dry-land soils determined by the sequential extraction methods of Hall et al. (1996) and Wenzel et al. (2001).
goethite and scorodite. Large portions of As were associated with Fe hydrous oxides in these soils, and As sorbed to Fe hydrous oxides was less stable in PLS than in DLS. During rice cultivation, the reductive dissolution and oxidative precipitation of Fe occur in the wet and dry periods, respectively. The consecutive wet-dry cycles lead to the for mation of more poorly crystalline Fe hydrous oxides and, consequently, a higher portion of As associated with them. Accordingly, it is suggested that As becomes more labile and, thus, poses a higher risk after longterm rice cultivation. Further studies will be conducted to investigate agronomic strategies to reduce the availability of soil As to rice plants under the influences of different soil properties and climate conditions.
in DLS. On the contrary, the relative contents of the W4 and W5 phases (i.e., bound to crystalline Fe and Al hydrous oxides and residual phases) were 28.8% and 19.0% in PLS, which were lower than their counterparts (i.e., 36.8% and 21.8%) in DLS. The solubilities of the different As fractions follow the order of the extraction sequence, i.e., the latter the fraction, the lower the solubility (Larios et al., 2013; Mihaljevi�c et al., 2003). Despite the differences in the extraction schemes and defined As phases of the Hall and Wenzel methods (Table 1), similar trends were observed in both soils. The H5 and W5 fractions (i.e., 17.4% and 19.0%) in PLS were less than their counterparts (i.e., 20.3% and 21.8%) in DLS. It is also worth noting that the residual fractions were close to the corresponding contents of scor odite, i.e., 15.3% and 23.2% in PLS and DLS, respectively, determined by As K-edge EXAFS. Thus, scorodite may constitute the most recalci trant As phase, whose relative content in PLS was lower than in DLS. Meanwhile, the sum of H2 and H3 fractions in each soil was close to the corresponding sum of W2, W3 and W4. Thus, the As fractions of these two methods consistently indicated that As associated with noncrystal line and crystalline Fe hydrous oxides was predominant in both soils. The relative content of As bound to noncrystalline Fe hydrous oxide in PLS was higher than that in DLS, while those of As bound to crystalline Fe hydrous oxides and residual phases were lower in PLS than in DLS. The relatively high availability of As in PLS may be attributed to the increased amount of poorly crystalline Fe hydrous oxides, resulting from the consecutive reductive dissolution and precipitation of Fe along continuous wet-dry cycles, as discussed in previous sections (Thompson et al., 2011; Vogelsang et al., 2016; Winkler et al., 2018). Poorly crys talline Fe hydrous oxides exhibit higher affinity toward As than their crystalline counterparts because of their high surface area and surface reactivity (Campbell and Nordstrom, 2014). Combining the results of As fractionation with those of As speciation by EXAFS, it is concluded that As was more labile in PLS than in DLS.
Acknowledgment This work was financially supported by the Ministry of Science and Technology, Taiwan (Grant No. MOST 103-2313-B-002-024-MY3), Environmental Protection Administration, Taiwan (Grant No. EPA-1051603-02-01) and National Taiwan University (Gant No. 103R7748). The authors are grateful to NSRRC for providing beamtime to conduct As XAS measurements and to Dr. Shih-Chang Weng for the assistance in the XAS measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jenvman.2019.109768. References Alcobe, X., Bassas, J., Tarruella, I., Roca, A., Vinals, J., 2001. Structural characterization of synthetic beudantite-type bhases by Rietveld refinement. Mater. Sci. Forum 378–381, 671–676. Bluteau, M.-C., Demorpoulos, G.P., 2007. The incongruent dissolution of scorodite solubility, kinetics and mechanism. Hydrometallurgy 87, 163–177. Burton, E.D., Johnston, S.G., Kocar, B.D., 2014. Arsenic mobility during flooding of contaminated soil: the effect of microbial sulfate reduction. Environ. Sci. Technol. 48, 13660–13667. Campbell, K.M., Nordstrom, D.K., 2014. Arsenic speciation and sorption in natural environments. Rev. Mineral. Geochem. 79, 185–216. Chang, T.K., Shen, C.C., Chen, S.K., Lo, Y.C., Cheng, B.Y., Shyu, G.S., Lin, S.C., 2010. Identifying pollution source of paddy soils by using lead isotope ratio measurements at Guandu, Taipei. J. Taiwan Agric. Eng. 56, 1–10.
4. Conclusions This study characterized As speciation in a paddy land and its adja cent dry land to provide the required knowledge to predict and control the environmental risk of As in paddy soils. The predominant As species in these soils were As(III)- and As(V)-sorbed ferrihydrite, As(V)-sorbed 6
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