Organic carbon nature determines the capacity of organic amendments to adsorb pesticides in soil

Journal of Hazardous Materials 390 (2020) 122162

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Organic carbon nature determines the capacity of organic amendments to adsorb pesticides in soil

T

Carlos García-Delgadoa,b, Jesús M. Marín-Benitoa, María J. Sánchez-Martína, M. Sonia Rodríguez-Cruza,* a b

Institute of Natural Resources and Agrobiology of Salamanca (IRNASA-CSIC), 40-52 Cordel de Merinas, 37008, Salamanca, Spain Department of Geology and Geochemistry, Autonomous University of Madrid. 28049, Madrid, Spain

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Editor: Deyi Hou

The spread of organic pollutants from soil to other environments is one important source of environmental pollution. The addition of organic amendments to soil is an interesting strategy to control pollutants leaching. However, the contribution of different carbon types of organic amendments to organic pollutants adsorption is not clear. Hence, the objective of this work was to determine the role of carbon types of organic amendments into the adsorption of four herbicides. To this extent, organic amendments were characterized by elemental analysis and 13C-NMR and adsorption–desorption isotherms of herbicides by the organic amendments and two soils amended with them were obtained. Adsorption coefficients were correlated with the organic carbon content of the organic amendments and the adsorption process was enhanced by the hydrophobicity of herbicides and the aliphatic and aromatic carbon of amendments. Organic amendments increased the adsorption of herbicides by soils but it is not possible to extrapolate results from one soil to another because organo-mineral interactions between soils and organic amendments can modify this process. Desorption isotherms of herbicides from organic amendments and/or amended soils presented hysteresis indicating the irreversible adsorption of herbicides. Desorption results indicated, the abundance of O-alkyl and N-alkyl groups in organic amendments enhanced the hysteresis in amended soils.

Keywords: Organic pollutants Soil Adsorption-desorption Pollution Carbon type

⁎

Corresponding author. E-mail address: [email protected] (M.S. Rodríguez-Cruz).

https://doi.org/10.1016/j.jhazmat.2020.122162 Received 20 December 2019; Received in revised form 20 January 2020; Accepted 21 January 2020 Available online 21 January 2020 0304-3894/ © 2020 Elsevier B.V. All rights reserved.

Journal of Hazardous Materials 390 (2020) 122162

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1. Introduction

Nahum et al., 2007) with the consequent difficulty to predict their effectiveness to adsorb pesticides or other pollutants (Marín-Benito et al., 2012a, b). Although organic amendments have been proved to be effective adsorbents of pesticides (Rojas et al., 2013), few studies have been carried out to evaluate the functional groups involved in the adsorption process (Wang et al., 2019; Xing, 2001). Therefore, research is required to determine the role of the structural carbon of the organic amendments responsible for the adsorption. Accordingly, the aim of this work was to determine the role of the functional groups of organic carbon (OC) coming from organic amendments into the adsorption process of herbicides by amendments, and by two unamended and amended soils with different texture. Analysis of amendments by cross-polarization and magic angle spinning nuclear magnetic resonance (CP-MAS 13C-NMR) was carried out to determine the distribution of structural carbon assigned to different groups. The adsorption parameters of herbicides by organic amendments and soils were obtained from the adsorption-desorption isotherms. This work enables a better understanding of the adsorption behaviour of herbicides by organic amendments and amended soils addressed to get an adequate selection of organic amendments in order to minimize the mobility of pesticides (or other organic pollutants) or to design new effective biosorbents or barriers.

Modern intensive agriculture involves the use of pesticides to increase the yield of crops. However, the environmental fate of these agrochemicals is not limited to agricultural soil and they can reach surface water or groundwater (Herrero-Hernández et al., 2017; Li et al., 2014; Pascual Aguilar et al., 2017; Sousa et al., 2018; Van Bruggen et al., 2018) due to rain and/or irrigation as well as further leaching process. Once a pesticide is applied and reaches the soil, a dynamic equilibrium between solid and liquid phases of the soil is established. The remaining pesticide in solution depends on the adsorption capacity of the solid phase of soil. The soil components that enhance the adsorption capacity for organic pollutants are clays, oxi-hydroxides of iron and manganese, and organic matter (OM). This last fraction has a clear role on organic pollutants adsorption because of its high reactivity (Stevenson, 1994). The presence of a great number of functional groups bestows the soil humic fraction a high affinity for organic pollutants with different characteristics as it has been previously reported (GarcíaDelgado et al., 2017; Hwang et al., 2015; Murano et al., 2018). The adsorption of pesticides by soils modifies their environmental behaviour because this process is key factor to pesticide leaching, (bio) degradation, volatilization, bioavailability and ecotoxicity to non-target organisms, including human beings (Álvarez-Martín et al., 2016a, b; Ogungbemi and van Gestel, 2018). One of the potential ways to minimize the mobility of pesticides through soil profile (Marín-Benito et al., 2018a, b) and to simultaneously enhance the soil quality (López-Rayo et al., 2016) takes place through the application of organic amendments (Keesstra et al., 2019). This common agricultural practice produces many benefits on physicochemical and biological soil properties such as the increase of water retention capacity, micro and macro aggregates, nutrient status, microbial biomass and activity furthermore this practice increases the capacity for carbon sequestration (García-Delgado et al., 2018; Hernandez et al., 2017; Marín-Benito et al., 2018a, b; Novara et al., 2019; Sarkar et al., 2018). Hence, it is considered as a friendly agricultural practice that conserves the health and productivity of the soil. The raw materials of organic amendments come from multiple sources. They usually come from by-products or wastes from agriculture, gardening, animal breeding, food industry, sewage sludge or urban solid wastes among others. A common and recommended manipulation of these organic materials involves their composting to stabilize the OM and microbiological composition (Moreno-Casco and Moral-Herrero, 2008) previous to the application to soil. The use of wastes and by-products into agriculture helps to minimize the disposal of organic wastes into landfills with the consequent environmental benefits and activation of circular economy. Many references report the ability of organic amendments to adsorb pesticides or other organic and inorganic pollutants (Álvarez-Martín et al., 2016a; Frutos et al., 2016; García-Delgado et al., 2017; MarínBenito et al., 2018a,b; Parolo et al., 2017). The pesticide adsorption capacity of the organic amendments depends on the pesticide dose employed and its physicochemical properties, mainly its hydrophobic character. As well as the physicochemical characteristics of the adsorbent greatly influence the reactivity of these organic amendments in the soil environment and they are determined by the number and type of functional groups (Rojas et al., 2013). It is known that the adsorption process is regulated through different interaction mechanisms (π–π interaction, Van der Waals forces, hydrogen bonds, hydrophobicity or polar interactions) between the functional groups of pollutant and those of the OM of organic amendments (Sophia and Lima, 2018; Wang et al., 2019). On the other hand, the OM of these amendments can also establish interactions with clay minerals of soils and alter their further interactions with pollutants when they reach the soil (Cornejo and Hermosín, 1996). However, the nature and composition of the OM of amendments, the main responsible of their adsorption capacity, is variable (Zmora-

2. Materials and methods 2.1. Pesticides Chemical structures and physicochemical characteristics of the four herbicides used are given in Table S1 (in Supplementary Material). Analytical standards (purity > 98.9 %) of triasulfuron (TSF) (1-[2-(2chloroethoxy) phenylsulfonyl]-3-(4-methoxy-6-methyl-1,3,5-triazin-2yl) urea), chlorotoluron (CTL) (3-(3-chloro-p-tolyl)-1,1-dimethylurea), flufenacet (FNC) (4´-fluoro-N-isopropyl-2-[5-(trifluoromethyl)-1,3,4thiadiazol-2-yloxyl]acetanilide) and prosulfocarb (PSC) (S-benzyl dipropyl(thiocarbamate) were provided by Sigma-Aldrich Química SA (Madrid, Spain). Labelled [Ring-U-14C]-PSC (specific activity 3.16 MBq mg−1, 94.8 % purity) was supplied by IZOTOP (Budapest, Hungary). 2.2. Soils and organic amendments Two different soils were sampled from the Muñovela experimental farm belonging to the Institute of Natural Resources and Agrobiology of Salamanca (IRNASA-CSIC), Spain. The soil samples (S1 and S2) were collected from the surface horizon (0–20 cm), air dried and sieved to 2 mm. The pH and electrical conductivity EC was determined in soil/ water suspension 1/2.5 w/v and particle size distribution was determined using the pipette method. OC and N content was determined on an element analyzer LECO CN628 Saint Joseph, MI, USA. The dissolved organic carbon content DOC was determined in soil extracts 1/ 2 w/v ratio in deionized water after soil shaking 24 h at 20 °C, centrifugation 20 min at 10,000 rpm, and filtering (Minisart NY 25 filter 0.45 μm, Sartorius Stedim Biotech, Germany). DOC analysis was carried out on a LECO CN628 (Saint Joseph, MI, USA) elemental analyzer. Inorganic carbon was determined as CaCO3 with a Bernard calcimeter. Four organic amendments, spent mushroom substrate (SMS), green compost (GC), manure (M) and sewage sludge (SS) from different origin and one humic acid (HA) (Fluka Química, Madrid, Spain) were selected. SMS was supplied by Sustratos de la Rioja S.L. (Pradejón, Spain). SMS is compost produced with the residual substrate waste of Agaricus bisporus cultivation. The substrate of A. bisporus is a pasteurized mixture of cereal straw and poultry litter, urea and gypsum, which was further composted for several weeks under aerobic conditions to obtain a composted SMS. GC formed by composted vegetal residues was provided by the nursery El Arca (Salamanca, Spain). M is a commercial pelletized manure for organic fertilization (3-4-9) provided by Timac 2

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AGRO España S.A. (Orcoyen, Spain). SS from urban wastewater treatment plant was stabilized by anaerobic digestion (Aqualia SA, Salamanca, Spain). The organic amendments were air dried and sieved to 2 mm. The pH and electrical conductivity EC was determined in amendment/water suspension 1/2.5 and 1/5 w/v, respectively. The dissolved organic carbon content DOC was determined in soil extracts 1/100 w/v ratio in deionized water after soil shaking 24 h at 20 °C, centrifugation 20 min at 10,000 rpm, and filtering Minisart NY 25 filter 0.45 μm, Sartorius Stedim Biotech, Germany. DOC analysis was carried out on a LECO CN628 Saint Joseph, MI, USA elemental analyzer. Total carbon, nitrogen, hydrogen and sulfur were analyzed using combustion LECO CHNS-932 analyzer, USA. Ash percentage was determined by weight difference after ignition at 540 °C for 24 h. Oxygen contents were calculated as the difference between 100 % and the sum of carbon, hydrogen, nitrogen, sulfur, and ash contents of the dry materials. The polarity index (PI) of the sorbents was calculated from the atomic ratios (O + N)/C (Marín-Benito et al., 2012; Xing, 2001) Amended soils were prepared by uniformly mixing soils (S1 or S2) with SMS, GC, M and SS at a rate of 10 % w/w. 2.3. Characterization of organic amendments by resonance

13

After adsorption equilibrium had been reached, 5 mL was removed from the solution and immediately replaced by 5 mL of CaCl2 0.01 M. The resuspended samples were shaken for 24 h at 20 ± 2 °C, whereupon the suspensions were centrifuged and the desorbed pesticide was measured. This desorption procedure was repeated four times for each sample. The amount of compound adsorbed by the adsorbent at each desorption stage was calculated as the difference between the initial amount adsorbed and the amount desorbed. All experiments were performed in duplicate. 2.5. Herbicide analysis TSF, CTL and FNC were determined by HPLC-DAD-MS (Waters, Milford, MA, USA), using a Phenomenex Luna PFP(2) (3 μm, 150 × 4.6 mm) column at ambient temperature. The mobile phase was acetonitrile:water + 1 % formic acid (70:30). The flow rate of the mobile phase was 0.4 mL min−1, and the sample injection volume was 10 μL. The herbicides were identified on the basis matching of UV spectra (200–400 nm) and the retention times (TSF 6.2 min, CTL 7.0 min, FNC 11.0 min) with those of commercial available standards (Sigma-Aldrich Química S.A., Madrid, Spain). Quantification involved monitoring the positive molecular ion [m/z] 402.8 [M+H]+ (TSF), 213 [M+H]+ (CTL) and 364 [M+H]+ (FNC) after applying an optimized cone voltage of 20 V. The 14C-PSC equilibrium concentration was determined, per duplicate, by mixing 1 mL of supernatant solution with 4 mL of scintillation liquid, and the activity of the pesticide was measured in disintegrations per minute (dpm) on a Beckman LS6500 Liquid Scintillation Counter (Beckman Instruments Inc., Fullerton, CA, USA) (Rodríguez-Cruz et al., 2012).

C nuclear magnetic

Organic amendments and HA were analyzed by cross-polarization and magic angle spinning nuclear magnetic resonance (CP-MAS 13 CNMR) to determine the most important functional groups contained in the OC. All spectra were produced using a Broker AV-400-WB computer in a triple probe of 4-mm channel, with rotors of ZrO and cap of Kel-F at room temperature. The speed was set at 10 kHz. The pulse sequence used was cross-polarization (CP-MAS) 1H–13C with a spectral width of 35 kHz, a contact time of 3 ms and a relaxation time of 2 s, with decoupling of 1H like tppm 15 at 83 kHz. The chemical shift was established in ppm using trimethylsilane as an initial reference. The spectra were divided into five intervals according to theoretical chemical shifts to identify the different functional groups (Stevenson, 1994): 0–45 ppm is usually associated with aliphatic C chains such as alkanes or fatty acids; 45–60 ppm with N-alkyl such as amines, amino acids, peptides and proteins; 60–110 ppm with O-alkyl groups in carbohydrates; 110–160 ppm with C of aromatic groups; 160–220 with carboxylic and carbonyl groups.

2.6. Data analysis The data obtained in the adsorption-desorption experiments were fitted to Freundlich model according to the equations: Cs = Kf Ce nf or Cs = Kfd Cenfd for the adsorption and desorption where Cs is the amount of herbicide adsorbed at equilibrium (μg g−1), Ce is the equilibrium solute concentration (μg mL−1), Kf or Kfd (μg1−nf g−1 mLnf) are the Freundlich adsorption or desorption coefficients and nf or nfd are the non-linearity coefficients. Distribution coefficients (Kd, mL g−1) were also determined from the relationship between Cs and Ce for a Ce of 10 μg mL−1 because isotherms were nonlinear. Soil OC – water partitioning coefficient (Koc) was calculated as the ratio of the mass of Kd herbicide adsorbed per unit of OC (K oc = 100 % OC ) The univariate statistical tests were performed by IBM SPSS Statistics v24 software whereas multivariate statistical tests were performed by Past v3.23 software. Analysis of variance (ANOVA) was performed to determine significant differences between adsorption constants. Means were compared by Tukey or Games-Howell post-hoc test (p < 0.05), according to previous Levene variance homogeneity test. The correlations between adsorption - desorption constants and OC composition were determined by Pearson´s test. In order to determine the role of OC composition on herbicides adsorption, Principal Components Analysis (PCA) was performed on matrix containing i) the percentage of each type of carbon quantified by CP-MAS 13C-NMR for each organic material and the adsorption constants of each herbicide, ii) the increase of percentage of each type of carbon quantified by CPMAS 13C-NMR for each amended soil and the adsorption constants of each herbicide and iii) the increase of percentage of each type of carbon quantified by CP-MAS 13C-NMR for each amended soil and the desorption hysteresis value of each herbicide. One-way and two-way PERMANOVA analysis was used to determine the significance of soil type, organic amendment and its interaction.

2.4. Adsorption-desorption isotherms The adsorption isotherms of herbicides by four organic amendments, one HA, two unamended and amended soils were obtained using the batch equilibrium technique. Duplicate soil samples (5 g) or organic materials (0.1 g) were equilibrated with 10 mL of CaCl2 0.01 M in MilliQ ultrapure water solution of TSF, CTL and FNC at concentrations of 1, 5, 10, 15, 20, and 25 μg mL−1 and 0.25, 0.5, 1, 2.5, 5 and 10 μg mL−1 for PSC, according to their solubility range. An activity of 100 Bq mL−1 was used for 14C-prosulfocarb. The suspensions were shaken at 20 ± 2 °C for 24 h in a thermostated chamber, with intermittent shaking for 2 h at 3-h intervals. Preliminary experiments revealed that contact for 24 h was long enough for attaining equilibrium. The suspensions were subsequently centrifuged at 5045 g for 10 min, and the equilibrium concentration of each herbicide was determined. The amount of pesticide adsorbed was calculated as the difference between that initially present in solution and that remaining after equilibration with the soil. Calculations were based on the assumption that no degradation of herbicides occurred during adsorption studies. The desorption isotherms of the herbicides were obtained from samples initially treated with the highest concentration solution of each herbicide (25 μg mL−1 for TSF, CTL and FNC and 10 μg mL−1 for PSC).

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Table 1 Characteristics of organic amendments (spent mushroom substrate (SMS), green compost (GC), manure (M) and sewage sludge (SS)), and humic acid (HA).

SMS GC M SS HA a b

pH

C (%)

H (%)

N (%)

S (%)

O (%)

Ash (%)

PIa

DOCb (%)

7.9 7.2

26.4 23.6 18.5 28.9 45.8

3.5 2.9 2.9 5.0 4.2

2.4 1.9 2.0 4.3 0.7

1.0 0.2 0.7 0.9 1.1

22.9 12.8 27.5 12.9 12.7

43.8 58.7 48.4 48.1 35.6

0.96 0.62 1.59 0.59 0.29

1.29 0.69 1.32 1.18 1.99

7.6

Table 3 Characteristics of unamended and amended soils.

PI, polarity index (O + N)/C. DOC, dissolved organic carbon.

3. Results and discussion

pH

ECa (dS m-1)

OCb (%)

DOCc (g kg-1)

Sand (%)

Silt (%)

Clay (%)

S1 S1-SMS S1-GC S1-M S1-SS

7.36 7.01 7.32 7.41 7.39

0.23 2.36 0.87 3.55 1.25

1.2 2.4 2.1 2.1 2.4

0.112 0.848 0.343 0.898 0.734

58

25

17

S2 S2-SMS S2-GC S2-M S2-SS

7.61 6.83 7.23 7.66 7.47

0.18 3.78 1.63 8.95 0.96

0.9 4.1 2.5 3.0 1.7

0.139 1.257 0.507 1.317 1.165

81

6

13

a b

3.1. Characterization of organic materials and soils

c

Table 1 shows a basic characterization of the organic materials. The C content of organic amendments ranged between 18.5 % and 26.4 %. The other organic material, purified HA, showed the highest % C (45.8 %). In the case of O %, HA was in the range of the other organic materials (between 12.7 % and 27.5 %). The PI showed HA as the most hydrophobic material whereas M was the most polar amendment. This index was also supported by the results of CP-MAS 13C-NMR (Table 2). HA was the material with higher aliphatic-C content and lower O-alkyl and N-alkyl content, whereas M contained the lowest aliphatic-C content and the highest O-alkyl content. In fact, some correlations were found between carbon composition and PI of organic materials. PI was positively correlated with O-alkyl (r = 0.928, p < 0.05) and with O % (r = 0.937, p < 0.05). In addition, O-alkyl was negatively correlated with % C (r = -0.903, p < 0.05) or aliphatic-C (r = -0.919, p < 0.05) which are correlated between them (r = 0.965, p < 0.01). The chemical characterization of organic amendments was similar to the interval of values published (Zmora-Nahum et al., 2007) with some exceptions such as the lowest aromatic-C of M and the highest aliphatic-C of GC in this work. The main characteristic of unamended and amended S1 and S2 are included in Table 3. S1 and S2 were slightly basic and presented low electric conductivity. These soils present different OC content (1.2 % and 0.9 %, respectively) and texture. S2 was loamy sand whereas S1 was sandy loam with a silt + clay content twice higher than S2. The OC contents of both soils increased between 1.75–2 times (S1) and between 1.9–4.5 times (S2) after amendment.

indicate different mechanisms of adsorption in function of the adsorbent and the adsorbate. In general, a lower water solubility or a higher hydrophobic character indicated by the Kow value of the herbicide corresponded to a higher adsorption. However, some exceptions were found, such as the adsorption behaviour of TSF in SS with higher Kf and Kd values than those of CTL and FNC. The highest adsorption constants, Kf or Kd, in all the materials were obtained for the herbicide PSC with the lowest water solubility and the highest Kow. Previous works reported similar results for other pesticides or organic pollutants and adsorbents (ÁlvarezMartín et al., 2016a; Marín-Benito et al., 2012a). They indicate the crucial role of organic pollutant solubility and the chemical characteristics of organic amendments on the adsorption mechanisms. The Kf values of the four herbicides by HA were significantly higher than those by the other organic amendments (SMS, GC, SS and M). In general, this trend was observed for the Kd and Koc values. This difference between Kf values of HA and those of the other materials was especially wide for TSF whose Kf values were 21, 55, 27, and 3 times higher than the Kf values of SMS, GC, M and SS, respectively. The difference between the Kf value of HA and the other materials decreased when the water solubility of the herbicides decreases. The Kf of HA for CTL was 6–21 times higher than that of the other materials, and for FNC and PSC the ranges were 5–9 and 1.5–2.3 times higher, respectively. Hence, not only the C content explains their adsorption capacity, but also the water solubility of the organic pollutants seems to be a key factor in the adsorption capacity of these organic materials. Significant correlations were found between Kf and % C of organic materials for TSF (r = 0.962, p < 0.01), CTL (r = 0.890, p < 0.05), FNC (r = 0.939, p < 0.05) and PSC (r = 0.909, p < 0.05). Therefore, the C content seems to be a good indicator of the capacity of organic amendments to adsorb organic pollutants. James et al. (2019) reported similar conclusion and highlighted the benefit for the herbicide leaching reduction. Accordingly, the Koc values obtained were closer than the Kf values, but the coefficients of variation (CV) among them were also wide, varying between 1.2 % and 21.6 % for TSF and PSC, respectively. Variations in the Koc values indicate the influence of the organic material chemical structure on the adsorption or other factors as reported for adsorption of pesticides from different organic residues (Marín-Benito et al., 2012b). A multiple regression model was tested with % C, PI and Kow to evaluate the different factors influencing the adsorption related to properties of herbicides and organic amendments. The resulting equation (Kf = - (440 ± 75) + (16.45 ± 1.71) %C + (87.33 ± 35.38) PI + (0.009 ± 0.001) Kow; R2 = 0.894, p < 0.001), indicated that chemical composition of the organic materials expressed as PI and % C were the most important factors together with the Kow of herbicides to predict the Kf adsorption parameters. Therefore, the model discloses

3.2. Adsorption – desorption of pesticides by organic amendments In Table S2, (in Supplementary Material) the adsorption and desorption data of pesticides by organic materials are reported. The adsorption isotherms (Fig. 1) fitted the Freundlich model with r2 values in the range 0.94–1.00, except in the case of TSF adsorbed by M (r2 = 0.82). The adsorption isotherms were of type S (nf > 1), L (nf < 1) or even close to linear (type C, nf = 1) for PSC. These results Table 2 Carbon distribution of organic materials calculated by relative areas of the chemical shift regions (ppm) in Cross-polarization and magic angle spinning 13 C-nuclear magnetic resonance (CP-MAS 13C-NMR) spectra. Organic material

Alkyl-C 0–45

N-alkyl-C 45–60

O-alkyl-C 60–110

Aromatic-C 110–160

Carboxyl/ carbonyl-C 160–220

SMS GC M SS HA

28.2 35.5 25.7 46.7 82.2

4.1 3.4 0.9 0.1 0.6

42.2 33.8 56.9 17.3 2.9

12.5 12.4 5.4 9.0 10.1

13.1 14.9 11.1 26.9 4.3

EC, electrical conductivity. OC, organic carbon. DOC, dissolved organic carbon.

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Fig. 1. Adsorption-desorption isotherms of triasulfuron (A), chlortoluron (B), flufenacet (C) and prosulfocarb (D) for spent mushroom substrate (SMS), green compost (GC), manure (M), sewage sludge (SS) and humic acid (HA). Closed symbols and continuous line correspond to adsorption and open symbols and dashed line correspond to desorption. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

that the polarity of organic pollutants and adsorbents, and carbon content of adsorbents are important factors to explain the adsorption. PCA was performed (Fig. 2) in order to visualize the relation between C composition of organic materials and their adsorption affinity for each herbicide (Kf). The distribution of components revealed the positive relation between aliphatic-C and aromatic-C and Kf of the four herbicides because of the positive values of their respective loading factors in component 1. In contrast, O-alkyl-C and to a lesser extent Nalkyl-C, carbonyl-C and carboxyl-C, that are more related to the second component, are negatively related to high adsorption constants (Kf). The distribution factors in PCA diagram indicated the positive effect of hydrophobic groups (aliphatic and aromatic-C) in adsorption of herbicides while polar groups produced negative effects, mainly O-alkyl. The low loading factor of carbonyl-C and carboxyl-C in component 1 might be related to the balance between the negative effect of them in herbicides adsorption in agreement with their polarity and their high relation with aliphatic-C (positive effect on herbicides adsorption) from aliphatic acids (Frutos et al., 2016). The organic amendments are clearly distributed in three zones of the diagram. HA, which is in the right zone, is clearly related with its high aliphatic and aromatic character and high adsorption capacity.

SMS, GC and M are in the left zone that is related to O-alkyl and N-alkyl carbon. SS is at the bottom center zone closely related to carbonyl-C and carboxyl-C and relative balance between hydrophobic groups (aliphatic and aromatic) and O-alkyl-C. PERMANOVA analysis highlights the significant differences between the three clusters (p = 0.002) or even four clusters (p < 0.001) extracting SMS from the initial cluster formed by SMS, GC and M denoted in Fig. 2 that represent the hierarchical clustering. Despite additional factors such as organo-mineral interactions governed the adsorption (Shi et al., 2018), the content of aliphatic- and aromatic-C in humic substances indicated an important role on organic pollutants adsorption. The desorption isotherms (Fig. 1) were performed by desorption from samples initially treated with 25 μg mL−1 of TSF, CTL and FNC and 10 μg mL−1 of PSC. The desorption constants from Freundlich equation were obtained and hysteresis coefficients (H) were determined (H = nf / nfd) (Table S2). The desorption isotherms showed hysteresis as the desorption data did not match with those of the adsorption isotherms, and the H coefficients obtained were constantly > 1 for all organic materials, indicating that the desorption process was not fully reversible. The percentage of desorbed herbicides were low or very low for TSF (2%–16%) and PSC (3%–17%) and higher in the case of CTL (27 Fig. 2. Principal component analysis (PCA) of organic materials (spent mushroom substrate (pink square), green compost (green diamond), manure (violet star), sewage sludge (blue triangle) and humic acid (orange dot)) showing loading scores for Freundlich adsorption coefficients of triasulfuron (Kf_T), chlorotoluron (Kf_C), flufenacet (Kf_F) and prosulfocarb (Kf_P) and relative areas of the chemical shift regions in CP-MAS 13CNMR spectra with respect to carbon content of organic materials. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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Fig. 3. Adsorption-desorption isotherms of triasulfuron (A), chlortoluron (B), flufenacet (C) and prosulfocarb (D) for S1 unamended soil and S1 amended with spent mushroom substrate (SMS), green compost (GC), manure (M) and sewage sludge (SS) at 10 % dose. Closed symbols and continuous line correspond to adsorption and open symbols and dashed line correspond to desorption. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

%–59%) and FNC (21 %–67%). Thus, in the desorption process, the Kow of herbicides was not a key factor. The molecular interactions between the organic materials and the herbicides seem to be more important. In this sense, despite the lower Kow of TSF with respect to the other herbicides, the H values of TSF were higher than those of the other herbicides. In contrast, previous work reported a significant correlation between Kow of eight fungicides and remaining adsorbed amounts in organic adsorbents derived from SMS (Marín-Benito et al., 2012b). However, these authors also reported the importance of chemical structure of the fungicides and the polarity of the organic materials in the adsorption – desorption processes.

(r ≥ 0.994, p < 0.001) and in S2, Kf and Kd (r ≥ 0.884, p < 0.05) and for FNC in S1, Kf (r = 0.878, p < 0.05) were significant. In amended soils, the Kf coefficients increased 0.79–4.79, 1.27–2.12, 1.13–1.42 and 1.39–1.58 times for TSF, CTL, FNC and PSC, respectively, with respect to unamended S1, and 1.50–16, 1.46–3.93, 2.61–3.93 and 2.09–2.63 times, respectively, with respect to unamended S2. The results indicate different increases in the adsorption capacity of the amended soils for the herbicides with respect to the unamended soils S1 and S2. In fact, the isotherm type was not always the same for one herbicide and organic amendment in S1 and S2. For TSF, isotherms type S were observed in S1-SMS or S1-M, whereas in S2SMS or S2-M they were type L, indicating an increase or decrease of the adsorption of this herbicide when its concentration increased. It should be also noted that increases in adsorption were higher in the loamy sand soil S2 with 6 % and 13 % of silt and clay fractions than in the sandy loam soil S1with 25 % and 17 % of silt and clay fractions for all the herbicides. Different interactions between the components of organic amendments and soils could occur. In fact, the importance of the interactions of soil OM and clay minerals on the behaviour of organic compounds in soils has been widely reported (Cornejo and Hermosín, 1996). New adsorption surfaces could be formed in S1 and S2 depending on the interactions between OC of the amendments and silt and clay fractions promoting new sites to increase the adsorption capacity of soil or inhibiting the adsorption by the organic amendments. This could produce different effects on the herbicides´ adsorption and explain how that different organic amendment reaches a maximum increase of Kf for each herbicide in S1 and S2. The highest increase of Kf was obtained for TSF by M in S1 and by SMS in S2, for CTL by SMS in S1 and S2, for FNC by SMS and M in S1 and by M in S2, and for PSC by SMS in S1 and S2. Therefore, these results were not the same for organic amendments on their own (Tables S2) where the highest Kf for TSF, CTL, FNC and PSC were achieved by SS, SMS and M, SMS, and SS, respectively. It is noteworthy the high adsorption of TSF in SS, the decrease of Kf in S1-SS and the low increase in S2-SS with respect to unamended soils. Hence, the good adsorption properties of one organic amendment do not guarantee good results in soil conditions. On the other hand, the normalized adsorption coefficients to OC content (Koc) varied in a wide range for the most soluble and the least

3.3. Adsorption – desorption of pesticides by unamended and amended soil The adsorption isotherms of unamended S1 and S2 soils (Figs. 3 and 4) fitted the Freundlich model with r2 values in the range 0.95–1.00. Tables S3 and S4 (in Supplementary Material) show the adsorption and desorption coefficients obtained. The PSC isotherms were close to linear (type C), while TSF, CTL and FNC isotherms were of type S or L for both S1 and S2. Therefore, the herbicides’ adsorption increased (TSF) or decreased (TSF, CTL and FNC) when their concentrations increased while PSC adsorption was less affected by its concentrations in solution. The adsorption by soils was also related to the water solubility of the herbicides, the highest water solubility, the lowest adsorption, as observed previously for organic amendments. However, the adsorption capacity was different for each soil and the adsorption coefficients for S1 were higher than for S2. There was a significant correlation between the water solubility of the herbicides and the ratio Kf S1 / Kf S2 (r = 0.98, p < 0.01), highlighting the important role of herbicide solubility in soil adsorption. The result agreed with the higher OM and clay contents of S1 comparing with those of S2. These two factors were described as the main responsible for pesticide adsorption by soils (Ren et al., 2018; Vitoratos et al., 2016). The application of organic amendments increased the OC content of two soils and the adsorption coefficients of herbicides as expected due to the positive effect of the amendments as previously indicated, and in general reported (James et al., 2019; Marín-Benito et al., 2013, 2018b), except for TSF by S1-SS. However, only the correlations between the OC content and the adsorption constants for PSC in S1, Kf and Kd 6

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Fig. 4. Adsorption-desorption isotherms of triasulfuron (A), chlortoluron (B), flufenacet (C) and prosulfocarb (D) for S2 unamended soil and S2 amended with spent mushroom substrate (SMS), green compost (GC), manure (M) and sewage sludge (SS) at 10 % dose. Closed symbols and continuous line correspond to adsorption and open symbols and dashed line correspond to desorption. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

hydrophobic herbicide (CV was 60%–90%) in S1 and S2 indicating the influence of the OC nature of the amendments, while for CTL, FNC and PSC the soil OC contents seemed to affect more to the adsorption than the nature of the amendment as indicated by the CV of Koc values (Tables S3 and S4) (18%–20%, CTL and FNC, and 11%–25%, PSC). This fact corroborates the influence of hydrophobic character of herbicides on their adsorption on amended soils as indicated above for the organic amendments. PCA performed with the increase of the percentage of each C type (CP-MAS 13C-NMR) and the adsorption capacity of the amended soils (Fig. 5) corroborated previous results and showed that organic amendments did not produce the same effect in the two tested soils. In fact, PERMANOVA analysis reported significant differences between S1 and S2 (p < 0.001), and interaction between organic amendments and soils (p < 0.001). Similar conclusion was extracted from the hierarchical clustering, where two clear clusters were formed by each soil. All of these findings support the necessity of carrying out tests at laboratory scale to determine the real effect of organic amendments in the target soil. The only study of adsorption capacity of organic amendments was not enough to determine the efficiency of these materials in soil conditions. Moreover, in the clusters, it is clear how the

organic amendments were grouped in different way for each soil. SMS, GC and SS formed a sub-cluster in S1 and M another one. In S2, SMS and GC formed a sub-cluster and SS and M formed another one (Fig. 5). Therefore, it is clear that organic amendments applied to different soils can increase the adsorption capacity of amended soils to a different extent because different organo-mineral interactions between pristine soil and organic amendments. Figs. 3 and 4 show the desorption isotherms of herbicides from unamended and amended S1 and S2, and Tables S3 and S4 summarize their respective desorption and H coefficients. The desorption of TSF, CTL and FNC in S1 was lower than in S2, in agreement with the higher OM and clay content of S1. In the case of PSC, desorption was similar in both soils, despite their different characteristics. It was due probably to the high hydrophobicity of PSC that favours its adsorption (Cristale et al., 2017; Marin-Benito et al., 2012b). There was hysteresis in both unamended soils and for the four herbicides, which indicates the incomplete reversibility of the adsorption process. Barba et al. (2019) reported the quick formation, less than 24 h, of non-extractable PSC residues in unamended and GC-amended soil. The H values of TSF in S1 and in S2 were the highest and corresponded to low percentages of desorption (0.7% in S1 and 15.7% in S2). The lack of correlation Fig. 5. Principal component analysis (PCA) of S1 (blue) and S2 (red) amended soils with spent mushroom substrate (square), green compost (diamond), manure (star) and sewage sludge (triangle) showing loading scores for Freundlich adsorption coefficients of triasulfuron (Kf_T), chlorotoluron (Kf_C), flufenacet (Kf_F) and prosulfocarb (Kf_P) and increase of relative areas of the chemical shift regions in CP-MAS 13CNMR spectra with respect to unamended S1 and S2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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Fig. 6. Principal component analysis (PCA) of S1 (blue) and S2 (red) amended soils with spent mushroom substrate (square), green compost (diamond), manure (star) and sewage sludge (triangle) showing loading scores for hysteresis coefficients of triasulfuron (H_T), chlorotoluron (H_C), flufenacet (H_F) and prosulfocarb (H_P) and increase of relative areas of the chemical shift regions in CP-MAS 13C-NMR spectra with respect to unamended S1 and S2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

between the Kow of herbicides and the desorption coefficients suggests that different factors such as soil characteristics or chemical structure of herbicides are responsible for the interaction between them to control the adsorption – desorption processes. The Kfd of the amended S1 was higher than that of S1 for TSF, CTL and PSC with the exception of S1-SS for TSF and CTL that reduced and did not modify the Kfd, respectively. For FNC, the values of Kfd were lower for all the amended S1 than for the unamended S1 (Table S3). The opposite trend was observed for amended S2 where desorption of the herbicides was reduced with respect to unamended S2 (Table S4). Thus, these results highlighted the importance of the interaction soil – organic amendment – pollutant and the necessity of testing previous field application. The irreversibility of herbicides adsorption enhances the usefulness of organic amendments to control pollutants leaching. Therefore, the analysis of the OC composition of the organic amendments to determine the main factors that enhance the desorption hysteresis could be useful to select appropriate organic amendments. PCA was performed with the increase of the percentage of each C type (CP-MAS 13C-NMR) and the H coefficients of amended soils (Fig. 6). PCA showed that component 1 was strong and positively related to aliphatic and aromatic-C whereas the hysteresis of CTL and FNC was negatively related. These results indicated the low or even negative effects of hydrophobic C in the hysteresis of CTL and FNC. The opposite trend was observed for PSC, where the hysteresis was clearly oriented to the positive zone of component 1 and therefore positively related to the hydrophobic character of C. Therefore, Van der Vaals and π – π interactions seems to be the main adsorption mechanisms for PSC by OC. The adsorption of another hydrophobic pesticide, chlorpyrifos, in soil was closely related to the hydrophobic character of soil OM (Parolo et al., 2017). The component 2 was positively related to the functional groups N-alkyl, O-alkyl and hysteresis of CTL and FNC and negatively related with carboxyl and carbonyl groups. Hence, N-alkyl and O-alkyl groups had an important role in the irreversible immobilization of CTL and FNC probably due to the interaction of these herbicides with OC by hydrogen bound or by interactions between polar groups. The low loading factors of the hysteresis of TSF in component 1 and 2 suggest that other factors such as pH or clay content controlled its adsorption – desorption (Han et al., 2019). PERMANOVA analysis indicated significant differences (p < 0.050) between organic amendments applied to soils, but a lack of differences between S1 and S2 (p = 0.516). However, there was interaction between organic amendments and soils (p = 0.014) indicating the importance of testing organic amendments with the target soil and pollutants. For instance, S1-SMS and S1-GC were located in the negative zone of component 1 whereas S2-SMS and S2-GC were located in the positive zone of component 1.

4. Conclusions The application of organic amendments in soils as adsorbents of organic pollutants such as pesticides is an effective way to immobilize these compounds in the soil and prevent ground water pollution. In this paper four organic amendments from different raw materials together with a commercial humic acid (HA) as a major fraction of soil OM were selected to determine the adsorption capacity of four herbicides with different structure. The herbicides were representative of different chemical groups such as sulfonylurea (triasulfuron), urea (chlorotoluron), oxyacetamida (flufenacet) and thiocarbamate (prosulfocarb). Results indicated that the adsorption capacity of organic amendments depends on their own chemical composition and structure and on external factors such as polarity of pollutants and soil properties. The key factors to guarantee an efficient adsorption of organic pollutants by organic amendments are their OC content and structural C type and moreover, the hydrophobicity of pollutants. The selection of organic amendments rich in carbon and aliphatic and aromatic structures optimizes the adsorption of pollutants. However, it is necessary to have an adequate presence of O-alkyl and N-alkyl groups to enhance hysteresis of some pollutants. Additionally, this work demonstrates the necessity of performing laboratory tests to determine the interactions pollutant organic amendment - soils before field application takes place in order to achieve an optimized organic pollutants immobilization. CRediT authorship contribution statement Carlos García-Delgado: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. Jesús M. Marín-Benito: Conceptualization, Methodology, Validation, Writing - original draft, Writing - review & editing, Visualization. María J. Sánchez-Martín: Conceptualization, Funding acquisition, Methodology, Project administration, Validation, Visualization, Writing - original draft, Writing review & editing. M. Sonia Rodríguez-Cruz: Conceptualization, Methodology, Validation, Writing - original draft, Writing - review & editing, Visualization, Supervision. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgements This work was funded by the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) and the Regional government, Junta de Castilla y Leon as part of the projectsAGL2015-69485-R and 8

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CSI240U14. C.G.D. and J.M.M.B thanks MINECO for their “Juan de la Cierva-Formación” JCFI-205-23543 and “Juan de la CiervaIncorporación” IJCI-2014–19538 postdoctoral contracts, respectively. We thank Prof. Enrique Eymar for his assistance with 13C-NMR CPMAS and elemental analysis.

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