Synthesis of acid treated carbonized mandarin peel for purification of copper

Technological growth has led to a negative effect on the environment. Inorganic and organic pollutants cause environmental problems due to the industrial effluent of the mining, paint, leather and textile processes (Enniya et al. 2018). Industrial pollution can be categorized as soil, air, and water pollution. Especially, heavy metal contamination in water is crucial since they are non-biodegradable and affect the living organism. Various methods such as precipitation, coagulation and flocculation (Yi et al. 2017), reverse osmosis (Amuda & Amoo 2007), membrane filtration, ion exchange (Kurniawan et al. 2006), and adsorption (Thuan et al. 2017) can be used for heavy metal removal from wastewater. Adsorption is one of the effective, low-cost, modular processes for wastewater treatment. On the other hand, the performance of the method is limited by the performance of the adsorbent. Recently, studies on the development of reusable bio-based waste adsorbent have attracted attention. The carbon-based materials produced from the biological feedstock are defined as non-toxic and low-cost adsorbent sources (Hashemian et al. 2014). The efficiency of these materials can be improved by increasing their surface area, porous structure, and functional groups (Yahya et al. 2015; Thuan et al. 2017; Saidi et al. 2019). Many vegetable foods can be converted to effective adsorbents. In the literature, the peel of apple (Enniya et al. 2018), banana (Thuan et al. 2017), coconut (Gratuitoa et al. 2008), Litchi (Yi et al. 2017), orange (Hashemian et al. 2014), sugarcane bagasse (Foo et al. 2013), and mandarin (Koyuncu et al. 2018) is reported to be a good candidate for an effective carbon-based adsorbent.

According to the data of the United Nations Food and Agriculture Organization (FAO) 2013, 21 million tons of mandarin are produced in a year. Turkey is the biggest supplier of mandarins with a production capacity of 872 thousand tons. The amount of waste peel in the mandarin is approximately 8–14% by weight. Utilizing waste peels as valuable substances and reusing them as useful products positively affects the supplier economy. Valuable organic essences, oil species, and some other chemicals can be produced by the regeneration of these peels. These chemicals can be used in the cosmetic and pharmaceutical industries. However, after the removal of these important chemicals, mandarin peel residue still has a vast amount of biomass. The remaining biomass can be used as an effective adsorbent owing to the lignocellulosic and phenolic content (Mudyawabikwa et al. 2017; Koyuncu et al. 2018). In the literature, peels are converted to activated carbon by using a series of difficult, long and expensive processes. Thereby, the cost of the adsorbent increases depending on the process difficulty (Dod et al. 2012; Azharul-Islam et al. 2017; Shakoor & Nasar 2017). In this study, an alternative technique is proposed to produce a carbon-based adsorbent from the mandarin peel. In this study, the efficiency of the acid-treated carbonized mandarin peel (CMP) was tested to remove copper from a simulated copper-incorporated wastewater for the first time. The effects of initial copper concentration, solution pH, adsorbent dosage and contact time on copper removal were investigated.

Copper sulfate pentahydrate was provided from Merck Chemicals, Turkey. Sulfuric acid (H₂SO₄) and sodium hydroxide (NaOH), which were used to convert the mandarin peels into functionalized carbon adsorbent, were provided from Merck Chemicals, Turkey.

The mandarin peel was washed with distilled water and dried in an oven (Santez SE-45F) at 100 °C for 24 hours. The functionalization was carried out in a hydrothermal reactor at 105 °C with 1 M of H₂SO₄ solution for four hours. The acid treated carbonized mandarin peels (CMP) were dried in an oven at 70 °C for one day. Following the drying process, CMP was neutralized with 2 M NaOH solution. After the neutralization, CMP was dried in a vacuum oven at 70 °C for 2 hours.

In order to identify the active functional groups in CMP, Fourier-Transform Infrared (Perkin Elmer- Spectrum 100) spectra were studied between the wavelengths of 600–4,000 cm⁻¹. The thermal behavior and the degradation period of CMP were determined using Thermogravimetric Analysis (TGA-Mettler Toledo) in the temperature range of 25–800°C with a heating rate of 20°C/min.

Figure 1 shows the FTIR spectra of CMP adsorbent. The peak at 3,360 cm⁻¹ is assigned to the presence of -OH groups. The alkene stretching vibration of C = C is observed at 1,642 cm⁻¹. The peaks at 1,229 cm⁻¹ and 1,218 cm⁻¹ should be assigned to the C-O stretching of ether and ester groups, respectively. The peak at 1,100 cm⁻¹ is corresponding to the C-H stretching vibration (Rout et al. 2015; Zhang et al. 2014; Rattanapan et al. 2017).

The thermal stability of the CMP was determined by means of the TGA test. Figure 2 shows the weight loss of CMP depending on the temperature. It is clear from the figure that the degradation of CMP occurs in three steps. The degradation before the temperature of 100–105°C is corresponding to the moisture inside the adsorbent. The first decomposition temperature from 100 °C to 200 °C is due to the volatile organic compounds in CMP. The second decomposition temperature from 200 °C to 450 °C is corresponding to the decomposition of the cellulose and hemicellulose in the adsorbent. The last degradation temperature from 450 °C to 700 °C should be attributed to the degradation of lignin (Gaur et al. 2018; Sabela et al. 2019). After 700 °C, there is no significant weight loss.

Contact time is one of the effective parameters of the low-cost adsorption process. It directly affects the adsorption performance. For a high-performance and cost-effective process, it is desired to complete the adsorption process in a short adsorption time. Figure 3 shows the effect of contact time on copper removal; when the metal concentration was 5 mg/L, the adsorbent concentration was 3.75 g/L and the solution pH value was 7.

The copper removal was 87.51% in the first 90 minutes and then reached 100% of removal within 150 minutes. As shown in the figure, the removal of heavy metal ions showed a significant increase up to 90 minutes. After 90 minutes, the adsorption layer became saturated. The driving force between the adsorbent and metal ions maximized the mass transfer (Souza et al. 2018). The optimum contact time was determined as 150 minutes by providing a dynamic balance.

Heavy metal concentration is an important parameter for mass transfer during the adsorption process. (King et al. 2006; Mobasherpour et al. 2014). The appropriate metal concentration leads to an increase in the adsorption efficiency by increasing the interaction between the adsorbent surface and metal (Semerciöz et al. 2017). In this study, the effect of initial metal concentration on the removal of copper from a copper-water solution was evaluated. Experiments were conducted with a constant adsorbent concentration of 3.75 g/L when the solution pH was 7 and the contact time was 150 minutes. The initial metal concentrations were changed from 5 mg/L to 15 5 mg/L and 25 mg/L. The effect of metal concentration on removal is illustrated in Figure 4.

Figure 4 shows that 100% of copper removal was achieved at low metal concentrations (5 mg/L). As the metal concentration was increased from 5 mg/L to 25 mg/L, the removal gradually decreased from 100% to 92.59%. The decrement should be attributed to the saturation of the active sites of the adsorbent with the metal (Moghadam et al. 2013; Semerjian 2018). Therefore, increasing metal concentration in the solution caused a decrease in the removal.

The adsorbent dosage or adsorbent concentration is another important parameter for the performance of the adsorption process. An increase in the adsorbent dosage directly affects the total cost of the adsorption process and recycling. Figure 5 displays the effect of adsorbent concentration on the copper removal process when the metal concentration was 5 mg/L, the solution pH value was 7 and the contact time was 150 minutes.

The removal enhanced from 76.47% to 100% when the adsorbent dosage increased from 0.625 g/L to 3.75 g/L. The concentration of the adsorbent determines the number of binding sites for adsorption. As the amount of the adsorbent increases in the adsorption media, the total surface area and the empty adsorption sites increase (Tadepalli et al. 2016). Therefore, adsorption performance increases as it was also obtained in the present study.

The surface charge of the adsorbent affects the ion concentration and the degree of ionization of functional groups (carboxyl, hydroxyl, amino) in the adsorbent (Othman et al. 2012). The active surface sites of the adsorbent also change with the pH of the media. Since the adsorption performance is directly related to the active sites on the adsorbent, the varying pH also affects the adsorption performance (Ali et al. 2016). The effect of solution pH (pH:3, pH:5, pH:7) on the removal of Cu was investigated when the metal concentration was 5 mg/L, the adsorbent dosage was 3.75 g/L and the contact time was 150 minutes. The removal results are illustrated in Figure 6. As shown in the figure, 82.61% of copper removal was obtained when the pH of the solution was 3. At the same conditions (adsorbent dosage, copper concentration and contact time), 100% of removal was achieved when the pH was 7. In a strongly acidic environment, the adsorbent surface is positively charged while in a basic environment the adsorbent is charged negatively. This difference affects the adsorbent surface chemistry and leads to a significant change in the adsorption efficiency. As the pH decreases, the concentration of H₃O⁺ ions increases and the adsorption of copper ions decreases. Similar results have also been obtained and reported by Ali et al. (2016).

The adsorption isotherm shows the distribution of adsorbate molecules between liquid and solid phases while the adsorption process is in an equilibrium state. The different isotherm models and isotherm data can be used to determine a suitable model for design purposes (Ali et al. 2016). Adsorption isotherms are used to optimize how the adsorbent interacts with the adsorbate. It gives information about the adsorption mechanism and surface properties of adsorbents. In the present studies, Langmuir (Figure 7(a)) and Freundlich (Figure 7(b)) isotherms were studied by using the removal data obtained from the different metal concentrations (5, 15 and 25 mg/L), a constant adsorbent dosage of 3.75 g/L, a constant adsorption time of 150 minutes, and a constant solution pH of 7. The isotherm data are shown in Table 1. According to the R² values, the most suitable isotherm was determined as the Freundlich isotherm. Therefore, it can be evaluated that the adsorption processes through the CMP occurred on heterogenous surfaces.

Kinetic models help to estimate the rate constants of adsorption and to explain the mechanism of the adsorption. In the present study, kinetic studies were performed using the metal concentration of 5 mg/L. In this concentration, the highest copper removal was achieved and with the highest adsorption rate. In order to estimate the adsorption mechanism, pseudo-first-order and pseudo-second-order models were applied. The kinetic model parameters are shown in Table 2 (C₀: 5 mg/L, adsorption dose: 3.75 g/L, pH: 7).

For determining the appropriate kinetic model, both the experimental and calculated q_e values accompanied with the R² value should be taken into account. Figure 8 shows the kinetic plots. According to the results obtained in this study, the pseudo-second order kinetic was suitable. The kinetic model was acceptable since the q_e value that was calculated from the figure and obtained from the experiment were close to each other. The pseudo-second order kinetic model confirmed that the adsorption rate was independent from the adsorbate concentration and was dependent on the adsorption capacity (Lim et al. 2017).

In this study, the removal of copper metal from wastewater by using acid-treated carbonized mandarin peels was investigated. The highest copper removal of 100% was achieved when the copper concentration was 5 mg/L, the adsorbent dosage was 3.75 g/L, and the solution pH was 7. After the adsorption test, the adsorbent was treated with 1 M HCl for four hours, dried and used for the same condition (5 mg/L Cu, 3.75 g/L adsorbent dosage, 7 solution pH). As a result, 94% of removal was achieved with regenerated adsorbent. Table 3 gives the adsorbents and experimental parameters used in copper removal. In the literature, there are no reported studies on heavy metal removal using the acid-treated mandarin peel. Therefore, the adsorbent results obtained from different fruit peels are included in the table.

In the present study, a new acid-treated and factionalized carbonized adsorbent was synthesized from the mandarin peels and used for the adsorption of simulated copper solution. As a result, it was obtained that the CMP was very effective to remove copper from simulated metal-containing water solution. The highest removal of 100% was obtained when the copper concentration was 5 mg/L, the adsorbent dosage was 3.75 g/L, and the solution pH was 7. After HCl regeneration, the adsorbent maintained 94% of its activity. The Freundlich isotherm was more appropriate to explain the removal of copper from the solution. The kinetic of the heavy metal adsorption onto CMP was obtained as pseudo-second-order. As a result, it can be evaluated that the adsorbent synthesized from the waste mandarin peels is an effective candidate to be used as a low-cost adsorbent.