Orange peel activated carbon produced from waste orange peels for adsorption of methyl red

Adsorption is a popular and efficient physical method of separation that may be used to get rid of or reduce the concentration of a wide variety of dissolved contaminants (organics and inorganics) in an effluent (Ali et al. 2012). It has recently come to light that activated carbon, also known as AC, is a well-known adsorbent that can be utilized effectively for the removal of a wide variety of contaminants from air, soil, and liquids. This development is very noteworthy. The majority of adsorption takes place on the pore walls of particles, which is why sorbents are typically classified as porous solids (Barna et al. 2013). AC, silica gel, activated alumina (which can absorb moisture), zeolites and molecular sieves, and synthetic resins are some examples of materials that can operate as adsorbents. AC is particularly effective at removing organic compounds. When it comes to removing organic, inorganic, and biological pollutants, AC is the most effective adsorbent that is of importance in the treatment of water and wastewater (Bharathi & Ramesh 2013). In recent years, it has been used more frequently for the purpose of preventing environmental pollution, and anti-pollution legislation has led to an increase in sales of air conditioners that are designed to reduce pollutants in the air and water (Cardoso et al. 2011).

AC is a processed type of microcrystalline, non-graphitic carbon having an internal porosity. Because of its high porosity, large surface area, and strong surface reactivity, AC is a very versatile material (Bokil et al. 2020). AC's high specific surface area ranging from 450 to 2,000 m²g⁻¹ facilitates the physical adsorption of gases, vapors, and dissolved or dispersed compounds from liquids (Hameed et al. 2007). It possesses a huge number of very tiny holes, which gives it a vast inner surface and excellent adsorption characteristics. In water and wastewater treatment, they effectively absorb organic, inorganic, microbiological, and biological pollutants (Akar et al. 2009; Balci et al. 2011).

The synthesis of ACs from biomass is an attractive technique to get carbonaceous compounds from food industry waste (Bokil & Rai 2016). Carbon materials are employed widely in numerous processes (Kadirvelu et al. 2001). The cost of using biowaste is rising. Using difficult-to-dispose-of waste biomass can benefit the environment. ACs from biomass are efficient and cost-effective compared with commercial carbons. Natural resources are widely employed as carbonaceous precursors (Topare & Bokil 2021). Orange peels are one of the waste materials that are utilized as precursors in the production of ACs. The present study involved the evaluation of different conditions (adsorbent dosage (0.25–1.25 g/L), MR concentration (100–400 mg/L), temperature (40–60 °C), contact time (10–60 min), and pH (3–11) in the MR adsorption efficiency). The data demonstrate that the orange peel-derived material possesses excellent adsorption capabilities, making it a promising option for actual use in wastewater treatment.

The unprocessed components (orange peels), which are necessary for the production of AC, will be procured from local juice shops. After collecting orange peels, they were first washed to remove any dirt, then dried, then crushed using an electrical grinder, and then sieved using a mesh size of 60. The production of AC requires the use of a chemical activation agent, and several acids have been investigated and reported as potential agents. The glassware was cleaned with distilled water, and all of the chemicals that were utilized were of an analytical (AR) quality. The materials were carbonized at a temperature of 250 °C for 2 h before being allowed to cool at ambient temperature. After collecting and cleaning the samples, 30 g of each were combined with 30 ml of zinc chloride (ZnCl₂) at a 1:1 ratio and kept in a SS container. After heating in a muffle furnace at a temperature of 500 °C for 1 h, the container made of SS was removed. The AC samples were washed to remove any remaining residue from the preparation process. The procedure of washing was continued until a pH level of 7 was reached. After that, the samples were placed in an oven set at 100 °C in order to remove any trace of moisture.

AC made from orange peels was the adsorbent that was utilized in this experiment. The physicochemical properties (typical properties, proximate and ultimate analysis) of orange peel AC are utilized for the purpose of determining its characterization (Armagan et al. 2003; Topare et al. 2020). SEM is used to examine the surface morphology of orange peel AC samples. Using CuKα as the radiation source, XRD was performed using an X-ray diffractometer. The functional groups were identified by recording FTIR spectra in the region of 4,000–500 cm⁻¹.

Brunauer–Emmett–Teller (BET) analysis was performed using Quantachrome Instruments v3.01 (Autosorb iQ Station 1). The BET surface area, pore volume, and pore diameter of the orange peel AC were measured by nitrogen adsorption methods. A thermogravimetric method, or ‘loss on drying’, is typically used to determine the moisture content. In this method, the sample is heated, and the weight loss from moisture evaporation is measured. The combination of a drying oven and a balance, along with a moisture analyzer, is a common moisture analysis technology. A small quantity of AC was weighed and then transferred to a glass dish. After that, it was heated for 2 h in an oven at a temperature of more than 100 °C. The glass dish was taken out of the oven and allowed to cool. The weight of the dried sample was determined after it had cooled. According to Ashtaputrey & Ashtaputrey (2016), the equation was used to calculate the moisture content (wt.%).

When adsorption reaches equilibrium, the isotherm reflects how molecules are divided across the liquid and solid phases (Lopicic et al. 2021; Sivakumar et al. 2022). At a range of temperatures, a study of adsorption isotherms was conducted using the well-known Langmuir and Freundlich isotherms (Babatunde et al. 2016). The Langmuir model is one that is utilized well in various monolayer adsorption procedures. This model operates under the presumption that adsorptions take place on the adsorbent at particular homogeneous spots. The Freundlich model acknowledges the existence of heterogeneity on the surface and makes the assumption that adsorption takes place at locations that have varying amounts of adsorption energy. The R² values of the correlation coefficients are used while assessing the value of the isotherm equation.

Methyl red (MR) adsorption onto orange peel AC has been investigated. The concentration of MR, the adsorbent dosage used, solution pH, and adsorption temperature are the primary variables affecting the efficiency of MR removal. Adsorption efficiency was observed to increase with increasing solution temperature (60 °C), contact time (60 min), and adsorbent dose (1 g/L), but to decrease with increasing initial dye concentration. Increases in pH led to more MR removal, with optimal adsorption achieved at pH 11. Langmuir and Freundlich's isotherms were used to fit equilibrium data, with Langmuir providing a better fit with 111.11 mg/g at 60 °C. The results of the study that compared orange peel AC with CAC demonstrate that orange peel AC provides a higher percentage removal of MR than CAC. According to the findings of this research, AC from orange peels is capable of functioning as a useful material for the removal of MR as an adsorbent.

The authors would like to express their gratitude to the Dr. Vishwanath Karad Research Funding Scheme, MITWPU, Pune, India for their support of a minor research project (Ref.: VCS/GC/02-2022).

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

The authors declare there is no conflict.