Obtaining adsorbents from acid and acid-thermal activation of bentonite for chlorothalonil pesticide removal

Montmorillonite (Mt) is the most widely known and applied clay mineral. It is of type 2:1 of the phyllosilicates family, where each layer has two silica tetrahedral sheets, and between them an alumina octahedral sheet (Mokaya & Jones 1995). Mt is dominant in bentonites and determines their defining properties, such as high surface area, cation exchange capacity, and surface acidity. Bentonites are widely used as catalysts, as geo-environmental barriers and liners (e.g. landfills), for wastewater treatment, and for oil-spill cleanup (Tsai et al. 2004).

One of the most common chemical modifications of clay minerals, used for both industrial and scientific purposes, is their acid activation. This consists of the reaction of clay minerals with a mineral acid solution, usually HCl or H₂SO₄. The main task is to obtain partly dissolved material of increased specific surface area, porosity and surface acidity (Vengris et al. 2001; Carrado & Komadel 2009; Daou et al. 2017). Acid activation reduces or eliminates most of the oxidic minerals, including calcium, magnesium, and alkali metal oxide, from the clay material relative to SiO₂ (Bergaya & Lagaly 2001; Chaisena & Rangariwatananon 2005). It also generally increases the surface area and adsorption capacity (Rusmin et al. 2016; Boudouara et al. 2017).

When bentonite is subjected to thermal treatment, its surface properties are also significantly changed. The specific surface area (SSA) of bentonite increases at 100 °C due to the removal of physio-adsorbed water and volatile impurities (Toor et al. 2015). However, a further increase of temperature (up to 500 °C) reduces the significant SSA (Bojemueller et al. 2001), caused by the dehydroxylation of kaolinitic minerals followed by montmorillonitic minerals (Nones et al. 2015; España et al. 2019).

The combined application of thermal and acid treatment was also found effective in increasing the surface area of clay minerals (Toor 2010; Rusmin et al. 2016; Yaghoobi-Rahni et al. 2017). In most of the cases, this would cause a significant difference in the pore size distribution of the adsorbent and impact its interaction with contaminant molecules or ions. For example, kaolinite modified by thermal treatment followed by acid activation showed a slight increase in specific surface area and cation exchange capacity (CEC) (Suraj et al. 1998; San Cristóbal et al. 2009; Toor 2010). Similarly, acid activated palygorskite displayed increased SSA following thermal treatment due to loss of mesopores and consequent development of micropores (Gonzalez et al. 1990).

The massive use of pesticides in recent decades has caused serious damage in nature. Many residues and metabolites of these phytosanitary products are found in most compartments of the environment, especially in surface waters (Gonzalez-Pradas et al. 2005). Chlorothalonil is one of the pesticides used in agricultural fields as an inhibitor of spore germination, acting on various enzymes and on the metabolism of fungi. It can seep into the aquatic environment by means of spray and runoff (Fletcher et al. 1994). Exposure to chlorothalonil is possible through ingestion of contaminated food or water (Hayward et al. 2010). Chlorothalonil has been classified by the U. S. Environmental Protection Agency (EPA) as a probable carcinogen, regardless of the route of exposure, with very high toxicity to fish and aquatic vertebrates (U.S. EPA 1999). Numerous water treatment processes such as membrane separation, photocatalytics, oxidation, and adsorption have been used for pesticide removal (Shen et al. 2012; Zuo et al. 2012; Mehta et al. 2017; Khairy et al. 2018; Mohammadi & Sheibani 2019). In general, adsorption is one of the favoured treatment processes because of its simplicity, high efficiency, and low cost (Djebri et al. 2017).

The focus of this work is partially structural modification of local bentonite in order to enhance its adsorption capacity for chlorothalonil pesticide. For this, in the first step, the purified bentonite was activated by acid leaching with sulfuric acid at different concentrations. The second step included two successive operations: bentonite activation by sulfuric acid attack was followed by thermal treatment at two temperatures, 100 and 200 °C. The effect of the experimental conditions on the behavior of adsorption was studied. The adsorption data were analyzed by the traditional models and the thermodynamic parameters were also calculated.

Bentonite used in this study was purchased from Mostaganem. This material is commercialized as industrial charge bentonite without additives by ENOF Company, Algeria. Before the experiments, the samples were purified and sieved at 80 μm. The cation exchange capacity (CEC) of natural bentonite was determined to be 48 meq/100 g by applying the conductometric titration method (Agouborde & Navia 2009). The point of zero charge (PZC) of purified bentonite, as determinated by the solid addition method (Srivastava et al. 2006), was equal to 6.8.

Chlorothalonil (Chl) is a fungicide, and was obtained from Syngenta Protection of Plants S.A, Bale, Switzerland. It contains 400 g/L of chlorothalonil in the form of a concentrated suspension with some impurities. The chemical formula of Chl is C₈Cl₄N₂, and its molecular weight is 265.93 g/mol. The IUPAC name of Chl is 2,4,5,6-tetrachloro-1,3-benzenedicarbonitrile and its N° CAS is 1897-45-6. The solubility of chlorothalonil in water is 0.6 mg/L at 20 °C. The structure of the Chl molecule is presented in Figure 1.

The bentonite samples were treated with different sulfuric acid normalities of 1 N, 3 N and 6 N. A sample amount of 5 g was added to 100 mL of H₂SO₄ solution at a temperature of 70 °C for four hours' contact time (Amari et al. 2010; Tomic et al. 2011; Khoualdia et al. 2017). The solids obtained after drying were designated as BA-1, BA-3 and BA-6.

Purified bentonite (5 g) was treated in 100 mL of 1N sulfuric acid solution under reflux condition at 70 °C for 4 h. The acid-activated bentonite was then separated from the liquid by centrifugation followed by thorough washing with distilled water. After drying at 60 °C, BA was subjected to thermal activation by heating in a muffle furnace at 100 and 200 °C for 30 minutes (Toor et al. 2015; Rusmin et al. 2016). The final product obtained after both the acid and thermal treatments was denoted as BAT-100 and BAT-200.

X-ray diffraction (XRD) patterns of powdered samples were collected by INEL CPS 120 diffractometer using Coλ_α (λ = 0.178 nm) radiation operating at 25 mA and 40 kV with fixed slit. Specific surface area tests were determined by BET method via Quantachrome instruments using adsorption nitrogen at −196 °C and by Methylene blue adsorption method (Chen et al. 1999; Santamarina et al. 2002). The main characteristics of the sorbents obtained, such as PZC and specific surface area, are summarized in Table 1. The absorbance measurements of chlorothalonil were undertaken with a spectrophotometer VIS 7220 G, Biotech Engineering Management at the λ_max = 360 nm.

The chemical analysis of raw bentonite was performed by X-fluorescence (XRF 9900, Thermo Instruments). The result of this analysis revealed that silica (64.22%), alumina (11.62%) and lime (9.33%) are the main oxides of the bentonite with the existence of other oxides in very small amounts such as Fe₂O₃ (4.88%), TiO₂ (1.06%), Na₂O (3.38%) and P₂O₅ (0.03%). According to the results mentioned in Table 1, the pH_PZC values of acid activated bentonite were almost identical at around 4, but this value was below to that of purified bentonite (6.8). The values of pH_PZC for BAT-100 and BAT-200 were equal to 3.7 and 3.8, respectively. We can conclude that acid attack and thermal activation considerably reduce the pH_PZC value of raw bentonite. However, the activated bentonite samples develop positive electrical charges at their surfaces at pH values below 4 while these charges become negative above pH 4.

The specific surface areas, S_BET, of acid activated bentonites are almost identical at around 80 m²/g, which is higher than that of purified bentonite (59 m²/g). That means the acid attack enhanced the specific surface area of bentonite and created microporosity. The average value of the S_MB (292 m²/g) of the activated bentonite proves the existence of a mesoporosity at the surface of samples. In the case of thermal activation, the calcination of bentonite at 100 or 200 °C affects the structure of the montmorillonite. The surface area of the BAT increased with the temperature increase up to 100 °C and then gradually decreased beyond 100 °C. Thermal activation under a high temperature can remove water molecules and other impurities. The initial increase in the surface area with temperature is due to the removal of adsorbed and hydrated water molecules, and volatile organic compounds attached on the surface of the raw bentonite, while calcinations at a higher temperature can alter the chemical and physical properties of the bentonite (Purkait et al. 2007; Toor et al. 2015).

The XRD patterns of purified and activated bentonite are shown in Figures 2 and 3, respectively. The bentonite sample contains some mineral phases such as montmonrillonite (M), kaolinite (K), calcite (C), quartz (Q) and dolomite (D). The characteristic peak d₀₀₁ of montmorillonite appears at 2θ = 5.5°, kaolinite is observed at 2θ = 10° and the peak of calcite appears at 2θ = 31°. The bentonite samples treated by sulfuric acid show no significant change in the mineral composition of the purified bentonite except for a decrease in the intensity of the peak of montmorillonite. The concentrations of acid used contributed to partially attack the structure of the clay by dissolving the octahedral sheets. Similar results were found by activating bentonite with sulfuric acid (D'Amico et al. 2014). Tomic et al. (2011) observed no significant change in the crystallinity of the main d₀₀₁ peak of Aleksinac bentonite treated with 6M of H₂SO₄; at the same time, they found that the indexed peak (001) in Petrovac bentonite was reduced with increased H₂SO₄ concentrations (6M).

The XRD patterns of acid-thermal activated bentonite samples are illustrated in Figure 4. The sample treated at 100 °C after acid attack presents no notable modification in the intensity of the main peak d₀₀₁ of the montmorillonite, while in the BAT-200 sample we see clearly the decrease in intensity and crystallinity of the characteristic peak of montmorillonite.

The effect of pH is among the parameters that affect the adsorption capacity; it can change the electric surface charge of adsorbents. In this study the evolution of pH was followed by adsorption of chlorothalonil onto activated bentonite samples, for an initial concentration of about 30 mg/L at room temperature. Figure 5 shows the adsorbed amount of Chl onto acid activated bentonites related to the change in pH of the solution. The first thing noticed is that the amount of Chl adsorbed by the three adsorbents is maximum at around pH 2 and 3. Then it begins to decrease gradually until pH 5 for BA-3 and BA-6, while for BA-1 the adsorbed amount decreases to pH 7. Above these pH values, there is a slight increase in the adsorbed amount, and then it decreases.

These results can be explained by the fact that chlorothalonil is an anionic pesticide in solution, and in acid medium the number of protons, H⁺, becomes higher, which makes the charge positive on the surface of the material. However, the chlorothalonil molecules will be adsorbed by these same positive sites of treated bentonite. Figure 6 also presents the same behavior of acid–thermal activated bentonite against the evolution of pH; that is, the adsorbed amount of Chl was optimal at pH range 2–3 and decreased continually until pH 9. Previous studies have been conducted by Sahnoun et al. (2016) on the adsorption of 2,4,5-trichlorophenol pesticide by clay treated at 2N H₂SO₄, and they have found that the pH maximum noted was pH = 4.

Research into the optimum contact time is the aim of the kinetic adsorption study, and to determine the process mechanism, for this a series of samples were prepared in the time range of 5–180 min. The concentration of Chl was fixed at 60 mg/L in 20 mL volume of solution, into which 0.1 g of adsorbent was added.

The results are registered in Figure 7, and show that the removed amount of chlorothalonil varied with the contact time, a maximum efficiency was obtained after 60 min for all acid activated bentonites and the equilibrium was reached at 45 min for BAT-100 and BAT-200. Note that the kinetics of pesticide adsorption on the clays studied have the same shape from the first minutes of contact, followed by an increase until a state of equilibrium is reached. The first stage is fast and corresponds to the transfer of external mass while the second stage is slow, linked to the phenomenon of diffusion (internal mass transfer).

To evaluate the adsorption rate, the adsorption kinetics were examined by both models, pseudo first-order and pseudo second-order. According to the results as listed in Table 2, it is clear that the pseudo second-order model describes very well our experimental data because the correlation coefficients R² are near to 1 and the experimental adsorbed amount of Chl for all samples is almost identical to that calculated. On the other hand, the equations of the pseudo first-order are very far from giving good straight lines.

The adsorption isotherms were obtained at different initial concentrations (100–300 mg/L) during 2 h at three temperatures of 20, 30 and 40 °C. The isotherms are formed by plotting adsorbed amounts of the pesticide versus equilibrium concentrations. Figure 8 shows the adsorption of Chl by the activated adsorbents at room temperature (20 °C). This figure indicates that the adsorbed amount of Chl onto BA materials increases in parallel with the equilibrium concentrations. Using the classification of Giles et al. (1960), the experimental isotherm obtained is of type S. This type of isotherm suggests a cooperative adsorption where the adsorbed molecules promote adsorption from the other molecules. The maximum amount of Chl adsorbed is registered at 56.40 mg/g, attributed to BA-6, but the difference in the adsorbed amount is very small between the three solids chemically treated. The acid-thermally modified adsorbents adsorb less than the adsorbents treated with acid. It was noted that the Chl amounts adsorbed onto BAT-100 and BAT-200 were 52.29 mg/g and 48.34 mg/g, respectively.

The adsorption of Chl by BA-1, BAT-100 and BAT-200 at temperature 30 and 40 °C was examined (Figure 9). The adsorption capacity slightly decreases with increased temperature only in the case of BA-1, but it increases with increased temperature for BAT-100 and BAT-200. For comparison, the adsorption capacity of the Chl by the acid-activated bentonite was more than that for acid-thermal activated bentonite at the three temperatures studied.

Two isotherm models are widely used to describe experimental adsorption data. These are the Freundlich equation and the Langmuir equation. As illustrated in Table 3, the correlation coefficients, R², for the Langmuir isotherm model were very low to the unit for acid treated samples, and were above 0.94 for acid-thermal activated samples, but we noted also that the maximum calculated amount of Chl adsorbed was very high compared to that tested. While for the Freundlich model the regression coefficients were greater than 0.91 for all samples. This result revealed that the number of adsorption sites on activated bentonite was unlimited and that the pesticide molecules may form poly-molecular layers on the heterogeneous adsorbent surface. The 1/n values observed were all less than unity, which proves the adsorption of Chl was favorable. It should be noted that the plots of the amount of Chl adsorbed against the equilibrium Chl concentration were well fitted by the Freundlich equation.

The objective of the thermodynamic study was to calculate the heat adsorption of pesticide onto treated bentonites. For this, we carried out the reaction of adsorption at 20, 30 and 40 °C. The values of the heat adsorption (ΔH°, ΔS° and ΔG°) in the range of initial concentrations of Chl of 100, 200 and 300 mg/L are reported in Table 4. The values of enthalpy and entropy are positive for all samples, proving that we are in physical adsorption and endothermic reaction, and the molecules disorder was located at the solid/liquid interface. It is noted that the BAT-200 adsorbent presents great values of enthalpy and entropy changes compared to the others adsorbents.

The free energy ΔG° values were negative for BAT-100 and BAT-200, which means the adsorption reaction of Chl by acid-thermal activated bentonites was spontaneous and the spontaneity increases with increasing temperature, while the adsorption of Chl on BA-1 is favorable only at room temperature. This fact was confirmed by the lower removal of Chl concentrations from the aqueous phase at 30 and 40 °C (Figure 9). These findings have been reported in the works of Gil et al. (2013), which removed methylene blue using acid and thermal activated clay minerals.

The aim of this work was to study the adsorption of the pesticide chlorothalonil on bentonite treated with acid attack at different concentrations for the first time, then the acid activation was combined with thermal treatment at 100 and 200 °C.

The acid and thermal activation of bentonite affects the structure of clay minerals, especially montmorillonite and kaolinite. Therefore the specific surface area was enhanced with increasing sulfuric acid concentration and decreased when the temperature of treatment increased.

The amount of Chl uptake increased with increasing initial Chl concentration and contact time, and decrease with pH of the solution. Also, the adsorption efficiency of the Chl onto the acid-activated bentonites was greater than that with acid-thermal activated bentonites. The maximum Chl adsorption onto treated clay was rapidly attained within 60 min. Kinetic data tend to fit well with the pseudo-second order rate expression. The adsorption equilibrium Chl/bentonite system is most suitably described by the Freundlich model. The adsorption follows a spontaneous, endothermic and physical adsorption. The study shows that acid activated bentonite or acid-thermal activated bentonite can be used as a cheap and efficient adsorbent for removing pesticide from water and wastewaters.

All relevant data are included in the paper or its Supplementary Information.