1. HUMUS – PESTICIDE INTERACTIONS IN SOIL AND THEIR MECHANISMS 1

2. Background Adsorption by organic matter is a key factor in the behaviour of many pesticides in soil, including bioactivity, persistence, biodegradability, leach ability and volatility. Soils that are black (Mollisols) have higher organic matter content than those which are lighter(Alfisols), and pesticide application rate must often be adjusted upward on the darker soils to achieve the desired results. 2

3. Factors affecting pesticide adsorption by SOM: 1.Physical-chemical characteristics of the adsorbent (humic colloids) Humic acid, Fulvic acid, colour, mol. wt. and functional groups, etc. 2. Nature of the pesticides Chemical and physical properties of the pesticides 3. Properties of the soil system Mineral composition, pH, kinds and amount of exchangeable Cations, etc. 3

4. Basics of humus chemistry in relation to pesticide behaviour A.Organic matter versus clay as adsorbent: Organic matter and clay are the soil components most often implicated in pesticide adsorption. However, individual effects are not as easily ascertained as might be supposed, for the reason that, in most soils, organic matter is intimately bound to the clay, probably as clay-metal- organic complex. Thus, two major types of adsorbing surfaces are normally available to the pesticide, namely, clay-humus and clay alone. 4

5.  Accordingly, clay and organic matter function more as a unit than separate entities and the relative contribution of organic and inorganic surfaces to adsorption will depend upon the extent to which the clay is coated with organic substances  Bailey and White demonstrated that the adsorption capacity of clays for herbicide followed the order of montmorillonite > illite > kaolinite  Comparative studies between known clay minerals and organic soils suggest that, most, but not all, pesticides have a greater affinity for organic surfaces than that of mineral surfaces 5

6. B. Qualitative differences in soil organic matter The fact that soil differ greatly in their organic matter content is widely known, but it is not generally appreciated that major qualitative differences also exist, not only with respect to the known classes of organic compounds (lipids, carbohydrates, proteins),but so called humic substances (humic acid, fulvic acid etc.) The humic material of grassland soil is dominated by humic acids; that in forest soil it is relatively rich in fulvic acids. 6

7.  Differences in organic matter compositions may have implications with respect to correlation studies of herbicide retention with organic matter content.  Haye’s et al., suggest that fulvic acids may be less effective in adsorbing s-triazine herbicide than humic acid.  Fulvic acids, by virtue of their high functional group content, they may act as a transporting agents for certain pesticides in soil and natural water and catalyze the chemical decomposition of certain herbicides. 7

8. C. Potential chemical reactions involving pesticides and organic substances There seems little doubt but that the organic fraction of the soil has the potential for promoting the non-biological degradation of many pesticides. Organic compound containing nucleophilic reactive groups of the type believed to occur in humic and fulvic acids (e.g. COOH, phenolic- and enolic-, heterocyclic-, and aliphatic-OH, semiquinones and others) are known to produce chemical changes in a wide variety of pesticides. Basic amino acids and similar compounds are able to catalyze the hydrolysis of organophosphorus esters as well as the dehydrochlorination of DDT and lindane. 8

9. Chemical binding of pesticides and their decomposition products Substantial evidence exist to indicate that pesticide-derived residues can form stable chemical linkages with components of soil organic matter and that such binding greatly increases persistence of the pesticide residues. Two main mechanism can be envisioned: 1.Direct chemical attachment of the residues to reactive sites on colloid organic surfaces, and 2.Incorporation in to the structures of newly formed humic and fulvic acids during the humification process. Those pesticides which are basic in character, such as s-triazines, have the potential for forming a chemical linkage with carbonyl constituent of soil organic matter. 9

10. Adsorption mechanisms Bonding mechanism for the retention of pesticides by organic substances in soil includes,  Ion exchange  Protonation  H-bonding  Van der Waal’s forces and  Ligand exchange (coordination through an attached metal ion) 10

11. Ion exchange and Protonation Like the secondary silicate minerals soil organic colloids are negatively charged, although positive spots may present under some conditions through amino groups. Adsorption of pesticides by this mechanism is largely restricted to those types which exist as cations (RNH 3 ) or which can become positively charged through protonation. (OM) - COO - + RNH 3 + → (OM) - COO -. H 3 NR 11

12. Anion exchange may be possible in some cases through interaction of anionic pesticide to positively charged spots on organic surfaces. Diquat and Paraquat, being divalent, have the potential for reacting with more than one negatively charged sites on soil humic colloids, such as through two COO - ions. 12

13. Hydrogen bonding For anionic pesticides, such as the phenoxyalkanoic acids, repulsion by the predominantly negatively charged surface of organic colloids may occur. Positive adsorption of anionic herbicide at pH values below their pKa values can be attributed to adsorption of the unionized form of the herbicide to organic surfaces, such as by hydrogen bonding between the COOH group and C=O or NH group of organic matter. The great importance of H- bonding is suggested, with multiple sites being available on both herbicide and organic matter surfaces. 13

14. Van der Waal’s Forces and Ligand exchange Other adsorption mechanisms include Van der Waal’s Forces (physical adsorption), ligand exchange (-M z+ ………….O=C), and for herbicides containing an ionizable COOH group, a salt linkage through a divalent cation on the organic exchange site. Vander waals forces(physical adsorption) - Attraction of intermolecular forces between molecules Ligand exchange - ligand in a compound is replaced by another Salt linkage - through a divalent cation on the organic exchange site. 14

15. Pi bonding Pi- (π) bonding results from the overlap of bonding orbitals perpendicular to the aromatic ring. These bonds are believed to be involved in the binding of alkenes, alkynes and aromatic compounds to soil organic matter. 15

16. Hydrophobic Bonding Hydrophobic sorption has been proposed as a mechanism for retention of non polar organic compounds by soil organic matter. Hydrophobic bonding increases as the solute becomes more and more non polar, or as a water solubility decreases. Active surfaces for hydrophobic bonding include fats, waxes and resins. 16

17. There are two thoughts as to the nature of the sorption interaction. One holds that retention of the solute occurs through “adsorption” onto hydrophobic surfaces of the organic matter and results from a “squeezing-out” of the molecule from solution and its accumulation on at the solid interphase where the competition with the solvent is minimum. The second scheme is that bonding occurs through “partition”, in which the case the sobbed material penetrates into interior of the organic phase. 17

18. Interactions Between The Organic Matter In Soil And A Chloro-s triazine Bonding of triazines with soil organic matter appears to include several mechanisms. Under pH conditions found in soils (pH 4–8), relatively strong H- bonds form between triazine ring N atoms (triazine is the bond acceptor) and acidic carboxylic, phenolic, and the amide organic functional groups in the soil organic matter (Kalouskova, 1989). 18

19. 19 Protons of the triazine amine groups form H-bonds with electronegative centers in the organic matter, principally the quinone, ketonic, and aldehyde groups. Such H- bonding is favored when the system pH is above the pKa of the triazine and below the pKa of the acidic functional group.

20. DYNAMIC OF HERBICIDES IN SOIL AND SOIL MODIFIED WITH CLAY AND /OR HUMUS 20 Copaja, S.V. and Sepulveda, C., 2022. DYNAMIC OF HERBICIDES IN SOIL AND SOIL MODIFIED WITH CLAY AND/OR HUMUS. Journal of the Chilean Chemical Society, 67(3), pp.5587-5594.

21. Table 1:-Atrazine % values at time zero and at the end of the study for the different soil samples. 21

22. Table 2:-Trifulaline % values at time zero and at the end of the study for the different soil samples. 22

23. Bonfleur, E.J., Kookana, R.S., Tornisielo, V.L. and Regitano, J.B., 2016. Journal of agricultural and food chemistry, 64(20), pp.3925- 3934. 23 Organomineral Interactions and Herbicide Sorption in Brazilian Tropical and Subtropical Oxisols under No-Tillage

24. Table 3:- Sorption Coefficients (Kd) and Sorption Coefficients Normalized to Soils OC Contents (Koc) for Alachlor, Bentazon, and Imazethapyr 24

25. Table 4:-Simple Linear Correlation Coefficients and Significance between the Physical and Chemical Soil Attributes, Quality Indicators of SOM, Sorption Coefficients (Kd) and Sorption Coefficients Normalized to Soils OC Contents (Koc) for Alachlor, Bentazon, and Imazethapyr 25 Kd = partition coefficients punctual (Kd = S/Ce, L kg−1 ) and Koc = partition coefficient normalized to the SOC content (Koc = 100 Kd/ OC, L kg−1 ).

26. Table 5:-AT adsorption (%) related to the largest loading (30 mg L1 ) and Freundlich parameters (Kf and nf) for clays and their complexes with HA. 26 Besse-Hoggan, P., Alekseeva, T., Sancelme, M., Delort, A.M. and Forano, C., 2009. Atrazine biodegradation modulated by clays and clay/humic acid complexes. Environmental Pollution, 157(10), pp.2837-2844.

27. Table 6:-Adsorption and Desorption K f (µmol 1-1/n L 1/n kg -1 ) and 1/n Parameters Obtained from the Linearized Freundlich Equation for Unmodified Soil and Humic Acid Soil (Soil-HA) a 27 The adsorption affinity order evaluated as a function of the K f values was AT-OH > AT > DIA > DEA, while desorption followed the order DEA > DIA ∼ AT > AT-OH. Abate et al.2004 Journal of agricultural and food chemistry, 52(22),6747-6754.