Organic carbon nature determines the capacity of organic amendments to adsorb pesticides in soil
Graphical abstract
Introduction
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 (García-Delgado 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; Marín-Benito 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-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.
Section snippets
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-(2-chloroethoxy) phenylsulfonyl]-3-(4-methoxy-6-methyl-1,3,5-triazin-2-yl) urea), chlorotoluron (CTL) (3-(3-chloro-p-tolyl)-1,1-dimethylurea), flufenacet (FNC) (4´-fluoro-N-isopropyl-2-[5-(trifluoromethyl)-1,3,4-thiadiazol-2-yloxyl]acetanilide) and prosulfocarb (PSC) (S-benzyl
Characterization of organic materials and soils
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
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
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
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 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 Cierva-Incorporación” 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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