2,4-Dichlorophenoxyacetic acid adsorption on adsorbent prepared from groundnut shell: Effect of preparation conditions on equilibrium adsorption capacity

2.1 2,4-Dichlorophenoxyacetic acid

Technical-grade (98% pure) 2,4-D (Sigma–Aldrich, Malaysia) was used as obtained in this study. Milli-Q water (conductivity of Milli-Q water = 18.2 MΩ·cm at 25 °C) was used for the preparation of all the solutions.

Methods of preparation of adsorbents from groundnut shell and its characterization are explained below.

2.2 Groundnut shell ash (GSA)

Groundnut shells were sun dried in a farm for 2 weeks. They were then grinded and passed through a British Standard Specification (BSS) 85-mesh sieve. Powdered shells were ignited in air under constant atmospheric conditions. The groundnut shell ash thus produced was allowed to cool at 30 °C and packed in air-tight polythene bags. They were manually cleaned for dust and soil traces to avoid contamination by foreign particles. The GSA was then sieved using a BSS 25-mesh sieve to separate bigger-sized and unburned particles. The GSA particles were rinsed almost six times with Milli-Q water until the water became clear, and then dried in an oven at 110 °C for 24 h.

2.3 Groundnut shell char (GSC)

Well-dried groundnut shells were grinded and passed through a BSS 85-mesh sieve. The powdered shells were pyrolyzed in a vertical downdraft two-stage fixed-bed biomass reactor (internal diameter 44 mm and thickness 4 mm) under a steady flow of nitrogen (300 cm³ min⁻¹) at 650 °C with heating rate of 20 °C min⁻¹. Inert nitrogen gas was injected downward. The pyrolysis temperature was maintained for 2 h. Char was collected from the first stage of the reactor once it cooled down.

2.4 Activated carbon from groundnut shell (GSAC)

The char produced by pyrolysis of GS (i.e., GSC) was soaked in potassium hydroxide solution (KOH) with an impregnation ratio of 2.5:1 (KOH : char wt.%) for 24 h and dehydrated in an oven overnight at 110 °C. Char is activated with pure nitrogen flow (150 cm³ min⁻¹ at 800 °C for 2 h) with a heating rate of 5 °C min⁻¹ in a tubular reactor. The GSAC was then cooled down to room temperature under a constant flow of nitrogen, and washed with 0.1 M hydrochloric acid first and then with Milli-Q water until the pH of the washing solution is between 6.5 and 7 (W.Tan et al., 2007).

2.5 Characterization of adsorbents

All adsorbent samples were hand ground with a ceramic mortar and pestle, allowing them to pass through a 200-mesh sieve, and dried overnight at 105 °C prior to characterization and batch studies.

A detailed physiochemical characterization of GSA, GSC, and GSAC was then carried out. Proximate analysis was carried out using the gravimetric method. The BET surface area was measured by Brunauer–Emmett–Teller (BET) method (ASAP-2010 Micromeritics). Prior to analysis the sample was degassed at 623 K at 10⁻⁵ Torr overnight. FTIR spectra were obtained using Perkin Elmer Spectrum One; CHNS analysis was performed on an Elemental Analyzer (vario MACRO Cube, Elementar, Munich, Germany), and the SEM images were obtained using JSM 6380A (JEOL, JAPAN). For the XRF analysis, PAN analytical Model No. PW2403 was used.

2.6 Batch adsorption study

A stock solution of 2,4-D was prepared by adding 400 mg of 2,4-D in 1000 ml Milli-Q water; all further dilutions up to 50 mg/L were obtained using this stock solution. All preweighted samples of the adsorbents (GSA, GSC, and GSAC) were added to 25 ml of 2,4-D solution of different initial concentrations, which were taken in a 50-ml glass vial. Studies on the effect of dose, initial concentration, and contact time variation were then carried out. Each vial was agitated in a temperature-controlled water bath shaker for a variable period at 30 °C. After agitation, the solution was filtered using Whatman filter paper No. 41. The concentration of 2,4-D before and after adsorption was measured using a UV–VIS spectrophotometer (Shimadzu UV 1800 Japan Inc.) at 283 nm (Trivedi et al., 2016a). Each experiment was performed in triplicate under the same conditions and average values were recorded. The amount of adsorption equilibrium was calculated using the following equation:

where C_o C_t and C_e (mg/L) are the liquid-phase concentrations of 2,4-D at initial, at time t and equilibrium, respectively; v(L) is the volume of the solution; and w(g) is the mass of dry adsorbent used. For the kinetic study, concentrations of 2,4-D at time t and equilibrium are denoted as q_t and q_e (mg/g), and are calculated as follows:

The percentage of 2,4-Dadsorbed is calculated using the following equation:

3.1 BET surface area

The BET surface area of GSA, GSC, and GSAC was found to be 8, 43 and 709 m²/g, respectively. Subsequent difference in surface areas is due to the temperature conditions, the presence or absence of oxygen and KOH activation. Gergova et al. (1994) also observed that temperature treatment and KOH activation has strong effect on the BET surface area of adsorbent prepared using GS. At higher temperatures (e.g., 650 °C), volatile matter and tars present in the biomass are released, leaving the porous carbon structure behind. In the case of GSAC, KOH intercalates into char and at higher temperatures alkali metals such as potassium have tendency to relocate, thus leaving the porous structure behind (Hameed et al., 2009; Marsh et al., 2006). This process increases the surface area of these molecules by several folds; in this study the BET surface area of GSAC was found be 14 times higher than that of GSC and 78 times higher than that of GSA.

3.2 FTIR

The surface groups of GSA, GSC, and GSAC were studied using FTIR spectroscopy (Fig. 1). In the spectra of GSA BA (before adsorption) a distinct peak at 1468 cm⁻¹ is noted, which is associated with the aromatic C⚌C bond; the band at 1119 cm⁻¹ is attributed to the C—OH stretching, and the signal at 618 cm⁻¹ is attributed to the C—O—H twist bending vibration (Foo, 2010).

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Figure 1

FTIR spectra of 2,4-D, before adsorption (BA) and after adsorption (AA) for GSA, GSC, and GSAC.

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The spectra of 2,4-D adsorbed on GSA AA (after adsorption) are shown in the coupled graph, which clearly shows the disappearance of peaks at 1745 cm⁻¹ and 1111 cm⁻¹, and the appearance of new bands at 1626 cm⁻¹ and 1369 cm⁻¹. This shifting of peak is due to donation of electron by 2,4-D on GSA surface. These new bands are due to C⚌O vibration of the 2,4-D anion, confirming the presence of 2,4-D in the anionic form. Similar results were obtained previously for adsorption of 2,4-D on hydrotalcite (Deokar et al., 2016b).

The FTIR spectra of 2,4-D show a peak at 1732 cm⁻¹, which indicates the presence of —C⚌O of the carboxyl group(Pavlovic et al., 2005). The antisymmetric and symmetric vibrations of C—O—C are represented by the bands at 1311 cm⁻¹ and 1089 cm⁻¹, respectively, whereas O—H deformation coupled with C—O stretching is indicated by the band at 1234 cm⁻¹ (Pavlovic et al., 2005). The peaks at 1475 cm⁻¹ and 1435 cm⁻¹ correspond to the C⚌C vibrations of the aromatic ring and CH₂ vibrations of alkanes, respectively (Pavia et al., 2008). The peak at 693 cm⁻¹ indicates C—Cl stretching (Salman et al., 2011).

Similar functional groups were identified on the surface of GSC (BA), GSC (AA), GSAC (BA) and GSAC (AA). The sharp distinct peak on GSAC at 1369 cm⁻¹ indicates the presence of 2,4-D. Because of the removal of nonvolatile matter and organic components at high temperature, the FTIR spectra of GSC and GSAC show less number of peaks.

3.3 SEM images

From the SEM images, it can be seen that GSA, GSC, and GSAC have a very rough surface. The surface pores are easily visible in the SEM images of GSAC, which are possible sites for 2,4-D adsorption. High-resolution SEM images of each ash sample showed different types of surfaces. The surface of GSA (Fig. 2a) demonstrated a rough surface having high surface irregularities with limited pores. Fig. 2b shows the SEM image of GSC demonstrating the irregularities and deep pores (Fig. 2c). The GSAC micrograph shows well-developed pores having a structured and repetitive pattern, which is indicative of the high surface area (Fig. 2c).

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Figure 2a

Scanning electron micrograph of GSA.

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Figure 2b

Scanning electron micrograph of GSC.

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Figure 2c

Scanning electron micrograph of GSAC.

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3.4 Effect of chemical activation

Several studies have indicated that activation of KOH at high temperatures (750–900 °C) leads to the development of highly porous surface. The increase in temperature of the reacting mixture above 700 °C produces activated carbon with more surface area (Marsh et al., 2006). Based on literature reports (Hameed et al., 2009), the KOH-to-char ratio and temperature were selected as 2.5 and 800 °C, respectively. In the BET surface area analysis, the effect of temperature and KOH was significantly shown by the increased surface area of GSAC, compared with that of GSC and GSA. SEM results also confirm the presence of pores in the case of GSAC.

3.5 Effect of adsorbent dosage

The effects of adsorbent dosage were studied by varying the dosages of GSA, GSC, and GSAC and keeping the concentration of 2,4-D fixed at 100 ppm. Dose of GSA was varied from 1 g to 6 g. It was observed that with the increase in adsorbent dose above 4 g, the percentage removal remained constant at 70% due to saturation of the surface area and attainment of equilibrium. Therefore, for all further experiments, 4 g GSA per 25 ml of 100 ppm 2,4-D solution was taken as the dosage. The need for such a high amount of adsorbent is due to the less BET surface area available for adsorption (8 m²/gm). Similarly, optimized GSC and GSAC doses were found to be 1 gm and 15 mg, respectively, per 25 ml of 100 ppm 2,4-D solution (Fig. 3).

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Figure 3

Effect of doses of GSA, GSC, and GSAC on the adsorption of 2,4-Dichlorophenoxyacetic acid.

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Adsorption efficacy depends on the chemical composition of the adsorbent. XRF analysis indicated that GSA and GSAC contain large amounts of silica. It has been found that silica is unfavorable for adsorption of anionic compounds because of its structure. The surface is relatively more negative when the proportion of silica in adsorbent is higher. Thus, an adsorbent with lesser silica content is a better for adsorption of anionic species such as 2,4-D. By contrast, positive centers such as Al₂O₃ favor the adsorption of these compounds. A previous study confirmed that 2,4-D adsorption does not occur via ligand exchange (Goyne et al., 2004), but rather via electrostatic interaction. GSA and GSAC have higher silica content than GSC, which deters the adsorption of 2,4-D moiety onto their surfaces; therefore, the percentage removal on these two molecules is lesser than on GSC.

3.6 Effect of contact time

To study effect of contact time, batch adsorption studies were performed in separate flasks and the samples were withdrawn at time intervals of 3, 6, 10, 30, 60, 120, 240, and 480 min. A solution of 100 ppm was used in this case and adsorbent doses were the optimum doses obtained from the dose study (Trivedi et al., 2016a). Experimental data have been fitted to pseudo-first-order and pseudo-second-order kinetics models and their kinetic constants are listed in Table 3. The pseudo-second-order model showed the best fit with the experimental data (R² = 0.99) for GSA, GSC, and GSAC, and these results are also presented in Table 3

The effect of time on adsorption is interesting as in 3 min 43% removal was achieved by GSA, which is more than 50% of equilibrium value, whereas in 60 min 70% removal was accomplished. During the first 3 min of adsorption, the driving force is very high, which can be seen from the rate constant k₂ (g/mg/min) value (104 × 10⁻³ for GSA). The equilibrium time taken for removal of 2,4-D on GSA, GSC and GSAC was found to be 60, 120, and 240 min, respectively (Fig. 4). Initial rates of adsorption for GSC and GSAC were lesser as compared to GSA. It is clearly reflected in rate constants values (14.2 × 10⁻³ for GSC, and 0.3 × 10⁻³ for GSAC). The higher silica content in biomass ash is responsible for faster kinetic rate (Deokar et al., 2016b). Hence, GSA exhibits faster rate as compared to GSC and GSAC. The order of time taken to reach equilibrium follows the following trend: GSA < GSC < GSAC.

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Figure 4

Effect of contact time of (GSA, GSC and GSAC) on 2,4-Dichlorophenoxyacetic acid removal.

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Literature reports that carbon fraction in the adsorbent determines the capacity of adsorption, whereas silica fraction determines the kinetics (Deokar et al., 2016b). GSAC with higher surface area and carbon content offers more binding sites, and hence it takes more time to occupy all the available binding sites and to reach equilibrium.

3.7 Effect of initial concentration of adsorbate

The effect of initial concentration was studied by varying 2,4-D concentration from 50 mg/L to 400 mg/L with an optimum dose of GSA, GSC, and GSAC, respectively. Results of the adsorption study presented in Fig. 5 show that with the increase in the initial concentration, the % removal decreases. Experimental data obtained were fitted to Langmuir, Freundlich, and Temkin isotherms (Table 4). The best fit was obtained for the Langmuir isotherm, suggesting monolayer adsorption in which the number of adsorption sites remains constant; consequently, the % removal decreases along with the increase in initial concentration.

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Figure 5

Effect of initial of concentration of 2,4-D on adsorption by GSA, GSC, and GSAC.

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GSC showed the maximum % removal due to its high carbon content and low surface pH (8.5), compared with GSA (10.5) and GSAC (9.7), which allows negatively charged 2,4-D anions to bind to its surface and enable maximum removal.

Table 5 compares the Langmuir capacities of some other adsorbents reported in the literature with those of GSA, GSC, and GSAC. From the table it can be seen that the adsorption capacity of GSAC (250 mg/g) is comparable with the highest reported values, namely those of langsat empty fruit bunch activated carbon (LEFBAC; 261 mg/g) and pumpkin seed hull (260 mg/g). Recently, biomass ashes were evaluated as low-cost adsorbents, which can serve dual purpose: removal of pesticide and as a source of supplementary micronutrients, although their adsorption capacities are very low (Deokar and Mandavgane, 2015; Trivedi et al., 2016a).

3.8 Mode of application of GSA, GSC, and GSAC

In this study, three adsorbents were prepared with different methods using GS. Although GSA has low adsorption capacity, it is a rich source of micronutrients such as Ca, Mg, K, and Si. Some studies have suggested that using RHA, BFA, and MPA on farmlands act as a source of micronutrient, thus increasing crop yield (Deokar and Mandavgane, 2015; Thind et al., 2012; Trivedi et al. 2016a,b). GSA is cheap and does not involve any sophisticated process for production. It is generally obtained as by-product from industrial boilers and can be directly spread on farmland to adsorb 2,4-D. However, syntheses of GSC and GSAC are both energy intensive and costly. Although GSC and GSAC cannot be applied on the farmland, they can still be used as efficient adsorbents to treat water bodies contaminated with 2,4-D in either batch or continuous operation. This point can be illustrated by considering a case of treating 1 m³ of water containing 400 ppm of 2,4-D using these adsorbents (batch processing). The amount of GSA, GSC, and GSAC required for this purpose will be 460, 130, and 1.6 kg, respectively. Because the amount of adsorbent required in the case of GSAC and GSC is less than that of GSA, the size of adsorption equipment will follow the order GSA > GSC > GSAC for treating the same solution.

3.9 GSA utilization in farmland

The Central Insecticides Board and Registration Committee (CIBRC, Government of India, 2014)(2014) has provided clear guidelines regarding the use of approved registered herbicides. 2,4-D in salt and ester form is recommended for wheat, maize, sugarcane, rice, sorghum, citrus, etc., to control different weed species. The pesticide sprayed, however, runs off from the farmland and leaches into groundwater though soil. To avoid this, based on our batch study results, we recommend spreading 574 kg of GSA/hectare [Langmuir capacity of GSA is 0.87 mg/g that means 0.87 mg of 2,4-D is adsorbed per gram of GSA. Hence to remove 0.5 kg of 2, 4-D; (0.5/0.00087 = 574) amount of GSA required per hectare is 574 kg] for wheat and maize. Based on composition [CaO (112 g/kg) K₂O (57 g/kg),and P₂O₅ (33 g/kg], 574 kg of GSA will supplement 64 kg of CaO [0.112 ∗ 574 = 64], 32 kg of K₂O [0.57 ∗ 574 = 32], and 21 kg of P₂O₅ [0.33∗574 = 21]_, which would significantly improve crop yield; moreover, as soon as 2,4-D comes in contact with GSA, equilibrium is reached almost instantly and 2,4-D will get adsorbed. Thus, GSA acts as a soil-protecting layer and as a barrier between 2,4-D and soil, thus preventing the chemical’s direct contact with the soil. The recommended GSA dosage is based on the assumption that the entire 2,4-D sprayed is runoff. The quantity of GSA spread will form a layer on soil surface, which will act as an adsorbent of 2,4-D. Table 6 presents the recommended dosages of GSA dispersal/hectare, according to our study results, for different crops and fruits being cultivated in India. It is also recommended that GSA should be spread before each spraying of 2,4-D.