Synthesis and characterization of activated carbon from red pumpkin skin for the removal of ionic dyes

The environment is the basis of human survival and the guarantee of sustainable human development. Environmental protection has unquestionably become a development strategy for all countries in the world. All countries must fulfill their environmental management obligations to plan for economic development, social progress and environmental protection (Jianping et al. 2013).

Many reports from different sources (public. private. regional or global) illustrate the continuing deterioration of environmental conditions. At the beginning of the 21st century, environmental changes are particularly dangerous and there are irreversible risks in many aspects: technological developments since the industrial revolution, which allowed man to exert a great influence on natural resources and ecosystems, increasing human population, which has led to a significant increase in human population density in many parts of the world, and the increase in resource and energy consumption since the industrial revolution which have accompanied economic growth and rising living standards in some parts of the world (Stephens et al. 2020).

A significant increase in the proportion of the world's population requires an increase in food production and energy consumption, including an increase in human activities, which will have a negative impact on ecosystems (Wright et al. 2011; Manisalidis et al. 2020).

Water pollution is a global challenge that has increased in both developed and developing countries, undermining economic growth as well as the physical and environmental health of billions of people, which requires continuous monitoring and assessment. It can be severely degraded as a result of contamination by pollutants, leading to a series of health and ecological effects. Many human activities, such as industrial processes, result in water pollution that can create toxic effluents. Oil spills, accumulation of plastics and bioaccumulation of persistent organic chemicals are major causes of severe environmental degradation (Ruthiraan et al. 2017).

Dyes are a type of organic pollutants widely used by textile industries, which generate considerable amounts of colored, toxic and even carcinogenic waste products, posing a serious threat to aquatic organisms. The main reason is the non-biodegradable nature of most dyes and resistance to aerobic digestion (Cuong Nguyen et al. 2021).

The modern world needs the textile industry to be used in daily life as clothes and carpets and in architecture as an immunization against weather phenomena. Currently, textile and dyeing factories in many developing countries have not fully treated their wastewater; they contain insoluble dyes that have serious toxic effects on human health and the environment. Therefore, textile industries are considered to be the largest sources of water pollution (Ozdemir et al. 2013; Masood et al. 2014).

Wastewater treatment has always been a major concern in developed countries. Several efficient and economical methods have been used for dye treatment such as hybrid, biological and physico-chemical treatments (Han et al. 2015). Adsorption is considered as an inexpensive and highly effective technique among the above methods to remove organic pollutants from wastewater and produce high-quality treated effluent. This method is widely employed using different adsorbents such as activated carbon obtained from several plant materials rich in carbon such as wood, olive kernels, peanuts, etc (Kouadio et al. 2019).

Activated carbon adsorption has gained importance as an alternative tertiary wastewater treatment and purification process. When the quality of the effluent after secondary treatment is not at the required level, tertiary treatment will be necessary. Tertiary treatments are advanced treatments. They are considered for discoloration because primary and secondary treatments are not particularly effective in this area. Secondary treatments can remove approximately 50% of the dye.

Adsorption techniques are simple, minimize the use of chemicals and the production of biological sludge, and are easy to operate (Collivignarelli et al. 2019; Gurav et al. 2021).

The objective of the present study is to produce activated carbon from red pumpkin skin agricultural waste by chemical activation using zinc chloride for the removal of two types of dyes.

To achieve this goal, pyrolysis of red pumpkin skin, followed by chemical activation, was used to obtain an eco-friendly activated carbon as a biosorbent for methylene blue and Congo red removal.

In chemical activation, red pumpkin skin is impregnated with an activating reagent such as zinc choloride (ZnCl₂) and the impregnated material is heated in an inert atmosphere, this method is more advantageous due to its higher yield, simplicity, the lower temperature and the shorter time necessary to activate the material and the good development of the porous structure.

Physicochemical methods were used to characterize the new biosorbent such as Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET). The efficiency of activated carbon to remove dyes was studied as a function of contact time, initial dye concentration, biosorbent dose, pH effect and temperature. In addition, Langmuir and Freundlich isotherms as well as pseudo-first-order and pseudo-second-order adsorption kinetics were investigated in this study.

Congo red (CR) was purchased from Biochem: Chemopharma (France). Methylene blue (MB), hydrochloric acid (HCl), sodium hydroxide (NaOH) and potassium nitrate (KNO₃) were purchased from Sigma Aldrich (Germany) and the ZnCl₂ was supplied by Honeywell laboratory – Riedel de Haen.

The red pumpkin skin was collected from kitchen waste. This was washed several times with tap water and then with distilled water. The sample was dried in an oven for 24 hours at a temperature of 105 °C. After drying, the material underwent grinding and then sieving to obtain regular sizes with dimensions less than 0.5 mm (Khelifi et al. 2016). For chemical activation of the activated carbon, 20 g of dried red pumpkin skin powder was well mixed with 100 mL of ZnCl₂ solution of concentration 0.4 g/L at impregnation rate of 2. The mixture was placed in an oven at 30 °C for 24 hours and then at 110 ° C until obtaining a dry mixture. The product obtained was stored in a desiccator before pyrolysis (Ahmed & Dhedan 2012).

The pyrolysis is used to describe the production of activated carbon precursor. By definition pyrolysis is a process of chemical decomposition of organic materials at high temperatures in the absence of oxygen. Because no oxygen is present, combustion does not occur. Chemical treatment and pyrolysis have an important role in the development of the porosity and texture of an adsorbent. The adsorption power is improved due to the increase in the dimension of the pores on the surface and in depth. ZnCl2 applied to skin of the red pumpkin gives for example a plastic paste. This mass is then heated in the absence of oxygen (pyrolysis) at temperatures between 400 and 500 °C. This agent can develop the micropores and mesopores by enlarging the pore size. Finally, the product must be washed to remove the activating agent, which can often be reused or recycled, although it may partially volatilize during the activation process (Li et al. 2018; Lee et al. 2019), The pyrolysis is the second step of the activated carbon manufacturing process. The dry sample obtained after impregnation was placed in a muffle oven at 400 °C with a heating rate of 2.66 °C/min for two hours. At the end, the obtained activated carbon was cooled and washed thoroughly with distilled water to neutral pH and dried at 105 °C in an oven (Mamane et al. 2016).

The isoelectric pH or zero charge point of activated carbon is the pH of an aqueous solution at which the activated carbon has zero charge potential on its surface. To do this, in 250 mL beakers, 0.1 g of activated carbon was placed in 20 mL of KNO₃ solution (0.1 N), left in contact for 24 hours at a stirring speed of 250 rpm and at ambient temperature. The initial pH was adjusted from 2 to 12 by adding HCl and NaOH (0.1 M). The pH_f was measured after 24 hours. The intersection with the abscissa axis of the graph of ΔpH as a function of pHi gives the pHpzc (Torres-Pérez et al. 2015; Ntakirutimana et al. 2019).

This technique is used to determine the functional groups on the surface of activated carbon For this. a small amount of the prepared material was taken and then analyzed in the 4,000–400 cm⁻¹ range. The analysis was carried out by Bruker spectroscopy.

The crystalline or amorphous state of the prepared adsorbent was identified by XRD using Bruker D8 Advance ECO A25 X-ray diffractometer. A simple quick scan will show that if there are characteristic peaks, the state is crystalline; if not, it is amorphous.

Several image acquisitions were performed using an FEI Quanta 250 Environmental Scanning Electron Microscope in the ‘morphological contrast’ mode of the two samples to highlight their geometric topography on many surfaces at different magnifications from 400X to 24000X. By low pressure mode 60 Pa with electronic element beam from 10 to 15 kV and WD working distance of 10.6A 10.8 mm.

The BET surface was obtained according to the BET equation to the adsorption of N₂ at 77 K using the ASAP 2010 surface analyzer (accelerated surface porosimetry analyzers). The sample is exposed to a stream of nitrogen (N₂). at various pressures. This gas is adsorbed on the surface and in the pores of the particles and therefore the pressure of the gas decreases. From this pressure variation, the adsorbed volumes are determined at each pressure used, then the gas adsorption-desorption isotherms are plotted. The software coupled to the ASAP2010 analyzer provides access to the specific surface area by a calculation based on the theory of Brunauer, Emmett and Teller, known as the BET method.

Stock solutions of cationic and anionic adsorbent (MB and CR) were prepared at a dilution of 1,000 mg per litre of distilled water. These dyes are very soluble in water. The characteristics of these adsorbates are presented in Table 1.

Different impregnation rates of ZnCl₂ to red pumpkin skin were studied to calculate the activated carbon yield. From Table 2, it can be concluded that the activated carbon yield was strongly affected by the chemical impregnation ratio. The yield of the activated carbons was proportionally enhanced by increasing the impregnation ratio of activation agent. It varied largely from 30 to 70.79%. It was concluded that a larger activation ratio and specific surface area resulted in a greater yield. The impregnation ratio corresponding to the maximum yield was 2. The carbonization of red pumplin skin without ZnCl₂ impregnation resulted in relatively low yield of 30% because a large amount of carbons were removed as CO, CO₂, CH₄ and tar coupling with O and H atoms. On the contrary, carbonization with ZnCl₂ impregnation rate resulted in higher yield at 70.79% since ZnCl₂ selectively separated H and O from red pumpkin skin as H₂O and H₂ rather than hydrocarbons, CO or CO₂ (Karimnezhad et al. 2014). At high impregnation ratio greater than 2, the yield was affected by the chemical reactions between raw materials; therefore the carbon burn-off was increased and the yield was decreased. Similar result has been reported by Wang et al. 2009; Yorgun et al. 2009.

The ability of porous materials to absorb moisture is 25–30%. These have no effect on the performance of the adsorbents in wastewater treatment but rather affect the weight of activated carbon required to perform the adsorption experiment. Samples must be dried after the purification process and stored in sealed and leak-proof containers in a dry place (Yakout et al. 2015). From these results. the moisture content of the biosorbent was found to be 7.6%. This proves that the results obtained comply with the moisture content conditions and the studies carried out by (Olowoyo & Orere 2012; Ekpete et al. 2017).

The ash content in carbon represents the percentage of dry residue after combustion. In other words in the deep carbonization heat treatment process, all VOCs were removed only inorganic material remained. It should be noted that the activated carbon has the lowest ash content at 29.46 (%). This value is lower than those found in some types of activated carbon (Cruz et al. 2012; Veena Devi et al. 2012).

The density test allows the determination of the quality of the activated carbon and the presence of fibres in the precursors. The higher the fibre content, the more the bulk density increases (Yakout et al. 2015). On the other hand, the American Society for Water Work has set a minimum value for the bulk density value of 25 mg/mL so that the activated carbon is of high quality and of scientific value. Experimental results of the bulk density and compaction of activated carbon can provide information about the porosity (Olowoyo & Orere 2012). Table 2 shows the conformity of the values obtained for the bulk density of the adsorbent with those of the values cited in the literature. It was estimated to be 0.420 g/cm³ (Chan et al. 2011). The packed density value of activated carbon is 0.638 g/cm3. Therefore, It can be seen that the prepared activated carbon has a high porosity index of 34.34% because of the large difference between the bulk density and the packed density. This leads to conclude that the activation by ZnCl₂ is certainly effective in increasing the density Isoelectric pH.

Knowledge of the zero charge point (pH_pzc) is very important information in the adsorption process. The zero charge point estimates both the internal and external surface charge of the particles. If the pH of the solution is above the zero point (pH > pH_pzc), the carbon appears as an anion. and the carbon carries a positive charge if it is lower (pH < pH_pzc). From this, it can be concluded that the positively charged carbon adsorbs the anions in solution while the negative carbon adsorbs cations (Shafeeyan et al. 2011). The pH_pzc value of the obtained material corresponds to the intersection point between the curve obtained and the coordinate axis. As shown in the Figure 1, the pH_pzc value of the activated carbon is close to the neutral (pH_pzc = 6.61), this value is due to the salt treatment by ZnCl₂ that the biomaterial underwent.

Analysis was performed to find out the functional groups present on the surface of activated carbon. The FTIR of the prepared sample is represented in Figure 2. Peaks found at 3,792.41, 3,731.87 and 3,447.13 cm⁻¹ are attributed to the vibrations of hydroxyl groups (OH⁻). The absorption peaks at 2,305.67, 2,101.66 and 2,001.78 cm⁻¹ are attributed to the C ≡ C vibration. The peak at 1,588.02 cm⁻¹ indicate the presence of a double bond C = C in the aromatic ring. The peaks at 1,000–1,300 cm⁻¹ correspond to C–C and C–O groups (alcohols, phenols, acidic and ethers). Finally, the absorption peaks at 717.67 and 573.57 cm⁻¹ are attributed to CH₂ oscillation and C–O stretching vibrations.

Similar results was found by Shaarani & Hameed 2011; Gao et al. 2013; Kan et al. 2017).

Figure 3 shows the X-ray diffraction XRD characteristics of activated carbon. Broad peaks were observed and the almost complete absence of sharp peaks, which is a good and useful property that indicates the obtained activated carbon is a good adsorbent material (Hidayu & Muda 2016). The presence of large peaks at 2θ = 23.14–26.037° and 44.89–58.5° indicates the existence of amorphous carbon structure and irregular accumulation by carbon rings. This is very useful to produce a well-defined adsorbent material. As for the presence of peaks at 2θ = 31.394 and 33.42° is due to the presence of zinc which was used as a chemical activating agent of the raw material (Uçar et al. 2015).

Figure 4 shows the SEM micrograph of the prepared adsorbent. It indicates the presence of a large number of pores randomly distributed on sample surface with an average diameter of 666.8 nm in image (a) and (772.9–701.8) nm in image (b) and (c) respectively. These pores indicate that the activated carbon has a relatively high surface area (Hidayu & Muda 2016).

The BET specific surface area of the adsorbent is an important and useful factor. This is due to its influence in the reaction or carbonization process, where there is a possibility of opening the restricted pores (Kılıç et al. 2012). Figure 5 shows the nitrogen adsorption isotherms at 77 K on the new adsorbent. The maximum BET area of AC reached 36.52 m²/g and the total pore volume was 0.0106 cm³/g. with an average pore size of 12.12 Ẳ. The activated carbon used in this study has a high surface area compared to previous studies. Table 3 shows the different surface areas of the materials previously studied.

The contact time is an important parameter. it expresses the time during which the adsorbent is saturated with adsorbate (Almeida et al. 2009). To determine the contact time necessary to reach the adsorption equilibrium of MB or CR, experiments were carried under the following conditions: volume of 20 mL of dyes initial concentration of 20 mg/L. pH of the solution, temperature of 20 °C, mass of the adsorbent of 0.015 g and a stirring speed of 200 rpm. The samples were taken at specific time intervals after filtration, the solutions were analyzed by UV-Vis spectrophotometer at a wavelength λ_max = 662 nm for the BM and 550 nm for the CR.

Figure 6 shows that the adsorption kinetics of MB and CR can be elucidated in two steps: the first is the accelerated step in which the adsorption rate increases rapidly up to 40 min for the anionic dye and 120 min for the cationic dye due to the availability of vacant adsorption sites on the prepared activated carbon. The second step is a slow phase evolving towards pore saturation to stabilize at 180 min for MB and 80 min for CR with an adsorption rate of 99.52% and 99.64% for MB and CR respectively.

The dye initial concentration in the solution is considered to be one of the defensive forces that prevent mass transfer between the solid and aqueous phase (Mall et al. 2005).

In order to study the effect of the initial concentration on the adsorption of MB and RC by the prepared activated carbon. The experiments were investigated by varying the concentration of the adsorbate in a range from 10 to 100 mg/L. The other parameters were kept constant as shown in Figure 7. It was noticed that for MB, the elimination rate increased with the initial concentration until reaching a maximum. up to 40 mg/L, an inverse correlation was observed with the percentage removal of MB from the aqueous solution. This is because there is not a sufficient amount of activated carbon with active sites available to absorb all MB molecules in solution. Furthermore, the results indicate that there is not an optimal amount of adsorbent for dye removal from aqueous solution, which depends on the initial dye concentration and other variable parameters, such as temperature and the pH. In the case of CR, it was noticed that the rate of elimination increased with the initial concentration until reaching a plateau at 70 mg/L where the rate of elimination no longer varies with the initial concentration of CR. This is due to the saturation of the active sites and therefore additional activated carbon must be added to the CR solution to increase the percentage removal of CR from the aqueous solution. These results are in agreement with the work carried out by (Arifur Rahman et al. 2012; Gupta et al. 2013).

The practicality of the sorptive removing process of contaminants vastly depends on the adsorbent and its physicochemical (adsorbate-adsorbent interactions, sorbent surface area, contact time, pH, temperature, particle size, etc.).To determine the mass of activated carbon required to remove the two dyes: MB and CR, experiments were conducted at varied mass of adsorbent in the range of 0.001–0.5 g, while keeping the other parameters constant. As shown in Figure 8, very fast adsorption kinetics was observed which increases the percentage removal of both dyes with the increase in adsorbent dose up to 0.05 g for MB and 0.01 g in the case of CR, then the removal rate remained unchanged. At equilibrium time, the % of dye removal of MB was of 99.71% and of 98.02% for CR. This can be explained by the large number of vacant sites favoring the adsorption process. These results allowed to conclude the existence of a direct relationship between the mass of the adsorbent and the percentage of removal. Similar results were found by the study conducted by (Cherifi et al. 2013).

Several studies conducted by Mall et al. 2005; Oh et al. 2012; Sánchez Orozco et al. 2018 proved that the pH value is the main factor affecting the adsorption process. It decomposes the functional groups of the adsorbent's active sites and affects its surface charge. In addition, it has a significant effect on the degree of ionization of the dye (Pavan et al. 2008). The effect of pH on the dye removal rate was performed in the range [2–12]. The other parameters were kept constant. In general, the activated carbon is defined as an amphoteric material, which possess both acidic and basic surface functional groups on their surface and show positively or negatively charged character depending on the solution pH. The attraction of activated carbon with anionic or cationic materials is mainly related with the surface characteristic. At the pH_pzc, the charge of surface is neutral, so the surface charge of activated carbon at the pH value of 6.61 is neutral. At the pH values below 6.61, the surface of activated carbon will be positively charged. On the other hand, it will be negatively charged at pH values above 6.61. The results found are shown in Figure 9. This shows that MB adsorption on activated carbon had no significant effect in the pH range studied. MB adsorption on activated carbon increased slightly reaching 99.86%. This is due to the electrostatic attraction between MB and activated carbon because MB molecules are cationic at the pH above 6.61 which is negatively charged.

As for the elimination of CR, at the pH values below 6.61, the surface of activated carbon is positively charged, and the dye adsorption process is mainly affected by pH. The removal rate of CR in the acid range (2–4) is greatly improved, reaching 99.93%, and then it begins to decrease gradually until it stabilizes in the basic medum (9–12) reaching 96.6%. Figure 9. This shows that MB adsorption on activated carbon had no significant effect in the pH range studied. MB adsorption on activated carbon increased slightly reaching 99.86%. This is due to the electrostatic attraction between MB and activated carbon because MB molecules are cationic at the pH above 6.61, which is negatively charged.

As for the elimination of CR, at the pH values below 6.61, the surface of AC is positively charged, the dye adsorption process is mainly affected by pH. The removal rate of CR in the acid range (2–4) is greatly improved, reaching 99.93%, and then it begins to decrease gradually until it stabilizes in the basic medum (9–12) reaching 96.6%. This can be explained as in the aqueous solutions, CR exists in dissociated form as anionic dye ions. In acidic medium a significantly high electrostatic force of attraction exists between the positively charged surface of activated carbon and the anionic dye, hence enhancing the dye uptake. However, with the increase of pH, the positively charged sites of the adsorbent get decreased and the surface of the adsorbent became negatively charged (Bestani et al. 2008; Ai et al. 2011; Banerjee & Chattopadhyaya 2013; Falyouna et al. 2022a, 2022b).

The study of the isotherms expresses the interaction mode between the adsorbent and the adsorbate. It also provides the relationship between the dye concentration in the solution and the amount of dye adsorbed at equilibrium (Pathania et al. 2017). There are many models available for the study of the adsorption isotherms. The two most common isotherms models in this study are the Langmuir and Freundlich isotherms.

According to the obtained value of R_L, the adsorption process is judged as follows: if R_L = 0 the adsorption is irreversible. R_L = 1 is linear (0 < R_L < 1) the adsorption is favorable and R_L > 1 the adsorption is unfavorable (Khozemy et al. 2020). For an initial concentration of 20 mg/L for both MB and CR. The R_L values calculated in this study are of 0.0041 and 0.0411 for MB and CR respectively. These values indicate a favorable adsorption by the prepared adsorbent (Robati 2013). It also reveals a very strong interaction between activated carbon and dye molecules (Ayawei et al. 2017).

K_f represents the adsorption capacity and n is the adsorption force (Simonin 2016). According to Table 4. values of n > 1 were found that allows to conclude that the adsorption of the two dyes is favorable onto the prepared activated carbon. Based on the correlation coefficient and the Akaike Information Criterion (AIC) values found, the Langmuir isotherm model is the most appropriate to describe the adsorption of the two types of dyes by the prepared activated carbon because it reaches the largest of R2 values and the lowest AIC values (Table 4). Moreover, the experimental and the calculated adsorption capacities by the Langmuir model are closer than those obtained by the Freundlish model that is less suitable to describe the adsorption of anionic and cationic dye (Figure 11).

To this end, in this study two models were tested for dye removal on the prepared activated carbon: the pseudo-first-order and pseudo-second-order models:

The values of the coefficients, K₁ and qe are calculated from the slope and the intercept of ln (q_e-q_t) versus time curve (Figure 12). The results are presented in Table 5. Based on the results found, it was noticed from the negative values of the AICs and the very low correlation coefficients, which showed a large difference between the experimental and calculated adsorption capacities (q_m), that the pseudo-first-order model is unsuitable for the two dyes studied. Similar results were found by Panda et al. 2009; Karaçetin et al. 2014.

From Figure 13, the value of q_e is derived from the relationship between the reciprocal of the slope of the curve t/q_t versus t and K₂ is calculated from the intercept of the curve. All experimental results are depicted in Table 5. It was noticed that the correlation coefficients for the two dyes are close to 1 and the AIC values were practically similar. The maximum experimental and calculated adsorption capacities are very close, which allow to deduce that the adsorption of MB and CR was described very well by the pseudo-second-order kinetics. The results obtained are comparable with those obtained by Karaçetin et al. 2014.

Temperature is an important parameter in the adsorption process. To understand the influence of this parameter on the adsorption of the anionic dye on our activated carbon, experiments were performed in a range of temperature from 20 to 80 °C, maintaining the other parameters constant. The results reflected the exothermic nature of the two dyes removal process from water by the prepared activated carbon. As shown in Figure 14, the removal efficiency decreased by increasing temperature. This decrease can be explained by the fact that the increase in temperature induces the increase on both mobility of the particles of dye in solution and diffusion of dye on the active surface of the carbon. Similar results have been found when a cationic dye was removed from an aqueous solution using an arginine-modified activated carbon. Maximum adsorption was obtained at a temperature of 20 °C for the two pollutants with a dye removal of 99.95% for MB and 99.98% for CR.

Enthalpy and entropy values were obtained from the linear plot of the variation of Ln(K_e) against 1/T (Figure 15), ΔH°/R and ΔS°/R are respectively the slope and the intercept of the Equation (16), where R is the thermodynamic parameter 8.314 (J/mo.K).

The values of the thermodynamic adsorption parameters of the two pollutants are grouped in Table 6.

The negative values of ΔG° and ΔH° < 0 calculated at equilibrium indicate that the adsorption is spontaneous, physisorption process of type and exothermic for both. Moreover, the negative value of ΔS° indicates that the adsorption occurred leading to a smaller disorder degree in the whole system (Han et al. 2015; El Alouani et al. 2019).

This work was focused on the use of the red pumpkin skin as an adsorbent to remove anionic and cationic dyes contained in industrial wastewater. The study of parametric effects showed that the contact time reached 180 min for MB and 80 min for CR. The dye removal efficiency is inversely proportional to the initial dye concentration. The percentage of dye removal increases with the mass of the adsorbent up to 0.05 g for MB and 0.01 g for CR. The influence of pH value proved that BM is adsorbed in a basic medium and CR is adsorbed in an acidic medium. The Langmuir model gave a good description for the modeling of MB and CR adsorption isotherms. The modeling of the adsorption kinetics is perfectly described by pseudo-second-order kinetics with a correlation coefficient of 0.999 for both dyes.

Data cannot be made publicly available; readers should contact the corresponding author for details.