Role of nano-magnetized Citrus X sinensis (orange) peels for decontamination of pendimethalin from aqueous resources: a comparative study of untreated and carbonaceous orange peels with nano-magnetized peels

In the present era, pesticide contamination of groundwater, surface water and soils is a major problem across the world as many of these compounds are hazardous for both human and environmental health. Pesticide usage in agriculture and home pest control operations is increasing and thus damaging the water resources day-by-day (Memon et al. 2008). Wind erosion, industrial discharges, surface runoff, leaching and variety of other sources all contribute to this pollution. As a result, pesticides are found in water bodies in many countries throughout the world. Residual pesticides are particularly a strong family of water contaminants since they are often non-biodegradable. Pesticides are also carcinogenic by nature. As a result of their toxicity, pesticides and their breakdown products pose a potential threat by polluting the environment. Pesticides are employed in agricultural crops globally in an estimated 2.5 million tonnes per year resulting in higher amount of residual pesticides in runoff water. In most studies, fewer than 0.3% of the administered pesticides reach the target insect, implying that 99.7% of them end up elsewhere in the environment (Bulgariu et al. 2019).

Pendimethalin is a chlorinated herbicide that is used in more than 80 countries and is arguably the most widely used herbicide in the world. Pendimethalin is a dinitroaniline herbicide, which works as a microtubule disruptor in plants, which stops elongation and cell division. It is practically non-volatile, with a half-life of 30–90 days in soil; however, this varies depending on environmental conditions such as moisture content, pH, microbial activity and temperature. It is used before emergence on maize, rice and cereals, as well as before sowing cotton, bean, groundnuts and soya bean with shallow soil absorption. Pendimethalin is transmitted to surface and subsurface water bodies due to its high persistence and mobility, and has been identified in ground water, drinking water, high alpine lakes, rain water and rivers (Ayuba & Nyijime 2021a, 2021b).

A number of techniques have been employed for the treatment of water contaminated with pendimethalin. These include sedimentation, coagulation, microfiltration, ultra-centrifugation and flocculation (Jabłońska 2012). However, these procedures are linked to a high level of risk, operation expenses and sludge production which need further treatment. Adsorption involving fruit peels as adsorbents is an effective technique for treating inorganic- and organic-pollutant-contaminated water (Moradi et al. 2014). Agricultural-based materials as adsorbents are a possible option for wastewater treatment because they have various benefits, which include abundant availability, low manufacturing costs, capacity to reuse, high removal efficiency and easy preparation stages (Kushwaha et al. 2013). More efficient and readily accessible biomaterials, such as orange peels, are available as waste from companies and juice stores in enormous quantities (Biswas et al. 2008). Adsorption of metals and hazardous materials has done by using activated carbon produced from orange peels as an adsorbent (Ren et al. 2011; Salman et al. 2011). As well as, the chemical treatment of orange peels caused an increase in their efficiency for the uptake of contaminants from water. For the first time, nano-magnetic Citrus X sinensis peels (NM-CXSP) are employed to mitigate pendimethalin from aqueous resources in this study. Batch sorption/isotherms investigations are used to compare this nano-magnetized bio-sorbent to untreated Citrus X sinensis peels (U-CXSP). To study the kinetics of the adsorption phenomena, pseudo-first-order, pseudo-second-order and intraparticle diffusion models were used. This work is a follow-up to our previously published work (Asghar et al. 2024), in which we removed chlorpyrifos using orange peels.

All reagents/chemicals (NH₃, NaOH, HCl, FeCl₂, FeCl₃, NH₃) of analytical grade and adsorbate (pendimethalin) were procured from Sigma Aldrich and Merck.

Citrus X sinensis peels (biowaste) were collected from several marketplaces in city Lahore, Pakistan. The peels were cleaned and chopped into tiny pieces and washed with tap water. Peels were sun-dried for 2 days before being further dried at 120 °C for 1–2 days in oven. Dried peels were ground into powder form and sieved using a 100 BSS (British Standard Sieves) sieve. The prepared U-CXSP is employed for further analysis/treatment.

To make activated carbon Citrus X sinensis peels (AC-CXSP) from raw Citrus X sinensis peels, the dried pieces were placed in a traditional mud pot. Two holes were drilled in the lid, one of which was filled with a tube attached to a nitrogen cylinder and clayed in place. The lid placed on top of the jar to make it airtight and sealed in place with clay. Nitrogen flow was switched on, and nitrogen was allowed to flow for 3 min through the vessel. During the carbonization process, once the nitrogen flow was turned off, the second hole was quickly filled with mud preventing the vessel from entering air.

The co-precipitation technique was used to make the NM-CXSP adsorbent. Orange peels (30 g) were placed in a flask containing 500 mL of water. FeCl₂·2H₂O (36 g) and FeCl₃·6H₂O (30 g) solutions (prepared in 500 mL separately) were added drop by drop via a separating funnel. At 60 °C, the solution is agitated continuously for 3–4 h. The pH of the solution was increased to 10 after complete addition by adding ammonia solution (10%) drop by drop. The solution was filtered, and the precipitates were washed in water until they became neutral. Precipitates were dried at 90 °C before being heated in furnace at 550 °C.

The range of the FTIR spectrometer (Spectrum-2) with ATR assembly of Perkin Elmer FTIR was tuned between 4,000 and 350 cm⁻¹ in a transmission mode using 10 kPa of compressed KBr pellets for functional groups analysis. Magnetic properties were measured at 25 °C in a magnetic field of 15,000 Oe using a quantum design VSM magnetometer. A Quanta 200 FEG scanning electron microscope (SEM) was used to examine the morphology of adsorbents. The EDX method was used to examine S50 FEI for elemental configuration of adsorbents. The structural study of adsorbents was performed using a PANAlytical eXpert Pro DY3805 Powder XRD.

Pendimethalin (0.1 g) was dissolved in 1 L of acetone:water (30:70) combination to make a stock solution (1,000 ppm). With the help of dilution formula, this stock solution was used for further experiments.

Here, ‘q_e’ indicates the adsorption capability of adsorbent, ‘C_i’ indicates the starting concentration of PDM solution, ‘C_e’ indicates the concentration of PDM at equilibrium. ‘V’ indicates the solution volume and ‘m’ represents the amount of adsorbent used.

The impact of the adsorbent dose was investigated by altering the dose between 0.1 and 0.5 g, and added to a 25-mL solution in 100-mL conical flasks of 0.1 ppm solution of PDM at constant temperature and pH. The effect of contact time was studied by taking 0.1 g of adsorbent in 100-mL flask containing 25-mL solution of 1 ppm pendimethalin. The contact time was varied between 10 and 70 min at a constant temperature and pH. The kinetic data of the study were used to determine the reaction kinetics based on pseudo-first-order, pseudo-second-order and intraparticle diffusion. Effect of initial concentration on adsorption was studied by taking solutions of different concentrations of PDM solution in different flasks for 30 min at constant temperature and pH. Adsorption isotherms (Langmuir and Freundlich) were carried out to investigate the efficiency of each adsorbent separately. The pH of PDM solution was varied between (2 and 10) to check its effect on adsorption using 0.1-M NaOH and HCl solutions. The mixtures were stirred continuously for 30 min at constant temperature. Temperature effects on adsorption were investigated by varying the temperature from 25 to 50 °C.

The starting concentration of PDM solution can also affect the adsorption phenomenon, it was investigated by varying the concentration of pendimethalin in between 5 and 200 ppm. All the experiments were done by keeping the adsorbent dosage constant at 0.1 g with continuous stirring for 30 min at constant temperature, i.e., 25 °C. At the start, increase in concentration causes an increase in adsorption percentage of up to 80 ppm solution of pendimethalin. After this concentration, further increase in concentration did not cause a significant increase in percentage adsorption. This is attributed to the fact that all the active sites available for the adsorbate molecules became occupied up to 80 ppm due to which there is no significant change observed in percentage adsorption after 80 ppm of concentration.

The separation factor describes the major aspect of the Langmuir isotherm ‘R_L’ (R_L = 1/(1 + K_L C_o), a dimensionless constant. Irreversible adsorption is observed by R_L = 0, favorable adsorption is observed when 0 < R_L > 1, linear adsorption is denoted by R_L = 1 and unfavorable adsorption is denoted by R_L > 1 (Sheng et al. 2014). In our study, the computed values of R_L were in the range of 0 < R_L > 1, showing that PDM adsorption on each adsorbent is favorable.

Table 2 shows the comparison of adsorption capacity of U-CXSP, AC-CXSP and NM-CXSP for pendimethalin with other reported low-cost adsorbents.

The impact of contact time on pendimethalin adsorption was studied by varying the contact period from 10 to 70 min. The reactions were carried out with a constant adsorbent dosage and pesticide solution concentration, i.e., 0.1 g and 0.1 ppm respectively. It is found that increasing the contact duration increased the adsorption percentage up to 30 min. The increase in adsorption percentage was ascribed to the fact that as the contact time was increased the adsorbate molecules have a better probability of being adsorbed by the adsorbent surface as time goes on (Kumar and Philip 2006). After 30 min of contact time, the chances of the adsorbate molecules to make contact with the active sites becomes less as all the available active sites of adsorbents became occupied and there are chances of repulsive forces between the adsorbate molecules and the surface of adsorbent (Sheng et al. 2014; Enniya et al. 2018).

Based on kinetics, isotherm and characterization, the adsorption mechanism for this study is provided. Adsorption obeyed second-order kinetics at low (starting) concentrations of adsorbate, as evidenced by the strong relationship between the estimated value of adsorption capacity (q_cal) for pseudo-second-order kinetics and the experimental value (q_exp). The FTIR study (Abdelhameed et al. 2020) verified that the free hydroxyl and carboxylic groups on the surface of the precursor adsorbent (U-CXSP) cause the adsorption of pendimethalin. Research showed that the carbonaceous and magnetized material made from orange peels improved the sorbent's efficacy in absorbing pendimethalin by increasing its surface area. The existence of many microscopic nanoparticles is what causes the rise in NM-CXSP adsorption efficiency. FTIR, VSM and SEM-EDX all indicated the successful formation of NM-CXSP.

Reusability of the used adsorbents is an important aspect of any cost-effective method. Using a vortex machine, the adsorbents were treated with acetone (2.0 mL) and double distilled water, respectively, to test the adsorption–desorption cycles of the materials for the absorption of pendimethalin. After that, the adsorbents were magnetically collected for further use. In order to assess the adsorbents' efficiency for the adsorption of pendimethalin, Table 4 displays the results from adsorption–desorption cycles. For four cycles, the values for adsorption and desorption were kept within the ranges of 74.87–75.71% and 63.28–64.65%. The outstanding efficiency and reusability of the adsorbents were demonstrated by the experimental findings, which revealed that the adsorption–desorption values were nearly similar. Therefore, it was discovered that the adsorbents employed in this investigation were both efficient and sustainable in cleaning the wastewater containing pendimethalin.

This study showed that residual pendimethalin present in water bodies as pollutant can successfully be removed by utilizing the biowaste obtained from Citrus X sinensis (orange) peels. This is a sustainable approach for the treatment of water resources which utilized low-cost ecofriendly bio-sorbents. The activated carbon and magnetized peels showed a better efficiency for the uptake of pendimethalin as compared to simple fruit peels. The prepared adsorbents were characterized using SEM, EDX, FTIR, VSM and XRD. All techniques confirmed the formation of nano-magnetized adsorbent. Biosorption studies were carried out by varying different parameters and it was seen that the adsorbents show maximum adsorption with an adsorbent dose 0.4 g, time of contact of 30 min, initial concentration of PDM 80 ppm, pH of 6 and temperature of 25 °C. The results showed that the removal efficiency of U-CXSP increased from 97 to 114 mg/g and to 111 mg/g for AC-CXSP and NM-CXSP adsorbents, respectively. According to the results, the pseudo-second-order kinetic model has the best fit to the experimental data, with a higher correlation coefficient (R²) than the pseudo-first-order kinetic model. The chemisorption-based pendimethalin adsorption on adsorbents is shown by the best-fit pseudo-second-order equation. Hence, the present work shows Citrus X sinensis adsorbents to be promising biomaterials for the removal of pendimethalin from aqueous resources.

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

The authors declare there is no conflict.