Removal and extraction efficiency of Quaternary ammonium herbicides paraquat (PQ) from aqueous solution by ketoenol–pyrazole receptor functionalized silica hybrid adsorbent (SiNPz)
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Pesticides and herbicides have been used extensively in agricultural practices to control pests and increase crop yields. Paraquat (PQT2+, 1,1-dimethyl-4,4-dipyridinium chloride) is one of the herbicide that belois classified as bipyridines and is used over the world. The objective of this study is to use ketoenol–pyrazole receptor functionalized silica hybrid as adsorbent for removal PQT2+ from aqueous solution. The adsorbent was synthesized, and characterized using scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), Thermal analysis and other techniques. Different experimental parameters such as the effect of the amount of adsorbent, solution pH and temperatures and contact times were studied. Pseudo-order kinetics models were studied, and our data followed a pseudo second order. Experimental data were analyzed for both Langmuir and Freundlich models and the data fitted well with the Langmuir isotherm model. To understand the mechanism of adsorption, thermodynamic parameters like standard enthalpy, standard Gibbs free energy, and standard entropy were studied. The study indicated that the process is spontaneous, exothermic in nature and follow physisorption mechanisms. The novelty of this study showed surface of pyrazol-enol-imine-substituted silica (SiNPz) has the ability to highlight the surface designed for efficient removal of PQT2+, from aqueous solutions more than other studies. The study also showed that ketoenol–pyrazole receptor can be regenerated in five cycles using HNO3 without affecting its adsorption capacity.
KeywordsKetoenol–pyrazole receptor Adsorption Paraquat Kinetics Isotherm
PQT2+, 1,1-dimethyl-4,4-dipyridinium chloride
scanning electron microscopy
nuclear magnetic resonance
high-performance liquid chromatography
Langmuir affinity constant
adsorption capacity at equilibrium (mg/g)
amount of adsorbate per unit mass of adsorbent (mg/g)
pseudo-second-order rate coefficient, the amount adsorbed per unit mass at equilibrium and at any time qe and qt
intra-particle diffusion rate constant
- C (mg g−1)
a constant proportional to the thickness of the boundary layer
standard free energy
- R (8.314 J/K.mol)
ideal gas constant
Pesticides have been used in agriculture to overcome pests and increase crop yields. They are used to reduce weeds, insecticides and fungicides. The amount of these pesticides that needed are not well known and most of the farmers exceeded the required quantity . Most industries and food processing companies are always releasing some pesticides through their effluents . Pesticides are organic compounds and they affect the environment in different ways. There are different types of pesticides categories including organophosphates, carbamates, substituted urea compounds, organochlorines, and pyrethroids. Due to their dangerous effect and toxicity in the environment, different research areas are involved to get rid of them from the environment .
Lately, agricultural types in Palestine are aiming to avoid low plant development and increase the production and the quality of the products. These changes help to introduce higher levels of herbicides in the agricultural ecosystem . The main output of these agricultural practices is the contamination of soils and waters, which leads to degrade the soil–water–plant system and bioaccumulate herbicide residues.
Paraquat (PQT2+, 1,1-dimethyl-4,4-dipyridinium chloride) is a herbicide and belongs to the class of the bipyridines. It is one of the most widely used herbicides in the world and forbidden in some countries. The advantages of paraquat over other herbicides is very quick and non-selective action to kill green plant tissue upon usage .
In the last years, several studies have been given to PQT2+, mainly due to the high rate of poisoning and fatalities attributed to it .
Several studies have been carried out for the removal of PQT2+ from aqueous medium and wastewater. One of them is related to the oxidation of PQT2+, which emphasize the destruction of the structure of the pesticide . In this study, several reagents can be used for this study and can be enhanced by applying ultraviolet radiation, which increases the formation of free radicals. Some disadvantages of this experiment are the production of toxic substances if the degradation process was not carried right [6, 7, 8].
Another method of removal of PQT2+ is adsorption on solid adsorbents using different substrates and nanomaterials. Various adsorbents such as activated carbon , biological tissues  and modified materials  have been employed for the adsorption of PQT2+ from aqueous solutions. In previous studies, sawdust and peanut shell powder were explored as adsorbents for the removal of phosphorus and other dyes from aqueous solutions [12, 13, 14, 15, 16].
Pesticides and herbicides are determined using instrumentation such as gas chromatography (GC) and high-performance liquid chromatography (HPLC) . Their degradation is involved by crops through their metabolites . There are several methods that are used for determination of pesticides like nanotechnology-based protocols were used to investigate these problems . Some examples like, some metals and silica nanoparticles are used for such studies . In this study, the ability of pyrazole and its derivatives to play as ligands with sp2 hybrid nitrogen donors have been the study areas of several scientists. This is shown in different research and published papers in this field [20, 21]. Besides that, ketoenol moiety has an important type of ligand in view of its distinct structural characteristics and high synthetic utility . Research on β-ketoenol derivatives and their metal complexes have been studied by a number of phenomena’s such as their important practical application.
This kind of molecules have two possibilities of coordination sites and can act as a uni- or bidentate ligand or coordinate to the metal atom through monoionic or neutral form. Sometimes they form a bridge between two metal atoms. It is obvious that will lead for the possibility to opens this kind of ligand to be grafted onto silica gel and increase adsorption capacity toward heavy metals or other contaminates of interest.
The goal of this study is to report the investigation of the fabrication of highly branched adsorbent and chelated material using covalent immobilization of a prepared mixed ligand (β-ketoenol–pyrazole) onto silica particles to study the adsorption of PQT2+ from aqueous solutions.
Materials and methods
The solvents and chemicals used in this study were purchased from Aldrich, USA. All of them with high purity. Silica gel (E. Merch) with a particle size in the range of 70–230 mesh, median pore diameter 60 Å, was activated using heat at 160–170 °C within 24 h. The salivating agent 3-aminopropyltrimethoxysilane (Janssen Chimica) was very pure. All the characterization of the samples was described and reported in our previous study for the removal of heavy metals . Paraquat dichloride was purchased from (Fluka, Steinheim, Germany).
Synthesis of (2Z)-1-(1,5-dimethyl-1H-pyrazole-3-yl)-3-hydroxybut-2-en-1-one
As we reported in previous study , amount of ethyl 1,5-dimethyl-1H-pyrazole-3-carboxylate (30 mmol) dissolved in 30 mL of toluene and added to a suspension of sodium (52.5 mmol) in 50 mL of anhydrous toluene. Acetone (2.5 g; 42.5 mmol) dissolved in 10 mL of toluene was added at very low temperature. The final solution was shacked vigorously at room temperature for 48 h.
Synthesis of 3-aminopropylsilica (SiNH2)
To accomplish this synthesis, reaction between the silylating agent and silanol groups on the silica surface was occured. An amount of activated silica gel SiO2 (30 g) mixed with about 150 mL of dried toluene was refluxed and stirred under nitrogen atmosphere for about 2 h. After that, 10 mL of aminopropyltrimethoxysilane was added dropwise to the suspended solution and the final mixture was refluxed for 2 days. The precipitate was filtered and washed several times with both toluene and ethanol. The solution was extracted using a mixture of ethanol and dichloromethane (1:1) for about 12 h to separate all residues (Scheme 1). In this stage we named the immobilized silica gel SiNH2 which was dried at room temperature .
Synthesis of pyrazol-enol-imine-substituted silica (SiNPz)
To prepare and synthesizedf SiNPz, amount of 3-aminopropylsilica (SiNH2) (5 g) and (2Z)-1-(1,5-dimethyl-1H-pyrazol-3-yl)-3-hydroxybut-2-en-1-one (3 g) were dissolved in 60 mL of dry diethyl ether. The mixture was stirred mechanically for 24 h at room temperature. As mentioned before, the solution was filtered and Soxhlet extracted using acetonitrile, methanol and dichloromethane for 12 h, respectively. Final product was dried at 70 °C for 24 h.
The elemental analysis for the synthesis of SiNH2 showed 4.46% of carbon and 1.66% of nitrogen. While the synthesis of SiNPz showed 9.73% of carbon and 2.8% of nitrogen. The variation of carbon and nitrogen between the two samples indicating the variation of organic moieties. The increase of both nitrogen and carbon in the second sample (SiNPz) indicated that the (2Z)-1-(1,5-dimethyl-1H-pyrazol-3-yl)-3-hydroxybut-2-en-1-one was attached to SiNH2.
All NMR, FT-IR, SEM, surface pore volume and thermal analysis were done for the sample prepared with SiNH2 and SiNPz and the sample was used for the application of studying the efficiency of removing heavy metals from aqueous solution .
Measurements of PQ in water
In our study for determination PQ in solution, a sensitive method was used and reported by Rai et al. [23, 24]. Where sodium borohydride is used as reducing reagent for the reduction of PQ to form a stable blue colored free radical ion. The advantages of the method are simple, reproducible, nontoxic reducing agent and excellent stability of the blue free radical ion.
In summary, 1000 mg L−1 -aqueous solution of paraquat (PQT) was prepared by dissolving 69.1 mg of paraquat dichloride (Aldrich, USA) in deionized water to make 50 mL of solution in a volumetric flask. Different working standard solutions and calibration curves were prepared by appropriate dilution from the stock solution depending on the experiment.
The absorption spectra of the blue colored solution showed maximum absorbance at 600 nm while the reagent blank had a negligible absorbance at this wavelength.
The reproducibility of the method was studied by replicate analysis of 3.0 µg of PQ in 10 mL solution for 5 days. The SD and relative SD of absorbance values were found to be ± 0.0053 and 1.47% respectively.
The adsorption kinetics experiments were studied as follow: (50 mg/L, 100 mg adsorbant and agitation speed of 300 rpm). The studies on the adsorption using the SiNPz adsorbent have indicated that the adsorption showed very fast and increased slowly after 50 min up 200 min. The samples were drawn from the beaker by a pipet of 10 mL at different interval times of 1, 5, 10, 30, 60, 90, and 180 min. Each sample was filtered with filter paper of 45 µm and analyzed using the spectrophotometer (Hitachi UV-1500A) at 600 nm. Both the effect of temperatures (15, 25, 35 and 45 °C) and the pH 2, 4, 6, 10 and 12) were studied. Each time we study one parameter we keep the other constant. This experiment was done with repletion of 3 times and the average was used when we analyzed the data.
In each experiment, about 100 mg of SiNPz adsorbent was placed into a shaker bath at 25 ± 0.1 °C and initial pH of 11.0 for all experiments. Isotherm experiments were handled by shaking (at 300 rpm) with a known volume (50 mL) of paraquat solutions at different initial concentration and specified contact time. The concentration of paraquat was analyzed at the end of each contact period and the measurements were repeated 3 times.
Results and discussion
The parameters affecting the adsorption of paraquat, such as dosage, initial concentration, pH, and temperature, were studied. In our study, for those parameters, we kept all variables constant except the one we want to study.
Investigation of adsorption parameters
pH effect on PQT2+ adsorption
From (Fig. 1), the amount of adsorption was very small at pH = 2 and increased when the solution become basic (as pH increases).
The small amount of adsorption at low pH (< 2) was due to the high mobility of H+ and adsorbed over PQT2+ and this lead to the increased of the adsorbed amount of cationic paraquat which was increased in response to the increasing number of negatively charged sites that exist due to the loss of H+ from the surface .
Temperature effect on PQT2+ adsorption
Concentrations effects on PQT2+ adsorption
To understand the adsorption capacity, we have to design an experiment at a specific temperature to remove paraquat from the aqueous solution.
There are several isotherm models like Langmuir, Freundlich, BET, etc. which can be applied at all temperatures. All of these models have equations that can be used, and the data will be fit into these equations. One of the factors that can lead to the type of isotherm model is the correlation coefficients, R2 .
Parameters in Langmuir and Freundlich adsorption isotherm models of paraquat onto ketoenol–pyrazole at 298 K
Langmuir isotherm parameters
Freundlich isotherm parameters
Presenting the experimental data through kinetics equations like the Lagergren pseudo-first-order model, the pseudo-second-order model will describe the mechanism of adsorption and degradation of paraquat in aqueous solution. Such studies give information about the possible mechanism of adsorption of paraquat and different transition states on the final complex of paraquat and the adsorbent. From the reactions parameters like rate constants and adsorption capacity factors, one can have an idea about the adsorption dynamics and this will help the industry for other applications.
The adsorption experimental data of paraquat by the SiNPz were analyzed using the most common kinetic models to understand the nature of the adsorption process.
Pseudo first order and pseudo second order kinetic model parameters for PQT2+ adsorption by SiNPz
Pseudo first order
qe (exp) 17.32
Pseudo second order
From Table 2, the calculated values of the amount adsorbed at equilibrium (qe, calc) were far from the experimental values (qe, exp) for the pseudo -first order which means that the adsorption cannot be represented by this model.
The pseudo-second-order equation was also used for describing the adsorption of the paraquat by SiNPz .
In case of pseudo-second order, it is clear from Table 2, that both the correlation coefficient R2 which is very close to one and the values for both (qe Calc) and (qe Exp) were very close and this indicates that the adsorption followed pseudo -second order.
Adsorption rate-controlling mechanism
The sorption of paraquat by SiNPz is a very complex process where both characteristics of both (adsorbate and adsorbent) plays an important role. Different factors will be involved in this process: bulk solution will be involved when adsorbate diffused from the solution to the boundary surface of the solution surrounding SiNPz. Other phenomena are film diffusion when paraquat diffuse through the film surrounding SiNPz. Finally, what we called pore diffusion when paraquat finds pores inside SiNPz. Usually, the slowest one will control the adsorption.
The values of the calculated thermodynamic parameters of PQT2+ adsorption
ΔS° (J/mol k)
From Fig. 9, both ΔH and ΔS can be calculated from the slope and the intercept of the straight line. The ΔH values was +22.56 kJ/mol, for the adsorption of paraquat by SiNPz from the aqueous solution. This positive value indicates the endothermic nature of the adsorption of paraquat by SiNPz, which confirmed our previous study of the effect of temperature that adsorption increased when temperature increased. Also, the value of ΔH suggests a strong affinity between paraquat by SiNPz and the physical nature of the adsorption. The low value of ΔS, 0.114 J/mol K, suggested very low of randomness at the SiNPz/solution interface during the adsorption and immobilization of paraquat.
The ΔG values were negative which indicates that the adsorption of paraquat by SiNPz was favored and spontaneous. The negative values of ΔG, and positive values of ΔH, and ΔS suggested that the adsorption of paraquat process is an entropy-driven process.
Regeneration of adsorbent
In order to make the adsorption more environmentally friendly, regeneration experiment was studied. Regeneration is an important factor to determine the cost-effectiveness and the possibility of reuse several times. The main important factor is the possibility to reuse paraquat that has been adsorbed for other things.
Thermodynamics study showed that the adsorption process is governed by physisorption, indicating a week force between adsorbent and adsorbate. This means that the regeneration process is feasible. Also, from the study of pH effect on adsorption, the removal efficiency increased as pH increased. In this case, decreasing pH will enhance the desorption process. This suggests that washing the contaminated ketoenol pyrazole with an acid like HNO3 is more efficient than with basic solution.
Pesticides have been used extensively in agriculture to control pests and increase crop yields. They are used to control weeds, insecticides and fungicides. This study approved that SiNPz receptor could be used as an adsorbent for paraquat from aqueous solution in a short time with high removal efficiency. The optimization parameter for adsorption like, pH, temperature, and dosage were studied and found to playan important role in the capacity of adsorption increased with increasing temperatures and pH. The adsorption isotherm was studied, and the data were best fitted with the Langmuir model. The data were fitted to both pseudo–first order and pseudo-second order and the results fitted much better to pseudo-second order using both correlation coefficient R2 and qe experiment was very closed to the calculated one. Thermodynamics study showed that the adsorption is spontaneous and exothermic with physisorption nature of the adsorption process.
The regeneration studies confirmed that the adsorbent can be reused for several times with adsorption capacity of more than 87%. This means that the adsorption process is efficient simple and cost-effective and can be used in large-scale industry.
The authors would like to thank the scientific research at An-Najah National University for their financial support under Project # ANNU-1718-Sc020. This funding helps in analysis the results outside and purchasing chemicals. They also, like to thank the department of chemistry at Mohammed Premier and An-Najah National Universities for their help and using the instrumentation over there.
SJ wrote the manuscript. GH did most of the adsorption experiment, ST and SR did the preparation and characterization of the adsorbent. OH and DJ helped in editing the English language beside adding some paragraphs to the text. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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