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Potential Contributions of Smectite Clays and Organic Matter to Pesticide Retention in Soils

Profile image of Brian TeppenBrian Teppen
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27 citations

2001, Journal of Agricultural and Food Chemistry

https://doi.org/10.1021/JF001485D
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9 pages

Abstract

Soil organic matter (SOM) is often considered the dominant sorptive phase for organic contaminants and pesticides in soil-water systems. This is evidenced by the widespread use of organic-matternormalized sorption coefficients (K OM ) to predict soil-water distribution of pesticides, an approach that ignores the potential contribution of soil minerals to sorption. To gain additional perspective on the potential contributions of clays and SOM to pesticide retention in soils, we measured sorption of seven pesticides by a K-saturated reference smectite clay (SWy-2) and SOM (represented by a muck soil). In addition, we measured the adsorption of atrazine by five different K-saturated smectites and Ca-saturated SWy-2. On a unit mass basis, the K-SWy-2 clay was a more effective sorbent than SOM for 4,6-dinitro-o-cresol (DNOC), dichlobenil, and carbaryl of the seven pesticides evaluated, of which, DNOC was sorbed to the greatest extent. Atrazine was sorbed to a similar extent by K-SWy-2 and SOM. Parathion, diuron, and biphenyl were sorbed to a greater extent by SOM than by K-SWy-2. Atrazine was adsorbed by Ca-SWy-2 to a much lesser extent than by K-SWy-2. This appears to be related to the larger hydration sphere of Ca 2+ (compared to that of K + ) which shrinks the effective size of the adsorption domains between exchangeable cations, and which expands the clay layers beyond the apparently optimal spacing of ∼12.2 Å for sorption of aromatic pesticide structures. Although a simple relation between atrazine adsorption by different K-smectites and charge properties of clay was not observed, the highest charge clay was the least effective sorbent; a higher charge density would result in a loss of adsorption domains. These results indicate that for certain pesticides, expandable soil clays have the potential to be an equal or dominant sorptive phase when compared to SOM for pesticide retention in soil.

Key takeaways
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  1. Expandable smectite clays can dominate pesticide retention compared to soil organic matter (SOM).
  2. K-SWy-2 clay adsorbs DNOC approximately 14 times more effectively than SOM at relative concentrations.
  3. Atrazine sorption is similar between K-SWy-2 clay and SOM, revealing complex interactions.
  4. Pesticide sorption varies significantly with clay type, saturating cation, and pesticide structure.
  5. Approximately 4.5 billion pounds of pesticides are used annually in the U.S., influencing environmental fate.

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The soil pH in uences the pesticide sorption activity and is mainly affected by its acidity (pKa) or base (pKb) constants (Hornsby et al. 1996), and several authors (Kah and Brown 2006, 2007; Hyun et al. 2003) have observed a correlation between ionizable pesticide sorption and soil pH, as the relative quantities of weakly basic and acidic pesticides in ionic form depend on the pH of the soil system. However, the adsorption of non-ionic molecules by hydrophobic bonding is pH independent (Weber and Miller 1989). In this study, six herbicides were examined: pendimethalin, aclonifen, urochloridone, S-metolachlor, pethoxamid and dimethenamid-p. These herbicides are often used for pre-and post-emergence purposes to control a variety of annual grasses and broad-leaf weeds. Pendimethalin degradation was previously studied by Ramakrishna et al. (2008) and observed a half-life (DT 50) of 90 days in the eld. It is usually degraded through photodegradation, volatilization, or biodegradation. Pendimethalin is strongly adsorbed to OM and clay minerals and is therefore slightly mobile in soil (Mohan et al. 2007; PPDB 2019). Aclonifen is a low water-soluble (1. 4-2.5 mg L − 1), non-ionizing herbicide and moderately sorbed on soil components (Gouy et al. 1999). Vischetti et al. (2002) studied sorption and degradation of aclonifen and found low degradation and high soil adsorption. Flurochloridone persistence was measured in eld conditions (Vouzounis and Americanos 1995). It is characterized by low volatility, low soluble in water, and is relatively persistent in soil environments (Klíčová et al. 2002). Very little is known about urochloridone adsorption and dissipation in laboratory conditions. However, its toxicological studies in the soil environment have been reported by EFSA (2010). S-metolachlor sorption was measured by Wu et al. (2011) as with many other hydrophobic and non-ionic herbicides, s-metolachlor has a high a nity for the organic matter fraction of soils and its sorption rate is mainly driven by the organic matter content (Weber et al. 2004). Si et al. (2009) found a signi cantly negative relationship between OM and s-metolachlor degradation. Pethoxamid is known to be non-persistent and moderately soluble in water with low soil adsorption (Jursík et al. 2015; PPDB 2019). Several studies have documented its e cacy and selectivity, as well as soil phytotoxic activity under different moisture conditions (Jursík et al. 2015; Dhareesank et al. 2005). Pose-Juan et al. (2018) were investigated pethoxamid dissipation in amended and unamended soil. Dimethenamid-p is characterized as having low volatility, high solubility in water, and is non-persistent in soil environments (PPDB 2019). It has a calculated DT 50 value of about 11.1 days in Haplic Chernozem under eld conditions (Kočárek et al. 2018). The aim of this research was: (i) to characterize the adsorption and degradation of herbicides in three soils concerning different properties; (ii) identify main soil parameters that in uence adsorption and degradation employing a correlation analysis, and (iii) to establish how the degradation and sorption processes

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  56. Weissmahr, K. W.; Hilderbrand, M.; Schwarzenbach, R. P.; Haderlein, S. B. Laboratory and field scale evaluation of geochemical controls on groundwater transport of nitroaromatic ammunition residues. Environ. Sci. Tech- nol. 1999, 33, 2593-2600.

FAQs

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What specific role do smectite clays play in pesticide retention?add

Smectite clays significantly enhance pesticide retention, with K-SWy-2 clay showing 390 mg/kg for carbaryl, substantially more than SOM's 11 mg/kg.

How does the type of exchangeable cation affect pesticide adsorption?add

Pesticide adsorption varies dramatically; K-SWy-2 is approximately seven times more effective for atrazine than Ca-SWy-2, highlighting cation influence on sorption.

What are the main factors influencing pesticide sorption by clays?add

Pesticide structure, clay type and layer charge, as well as saturating cations, greatly influence sorption effectiveness and mechanisms.

What evidence supports the dual role of clays and organic matter in sorption?add

Sorption coefficients indicated K-SWy-2 outperformed SOM for specific pesticides, showcasing clays' significant contribution alongside organic matter.

How do molecular characteristics of pesticides affect their retention by clays?add

Planar and electron-withdrawing substituents enhance binding efficiency with clays, facilitating complex formations that improve sorption rates.

About the author
Brian Teppen
Michigan State University, Faculty Member
Papers
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Effects of Clay Content in Soil on Pesticides Sorption Process
Sylvia Copaja

Journal of the Chilean Chemical Society, 2021

Intensive application of pesticides in the agricultural sector and for domestic purposes has resulted in increased usage over the years. Pesticides are used to control pest, diseases and weeds in agricultural and urban areas, but their persistence in the environment has resulted in human poisoning, health risk problem and environmental pollution due to their ability to permeate the soil surface, groundwater systems and water surface bodies. Sorption of two organophosphorus pesticides: Diazinon and Clorpyriphos was studied in a soil (S) and in soil modified with clay addition. Soil Alhue, VI region, Chile, was spiked with 1% of either Montmorillonite (M) or Kaolinite (K) (S1). In addition, organic matter (OM), in the soil was eliminated and this soil (S2), was spiked again, with 1% of both clays. Batch sorption and kinetics experiments were conducted to obtain the retained amounts onto the soil samples. Pesticides were quantified by high performance liquid chromatography (HPLC). The results showed that for both pesticides the sorption order is: S1-M ˃ S1-K > S1. For soil without OM (S2) the order is the same, although the adsorption was lower. The adsorption isotherms were expressed by the Langmuir and Freundlich models. The negative Gibb's free energy change (ΔG°) values obtained suggest that the adsorption of both pesticides, is an exothermic process. Clay addition to the soil, increased the adsorption processes, generating an increase in the amount of pesticide retained in the soil and partially avoiding possible contamination of the aquifers. The difference found in relation to the behavior of both clays on the soil, can be explained considering its structure and its octanolwater partition coefficient (kow).

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Effects of organic matter and clay content in soil on pesticide adsorption processes
Jelena Gajic Umiljendic

Pesticidi i fitomedicina, 2009

The effect of organic matter and clay content on the adsorption of atrazine, acetochlor, clomazone, pendimethalin and oxyfluorfen in soil samples was studied. In order to determine whether and to what degree different soil properties affect the process of determination of selected pesticides, three soils with different clay and organic matter contents were used. An optimized liquid-solid extraction procedure followed by SPME measurement was applied to analyze the selected pesticides in soil samples. Detection and quantification were done by gas chromatography-mass spectrometry (GC/MS). Relative standard deviation (RSD) values for multiple analyses of soil samples fortified at 30 ?g/kg of each pesticide were below 19%. Limits of detection (LODs) for all compounds studied were less than 2 ?g/kg. The results indicate that soils with different physic-chemical properties have different effects on the adsorption of most pesticides, especially at higher concentration levels.

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Sorption and desorption of atrazine and diuron onto water dispersible soil primary size fractions
Arturo A Keller

Water Research, 2009

In this study, a low energy separation method was employed to separate water dispersible clay-, silt-, and sand-sized fractions. The batch equilibrium method was used to conduct atrazine and diuron sorption/desorption experiments with the bulk soils and their size fractions separately. A Freundlich sorption model provided the best fit for all sorption and desorption data. A mass balance calculation, taking into account the pesticide concentration differences in the size fraction and bulk soil, showed that pesticide sorption onto the different size fractions reproduces well the total amount of the pesticide sorbed onto the bulk soils. Due to their higher soil organic carbon content, the clay fractions were much more effective sorbents for the pesticides than the bulk soils, silt, and sand fractions. For all soils, the amount of the pesticide sorbed onto the clay fractions was more than 20% of the total amount of the pesticide sorbed by the bulk soils even though the clay fractions in these soils were only 5.3-14.0% (by weight). The clay fractions had the highest desorption hysteresis among all size fractions and the bulk soils, followed by silt fractions, implying the clay fractions had the strongest bound and least desorbable pesticide molecules. Our results suggest that attention should be paid to the pesticide sorbed to the smallest colloids, the water dispersible fraction, which can be potentially mobilized under field conditions, leading to wide spreading of contamination.

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Mineral contributions to atrazine and alachlor sorption in soil mixtures of variable organic carbon and clay content
tim grundl

Journal of Contaminant Hydrology, 1993

A sediment mixing approach was taken to systematically vary the organic carbon (oc) and clay content (cm) of a suite of organic-poor, clay-rich sediments. Organic carbon content ranged from 3.2% to 0.4% and clay content ranged from 24% to 51%. Atrazine and alachlor were shown to sorb to both natural organic carbon and clay minerals. Partition coefficients to natural organic carbon (Koc) were found to be 217 and 412 L/kg organic carbon for atrazine and alachlor, respectively. Partition coefficients to the clay fraction were found to be 3.5 and 4.9 L/kg clay for atrazine and alachlor, respectively. When expressed in terms of surface area, the partition coefficients to clay for atrazine and alachlor were 1.80.10-5 and 2.51.10-5 L/m 2 clay, respectively. Critical cm/oc ratios at which mineral phase sorption accounts for 50% of the total are defined. Implications for the modelling of herbicide movement in the subsurface if mineral phase sorption is ignored is discussed.

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Sorption of two polar herbicides in soils and soil clays suspensions
Rafael Celis

Water Research, 1997

Adsorption of the polar herbicides thiazafluron (l,3-dimethyl-l-(5-trifluoromethyl-l,3,4-thiadiazol-2-yl)urea) and metamitron (4-amino-4,5-dihydro-3-methyl-6-phenyl-l,2,4-triazin-5-one) in the aqueous su,;pension of five soils of southern Spain, their respective clay fractions (with diverse organic carbon content and clay mineralogy) and model pure clay minerals has been monitored as an integrated study to assess the role of the diverse colloidal soil components and their solid/solution ratio, as relevant to the transport of contaminants by particulate matter in water. Adsorption isotherms obtained were analysed and fitted to the logarithmic form of the Freundlich equation and adsorption coefficients Kr calculated. Thiazafluron adsorbs on soils, soil clays and model mineral sorbents to a higher extent than the herbicide metamitron due to their different molecular structure. The sorption of both herbicides in clay fractio~as increases with decreasing solid/solution ratio. The highest Kf value at high solid/solution ratio for both herbicides is found in a saline soil with its clay fraction predominantly composed of an altered illite mineral which behaves as a montmorillonite. Thiazafluron and metamitron also show the highest adsorption capacity (at low solid/solution ratio) on a predominantly montmorillonitic clay fraction of low cation exchange capacity (CEC), whereas low adsorption is found on a montmorillonitic clay fraction of high CEC. The negative influence of the clay CEC is confirmed in adsorption studies on pure clay minerals suspensions. The sorption of both herbicides by soil clays after removing organic matter (OM), shows that contribution of the colloidal OM is very low for thiazafluon, although rather important for metamitron. The influence of the different nature of the OM associated to the clay fractions of diverse soils is suggested. The mineral components of the soil clays, especially expandable layer silicates such as montmorillonite and a type of altered illite, are revealed to be responsible for the adsorption and hence the transport of these polar herbicides by waters in contact with soils or fine-size soil separates. Not only the relative amounts of the organic and inorganic components are important, but also the surface properties and the accessibility of the functional active groups of the herbicide molecule to those surfaces.

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    Acetamiprid, carbendazim, diuron and thiamethoxam sorption in two Brazilian tropical soils
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    Journal of Environmental Science and Health, Part B, 2007

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    Wolfgang Friesl-Hanl

    Journal of Soils and Sediments, 2012

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    Slow-release formulations of the herbicide MCPA by using clay-protein composites
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    Pest Management Science, 2014

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