Comparative adsorption of selected pesticides from aqueous solutions by activated carbon and biochar

Water pollution consequent to pesticides addition is an essential environmental problem. The huge increase in world population during the 20th century would not have been possible without a simultaneous increase in food production. Almost one-third of agriculture production depends on pesticides utilization, otherwise an estimated 78% of fruit, 54% of vegetable and 32% of cereal crop production would be lost (Tudi et al. 2021). The increased use of pesticides in the agricultural sector and domestic applications, its prevalent mishandling (Thuy et al. 2012a, b), and its toxicity and carcinogenicity (Hussain et al. 2002) leads to production of toxic pollutants, contaminating the natural environment and causing human health risk (Hoai et al. 2011). Approximately, 2 million tonnes of pesticides are utilized annually worldwide (Sharma et al. 2019), however, it was only anticipated to be 5–7 metric tonnes with active substances annually throughout Europe (Kristoffersen et al. 2008). According to the United States Geological Survey (USGS), in US streams more than 90% of water samples were contaminated with a variety of pesticides. Similarly, Mazlan et al. (2017) reported 37,000–500,000 m² of wetlands contamination in Canada by herbicides (Mazlan et al. 2017). Another illustration of the level of pesticide contamination in Turkey's West Mediterranean region is 53,349.5 g/ha, or an estimated 79% of the available pesticides (Yilmaz 2015). These pesticides percolate deep down in to the soil, contaminate the subsoil water, leaching into ground and surface water, and deteriorate water quality (Hussain et al. 2002). Since a few decades ago, remedial studies have gained attention in order to meet set drinking water requirements (Somashekar et al. 2015). Numerous research has been carried out to develop efficient techniques for the elimination of organic pollutants from water used for drinking purposes (Kubo et al. 2007; Ahmad et al. 2014) including precipitation, reverse osmosis, adsorption, ion exchange, incineration, air stripping, and chemical treatment. Amongst them, adsorption onto carbonaceous materials AC and BC as remedial strategy for pesticides' removal (Kyriakopoulos & Doulia 2006; Areerachakul et al. 2007; Mohan et al. 2014) has achieved environmental significance since it can efficiently remediate pollutants from both gaseous streams and aqueous environments (Aksu & Kabasakal 2004; Kah & Brown 2007). Adsorption as a surface phenomenon, its adsorption extent and rate for a specific adsorbent are influenced by the adsorbent physicochemical characteristics: pore size, elemental composition, surface chemistry and surface area (Liu et al. 2015; Khan et al. 2024a). During the adsorption process the pollutant adheres to the adsorbents surface due to electrostatic and hydrophobic interactions between the adsorbent and adsorbate (Al-Ghouti & Al-Absi 2020). For a long time, carbonaceous materials have been utilised as a good sorbent for remediation of organic and inorganic pollutants in water and soil medium (Ambaye et al. 2020).

During the past few decades, a number of studies have been conducted on biochar and activated carbon as promising adsorbents for pesticides remediation from water (Binh & Nguyen 2020). However, research on the optimal adsorption conditions, isotherm, and kinetics of a mixture of pesticides by the adsorbents AC and BC in aqueous solution is rarely investigated. Therefore, the aim of the study was to investigate the adsorption efficiency of BC and AC for four pesticides in aqueous solution.

The pesticides atrazine (C8H14ClN5), chlorothalanil (C₈Cl₄N₂₎, β-endosulfan (C9-H6-Cl6-O3-S) and α-endosulfan (C9 H6 Cl6 O3 S), were used as adsorbate and were purchased from the local market. Initially, 1,000 μg L⁻¹ stock solution of all the four pesticides was prepared, from which a further 12 μg L⁻¹ working solution was prepared and used in the experiment. For pH adjustment, diluted HCl and NaOH were used, and buffer solutions of pH 3, 5, 7, 9 and 11 were used to sustain the specific pH of the solution. BC was derived from a hard wood (Dalbergia sissoo) through the process of pyrolysis under a continuous flow of nitrogen (N2) for 6 h at 500 °C of static temperature (Khan et al. 2015), and AC applied in these experiments was purchased from Norit Nederland BV, Amersfoort. Analytical grade chemicals (N-Hexane, dichloromethane, NaOH, HCl, Na₂SO₄) were used in the study. Samples was prepared in deionized water. Prior to starting each experiment, the glass-wares utilized were washed with dichloromethane and frequently rinsed with distilled water. The physical and chemical properties are provided in Table 1.

For pH measurements, a pH meter (Mettler–Toledo) was used. Gas chromatographic mass spectrophotometer (GC-MS Shimadzu, 2010) was used for determination of pesticides. BC and AC spectral properties were assessed through Fourier Transform Infrared Spectroscopy (FTIR) (Shimadzu IR Prestige-21), and X-ray diffraction analysis (XRD) (JDX-3532) was used to examine spectral characteristics of AC and BC, while scanning electron microscopy (SEM) (JSM 5910 JEOL) was used to examine the morphology of the adsorbent surface in the selected materials.

Experiments were repeated three times and the average value of the obtained data were recorded. The studied parameters were adsorbent dose, pH, adsorbate initial concentration, particle size, temperature, contact time and rotation per minute (rpm).

For pesticides' extraction from the filtrate of the batch adsorption, liquid-liquid extraction method was employed as the best suited extraction method (AOAC 2000). Briefly, 40 mL of filtrate, 12 mL of the solvent N-hexane and 4 g NaCl was placed into a conical flask and shaken in a horizontal shaker for 1 hour at a rate of 80 rpm (Bremon, POB 105363, Germany). For pH adjustment a few drops of phosphate buffer and diluted HCl and NaOH were added when needed. A separating funnel was used to separate the mixture until formation of two distinct phases. The parted N-hexane layer (filtrate) was rinsed twice through the addition of 2.4 mL hexane solvent. A further small amount of sodium sulphate (Na₂SO₄) was added to the separated layer of N-hexane in order to remove water particles. Following separation by a separating funnel, the contents were evaporated at a temperature of 25 °C using a rotary evaporator to reduce the volume up to 1 mL. The extraction and the analysis process precision and accuracy were checked by implying sample blanks and reference materials.

The saturated organic solvent was collected in a 1 mL GCMS vial and placed in the GC-MS agilent's port for determination of the pesticides by gas chromatograph. A gas chromatograph mass spectrophotometer was used for the analysis, equipped with a split-less mode injector system, a flame photometric detector and a TRB 5 MS capillary column with 30 m length, 0.25 mm internal diameter and 0.25 μm stationary film thickness prepared from Phenomenex. Temperature of the oven was initially maintained at 50 and 25–125 °C for 1 min, 125–200 °C at a rate of 10 °C/min, and 200–270 °C at a rate of 5 °C /min; a 1 μL sample was then injected under split mode (split ratio 10:1). For one GC run the total time was 27.5 min. The ultra-pure helium was passed through a molecular sieve trap and used the trapped oxygen as the carrier gas. The velocity was kept constant at 40 cm/sec and the temperature of the injection port was maintained at 250 °C and used in a split-less mode injecting system. The detector temperature was held at 270 °C. A hydrogen generator instrument was applied for supplying hydrogen gas for the flame photometric detector (FPD) at a flow rate of 69.0 mL/min.

To ensure quality control, gauging accurateness and precision standards solution and reagent blanks was involved in every batch experiment. No contamination of the glassware was recorded per se. The results of the standard solution were also found to be satisfactory (97–99%). Experiments were performed in triplicate and the obtained results were expressed as mean of the corresponding triplicates. For statistical analysis, Origin Pro 2016 was applied to prepare graphs.

Adsorbent dosage is an important parameter in the adsorption process. This experiment was performed at 12 μg L⁻¹ of stable initial concentrations, contact time of 1 h, temperature of 25 °C and at stable pH of 07. Table 2 makes it evident that the quantity of adsorbed pesticides is significantly influenced by the dose of the adsorbent. The results show that the adsorption rate increased positively with the increases in mass of the adsorbent from 20 to 200 μg L⁻¹. The percentage removal of pesticides was observed for atrazine from 91.75 to 99.9 and 75.8–98.3%, chlorothalanil from 81.6 to 90 and 72.4–96.6%, β-endosulfan from 75.8 to 85.8 and 71.6–94.1%, α-endosulfan from 73.3 to 82.5 and 69.1–92.5% for AC and BC, respectively. One possible explanation could be the increase in the number of existing sorption positions along with the intensification in the amount of solid mass in the solution (Gupta et al. 2011b). The results may also be attributed to the competition among water molecules and pesticides towards the adsorbent surface. As the adsorbent dose increases the active sites and surface area are abundantly exposed to the pesticides, leading towards reduction in competition between water molecules and pesticides, enhancing the capacity of adsorption. On the other hand with the reduction of adsorbent dose, competition was boosted up among water molecules and pesticides for the surface functional group of the adsorbent, resulting in a reduction in capacity of pesticides' adsorption (Ahmad et al. 2014). Other adsorption studies which employed biochar and various other adsorbents for the removal of pesticides report equivalent effects (Okoya et al. 2020). Suo et al. (2018) reported that an increase in the dosage of adsorbent enhances the capacity of adsorption because the available surface area for adsorption increases positively.

Table 4 summarizes the thermodynamic parameters values at different temperature which signify the adsorption process sensitivity towards temperature. The free energy (ΔG) negative value at every temperature shows the practicability and spontaneity of the adsorption process. It is also identified that the variation in free energy decreases as the temperature increases, exhibiting an enhancement in adsorption with the increase in temperature (Gupta & Ali 2008). The extra release in energy resulted in negative values during the pesticides and the adsorbent surface interaction. Without any doubt, the ΔH value indicated the collective impact of exothermic adsorptions process and endothermic collapse of hydrogen bonds. Endothermic adsorption processes were prevailed by exothermic processing, resulting in negative values of ΔH, supporting the exothermic nature of the process. The obtained entropy positive values (ΔS) indicate amplified randomness at the interface of the solid solution throughout pesticides fixation on the adsorbents active sites and imitates the adsorbent materials affinity towards pesticides (Gupta et al. 2011a).

The current study investigated pesticides' adsorption onto AC and BC derived from hardwood by conducting batch experiments. Adsorption of pesticides is strongly dependent upon adsorbent initial concentration, contact time, temperature and solution pH. Under the optimal condition of pH 7, contact time 60 min, agitation speed 180 rpm and 12 μg L⁻¹ of initial pesticides concentration the adsorbents offered commendable capacities of adsorption for removal of model pesticides used. pH was considered as a key factor for removal of pesticides from aqueous solution and therefore it was observed that pesticides were easily decomposed at neutral pH. Adsorption of pesticides on adsorbents includes diffusion and surface adsorption phenomenon, which become more effective with the increase in temperature. Both the textural and surface chemistry of the adsorbents were favorable for the bulky adsorbates greater adsorption capacity. Reducing tendency in the adsorption proficiency of the adsorbents is due to the pesticide solubility. Adsorption equilibrium data follow the Langmuir adsorption isotherm model, while the kinetics data were better described by the pseudo-second-order model. The thermodynamics studies confirm the exothermic, spontaneous and random nature of the experimental process. This study successfully ascertained that both the adsorbents are eco-friendly and economical for the remediation of pesticides in aqueous solution and water containing bodies.

No funding was available for this work

The authors declare that they have no known competing interests that could have appeared to influence the work reported in this paper.

Dr Sardar Khan designed the study. Dr Kalsoom performed laboratory experiments. The first draft of the manuscript was written by Dr Kalsoom. Statistical techniques were applied by Dr Afsar Khan. Figure and graphs were prepared by Mr Zar Ali Khan. Dr Nisar Muhammad was involved in graphing of the manuscript, Dr Fariha Jabeen, Muhammad Ziad contributed in the revision of this manuscript.

All relevant data are available upon request.

The authors declare there is no conflict.