Environ Sci Pollut Res DOI 10.1007/s11356-014-3958-5
RESEARCH ARTICLE
Effect of sample pretreatment on the fractionation of arsenic in anoxic soils Guanxing Huang & Zongyu Chen & Jichao Sun & Fan Liu & Jia Wang & Ying Zhang
Received: 8 October 2014 / Accepted: 4 December 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Using by sequential extraction procedures to obtain the chemical forms of arsenic in soils can provide useful information for the assessment of arsenic mobility and bioavailability in soils. However, sample pretreatments before the extraction probably have some effects on the fractionation of arsenic in soils. Impact of sample pretreatments (freeze-drying, oven-drying, air-drying, and the fresh soil) on the fractionation of arsenic in anoxic soils was investigated in this study. The results show that there are some differences for arsenic fractions in soils between by drying pretreatments and by the fresh soil, indicating that the redistribution among arsenic fractions in anoxic soils occurs after drying pretreatments. The redistribution of arsenic fractions in anoxic soils is ascribed to the oxidation of organic matter and sulfides, the crystallization of iron (hydr)oxides, the ageing process, and the diffusion of arsenic into micropores. The freeze-drying is the best drying method to minimize the effect on the fractionation of arsenic in anoxic soils, while air-drying is the worst one. Drying pretreatments are not recommended for the fractionation of arsenic in anoxic soils with high concentration of iron.
Keywords Arsenic . Sample pretreatment . Sequential extraction procedures . Fractionation . Anoxic soils
Responsible editor: Zhihong Xu G. Huang : Z. Chen : J. Sun : F. Liu : J. Wang : Y. Zhang Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China G. Huang (*) Hebei Key Laboratory of Groundwater Remediation, Shijiazhuang 050061, China e-mail: [email protected]
Introduction The environmental geochemical behavior of arsenic depends critically on the form in which it occurs. Therefore, it is untenable that the use of total arsenic concentration as a criterion to assess the potential effect of soil contamination implies all forms of arsenic which have an equal impact on the environment. In order to solve this problem, the methods of sequential extraction for arsenic fractionation in soils have been proposed, which can provide useful information for the assessment of arsenic mobility and bioavailability in soils (McLaren et al. 1998). In sequential extractions, a series of reagents are applied to the same sample to subdivide the total arsenic content. Based on the differences of reagents, many sequential extraction procedures have been developed by the researchers (Manful 1992; McLaren et al. 1998; Keon et al. 2001; Wenzel et al. 2001; Larios et al. 2012a; Javed et al. 2013). Among them, the procedure proposed by Wenzel et al. (2001) has become one of the most widely used methods for the fractionation of arsenic in soils (Tang et al. 2007; Marabottini et al. 2013; Kim et al. 2014) due to the good selectivity and short time for extraction. To date, these procedures have been strongly criticized (Young et al. 2005; Bacon and Davidson 2008; Larios et al. 2012b), and some defects of them such as redistribution of analytes among phases during extraction, nonselectivity of extractants for target phases, and incomplete extraction have been highly concerned (Howard and Vandenbrink 1999; Bacon and Davidson 2008), while the effect of sample pretreatment before extraction on arsenic fractionation has received little attention. It is known that various drying methods such as air-drying, oven-drying, and freeze-drying for sample pretreatment are commonly used before the extraction of trace element fractionation (Bordas and Bourg 1998; Zhang et al. 2001; Juhasz
Environ Sci Pollut Res
et al. 2008; Yan et al. 2013; Liang et al. 2014). Generally, airdrying is the most convenient, without any equipment, but needs more time, oven-drying needs less time but has the high temperature, and freeze-drying needs less time and low temperature, but the operation is relatively complicated. Due to the convenient operation, air-drying is the most commonly used method (Ko et al. 2012; Li et al. 2013; Liang et al. 2014). However, drying methods for sample pretreatment may be the result in significant difference in element fractionation due to the modification of physicochemical properties such as redox condition and temperature, especially for anoxic soils. Therefore, the ideal way for element fractionation is to work with fresh soils (Száková et al. 2009), but it is not always possible; conservation of samples is sometimes necessary (Bordas and Bourg 1998). Another reason is that the reproducibility of analysis on wet (fresh) samples was considerably poorer than that for dried samples (Baeyens et al. 2003). To date, the effect of sample pretreatment on the fractionation of metals in soils or sediments has been concerned (Bordas and Bourg 1998; Zhang et al. 2001; Claff et al. 2010), to minimize the effect of sample pretreatment on the fractionation of metals in sediments; freeze-drying and air-drying are recommended, while oven-drying is not (Bordas and Bourg 1998). However, arsenic is one of the redox-sensitive metalloids, the effect of sample pretreatment on the fractionation of which has been ignored. The aims of this study were to investigate the impact of sample pretreatments such as air-drying, oven-drying, and freeze-drying on the fractionation of arsenic in anoxic soils by using Wenzel’s sequential extraction procedure (Wenzel et al. 2001) and to compare with the analysis of fresh samples to determine which one is the best drying method to minimize the effect on arsenic fractionation in anoxic soils.
Materials and methods Soil sampling and characterization Soils (0–10 cm) were taken from a site of the Pearl River Delta region in South China. Soil samples were air-dried, ground, Table 1
then passed through a 2-mm mesh sieve, and finally pooled into one composite sample and mixed thoroughly to ensure homogeneity. The distribution of soil particle size was determined by the sedimentation method. Soil pH was measured with a glass electrode using a 1:5 (w/w) suspension of a soil to water ratio. The total iron oxides and aluminum oxides were extracted by the HCl-HNO3-HClO4-HF method (Lu 2000). The free iron oxides and the amorphous iron oxides were extracted by the sodium dithionite-sodium citrate with bicarbonate buffer method and the acidic ammonium oxalate method, respectively. The soil organic matter was measured by the K2Cr2O7 method. The cation exchange capacity of the soil was determined by the ammonium acetate method (Pansu and Gautheyrou 2006). The specific surface area of the experimental soil was measured by the BET method multiple point technique (Ladeira and Ciminelli 2004). The background value (total concentration) of As in soil was obtained by digesting soil sample with 9:1 (v/v) HNO3/HF. The details are shown in Table 1 in Huang et al. (2013).
Soil preparation and handling Three samples were taken from the composite soil with each of 100 g. Two samples were spiked with 70 ml of NaAsO2 and Na3AsO4 solution (0.0019 mol/l of As) to artificially increase the total As concentration in the samples by about 100 mg/kg, respectively, and are hereby denoted as SAs(III) and SAs(V). The other sample was removed 2 % weight of soil, then added 2 % weight of iron powder, and finally mixed thoroughly. This sample was also spiked with NaAsO2 solution to artificially increase the total As concentration in the sample by about 100 mg/kg and is hereby denoted as Siron. These soils were mixed thoroughly and maintained the soil moisture at approximately 150 % of water-holding capacity (flooded) in order to simulate the anoxic condition. All soils were sealed and stored in an incubator at 25 °C. After 6 months, these soils were pretreated by four methods. The first one was air-dried at 25 °C for 10 days, the second one was freeze-dried for 24 h, the third one was oven-dried at 105 °C for 24 h, and the fourth one was fresh soil and analyzed directly without pretreatment.
Sequential extraction procedure for As fractionation in soils
Fraction
Extractant
Extraction condition
Soil solution ratio
F1 F2 F3
(NH4)2SO4 (0.05 M) (NH4)H2PO4 (0.05 M) NH4-oxalate buffer (0.2 M); pH 3.25
4 h shaking, 20 °C 16 h shaking, 20 °C 4 h shaking in the dark, 20 °C
1:50 1:50 1:50
F4 F5a
NH4-oxalate buffer (0.2 M); + ascorbic acid (0.1 M) pH 3.25 Aqua regia
30 min in a water basin at 96±3 °C in the light Digestion
1:50 1:100
Modified after Wenzel et al. (2001) a
Extraction methods for total As concentration and F5 are the same
Environ Sci Pollut Res
Sequential extraction procedure The fractionation of As was determined on different pretreated (air-dried, oven-dried, and freeze-dried) samples and the fresh samples. The dried samples were ground and mixed thoroughly, and 1-g subsamples weighted into acid-washed 50-ml centrifuge tubes. The fresh samples were directly weighted representative 1-g (equivalent of dry weight) subsamples into acid-washed 50-ml centrifuge tubes before immediate commencement of the sequential extraction procedure. Duplicate subsamples were weighed out for both the dried and fresh samples. The sequential extraction procedure involved five steps outlined in Table 1, which is modified after Wenzel’s procedure (Wenzel et al. 2001). The five steps were assumed to correspond respectively to nonspecifically sorbed As (F1), specifically sorbed As (F2), As associated with amorphous and poorly crystalline hydrous oxides (F3), As associated with well-crystallized hydrous oxides (F4), and residual As (F5). Extracts were filtered through 0.45-μm filter membranes prior to As analysis.
Analysis and quality control For the sample analysis, all reagents were analytical grade or better. The contents of As in extracts were directly determined using a hydride generation atomic fluorescence spectrophotometer (HG-AFS, AFS-610A model, Beijing BRAIC Instrumental Company, Beijing, China). Duplicate samples were measured and the relative standard deviations were less
than 5 %. A standard reference soil GSS-16 (National Center for Standard Reference Material, Beijing, China) was used for quality control of the acid digestion. The recovery for As in the standard reference soil GSS-16 was 98.7 %. The overall recovery of As using the fractionation procedure, as determined by comparing the sum of As determined in all five fractions with a single total As determination, was found to be within the range of 96.3–109.5 %.
Results and discussion Impact of sample pretreatment on five fractions of As in soils F1 fraction The nonspecifically sorbed fraction (F1) corresponds to As in soils most readily released into other environments such as groundwater and crops. Accordingly, changes to this fraction are critical for the ecological risk assessment of As in soils (Tang et al. 2007). F1 fractions in these anoxic soils are affected by the freeze-dried pretreatment, with both of SAs(V) and SAs(III) exhibiting an increase in F1 fraction, except in Siron (Fig. 1). This increase may be due to the oxidation of arsenopyrite and the decomposition of organic matter. It is known that arsenopyrite generally occurs in anoxic sediments (Bostick et al. 2004) such as anoxic soils in this study. The
Fig. 1 Impact of pretreatment of drying on five fractions of As in anoxic soils (SAs(III) (black box), Siron (blue box), SAs(V) (red box))
Environ Sci Pollut Res
process of freeze-dried pretreatment is usually accompanied by the oxidation. Therefore, the oxidation of arsenopyrite will occur during the process of dry pretreatments as follows (Walker et al. 2006): 4FeAsS + 11O 2 + 6H 2 O = 4Fe 2+ + 4H3AsO3 +4SO42− (1). This mechanism is one of the possible reasons for the increase of F1 fraction in SAs(V) and SAs(III) by the freezedried pretreatment. The other is the decomposition of organic matter. Hlavay et al. (2004) reported that the drying of soils aids in the decomposition of organic materials and may cause more residual forms of organic matter to become more available, which can then be removed during an extraction targeting mobile fraction of metal(loid)s so-called labile fraction or nonspecifically sorbed fraction (Klitzke and Lang 2007). Therefore, the decomposition of organic matter is also one of the possible reasons for the increase of F1 fraction in SAs(V) and SAs(III) by the freeze-dried pretreatment. Differing from in SAs(V) and SAs(III), the freeze-dried pretreatment has little effect on the F1 fraction in Siron (Fig. 1), which is likely due to the immobilization of As oxyanions by ferric hydroxides. As mentioned previously, Siron is characterized by the high content of iron, and ferric hydroxides will be formed in Siron during the process of freeze-drying. In this condition, the mobile fraction of As oxyanions mobilized from the oxidation of arsenopyrite and the decomposition of organic matter is likely to be immobilized by the ferric hydroxides and co-precipitated with the ferrosoferric hydroxides such as [Fex2+Fe3+(OH)3x+](AsO4, AsO3)x (Kim et al. 2002). Therefore, no significant change of F1 fraction by the freezedried pretreatment in Siron may be due to the rough equilibrium of As mobilization and immobilization. As mentioned above, if there is no other factor, theoretically, it should be shown an increase of F1 fraction in anoxic soils by the oven-dried and air-dried pretreatments due to the mobilization of As caused by the oxidation of arsenopyrite and the decomposition of organic matter. Indeed, many previous studies had already shown that oven-drying and air-drying the soils result in some proportion of metals such as Pb and Mn being released in the mobile fraction (Rapin et al. 1986; Tome Jr. et al. 1996; Bordas and Bourg 1998). However, differing from the freeze-dried pretreatment, the fact is that oven-drying and air-drying have little effect on the F1 fraction in all soils in this study (Fig. 1). The reason may also be due to the equilibrium of As mobilization and immobilization. It is known that the air-dried pretreatment usually needs 10 days or more. It is also known that the mobile fraction of As such as F1 fraction decreases in soils with increasing contact time, which is socalled ageing process (Tang et al. 2007; Juhasz et al. 2008; Quazi et al. 2011). Therefore, we infer that little change of F1 fraction by the air-drying may be due to the equilibrium of As mobilization triggered by the oxidation of arsenopyrite and the decomposition of organic matter and As immobilization resulted from the ageing process. In addition, it is known that
adsorption capacity of As onto iron and aluminum (hydr)oxides increases with the increase in temperature and shows an endothermic process (Banerjee et al. 2008; Zha et al. 2013). This indicates that the adsorption of As onto soils is strengthened during oven-drying with high temperature and occurs in the immobilization of As. Therefore, we can conclude that little change of F1 fraction by the oven-drying is likely due to the equilibrium of As mobilization caused by the oxidation of arsenopyrite and the decomposition of organic matter and As immobilization triggered by the high temperature. F2 fraction Apart from F1 fraction, the specifically sorbed As (F2) is also one of the main proportions of bioaccessible soil As (Tang et al. 2007). Contrary to F1 fraction, this fraction is shown little difference in SAs(V) and SAs(III) after freeze-drying, but it is obviously influenced by freeze-drying in Siron and shows an increase (Fig. 1). Also, there is an increase of F2 fraction in Siron after the oven-dried and air-dried pretreatments. This increase in Siron may be due to the mobilization of iron and its (hydr)oxides. As mentioned previously, some iron powder was added into Siron before incubation. As a result, more iron (hydr)oxides such as amorphous forms should be formed in Siron than in SAs(V) and SAs(III), and bound a large proportion of As in Siron, which is evidenced by the high proportion of F3 fraction in Siron (Fig. 2). It is known that part of iron (hydr)oxides usually bound to organic matter through functional groups (Mikutta and Kretzschmar 2011; Neubauer et al. 2013). Therefore, it is possible that the mobilization of iron and its (hydr)oxides occurs during the process of drying due to the oxidation/decomposition of organic matter, which is evidenced by Rechcigl et al. (1992) who reported that the drying increased the extractable Fe in soil samples. According to the above discussion, we can conclude that the increase of F2 fraction in Siron by drying is likely due to the mobilization of iron (hydr)oxides accompanied by the mobilization of As, which is supported by an decrease of As fraction associated with amorphous hydrous oxides (F3) in Siron by drying (Fig. 1). Differing from the freeze-drying, oven-drying and airdrying have significant effect on the F2 fraction in SAs(V) and SAs(III) and show a decrease (Fig. 1). For air-drying, this decrease may be due to the ageing process. As mentioned above, the air-dried pretreatment in this study lasted for 10 days. It is known that the mobilizable fraction of As such as F2 fraction decreases in soils with increasing contact time due to the ageing process (Tang et al. 2007; Juhasz et al. 2008). For oven-drying, the decrease of F2 fraction in SAs(V) and SAs(III) may be due to the diffusion of As into micropores, because the adsorption of As onto soil materials such as iron and aluminum (hydr)oxides increases with the increase in
Environ Sci Pollut Res
A 100% 80% 60%
F1 F2 F3 F4 F5
40% 20% 0% Fresh soil Freeze-dried Oven-dried
Air-dried
B 100% 80% 60% F1 F2 F3 F4 F5
40% 20% 0% Fresh soil Freeze-dried Oven-dried
Air-dried
C
100% 80% 60% F1 F2 F3 F4 F5
40% 20% 0% Fresh soil Freeze-dried Oven-dried Air-dried
Fig. 2 Fractionation of As in soils by the fresh one and after the pretreatment of drying (a SAs(III), b Siron, c SAs(V))
temperature and shows an endothermic process (Banerjee et al. 2008; Zha et al. 2013). In addition, it is known that metal fraction associated with the weakly bound soil fractions decreases with increasing temperature due to the diffusion of metal ions into micropores of clay minerals (Barrow 1992; Ma et al. 2006). Therefore, we can infer that the diffusion of As from inner-sphere surface into micropores of clay minerals is strengthened during the oven-drying with high temperature, which is responsible for the decrease of F2 fraction in SAs(V) and SAs(III) by oven-drying. F3 fraction F3 fraction is associated with amorphous and poorly crystalline hydrous oxides. There is some or slight effect of drying
pretreatments on F3 fraction in SAs(V) and SAs(III) with exhibiting a small increase; except the pretreatment of airdrying on F3 fraction in SAs(III), the effect is negligible (Fig. 1). Compared with F2 fraction, we infer that the slight increase of F3 fraction in SAs(V) and SAs(III) may be mainly from F2 fraction. This change was also observed by Tang et al. (2007) who reported that the proportions of As in F3 fraction increased and in F2 fraction decreased in soils with contact time. Differing from SAs(V) and SAs(III), drying pretreatments have significant effect on F3 fraction in Siron with exhibiting a great decrease (Fig. 1), especially for air-drying. This decrease is likely due to the crystallization of hydrous iron oxides. As mentioned previously, more iron (hydr)oxides such as amorphous and poorly crystalline forms are formed in Siron than in SAs(V) and SAs(III) and bound a large proportion of As in Siron (Fig. 1). It is known that the crystallization of hydrous iron oxides generally accelerated during the process of drying (Rapin et al. 1986; Bordas and Bourg 1998; Thompson et al. 2006). In addition, the oxidation of organic matter occurs during the process of drying, which also favors facilitating the crystallization of iron (hydr)oxides, because high concentration of organic matter usually inhibits the crystallization of iron (hydr)oxides (Schwertmann 1966; Cornell and Schwertmann 1976; Kodama and Schnitzer 1977). As a consequence, the fraction of As associated with amorphous and poorly crystalline hydrous oxides (F3) transforms into the fraction associated with well-crystallized hydrous oxides (F4) by drying (Ford 2002). Therefore, the crystallization of hydrous iron oxides caused by drying is mainly responsible for the decrease of F3 fraction in Siron. It is worth mentioning that freeze-drying has less effect on the F3 fraction in Siron than oven-drying and air-drying.
F4 fraction F4 fraction shows a great increase in soils by drying pretreatments except in SAs(V) and SAs(III) by the freeze-drying pretreatment. Contrary to other fractions, it can be concluded that the increase of F4 fraction in Siron is mainly derived from the F3 fraction, while that in SAs(V) and SAs(III) is mainly originated from the F2 fraction (Fig. 1). As mentioned above, the increase of F4 fraction in Siron is mainly ascribed to the crystallization of iron and aluminum (hydr)oxides and the oxidation of organic matter (Kodama and Schnitzer 1977; Thompson et al. 2006), while ageing process and diffusion of As from inner-sphere surface into micropores of clay minerals triggered by high temperature are responsible for the increase of F4 fraction in SAs(V) and SAs(III) (Ma et al. 2006; Tang et al. 2007). It is worth mentioning that freeze-drying has less effect on the F4 fraction in all soils in this study than ovendrying and air-drying.
Environ Sci Pollut Res
F5 fraction F5 fraction shows a significant increase in Siron by freezedrying. Compared with other fractions, we infer that the increase of F5 fraction in Siron by freeze-drying is also mainly derived from the F3 fraction (Fig. 1). The little decrease of F5 fraction in Siron by oven-drying and air-drying and little increase of F5 fraction in SAs(III) by freeze-drying may be ascribed to the different recoveries in the residual fraction (Claff et al. 2010). As mentioned previously, F5 fraction is extracted by the aqua regia (Table 1), while aqua regia digests do not completely recover trace elements associated with silicates (Chen and Ma 1999), indicating that some variation in the F5 fraction is to be expected. However, most of F5 fraction shows an obvious decrease in soils by drying pretreatments in this study, especially for SAs(V) by air-drying. This decrease may be due to the oxidation of some residual fraction such as recalcitrant organic matter and sulfides. It is known that F5 fraction is associated with a range of recalcitrant materials such as sulfides (FeAsS), recalcitrant clays, and organic matter. In addition, as mentioned above, the soils in this study are anoxic and flooded with water, and the oxidation of recalcitrant organic matter and sulfides occurs during the process of drying pretreatments. As a result, F5 fraction of As in soils is mobilized by drying pretreatments due to the oxidation of recalcitrant organic matter and sulfides. Therefore, the significant decrease of F5 fraction in SAs(V) and SAs(III) by drying pretreatments is likely attributed to the oxidation of recalcitrant materials. It is worth mentioning that freeze-drying has less effect on the F5 fraction in SAs(V) and SAs(III) than oven-drying and airdrying, while freeze-drying has more effect on the F5 fraction in Siron than oven-drying and air-drying (Fig. 1).
of oven-drying and air-drying (Fig. 3), indicating that freezedrying is the best drying method to minimize the effect on As fractionation in these anoxic soils. In contrast, the correlation coefficients of air-drying in all soils are lower than those of freeze-drying and air-drying (Fig. 3), indicating that air-drying is the worst pretreatment for As fractionation in these anoxic soils. In addition, it is worth mentioning that the correlation coefficients of freeze-drying in SAs(V) (Fig. 3c, r=0.995, p
RESEARCH ARTICLE
Effect of sample pretreatment on the fractionation of arsenic in anoxic soils Guanxing Huang & Zongyu Chen & Jichao Sun & Fan Liu & Jia Wang & Ying Zhang
Received: 8 October 2014 / Accepted: 4 December 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Using by sequential extraction procedures to obtain the chemical forms of arsenic in soils can provide useful information for the assessment of arsenic mobility and bioavailability in soils. However, sample pretreatments before the extraction probably have some effects on the fractionation of arsenic in soils. Impact of sample pretreatments (freeze-drying, oven-drying, air-drying, and the fresh soil) on the fractionation of arsenic in anoxic soils was investigated in this study. The results show that there are some differences for arsenic fractions in soils between by drying pretreatments and by the fresh soil, indicating that the redistribution among arsenic fractions in anoxic soils occurs after drying pretreatments. The redistribution of arsenic fractions in anoxic soils is ascribed to the oxidation of organic matter and sulfides, the crystallization of iron (hydr)oxides, the ageing process, and the diffusion of arsenic into micropores. The freeze-drying is the best drying method to minimize the effect on the fractionation of arsenic in anoxic soils, while air-drying is the worst one. Drying pretreatments are not recommended for the fractionation of arsenic in anoxic soils with high concentration of iron.
Keywords Arsenic . Sample pretreatment . Sequential extraction procedures . Fractionation . Anoxic soils
Responsible editor: Zhihong Xu G. Huang : Z. Chen : J. Sun : F. Liu : J. Wang : Y. Zhang Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China G. Huang (*) Hebei Key Laboratory of Groundwater Remediation, Shijiazhuang 050061, China e-mail: [email protected]
Introduction The environmental geochemical behavior of arsenic depends critically on the form in which it occurs. Therefore, it is untenable that the use of total arsenic concentration as a criterion to assess the potential effect of soil contamination implies all forms of arsenic which have an equal impact on the environment. In order to solve this problem, the methods of sequential extraction for arsenic fractionation in soils have been proposed, which can provide useful information for the assessment of arsenic mobility and bioavailability in soils (McLaren et al. 1998). In sequential extractions, a series of reagents are applied to the same sample to subdivide the total arsenic content. Based on the differences of reagents, many sequential extraction procedures have been developed by the researchers (Manful 1992; McLaren et al. 1998; Keon et al. 2001; Wenzel et al. 2001; Larios et al. 2012a; Javed et al. 2013). Among them, the procedure proposed by Wenzel et al. (2001) has become one of the most widely used methods for the fractionation of arsenic in soils (Tang et al. 2007; Marabottini et al. 2013; Kim et al. 2014) due to the good selectivity and short time for extraction. To date, these procedures have been strongly criticized (Young et al. 2005; Bacon and Davidson 2008; Larios et al. 2012b), and some defects of them such as redistribution of analytes among phases during extraction, nonselectivity of extractants for target phases, and incomplete extraction have been highly concerned (Howard and Vandenbrink 1999; Bacon and Davidson 2008), while the effect of sample pretreatment before extraction on arsenic fractionation has received little attention. It is known that various drying methods such as air-drying, oven-drying, and freeze-drying for sample pretreatment are commonly used before the extraction of trace element fractionation (Bordas and Bourg 1998; Zhang et al. 2001; Juhasz
Environ Sci Pollut Res
et al. 2008; Yan et al. 2013; Liang et al. 2014). Generally, airdrying is the most convenient, without any equipment, but needs more time, oven-drying needs less time but has the high temperature, and freeze-drying needs less time and low temperature, but the operation is relatively complicated. Due to the convenient operation, air-drying is the most commonly used method (Ko et al. 2012; Li et al. 2013; Liang et al. 2014). However, drying methods for sample pretreatment may be the result in significant difference in element fractionation due to the modification of physicochemical properties such as redox condition and temperature, especially for anoxic soils. Therefore, the ideal way for element fractionation is to work with fresh soils (Száková et al. 2009), but it is not always possible; conservation of samples is sometimes necessary (Bordas and Bourg 1998). Another reason is that the reproducibility of analysis on wet (fresh) samples was considerably poorer than that for dried samples (Baeyens et al. 2003). To date, the effect of sample pretreatment on the fractionation of metals in soils or sediments has been concerned (Bordas and Bourg 1998; Zhang et al. 2001; Claff et al. 2010), to minimize the effect of sample pretreatment on the fractionation of metals in sediments; freeze-drying and air-drying are recommended, while oven-drying is not (Bordas and Bourg 1998). However, arsenic is one of the redox-sensitive metalloids, the effect of sample pretreatment on the fractionation of which has been ignored. The aims of this study were to investigate the impact of sample pretreatments such as air-drying, oven-drying, and freeze-drying on the fractionation of arsenic in anoxic soils by using Wenzel’s sequential extraction procedure (Wenzel et al. 2001) and to compare with the analysis of fresh samples to determine which one is the best drying method to minimize the effect on arsenic fractionation in anoxic soils.
Materials and methods Soil sampling and characterization Soils (0–10 cm) were taken from a site of the Pearl River Delta region in South China. Soil samples were air-dried, ground, Table 1
then passed through a 2-mm mesh sieve, and finally pooled into one composite sample and mixed thoroughly to ensure homogeneity. The distribution of soil particle size was determined by the sedimentation method. Soil pH was measured with a glass electrode using a 1:5 (w/w) suspension of a soil to water ratio. The total iron oxides and aluminum oxides were extracted by the HCl-HNO3-HClO4-HF method (Lu 2000). The free iron oxides and the amorphous iron oxides were extracted by the sodium dithionite-sodium citrate with bicarbonate buffer method and the acidic ammonium oxalate method, respectively. The soil organic matter was measured by the K2Cr2O7 method. The cation exchange capacity of the soil was determined by the ammonium acetate method (Pansu and Gautheyrou 2006). The specific surface area of the experimental soil was measured by the BET method multiple point technique (Ladeira and Ciminelli 2004). The background value (total concentration) of As in soil was obtained by digesting soil sample with 9:1 (v/v) HNO3/HF. The details are shown in Table 1 in Huang et al. (2013).
Soil preparation and handling Three samples were taken from the composite soil with each of 100 g. Two samples were spiked with 70 ml of NaAsO2 and Na3AsO4 solution (0.0019 mol/l of As) to artificially increase the total As concentration in the samples by about 100 mg/kg, respectively, and are hereby denoted as SAs(III) and SAs(V). The other sample was removed 2 % weight of soil, then added 2 % weight of iron powder, and finally mixed thoroughly. This sample was also spiked with NaAsO2 solution to artificially increase the total As concentration in the sample by about 100 mg/kg and is hereby denoted as Siron. These soils were mixed thoroughly and maintained the soil moisture at approximately 150 % of water-holding capacity (flooded) in order to simulate the anoxic condition. All soils were sealed and stored in an incubator at 25 °C. After 6 months, these soils were pretreated by four methods. The first one was air-dried at 25 °C for 10 days, the second one was freeze-dried for 24 h, the third one was oven-dried at 105 °C for 24 h, and the fourth one was fresh soil and analyzed directly without pretreatment.
Sequential extraction procedure for As fractionation in soils
Fraction
Extractant
Extraction condition
Soil solution ratio
F1 F2 F3
(NH4)2SO4 (0.05 M) (NH4)H2PO4 (0.05 M) NH4-oxalate buffer (0.2 M); pH 3.25
4 h shaking, 20 °C 16 h shaking, 20 °C 4 h shaking in the dark, 20 °C
1:50 1:50 1:50
F4 F5a
NH4-oxalate buffer (0.2 M); + ascorbic acid (0.1 M) pH 3.25 Aqua regia
30 min in a water basin at 96±3 °C in the light Digestion
1:50 1:100
Modified after Wenzel et al. (2001) a
Extraction methods for total As concentration and F5 are the same
Environ Sci Pollut Res
Sequential extraction procedure The fractionation of As was determined on different pretreated (air-dried, oven-dried, and freeze-dried) samples and the fresh samples. The dried samples were ground and mixed thoroughly, and 1-g subsamples weighted into acid-washed 50-ml centrifuge tubes. The fresh samples were directly weighted representative 1-g (equivalent of dry weight) subsamples into acid-washed 50-ml centrifuge tubes before immediate commencement of the sequential extraction procedure. Duplicate subsamples were weighed out for both the dried and fresh samples. The sequential extraction procedure involved five steps outlined in Table 1, which is modified after Wenzel’s procedure (Wenzel et al. 2001). The five steps were assumed to correspond respectively to nonspecifically sorbed As (F1), specifically sorbed As (F2), As associated with amorphous and poorly crystalline hydrous oxides (F3), As associated with well-crystallized hydrous oxides (F4), and residual As (F5). Extracts were filtered through 0.45-μm filter membranes prior to As analysis.
Analysis and quality control For the sample analysis, all reagents were analytical grade or better. The contents of As in extracts were directly determined using a hydride generation atomic fluorescence spectrophotometer (HG-AFS, AFS-610A model, Beijing BRAIC Instrumental Company, Beijing, China). Duplicate samples were measured and the relative standard deviations were less
than 5 %. A standard reference soil GSS-16 (National Center for Standard Reference Material, Beijing, China) was used for quality control of the acid digestion. The recovery for As in the standard reference soil GSS-16 was 98.7 %. The overall recovery of As using the fractionation procedure, as determined by comparing the sum of As determined in all five fractions with a single total As determination, was found to be within the range of 96.3–109.5 %.
Results and discussion Impact of sample pretreatment on five fractions of As in soils F1 fraction The nonspecifically sorbed fraction (F1) corresponds to As in soils most readily released into other environments such as groundwater and crops. Accordingly, changes to this fraction are critical for the ecological risk assessment of As in soils (Tang et al. 2007). F1 fractions in these anoxic soils are affected by the freeze-dried pretreatment, with both of SAs(V) and SAs(III) exhibiting an increase in F1 fraction, except in Siron (Fig. 1). This increase may be due to the oxidation of arsenopyrite and the decomposition of organic matter. It is known that arsenopyrite generally occurs in anoxic sediments (Bostick et al. 2004) such as anoxic soils in this study. The
Fig. 1 Impact of pretreatment of drying on five fractions of As in anoxic soils (SAs(III) (black box), Siron (blue box), SAs(V) (red box))
Environ Sci Pollut Res
process of freeze-dried pretreatment is usually accompanied by the oxidation. Therefore, the oxidation of arsenopyrite will occur during the process of dry pretreatments as follows (Walker et al. 2006): 4FeAsS + 11O 2 + 6H 2 O = 4Fe 2+ + 4H3AsO3 +4SO42− (1). This mechanism is one of the possible reasons for the increase of F1 fraction in SAs(V) and SAs(III) by the freezedried pretreatment. The other is the decomposition of organic matter. Hlavay et al. (2004) reported that the drying of soils aids in the decomposition of organic materials and may cause more residual forms of organic matter to become more available, which can then be removed during an extraction targeting mobile fraction of metal(loid)s so-called labile fraction or nonspecifically sorbed fraction (Klitzke and Lang 2007). Therefore, the decomposition of organic matter is also one of the possible reasons for the increase of F1 fraction in SAs(V) and SAs(III) by the freeze-dried pretreatment. Differing from in SAs(V) and SAs(III), the freeze-dried pretreatment has little effect on the F1 fraction in Siron (Fig. 1), which is likely due to the immobilization of As oxyanions by ferric hydroxides. As mentioned previously, Siron is characterized by the high content of iron, and ferric hydroxides will be formed in Siron during the process of freeze-drying. In this condition, the mobile fraction of As oxyanions mobilized from the oxidation of arsenopyrite and the decomposition of organic matter is likely to be immobilized by the ferric hydroxides and co-precipitated with the ferrosoferric hydroxides such as [Fex2+Fe3+(OH)3x+](AsO4, AsO3)x (Kim et al. 2002). Therefore, no significant change of F1 fraction by the freezedried pretreatment in Siron may be due to the rough equilibrium of As mobilization and immobilization. As mentioned above, if there is no other factor, theoretically, it should be shown an increase of F1 fraction in anoxic soils by the oven-dried and air-dried pretreatments due to the mobilization of As caused by the oxidation of arsenopyrite and the decomposition of organic matter. Indeed, many previous studies had already shown that oven-drying and air-drying the soils result in some proportion of metals such as Pb and Mn being released in the mobile fraction (Rapin et al. 1986; Tome Jr. et al. 1996; Bordas and Bourg 1998). However, differing from the freeze-dried pretreatment, the fact is that oven-drying and air-drying have little effect on the F1 fraction in all soils in this study (Fig. 1). The reason may also be due to the equilibrium of As mobilization and immobilization. It is known that the air-dried pretreatment usually needs 10 days or more. It is also known that the mobile fraction of As such as F1 fraction decreases in soils with increasing contact time, which is socalled ageing process (Tang et al. 2007; Juhasz et al. 2008; Quazi et al. 2011). Therefore, we infer that little change of F1 fraction by the air-drying may be due to the equilibrium of As mobilization triggered by the oxidation of arsenopyrite and the decomposition of organic matter and As immobilization resulted from the ageing process. In addition, it is known that
adsorption capacity of As onto iron and aluminum (hydr)oxides increases with the increase in temperature and shows an endothermic process (Banerjee et al. 2008; Zha et al. 2013). This indicates that the adsorption of As onto soils is strengthened during oven-drying with high temperature and occurs in the immobilization of As. Therefore, we can conclude that little change of F1 fraction by the oven-drying is likely due to the equilibrium of As mobilization caused by the oxidation of arsenopyrite and the decomposition of organic matter and As immobilization triggered by the high temperature. F2 fraction Apart from F1 fraction, the specifically sorbed As (F2) is also one of the main proportions of bioaccessible soil As (Tang et al. 2007). Contrary to F1 fraction, this fraction is shown little difference in SAs(V) and SAs(III) after freeze-drying, but it is obviously influenced by freeze-drying in Siron and shows an increase (Fig. 1). Also, there is an increase of F2 fraction in Siron after the oven-dried and air-dried pretreatments. This increase in Siron may be due to the mobilization of iron and its (hydr)oxides. As mentioned previously, some iron powder was added into Siron before incubation. As a result, more iron (hydr)oxides such as amorphous forms should be formed in Siron than in SAs(V) and SAs(III), and bound a large proportion of As in Siron, which is evidenced by the high proportion of F3 fraction in Siron (Fig. 2). It is known that part of iron (hydr)oxides usually bound to organic matter through functional groups (Mikutta and Kretzschmar 2011; Neubauer et al. 2013). Therefore, it is possible that the mobilization of iron and its (hydr)oxides occurs during the process of drying due to the oxidation/decomposition of organic matter, which is evidenced by Rechcigl et al. (1992) who reported that the drying increased the extractable Fe in soil samples. According to the above discussion, we can conclude that the increase of F2 fraction in Siron by drying is likely due to the mobilization of iron (hydr)oxides accompanied by the mobilization of As, which is supported by an decrease of As fraction associated with amorphous hydrous oxides (F3) in Siron by drying (Fig. 1). Differing from the freeze-drying, oven-drying and airdrying have significant effect on the F2 fraction in SAs(V) and SAs(III) and show a decrease (Fig. 1). For air-drying, this decrease may be due to the ageing process. As mentioned above, the air-dried pretreatment in this study lasted for 10 days. It is known that the mobilizable fraction of As such as F2 fraction decreases in soils with increasing contact time due to the ageing process (Tang et al. 2007; Juhasz et al. 2008). For oven-drying, the decrease of F2 fraction in SAs(V) and SAs(III) may be due to the diffusion of As into micropores, because the adsorption of As onto soil materials such as iron and aluminum (hydr)oxides increases with the increase in
Environ Sci Pollut Res
A 100% 80% 60%
F1 F2 F3 F4 F5
40% 20% 0% Fresh soil Freeze-dried Oven-dried
Air-dried
B 100% 80% 60% F1 F2 F3 F4 F5
40% 20% 0% Fresh soil Freeze-dried Oven-dried
Air-dried
C
100% 80% 60% F1 F2 F3 F4 F5
40% 20% 0% Fresh soil Freeze-dried Oven-dried Air-dried
Fig. 2 Fractionation of As in soils by the fresh one and after the pretreatment of drying (a SAs(III), b Siron, c SAs(V))
temperature and shows an endothermic process (Banerjee et al. 2008; Zha et al. 2013). In addition, it is known that metal fraction associated with the weakly bound soil fractions decreases with increasing temperature due to the diffusion of metal ions into micropores of clay minerals (Barrow 1992; Ma et al. 2006). Therefore, we can infer that the diffusion of As from inner-sphere surface into micropores of clay minerals is strengthened during the oven-drying with high temperature, which is responsible for the decrease of F2 fraction in SAs(V) and SAs(III) by oven-drying. F3 fraction F3 fraction is associated with amorphous and poorly crystalline hydrous oxides. There is some or slight effect of drying
pretreatments on F3 fraction in SAs(V) and SAs(III) with exhibiting a small increase; except the pretreatment of airdrying on F3 fraction in SAs(III), the effect is negligible (Fig. 1). Compared with F2 fraction, we infer that the slight increase of F3 fraction in SAs(V) and SAs(III) may be mainly from F2 fraction. This change was also observed by Tang et al. (2007) who reported that the proportions of As in F3 fraction increased and in F2 fraction decreased in soils with contact time. Differing from SAs(V) and SAs(III), drying pretreatments have significant effect on F3 fraction in Siron with exhibiting a great decrease (Fig. 1), especially for air-drying. This decrease is likely due to the crystallization of hydrous iron oxides. As mentioned previously, more iron (hydr)oxides such as amorphous and poorly crystalline forms are formed in Siron than in SAs(V) and SAs(III) and bound a large proportion of As in Siron (Fig. 1). It is known that the crystallization of hydrous iron oxides generally accelerated during the process of drying (Rapin et al. 1986; Bordas and Bourg 1998; Thompson et al. 2006). In addition, the oxidation of organic matter occurs during the process of drying, which also favors facilitating the crystallization of iron (hydr)oxides, because high concentration of organic matter usually inhibits the crystallization of iron (hydr)oxides (Schwertmann 1966; Cornell and Schwertmann 1976; Kodama and Schnitzer 1977). As a consequence, the fraction of As associated with amorphous and poorly crystalline hydrous oxides (F3) transforms into the fraction associated with well-crystallized hydrous oxides (F4) by drying (Ford 2002). Therefore, the crystallization of hydrous iron oxides caused by drying is mainly responsible for the decrease of F3 fraction in Siron. It is worth mentioning that freeze-drying has less effect on the F3 fraction in Siron than oven-drying and air-drying.
F4 fraction F4 fraction shows a great increase in soils by drying pretreatments except in SAs(V) and SAs(III) by the freeze-drying pretreatment. Contrary to other fractions, it can be concluded that the increase of F4 fraction in Siron is mainly derived from the F3 fraction, while that in SAs(V) and SAs(III) is mainly originated from the F2 fraction (Fig. 1). As mentioned above, the increase of F4 fraction in Siron is mainly ascribed to the crystallization of iron and aluminum (hydr)oxides and the oxidation of organic matter (Kodama and Schnitzer 1977; Thompson et al. 2006), while ageing process and diffusion of As from inner-sphere surface into micropores of clay minerals triggered by high temperature are responsible for the increase of F4 fraction in SAs(V) and SAs(III) (Ma et al. 2006; Tang et al. 2007). It is worth mentioning that freeze-drying has less effect on the F4 fraction in all soils in this study than ovendrying and air-drying.
Environ Sci Pollut Res
F5 fraction F5 fraction shows a significant increase in Siron by freezedrying. Compared with other fractions, we infer that the increase of F5 fraction in Siron by freeze-drying is also mainly derived from the F3 fraction (Fig. 1). The little decrease of F5 fraction in Siron by oven-drying and air-drying and little increase of F5 fraction in SAs(III) by freeze-drying may be ascribed to the different recoveries in the residual fraction (Claff et al. 2010). As mentioned previously, F5 fraction is extracted by the aqua regia (Table 1), while aqua regia digests do not completely recover trace elements associated with silicates (Chen and Ma 1999), indicating that some variation in the F5 fraction is to be expected. However, most of F5 fraction shows an obvious decrease in soils by drying pretreatments in this study, especially for SAs(V) by air-drying. This decrease may be due to the oxidation of some residual fraction such as recalcitrant organic matter and sulfides. It is known that F5 fraction is associated with a range of recalcitrant materials such as sulfides (FeAsS), recalcitrant clays, and organic matter. In addition, as mentioned above, the soils in this study are anoxic and flooded with water, and the oxidation of recalcitrant organic matter and sulfides occurs during the process of drying pretreatments. As a result, F5 fraction of As in soils is mobilized by drying pretreatments due to the oxidation of recalcitrant organic matter and sulfides. Therefore, the significant decrease of F5 fraction in SAs(V) and SAs(III) by drying pretreatments is likely attributed to the oxidation of recalcitrant materials. It is worth mentioning that freeze-drying has less effect on the F5 fraction in SAs(V) and SAs(III) than oven-drying and airdrying, while freeze-drying has more effect on the F5 fraction in Siron than oven-drying and air-drying (Fig. 1).
of oven-drying and air-drying (Fig. 3), indicating that freezedrying is the best drying method to minimize the effect on As fractionation in these anoxic soils. In contrast, the correlation coefficients of air-drying in all soils are lower than those of freeze-drying and air-drying (Fig. 3), indicating that air-drying is the worst pretreatment for As fractionation in these anoxic soils. In addition, it is worth mentioning that the correlation coefficients of freeze-drying in SAs(V) (Fig. 3c, r=0.995, p
