Production of activated carbon from food wastes (chicken bones and rice waste) by microwave assisted ZnCl2 activation: an optimized process for crystal violet dye removal

Abstract A major worldwide challenge that presents significant economic, environmental, and social concerns is the rising generation of food waste. The current work used chicken bones (CB) and rice (R) food waste as alternate precursors for the production of activated carbon (CBRAC) by microwave radiation-assisted ZnCl2 activation. The adsorption characteristics of CBRAC were investigated in depth by removing an organic dye (crystal violet, CV) from an aquatic environment. To establish ideal conditions from the significant adsorption factors (A: CBRAC dosage (0.02–0.12 g/100 mL); B: pH (4–10); and C: duration (30–420), a numerical desirability function of Box-Behnken design (BBD) was utilized. The highest CV decolorization by CBRAC was reported to be 90.06% when the following conditions were met: dose = 0.118 g/100 mL, pH = 9.0, and time = 408 min. Adsorption kinetics revealed that the pseudo-first order (PFO) model best matches the data, whereas the Langmuir model was characterized by equilibrium adsorption, where the adsorption capacity of CBRAC for CV dye was calculated to be 57.9 mg/g. CV adsorption is accomplished by several processes, including electrostatic forces, pore diffusion, π-π stacking, and H-bonding. This study demonstrates the use of CB and R as biomass precursors for the efficient creation of CBRAC and their use in wastewater treatment, resulting in a greener environment. NOVELTY STATEMENT The novelty of this research work relates to converting food wastes (mixture of chicken bones and rice waste) into activated carbon via microwave assisted ZnCl2 activation. Moreover, the produced activated carbon was successfully applied as a potential adsorbent for removal of a toxic cationic dye; namely, crystal violet (CV) from aqueous environment.


Introduction
A significant global challenge is the ever-increasing disposal of municipal solid waste (MSW) in urban areas (Khan et al. 2022).Food waste (FW), which is produced annually at an exponential pace and is recycled at a low rate, is one of the main organic parts of MSW (Yang et al. 2022).Anaerobic digestion (Mannarino et al. 2022;Zhang et al. 2022), aerobic composting (Liu et al. 2022), landfill (Mathioudakis et al. 2022), incineration (Escamilla-Garc� ıa et al. 2020), and open dumping (Mohan and Joseph, 2021) are the primary industrial techniques for recycling food waste currently.Although the aforementioned approach to large-scale industrialization can be used to treat food waste, there are some critical shortcomings, including the widespread occupation of land, high equipment investment costs, low product profit margins, the ease with which secondary pollution like greenhouse gases arise, and waste leachate (Siddiqua et al. 2022).As a result, recycling food waste through chemical conversion processes is crucial for environmental sustainability.
Microwave (Liu et al. 2014), torrefaction (Singh and Yadav, 2021), co-hydrothermal carbonization (Alshareef et al. 2022), and pyrolysis (Yang et al. 2023: Elkhalifa et al. 2019) are examples of traditional methods for converting food waste.Therefore, it is crucial to produce high-quality carbonaceous materials from food waste.One of the most popular meals in the world is chicken with rice, and in Malaysia, this dish is popular at fast-food restaurants and is called "nasi ayam".Large amounts of leftover rice and chicken bones from this meal make disposal difficult and it is a significant obstacle for urban government.Additionally, it could contaminate the air, soil, and water.Therefore, it is crucial and practical for sustainable development to utilize these wastes as a carbon source for the creation of high-performance activated carbon.The versatility and widespread use of activated carbon (AC) as an adsorbent and catalyst in numerous industries, including pharmaceuticals, food processing, wastewater treatment, and air pollution, can be assigned to its ideal characteristics, including pore size distribution, high surface area, and changeable surface properties (Reza et al. 2020;Duan et al. 2021).
Environmental pollution, particularly water pollution, is one of the most serious issues in nowadays, affecting water quality and limiting its usage (Najemalden et al. 2018;Siddiqua et al. 2022;Smadi et al. 2023;).Organic dyes are nowadays widely employed as coloring agents in a wide range of industries, including leather, medicine, paper, skincare products, plastics, and textiles (Shayesteh et al. 2016;Samsami et al. 2020;Beigi et al. 2023).The release of organic dyes such as crystal violet (CV) into bodies of water without pretreatment endangers aquatic species and causes various human health problems such as mutagenicity, significant eye impairment, and cancer (Suhaimi et al. 2022).Pretreatment of industrial wastewater containing organic dyes (such as CV dye) is critical for protecting human health and ecosystems and should not be overlooked.To remove organic dyes from industrial wastewater, several technologies such as coagulation (Obiora-Okafo et al. 2022), electrochemical decomposition (Tang et al. 2022), photolysis (Anisuzzaman et al. 2022), membrane filtration (Ma et al. 2022), and adsorption (Sultana et al. 2022) have been adopted.The majority of these methods are limited and have limitations such as secondary waste creation, cost, and complexity.Because of its distinctive properties such as cheap cost, high efficiency, promise, and convenience of use in industrial applications, adsorption technology has been embraced as a critical method of removing harmful contaminants (Hamzezadeh et al. 2022;Jawad et al. 2022).The Box-Behnken design (BBD), which has been employed in the optimization of the adsorption process, is one of the experimental designs adopted by the response surface methodology (RuiHong et al. 2023).BBD is ideally suited to minimize the total number of experiments and, as a consequence, the consumption of chemical reagents without losing the accuracy of the findings (RuiHong et al. 2023).
The purpose of the study is to develop and describe the properties of activated carbon obtained by microwaveassisted ZnCl 2 activation of food waste, particularly chicken bones (CB) and rice (R).Crystal violet (CV), an organic pollutant, was tested for removal from water using the resultant activated carbon (CBRAC).An essential part of this study is the use of food waste as a precursor for the creation of activated carbon since it offers a practical and affordable way to reuse and recycle food waste while producing a useful good that can be utilized for treating water pollution.BBD including a numerical desirability function was utilized to determine the ideal values for the adsorption variables (CBRAC dosage, pH, and duration).The kinetic and isothermal characteristics of CV adsorption on CBRAC were also carefully investigated.An appropriate mechanism was designed for the adsorption of CV dye onto the CBRAC surface.

Materials
Food waste (CB and R) from a restaurant nearby in Shah Alam, Malaysia, was employed as a precursor for the generation of AC.The CV dye (MW: 408 g/mol; k max : 584 nm) was purchased from R&M Chemicals.Without additional purification, R&M Chemicals was utilized to acquire hydrochloric acid (HCl), zinc chloride (ZnCl 2 ), sodium chloride (NaCl), and sodium hydroxide (NaOH), which were all used in the present investigation.

CBRAC preparation
Initially, a 2:1 impregnation ratio (W/W) of ZnCl 2 solution was used for treating the CBR.The impregnated sample was subsequently transferred to an adapted microwave oven with an adjustable radiation controller after overnight drying in an oven at 100-105 � C. The process took place at 600 W for 15 min with N 2 (99.99%) flowing at a flow rate of 150 mL/min.The microwaved sample was then washed with distilled water multiple times to eliminate any chemical residue, and it was dried at 105 � C.After passing through a sieve with a mesh to get a uniform particle size (250 lm), the produced activated carbon (CBRAC) was preserved in a sealed container for subsequent use.In the Supplementary Data (Text S1) section, there is information about CBRAC characterization, including the methods employed.

Experimental design
With a small number of intended tests, RSM is a helpful approach for the multivariate optimization of the CV adsorption procedure by CBRAC.The key advantage of the BBD matrix is that it helps prevent tests from being conducted in challenging circumstances, which might provide unsatisfactory findings.In the present research, BBD was used to analyze the impact of the three individual parameters (CBRAC dosage, pH, and time) on CV removal.In the current study, BBD was utilized to construct examinations using the Design-Expert program (version 13, Stat-Ease).
The range and grades of the CV adsorption factors are listed in Table 1.The parameters and their range were chosen for the study that depend on the preliminary tests.To anticipate the optimum operating characteristics, the following formula (Equation 1) approximates the connection between the output (CV removal) and inputs (factors).
where Y is the estimated response; b 0 ¼ the regression coefficient of intercept, whereas the regression coefficients for linearity, squaring, and interactivity are b i , b ii , and b ij , respectively; X i and X j are the adsorption factors.Table 2 contains a list of BBD tests for CV with the produced adsorbent.The CV removal experiments were started by pouring 100 mL of the CV solution into a conical flask with the known quantity of CBRAC.After agitating the solutions in the flasks at a rate of 80 rpm in a water bath shaker for a set period of time, where the CBRAC was extracted using a 0.45 lm syringe filter.The quantity of the residual CV in solution was determined by UV-vis (HACH DR 2800) at k max ¼ 584 nm.The efficiency of CV removal was assessed using Equation 2.
where C o and C e refer to the initial and final concentrations (mg/L) of CV, respectively.

Adsorption study of CV on CBRAC
The ideal adsorption factor values are identified using the numerical desirability function and were then used in batch equilibrium tests.In line with the desirability function (see Fig. S1), the CBRAC dosage of 0.118 g, pH of 9.0, and duration of 408 min are the best settings for achieving the highest CV elimination (90.06%).The adsorption equilibrium tests with a range of initial CV values (60-200 mg/L) were conducted using these optimum variable data.The CV dye batch tests were completed using the same approach as explained in section 2.3 earlier.The adsorption capacity (q e , mg/g) of the adsorbent has been identified using the formula below (Equation 3).
The CV solution volume is denoted by V(L), and the adsorbent weight is denoted by W(g).

Characterization of CBRAC
The CBRAC surface area and porosity play crucial roles in the uptake of CV dye.For CBRAC, Table 3 lists its surface characteristics such as specific surface area and pores along with the elemental content of CBRAC.The elemental examination demonstrates that the C, O, H, and N ratios in the CBRAC were 42.72%, 51.16%, 2.46%, and 3.66%, respectively.According to calculations, CBRAC has a total pore volume and specific surface area of 0.09743 cm 3 /g and 150.2 m 2 /g, respectively.According to IUPAC (Sing, 1985), the results of the average pore diameter (3.2 nm) show that CBRAC has a porous structure (pore diameters ranging from 2.0 nm to 50 nm).Figure 1 provides the N 2 adsorption and absorption isotherms of CBRAC. Figure 1 represents that the CBRAC  adsorption isotherm is a type IV system and reveals that the structure of the substance contains mesopores (Sing, 1985).
Additionally, the H3 hysteresis loop that occurs in the relative pressure (P/P o ) range suggests that CBRAC contains slit pores (Appiah-Ntiamoah et al. 2023).Due to its high porosity and sizable specific surface area, synthesized CBRAC often contains a significant number of active sites that may interact with CV dye molecules in a useful manner.
Using XRD measurements, information regarding the crystalline features of CBRAC was characterized.The X-ray patterns of CBRAC are shown in Figure 2. It is evident that the CBRAC sample had clear crystal reflections close to 15.3 � , 22.15 � , 31.5 � , 34.13 � , and 36 � .These findings show that the CBRAC's hydroxyapatite crystal structure corresponds to the diffraction peaks (Côrtes et al. 2019).Additionally, according to the data in the CBRAC spectrum, there are several peaks at 2theta ¼ 32 � , 34.5 � , 37 � , 47.8 � , 57 � , and 63.1 � , which are linked to the activation agent (ZnCl 2 ) used in the synthesis of CBRAC and are related to ZnO (Hadi et al. 2021).
FTIR spectroscopy provides insight into the surface chemical characteristics of CBR, CBRAC, and CBRAC-CV interactions.The FTIR spectra for CBR, CBRAC, and CBRAC-CV are shown in Figure 3(a-c).The wide band (at 3500 cm −1 ) in the spectra of CBR (Figure 3 SEM-EDX analysis was used to evaluate the structural characteristics and chemical composition of CBRAC both before and during its capture of CV particles.Figure 4 shows SEM images of the CBRAC before and after CV particles have been adsorbed onto its surface in (a) and (b), respectively.The CBRAC slit-shaped morphology is seen in Figure 4(a).The primary components of CBRAC are nitrogen, oxygen, and carbon, according to the EDX test.The morphological properties of CBRAC were visibly altered after CV dye adsorption (Figure 4(b)), becoming more compact and with a clear decrease in porosity and fractures, showing CV dye loading on the surface of CBRAC.EDX analysis verified the existence of carbon, oxygen, and nitrogen atoms in CBRAC following CV dye adsorption.

Statistical evaluation
Analysis of variance (ANOVA) was used for verifying the results.The ANOVA data are shown in Table 4.The model's F-value of 45.03 for CV removal (%) by CBRAC shows that has statistical strength ( € Ozc ¸elik et al. 2022).The models are statistically significant, as evidenced by the R 2 of 0.98.This result also demonstrates the confidence link between the real and anticipated CV color removal data.The factor is regarded as significant in eliminating CV when the p-value is less than 0.05 (Kwikima et al. 2022).The key terms are found to be as follows: A, B, C, A 2 , AB, and AC.After eliminating those meaningless variables, a final regression equation (Equation 4) was constructed.

CV removal %
The diagnostic graphs, which consisted of a normalized probability plot for student residuals and an estimated vs actual value plot, show the range of reliability of the generated model (Tamjid Farki et al. 2023).The normalized probability residuals (Figure 5(a,b)) show that the points were plotted in alignment with a straight line.As a result, errors are routinely fixed and the data is reliable (Niyitegeka et al. 2023).Figure 5(c,d) show diagnostic schematic diagrams of actual vs expected data.As seen in Figure 5(c,d), the experimental findings are quite comparable to the simulated data, which verify the models' feasibility (Belachew and Hinsene, 2022).

Effects of interactive variables
Using 3D and 2D graphs, it is simple to illustrate how the examined chemicals interact with one another to affect CV adsorption.The combined effects of pH and CBRAC dosage on CV removal performance were plotted in both 3D and 2D, demonstrating that pH has a substantial impact on CV  removal as seen by the enhanced CV removal effectiveness observed with increasing pH from 4 to 10.The pH pzc value of CBRAC was determined from Figure 6(e) to be 6.0.The CBRAC surface can be protonated at pH levels below 6 and negatively charged at pH levels above pH pzc , depending on the pH pzc of the adsorbent.Accordingly, the CBRAC surface becomes negatively charged at higher pH values.This means that at higher pH levels, the CBRAC surface becomes negatively charged, resulting in improved adsorption of dye cations by electrostatic forces (Equation 5).
The findings shown in Figure 6(a,b) amply demonstrate that the uptake efficiency is improved by raising the CBRAC dosage from 0.02 to 0.12 g/L.This interpretation may be brought about by the increased surface area and the possibility that a higher CBRAC dosage will result in more accessible adsorption sites.Time has a substantial impact on CV removal, where greater CV removal efficacy occurs with increasing time from 30 min to 420 min, according to 3D and 2D plots that show the combined effects of duration with a dosage of CBRAC on performance (see Figure 6(c,d)).This could relate to the longer time that allows for more interaction between the adsorbent and CV as well as the accumulation of CV dye molecules in the pores of CBRAC, which improves the ability to bind CV onto CBRAC.

Adsorption study
A thorough investigation was done that relates how time and a variety of dye concentrations (60, 80, 100, 150, and 200 mg/L) affect the CBRAC adsorbent's capacity to adsorb CV.The results are shown in Figure 7(a) where other crucial ideal parameters i.e., CBRAC dosage of 0.118 g/100 mL, a pH of 9.0, and a temperature of 25 � C were left unmodified.When the dye concentration varied from 60 to 200 mg/L, the findings demonstrated an improvement in the adsorption capacity (33.33 to 55.36 mg/g), as shown in Figure 7(a).As a result, a larger concentration has a stronger driving force to overcome the mass transfer constraint, which causes the adsorbent to absorb more CV (Ma et al. 2020).

Adsorption kinetics
The dynamics of the adsorption of CV by CBRAC offer light on the adsorption process and may be used for estimating the rate of decolorization of CV from an aqueous medium.The pseudo-first-order (Lagergren 1898) and pseudo-second-order (Ho and McKay, 1998) were implemented to determine the rate limiting step of adsorption.Three adsorption isotherms' nonlinear formulae with associated parameters can be found in Table S1.Although the R 2 values for PFO and PSO are close to one another, Table 5 shows that the adsorption of CV by CBRAC fits the PFO model owing to the derived q e values and the experimental q e values concur satisfactorily.The results above demonstrate that CV has been physically adsorbed onto CBRAC (Hashem et al. 2023).

Adsorption isotherms
The specific type of relationship between CV and the adsorbent (CBRAC) was examined, and isotherms were used to calculate the amounts of CV adsorbed on CBRAC at equilibrium.The Freundlich, Langmuir, and Temkin equilibrium systems (Freundlich 1906;Langmuir 1918;Temkin 1940) were implemented to determine the equilibrium nature of the adsorption process.Three adsorption isotherms' nonlinear formulae with associated parameters can be found in Table S1.The model variables already outlined were thoroughly assessed and gathered in Table 6 and Figure 7(b).Due to the greater regression coefficient (R 2 ) of the Langmuir isotherm than that of the Temkin and Freundlich models, the adsorption process, i.e., the adsorption of CV by CBRAC corresponds to the Langmuir model.These findings point to the surface coverage of CV as a single-layer onto CBRAC throughout the adsorption process (Chen et al. 2023).Additionally, the CBRAC adsorption capacity (q max of CBRAC ¼ 57.86 mg/g) was established, and this outcome was assessed using various adsorbents reported in the literature (see Table 7).These findings demonstrate that CBRAC exceeds most other adsorbents for its ability to remove toxic contaminants from water.

Adsorption mechanism of CV
Several interactions, including hydrogen bonding, p-p interaction, pore filling, and electrostatic interaction, have been highlighted in the literature as major factors in the adsorption of cationic basic dyes on ACs (Hanafi et al. 2022;Jasri et al. 2023).Figure 8 exhibits the adsorption mechanism for CV adsorption using CBRAC.The CBRAC structure is porous according to the BET study, and CV has a longitudinal length of 1.4 nm (Hanafi et al. 2022), which strengthens the 0.95 potential of using CV molecules to fill the pore domains of the CBRAC structure as seen in Figure 8.The interaction between CV cations and the negatively charged surface of CBRAC at a pH higher than pH pzc of CBRAC improves the process of CV adsorption via electrostatic interactions.On the CBRAC surface, hydrogen-containing functional groups have the ability to intercalate with N atoms of CV species to create a hydrogen bond.The strong CV affinity on the CBRAC surface is also due to electron donor interactions (often referred to as "p-p stacking") between the hexagonal skeleton of CBRAC and the CV benzene rings (Hanafi et al. 2022).

Conclusion
The activated carbon (CBRAC) was successfully produced by microwave radiation-assisted ZnCl 2 activation of CB and R. When the following parameters were achieved, the greatest CV decolorization by CBRAC was found to be 90.06%, at dosage ¼ 0.118/100 mL, pH ¼ 9.0, and duration ¼ 408 min.The F-value of 45.03 for the model's CV elimination (%) using CBRAC demonstrates its statistical validity.The R 2 (derived from the BBD model) of 0.98 indicates that the model appears statistically significant.The equilibrium study revealed a strong correspondence with the Langmuir isotherms, confirming CV adsorption onto CBRAC as a monolayer form.The adsorption capacity of CBRAC for CV dye was calculated to be 57.9 mg/g.CV adsorption is accomplished by a number of processes, including electrostatic forces, pore diffusion, p-p stacking, and H-bonding.This study demonstrates the use of CB and R as biomass precursors for the efficient creation of CBRAC and their use in wastewater treatment, resulting in a greener environment.CBRAC is a promising new material with the potential to address water treatment.Its high surface area, porosity, and good water wettability make it an effective and efficient way to remove a wide range of pollutants like cationic dyes and heavy metals from water.

Figure 4 .
Figure 4. SEM images and EDX spectra of (a) CBRAC and (b) CBRAC after CV adsorption at a Magnification of 2.00 k times.
(a)) was clearly visible and was generated by the O-H and N-H vibrations of the hydroxyl and amine groups found in the compound mixture of CBR (Côrtes et al. 2019).In the aforementioned spectrum, several distinct peaks can be seen at 2920 cm −1 (C-H stretching), 2341 cm −1 (C-C stretching), 1650 cm −1 (C ¼ O stretching), 1560 cm −1 (C ¼ C), 1033 cm −1 (C-O elongation), and 560 cm −1 (PO 4 −3 phosphate group) (Côrtes et al. 2019).The FTIR spectrum (Figure 3(b)) of the generated CBRAC shows a significant change compared to CBR, where most of the peaks disappeared, indicating that during the process of carbonization and activation, chemical bonds were broken and activated carbon was formed.After the adsorption of CV onto the surface of CBRAC (Figure 3(c)), the FTIR spectra showed the appearance of new bands, which reveal that the main functional groups of CBRAC were involved in the adsorption of CV.

Figure 5 .
Figure 5. (a) Normal probability plot of residuals for CV removal (%) by CBRAC; (b) plot of the relationship between the predicted and actual values for CV removal (%) by CBRAC.

Table 1 .
Codes and actual variables and their levels in BBD.

Table 2 .
Matrix of BBD model and the related response (CV removal (%)).

Table 5 .
PFO and PSO kinetic parameters for CV adsorption by CBRAC.

Table 7 .
A Comparison of CV adsorption capacities by various AC adsorbents.

Table 6 .
the parameters of isotherm models and equilibrium parameters for CV adsorption on CBRAC.