Adsorption of pesticides onto natural clay mineral relies on the use of adsorbents with minimal treatment beyond their preparation to provide a narrow size distribution and homoionic form by exchanging the naturally occurring interlamellar cations (in the case of smectites) by some alkaline (Na+ or K+) or alkaline earth (Ca2+or Mg2+) cation. Additional modifications include organophilization, intercalation with metal polycations and pillaring. The adsorption capacity and strength of pesticides onto homoionic, organophilic and intercalated/pillared clay minerals depend on the chemical nature of the pesticide, surface area, and pore volume. Electrostatic interactions, hydrogen and coordinative bonds, surface complexations, and hydrophobic associations are the main interactions between pesticides and clay minerals.
The so-called Green Revolution dramatically increased agricultural productivity from the middle of the twentieth century to the present . However, this increase relied heavily on chemical fertilizers and a wide range of pesticides, especially herbicides. Consequently, herbicides’ contamination of soils, groundwater, and surface water is a concern of prime importance due to the severe effects of these compounds on humans, animals, and the ecosystem’s equilibrium .
Adsorption is among the most efficient technologies to prevent or remediate pollution from pesticides because it relies on low-cost materials such as biomaterials, aluminum, and iron oxides, or oxyhydroxides, zeolites, and clay minerals. Clay minerals exhibit properties such as high superficial area, high adsorption capacity, low cost, and ready availability that are valuable to the development of herbicide formulations with controlled releasing of active components , cleanup of contaminated soils, groundwater protection , and water treatment .
Clay minerals have gained interest because they are abundant in nature and are environmentally compatible. Montmorillonite (Mt), like other smectites, and vermiculite (Vt), have permanent negative charges generated by the isomorphic substitutions of Si 4+ by Al 3+ in the tetrahedral sheets and of Al 3+ by Mg 2 + in the octahedrons. Cations such as Na + , Ca 2+ , and Mg 2+ in the interlayer keep the electroneutrality. These permanent negative charges interact with cationic herbicides such as paraquat and diquat . Additionally, these interlayer cations are easily exchangeable, for instance, with organic quaternary ammonium salts to produce hydrophobic organoclays suitable for retaining neutral pesticides . Aluminum, Fe 3+ , Cr 3 + , Ti 4+ , Zr 3+ exchange with the interlayer cations and form polynuclear cationic species under hydrolysis, increasing the affinity towards anionic species . Alternatively, the suspension of polycations can be first prepared and then exchanged with the interlayer cation . The exchange of the interlayer cation by a polynuclear hydroxyl metal cation is a modification process named intercalating. Calcinating the intercalated clay minerals produces oxide pillars between the layers, increasing the basal spacing d(001), the specific surface area, and the microporosity, enhancing their adsorption capacity and affinity towards a wide variety of organic molecules, including the pesticides .
Excellent reviews recently addressed the interactions between organic compounds and clay minerals, aiming to develop controlled released herbicide formulation and water purification . A straightforward comparison of adsorbent efficiencies towards various compounds is possible only if parameters such a mass to volume ratio, kinetic, adsorption isotherms, and thermodynamic constants derived from adsorption experiments are presented.
Cation exchange, for instance, is the primary retention mechanism for cationic pesticides such as paraquat, diquat, and difenzoquat on natural and modified clays . For instance, adsorption isotherms of terbutryn (basic), dicamba (anionic), and paraquat on natural and modified clays from Morocco revealed that the natural clay efficiently adsorbed paraquat from aqueous solutions ( Table 1 ).
|Clay Mineral||Characterization Techniques||Compounds||Adsorbent Concentration (g L−1)/Contact Time (h)||Kinetic Evaluation||Models for Equilibrium Data Treatment||Removal (%) or Adsorption Capacity (Higher Results)||Reference|
|Mt||CEC, SSA, XRD||AT, 2,4-D, paraquat, metsulfuron methyl, glyphosate||0–8.1/6||No||Langmuir||Paraquat: 457 μmol g−1, Metsulfuron methyl: 56 μmol g−1, 2,4-D: negligible|||
|Bt, NSC||CEC, SSA, XRD, TG-DTA, organic carbon||Terbutryn, dicamba, paraquat||10/24||No||Langmuir, Freundlich||Paraquat: 100% (Bt), 47% (NSC), Dicamba: 30.6% (Bt), 15.2% (NSC), Terbutryn: 11.3% (Bt), 8.29% (NSC)|||
|Bt||CEC, SSA, XRD, XRF, TGA, FTIR||Paraquat||2.0/24||No||Langmuir||111 mg g−1 (403 µmol g−1)|||
|Mt||SEM, TGA, XRD, SSA, FTIR, elemental analysis, zeta potential||Paraquat||-/6||No||Langmuir||442 μmol g−1|||
|Bt, Sepiolite, Illite||CEC, SSA, XRD, TGA, organic carbon||Paraquat||2.0/24||No||Freundlich, Langmuir, Dubinin–Radushkevich||48 μmol g−1 (sepiolite), 212 μmol g−1 (illite), 165 μmol g−1 (Bt)|||
|Mt||SEM, FTIR, SSA, pHzpc||Ametryn||1.0/6||Yes||Freundlich, Langmuir, Temkin,||188.81 mg g−1 (831 µmol g−1)|||
|Bt||CEC, XRD, XRF, SSA, FTIR||Decis (deltametrin)||20/4||Yes||Freundlich, Langmuir||36.39–36.74 mg g−1
(72.0–72.7 µmol g−1)
|Mt, Vt||XRD, SSA, CEC, iron content||AT, DEA, DIA, HAT||10/24||No||Freundlich||AT, DIA, HAT: > 99.5% (Mt), DEA: 64-72% (Mt), HAT: > 90% (Vt), AT, DIA: ≈10% (Vt), DEA: negligible (Vt)|||
|Mt||pHzpc, FTIR, Mössbauer, XRD, Na, K, Ca, Mg||Glyphosate||6.0/24||No||Freundlich, Langmuir, one/two-sites Sips||85.64 mg g−1 (507 µmol g−1) at pH 7.0|||
|Mt||XRD, XPS, SSA, CEC, SEM, chemical composition||Glyphosate||10/24||No||Langmuir||4.0 ± 0.2 μmol m−2 (2.7 × 103) µmol g−1 at pH 4.0|||