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Vol. 8. Issue 5.
Pages 4713-4724 (September - October 2019)
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Vol. 8. Issue 5.
Pages 4713-4724 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.08.017
Open Access
Preparation of a montmorillonite-derived adsorbent for the practical treatment of ionic and nonionic pesticides
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S.F.A. Shattar, N.A. Zakaria, K.Y. Foo
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k.y.foo@usm.my

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River Engineering and Urban Drainage Research Centre (REDAC) Engineering Campus, Universiti Sains Malaysia, Seri Ampangan 14300 Nibong Tebal, Penang, Malaysia
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Tables (7)
Table 1. Selected molecular characteristics of 2,4-D and metolachlor.
Table 2. Surface physical properties of montmorillonite and HM.
Table 3. Adsorption isotherm parameters for the adsorption of 2,4-D and metolachlor onto HM at 30 °C.
Table 4. Comparative evaluation of monolayer adsorption capacities for 2,4-D and metolachlor onto different clay derivatives.
Table 5. Worldwide maximum concentration levels, MCLs (mg/L) of 2,4-D and metolachlor.
Table 6. Adsorption kinetic parameters for the adsorption of 2,4-D and metolachlor onto HM at 30 °C.
Table 7. Thermodynamic parameters for the adsorption of 2,4-D and metolachlor onto HM.
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Abstract

The preparation of a new montmorillonite-derived functionalized adsorbent (HM) via one-step modification of hexadecyltrimethylammonium bromide (HDTMA) for unique treatment of both ionic and non-ionic pesticides, 2,4-Dichlorophenoxyacetic acid and metolachlor has been attempted. Equilibrium data were simulated by the non-linear Langmuir and Freundlich isotherm models, while the adsorption kinetics were analyzed by the pseudo-first and pseudo-second order kinetic equations. Conformation analysis of the intercalated surfactant in the interlayer montmorillonite was verified by the morphological development and alteration of the surface chemistry via scanning electron microscopy, Fourier-transform infrared spectroscopy, and nitrogen adsorption-desorption curve. The adsorption data were well fitted to the Langmuir isotherm model, with the monolayer adsorption capacities for 2,4-D and metolachlor of 185.19 and 84.75 mg/g, respectively. The adsorption kinetic was best described by the pseudo-second-order kinetic model, implying the overall rate of the adsorption process was controlled mainly by chemisorption. Examination of the textural structure verified the presumption that the intercalation of HDTMA has been successfully attained by diminishing the pore development, while the alteration of FTIR bands signified the modification and development of new functionalities. Thermodynamic study illustrated that the adsorption interaction was spontaneous and endothermic in nature. The findings provided an invaluable insight into the novel preparation technique and the great feasibility for the practical treatment of 2,4-D and metolachlor contaminated water or agricultural discharge.

Keywords:
Adsorption
2,4-dichlorophenoxyacetic
Metolachlor
Montmorillonite
Pesticides
Abbreviations:
ACGIH
BET
2,4-D
FTIR
HDTMA
HM
IARC
ILO
IR
IUPAC
KBr
MCLs
NaCl
pHpzc
SWA
Full Text
1Introduction

Today, the development of low cost, eco-friendly and new emerging adsorbent as a potential substitution to the carbonaceous activated carbon, has attracted aesthetic attraction among the environmentalists [1]. Within the wide spectrum of natural adsorbents, natural montmorillonite, a clay mineral from the smectite group, comprises of octahedral sheet sandwiched between two tetrahedral sheets, has been proposed to be a potential attraction for the adsorptive treatment of different water pollutants. The tetrahedral sheets are occupied by Si(IV) or Al(III) as the central atom, to be partially balanced by other exchangeable cations with lower valency. These cations, mostly alkaline or alkaline-earth metals within the interlayer space of clay minerals, demonstrate high hydration energy to be an energetically, favourable process and essentially hydrophilic [2]. To render these hydrophilic phyllosilicates to be an organophilic phase, the introduction of cationic surfactants or quaternary ammonium salts with a long hydrocarbon chain are of special interest, due to greater retensive capability for a variety of hydrophobic contaminants. During the process, the alteration of surface properties, the transformation from hydrophilic to hydrophobic surface structure are usually expected to produce pronounce implications for the practical improvement on the adsorption capacity of the functionalized montmorillonite. One of the common surfactant is hexadecyltrimethylammonium (HDTMA) bromide, a tetra-substituted ammonium cation with permanently charged pentavalent nitrogen, and a long straight alkyl (C16) chain to impart a high degree of hydrophobicity. This quaternary ammonium cations could interact with clay minerals to expand the interlayer spacing, and form six predominant active sites: (a) surface aluminol and silanol groups, (b) isomorphic substitutions, (c) exchangeable cations, (d) hydrophobic silanol surfaces, (e) hydration shell surrounding exchangeable cations, and (f) hydrophobic site, contributing to the unique interactions between organic molecules with the clay minerals [3,4]. These modified clays have shown great feasibility for the effective control of a wide range of environmental contaminants, governed by the electrostatic attraction and hydrophobic interaction with the adsorbing molecules.

In particular, pesticide application has been defined to be the complementary practice for intensive agricultural production, to feed with the rising food demand, and the trend may intensify for the next 50 years [5]. These pesticides and their metabolites have been detected ubiquitously in the natural environment at the concentrations from ng/L to mg/L, and exceeding half of the detected substances have long been phased out of use, while another 10 to 20% are stable transformation products. An additional striking indication of wide spread pesticide migration was the detection at high altitude regions, illustrating the extremely sufficient persistence to carry them over hundreds of kilometers in the atmosphere [6]. Similarly, pesticides possess the ability to bioaccumulate, biomagnify, and bioconcentrated by up to 70,000 fold relative to the initial concentration [7], while repeated application of pesticides may led to the loss of biodiversity, increased pest resistance and pest resurgence, with an elevated risk of contamination of soils, groundwater and surface-water resources.

Within this framework, 2,4-dichlorophenoxyacetic acid (2,4-D) is a member of the phenoxy herbicide group, widely applied for the control of a wide range of broad leaf weeds. It is adsorbed by the roots, to be translocated to the different growing points to inhibit the normal growth of plants [8]. 2,4-D is water soluble, and exists in its neutral and anionic forms in aqueous solutions. Due to its low acid dissociation constant, pKa value of 2.73, 2,4-D appears predominantly in its anionic form in the natural environment, and the half-life of 2,4-D ranges from one to several weeks under aerobic conditions, and exceeding 120 days under anaerobic conditions [9]. Meanwhile, metolachlor is a non-ionizable substituted methoxyacetamide herbicide that control grasses and broadleaf weeds, with the half-life of 200 and 97 days, respectively, in highly acidic and alkaline waters. It shows a relatively high water solubility (530 mg/L at 20 °C) to hinder the elongation of roots and shoots by the inhibition of amino acid glutathione [3].

Owing to their physicochemical properties that are not degradable by the soil particles, high solubility and mobility, longer hydrolysis of half-life, and great tendency to move rapidly with the infiltrating water, these highly mobile herbicide compounds, may constitute a hazardous source of contamination to the water bodies [10]. The efficient route for the removal of both ionic and non-ionic herbicides from the contaminated water is of great concern, and one of the highly promoted adsorbent is the organoclays. Organoclays have been reported to induce a strong immobilisation effect on the ionic and nonionic chemicals with high water solubility. They have been widely applied as a possible carrier in the controlled release of bentazone and dicamba as the typical examples of the highly mobile and persistent acidic herbicides, with the total leaching losses from 55% to 90% [11]. The potential application of organoclays as a technical barrier for the preventive control of herbicide pollution at the point source to the ground water table has been documented.

The present work is devoted to the preparation of a new montmorillonite-derived functionalized adsorbent (HM) for the practical treatment of 2,4-D and metolachlor contaminated water. To develop an in-depth understanding of the structural modification, and adsorptive behaviour, distinct physio-chemical properties, and notably the morphological texture, surface functional properties and pore development were examined by scanning electron microscopy, Fourier-transform infrared (FTIR) spectroscopy, and nitrogen adsorption-desorption curve. The effects of initial concentration, contact time, and solution pH on the treatment process were evaluated. Moreover, the adsorption isotherms, kinetic modelling, and thermodynamics analysis were elucidated.

2Materials and methods2.1Adsobate

Pure analytical grade 2,4-D and metolachlor, with radiopurity >98% were selected as the model adsorbate. 2,4-D, a member of the ionic carcinogenic agent, with the molecular structure of C8H6Cl2O3 and annual usage of exceeding 46 million pounds/year, has been identified to be an active ingredient for more than 1500 herbicide products. Meanwhile, metolachlor, a non-ionizable substitute methoxyacetamide herbicide to be a frequently detected pesticide in surface and ground waters, at the concentrations above the European threshold for drinking water [12], mainly owing to the extremely high water solubility and long half-life in the natural environment. The selected molecular characteristics of 2,4-D and metolachlor are reported in Table 1. The standard stock solution was prepared by careful weighing and dissolving the solute molecule in deionized water, and working solutions were obtained by serial dilutions.

Table 1.

Selected molecular characteristics of 2,4-D and metolachlor.

Properties  2,4-D  Metolachlor 
Scientific name  2,4-dichlorophenoxyacetic acid  N-(2-ethyl-6-methylphenyll-N-(2-methoxy-l-methylethyl) acetamide 
Molecular formula  C8H6Cl2O3  C15H22ClNO2 
Structural formula 
 
 
Molecular weight (g/mole)  219.97  283.30 
pKa2.6  Non-ionized 
Log Kow2.83  2.90 
Solubility (mg/L)  620  480 
Melting point (°C)  140.5  −62.1 
Registration year  1948  1976 

pKa * = The negative base-10 logarithm of the acid dissociation constant (Ka) of a solution.

Kow = The ratio of the concentration of a chemical in n-octanol and water at equilibrium at a specified temperature.

2.2Adsorbent

The preparation of a new montmorillonite-derived functionalized adsorbent (HM) has been conducted by mixing the predetermined amount of bromide salt, C19H42NBr and montmorillonite with a chemical impregnation ratio, IR of:

whereWmontmorillonite (g) and WHDTMA (g) are the dry weight of montmorillonite and HDTMA powder and, respectively.. The mixtures was agitated at 70 °C for 3 h, and the modified sample was rinsed repeatedly with distilled water till it was free of bromide ions, by determination of silver nitrate, AgNO3 test.

2.3Physical and chemical characterization

The surface physical properties were examined by nitrogen (N2) adsorption-desorption curve at −196 °C (77 K) with the saturation pressure of 106.65 kPa using an automated gas sorption system (Micromeritics, ASAP 2020, USA). The morphological structure was evaluated using a scanning electron microscope (Leo Supra 35 V P Field Emission SEM), while the surface functional groups were detected using Fourier-transform infrared (FTIR) spectroscopy (Perkin Elmer, Model 2000 FT-IR, USA) at the scanning range of 4000 to 400 cm−1 by adopting the KBr pellet method. The zero-point-of-charge, pHpzc was performed by mixing 50 cm3 of 0.01 M sodium chloride (NaCl) solution with 0.15 g of HM. The solution pH was adjusted to a value from 2 to 12, and the final pH was measured after 48 h under agitation. The pHpzc is the point where pHinitial-pHfinal = 0.

2.4Batch adsorption experiments

The batch adsorption experiments were carried out in a set of 250 mL Erlenmeyer flasks containing 0.2 g of HM and 200 mL of 2,4-D or metolachlor solutions within the concentration range of 50–300 mg/L and 50–400 mg/L, respectively in a thermostated water bath shaker with an agitation speed of 120 rpm at 30 °C. The effect of solution pH was examined by adjusting the metolachlor solution within the pH range of 2–12, by the drop-wise addition of 0.1 M of hydrochloric acid or sodium hydroxide solution. The solution pH was measured using a pH meter (Accumet XL200, Fischer Scientific). The concentrations of 2,4-D and metolachlor were analyzed at predetermined time intervals using a double beam UV–vis spectrophotometer (Shimadzu, Model UV 1800, Japan) at 283 and 194 nm, respectively. The amounts of pesticide uptake at time t, qt (mg/g) and at equilibrium, qe, (mg/g), respectively were determined by:

where C0 (mg/L), Ct (mg/L), and Ce (mg/g) are the concentration of 2,4-D or metolachlor at initial, time t, and equilibrium, respectively. V (L) is the volume of solution and W (g) is the dry mass of adsorbent.

2.5Adsorption isotherm

Adsorption isotherm is a unique relationship for the operational design, upscaling and on-site practice of the adsorption systems [13]. In the present work, the most widely applied isotherm models, the Langmuir and Freundlich isotherm models have been adopted. Langmuir isotherm model [14] assumes monolayer adsorption onto a surface containing a finite number of adsorption sites, with no transmigration of adsorbate in the plane of the neighbouring surface, given by:

where Q0 (mg/g) and KL (L/mg) are the adsorbing isotherm constants related to the monolayer adsorption capacity and the energy of adsorption, respectively.

Freundlich model is an empirical equation based on sorption on a heterogeneous surface or surface supporting sites of varied affinities. It could be assumed that the stronger binding sites are occupied first, and the binding strength decreases with increasing the degree of site occupation. The Freundlich [15] isotherm model is derived as:

where KF (mg/g (L/mg)1/n) and n are the Freundlich isotherm constants related to the adsorption capacity and adsorption intensity, respectively. The applicability of the isotherm models was carried out by judging the correlation coefficients, R2 values, defined as:
where qe, meas, qe,calc and q¯e,calc are the measured, calculated and average mean of adsorptive uptake (mg/g), respectively.

2.6Adsorption kinetic

For interpretation of kinetic analysis, the kinetic data were simulated by the pseudo-first order and pseudo-second order kinetic equations. The Lagergren and Svenska [16] expression is given by:

where k1 is the pseudo-first order kinetic rate constant.

Conversely the pseudo-second order kinetic [17], equation is expressed by:

where k2 is the pseudo-second order kinetic rate constant. The applicability of the kinetic models was further ascertained by the correlation coefficient, R2, and the normalized standard deviation, Δq (%), defined as:
where qexp (mg/g) and qcalc (mg/g) are the experimental and calculated adsorption capacities, respectively.

3Results and discussion3.1Textural and surface characterization

Nitrogen adsorption-desorption study is a standard technique widely applied for the determination of porosity of the carbonaceous adsorbents [18]. The nitrogen adsorption-desorption plots of montmorillonite and HM is presented in Fig. 1(a). It can be found from Fig. 1(a) that the isotherm pertains an intermediate between type I and II isotherms as defined by the International Union of Pure and Applied Chemistry (IUPAC) classification. This type of isotherm is usually exhibited by a combination of microporous and mesoporous structures. The initial part of the isotherm represents micropore filling, and the slope of the plateau at high relative pressure is due to multilayer adsorption onto the external surface. Meanwhile, the desorption branch presents a hysteresis loop at high relative pressures, pointing to a considerable development of mesoporosity, with wide pores of 2–50 nm [19].

Fig. 1.

Nitrogen isotherm-desorption curves (a) and pore size distribution (b) of montmorillonite and HM.

(0.19MB).

Based on the assumption of cylindrical pores, the specific surface area (SBET) was calculated by the BET equation; the total pore volume (VT) was evaluated by converting the adsorption volume of nitrogen at relative pressure of 0.95 to equivalent liquid volume of the adsorbate, while the average pore size (r) was estimated by:

Detailed characteristic of the porosity of montmorillonite and HM obtained from the nitrogen adsorption isotherms are summarized in Table 2. The external and micropore surface areas deduced from the t-method were tabulated. From the data, it can be inferred that the BET surface area, Langmuir surface area and total pore volume of HM were greatly diminished from 164.79 to 68.54 m2/g, 206.45 to 85.33 m2/g, and 0.271 to 0.062 cm3/g, respectively, indicating the filling and obstructing of some internal layers of montmorillonite by HDTMA modification. These larger HDTMA cations may be subjected to compact packing in the interlamellar layer, and resulted in more serious pore blocking to inhibit the passage of nitrogen molecules [20].

Table 2.

Surface physical properties of montmorillonite and HM.

Properties  Montmorillonite  HM 
BET surface area (m2/g)  164.79  68.54 
Micropore surface area (m2/g)  38.48  13.28 
External surface area (m2/g)  126.31  55.26 
Langmuir surface area (m2/g)  206.45  85.33 
Total pore volume (cm3/g)  0.2708  0.0618 
Micropore volume (cm3/g)  0.0168  0.0084 
Mesopore volume (cm3/g)  0.2540  0.0534 
Average pore size (Å)  65.74  39.55 

Pore size distribution is an important property that determines the fraction of the total pore volume accessible to molecules of a given size and shape [21]. According to the classification of IUPAC-pore dimensions, the pores of adsorbents are grouped into micropore (d < 2 nm), mesopore (d = 2–50 nm) and macropore (d > 50 nm). The pore size distribution of HM was ascertained by Density Functional Theory model. Result (Fig. 1(b)) detected the sharpest peak at pore width between 20–100 Å, with an average pore size of 39.55 Å, which shows that a vast majority of the pores fall into the range of mesopore.

The scanning electron micrographs of the montmorillonite and HM are depicted in Fig. 2. The raw montmorillonite was massive, adhesive and contains small particles with irregular aggregates. During the modification, the undulating sheets of raw montmorillonite has been significantly improved, with the presence of a series of porosities around the surface. The FTIR spectra of the montmorillonite and HM are shown in Fig. 3. The region between 3617-3625 cm−1 is related to the vibration of the structural OH of AlAlOH and AlMgOH. The fundamental of the progressive water reduction with the loading of surfactant could be observed at the broad band at 3431/3400 cm−1, mainly ascribed to the symmetric (OH) vibration of H-bonding, bonded to the structural SiOAl. The alteration of the wavenumber reflected weakening of the interaction, and disturbance of the H-bond network, induced by the intercalation of HDTMA molecules. The CH2 asymmetric stretching at 2924 cm−1, and the symmetric stretching at 2853 cm−1 could be attributed to the gauche-/trans-conformation of the surfactant. A sharp narrow single peak at 1470 cm−1 is associated with the parallel arrangement of the methylene chains with a hexagonal phase. The signal at 1043 cm−1 is related to the stretching of Si-O, while the broad band at 797 cm−1 indicates the presence of cristobalite structure. The sharp peak at 721 cm−1 could be originated from the interlayer surfactant molecules, pointing to the rocking vibration of methylene group (CH2), and the signals at 526 and 470/467 cm−1 are identical to the AlSiAl and SiOFe of the clay structure.

Fig. 2.

Scanning electron micrographs of (a) montmorillonite and (b) HM.

(0.58MB).
Fig. 3.

Fourier-Transform Infrared Spectroscopy of montmorilloite and HM.

(0.12MB).
3.2Adsorption studies3.2.1Effect of chemical impregnation ratio

The major effects of chemical impregnation ratio (IR) on the adsorptive uptake of 2,4-D and metolachlor is displayed in Fig. 4. It can be observed that augmenting the IR from 1:1 to 1:3 showed an enhancement of the adsorptive uptake of 2,4-D and metolachlor from 71.32 to 89.14 mg/g, and from 69.72 to 81.21 mg/g, respectively. Beyond the value, subsequent increase in IR illustrated a gradual decrease in the adsorptive uptake. It was presumed that when HDTMA was conjugated with motmorillonite, the HDTMA molecules were initially adsorbed into the interlayers of montmorillonite by cation exchange process. As the impregnation ratio increased, the HDTMA molecules were adsorbed to the external surface via both cation exchange and hydrophobic bonds [22], to significantly amend the original surface properties of the montmorillonite. Importantly, the hydrophobicity of the montmorillonite surface has been drastically improved, that is beneficial for the conjugation of 2,4-D and metolachlor.

Fig. 4.

Effects of chemical impregnation ratio on the adsorptive uptakes of 2,4-D and Metolachlor.

(0.17MB).

However, as the concentration of HDTMA was greater than its critical concentration, these surfactant molecules were expected to form aggregates of micelles on the montmorillonite to increase the solubility of the herbicide molecules [23]. Similar synergistic effect on the solubility of these organic herbicides by the intercalation of cationic surfactants has been reported by the previous researches [24]. It could be found that when the IR was greater than 1:3, the removal efficiency was gradually decreased, mainly attributed to the electrostatic neutralization of the agglomerations of positive charge HDTMA micelles and negatively charged montmorillonite. This agglomeration would reduce the specific surface area, shielding the surface active sites, to offset the adsorptive uptake of 2,4-D and metolachlor. Similar finding has been reported by Ruan et al. [25] on the adsorption of 2,4-dichlorophenol onto organobentonite.

3.2.2Effect of contact time and initial concentrations

The adsorptive uptakes of 2,4-D and metolachlor onto HM were examined at the concentration ranges of 50–300 and 50–400 mg/L, respectively as illustrated in Fig. 5. Initially, the adsorption process increased sharply, and it decreased gradually until the equilibrium was attained. At this point, the amount of pesticide desorbing from the HM surface is in a state of dynamic equilibrium with the amount of pesticide being adsorbed onto the adsorbent. This phenomenon could be attributed to the availability of larger number of vacant surface sites for the adsorption process and thereafter, the availability of the adsorption sites was gradually reduced till the saturation was reached [26]. Initial concentration plays an important role to overcome the mass transfer resistance of the adsorbate molecules onto the solid adsorbent. Increasing the initial concentration of 2,4-D and metolachlor from 50 to 300 mg/L, and from 50 to 400 mg/L, showed a greater adsorptive uptakes from 47.44 to 176.23 mg/g, and from 7.23 to 38.69 mg/g, respectively, due to a higher driving force to overcome the mass transfer resistance between the bulk and the liquid surface [27].

Fig. 5.

Effect of contact time and initial concentrations on the adsorptive uptakes of 2,4-D and metolachlor onto HM at 30 °C.

(0.35MB).
3.2.3Effect of solution pH

Solution pH affects the adsorption process by regulating the surface charge and the degree of ionization of the adsorbate molecules [28]. The adsorption behavior of 2,4-D and metolachlor over a broad pH range of 2–12 is depicted in Fig. 6. Decreasing solution pH from 12 to 2 exerted an enhancement of the adsorption uptake of 2,4-D from 26.87 to 170.23 mg/g, mainly ascribed to the changing surface functionality of the 2,4-D molecules. The result was supported by previous researches [29,30], which identified that 2,4-D is a weakly acidic herbicide, with the pKa of 2.73. It could be easily dissociated in water in the form of anionic ions, according to the reaction:

Fig. 6.

Effect of solution pH on the adsorptive uptakes of 2,4-D and metolachlor onto HM at 30 °C.

(0.06MB).

These protonated species are dominant at the solution pH below 2, and the deprotonated form are predominant at the solution pH above 4.5. Therefore, at a higher solution pH, 2,4-D is highly dissociated, with greater proportion in the ionized form of 2,4-dichlorophenoxyacetate ion, to induce repulsion or dispersion between the 2,4-D ions with the HM surface lowering the adsorptive uptake of 2,4-D. Additionally, at the strong basic pH, the high mobility of OH ions may be competing with 2,4-dichlorophenoxyacetate anion for the surface active sites [31]. At pH 4.2 (pHpzc), the surface charge of HM was essentially neutral, and at pH below 4.2, HM was positively charged, and negatively charged at the solution pH greater than 4.2. In this sense, the formation of electric double layer could amend the polarity of HM at the acidic condition, and the adsorptive uptake of 2,4-D increased.

During this stage, the intercalation of the surfactant molecules into the montmorillonite surface may occupy the interlayer spaces, external surfaces and pores within the ‘house of cards’ structure as suggested by He et al. [32], that are oriented outward from the adsorbent surface. This hydrophobic interaction between the surfactant tails and nonpolar part of the herbicide has been proposed to be the adsorption mechanism of 2,4-D onto HM [33]. Meanwhile, solution pH did not show appreciable effect on the adsorption of metolachlor onto HM, which could be associated with the non-ionizable nature of metolachlor in the studied pH. These metolachlor molecules are in the non-dissociated form, with neutral molecules as the major form. However, slightly decrease in the adsorption of metolachlor from 107.52 mg/g to 97.29 mg/g could be observed as the solution pH increased from 2 to 4, due to presence of excess H+ ion which accelerates the removal of metolachlor anions in the aqueous solution. Moreover, the zero point of charge, pHzpc of HM was identified to be 4.2. At the acidic pH, the surface of HM was positively charged, and this suggested that the major interactions between HM and metolachlor at pH < 4 was governed mainly by the electrostatic interaction between the positive charge of adsorbent surface and electron rich region in the adsorbed molecules. At pH = 4.2, the surface of HM was essentially neutral, leading to the weakening of the interactions with the metolachlor molecules, to decrease the degree of adsorption. The result was in agreement with the phenomenon reported by Otero et al. [34] for the adsorption of metolachlor onto the dodecylsulfate and tetradecanedioate anions intercalated double hydroxides.

3.3Adsorption isotherm

Adsorption isotherm explains the specific relation between the adsorbate concentration in the bulk and at the interface [35]. The isotherm constants obtained from the non-linear isotherm models are presented in Table 3. Results revealed that the adsorption of 2,4-D and metolachlor onto HM were best described by the Langmuir isotherm model, with the monolayer adsorption capacities for 2,4-D and metolachlor of 185.19 and 84.75 mg/g, respectively. The better fit obtained with the Langmuir isotherm model indicated the adsorbent surface was energetically homogenous, with monolayer coverage of herbicide molecules at the outer surface of HM. Similar observation has been found for the removal of metolachlor onto the layered double hydroxides intercalated with dodecylsulfate, and tetradecanedioate anions intercalated double hydroxides [36], 2,4-D onto surfactant modified tectosilicates and phyllosilicates [37], and 2,4-D onto organo-palygorskite [33].

Table 3.

Adsorption isotherm parameters for the adsorption of 2,4-D and metolachlor onto HM at 30 °C.

Adsorbate  Langmuir isotherm modelFreundlich isotherm model
  Q0 (mg/g)  KL (L/mg)  R2  KF (mg/g).(L/mg)1/n  n  R2 
2,4-D  185.19  0.128  0.999  42.20  3.064  0.914 
Metolachlor  84.75  0.002  0.999  0.42  1.295  0.995 

A specific point to be highlighted in this study was the greater adsorptive uptake for 2,4-D than metolachlor, that could be attributed to the smaller molecular structure of 2,4-D, which could penetrate easily into the pores, and the presence of Cl groups of the 2,4-D molecules, has led to a well pronounce electron withdrawing property, with a greater adsorption affinity [38]. A comparative of the monolayer adsorption capacities of 2,4-D and metolachlor onto different clay derivatives is provided in Table 4[33,36,37,39–42]. The adsorbent prepared in this work showed comparative adsorptive performance as compared with previous researches, indicating the high feasibility for the real practical applications.

Table 4.

Comparative evaluation of monolayer adsorption capacities for 2,4-D and metolachlor onto different clay derivatives.

Herbicide  Adsorbent  Activating agent  Adsorption capacity (mg/g)  Reference 
2,4-D  Montmorillonite  HDTMA  185.19  Present study 
  Palygorskite  Octadecyl trimetylammonium bromide  42.02  [33] 
  Palygorskite  Dioctadecyl dimethylammonium bromide  25.77   
  Montmorillonite  Dioctadecyltrimetylammonium ion  161.29  [37] 
  Zeolite  Cetyltrimethylammonium bromide  45.87  [39] 
  Bentonite  Cetyltrimethylammonium bromide  91.91   
  Sepiolite  N-Cetylpyridinium ion  7.62  [40] 
  Sepiolite  Dedocylammonium ion  4.58  [41] 
  Vermiculate  Decylammonium chloride  6.04  [42] 
Metolachlor  Montmorillonite  HDTMA  84.75  Present study 
  Organosilica  Benzene  0.21  [36] 
  Organosilica  Ethane  0.08   

Accordingly, the pesticide contamination in the surface water or groundwater systems has been highlighted by the worldwide regulatory jurisdictions, with the primary aim on ecosystem and public health conservations. These regulatory jurisdictions have promulgated the standard values for pesticides in residential soil, air, drinking water, and agricultural commodity, with more than 19,400 pesticide soil regulatory guidance values and 5400 pesticide drinking water maximum concentration levels (MCLs) have been regulated by 54 and 102 nations, respectively [43]. A summary of the fragmented policies, controlling rules and legislations on the maximum allowable concentrations (MCLs) for 2,4-D and metolachlor is given in Table 5. The statistical information reveals that 2,4-D is the most frequently regulated pesticides in drinking water with 180 MCLs, including 59 U.S. MCLs and 121 worldwide MCLs. However, as the fourth most widely applied agricultural pesticides at the United States and Group C-possible human carcinogen, metolachlor has not been evaluated by the International Agency for Research on Cancer (IARC), American Conference of Industrial Hygienists (ACGIH), Safe Work Australia (SWA), International Labour Organization (ILO), or the German Research Foundation [44]. The data compilation demonstrated that these regulated standard value may vary by 5 to even 7 orders of magnitude, or have not been derived comprehensively enough for human health risk and exposure pathways control. In conjunction with this present study, the newly develop functionalized adsorbent, HM with the monolayer adsorption capacity for 2,4-D and metolachlor of 185.19 and 84.75, respectively, could be integrated to be a viable, eco-friendly, and low cost solution for the effective treatment of ionic and non-ionic pesticides contaminated agricultural run-off in the near future.

Table 5.

Worldwide maximum concentration levels, MCLs (mg/L) of 2,4-D and metolachlor.

Pesticides  Worldwide jurisdictions/Country  Maximum concentration levels, MCLs (mg/L) 
2,4-D  Italy DoD Purchased Water  0.0001 
  Republic of Belarus  0.001 
  Republic of Sudan  0.02 
  Gulf Standardization Organization (GSO)  0.03 
  World Health Organization (WHO)  0.03 
  Russian Federation SanPin  0.03–1.00 
  New Zealand National Standard  0.04 
  U.S. Environmental Protection Agency  0.07 
  Food and Drug Administration  0.07 
  State of Oregon, United States  0.07–0.70 
  Hashemite Kingdom of Jordan  0.09 
  U.S. Millitary  0.14–1.40 
2,4-D  United Mexican States, Socialist Republic of Viet Nam  30.00 
  Antigua and Barbuda, Australia (Commonwealth of Australia, Australian Capital Territory State of Tasmania, State of New South Wales, State of Northern Australia, State of Queensland, State of Western Australia), Commonwealth of Bahamas, Belize, Kingdom of Bhutan, Federative Republic of Brazil, San Paolo (Brazil), Kingdom of Cambodia, People’s Republic of China, Arab Republic of Egypt, State of Kuwait, Malaysia, Kingdom of Morocco, Republic of Albania, Chile, Costa Rica, Cuba, Fiji, Guatemala, Honduras, Kazakhstan, Kiribati, Latvia, Nauru, Nicaragua, Peru, Philippines, Singapore, Uganda, Vanuatu, Islamic Republic of Pakistan, State of Qatar, Saint Lucia, Syrian Arab Republic, Kingdom of Tonga, Tuvalu, Eastern Republic of Uruguay, Bolivarian Republic of Venezuela, Eastern Republic of Uruguay, East Africa Community, Palestine, Japan  0.03 
2,4-D  Province of Quebec, Canada, Republic of Italy, Taiwan, United States (State of Alaska, State of Arizona, State of Arkansas, State of California, State of Colorado, State of Connecticut, State of Delaware, State of Florida, State of Hawaii, State of Idaho, State of Illinois, State of Indiana, State of Iowa, State of Kansas, Commonwealth of Kentucky, State of Louisiana, State of Maine, State of Maryland, Commonwealth of Massachusetts, State of Michigan, State of Minnesota, State of Missouri, State of Montana, State of Nebraska, State of Nevada, State of Hampshire, State of New Jersey, State of New Mexico, State of New York, State of North Carolina, State of North Dakota, State of Ohio, State of Oklahoma, Commonwealth of Pennsylvania, Rhode Island, State of South Carolina, State of South Dakota, State of Tennessee, State of Texas, State of Utah, State of Vermont, Commonwealth of Virginia, State of West Virginia, State of West Wisconsin, State of Wyoming)  0.07 
2,4-D  Canada (Province of Alberta, Province of British Columbia, Province of Newfoundland and Labrador, Northwest Territories, Canada Nova Scotia, Province of Ontario, Province of Prince Edward Island, Yukon, Province of Saskatchewan), Republic of Colombia, Indonesia, Palau, Serbia, Unincorporated Territory of Guam  0.10 
Metolachlor  World Health Organization (WHO)  0.01 
  Republic of Uganda  0.01 
  Commonwealth of Massachusetts  0.10 
  U.S. Millitary  0.93–2.80 
3.4Kinetic modelling

The equilibrium data were fitted with respect to the pseudo-first order and pseudo-second order kinetic equations. The plot of ln (qe-qt) versus t gave the slope of k1 and the intercept of ln qe, and the plot t/qt versus t gave the slope and intercept of ln 1/qe and 1/k2qe2, respectively. The results are tabulated in Table 6. The adsorptive uptakes of 2,4-D and metolachlor onto HM were best represented by the pseudo-second order kinetic model, with the correlation coefficient, R2 ranged from 0.998 to 0.999, and from 0.989 to 0.999, respectively. Besides, the experimental qe values showed high agreement with the calculated values obtained from the linear plots, indicating great suitability of the pseudo-second order kinetic model to describe the adsorption process, with the lowest normalized standard deviation, Δq (%) values that ranged between 0.43% to 7.21% and between 0.29% to 5.46%, for 2,4-D and metolachlor respectively. A better fit to the pseudo second-order kinetic equation suggested that the adsorption rate is dependent on the availability of the adsorption sites, rather than the concentrations of the herbicide present in the aqueous solution. This suggested that the adsorption process was controlled mainly by the chemisorption process that involved valency forces through electron sharing or exchange between the adsorbent and adsorbate molecules [45]. The result were supported by Njoku et al. [46] for the adsorption of 2,4-D onto pumkin seed hull derived activated carbon and Otero et al. [36] for the adsorption of metolachlor onto periodic mesoporous silica.

Table 6.

Adsorption kinetic parameters for the adsorption of 2,4-D and metolachlor onto HM at 30 °C.

C0(mg/L)qe,exp(mg/g)Pseudo-first-order kinetic modelPseudo-second-order kinetic model
qe,cal (mg/g)  k1 (1/h)  R2  Δq (%)  qe,cal (mg/g)  k2 (g/mg h)  R2  Δq (%) 
2,4-D                   
50  47.44  24.14  5.414  0.811  49.12  46.73  0.116  0.999  1.50 
100  92.66  53.21  9.063  0.948  42.58  96.15  0.128  0.999  3.77 
150  129.94  56.67  8.191  0.879  56.37  131.58  0.157  0.999  1.26 
200  158.09  78.80  8.582  0.877  50.16  161.29  0.180  0.999  7.21 
250  168.77  78.36  6.278  0.834  53.57  169.49  0.193  0.999  0.43 
300  176.30  88.75  9.834  0.859  49.66  178.57  0.382  0.998  1.29 
Metolachlor                   
50  7.32  4.92  1.853  0.614  32.80  7.72  0.255  0.999  5.46 
100  13.57  8.33  1.763  0.723  38.64  13.77  0.166  0.999  1.47 
200  23.80  15.38  1.652  0.846  35.51  23.87  0.087  0.998  0.29 
300  29.97  18.58  1.518  0.794  38.00  30.77  0.059  0.991  2.67 
400  38.69  21.20  1.129  0.834  45.20  37.71  0.049  0.989  2.53 
3.5Adsorption thermodynamic

Thermodynamic is a critical aspect predicting the stability of the solid-liquid phase equilibrium, and is a basic requirement for the characterization of an adsorption system [47]. In this work, the values of enthalpy change (ΔH°) and entropy change (ΔS°) were determined from the slope and intercept of the van’t Hoff plot of ln kd versus 1/T, and the Gibbs free energy change (ΔG°) at the adsorption temperatures of 30, 40 and 50 °C, respectively was given by:

where R (8.314 J/mole K) and T (K) are the universal gas constant and absolute temperature, and Kd, is the distribution coefficient defined as:
where CAe (mg/L) is the equilibrium concentration of 2,4-D or metolachlor on HM.

The thermodynamic parameters are presented in Table 7. The negative ΔG° of −0.89, −2.51 and −3.83 kJ/mole for the adsorption of 2,4-D at the operating temperatures of 30, 40 and 50 °C ascertained the great feasibility and spontaneous nature of the adsorption process, with high preference of 2,4-D onto HM, and extra energy generation within the adsorption system. Conversely, the ΔG° for the adsorption of metolachlor was positive at 30 and 40 °C, and the ΔG° value turned negative at the higher operating temperature of 50 °C. This negative ΔG° is mainly related to the changing van der waals attraction forces between the metolachlor molecules with the HM surface at the higher operating temperature of 50 °C, supported by the lower viscosity but higher molecular motion of adsorbate molecules to enhance the pollutant uptakes. The results were in agreement with the findings reported for the acid activated langsat empty fruit bunch derived biosorbent [48]. The positive value of ΔS° showed the high affinity of HM for the adsorption of 2,4-D or metolachlor molecules, with structural alteration and increasing randomness between the solute and surface interaction during the adsorption process. The positive value of ΔH° represented endothermic nature of the adsorption interaction. This endothermic condition was attributed to the strengthening of the adsorptive forces between the binding sites for the 2,4-D or metolachlor species, and between the adjacent 2,4-D or metolachlor molecules on the adsorbed phase.

Table 7.

Thermodynamic parameters for the adsorption of 2,4-D and metolachlor onto HM.

Herbicide  ΔG° (kJ/mole)ΔH° (kJ/mole)ΔS° (kJ/mole K)
  30 °C  40 °C  50 °C 
2,4-D  −0.890  −2.514  −3.831  43.72  147.38 
Metolachlor  9.068  4.619  −0.713  149.00  461.64 
4Conclusion

The present study revealed the preparation of a new montmorillonite derivative, HM for the adsorptive removal of 2,4-D and metolachlor from the aqueous solution. The adsorption process was best described by the pseudo-second order kinetic model, indicating the adsorption rate was dependent mainly on chemisorption, the availability of the surface active sites rather than the concentrations of the pesticide molecules. Langmuir isotherm model provided a better correlation with the equilibrium data, suggesting that the adsorption of 2,4-D and metolachlor onto HM took place as monolayer adsorption on a surface that was homogenous, with a uniform surface affinity, recorded the monolayer adsorption capacities for 2,4-D and metolachlor of 185.19 and 84.75 mg/g, respectively. Thermodynamic analysis showed that the adsorption process was spontaneous and endothermic in nature. The findings illustrated the potential of montmorillonite derived functionalized adsorbent as a viable solution for the practical treatment of herbicides contaminated water. The on-site treatment of these pesticides by means of simple field test could be further evaluated.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgement

The authors acknowledge the financial support provided by the Ministry of Education Malaysia under the Malaysia Research University Network (MRUN) Collaborative Research Program, with the project title: Sustainable water resources management solutions to overcome national water security issues (Project No. 203/PREDAC/6720016) and Universiti Sains Malaysia under the USM Fellowship scheme.

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