Microwave-Driven Upcycling of Biomass and Soft Slaughterhouse Waste into Activated Carbon for Efficient Cr(VI) Removal

1. Introduction

The growing environmental crisis in recent years has intensified the need for sustainable waste management, leading to increased research interest in the development of innovative technologies for converting waste into high value-added materials. Agricultural residues biomass and slaughterhouse waste constitute significant sources of organic matter, which often remain underutilized or are disposed of in environmentally harmful ways. Their utilization to produce porous carbon materials represents an attractive approach within the framework of the circular economy [1,2].

Agricultural waste biomass originates from the by-products and residues of farming, livestock rearing, and food processing operations. These residues biomass create important environmental, economic, and social issues worldwide, as a considerable fraction of produced wastes [1]. However, instead of being discarded or burned, this renewable organic material also constitute a resource that can be valorized for bioenergy generation, compost production, soil mulching, and the development of high-value products such as biochar, bioplastics, single-cell protein, and bio-based chemical building blocks [1,3,4].

Slaughterhouse residues is another category of wastes which primarily composed of residues originating from the rumen, stomach, and intestinal contents. Large quantities of such by-products are generated annually, particularly by poultry processing industries, driven by the high consumption of poultry meat [2,5]. If discharged without proper treatment into municipal wastewater systems, these wastes can create serious environmental concerns because contains numerous toxic and persistent organic contaminants, including refractory and difficult-to-degrade compounds, as well as elevated concentrations of organic matter, fats, proteins, detergents, blood residues, and pathogenic microorganisms [5,6,7,8,9]. Therefore, the efficient treatment of slaughterhouse wastewater is essential for minimizing environmental impacts and promoting sustainable waste management practices.

Porous carbon materials with high specific surface area are widely used in applications such as energy storage (supercapacitors, batteries), catalyst support, and environmental remediation through pollutant adsorption [10]. Especially, biomass-derived activated carbons are considered highly promising because they originate from abundant renewable waste resources, possess well-developed surface areas, and exhibit favorable pore size distributions suitable for these applications [11,12,13,14,15]. Owing to these properties, such materials represent environmentally sustainable alternatives with considerable potential in adsorption technologies, catalysis, and energy-related applications. Many sources of biomass have been used to produce highly porous activated carbons via traditional activation methods using conventional pyrolysis [12,16,17,18,19,20].

However, such methods are characterized by long processing times leading to high energy consumption. For this reason, microwave assisted heating has attracted scientific attention in the last two decades for the efficient production of activated carbon. Microwave heating offers rapid and uniform energy transfer, enabling more efficient conversion of raw materials [13,21,22]. Many biomass wastes such as corn cob [21], macademia nuts [23], orange peel [24], straw maize [25], almond shells [26], rubber seed pericarp [22], wood [27], oil palm biodiesel solid residue [28] etc. have been used as precursors for producing activated carbons via microwave assisted pyrolysis, showing high surface area and advanced sorption properties in removal of dyes, phenols, NO₂, H₂S, heavy metals etc. [20,29].

Unlike lignocellulosic agricultural residues or animal bones, which possess an innate structural matrix favorable for direct carbonization, soft slaughterhouse structures (such as liver, lung, and heart) presents unique processing challenges due to its high moisture content, volatile organic load, and elevated lipid/protein ratio. Direct thermal treatment of such wet tissues often leads to uncontrolled decomposition and pore clogging. To overcome these limitations, structural stabilization and alternative carbonization pathways are required. Formaldehyde treatment serves as an effective pre-treatment step, cross-linking the polypeptide chains and stabilizing the tissue structure against premature thermal collapse [30]. Furthermore, microwave assisted hydrothermal treatment (HT) has emerged as a powerful alternative for wet biomass valorization. HT promotes the direct conversion of high moisture animal tissues into a stable, carbon rich solid intermediate (hydrochar) without the energy intensive demand of pre drying [31]. When sequentially combined with microwave-assisted ZnCl₂ chemical activation, this hybrid HT and pyrolysis approach offers a novel, low-energy pathway to transform complex soft structures into high-surface-area functional porous carbons.

In this context, the present study explores the microwave assisted ZnCl₂ activation process of aloe vera leaf industrial wastes, agricultural corn cob residues, and soft slaughterhouse waste to produce porous carbon materials with enhanced textural and adsorption properties. The relationship between precursor type, physicochemical properties, and adsorption performance is evaluated, highlighting the potential of these waste derived carbons as sustainable adsorbents for environmental remediation applications. The novelty of this work lies in the comparative valorization of both agricultural and slaughterhouse wastes through a rapid, low-temperature microwave pyrolysis process, combined with the investigation of their efficiency in hexavalent chromium (Cr(VI)) removal. Τhis is the first time that aloe vera waste leaves, corn cob and soft slaughterhouse residues have been tested as precursors to produce activated carbon using simple and rapid impregnation route of activated agent and microwave assisted pyrolysis. In addition, this is the first time that all these materials are proposed as efficient materials for Cr(VI) removal.

2. Materials and Methods

2.1. Materials

Aloe vera waste leaves (av) were provided from the Greek Industrial Company Hellenic Aloe, Ethnikis Antistaseos 21, Heraklion, Crete, 71306, Greece, whereas Corncob wastes (cc) were supplied by Agricultural Cooperative Agrinio Union, Agrinio, 30100, Greece. The specific av wastes consist of the outer peel of the leaf and some residues from the inner gel. These biomass wastes av and cc were washed several times with tap and deionized water, dried in 80 ^oC for 48 hours and were ground into fine powder using a household grinder. Slaughterhouse wastes include pork’s liver, heart and lung were offered from a local butchery, were washed with tap and deionized water, wiped with paper and air dried. Zinc chloride (ZnCl₂, 98%), ethanol (C₂H₆O, 99,8%), hydrochloric acid (HCl, 37%), and formaldehyde solution (CH₂OH, 37%) were purchased from Merck (Darmstadt, Germany) and used without further purification.

2.2. Preparation of Biomass Derived Porous Carbon Materials

Activated carbons, denoted as ACav and ACcc, derived from aloe-vera leaf wastes, and corncob residues, respectively, were produced using chemical activation procedure combined with microwave assisted pyrolysis. The biomass precursors were wet impregnated with ZnCl₂ aqueous solution at a mass ratio of biomass:ZnCl₂=1:2 and semidried at 80 ^οC for 30 min. The resulting mixture, in the form of a sludge, was pyrolyzed in microwave furnace (PYRO Advanced Microwave Muffle, Milestone) at temperatures ranging from 300 to 600 ^oC under argon flow. Each sample was heated until the target temperature was reached, subsequently the microwave irradiation switched off and the sample allowed to cool down naturally under argon flow. The solid products were washed with 1N HCl solution under stirring for 24 hours, following by several washings with deionized water and subsequently drying at 80 ^oC for 24 hours. The resulting carbon materials were designed as ACav-X and ACcc-X, where X corresponds to the pyrolysis temperature of each sample. The microwave irradiation time was 3, 5, 10, 15, 18 min for X: 253, 300, 400, 500 and 600 ^oC, respectively.

Figure 1. Aloe vera plant (a), aloe-vera leaf wastes after drying (b), after grounding into fine powder (c) and after activation and microwave assisted pyrolysis (ACav) (d).

Figure 1. Aloe vera plant (a), aloe-vera leaf wastes after drying (b), after grounding into fine powder (c) and after activation and microwave assisted pyrolysis (ACav) (d).

Figure 2. Corn cob wastes (cc) after drying (a), after grounding into fine powder (b) and after activation and microwave assisted pyrolysis (ACcc) (c).

Figure 2. Corn cob wastes (cc) after drying (a), after grounding into fine powder (b) and after activation and microwave assisted pyrolysis (ACcc) (c).

2.3. Preparation of Slaughterhouse Derived Porous Carbon Materials

For the synthesis of porous carbon using slaughterhouse wastes, the appropriate piece of the precursor was immersed in formaldehyde solution for 30 min following by air drying. Then the chemical modified precursor transferred to a Teflon autoclave container (100 ml), filled with 30 ml distilled water at a volumetric ratio of precursor:H₂O 1:1 and treated hydrothermally at 200 ^oC for 30 min in a microwave reactor (Milestone FlexiWAVE) and 1800 watt maximum power. After that, the hydrothermally treated precursor piece (sh-HT) was separated from the liquid and dried in air. Then, the air dried sh-HT turned into porous carbon using chemical activation procedure combined with microwave assisted pyrolysis following the same procedure that described section 2.2. The resulting carbon materials were designed as ACsh-X, where X corresponds to the pyrolysis temperature of each sample. The microwave irradiation time 10, 15, 18 min for X: 400, 500 and 600 ^oC, respectively.

Figure 3. Slaughterhouse wastes (sh) as received (a), after immersion in formaldehyde solution and air drying (b), after hydrothermal treatment (sh-HT) (c) and after activation and microwave assisted pyrolysis (ACsh) (d).

Figure 3. Slaughterhouse wastes (sh) as received (a), after immersion in formaldehyde solution and air drying (b), after hydrothermal treatment (sh-HT) (c) and after activation and microwave assisted pyrolysis (ACsh) (d).

2.4. Water Dispersibility/Stability

The dispersibility/stability of the activated carbons was evaluated by suspension tests in water. The samples were dispersed in water (0.025 g AC/10 ml H₂O) using ultrasonication for 30 min and subsequently left undisturbed for periods ranging from 1 to 24 h in a dark room, under constant overhead artificial lighting. Photographs were taken at specific time intervals to monitor sedimentation behavior and phase separation, using a smartphone camera (Xiaomi Redmi Note 13 Pro, operating at a high-resolution setting of 200 MP with the camera flash disabled). To eliminate variations in ambient lighting and camera positioning between consecutive measurements, a control sample containing pure water was photographed simultaneously inside the same frame as the carbon dispersions.

Image processing was performed using the ImageJ software v1.53e. The images were converted to 8-bit grayscale format, and identical regions of interest were defined for both the carbon dispersions and the pure water control. The Dispersibility Index (DI%) for each time interval (t) was calculated according to the following equation:

$$\mathrm{DI}\%={\displaystyle \frac{\mathrm{M}\mathrm{e}\mathrm{a}{\mathrm{n}}_{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}\left(\mathrm{t}\right)-\mathrm{M}\mathrm{e}\mathrm{a}{\mathrm{n}}_{\mathrm{C}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n}}\left(\mathrm{t}\right)}{\mathrm{M}\mathrm{e}\mathrm{a}{\mathrm{n}}_{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}\left(0\right)-\mathrm{M}\mathrm{e}\mathrm{a}{\mathrm{n}}_{\mathrm{C}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n}}\left(0\right)}}$$

(1)

where Mean_Carbon(t) and Mean_water(t) represent the mean gray values of the carbon dispersion and the pure water control at time (t), respectively, while Mean_Carbon(0) and Mean_water(0) represent the corresponding initial values at (t = 0). Based on this normalization, a DI% value of 100% indicates maximum, homogeneous dispersion (after sonication), whereas a decrease toward 0% shows progressive sedimentation.

2.5. Adsorption Experiments for hexavalent Chromium Removal

In order to study adsorption properties of the produced activated carbons, the highest surface area materials from ACav-X, ACcc-X and ACsh-X series, were used for the removal of hexavalent chromium from 100 ml aqueous solutions with different initial concentrations (i.e. ppm) at pH 3. The amount of the absorbent material was 18 mg and stirred for 24 hours in the Cr(VI) solution at 25 ^oC. The sampling and measuring were carried out according to the method described in our previous works [17,18,32].

2.6. Characterization Techniques

The physicochemical and structural properties of the synthesized materials were investigated using X-ray diffraction (XRD), Fourier transform infrared and Raman spectroscopies (FT-IR and Raman), and nitrogen adsorption–desorption porosimetry.

XRD measurements were carried out using a D8 Advance Bruker diffractometer equipped with Cu Kα radiation (λ = 1.54178 Å), operating at 40 kV and 40 mA, and fitted with a graphite monochromator on the secondary beam. Diffraction patterns were recorded over a 2θ range of 2–80°, with a step size of 0.02° and a counting time of 2 s per step.

FT-IR spectra were obtained using a JASCO FT/IR-6000 spectrometer within the wavenumber range of 4000–400 cm⁻¹ at a resolution of 4 cm⁻¹. The samples, in powder form, were mixed with KBr and pressed into pellets prior to analysis.

Raman spectroscopy measurements were performed using a Renishaw RM 1000 micro-Raman spectrometer, employing a 532 nm excitation laser (Nd:YAG source).

Textural characteristics were evaluated by nitrogen adsorption–desorption isotherms measured at 77 K using a Quantachrome Autosorb iQ analyzer. Prior to measurements, the samples were degassed at 150 °C for 20 h under high vacuum (10⁻⁶ mbar). The specific surface area was determined using the Brunauer–Emmett–Teller (BET).

Pore size distribution was derived using Density Functional Theory (DFT). The total pore volume was estimated from the amount of nitrogen adsorbed at relative pressure close to unity (P/P₀ ≈ 0.998), assuming complete pore filling.

Finally, the chromium ion concentration in solution was determined from UV–Vis absorption spectra. The measurements were carried out using a UV-2401(PC) Shimadzu spectrophotometer equipped with a halogen lamp and a quartz cuvette. Spectra were recorded over a wavelength range of 400–700 nm with a step size of 0.5 nm.

3. Results and Discussion

This section presents the results of the physicochemical and structural characterization of the prepared activated carbons. The materials were analyzed using nitrogen adsorption–desorption measurements, X-ray diffraction (XRD, )Fourier transform infrared (FT-IR) and Raman spectroscopies. The findings from each technique are discussed in the following subsections.

3.1. Nitrogen Adsorption-Desorption Results

Figure 1 presents the nitrogen adsorption-desorption isotherms (Figure 4a) and pore size distributions (Figure 4b) of activated carbons derived from aloe vera leaf wastes, in varied pyrolysis temperatures (400, 500 and 600 ^oC). In all cases, the adsorption-desorption isotherms correspond to type IV according to IUPAC classification, exhibiting hysteresis loop and indicating the presence of both micropores and mesopores. The S_BET specific surface area was determined to be 1120, 728 and 234 m²/g for ACav-400, ACav-500 and ACav-600, respectively. This finding can be attributed to structural shrinkage and partial collapse or widening of the pore network at elevated temperatures. The pore size distribution analysis using DFT calculations, reveals the presence of micropores with diameter 1.1 nm and mesopores with mean diameters ranging from 3 to 8 nm. Also, a significant reduction in cumulative pore volume is observed with increasing pyrolysis temperature, decreasing from 1.21 cm³/g for ACav-400 to 0.59 cm³/g for ACav-500 and 0.16 cm³/g for ACav-600. For the sample ACav-253 the adsorption-desorption isotherms indicate a nonporous or microporous material.

Similar results obtained also from the nitrogen adsorption-desorption isotherms (Figure 5a) and pore size distributions (Figure 5b) of the activated carbons derived from corncob wastes, in varied pyrolysis temperatures (300, 400, 500 and 600 ^oC). All isotherms are type IV, exhibiting hysteresis loop with microporous and mesoporous features. The specific surface area of the materials is increasing from 872 m²/g for ACcc-300 to a maximum of 1442 m²/g for ACcc-400 followed by a slight decrease to 1369m²/g and 1250 m²/g for ACcc-500 and ACcc-600, respectively. A similar trend is observed for the cumulative pore volume (Table 1), as determined by DFT analysis. Pore size distribution results confirm the coexistence of micropores with diameter 1 nm and mesopores with sizes ranging from 3 to 7 nm.

In the case of the carbon materials that are produced from slaughterhouse wastes (Figure 6a) the adsorption-desorption isotherms at lower pyrolysis temperatures (ACsh-400 and ACsh-500) as well as the product sh-HT derived from hydrothermal treatment at 200 ^oC, are shown characteristics of nonporous or macroporous materials. Only the material produced at 600 ^οC (ACsh-600) exhibits isotherms correspond to type I according to IUPAC classification which is characteristic for microporous materials and surface area 775 m²/g. However, a more careful examination of these isotherms (Figure 6b) shows the existence of hysteresis loop that reveals the coexistence of mesopores. Also, the DFT pore size distribution analysis confirms these findings showing the presence of 0.9 nm micropores and mesopores in the average size of 2 to 7 nm. This pronounced pore development for the slaughterhouse waste (where S_BET increases from 5 m²/g at 500 °C to 775 m²/g at 600 °C) can be attributed to the temperature dependent thermal degradation of animal fats and protein complexes. Below 600 °C, residual lipid fractions and undecomposed carbonaceous volatile matter probably remain trapped within the carbon matrix, completely blocking the microporous network suggesting that as the microwave pyrolysis temperature reaches 600 °C, intensive volatilization and sudden release of these trapped decomposition products occur.

Figure 5. a) Nitrogen adsorption-desorption isotherms and b) pore size distributions derived from DFT method for porous carbons ACcc-X.

Figure 5. a) Nitrogen adsorption-desorption isotherms and b) pore size distributions derived from DFT method for porous carbons ACcc-X.

Figure 6. a) Nitrogen adsorption-desorption isotherms and b) magnification of isotherms and pore size distribution derived from DFT method for porous carbon ACcc-600.

Figure 6. a) Nitrogen adsorption-desorption isotherms and b) magnification of isotherms and pore size distribution derived from DFT method for porous carbon ACcc-600.

It is observed that the textural properties of all AC samples are strongly influenced by the pyrolysis temperature. Comparing the materials that produced from different precursor source (av, cc, sh), we conclude that the significantly higher BET surface area observed for ACcc-400 (1442 m²/g¹), following by ACav-400 (1120 m²/g) and finally ACsh-600 (775 m²/g). The chemical activation with ZnCl₂ and microwave pyrolysis, effectively create micropore/mesopore structures, through dehydration and volatilization processes. The DFT results further confirm the presence of this dual porosity, which is advantageous for applications requiring both high surface area and efficient mass transport. Overall, the results suggest that in the case of biomass av and cc precursors, the lower pyrolysis temperatures (400 ^oC) are more favorable for producing activated carbons with enhanced textural properties, while for slaughterhouse waste at pyrolysis temperature lower than 600 ^oC it is not possible to form a porous structure.

3.2. Infrared Spectra

Infrared spectra of the produced carbon materials in comparison with the corresponding initial biomass and hydrothermally treated slaughterhouse wastes are presented in figures 7a, b and c, respectively. In all cases, the precursor materials exhibit the characteristic absorption bands of lignocellulosic and organic biomass structures, including -OH stretching vibrations (in the region ~3420-3350 cm⁻¹), aliphatic C-H stretching vibrations (at ~2930 and 2850 cm⁻¹), carbonyl related bands (in the region ~1735-1660 cm⁻¹), and C-O/C-O-C vibrations in the region ~1250-1000 cm⁻¹ [5,33,34]. These bands progressively decrease in intensity with increasing pyrolysis temperature, indicating the thermal decomposition of cellulose, hemicellulose, lipids, proteins, and other oxygenated surface functionalities, accompanied by the gradual formation of aromatic carbon structures.

In the case of aloe-vera leaf waste biomass (Figure 7a), the precursor’s (av) bands are flattening as the pyrolysis temperature increases, indicating the transformation of these groups into aromatic carbon structure. More specifically, in the spectra of ACav-400, 500 and 600, new bands are observed at 1580, 1210, 1123 and 800 cm^-1 which are characteristics in activated carbon’s infrared spectrum [17,18,32]. The bands at 1580 cm^-1 and 1123 cm^-1 contribute to the stretching vibrations of C=C and C-H bonds in aromatic carbon rings, respectively [35], whereas the band at 1210 cm-¹ corresponds to stretching vibration modes of C–O bonds [18,36]. The band at 800 cm-¹ is typically related to the aromatic structure (C-H out of plane bending or deformation modes) of the activated carbon skeleton, specifically the hydrogen atoms attached to the rings [36].

Similar results are also observed for the case of carbon materials derived from corn cob biomass (Figure 7b), where the intensity of the precursor’s (cc) bands decreases significantly, confirming also, the thermal degradation of hemicellulose, cellulose, and lignin components. As the pyrolysis temperature increases, these spectra become progressively simpler and are dominated by bands (1564, 1270 and 1162 cm^-1) related to the aromatic carbon structures. The persistence of weak oxygen-containing functional group bands at 1700 and 1605 cm^-1 (C=O in COOH and COO^- groups, respectively [37]) suggests that a limited number of surface oxygen functionalities remain on the activated carbon surface, which may contribute positively to adsorption-related applications.

The spectra of the slaughterhouse-derived carbons (Figure 7c) exhibit a more pronounced transformation due to the combined hydrothermal treatment and activation process. The Sh-HT precursor displays characteristic bands associated with residual fats, lipids, proteins, and nitrogen-containing compounds, including aliphatic C–H vibrations (2924 and 2853 cm⁻¹), carbonyl and aromatic-related bands (~1660 cm⁻¹), and C=N vibrations (~1535 cm⁻¹) [5]. Following pyrolysis, these bands progressively flatten, while a broad absorption band centered around 1050 cm⁻¹ develops, corresponding to C–O and C–O–C stretching vibrations typically observed in activated carbons [18,35,36]. Moreover, the band at 1660 cm⁻¹ shifts toward lower wavenumbers (1645 cm^-1 for ACsh-400 and 1625 cm^-1 for ACsh-500 and ACsh-600), suggesting the transformation of amide, ketone, and quinone groups into COO⁻ functionalities. The gradual decrement of the aliphatic C-H bands further confirms the thermal degradation of fatty chains and the development of a more aromatic carbon framework at higher pyrolysis temperatures.

Overall, the FT-IR results demonstrate that aloe-vera and corncob biomasses as well as slaughterhouse waste undergo substantial structural transformation during microwave-assisted pyrolysis, leading to the formation of activated carbons with predominantly aromatic character and reduced oxygen-containing functionalities.

3.3. Raman Spectra

The Raman spectra of the porous carbons derived from aloe vera leaf waste (ACav-X), corn cob waste (ACcc-X), and slaughterhouse residues (ACsh-X) are presented in Figure 8. All samples exhibit the characteristic D- and G-bands of carbonaceous materials, confirming the formation of partially graphitized porous carbons after ZnCl₂ activation and microwave pyrolysis at various temperatures (X). The D-band, located at 1348-1353 cm⁻¹, is associated with sp² hybridized carbon bonded with structural defects, whereas the G-band appearing at 1590-1615 cm⁻¹ corresponds to the in-plane vibration of sp² bonded crystalline carbon atoms [38,39,40]. The presence of these two bands indicates that the materials exhibit a mixed amorphous/graphitic structure typical of activated carbons produced from biomass precursors [41,42,43]. The intensity ratio of these bands, I_D/I_G, as well as the position of the G-band are some of the Raman features that characterize the changes in the graphitization degree of the carbon materials derived from biomass [38,40,44,45,46]. An increase in the I_D/I_G ratio suggests the progressive development and lateral expansion of graphene-like aromatic domains within the carbon matrix. This behavior indicates the growth and structural organization of polyaromatic clusters toward larger nanostructured carbon regions. In parallel, the shift of G-band from 1500 cm⁻¹ to 1600 cm⁻¹, indicates the enhancement of structural ordering of the aromatic carbon framework. This shift may be related to the formation of larger aromatic carbon structures and changes in the carbon bonding environment [40].

In all cases of ACav-X, ACcc-X and ACsh-X materials it is observed that I_D/I_G ratio is increased by increasing the pyrolysis temperature indicating the progressive formation and structural ordering of larger graphene-like aromatic carbon domains within the carbon matrix. In addition, the shift of the G-band from 1590 to 1615 cm⁻¹, 1595 to 1603 cm⁻¹ and 1580 to 1607 cm⁻¹ at higher pyrolysis temperatures, for ACav-X, ACcc-X and ACsh-X, respectively, also indicates structural rearrangement and partial graphitization of the carbon framework.

Figure 9 summarizes the effect of pyrolysis temperature on the I_D/I_G ratio for all prepared carbons where it is clearly observed that in all cases, increasing temperature resulted in higher I_D/I_G values. Overall, the Raman analysis demonstrates that both precursor type and pyrolysis temperature strongly affect the structural ordering and defect density of the produced activated carbons, which are critical parameters for adsorption and electrochemical properties.

3.4. X-Ray Results

Figure 10 presents the XRD patterns of the porous carbons prepared from aloe vera waste leaves (ACav-X), corn cob biomass (ACcc-X), and slaughterhouse residues (ACsh-X). All samples exhibit broad diffraction peaks centered approximately between 20° and 26° (2θ), which are characteristic for amorphous lignocellulosic structures and disordered graphitic carbon domains, respectively [39,47,48]. Amorphous lignocellulosic structures exhibit broad diffraction peaks centered at ~15 and 21^o [48], whereas a broad diffraction peak in the area near 26^o corresponds to the (002) reflection of low crystallinity carbon structures [43]. In all cases of AC carbons, the absence of sharp crystalline reflections indicates that the activation process promoted the formation of disordered porous carbons.

Specifically, in the case of ACav-X materials (Figure 10a), it is observed that there is a shift of the peak from 20^o to 26^o, as the temperature treatment increased from 253 to 600 °C, indicating the pronounced transformation of the lignocellulosic precursor av to amorphous carbon structures. Moreover, the slightly enhanced intensity and definition of the peak at 26^o that is observed for the higher pyrolysis temperature (600 ^oC) probably indicates the development of more condensed aromatic carbon structures due to increased carbonization under microwave irradiation. However, the persistence of broad peaks confirms that the material remained largely amorphous.

For the ACcc-X samples, the diffraction patterns (Figure 10b) exhibit the main diffraction peak centered at 23.4° for ACcc-300 and shifted toward 24.7° for ACcc-600. This slight shifting toward higher diffraction angles indicates slightly improved structural ordering and partial graphitization with increasing pyrolysis temperature. Also, in this case, the broad nature of the peaks demonstrates that the carbon framework still consisted mainly of disordered graphitic layers with limited crystallite growth.

For the slaughterhouse-derived carbons (Figure 10c), the hydrothermally treated precursor (Sh-HT) displayed a broad and weak diffraction pattern at ~20^o typical of poorly carbonized organic precursor. After activation and microwave treatment, all the ACsh-X samples exhibit broad peak shifted at 25.5°, indicating the formation of amorphous graphitic carbon domains. This peak position shifting demonstrates the increased aromatic condensation and structural organization which appears to be less dependent on pyrolysis temperature than in the ACav-X and ACcc-X materials.

Overall, the XRD analysis demonstrates that both the precursor type and pyrolysis temperature slightly affect the graphitic structure of the produced porous carbons. In all cases, microwave-assisted ZnCl₂ activation resulted in predominantly amorphous carbon materials with poorly ordered graphitic domains, as confirmed by the broad diffraction peaks in the 20–26° (2θ) region and the absence of sharp crystalline reflections. Increasing pyrolysis temperature led to the gradual transformation of lignocellulosic structures into more condensed aromatic carbon frameworks.

3.5. Dispersion/Stability in Water

The water dispersibility behavior of the produced activated carbons was monitored (inset photos in Figure 11) after sonication in water and left undisturbed for 24 h. Initially, all samples formed homogeneous suspensions. The gradual phase separation, which was observed with increasing standing time, accompanied by the formation of sediment at the bottom of the containers, was studied by calculating the dispersibility index (DI%) as described in the experimental section. Samples exhibiting delayed sediment formation likely possess higher dispersibility in water. Figure 11a, b and c present the DI% for the different materials, as a function of standing time.

In all cases, the homogeneous suspensions that initially (0 hours) obtained after sonication were referred to 100% DI. It is also observed that all the materials which are produced at the higher temperature (600 ^oC), exhibited the highest dispersibility 95-100% DI even after 24 hours standing time. At a little bit lower pyrolysis temperature (500 ^oC), only the material from aloe-vera precursor (ACav-500) exhibited the same behavior (100% DI after 24 h) whereas DI gradually decreased with increasing standing time, reaching to 43 and 52% for the materials ACcc and ACsh, respectively, indicating progressive particle sedimentation. By further decreasing the pyrolysis temperature to 400 ^oC, the ACav-400 shows stable dispersion in water for the first 3 hours standing, with 100% DI which reduced to 74% after 24 h (Figure 11a) whereas the lower DI value 60% was observed for 253 ^oC pyrolysis temperature. For ACcc materials, the DI slightly decreased (~10%) by decreasing the pyrolysis temperature from 500 to 400 and 300 ^oC reaching 36% DI after 24 h for ACcc-300 (Figure 11b). In case of ACsh-400 the DI value dramatically decreased to 38% after 3 hours, reaching to only 15% after 24 hours (Figure 11c).

Figure 11d shows the comparison of dispersibility index, after 24 hours standing, for materials produced using the three different precursors (av, cc and sh) as a function of the pyrolysis temperature. In all cases, it is observed that increasing pyrolysis temperature generally enhanced dispersion stability, whereas ACav-X materials exhibited the highest dispersion stability. Even the samples prepared at pyrolysis temperatures 300–500 °C, retained high DI values (60-75%) after 24 h. Corn cob-derived carbons (ACcc-X) showed similar trend at pyrolysis temperatures 300-500 ^oC, with ~ 40% DI after 24 h standing which excitingly increased to 100 % at 600 ^oC. In contrast, the slaughterhouse derived carbons (ACsh-X) displayed the lower dispersion stability (15%) at pyrolysis temperature 400 °C, with faster sedimentation and lower DI values after 24 h which also impressively reached 95% for the material produced at 600 ^oC.

The observed higher dispersion stability in all cases at higher pyrolysis temperatures cannot be only explained by surface chemistry effects, since FT-IR results indicate a decrease in hydrophilic surface functional groups with increasing temperature. In addition to changes in oxygen-containing functional groups, variations in particle size induced by microwave-assisted pyrolysis may also contribute to the observed behavior. Smaller particles are expected to exhibit lower sedimentation rates, resulting in enhanced suspension stability. Taken together, these findings suggest that increasing the microwave-assisted pyrolysis temperature may promote the formation of smaller particles. This could explain the improved dispersion stability despite the decrease in hydrophilic surface functionalities, as smaller particles are expected to exhibit lower sedimentation rates and remain suspended for longer periods.

3.6. Cr(VI) Removal Efficiency

In order to evaluate the adsorption performance of the microwave-produced activated carbons, the samples exhibiting the highest specific surface area were contacted with aqueous Cr(VI) solutions of different initial concentrations at pH 3 for 48 h at room temperature. Figure 12 presents the experimental isotherm results in the amount of Cr(VI) removed in each case. It is observed that specific surface area is affected the adsorption capacity of the materials and ACcc-400 material which exhibited the highest surface area (1442 m²/g), shows also the highest adsorption capacity, following by ACav-400 (1120 m²/g) and ACsh-600 (775 m²/g).

The Langmuir and Freundlich models (equations 2 and 3, respectively) [49,50] were used in order to explore the Cr(VI) removal trends by ACav-400, ACcc-400 and ACsh-600 materials and the results are presented in Figure 13.

$$\mathit{Langmuir}:{\displaystyle \frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}}+{\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}{\mathrm{K}}_{\mathrm{L}}}}$$

(2)

$$\mathit{Freundlich}:\mathrm{l}\mathrm{n}{\mathrm{q}}_{\mathrm{e}}=\mathrm{l}\mathrm{n}{\mathrm{K}}_{\mathrm{F}}+{\displaystyle \frac{1}{\mathrm{n}}}\mathrm{l}\mathrm{n}{\mathrm{C}}_{\mathrm{e}}$$

(3)

where C_e(mg/L) is the equilibrium concentration of the liquid phase, q_e(mg/g) is the absorbed amount in equilibrium, q_max represents the maximum adsorption capacity, K_L the Langmuir energy constant, K_F the Freundlich constant and n is the adsorption intensity.

The parameter values obtained from the fitting of the two models are summarized in Table 2. Based on the correlation coefficients (R²), the Langmuir model provides a better description of the Cr(VI) removal isotherm. The Langmuir model supposes that the adsorption sites on the adsorbent surface are uniformly distributed, energetically identical, and equally accessible for adsorption. In addition, the model considers that no interactions occur between the adsorbed molecules or ions. According to the Langmuir model, the maximum adsorption capacity was estimated to be ~112, 157 and 71 mg/g for ACav-400, ACcc-400 and ACsh-600, respectively. The maximum adsorption capacity appears to be linearly correlated with the specific surface area (Figure 14).

The adsorption capacity of the materials is much higher than other values that have been reported in literature for biomass derived AC which have been also produced via microwave assisted pyrolysis (Table 4). In addition, these values are among the highest for biomass derived activated carbons prepared using conventional pyrolysis [51,52].

Table 3. Adsorption capacity (q_max) in Cr(VI) for various activated carbons produced via microwave assisted pyrolysis activation, in literature.

Absorbent	Surface area (m2/g)	pH	qmax (mg/g)	Reference
ACArundo donax Linn	1332	5.3	15.07	[53]
ACLeucas aspera	916	2	30.3	[54]
ACpalm kernel	1256	3 (50oC)	19.1	[55]
ACEucalyptus sawdust	1429	3	11.5	[56]
ACpalm oil kernel shell	192.6	4.5	40.6	[57]
ACFicus carica	-	3	44.84	[58]
ACav-400	1120	3	112	This work
ACcc-400	1442	3	157	This work
ACsh-600	775	3	71	This work

Table 3. Adsorption capacity (q_max) in Cr(VI) for various activated carbons produced via microwave assisted pyrolysis activation, in literature.

Absorbent	Surface area (m2/g)	pH	qmax (mg/g)	Reference
ACArundo donax Linn	1332	5.3	15.07	[53]
ACLeucas aspera	916	2	30.3	[54]
ACpalm kernel	1256	3 (50oC)	19.1	[55]
ACEucalyptus sawdust	1429	3	11.5	[56]
ACpalm oil kernel shell	192.6	4.5	40.6	[57]
ACFicus carica	-	3	44.84	[58]
ACav-400	1120	3	112	This work
ACcc-400	1442	3	157	This work
ACsh-600	775	3	71	This work

The Gibbs free energy ΔG° was also calculated for the different initial concentrations using the equation 3 and the results are presented in Table 4.

ΔG⁰ = -RT ln(K)

(3)

where R is the gas constant (8.134 J/mol K), T the temperature in Kelvin and K a thermodynamic equilibrium constant equal with qe/Ce [59]. In all cases, Gibbs free energy has a negative value that confirms the spontaneous nature of the adsorption process. In addition, ΔG° values within the range of -20 to -80 kJ mol⁻¹ suggest a transition from physical adsorption to chemical adsorption mechanisms [60,61]. This implies that both physisorption and chemisorption contributed to the overall adsorption process.

Table 4. Gibbs free energy ΔG° for the different initial Cr(VI) concentrations.

Absorbent	Initial Cr(VI) (mg/g)	ΔG° (kJ/mol)
ACav-400	66.7	-27.789
	133.3	-23.981
	266.0	-20.106
	500.0	-17.823
ACcc-400	111.8	-23.972
	218.6	-21.920
	510.6	-18.905
ACsh-600	65.0	-23.658
	128.8	-20.378
	263.1	-18.042
	510.7	-16.395

Table 4. Gibbs free energy ΔG° for the different initial Cr(VI) concentrations.

Absorbent	Initial Cr(VI) (mg/g)	ΔG° (kJ/mol)
ACav-400	66.7	-27.789
	133.3	-23.981
	266.0	-20.106
	500.0	-17.823
ACcc-400	111.8	-23.972
	218.6	-21.920
	510.6	-18.905
ACsh-600	65.0	-23.658
	128.8	-20.378
	263.1	-18.042
	510.7	-16.395

4. Conclusions

In the present work, high surface area micro/mesoporous activated carbons were successfully synthesized from aloe vera industrial waste, corn cob agricultural residues, and soft slaughterhouse wastes (pork’s liver, heart, and lung). Using a rapid and energy-efficient microwave assisted pyrolysis process combined with ZnCl₂ chemical activation, the carbonization and activation time was drastically reduced to less than 20 minutes, demonstrating a sustainable route for biomass valorization.

The experimental findings demonstrate that the structural characteristics, surface chemistry, water dispersibility, and heavy metal adsorption efficiency are influenced by both the precursor type and the final pyrolysis temperature.

Nitrogen adsorption-desorption analysis revealed the formation of well-developed porous networks. Lower microwave pyrolysis temperatures (400 °C) were found to be optimal for agricultural biomass precursors, yielding specific surface areas of 1442 m²/g for corn cob and 1120 m²/g for aloe vera leaves. Conversely, soft slaughterhouse waste required higher pyrolysis temperature of 600 °C, following a microwave assisted hydrothermal carbonization (HT) pre-treatment, to efficiently unblock the porous network and achieve a significant surface area of 775 m²/g.

FT-IR, Raman, and XRD characterizations confirmed the progressive thermal decomposition of cellulose, hemicellulose, proteins, and lipids into predominantly amorphous aromatic carbon frameworks containing partially graphitized domains with low crystallinity.

Water dispersibility tests demonstrated that samples pyrolyzed at the highest temperature (600 °C) exhibited exceptional long-term stability (95-100% Dispersibility Index after 24 hours). This behavior indicates that higher microwave-assisted temperatures maybe promote particle size reduction, decreasing sedimentation rates despite the thermal reduction of hydrophilic surface functional groups.

The synthesized porous carbons displayed outstanding efficiency for the removal of toxic hexavalent chromium from aqueous solutions. The adsorption data perfectly fitted the Langmuir isotherm model, calculating maximum adsorption capacities of 157.23 mg/g, 112.35 mg/g, and 71.43 mg/g for corn cob, aloe vera, and slaughterhouse derived carbons, respectively. Thermodynamic evaluation yielded negative Gibbs free energy values, confirming the spontaneous nature of the adsorption process via combined physical and chemical uptake mechanisms.

Overall, this comparative study underscores the feasibility of the circular economy concept by upcycling different bio wastes into high performance materials. The novel two-step hybrid approach (microwave HT and microwave pyrolysis) developed for high-moisture soft animal structures successfully fills a major gap in slaughterhouse waste management, offering a highly competitive, green, and sustainable adsorbent for wastewater treatment.