Effects of soil properties and long aging time on the toxicity of exogenous antimony to soil-dwelling springtail Folsomia candida

Chemosphere 241 (2020) 125100

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Effects of soil properties and long aging time on the toxicity of exogenous antimony to soil-dwelling springtail Folsomia candida Xianglong Lin a, b, Zaijin Sun a, Jin Ma a, Hong Hou a, b, *, Long Zhao a, ** a b

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing, 100000, China College of Water Sciences, Beijing Normal University, Beijing, 100875, China

h i g h l i g h t s
Prediction models of Sb toxicity in Sb(III)-treated soil were developed basing soil properties.
pH and OM could well predicted Sb toxicity in Sb(III)-treated soil.
Sb(III) and Sb(V) exhibited no toxicity after aged for 365 d in the most soils.
pH was the most important factor controlling aging effects on Sb toxicity in Sb(III)-treated soil.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 July 2019 Received in revised form 8 October 2019 Accepted 9 October 2019 Available online 13 October 2019

The most existing studies on the toxicity of antimony (Sb) were performed in limited types of soil and after short aging time. Effects of soil properties and long aging time on chronic toxicity of Sb(III) and Sb(V) to model organism Folsomia candida were studied in the laboratory studies. The results showed that after the Sb(V)-treated soils were aged for 365 d, the Sb exhibited no toxicity to survival and reproduction even at the nominal highest concentration of 12,800 mg kg1 in ten types of soils with distinct differences in soil properties. In the Sb(III)-treated ten soils aged only for 30 d, the concentrations causing 50% mortality (LC50) and concentrations inhibiting 50% reproduction (EC50) were 1288 e3219 mg kg1 and 683e1829 mg kg1, respectively. The LC50 were higher than the highest test concentration and the EC50 significantly increased by 2.24e6.16 fold after the Sb(III)-treated soils were aged for 150 d, and soil pH was the most important single factor explaining the variance in aging effects. After the aging time was 365 d, similar with Sb(V)-treated soils, no toxicity were observed in the most Sb(III)treated soils, indicating the increasing aging effects with aging time. Regression analysis indicated that the OM and pH were the most important single factor predicting Sb toxicity to reproduction in Sb(III)treated soils aged for 30 and 150 d, respectively. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: T Cutright Keywords: Antimony toxicity Folsomia candida Soil properties Aging time Oxidation reaction

1. Introduction Metalloid antimony (Sb) occurs naturally in soil with common natural background concentrations of 0.3e8.6 mg kg1 (Pierart et al., 2015). Sb is a toxic element, and anthropogenic activities such as mining, smelting, military training, shooting ranges and extensive use of Sb-containing industrial products have remarkably

* Corresponding author. State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing, 100000, China. ** Corresponding author. E-mail addresses: [email protected] (H. Hou), [email protected] (L. Zhao). https://doi.org/10.1016/j.chemosphere.2019.125100 0045-6535/© 2019 Elsevier Ltd. All rights reserved.

elevated total soil Sb concentrations (Mubarak et al., 2015; Chai et al., 2016; Herath et al., 2017). Sb exists in four oxidation states (-III, 0, III, and V) in the environment, however, the Sb(V) is found mainly in soils due to the Sb(III) is easily oxidize in general aerobic soil (Wilson et al., 2010; Herath et al., 2017). The increasing release of Sb to soil have promoted the studies about Sb toxicity to ecological receptors (plants, microbes and invertebrates) in recent decades (Pierart et al., 2015; He et al., 2018; Shahid et al., 2019), which could provide bases for evaluating hazard and establishing soil environmental quality criteria of Sb. However, two main deficiencies existed. Firstly, these studies were all performed in artificial soil or limited types of natural soil, and thus did not quantify the effects of soil properties on Sb toxicity. As is well-known, the types of natural soils are various with distinct

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physicochemical properties mainly including the soil pH, adsorption phases (clay, organic matter and metal oxyhydroxides), and cation exchange capacity (CEC) (Criel et al., 2008). The Sb will exhibit different toxicity at the same total concentration in different soils due to soil properties strongly control the solid-solution partitioning and the oxidation states of Sb (Fan et al., 2013; Cai et al., 2016; He et al., 2018). Moreover, developing quantitative relationships between the toxicity values of Sb and soil properties is one of the indispensable steps in establishing soil-specific criteria (Oorts and Smolders, 2009; Wang et al., 2018). Secondly, the influences of long aging time on Sb toxicity have only been considered in few studies (Hammel et al., 1998; Oorts et al., 2008; Lin et al., 2019), and the relationship between soil properties and aging effects was not well understood due to the use of limited types of test soil. It has been widely accepted that aging the freshly metal-spiked soils for a relatively long time is ecologically important to gain reliable toxicity values and thus better extrapolate laboratory toxicity data to field conditions (Smolders et al., 2009, 2015). Especially, when soluble Sb(III) salt which is much more toxic than that of Sb(V) (Duester et al., 2011; He et al., 2018) is used as test compound, both the available fraction and redox state of Sb in soil will change during aging due to the timedependent oxidation process (Cai et al., 2016; Lin et al., 2019). Thus, studying toxicity of Sb in freshly soluble Sb(III)-spiked soil will greatly overrate Sb toxicity, because the Sb bioavailability is comparatively low and Sb(V) species dominate in realistic Sbcontaminated soil (Wilson et al., 2010; Ettler et al., 2010; Herath et al., 2017). Additionally, it should be noted that the aging effect is related to the soil properties and the decrease extent of toxicity during aging vary with different soils (Oorts et al., 2007; Smolders et al., 2015; Wang et al., 2018). The soil-dwelling springtails are typical invertebrates and abundant in various soil habitats worldwide, which perform crucial roles such as contributing to the decomposition processes in soil ecosystems (Coelho et al., 2015; Buch et al., 2016). The springtails, which contacted with the soil solid-aqueous phases through oral ingestion and the epidermis/ventral tube uptake, are susceptible to soil metals (Fountain and Hopkin, 2005). Furthermore, the springtails represent arthropod species with a different exposure route compared to earthworms and enchytraeids (ISO, 2014). As the most typical representative of springtails, parthenogenetic Folsomia candida has been extensively used as a model organism to assess metal toxicity due to its high sensitivity and easy cultivability in the laboratory (Fountain and Hopkin, 2005; ISO, 2014). So far, only a few authors reported the toxicity of Sb to soil-dwelling springtails without sufficient consideration of above-mentioned factors (Kuperman et al., 2006; An et al., 2013; Lin et al., 2019). This study was thus: (1) to develop the prediction models of exogenous Sb toxicity to Folsomia candida basing main soil properties. A range of representative Chinese soils with varied properties were used. (2) to further investigate the influence of long aging time on Sb toxicity and the relationship between soil properties and aging effects. The results supplemented toxicity data of Sb and provided insights into the toxicity evaluation of Sb-contaminated soil. 2. Materials and methods 2.1. Test soils, chemicals and organisms According to the major soil types and the distributions of soil pH and organic matter content of agricultural soils in China, the test soils were collected from ten provinces and municipalities: black soil (silty clay loam) from Hailun city in Heilongjiang province (HLJ), red soil (silty clay loam) from Jiangmen city in Guangdong

province (GD), paddy soil (silty clay loam) from Qiyang city in Hunan province (HUN), yellow-brown soil (silty clay loam) from Xuancheng city in Anhui province (AH), red soil (clay) from Yingtan city in Jiangxi province (JX), yellowish-red soil (loam) from Dali city in Yunnan province (YN), purple soil (loam) from Chongqing city (CQ), loessal soil (silt loam) from Xian city in Shanxi province (SX), fluvoaquic soil (silt loam) from Zhengzhou city in Henan province (HN), gray desert soil (loam) from Urumchi city in Xinjiang province (XJ). Every type of soil sample were collected from 0 to 20 cm deep. After air-dried, homogenized, ground and passed through a 2 mm mesh sieve, soil samples were stored in plastic bags at room temperature until the use. The soil physiochemical properties were determined according to the methods proposed by Lu (1999). Specifically speaking, the soil pH were measured using a microelectrode pH (soil-to-water ratio of 1:2.5). The organic matter (OM) content was determined using the potassium dichromate volumetric method. The EDTA-ammonium salt method was used to measure cation exchange capacity (CEC). The calcium carbonate (CaCO3) was measured using neutralization titrimetric method. The soil clay content (particle size <0.002 mm) was measured by pipette method. The amorphous Fe oxides (FeOX), Al oxides (AlOX), and Mn oxides (MnOX) were extracted by acidified ammonium oxalate buffer. The free soil Fe oxides (FeDCB), Al oxides (AlDCB), and Mn oxides (MnDCB) were extracted by dithioniteecitrateebicarbonate (DCB). The total Fe, Al, Mn contents were determined by inductively coupled plasma atomic emission spectrometer (ICPAES, RIS Advantage) after digestion of soils with a 3:1:1 (v/v/v) mixture of HF, HClO4, and HNO3 at 180  C for 45 min in a closed microwave digestion device (Fan et al., 2013). All analyses were performed in triplicate. All chemical reagents used in the analysis of soil properties were of analytical reagent grade. The main soil physiochemical properties were summarized in Table 1. Highly soluble Sb(III) salt (antimony potassium tartrate, KSbC4H4O7$1/2H2O) and sparingly soluble Sb(V) salt (potassium pyroantimonate, KSb(OH)6) were used as test chemicals, both which were used in the most previous studies on Sb toxicity. These two chemicals were of analytical grade with the purity of 98% and obtained from Beijing Chemical Co. (Beijing, China). The springtail Folsomia candida were originally obtained from Chinese Academy of Sciences and had been cultured in our laboratory for nearly four years. According to ISO 11267 (2014), the animals were cultivated in culture substrate (a moist mixture of plaster and charcoal) at constant 20 ± 1  C and a 16 h light/8 h dark regime. A small amounts of dry baker’s yeast were supplemented two times weekly, and distilled water was weekly added to maintain the moisture content of culture substrate. The adults were transferred to new culture substrate for laying eggs to obtain synchronized juveniles of 10e12 d old which will be used in the toxicity tests. 2.2. Soil spiking and aging Ten test soils were artificially spiked with Sb(III) aqueous solutions or Sb(V) suspensions. Only deionized water was added to the controls. The nominal total Sb(III) and Sb(V) concentrations were set up as 400e6400 mg kg1 and 800e12,800 mg kg1, respectively (The Sb pollution in the real soil environment soils usually show variable concentration and the high concentration of Sb often occur in the soils near mining and shooting ranges, so we investigated the collembolan assay using a range of Sb concentration including high Sb concentration). The soil humidity was adjusted to 55%e60% of the WHC by adding appropriate volumes of distilled water and then the soils were thoroughly mixed. Both the freshly Sb(III)- and Sb(V)-spiked soils were stored in open plastic bags and then aged for 365 d in the dark at 25  C in climate-controlled greenhouse until

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Table 1 The tested soil physicochemical properties. Items

HLJ

GD

HUN

AH

JX

YN

CQ

SX

HN

XJ

pH OMa (g kg1) CaCO3 (g kg1) Clayb (%) CECc (cmol kg1) Fetotald (g kg1) Mntotald (g kg1) Altotald (g kg1) FeOXe (g kg1) MnOXe (g kg1) AlOXe (g kg1) FeDCBf (g kg1) MnDCBf(g kg1) AlDCBf(g kg1)

5.78 54.71 0.44 32.8 33.4 28.96 0.62 17.38 4.35 0.59 1.92 7.42 0.52 1.67

4.94 46.10 0.80 39.9 18.0 32.92 0.16 12.37 7.87 0.12 1.03 20.52 0.03 1.37

6.60 27.61 0.51 33.7 13.9 33.48 0.49 9.57 5.46 0.54 1.29 23.34 0.49 2.61

5.29 21.88 0.73 29.6 14.4 33.38 0.42 13.05 1.98 0.44 1.30 23.93 0.43 2.86

4.91 10.11 0.79 38.6 12.1 37.4 0.26 9.96 3.45 0.14 2.12 23.04 0.16 3.77

6.93 26.52 5.77 24.7 16.1 31.37 0.2 12.32 5.99 0.19 0.78 15.18 0.09 1.03

6.40 17.00 0.76 17.9 24.3 33.45 0.45 29.73 2.95 0.27 0.57 9.09 0.17 0.40

8.25 14.60 73.88 15.1 10.5 27.71 0.69 20.43 0.52 0.35 0.70 6.58 0.37 0.52

8.35 12.18 84.86 11.1 10.1 23.61 0.57 19.16 0.93 0.28 0.47 7.38 0.31 0.52

8.17 14.22 35.69 19.0 10.6 28.92 0.82 44.78 0.59 0.38 0.62 4.03 0.34 0.58

Data represent the mean values. HLJ: Heilongjiang soil; GD: Guangdong soil; HUN: Hunan soil; AH: Anhui soil; JX: Jiangxi soil; YN: Yunnan soil; CQ: Chongqing soil; SX: Shanxi soil; HN: Henan soil; XJ: Xinjiang soil. a Organic matter. b < 0.002 mm. c Cation exchange capacity. d Total soil Fe, Mn and Al concentrations. e Amorphous Fe, Mn, and Al oxides. f Free Fe, Mn, and Al oxides.

the toxicity test. Given that Sb(III) is much more toxic than Sb(V), the Sb(III)-spiked soils were also aged for 30 and 150 d to better illustrate the influences of long aging time on toxicity effect. During the aging process, the soil moisture was maintained at 55%e60% of the WHC by weighing the soils and supplementing water every 2e3 d. 2.3. Toxicity tests According to ISO 11267 test protocol (ISO, 2014), ten juveniles of 10e12 d old were transferred to each glass jar containing 30 g humid Sb-treated and control soil (four replicates of each concentration treatment). The springtails were exposed to soils for 28 d under the conditions of 20 ± 1  C and a 16 h light/8 h dark regime. During exposure, the springtails were fed a few grains of dried yeast and deionized water was added to maintain the desired moisture content once each week. This process also involved aerating the soil in each jar. At the end of exposure, the jars were emptied into a 200 mL glass beaker, then 150 mL tap water and a few drops of black ink were added. The soil mixture was gently stirred and the water surface were photographed. The number of springtails in the pictures were then counted using the image J software. 2.4. Analysis of soil Sb After the aging, Sb-treated soil samples were collected, air-dried and ground to pass through a 0.149 mm sieve for following analysis of soil Sb. The measured total concentration instead of nominal total concentration was generally used to calculate metal toxicity threshold. Thus, a 100 mg dry soil sample was digested in a 3:1:1 (v/v/v) mixture of HF, HClO4, and HNO3 at 180  C for 45 min in a closed microwave digestion device (Fan et al., 2013). The total Sb concentration in each digest was determined by inductively coupled plasma atomic emission spectrometer (ICP-AES, RIS Advantage). Soil certified reference material (GBW07401, National Research Center for Certified Reference Materials, China) were used to ensure the precision of the analytical process. Measured total Sb concentrations in the reference material were within 10% of the certified concentrations. According to Liu et al. (2015), the water-soluble fraction of Sb in

each soil sample was extracted by adding 20 mL ultrapure water to 2 g dry soil sample and shaking the mixture for 2 h (200 rpm, 25  C). The mixture was centrifuged at 1000g for 10 min, and then passed through 0.45 mm filter membrane. Sb concentration in the extract was determined by inductively coupled plasma mass spectrometer (ICP-MS, Agilent 7500). The redox state of Sb in each Sb(III)-treated soil sample were determined according to Qin et al. (2009) and Yang et al. (2015). A 0.3 g dry soil sample was mixed with 3 mL of 0.1 mol L1 aqueous citric acid in a centrifuge tube. The mixture was shaken at 60  C and 200 rpm for 30 min and then centrifuged at 1000g for 10 min. After being passed through 0.45 mm filter membrane, the Sb(III) and Sb(V) concentrations in each extract was then determined using hydride generation atomic fluorescence spectrometer (HG-AFS, Titan Instrument Co. Ltd., Beijing, China). 2.5. Data analysis The EC50 (concentrations causing 50% inhibition of reproduction) and LC50 (concentrations causing 50% mortality) values are calculated in this study because these values are more robust and less influenced by experimental errors than the no-observable effect concentration (NOEC), EC10 and LC10 values (Oorts et al., 2007). The EC50 for the reproduction and LC50 for the survival and their 95% confidence intervals were calculated by the logistic sigmoidal nonlinear model (Van Ewijk and Hoekstra, 1993) using SigmaPlot 12.5 software:

 y ¼ k ð1 þ ðX=X0 ÞÞs where y is the number of juveniles or survived adults, x is the measured total Sb concentrations in soil (mg kg1), k is the number of juveniles or survived adults in the controls, s is the slope parameter, x0 is the EC50 or LC50 value. Pearson’s correlation analysis was used to determine the relationships between soil properties and toxicity values of Sb, and the relationships between soil properties and aging effects. The simple and multiple regression analysis were used to develop the prediction models of Sb toxicity values and aging effects basing main soil properties. One way analysis of variance (ANOVA) was

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used to evaluate the significant differences between control and each Sb concentration, and p < 0.05 was considered to be statistically significant. These analyzes were done using the SPSS 22.0 software. 3. Results 3.1. Toxicity of Sb(V) to springtails in soils After exposure of 28 d, the survived adults and the minimal reproduction numbers of springtails in the controls met the requirements in the ISO guidelines (ISO, 2014), and the coefficient of variation was less than 35% for the replicate control tests. In ten Sb(V)-treated soils aged for 365 d, the obvious decreases in survival (Fig. 1a) and reproduction (Fig. 1b) with increasing total Sb concentrations were not observed, indicating that the LC50 values for the survival and EC50 values for the reproduction were higher than the nominal highest concentration of 12,800 mg kg1. Consequently, the prediction model of Sb(V) toxicity could not be developed. 3.2. Toxicity of Sb to springtails in Sb(III)-treated soil 3.2.1. Differences in toxicity in ten soils aged for different time In ten Sb(III)-treated soils aged only for 30 d, as a whole, the survival and reproduction of springtails were reduced in a dosedependent manner in each soil. However, the response of springtails to increasing total Sb varied with different soils (Fig. 2a, d), and the calculated LC50 and EC50 values basing on the measured total Sb concentration ranged from 1288 to 3219 mg kg1 and from 683 to 1829 mg kg1, respectively (Table 2). When the soils were aged for 150 d (Fig. 2b, e), the Sb only caused significant decrease (p˂0.05) in survival at the nominal highest concentration of 6400 mg kg1 in JX, AH, YN and XJ soil compared to the controls, however, the LC50 values were higher than the highest concentration. Though the obvious increase of reproduction, the inhibition were still significantly (p˂0.05) at higher concentration in all soils, and the calculated EC50 values ranged from 1532 to 5596 mg kg1 (Table 2). When the soils were aged for 365 d (Fig. 2c, f), surprisingly, Sb was not toxic to both the survival and reproduction at any concentrations in each soil except in JX soil with the EC50 values of 2180 mg kg1.

3.2.2. Relationship between toxicity values and soil properties As shown in Table 3, the correlation analysis showed that in soils aged for 30 d, both the LC50 values for the survival and EC50 values for the reproduction positively correlated with soil OM (r ¼ 0.852 and 0.907, respectively, p˂0.01), indicating the decreasing toxicity with increasing soil OM. Additionally, the LC50 values positively correlated with FeOX (r ¼ 0.658, p˂0.05) and EC50 values positively correlated with CEC (r ¼ 0.655, p˂0.05). In soils aged for 150 d, the EC50 values positively correlated with pH (r ¼ 0.706, p˂0.05) and CaCO3 content (r ¼ 0.656, p˂0.05). Unfortunately, the relationship between toxicity values in soils aged for 365 d and soil properties could not be determined due to the unavailable toxicity values. Relationships between the log-transformed toxicity values and main soil properties were further analyzed using the regression analysis. As shown in Table 4, the results indicated that in soil aged for 30 d, the regression models with CEC as single factor explained 34% of the variance in EC50 values. The soil OM was the most important single factor in predicting soil Sb toxicity, which alone explained 72% of the variance in LC50 values and 78% of the variance in EC50 values. In soil aged for 150 d, the pH was the most important single factor in predicting EC50 values and could explain 33% of the toxicity variance, however, incorporating OM content into this regression models gave a significant improvement in predictability and could well explain 67% of the toxicity variance. As shown in Fig. S1, the measured values were found to well correlate (r2 ¼ 0.65e0.85) with the predicted toxicity values which were calculated using the regression models based on pH and OM. 3.2.3. Relationship between aging effects and soil properties As described above, the increasing aging time caused obvious increase of the LC50 values for the survival and EC50 values for the reproduction (Table 2), indicating significantly decreasing soil Sb toxicity. The aging effects varied from soil to soil, and the aging factor (AF) for the EC50 values in soils aged for 150 d (dividing the toxicity values in soil aged for 150 d by the values in soil aged for 30 d) ranged from the minimum value of 2.24 in acidic JX soil to 6.16 in alkaline HN soil. The correlation analysis showed that the aging factor positively correlated with soil pH (r ¼ 0.883, p < 0.01) and CaCO3 content (r ¼ 0.733, p˂0.05) (Table 3). The regression analysis further showed that the pH was the most important single factor in predicting aging factor and could alone explain 75% of the variance among ten soils (Table 4). Including the FeDCB in regression models

Fig. 1. Number of survived adults (a) and juveniles (b) of Folsomia candida after 28 d exposure to ten Sb(V)-treated soils aged for 365 d. Error bars indicate standard deviations.

X. Lin et al. / Chemosphere 241 (2020) 125100

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Fig. 2. Number of survived adults (a, b and c) and juveniles (d, e and f) of Folsomia candida after 28 d exposure to ten Sb(III)-treated soils aged for 30, 150 and 365 d. Error bars indicate standard deviations.

with soil pH offered additional improvement for predicting aging factor and could explain 85% of the variance (Table 4). 4. Discussion The toxicity values of metal vary greatly with the used evaluation endpoint of test organisms. As ecologically relevant endpoints,

the survival and reproduction of soil-dwelling Folsomia candida have been frequently used in the most studies (Fountain and Hopkin, 2005). The reproduction, considered as the most vital € nmark, function in the life cycle of living organism (Woin and Bro 1992), is a suitable indicator of metal toxicity (Crouau and Moïa, 2006; Buch et al., 2016). Consistent with previous study about the toxicity of Sb to springtails (Kuperman et al., 2006; An et al., 2013;

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Table 2 Summary of the LC50 values for the survival and EC50 values for the reproduction of Folsomia candida exposed to Sb(III)- and Sb(V)-treated soils. The values were calculated using the measured total Sb concentrations (mg kg1) after aging. Soils

EC50

LC50 30 d

Sb(III)

Sb(V)

a b

HLJ GD HUN AH JX YN CQ SX HN XJ all soils

150 d

2431 3219 2033 1772 1288 1338 1385 1491 1399 1484

(2031e2832) (3024e3416) (1544e2521) (1716e1829) (784e1792) (768e1908) (982e1788) (1463e1520) (1340e1457) (1234e1734)

a

d e e e e e e e e e

b

365 d

30 d

150 d

e e e e e e e e e e e

1829 (799e2860) 1822 (1015e2630) 885 (882e890) 824 (792e855) 683 (634e731) 817 (791e844) 878 (629e1126) 911 (347e1473) 909 (192e1623) 924 (523e1325)

4240 4521 4139 3646 1532 3633 3426 4889 5596 5268

pH 30 d-LC50 30 d-EC50 150 d-EC50 150 d-AF a

(4044e4437) (4265e4688) (3187e5092) (426e6867) (1255e1809) (2056e5209) (3021e3832) (4668e5110) (4535e6657) (4960e5576)

e e e e 2180 (2120e2240) e e e e e e

95% confidence intervals. The toxicity values were higher than the highest concentration.

Table 3 Correlation coefficients for the relationships (n ¼ 10) between soil properties and toxicity values (LC50 values for the survival and EC50 values for the reproduction) and aging factors (AF) in Sb(III)-treated soils aged for different time.

b

365 d

OM

CaCO3 b

0.706 0.883b

CEC a

0.852 0.907b a

FeOX 0.658

0.655a a

0.656 0.733a

Significant at the 0.05 level. Significant at the 0.01 level.

Table 4 The simple and multiple linear regressions of toxicity values (LC50 for the survival and EC50 for the reproduction) and aging factors (AF) in Sb(III)-treated soils basing the main soil properties. Age time

Regression equation (n ¼ 10)

adjusted-r2

p

30 d 30 d 30 d 150 d 150 d 150 d 150 d

LogLC50 ¼ 0.008OMþ3.046 LogEC50 ¼ 0.013CECþ2.790 LogEC50 ¼ 0.009OMþ2.784 LogEC50 ¼ 0.075 pH þ 3.097 LogEC50 ¼ 0.111 pH þ 0.007OMþ2.692 AF ¼ 0.930pH-1.934 AF ¼ 0.084FeDCBþ1.301pH-5.550

0.72 0.34 0.78 0.33 0.67 0.75 0.85

0.001 0.046 0.000 0.047 0.009 0.001 0.001

Lin et al., 2019), the present study showed that the reproduction was much more sensitive to soil Sb stress than that of survival by comparing the LC50 and EC50 values. Soil properties result in marked modification effects on metal toxicity to soil ecological receptors (plants, microbes and invertebrates). Relationships between the metal toxicity and soil properties have been extensively studied by developing the prediction models, which offer a practical method for determining soil-quality criteria (Oorts et al., 2006; Warne et al., 2008; Wang et al., 2018). The toxicity differences of Sb in different soils have only been reported by few authors using limited types of soils (Hammel et al., 1998; Tschan et al., 2010; Baek et al., 2014; Lin et al., 2019). In this study, the different Sb toxicity to typical soil invertebratedspringtails in ten Sb(III)-treated soils were not only associated with the differences in oxidation potentials but also sorption capacities of the test soils. Relationships between the Sb toxicity values in Sb(III)-treated soils and main soil properties indicated that the Sb toxicity significantly decreased with increasing soil pH and OM, and these two properties could well predict soil Sb toxicity. Baek et al. (2014) and Lin et al. (2019) also reported that the toxicity of Sb were lower in Sb(III)-treated soil with higher OM

content. Actually, it has been extensively believed that the pH and OM were the most important properties influencing and predicting the toxicity of other metals (Li et al., 2010, 2011; Duan et al., 2016; Wang et al., 2018; Zhang et al., 2019). In this study, the antimony potassium tartrate was used to supply Sb(III) and tartrate could chelate and ionize Sb (III) as an anionic complex. Both the unoxidized Sb(III) and oxidized Sb (V) existing as oxyanion in soil were not readily sorbed by the soil with a high pH, which was due to the high OH activity lead to strong competition between Sb and OH for binding mineral functional groups at high soil pH values (Hou et al., 2013; He et al., 2018). However, the oxidation efficiency of Sb(III) increase with increasing pH due to that OH ligands could donate electrons and increase metal basicity and reducibility (Leuz et al., 2006; Fan et al., 2014), which could explain our results to some extent given the much smaller toxicity of Sb(V). Also, the decreased Sb toxicity with increasing pH was also because the solubility and bioavailability of Sb in alkaline soils with high CaCO3 contents were also controlled by the Ca-antimonate precipitation (Ca [Sb(OH)6]2), especially at the higher Sb concentration, on the soil surface (Oorts et al., 2008; Wilson et al., 2010), which could be further supported by the discontinuous increase of water soluble Sb with increasing total Sb in alkaline SX, HN and XJ soils, and the water soluble Sb in these three soils were much lower than those in the other soils at the highest Sb concentration (Table S1). The Caantimonate precipitation could also explain why the toxicity values positively correlated with CaCO3 contents in this study. Soil OM with different functional groups such as carboxyl, phenol, and hydroxyl has a high capacity for complexing and tightly binding Sb(III) and Sb(V) (Wilson et al., 2010; Pierart et al., 2015; Cai et al., 2016). Additionally, the Soil OM plays an important role in promoting the oxidation of Sb(III) due to the quinone and/or disulfide functional groups are good electron acceptors and could take part in the oxidation of Sb(III) (Buschmann and Sigg, 2004). Thus the high OM facilitates the decrease of Sb toxicity in soil. Additionally, the CEC positively correlated with Sb toxicity values and could partly account for the variations in Sb toxicity. In several previous studies, the CEC was also found to be the important predictor of metal toxicity (Criel et al., 2008; Li et al., 2011; Duan et al., 2016). These are due to the CEC is regarded as an integrated measurement for the amount of sorption sites and incorporates the metal oxyhydroxides, organic matter and pH (Janssen et al., 1997; Criel et al., 2008). The freshly spiked soils cause sudden toxicity stresses to soil organisms and do not represent the realistic exposure scenarios of soil organisms to field-contaminated soils (Smolders et al., 2009, 2015). Hammel et al. (1998) and Lin et al. (2019) reported that after

X. Lin et al. / Chemosphere 241 (2020) 125100 Table 5 Mean Sb(V) percentages (%) in citric acid extracts of the ten Sb(III)-treated soils aged for different time (30, 150 and 365 d). Soils

HLJ GD HUN AH JX YN CQ SX HN XJ

400 mg kg1

1600 mg kg1

6400 mg kg1

30 d

150 d

365 d

30 d

150 d

365 d

30 d

150 d

365 d

99.2 85.3 99.3 99.1 68.0 98.3 99.5 99.3 99.6 99.7

99.8 99.2 99.6 99.6 98.1 99.5 100.0 99.8 99.9 100.0

99.7 99.6 99.7 99.7 99.3 99.6 99.9 99.9 99.8 100.0

97.6 78.4 76.3 63.6 27.5 61.6 91.3 75.8 69.2 74.8

100.0 98.3 96.9 99.1 60.6 97.4 99.4 98.4 97.7 97.4

100.0 99.2 99.3 99.5 95.2 98.9 99.7 99.4 98.9 99.2

69.2 43.9 41.9 26.2 11.3 33.1 38.5 15.3 10.3 16.0

94.3 77.6 78.2 51.6 16.6 50.6 72.9 74.7 85.0 70.4

98.1 99.3 94.0 80.5 32.5 84.7 90.7 88.9 98.1 91.9

the soil was aged for 180 d, the EC50 values in Sb(III)-treated soil (added into soil as KSbO-tartrate) were higher than the highest test concentration of 1000 and 3200 mg kg1, respectively. Similarly, in this study the toxicity of Sb significantly decreased with increasing aging time in Sb(III)-treated soils and Sb did not exhibit toxicity to the survival and reproduction even at the highest concentration of 6400 mg kg1 in the most soils when the aging time was 365 d, which was similar with the cases in Sb(V)-treated soils. Thus, it is clear that Sb toxicity will be greatly overestimated when using soluble Sb(III) salts as test chemicals without sufficient aging time. Also, the developed prediction model basing the toxicity values in soils aged for 30 and 150 d obviously could not be used to predict Sb toxicity in soil aged for 365 d. After the long aging process, the decreased toxicity of Sb in Sb(V)-treated soil mainly resulted from the reduced available fraction according to general cognition about aging effect (Smolders et al., 2009). However, referring to Cai et al. (2016) who investigated the kinetic process of Sb(III) oxidation and sorption, in Sb(III)-treated soils the aging process is multi-step including: (1) the adsorption of Sb(III) onto soil surface sites, (2) the oxidation of Sb(III) to Sb(V) at soil surface, (3) the release of generated Sb(V) from the surface into soil pore-water, and (4) the decrease of Sb(V) available fraction due to adsorption, diffusion, precipitation and so on. Among these processes, there is no doubt that the oxidation of Sb(III) to Sb(V) is especially important to lower the Sb(III) toxicity given the Sb(III) is much more toxic than Sb(V). The oxidation of Sb(III) in ten soils were observed in this study (Table 5). The results indicated that the percentages of Sb(V) gradually increased with aging time and almost all the Sb(III) were oxidized to Sb(V) in test soils even at the highest concentration of 6400 mg kg1 except in JX soil after the aging time was 365 d, which was consistent with the largest Sb toxicity in this soil. Additionally, the aging effects on Sb(III) differed among test soils. The aging factor significantly increased with soil pH and CaCO3, and the former could well explain the variance in aging factor. This was due to, as mentioned above, the higher pH contributed to the oxidation of Sb(III) (Leuz et al., 2006; Fan et al., 2014) and the produce of Ca-antimonate precipitation in alkaline soils (Oorts et al., 2008; Wilson et al., 2010), which again indicated that the vital role played by soil pH in influencing Sb(III) toxicity. In this study, the combination of pH and FeDCB could better explain the variance in aging factor better, which was due to Fe oxides facilitated Sb partitioning and enhanced the oxidation of Sb(III) (Wilson et al., 2010; Cai et al., 2016). These results indicated that the importance of relating the effects of aging on Sb toxicity to soil properties. 5. Conclusions The toxicity differences of Sb to soil invertebrates Folsomia

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candida in different soils were studied. In the Sb(III)-treated ten soils aged only for 30 d, the LC50 for the survival and EC50 for the reproduction were 1288e3219 mg kg1 and 683e1829 mg kg1, respectively. The EC50 values were 1532e5596 mg kg1 in the Sb(III)-treated soils aged for 150 d. The prediction models using main soil properties (including pH, CEC, and OM) were firstly developed basing the above-mentioned toxicity values in Sb(III)treated soils, which can be used to establish soil-specific guidance on soil Sb ecological criteria. Aging metal-spiked soils for long time is necessary to scientifically evaluate metal toxicity, and the effects of long aging time on Sb toxicity was clearly highlighted in this study. Though the Sb(III) was generally more toxic than Sb(V), however, it should be noted that the toxicity differences between Sb(III) and Sb(V) obviously decreased after aging for long time due to the occurrence of time-dependent oxidation reaction of Sb(III). The results in this study could provide the basis for assessing the long-term risks of realistic Sb-contaminated aerobic soil where the Sb(V) generally predominates. Declaration of competing interest The authors have no conflict of interest to declare regarding this article. Acknowledgments We thank the anonymous reviewers for providing very significant comments and improving our manuscript. This study was funded by the “Research on Migration/Transformation and Safety Threshold of Heavy Metals in Farmland Systems” (2016YFD0800400), National Key Research and Development Program of China. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.125100. References An, Y.J., Kim, S.W., Lee, W.M., 2013. The collembola Lobella sokamensis, juvenile as a new soil quality indicator of heavy metal pollution. Ecol. Indicat. 27, 56e60. Baek, Y.W., Lee, W.M., Jeong, S.W., An, Y.J., 2014. Ecological effects of soil antimony on the crop plant growth and earthworm activity. Environ. Earth. Sci. 71, 895e900. Buch, A.C., Niemeyer, J.C., Correia, M.E.F., Silva-Filho, E.V., 2016. Toxicity of mercury to Folsomia candida and Proisotoma minuta (Collembola: isotomidae) in tropical soils: baseline for ecological risk assessment. Ecotoxicol. Environ. Saf. 127, 22e29. Buschmann, J., Sigg, L., 2004. Antimony(III) binding to humic substances: influence of pH and type of humic acid. Environ. Sci. Technol. 38, 4535e4541. Cai, Y.B., Mi, Y.T., Zhang, H., 2016. Kinetic modeling of antimony (III) oxidation and sorption in soils. J. Hazard Mater. 316, 102e109. Chai, L.Y., Mubarak, H., Yang, Z.H., Yong, W., Tang, C.J., Mirza, N., 2016. Growth, photosynthesis, and defense mechanism of antimony (Sb)-contaminated Boehmeria nivea L. Environ. Sci. Pollut. Res. 23, 7470e7481. Coelho, C., Branco, R., Natal-Da-Luz, T., Sousa, J.P., Morais, P.V., 2015. Evaluation of bacterial biosensors to determine chromate bioavailability and to assess toxicity of soils. Chemosphere 128, 62e69. Criel, P., Lock, K., Eeckhout, H.V., Oorts, K., Smolders, E., Janssen, C.R., 2008. Influence of soil properties on copper toxicity for two soil invertebrates. Environ. Toxicol. Chem. 27, 1748e1755. Crouau, Y., Moïa, C., 2006. The relative sensitivity of growth and reproduction in the springtail, Folsomia candida, exposed to xenobiotics in the laboratory: an indicator of soil toxicity. Ecotoxicol. Environ. Saf. 64, 115e121. Duan, X.W., Xu, M., Zhou, Y.Y., Yan, Z.G., Du, Y.L., Zhang, L., Zhang, C.Y., Bai, L.P., Nie, J., Chen, G.K., Li, F.S., 2016. Effects of soil properties on copper toxicity to earthworm Eisenia fetida in 15 Chinese soils. Chemosphere 145, 185e192. Duester, L., Geest, H.G.V.D., Moelleken, S., Hirner, A.V., Kueppers, K., 2011. Comparative phytotoxicity of methylated and inorganic arsenic- and antimony species to Lemna minor, Wolffia arrhiza and Selenastrum capricornutum. Microchem. J. 97, 30e37.

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