Adsorption isotherms, degradation kinetics, and leaching behaviors of cyanogen and hydrogen cyanide in eight texturally different agricultural soils from China

Ecotoxicology and Environmental Safety 185 (2019) 109704

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Adsorption isotherms, degradation kinetics, and leaching behaviors of cyanogen and hydrogen cyanide in eight texturally different agricultural soils from China

T

Wenwen Zhoua, Yue Zhangb, Wei Lib, Haoran Jiab, Huajun Huangb, Baotong Lib,∗ a b

College of Food Science and Engineering, Key Laboratory of Crop Physiology, Ecology and Genetic Breeding, Jiangxi Agricultural University, Nanchang 330045, China College of Land Resources and Environment, Jiangxi Agricultural University, Nanchang, 330045, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Cyanogen Hydrogen cyanide Agricultural soil Adsorption Degradation Leaching

Cyanogen (C2N2) is a new and effective alternative soil fumigant to methyl bromide. The effects of soil properties on the fate of C2N2 and its degradation products, including hydrogen cyanide (HCN), are not fully understood. The objectives of this study were to determine the adsorption kinetics, adsorption isotherms, and degradation kinetics of C2N2 and HCN in texturally different soils and evaluate their leaching potentials using soil columns. Eight agricultural soils were collected throughout China: Luvisols (Hebei Province), Phaeozems (Heilongjiang Province), Gleysols (Sichuan Province), Anthrosols (Zhejiang Province), Ferralsols (Jiangxi Province), Lixisols (Hubei Province), Alisols (Shandong Province), and Plinthosols (Hainan Province). The adsorptions of C2N2 and HCN in C2N2-fumigated soils were positively correlated with organic matter and clay contents. For a C2N2 dose of 100 mg kg−1, the adsorptions of C2N2 and HCN were highest in Phaeozems and lowest in Gleysols according to their adsorption coefficients (15.744 and 3.119, respectively). No significant difference in the half-life of C2N2 and HCN was observed between sterilized and unsterilized soils, indicating that abiotic degradation was predominant in the degradation of C2N2 and HCN. After leaching, the residual C2N2, HCN, NH4+–N, and NO3−–N concentrations in C2N2-fumigated Phaeozems were highest within 15 cm of the soil surface (30, 20, 19.68, and 10.41 mg kg−1 soil, respectively). The results indicate that C2N2 and HCN have short lifetimes and low leaching potentials in agricultural soils, even under heavy rainfall conditions. The findings demonstrate that C2N2 and HCN resulting from fumigation will not accumulate in the soil and are not likely to contaminate groundwater.

1. Introduction Soil fumigation is extensively used to control pathogenic bacteria, fungi, nematodes, and insects in soil (Li et al., 2017b). Cyanogen (C2N2) and its degradation product, hydrogen cyanide (HCN), are new fumigants that have been proposed as alternatives to methyl bromide (Armstrong and Najar-Rodriguez, 2019; Hnatek et al., 2019; Stevens et al., 2019). A significant advantage of C2N2 as a fumigant is that, unlike methyl bromide or sulfuryl fluoride, C2N2 is not a greenhouse gas and does not deplete the atmospheric ozone (Brierley et al., 2018; Hall et al., 2017; Minini et al., 2017; Zakladnoy, 2018). The chemistry of C2N2 is well understood; it is a colorless gas with a boiling point of −21.2 °C. At low pH, C2N2 reacts to form derivatives of formic acid and HCN (Ren, 2002). In the past, fumigants were used without much consideration of their environmental impacts. However, in recent years, many studies have shown that fumigants can be detrimental to

beneficial soil microorganisms (Li et al., 2017a, 2017b; Thalavaisundaram et al., 2017). Few studies have evaluated the effects of soil properties on the adsorption of fumigants and particularly the environmental consequences of changes in these properties. Most previous studies on C2N2 as a soil fumigant have focused on its efficacy for removing soil-borne pathogenic microorganisms along with its distribution and emission characteristics (Caddick, 2004; Mattner et al., 2004). Waterford et al. (2011) demonstrated that C2N2 has great potential to replace methyl bromide for a range of targets, including insects, nematodes, pathogens, and weeds. O'Brien et al. (1999) tested the toxicity of C2N2 to the first instar larvae of G. leucolama using different types of soil. They found that for a C2N2 dose of 11.5 mg L−1, the mortality of G. larvae was 100% and the average headspace concentration of C2N2 was 1.11 mg L−1 in Pemberton loam at 23 h after application; in stark contrast, the mortality of G. larvae was zero and no fumigant was detected in peat soil at the same time point. Another

∗ Corresponding author. College of Land Resources and Environment, Jiangxi Agricultural University, 1225 Zhimin Avenue, Changbei Economic Development Zone, Nanchang, Jiangxi Province, 330045, China. E-mail address: [email protected] (B. Li).

https://doi.org/10.1016/j.ecoenv.2019.109704 Received 30 May 2019; Received in revised form 13 August 2019; Accepted 19 September 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.

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2.2. Soil fumigation and adsorption kinetics experiments

study found that both C2N2 and HCN residues in the soil decreased to indistinguishable levels within 48 h after C2N2 application (Mattner et al., 2003; Mattner et al., 2006). However, the effects of soil properties on the adsorption and degradation behaviors of C2N2 and HCN have rarely been reported. Research on the adsorption and degradation kinetics of soil fumigants is limited in China. For example, chloropicrin (CP), 1,3-dichloropropene (1,3-D), and dimethyl disulfide (DMDS) have been reported to promote nitrogen mineralization, thereby increasing ammonium nitrogen (NH4+–N) concentrations and N2O emissions of the soil (Yan et al., 2013). DMDS has a relatively long half-life (2.2–5.0 d) when applied to a range of soil types (Qin et al., 2016). DMDS degradation occurs in the soil mainly via biotic pathways, and the degradation rate is related to soil pH conditions (Han et al., 2017). To date, studies on the adsorption and leaching behaviors of C2N2 and HCN have not been reported in soils under subtropical conditions across China. To explore the effects of soil properties on the potential of fumigants to leach into groundwater, this study aimed to: 1) investigate the adsorption kinetics and isotherms of C2N2 and HCN in eight different soils after fumigation with C2N2; 2) determine the effects of soil properties on fumigant degradation and distribution in C2N2-fumigated soils; and 3) estimate the leaching potentials of C2N2 and HCN in C2N2-fumigated soils.

Potassium cyanide (KCN) was obtained from Foshan Chemical Co., Ltd. (Foshan, Guangdong, China). Copper sulfate (CuSO4) and 98% sulfuric acid (H2SO4) were purchased from Xishao Science Factory (Shantou, Guangdong, China). All chemicals were of analytical reagent grade. Gaseous C2N2 was prepared via the reaction of KCN and H2SO4 in the laboratory (Brotherton and Lynn, 1959). Soils were distributed into eight groups of 1-L Erlenmeyer flasks and sealed with septum inlets. Each group comprised three replicates of one soil. Each flask contained approximately 800 g of soil (60% load factor), except for S1 (Luvisols, ~1000 g) and S2 (Phaeozems, ~1000 g). C2N2 was applied to the soils at a typical field application rate (150 mg kg−1) via injection into the sealed flasks with an airtight syringe (Eisler, 1991; Ren, 2002). A volume of air equivalent to the dosage volume of C2N2 was then removed from the fumigation chamber to avoid changes in pressure. The flasks with fumigated soil were incubated for 48 h in the dark at 20 °C (O'Brien et al., 1999). At specified time points (0, 0.1, 0.2, 0.5, 1, 2, 4, 12, 24, and 48 h after application), 80 μL of air was withdrawn from the flasks through the sampling port and analyzed by gas chromatography-mass spectrometry (GC-MS). 2.3. Adsorption isotherm experiments Adsorption isotherm experiments were conducted according to the batch equilibrium method (Okada et al., 2016). To approach natural subsurface conditions, the experiments were carried out at a constant temperature of 25 °C ± 1 °C. Briefly, 1 mg of C2N2 was injected into 150-mL vials containing 10 g of soil, which was then shaken for 24 h as a pre-equilibration step. Different initial concentrations of C2N2 (C0) were added: 0.05, 0.1, 0.2, 0.5, 1, 2, and 5 mg L−1. Samples were shaken for another 24 h. The fumigant concentration in the headspace gas (80 μL) was then analyzed by GC-MS. After 96 h of equilibration, the concentrations of gas-phase C2N2 and HCN in the vials were measured. For each experiment, control vials containing unfumigated soil and blank vials containing no soil were also analyzed.

2. Materials and methods 2.1. Experimental soils Eight agricultural soils were collected from different regions across China: Shijiazhuang (Hebei Province, S1), Harbin (Heilongjiang Province, S2), Chengdu (Sichuan Province, S3), Xiangshan (Zhejiang Province, S4), Nanchang (Jiangxi Province, S5), Yichang (Hubei Province, S6), Weifang (Shandong Province, S7), and Haikou (Hainan Province, S8). In each region, surface soils were collected from a depth of 0–30 cm, air-dried, gently ground, and passed through a 0.25-mm mesh sieve. The eight soils (S1 to S8) were classified as Luvisols, Phaeozems, Gleysols, Anthrosols, Ferralsols, Lixisols, Alisols, and Plinthosols, respectively (Table 1), using the World Reference Base (WRB) for Soil Resources (L’Huillier et al., 1998). Soil texture was determined using the pipette method (Gee and Bauder, 1986). Soil organic matter (OM) content was determined using the dichromate digestion method (Nelson and Sommers, 1985). Soil bulk density was determined by the cutting ring method (Blake and Hartge, 1986). Soil pH was measured in a soil: H2O (1:2.5, w/v) solution. Nitrate nitrogen (NO3−–N) and NH4+–N concentrations were analyzed using the sodium salicylate method (Hofer, 2003) and cadmium reduction (Knepel, 2003), respectively. The basic physicochemical properties of the eight soils were diverse and typical (Table 1).

2.4. Degradation and leaching experiments Degradation experiments were conducted using sterilized and unsterilized soils. Sterilized soil samples were prepared by autoclaving twice at 120 °C for 30 min immediately before start of the experiment, and then handled aseptically to maintain sterility as long as possible during the experiment (Asma et al., 2019). The equipment and methods used to monitor C2N2 and HCN degradation in the C2N2-fumigated soils followed those developed previously (Wang et al., 2018; Waterford et al., 2011). Column experiments were conducted in the laboratory to investigate C2N2 and HCN leaching from the tested soils. The eight soils, loosely packed, were separately weighed and transferred into 980-mL

Table 1 Basic physicochemical properties of the selected soils. Soil sample

Location

Latitude/

Soil type

Soil texture

longitude S1 S2 S3 S4 S5 S6 S7 S8

Hebei Heilongjiang Sichuan Zhejiang Jiangxi Hubei Shandong Hainan

39°45′ N/117°32′ E 41°36′ N/127°53′ E 30°56′N/105°51′ E 29°14′N/121°48′ E 28°46′ N/115°36′ E 30°46′N/111°19′ E 35°06′ N/118°21′ E 19°32′N/110°10′ E

Luvisols Phaeozems Gleysols Anthrosols Ferralsols Lixisols Alisols Plinthosols

Silt loam Sandy loam Silt loam Loam Sandy loam Sandy loam Sandy loam Silt loam

Claya

BDa

(%)

(g·cm−3)

5.66 F 14.45 A 1.03 H 6.57 E 11.08 B 8.29 D 9.90 C 2.70 G

1.417 B 1.445 A 1.365 C 1.412 B 1.364 C 1.172 F 1.344 D 1.318 E

pHa

6.58 D 6.38 E 8.48 A 7.85 B 5.21 G 6.78 C 5.70 F 7.79 B

OMa

CECa

NH4+–Na

NO3−–Na

(%)

(cmol·kg−1)

(mg·L−1)

(mg·L−1)

0.48 E 4.64 A 0.17 G 1.66 B 0.35 F 1.20 C 0.22 G 0.88 D

26.15 B 30.36 A 25.40 C 12.90 E 15.83 D 12.19 F 11.99 F 11.00 G

7.07 C 9.68 A 1.87 G 5.68 D 0.80 H 2.77 E 2.32 F 7.43 B

0.71 E 7.24 A 0.92 E 0.73 E 3.30 D 4.10 C 5.47 B 0.62 E

BD is bulk density, OM is organic matter content, and CEC is cation exchange capacity. a Different uppercase letters indicate statistical significance at the P < 0.01 level using Duncan's multiple range test. 2

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respectively.

PVC columns (5 cm diameter × 50 cm height) equipped with gas sampling ports on the wall of the column. Eight sampling ports were positioned at 0, 5, 15, 20, 25, 30, 35, and 45 cm from the bottom of the column. Before the start of the experiment, saturated hydraulic conductivity (Ks) was determined using the constant-head method (Kanwar et al., 1989), and the soil was then oven-dried at 105 °C before weighing for soil bulk density measurement. Soil total porosity was calculated using soil particle density and bulk density (ASTM, 2010). C2N2 (100 mg L−1) was injected through a single injection port (2 mm × 2 mm) located 20 cm from the top of the column (Mattner et al., 2003). A flux chamber was sealed onto the top of the soil column to collect any C2N2 or HCN gas emitted from the soil. The experiments were conducted in a room held at 25 °C ± 1 °C. The S2 soil (Phaeozems) with high OM content was used to monitor the vertical distribution of C2N2 and HCN in the soil column. After C2N2 injection (100 mg L−1), 80 μL of gas was removed from the sampling ports at 0.5, 1, 2, 4, 12, 24, 48, and 96 h using a gastight syringe (Agilent Technologies, Santa Clara, USA). The gas samples were immediately injected into the gas chromatograph for analysis. To test the leaching behaviors of C2N2 and HCN in different types of soil, the soil columns were leached with 2 L of CaCl2 solution (21 mL h−1) after 100 mg L−1 of C2N2 injection. The leaching process lasted for 4 d when the concentrations of C2N2 and HCN in the leachate became undetectable. After leaching, the columns remained in the fume hood for another 4 d. To determine the concentrations of C2N2, HCN, NH4+–N, and NO3−–N residues in the soils, 10-g soil samples were taken from depths of 0–45 cm in the column at 5-cm intervals and transferred into 100-mL vials with clear headspace. The vials were sealed at the end of each experimental run with Teflon-faced butyl rubber septa and aluminum crimps. The soil samples were stored at −80 °C before extraction and analysis using previously developed methods (Guo et al., 2004; Wang et al., 2018; Zhang and Wang, 2007). Each treatment was replicated three times.

2.6. Data analyses The dosages and required volumes for the fumigant concentrations were calculated from Eq. (1) calibrated to the laboratory temperature and pressure (Ren et al., 2011):

T ⎛ 1.7 × 10 4 × C × V ⎞ ⎞ Vf = ⎛1 − 273 ⎠ ⎝ P×M×N ⎝ ⎠ ⎜

⎟

(1)

where Vf is the dosage volume of fumigant (mL); T is the temperature (°C); C is the intended concentration of fumigant (mg·L−1); V is the volume of the fumigation container (L); P is the pressure (mm Hg); M is the molecular weight of the fumigant; and N is the purity of the gas (%). For each replicate, a smooth curve was fitted to the C2N2 adsorption data, from which concentrations at any time could be estimated for statistical comparison. The concentration generally showed an initial rapid drop followed by a gradual decrease; thus, the data for each replicate were fitted by an equation [Eq. (2)] describing two-phase exponential decay (Pranamornkith et al., 2014):

C(t) = Co (Aexp ( −k1 t ) + (1 − A)exp( −k2 t ))

(2)

where C0 is the initial concentration of fumigant calculated from the injected volume of gas and the load factor; k1 is the exponential decay rate of the initial drop; k2 is the exponential decay rate of the subsequent gradual decrease, and A specifies the relative weightings given to each of these processes. A, k1, and k2 were estimated separately for each replicate. The adsorption of nonionic organic compounds by sediments and soils is commonly referred to as partitioning and is often described by a single partition coefficient Kd. Nonlinear isotherms are frequently described by the Freundlich equation [Eq. (3)] (Calderón et al., 2015; Dionisio and Rath, 2016; Grathwohl, 1990):

CS = Kf Cw1/ n

2.5. Sample analysis

(3) −1

where Cs (mg·kg ) is the concentration of fumigant adsorbed on the soil at adsorption equilibrium; Kf (mg1−1/n (L)1/n mg−1) is the Freundlich adsorption coefficient; Cw (mg·L−1) is the concentration of fumigant in aqueous solution at adsorption equilibrium; and 1/n is the Freundlich exponent, a constant that describes the shape of the adsorption curve. The degradation data of the fumigants in soils were fitted to a firstorder kinetic model [Eq. (4)] that has been used frequently in similar studies (O'Brien et al., 1999; Ren, 2002):

The C2N2 and HCN concentrations in the soils were analyzed using a headspace extraction method (Seto et al., 1993). An 80-μL headspace volume was sampled and injected into the gas chromatograph. The limit of detection and the limit of quantification for both C2N2 and HCN in the soil samples were 0.003 and 0.013 mg kg−1, respectively. The gas-phase C2N2 and HCN concentrations in the headspace of vials were analyzed by GC-MS (Agilent Technologies 7890B gas chromatograph equipped with a split/splitless injector in combination with an Agilent 5977B mass spectrometer. A SampleLock Syringe (Hamilton Company, USA) was employed for headspace sampling. The inlet liner (internal diameter = 1 mm) was held at 250 °C, and the injection was performed in split mode (split ratio = 100:1, the flow rate of helium = 1.0 mL min−1). The compounds were then separated on a strong polarity column (DB-WAX, length = 30 m, inner diameter = 0.25 mm, 0.25-μm coating) using the following temperature program: hold for 1 min at 70 °C and then heat at 10 °C·min−1 to 90 °C (total runtime = 3 min). After GC separation, C2N2 and HCN were ionized in positive electron impact mode. The mass spectrometer was operated at 70 eV. The temperatures of the ion source, quadrupole, and interface were set at 230 °C, 150 °C, and 270 °C, respectively. MS acquisition was performed in selected ion monitoring mode, which was implemented for quantitative purposes by monitoring m/z 12, 13, 26, 27, 52, and 53 (dwell time = 40 ms). To determine the soil concentrations of NO3−–N and NH4+–N, 10-g samples were extracted with 50 mL of 2 M KCl and shaken for 0.5 h. The samples were then filtered through a 0.45-μm filter connected to a vacuum pump. Filtered samples were refrigerated for up to one week until analysis on a Lachat flow-injected colorimeter to determine NO3⁻–N and NH4+–N concentrations using the sodium salicylate method (Hofer, 2001) and cadmium reduction method (Knepel, 2003),

Ct = CO e−kt ,

(4) −1

−1

where Ct (mg·kg ) and C0 (mg·kg ) are the concentrations of fumigant in the soil at incubation times t (h) and 0 (h), respectively; and k is the first-order rate constant (h−1). The half-life (t1/2) was calculated using Eq. (5):

t1/2 = ln2/ k

(5)

Data fitting (first-order model for degradation and Freundlich model for adsorption isotherms) was performed using OriginPro 8.0 (OriginLab Corp., Northampton, USA). Reported values are the means of three replicates. Differences between the means were statistically analyzed by Duncan's multiple range test, and relationships between the means were examined by Spearman's correlation analysis using SPSS Statistics 22.0 (IBM SPSS, Somers, USA). 3. Results and discussion 3.1. Adsorption kinetics of C2N2 and HCN The adsorption data of C2N2 and HCN were well fitted by the twophase exponential function, with an average mean squared error of 3

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Fig. 1. Adsorption behaviors of C2N2 (a) and HCN transformed from C2N2 (b) in the eight agricultural soils. Adsorption isotherms of C2N2 (c) and HCN transformed from C2N2 (d) in the eight soils. Values are the means ± standard error (n = 3).

play an important role in the adsorption of organic cations (Bakouri et al., 2007; Okada et al., 2016; Rojas et al., 2015; Spark and Swift, 2002); however, their importance in the adsorption of nonionic molecules in natural systems is difficult to assess. In the present study, the variations in adsorption kinetic parameters A and k2 between soils with different clay contents were much greater than those between soils with similar clay contents (Table 2). This was not the case for parameter k1, which reflects the difficulty in estimating the slope of the initial rapid decay that occurred in all fumigations. The decay rate for the later gradual drop in C2N2 concentration was 1.45 times greater in the S2 soil (k2 = 7.176) than in the S3 soil (k2 = 4.961). These decay rates are close to those expected because the C2N2 adsorption in the S2 soil was approximately two times over that in the S3 soil. Ren et al. (2011) also found that the initial C2N2 headspace concentrations decreased rapidly to 50%–60% of the original concentrations in the first hour in experiments with kiln-dried Douglas fir and a load factor of 20%. The reduced adsorption may be attributed to the complex formation of soil OM with organic chemicals or competition for adsorption sites. Similar results were reported that the competition of soil OM for adsorption sites occurred at the soil particle surface (Song et al., 2008; Tian et al., 2019; Zhang et al., 2010). A specifies the relative weighting given to each process, k1 is the exponential decay rate of the initial drop, and k2 is the exponential decay rate of the subsequent gradual decrease. Values are the means ± standard error (SE, n = 3). Different uppercase letters indicate statistical significance at the P < 0.01 level using Duncan's multiple range test. Similarly, different lowercase letters indicate statistical significance at the P < 0.05 level using Duncan's multiple range test.

0.9842 across all time points and test replications. Fig. 1a and b shows the average of the fitted curves for the eight soils and all data used to fit the curves. Both C2N2 and HCN reached adsorption equilibrium by 24 h after application. A study on timber fumigation found that the adsorption pattern of C2N2 on sawn timber, as measured by the concentration of gas in the headspace at any time compared to the initial concentration in the chamber, was mainly influenced by the load factor (Pranamornkith et al., 2014). The relative importance of the more rapid decay process (A = 0.249) was much smaller for the S2 soil (Phaeozems; clay = 14.45%) than the S3 soil (Gleysols; clay = 1.03%, A = 0.467; Tables 1 and 2). At 0.5 h, the C2N2 concentrations were typically around 51.2% of the initial concentration in the S3 soil but only 27.8% of the initial concentration in the S2 soil. At 48 h, the average C2N2 concentrations remaining in S3 and S2 were 0.2% and 0.04% of the applied dose, respectively (Table 2). Higher adsorption in the presence of higher clay content has been reported for other fumigants and their products (e.g., CP, 1,3-D, and DMDS). Indeed, clays and clay minerals Table 2 Adsorption kinetic parameters for C2N2 headspace depletion in the eight soils. Soil sample

S1 S2 S3 S4 S5 S6 S7 S8

Two-phase exponential function A

K1 ± SE

0.600 ± 0.032 0.249 ± 0.026 0.467 ± 0.039 0.612 ± 0.032 0.282 ± 0.033 0.340 ± 0.041 0.293 ± 0.029 0.444 ± 0.031

5.706 ± 0.937 0.102 ± 0.032 0.088 ± 0.021 5.410 ± 0.826 0.107 ± 0.035 0.109 ± 0.037 0.093 ± 0.027 0.092 ± 0.018

a

r2

K2 ± SE Aa Bb Bb Aa Bb Bb Bb Bb

0.086 ± 0.020 7.176 ± 0.779 4.961 ± 1.072 0.101 ± 0.024 6.015 ± 0.800 5.780 ± 1.026 5.596 ± 0.694 4.772 ± 0.756

Dd Aa BCc Dd Bb Bb BCbc Cc

0.9830 0.9882 0.9790 0.9858 0.9846 0.9777 0.9868 0.9881

4

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Table 3 Adsorption isotherm parameters in the eight C2N2-fumigated soils. Soil sample

S1 S2 S3 S4 S5 S6 S7 S8

Freundlich (C2N2)

a

Freundlich (HCN)

a

Kf ± SE

1/n

r2

Kf ± SE

1/n

r2

7.068 ± 0.098 De 15.744 ± 0.027 Aa 3.119 ± 0.087 Fg 7.108 ± 0.087 De 10.079 ± 0.120 Bb 8.481 ± 0.044 Cd 9.623 ± 0.053 Bc 5.880 ± 0.044 Ef

0.981 ± 0.016 0.951 ± 0.004 0.931 ± 0.029 1.052 ± 0.019 1.030 ± 0.012 0.977 ± 0.007 0.976 ± 0.006 0.977 ± 0.007

0.9995 0.9999 0.9984 0.9993 0.9997 0.9999 0.9999 0.9999

5.599 ± 0.144 Ee 14.260 ± 0.954 Aa 3.388 ± 0.110 Ff 7.192 ± 0.251 Dd 10.492 ± 0.090 BCb 9.710 ± 0.473 Cc 10.246 ± 0.289 Bb 5.316 ± 0.066 Ee

1.0631 ± 0.036 1.103 ± 0.024 1.005 ± 0.025 0.906 ± 0.026 0.845 ± 0.088 0.973 ± 0.032 0.964 ± 0.008 1.026 ± 0.009

0.9972 0.9986 0.9988 0.9988 0.9871 0.9979 0.9999 0.9998

active sites and thus displayed stronger di-n-butyl phthalate adsorption; in contrast, coarse sand fractions with lower soil OM and clay contents showed weaker di-n-butyl phthalate adsorption. The soil with the highest fumigant adsorption capacity possibly had the highest soil OM and clay contents.

3.2. Adsorption isotherms of C2N2 and HCN The Freundlich equation gave a good fit to the adsorption isotherms of C2N2 and HCN in all eight soils [correlation coefficient (r2) > 0.99; Table 3 and Fig. 1c and d]. The isotherms exhibited L-type (1/n < 1) patterns according to the classification of Giles et al. (1960). The position of the adsorption curve on the graph reflects the adsorption capacity of the fumigant in the soil; the higher the curve on the Y-axis, the more fumigant adsorbed. The Kf values of C2N2 ranged from 3.119 (S3) to 15.744 (S2), indicating that adsorption was greatest on the Phaeozems and lowest on the Gleysols. The adsorption amount decreased in the following order: S2 (Phaeozems) > S5 (Ferralsols) > S7 (Alisols) > S6 (Lixisols) > S4 (Anthrosols) > S1 (Luvisols) > S8 (Plinthosols) > S3 (Gleysols; P < 0.01). The same trend was observed for HCN adsorption, and the trends corresponded to the order of soil OM content. Similarly, other studies have found that the adsorption of organic chemicals in soils and sediments is primarily related to OM content (Li et al., 2019; Xiang et al., 2019; Zhang et al., 2019a). At each tested fumigant concentrations, the amount of fumigant adsorbed in low-OM soils was less than that adsorbed in high-OM soils. As the initial concentration of C2N2 increased, the amount of fumigant adsorbed increased for all soils (Fig. 1). Kf (mg1−1/n (L)1/n mg−1) is the Freundlich adsorption coefficient, 1/n is the Freundlich exponent, and r2 is the correlation coefficient. a Values are the means ± standard error (SE, n = 3). Different uppercase letters indicate statistical significance at the P < 0.01 level using Duncan's multiple range test. Similarly, different lowercase letters indicate statistical significance at the P < 0.05 level using Duncan's multiple range test. The average Kf of C2N2 was five times greater for S2 (OM = 4.64%; Kf = 15.744) than for S3 (OM = 0.17%; Kf = 3.119). Meanwhile, the average Kf value of HCN was 4.2 times greater for S2 (Kf = 14.260) than for S3 (Kf = 3.388; Table 3). In general, high-OM soil has been reported to have a larger number of functional groups capable of adsorbing fumigants than low-OM soil (Dionisio and Rath, 2016; Pantelelis et al., 2006; Wang et al., 2019). Differences in Kf values were correlated with differences in soil physicochemical properties (Table 4). In particular, the adsorption coefficient was dominated by OM (r2 > 0.82, P < 0.01) and clay (r2 > 0.92, P < 0.01), indicating that higher OM and clay contents may provide more active sites for C2N2 and HCN adsorption via hydrophobic interactions. Additionally, the adsorption coefficient Kf was stronger positively correlated with NH4+–N (Slope > 0, P < 0.01) and NO3−–N (Slope > 0, P < 0.01), and stronger negatively correlated with pH (Slope < 0, P < 0.01) (Table 4). In soil, as the humification of OM progresses, the contents of some polar and ionizable groups are increased (Calvet, 1989; Chen et al., 2017). These natural organic functional groups have a greater affinity for hydrophilic C2N2 and HCN, which may explain the higher adsorption of C2N2 and HCN on the soil with higher OM and clay contents. Our findings generally agree with the previous results (Xiang et al., 2019) that humic acid and clay fractions, which contained higher soil OM and clay contents, had more

3.3. Degradation kinetics of C2N2 and HCN The values of k, t1/2, and r2 for C2N2 and HCN in the sterilized and unsterilized soils are summarized in Table 5. No significant difference was observed in these values between unsterilized and sterilized soils (Fig. S1). The r2 values for C2N2 and HCN ranged from 0.8832 to 0.9979, indicating that the first-order kinetic model was a good fit for the degradation data of both C2N2 and HCN. The k values of C2N2 ranged from 0.022 to 0.050 h−1 in unsterilized soils and 0.024–0.052 h−1 in sterilized soils, whereas those of HCN varied from 0.014 to 0.026 h−1 in unsterilized soils and 0.017–0.030 h−1 in sterilized soils. However, the difference in k was not obvious within the OM content range of 0.22%–1.20% (Table 5). A stronger negative correlation between fumigant degradation rate and soil OM was observed (slope < 0, P < 0.01; Table 4). For example, within the pH range of 6.38–6.58, the degradation rates of C2N2 and HCN in unsterilized S1 soil (Luvisols; OM = 0.48%) were 2.08 and 1.73 times higher than those in unsterilized S2 soil (Phaeozems; OM = 4.64%), respectively. In contrast, other studies have found a positive correlation between soil OM content and fumigant degradation rate. For example, the degradation rates of 1,3-D and methyl iodide were reported to increase substantially with increasing soil OM content (Han et al., 2018). The degradation rate of a fumigant is also affected by other soil chemical and biological conditions, both of which could be influenced by OM (Huang et al., 2019). In particular, different types of soil are known to support various microbial communities. In situations where soil OM promotes the growth and activity of selected microorganisms (Whitman et al., 2016), the rates of C2N2 and HCN degradation may vary depending on the microorganisms present. The t1/2 of C2N2 ranged widely from 13.953 to 32.000 h in unsterilized soils, and 13.414–28.908 h in sterilized soils (Table 5). No significant difference in the t1/2 was observed between sterilized and unsterilized soils, indicating that abiotic degradation was predominant in the degradation of C2N2 and HCN. This fact could be attributed to the low bioavailability of C2N2 and HCN for microbial degradation because of their high adsorption affinity to OM and soil (Hnatek et al., 2019). The t1/2 values of HCN were 1.52–2.05 times greater than those of C2N2. Since both HCN and C2N2 are effective fumigants for multiple purposes (Hnatek et al., 2019; Stejskal et al., 2017), they may provide continuous control of pests after C2N2 degradation into HCN. 3.4. C2N2 and HCN leaching in the Phaeozems The vertical distributions of C2N2 and HCN in the S2 soil (Phaeozems) columns over time are shown in Fig. 2 (for a 100-mg·L−1 dose of C2N2). The highest C2N2 and HCN concentrations were detected 5

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Table 4 Results of correlation analysis between soil physicochemical properties and the Freundlich adsorption coefficients (Kf) and degradation kinetics coefficients (k) for C2N2 and HCN. Soil properties

Kf (C2N2) Slope ± SE

OM Clay pH NO3−-N NH4+-N

Kf (HCN) a

2.6274 ± 0.002 Aa 0.8047 ± 0.005 Cc −0.2035 ± 0.015 Ee 0.9652 ± 0.007 Bb 0.3022 ± 0.007 Dd

r2

Slope ± SE

0.9444 0.9224 0.4489 0.2187 0.1100

0.3632 ± 0.005 Cc 0.7646 ± 0.005 Bb −0.261 ± 0.004 Dd 1.1011 ± 0.080 Aa 0.2861 ± 0.007 Cc

k(C2N2) Slope ± SE

r2 0.8248 0.9573 0.5125 0.3587 0.0131

k(HCN) a

−0.0038 ± 0.001 Cc −0.0009 ± 0.0002 ABab −0.001 ± 0.0005 BCb 0.0005 ± 0.0002 Aa −0.0002 ± 0.0002 ABab

OM Clay pH NO3−-N NH4+-N

a

r2

Slope ± SE

a

r2

0.2183 0.1408 0.0105 0.0605 0.0117

−0.0014 ± 0.0004 Cc −0.0005 ± 8.8E-5 Bb 0.0001 ± 0.0001 ABa 0.0004 ± 0.0001 Bb 0.0004 ± 0.0002 Aa

0.1518 0.1758 0.0057 0.1513 0.0631

a Values are the means ± standard error (n = 3). Different uppercase letters indicate statistical significance at the P < 0.01 level using Duncan's multiple range test. Similarly, different lowercase letters indicate statistical significance at the P < 0.05 level using Duncan's multiple range test.

concentrations at 25 and 35 cm from the top of the column decreased rapidly. The results indicate that C2N2 and HCN were degraded rapidly throughout the soil column starting at 4 h after C2N2 injection. C2N2 and carbonyl sulfide were reported to be stable in a variety of soil types from New South Wales and Western Australia for 3–5 h, after which they were broken down to naturally occurring soil components such as H2S and CO2 (Ren, 2002).

15 cm from the top of the column shortly after C2N2 injection. The gas concentrations recorded at the injection port were highest at 0.5 h (30 mg L−1 for C2N2 and 21 mg L−1 for HCN), and both concentrations decreased to less than 0.2 mg L−1 at 96 h. Over time, the C2N2 concentrations decreased at the injection port while increasing in the other parts of the soil column. This trend is similar to the dispersion characteristics reported for other fumigants, including chloropicrin and 1,3D in soil columns (Gao et al., 2011). The concentrations of C2N2 and HCN gases above the injection port were significantly higher than those at or below the injection port within 4 h after C2N2 injection. However, the C2N2 gas concentration above the injection port decreased rapidly after 4 h. At 2 h after injection, the C2N2 gas concentration within 5 cm from the top of the column (4.02 mg L−1) was 7.16 times higher than that at 25 cm below the top; this difference was reduced to 1.90 times at 24 h, indicating that the initial vertical diffusion of C2N2 was mainly in the upward direction. The largest observed C2N2 concentrations were 1.7 mg L−1 (25 cm from the top of the column) and 0.05 mg L−1 (35 cm) at 4 h, while the largest HCN concentrations were 1.28 mg L−1 (25 cm from the top of the column) and 0.038 mg L−1 (35 cm) at 4 h. After 4 h, the C2N2 and HCN

3.5. Residual fumigants in soils after leaching Knowledge of soil hydrological properties is critical for understanding soil water movement and predicting soil parameters that affect agronomic and environmental practices in a region (Zhang et al., 2019b). Bulk density, porosity, and hydraulic conductivity changed significantly across the eight soil samples (Fig. S2). Soil OM had a stronger positive effect (Slope > 0, p < 0.01) on the tested soil hydrological properties in that it increased porosity and hydraulic conductivity while decreasing bulk density in the soil samples (Table S1). This result is consistent with a previous study (Bohara et al., 2019; de Souza et al., 2019) that found soil texture, porosity, and bulk density

Table 5 Degradation parameters of C2N2 and HCN in sterilized and unsterilized soils. Soil sample

S1 S1 S2 S2 S3 S3 S4 S4 S5 S5 S6 S6 S7 S7 S8 S8

(SS) (SS) (SS) (SS) (SS) (SS) (SS) (SS)

C2N2

a

HCN

a

k ± SE

t1/2 (h)

r2

k ± SE

0.050 ± 0.003 Aa 0.052 ± 0.003 Aa 0.024 ± 0.005 Ce 0.026 ± 0.003 Cc 0.040 ± 0.008 Bcd 0.043 ± 0.001 Bcd 0.022 ± 0.003 Ce 0.024 ± 0.001 Cc 0.038 ± 0.005 Bd 0.040 ± 0001 Bd 0.046 ± 0.023 Ab 0.048 ± 0.022 Ab 0.040 ± 0.005 Bcd 0.042 ± 0.003 Bcd 0.042 ± 0.005 Bcd 0.044 ± 0.004 Bc

13.953 ± 0.093 13.414 ± 0.087 28.908 ± 0.906 27.009 ± 0.355 17.488 ± 0.395 16.005 ± 0.322 32.000 ± 0.500 28.908 ± 0.696 18.247 ± 0.247 17.184 ± 0.141 15.030 ± 0.720 14.402 ± 0.663 17.332 ± 0.251 16.372 ± 0.127 16.507 ± 0.225 15.755 ± 0.207

0.9902 0.9934 0.9867 0.9893 0.9574 0.9615 0.9961 0.9925 0.9924 0.9976 0.9915 0.996 0.9857 0.9922 0.9938 0.9979

0.026 ± 0.006 0.030 ± 0.005 0.015 ± 0.001 0.019 ± 0.002 0.019 ± 0.003 0.023 ± 0.003 0.014 ± 0.008 0.017 ± 0.005 0.018 ± 0.008 0.026 ± 0.012 0.019 ± 0.001 0.022 ± 0.005 0.020 ± 0.001 0.024 ± 0.002 0.025 ± 0.008 0.029 ± 0.008

Aa Aa CDc Ded BCb BCc Dc Ed BCb ABb BCb CDc Bb BCbc Aa Aa

t1/2 (h)

r2

26.353 ± 0.683 23.117 ± 0.445 45.793 ± 3.830 37.296 ± 1.812 35.863 ± 0.607 29.712 ± 0.418 48.707 ± 2.914 40.859 ± 1.390 37.293 ± 1.811 26.431 ± 1.251 36.243 ± 2.676 32.283 ± 2.228 34.883 ± 2.024 28.553 ± 1.016 27.422 ± 0.937 23.667 ± 0.701

0.9846 0.9600 0.9698 0.9715 0.9299 0.9343 0.9593 0.9302 0.8832 0.9498 0.9855 0.9625 0.9552 0.8925 0.9821 0.9582

SS is the sterilized soil, k is the degradation rate constant, t1/2 is the half-life, and r2 is the correlation coefficient. a Values are the means ± standard error (SE, n = 3). Different uppercase letters indicate statistical significance at the P < 0.01 level using Duncan's multiple range test. Similarly, different lowercase letters indicate statistical significance at the P < 0.05 level using Duncan's multiple range test. 6

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Fig. 2. Leaching of C2N2 (a) and HCN (b) in the S2 soil (Phaeozems) columns and concentrations of C2N2 (c), HCN (d), NH4+–N (e), and NO3−–N (f) residues (on a dry soil basis) in the eight soil columns for a C2N2 dose of 100 mg L−1. Values are the means ± standard error (n = 3).

could greatly affect soil water dynamics. In addition, enhanced π-π interaction along with increased porosity might contribute to the adsorption of OM. Different fractions of soil were collected from all soils for 1–4 d after C2N2 injection. These fractions showed no residues of C2N2, HCN, NH4+–N, and NO3−–N at 4 d. The distributions of fumigant residues in soils at different depths showed that the surface-applied C2N2 was not distributed equally throughout the column length; instead, the concentrations varied with soil depth (Fig. 2c). In the columns of S1 (Luvisols), S2 (Phaeozems), S3 (Gleysols), and S4 (Anthrosols), the highest concentrations were recorded at a depth of 15 cm. In contrast, the highest concentrations in the column of S7 (Alisols) were observed at a depth of 10 cm, while the highest concentrations in the column of S5 (Ferralsols), S6 (Lixisols), and S8 (Plinthosols) were found at 0 cm. No residues were detected at depths greater than 35 cm. Thus, C2N2

remained only in the topsoil (depths up to 15 cm), indicating low fumigant mobility in the soil column. Fig. 2c, Fig. S2 and Table S1 indicate that the presence of soil OM and hydraulic conductivity in the leachate could facilitate the movement of C2N2, confirming the interaction between C2N2 and soil OM or hydraulic conductivity. This phenomenon might be related to the level of OM or hydraulic conductivity in the soils (McLaughlin and Johnson, 1997; Tian et al., 2019; Zhang et al., 2010). The residual concentrations of C2N2, HCN, NH4+–N, and NO3−–N remaining in the soil at the end of the leaching experiment are shown in Fig. 2c–f. Generally, the highest residues were recorded near the injection port, while the lowest residues were observed at depths of 35–45 cm and near the surface. Relatively high C2N2, HCN, NH4+–N, and NO3−–N concentrations were found within 15 cm of the soil surface after 100 mg L−1 C2N2 injection. The C2N2 and HCN concentrations in the S2 soil (Phaeozems) within 15 cm of the soil 7

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surface were 30 and 20 mg kg−1 soil, respectively; the C2N2 and HCN concentrations in the S7 soil (Alisols) within 10 cm of the soil surface were 18.32 and 6.32 mg kg−1 soil, respectively. In the S8 soil (Plinthosols), the concentrations of C2N2 and HCN on the soil surface (depth = 0 cm) were 20.19 and 6.29 mg kg−1 soil, respectively. However, no significant differences in NH4+–N, and NO3−–N concentrations were observed among these three soils. Data from Mattner's group indicate that 150 mg kg−1 C2N2 is sufficient to control all target soil organisms; assuming a 100% conversion of applied C2N2 (150 mg kg−1) to HCN, the concentration of HCN will be 78 mg kg−1, much lower than the maximum allowable concentration in residential soil (Mattner et al., 2003; Mattner et al., 2006). The results confirm that C2N2 and HCN will not accumulate in the environment and are not likely to contaminate groundwater. Kjeldsen (1999) showed that the behavior of cyanide compounds in soil and groundwater is governed by interacting chemical and microbial processes. Redox conditions and pH are important for the degradation and leaching of iron-cyanide complexes.

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4. Conclusions The results of this study indicate that soil physicochemical properties play a critical role in the fate of C2N2 and HCN after C2N2 application. The adsorptions of C2N2 and HCN were positively correlated with soil OM and clay contents. Abiotic processes were the predominant pathways involved in the degradation of C2N2 and HCN. Both C2N2 and HCN were mostly emitted and leached downward, leaving little residues in the soil. The results confirm that C2N2 and HCN will not accumulate in the soil and are not likely to contaminate groundwater. Further research that accounts for the greater complexity of the field environment compared to the laboratory to establish the minimum C2N2 dose that can be used with barrier films to effectively control pests and improve crop yields. Based on our results and the existing application methods in Europe, plastic covers should be mandatory after applying C2N2 in China. Notes The authors declare no competing financial interest. Acknowledgments This study was supported by the National Natural Science Foundation of China (grant Nos. 30960081 and 31560162) and the Science and Technology Project of Jiangxi Provincial Education Department (grant No. GJJ170310). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ecoenv.2019.109704. References Armstrong, J., Najar-Rodriguez, A., 2019. Efficacy of ethanedinitrile (EDN) as a fumigant for export logs. A report prepared for stakeholders in methyl bromide reduction. Quarantine Treatments and Market Access Specialist Quarantine Scientific Limited, pp. 1–32. Asma, B.S., Hanene, C., Pierluigi, C., Alberto, A., Rachid, S., Sami, F., 2019. Environmental fate of two organophosphorus insecticides in soil microcosms under mediterranean conditions and their effect on soil microbial communities. Soil Sediment Contam.: Int. J. 28, 285–303. ASTM, 2010. Standard Test Method for Density of Soil in Place by the Drive-Cylinder Method. Vol. D2937-10. Bakouri, E.I., Morillo, H., Usero, J., Ouassini, A., 2007. Removal of prioritary pesticides contamining r'mel ground water by using organic waste residues. 72, 197–207. Blake, G.R., Hartge, K.H., 1986. Bulk density. In: Methods of Soil Analysis. Soil Science Society of America, WI, pp. 363–375. Bohara, H., et al., 2019. Influence of poultry litter and biochar on soil water dynamics and nutrient leaching from a very fine sandy loam soil. Soil Tillage Res. 189, 44–51.

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