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    Adsorption of Pesticides onto Clay Minerals

    Subjects: Chemistry, Applied
    View times: 8
    Submitted by: Jorge Cesar Masini
    (This entry belongs to Entry Collection "Environmental Sciences ")


    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.

    1. Introduction

    The so-called Green Revolution dramatically increased agricultural productivity from the middle of the twentieth century to the present [1]. 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 [2][3][4][5][6].

    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 [7][8], cleanup of contaminated soils, groundwater protection [9], and water treatment [10][11][12].

    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 [13]. Additionally, these interlayer cations are easily exchangeable, for instance, with organic quaternary ammonium salts to produce hydrophobic organoclays suitable for retaining neutral pesticides [14]. 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 [15][16]. Alternatively, the suspension of polycations can be first prepared and then exchanged with the interlayer cation [2]. 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 [2][17][18][19].

    Excellent reviews recently addressed the interactions between organic compounds and clay minerals, aiming to develop controlled released herbicide formulation and water purification [7][9][20]. 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.

    2. Natural Homoionic Clay Minerals as Pesticide Adsorbents

    Cation exchange, for instance, is the primary retention mechanism for cationic pesticides such as paraquat, diquat, and difenzoquat on natural and modified clays [13][21][22][23][24][25]. 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 ).

    Table 1. Sorbent characterizations and sorption features of pesticides and metabolites onto natural clay minerals.
    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 [26]
    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) [21]
    Bt CEC, SSA, XRD, XRF, TGA, FTIR Paraquat 2.0/24 No Langmuir 111 mg g−1 (403 µmol g−1) [22]
    Mt SEM, TGA, XRD, SSA, FTIR, elemental analysis, zeta potential Paraquat -/6 No Langmuir 442 μmol g−1 [24]
    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) [27]
    Mt SEM, FTIR, SSA, pHzpc Ametryn 1.0/6 Yes Freundlich, Langmuir, Temkin, 188.81 mg g−1 (831 µmol g−1) [28]
    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) [2]
    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 [30]
    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 [31]
    CEC: Cation Exchange Capacity, FTIR: Fourier Transform Infrared, pHzpc: pH of zero point of charge, SSA: Specific Surface Area, SEM: Scanning Electron Microscopy, TGA: Thermogravimetric Analysis, TG-DTA: Thermogravimetric-Differential Thermal Analysis, XPS: X-ray Photoelectron Spectroscopy, XRD: X-ray Diffraction, XRF: X-ray Fluorescence, Bt: Bentonite, Mt: Montmorillonite, NSC: Non-swelling clay, Vt: Vermiculite, 2,4-D: 2,4-dichlorophenoxyacetic acid, AT: Atrazine, DEA: Deethylatrazine, DIA: Deisopropylatrazine, HAT: hydroxyatrazine.


    3. Organophilic Clay Minerals

    Exchange of the inorganic interlayer cations by cationic or zwitterionic surfactants keep apart the aluminosilicate layers, resulting in increased basal spacing and hydrophobicity, thus enhancing the affinity toward neutral organic compounds with poor water solubility or high KOW values [14][32]. Examples of organic cations appear in Figure 1, and some relevant applications to removing pesticides from aqueous media are highlighted in Table 2. Since log KOW is pH-dependent, proper pH control and reporting are mandatory in adsorption studies, especially for interpreting the adsorption behavior of ionizable pesticides. In their neutral forms, the magnitude of adsorption constants correlates with the KOW. For instance, in organophilic kaolinite and bentonite, the distribution coefficient (Kd) values increased in the order atrazine (log KOW = 2.61) < alachlor (log KOW = 3.52) < trifluralin (log KOW = 5.34) [33]. The size of the alkyl chain of organic cation and the amount incorporated into the clay gallery strongly affect the adsorbent’s hydrophobicity, enabling a fine-tuning in the treatment of the mineral phase to attend to the application needs.
    Minerals 11 01282 g001

    5. Concluding Remarks

    The use of raw or modified clay minerals for elaborating sorbents to environmental cleanup and reducing pesticide spills’ impacts continues to increase. Additionally to the impressive availability of image and analytical techniques for materials characterization and understanding the interaction mechanisms, the design of adsorbents and adsorption-based treatments still relies on kinetic and equilibrium experiments. This review proposed some guidelines for the adsorption experiments and commented on some of the published results. In summary, the analytical technique to measure the free concentrations must be selective and sensitive, having a wide linear dynamic range to enable reliable measurements under low and high degrees of site occupation. The adsorbent mass to solution ratio and the sensitivity of the analytical technique are parameters to consider aiming to obtain reliable measurements of free concentrations in a wide range of site occupations. Kinetic measurements should precede the construction of adsorption isotherms and thermodynamic experiments since both of them require measurements under equilibrium conditions. The availability of software packages favors the fitting of kinetic and adsorption isotherms by nonlinear regression analysis, thus minimizing the errors inherent to the linearization.
    In many cases, the comparison of adsorption constants and capacities does not consider the standard deviations of these parameters, thus making the comparison subjective and without statistical support. Finally, the experimental conditions should emulate as much as possible those found in the environment. Thus, the influence of ionic strength, pH, temperature, and natural organic matter, among others, on the adsorption parameters should be evaluated for each new clay mineral-based adsorbent proposed.

    This entry is adapted from 10.3390/min11111282


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