Sustainable Production of Porous Activated Carbon from Hydrothermally Carbonized Jamoya Fruit Seeds and Its Potential for Adsorbing the Azo Dye Carmoisine B

Abstract

Porous carbon materials can serve as effective and versatile adsorbents in water pollution management. This study presents a cost-effective and environmentally friendly method to produce porous carbon materials (JFS-PC) by exploiting Jamoya fruit seeds (JFS) as a precursor using a hydrothermal carbonization (HTC) process. HTC is a thermochemical process for the conversion of high moisture content biomass into carbon-rich materials. The process is performed in a temperature range of 180–250 °C during which the biomass is submerged in water and heated in a sealed environment under autogenous pressure. The adsorbents obtained were explored using different techniques viz. XRD, FTIR, FE-SEM, and surface area analyses to evaluate their characteristics that are beneficial for the adsorption process. Surface area analysis revealed that the developed activated carbon exhibits appreciable surface area (440.8 m²g⁻¹), with a mean pore diameter of 3.97 nm. Activated carbon was successfully tested on the removal of an azo dye, Carmoisine B (CB), from water systems. Isothermal and kinetic evaluation demonstrated that the dye adsorption agrees well with the Langmuir (R² = 0.993) and pseudo-second-order (R² = 0.998) kinetics models. The experiments were designed to investigate the influence of adsorbate concentration (1 × 10⁻⁴ and 2 × 10⁻⁴ mol L⁻¹), collision time (5–300 min), pH (2–12) of the solution, and temperature (25–45 °C) on the adsorption of the selected dye. The results revealed that pH influences the adsorption capacity of CB and showed maximum adsorption between pH 2 and 5. Experimentally, the CB isotherms showed maximum adsorption capacities of 169.0 mg g⁻¹, at 45 °C. Mechanisms indicate that the surface charge of the adsorbent, and structures of the adsorbate play key roles in adsorption. Thermodynamic parameters revealed an endothermic and a physisorption process supported by Van’t Hoff calculations. The study indicates that the developed porous carbon (JFS-PC) can be successfully used for the removal of CB from water systems. It also highlights the use of an inexpensive and renewable precursor for the development of porous carbon materials.

1. Introduction

The agricultural, industrial, pharmaceutical, and building sectors are actively involved in the use of a huge amount of natural resources and consequently produce a large amount of biomass waste that impacts the environment. Inadequate management of biomass waste has attracted the attention of the scientific community to develop new strategies for managing it, in order to promote sustainability [1]. The valorization of biomass waste resources by converting them into porous carbon materials has emerged as an effective approach for their application in water treatment in the recent years [2,3,4].

Biomass including agricultural waste [5], apricot stones [6], eucalyptus barks [7], almond shells [8], bamboo sawdust [9], Tectona grandis sawdust, stubble, fibers from the Adansonia digitata L. tree, and bamboo flower [10], were used in numerous studies for the production of porous carbons, mainly activated carbons (ACs). The hydrothermal conversion of biomass waste into porous materials has become the research focus in recent years due to its energy efficiency, usage of low temperature, and it not needing pre-drying features as compared to the pyrolysis, gasification, and torrefaction techniques. Moreover, water is used as a solvent during HTC which diminishes the use of hazardous chemicals and is thereby termed as an environmentally friendly process. No hazardous emissions of gasses are produced during the HTC process, which represents a benefit over other methods. Additionally, the carbons produced via the hydrothermal route are rich in carboxylic, ketonic, and phenolic groups [10,11,12] which are useful for adsorption applications.

Azo dyes have been widely exploited in various industrial processes for coloring leather, textiles, and as food colorants [13,14,15]. Due to the low production cost of azo dyes, they are extensively used and their production reached 8.8 × 10³ metric tons in 2023, and is anticipated to reach 11 × 10³ metric tons by 2029 [16]. Among these dyes, Carmoisine B (CB), anionic in nature, is widely used for dyeing purposes and as a food colorant [17]. However, CB has been reported to cause neurological, allergic, carcinogenic, and hyperactivity disorders in humans [13] and therefore disposal of untreated water into the water streams poses a substantial threat to the environment, such as deteriorating water quality, affecting aquatic life, and potentially posing harm to humans. Various techniques for the removal of dyes from water have been discussed in the literature, viz. membrane filtration [18], biological degradation [19], ion exchange [20], reverse osmosis [21], electrochemical oxidation [22], photocatalytic degradation [23], and adsorption [24]. Among these techniques, adsorption, using diverse adsorbents including low-cost biomass [10,18], hydrochars, and activated carbons fulfills the effective removal of dyes due to its ease of operation, lower selectivity towards the pollutants, and simplistic design [25,26,27,28,29].

The demand for efficient carbonaceous materials has boomed across the globe due to their multi-dimensional applications including energy storage, soil remediation, pharmaceutical delivery, and water treatment. Still, some drawbacks are related to the use of expensive and non-renewable precursors and high-energy processes in commercial production. These challenges underscore the need for the development of carbonaceous materials using energy-saving approaches and low-cost precursors, preferably renewable, such as biomass waste.

This work addresses the removal of dyes from wastewater by a diversity of adsorbents in its current state. The scalability of the adsorbent production method, for large-scale applications, is a limitation that has to be addressed further. Despite these challenges, the potential applications of the developed porous carbon materials in several water treatment scenarios, particularly in addressing dye contamination, are promising and warrant further investigation.

A promising source of waste biomass for porous carbon production and its utilization in dye removal is JFS. The Jamoya tree (Syzygium cumini), a member of the Myrtaceae family, belongs to the Indian sub-continent; however, it is cultivated in the tropical and sub-tropical regions of the world [30]. Jamoya fruits are abundantly available in the Asian region, holding commercial value for culinary purposes [31]. Conversely, the fruit seeds lack utility or commercial value, serving as a source of solid biomass waste. To date, no study is available on the production of hydrothermally derived porous carbon from waste biomass (JFS). Being an inexpensive material, the use of Jamoya fruit seeds for their transformation into ACs contributes to waste management too. The utilization of HTC as an energy-efficient process contributes to energy conservation. In addition, the incorporation of Jamoya seeds in a circular economy framework will create environmental benefits through a reduction in waste and the promotion of sustainable practices [32]. Converting this underutilized biomass into value-added products has significant economic implications, further opening new prospects for renewable materials toward the broader goals of sustainability.

This study presents the production of hydrothermally derived JFS powder porous green carbon (JFS-PC) to remove CB from aqueous solutions, crucial to expand research in nutrition and the textile industry. An intensive characterization of the prepared materials was carried out to explore their textural and chemical properties. The role of functional groups, the substituents groups attached to the dyes, surface area, and surface charge of the adsorbent was highlighted in the adsorption. Kinetic models, isotherms, and thermodynamics parameters were performed and calculated to provide insights into the mechanism, sample nature, and features influencing the adsorption of the selected dyes.

2. Materials and Methods

2.1. Materials

In this study, Jamoya fruit seeds were collected locally. The seeds were washed several times to remove the adhered dust and impurities and dried in an oven at 60 °C for 24 h. The seeds were then ground and sieved to obtain a 10–30 mesh size and kept in a sealed plastic vial until its utilization. A Carmoisine B (CB) was purchased from Tokyo Chemical Industry (TCI), Tokyo, Japan. The NaOH and HCl used for the pH adjustments were supplied by Merck, Darmstadt, Germany. Double distilled water was used for the preparation of solutions throughout the work.

2.2. Preparation of Porous Green Activated Carbon

Jamoya fruit seeds (JFS) underwent hydrothermal carbonization, at a temperature of 230 °C, for 4 h in a PTFE lined hydrothermal reactor (Parr, Moline, Illinois, United States of America (USA)). The obtained JFS-based porous carbon, named JFS-HC, was stored in sealed plastic vials for future utilization. Porous green activated carbon was prepared by exposing 5 g of hydrothermally carbonized JFS (JFS-HC) to a heating rate of 5 °C min⁻¹ until 850 °C in a tube furnace (Carbolite Gero, Hope Valley, United Kingdom) under CO₂ gas flow (400 mL min⁻¹) for 5 h [33]. The furnace was then cooled to room temperature under N₂ flow (100 mL min⁻¹), giving rise to a porous activated carbon (JFS-PC).

2.3. Proximate Analysis and pH at the Point of Zero Charge Determination

The proximate analysis of the biomass feedstock, derived hydrochar, and activated carbon provides significant information regarding the amount of volatile matter, ash, and moisture content. A high moisture content may reduce adsorption efficiency as it can block adsorption sites and may even interfere with the interactions between adsorbent and adsorbate. Volatile matter gives an indication of organic compounds decomposition. High ash indicates the presence of inorganic impurities which reduce adsorption efficiency. These analyses ensure the development of efficient porous carbon materials for specific applications. The volatile matter, ash, and moisture contents of JFS, JFS-HC, and JFS-PC were determined in accordance with the ASTM standard methods D3175-07 [34], E1755-01 [35], and D4442-07 [36], respectively. The pH_pzc of the samples was determined according to a method described by Carrott et al. [37].

2.4. Instrumental Methods

The ultimate analysis of the samples was performed primarily to confirm the carbonaceous nature of the precursor, ensuring the development of porous materials with high carbon content, which is generally beneficial for adsorption purposes. The analysis was performed using a CHNS analyzer (Vario Micro, Elementar, New York, NY, USA). Residual concentrations of CB in the solutions were determined using UV-Vis spectrophotometry (UV-1800, Shimadzu, Kyoto, Japan) at 518 and 521 nm, respectively. JFS, JFS-HC, and JFS-PC were evaluated by Fourier transform infrared spectroscopy (Spectrum Two, Perkin Elmer, Shelton, CT, USA) to evaluate the group functionality on their surface. Analyses were performed in the spectral region of 4000 to 400 cm⁻¹. N₂ adsorption–desorption measurements of the samples were taken using surface area analyzers at 77K (NOVA II and Autosorb iQ, Quantachrome, Boynton Beach, FL, USA). BET analysis was undertaken to determine the surface area, whereas the pore volume and average pore diameter of the samples were evaluated using the BJH method. X-ray diffraction (XRD) analysis was performed using a powder X-ray diffractometer (DMax-2200, Rigaku, Tokyo, Japan) equipped with copper radiation (Cu-Kα) working at 40 kV/40 mA. A 2θ range of 5–80°, with a scanning step of 0.02°, was selected for the analysis. Scanning electron microscopy (SEM) to evaluate the morphology of the samples was carried out using an EVO 18 Special Edition, Zeiss, Oberkochen, Germany microscope with an acceleration voltage ranging from 5 to 15 kV.

2.5. Batch Dyes Adsorption Experiments

Adsorption experiments in a one-component system were carried out as follows: 0.01 g adsorbent was added to 10 mL of dyes solution, with varying concentrations at the natural pH of the dye (5.3). The suspensions were left under agitation to equilibrium in a water bath shaker (JSGW, Ambala, India) with a temperature control feature at 25 °C. After reaching equilibrium, the mixture was separated using a Whatman filter paper, and the residual concentration of dyes that remained in the solution was evaluated using a UV-Vis spectrophotometer. UV-Vis spectrophotometry was used to measure dye concentration, with no sample interference since the solutions were synthetic and contained only CB dye. The instrument was regularly calibrated and blank (a blank was prepared with the adsorbent and distillated water) corrections along with replicate measurements minimized deviations. No significant absorbance variations were observed, ensuring the precision of dye concentration estimates. The equilibrium adsorption capacity (q_e) was calculated using the following formula:

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{e}}}{\mathrm{m}}}V$$

(1)

2.6. Statistical Analysis

The experimental data for the adsorption models and kinetics of the adsorption process were analyzed using Microsoft Excel to determine the correlation coefficients (R²), which describe the goodness of fit. The experiments were performed in triplicate, and the R² values were used to assess the accuracy and reliability of the data.

3. Results and Discussion

3.1. Characterization of JFS, JFS-HC, and JFS-PC

The moisture, ash, and volatile matter contents, presented in Table 1, were found to be decreased upon hydrothermal carbonization and further activation process. The low moisture content could be ascribed to the carbonization of JFS at higher temperature while preparing the JFS-HC. The low volatile matter observed for the JFS-HC indicates the reduced particle density, which in turn is beneficial for applications in adsorption ultimate analysis (Table 1), showed a substantial increase in the carbon content of the JFS-PC, indicating that JFS can be utilized as an effective precursor for the production of porous carbons, mainly ACs. A comparison of ultimate analysis with other ACs available in the literature is provided in Table 1, which ensures the efficiency of developed porous carbon. The point of zero charge was determined in accordance with the method suggested by Carrott et al. [37] and was found to be 5.2, which reveals that the surface of JFS-PC is positively charged below pH 5.2, which will be beneficial for the interaction with contaminants anionic in nature, whereas it attracts cationic contaminants above pH 5.2. Therefore, it can be said that at pH < pHpzc, anionic pollutants are readily adsorbed by the developed ACs, whereas they favor the adsorption of cationic pollutants at pH > pHpzc.

Figure 1 depicts the FTIR spectra of JFS, JFS-HC, and JFS-PC. JFS showed a peak near 3400 cm⁻¹, ascribed to –OH stretching of hydroxyl groups. The band near 2900 cm⁻¹ represents the C–H stretching vibration in the sample. The peak at 1587 cm⁻¹ is attributed to the aromatic COOH and C–C groups in JFS. A peak at 1118 cm⁻¹ refers to C–O–C stretching, confirming the presence of hemicellulose and cellulose in JFS [38]. A peak near 700 cm⁻¹ represents the CH bending vibrations in JFS. JFS-HC showcases all the distinctive peaks of JFS with minor band shift and decrease in intensity; however, upon hydrothermal carbonization, few new peaks between 1600 and 1400 cm⁻¹ emerged which may be attributed to the increase in the oxygenated functional groups during HTC. Interestingly, JFS-PC showed a smaller number of peaks which may be a consequence of the submission to a high temperature during activation. The peaks at 3450 and 2930 are assigned, respectively, to the OH and C–H stretching in the sample. The peaks at 1635 and 605 cm⁻¹ are imputed to the aromatic COOH and C–H bending vibrations [39,40,41].

Figure 2 shows the XRD patterns of JFS-PC in comparison to JFS-HC and JFS samples. Two broad peaks at 2θ = 14° and 22.1°, attributed to (101) and (002) plane, were present in the case of JFS, which are the characteristic peaks of cellulose. A similar pattern was obtained with JFS-HC, indicating the retention of the original cellulosic structure of JFS despite undergoing HTC. In the case of JFS-PC, the characteristic peaks of cellulose become broader, indicating the formation of amorphous carbon material. Another strong peak seen at 2θ = 43° corresponds to (100) plane of graphite crystals, attributed to the interlayer condensation of graphite layers [42,43].

Figure 3 shows the SEM micrographs of JFS, JFS-HC, and JFS-PC, indicating that JFS has a smooth surface which converts to a rough surface with some pores and cracks upon HTC. Also, the formation of sphere-like structures was seen on the surface of JFS-HC (Figure 3b). The surface of the JFS-PC showed a porous surface with retention of the spherical structures (Figure 3c). The spherical structures indicate the hydrophobic nature of the prepared JFS-PC, which is beneficial for adsorption applications.

The N₂ adsorption–desorption isotherm shows a Type I curve with a short plateau ending at p/p° < 1, and an upward deviation is observed in Figure 4, revealing wider micropores or mesopores contributing to the pore filling. The presence of a H4 hysteresis loop indicates that the capillary condensation is responsible for pore filling, and slit-shaped pores have been formed [44]. This is further confirmed by the textural properties (Table 2), which showed that the average pore diameter of JFS-PC is 3.97 nm, indicating the development of mesopores or wider micropores, facilitating the molecular diffusion of dye to the internal sites. Moreover, JFS-PC exhibited a high surface area (440.8 m² g⁻¹) and pore volume (0.437 cm³ g⁻¹), both of which enhance the material’s adsorptive performance. The adsorbent with a higher surface area can enhance the removal potential of JFS-PC owing to its ample active surface sites. In addition, the mesoporous structure of JFS-PC could give rise to the capability of adsorbing CB. The surface area of JFS and JFS-HC samples was also analyzed, and data presented in Table 2 reveals that both materials possess very low surface area when compared with JFS-PC, hence full isotherm was not given.

3.2. Adsorption Studies

3.2.1. Effect of Contact Time, Initial Dye Concentration, and Solution pH

Figure 5 shows the adsorption equilibrium time profile for the removal of CB dye by JFS-PC at different initial dye concentrations (1 × 10⁻⁴ and 2 × 10⁻⁴ mol dm⁻³). The equilibrium time, representing the time span necessary to attain equilibrium, was found to be 180 min. At this point, the dye desorption from JFS-PC reaches dynamic equilibrium with the adsorption process. Moreover, the obtained curves level off between 150 and 180 min, indicating monolayer adsorption. It is noteworthy that adsorption occurs rapidly initially and then slowly attains equilibrium. This may be attributed to the available vacant sites on JFS-PC initially which become saturated over time, resulting in increased time period [45]. Figure 5 also reveals that the adsorption of CB increases as its initial concentration increases. It may be attributed to the increase in collisions between dye and JFS-PC molecules at higher concentrations. Moreover, an increase in the driving force of concentration gradient reduced the mass transfer resistance of CB between the liquid and the JFS-PC adsorbent [46].

Solution pH directly affects the adsorption of dyes and therefore it is crucial to investigate its impact on the process. Figure 6 displays the experimental results concerning the effect of the pH of the solution on the adsorption of CB by JFS-PC. It was seen that the extent of removal of the dye was highly influenced, with varying pH due to its effect on the binding sites of the adsorbent and the ionization of CB molecules. The adsorption capacity of JFS-PC decreases with an increase in pH from 2 to 12; however, a minor difference in the adsorption capacity was observed between pH 2 and 5. Since JFS-PC exhibits enhanced adsorption of CB under pH levels between 2 and 5, the natural pH (5.3) of the dye solution was selected as optimum pH for the experimental investigations. In the acidic pH range, the surface of the JFS-PC is positively charged (pHpzc—5.5), thereby attracting the negatively charged molecules of CB. The higher adsorption of the dye is also supported by the increased protonation. This promotes the dye’s affinity for active sites and accelerates the diffusion process. Meanwhile, in an alkaline pH range, protonation is reduced, and the hydroxyl ion concentration dominates in the solution. The OH¯ ions compete with the dye molecules for the adsorption sites, thereby retarding and reducing the adsorption of CB. The surface of the JFS-PC is also negatively charged in the alkaline pH range, resulting in reduced adsorption owing to the repulsion of the dye molecules.

3.2.2. Adsorption Isotherms and Thermodynamics Studies

The relationship between the equilibrium concentration of CB and the equilibrium adsorption capacity of JFS-PC at different temperatures (25, 35, and 45 °C) was investigated, and the experimental data were fitted and analyzed using the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D-R) models. The associated equations and the corresponding parameters for the above models are given in Table 3. As depicted in Figure 7 and Table 4, the adsorption capacity significantly increased and then gradually reached equilibrium with an increasing concentration of CB at 25, 35, and 45 °C. Figure 7 shows that CB adsorption increased with increasing temperature. As shown in Figure 8a–d and Table 4, the Langmuir adsorption capacity closely related to the experimental values obtained at 25, 35, and 45 °C (0.310, 0.324, and 0.334 mmol g⁻¹, respectively), and the co-relation coefficient (R² = 0.993 to 0.998) for the Langmuir model was higher than that of all the other applied models. This indicates that the JFS-PC surface has uniform adsorption energy for the modeled dye. The adjustment made with the DR equation was the one that gave a lower correlation coefficient. Consequently, the dye adsorption process can be best described through a monolayer adsorption mechanism. A comparison of the adsorption capacity and best-fitting isotherm model of JFS-PC with other activated carbons, available in the literature, is provided in Table 5.

Figure 9 presents the Gibb’s free energy (ΔG°), entropy (ΔS°), and enthalpy (ΔH°) values obtained through the calculation of the Van’t Hoff equation (Table 3), and the obtained parameters are given in Table 6. The positive ΔH° value and negative ΔG° value suggest that the adsorption of CB by JFS-PC is an endothermic and spontaneous process [47]. Meanwhile, the gradual increase in values of free energy with increasing temperature indicates that high temperatures favor the adsorption of CB by JFS-PC [48]. The positive ΔS° value indicates an increase in the degree of disorder at the solid–liquid interface during the adsorption process.

3.2.3. Mechanism

The surface of JFS-PC exhibits ample adsorption sites which facilitate the binding of CB. Point of zero charge (pHpzc) investigations of JFS-PC revealed that the JFS-PC’s surface exhibits a positive charge when subjected to acidic pH conditions. Consequently, this surface with positive charge has the ability to form electrostatic interactions with CB dye molecules [49], facilitating the binding to JFS-PC. The molecules of CB dye have a negatively charged structure due to the presence of sulphonate and hydroxyl groups in its structure which also enhances the extent of binding through electrostatic attraction of CB with the adsorbent below pH 5.2. The surface of JFS-PC has different functional groups, including carboxylic and –OH, which can serve as active sites for binding with SO₃⁻Na⁺ and N⁺ of the CB dye [50]. This binding is supported via different mechanisms viz. electrostatic interactions and H-bonding. In addition, the contaminants having a benzene ring generally have strong affinity to adsorb onto the surface of carbon rich materials owing to the interactions between C=C of JFS-PC and the benzene ring of the CB via $\pi$-$\pi$ interactions [51].

The porous character of JFS-PC is crucial for the adsorption of the dyes from water. The high surface area (440.8 m²g⁻¹) of JFS-PC and high porosity (0.437 cm³g⁻¹) allows the molecules of CB to adsorb on the JFS-PC surface. Therefore, the aforementioned mechanisms may be attributed to the higher adsorption of CB onto the JFS-PC surface. A pictorial representation for the possible mechanisms, responsible for the adsorption of CB onto JFS-PC, is shown in Figure 10.

3.2.4. Kinetic Investigations

To understand the adsorption mechanism of JFS-PC for CB, an in-depth analysis of adsorption kinetics was carried out and the details with corresponding parameters for the applied models (pseudo-first-order (PFO), pseudo-second-order (PSO), and Elovich models) are given in Table 3. As depicted in Figure 11a–c, and Table 7, the experimental data were fitted using PFO, PSO, and Elovich models. The fitting correlation coefficient (R² ≥ 0.9861) of the PSO model was higher than those obtained with the PFO and Elovich models (R² ≥ 0.7229), and the experimental adsorption values were similar to the calculated values. This reveals that the adsorption kinetics of CB on JFS-PC is in close agreement with the PSO model.

The involved mechanism and the rate governing step were further elucidated by applying the intraparticle diffusion model (IPD) to the experimental data. The intraparticle diffusion model in Figure 11d and Table 7 shows the multi-stage sorption of CB onto developed JFS-PC. The first stage describes the transfer of CB from the bulk of the solution to the JFS-PC’s surface, whereas the second stage denotes the penetration of CB molecules into the JFS-PC’s pores. In addition, the line in Figure 11d does not pass by the origin, affirming that some other mechanism along with IPD is involved in the CB adsorption by JFS-PC [52]. A comparison of kinetic studies of the current work with the results obtained on the use of other activated carbons for the removal of dyes is provided in Table 5. Maximum research works indicate that the pseudo-second-order model is the best fit to describe the kinetics, which is in line with the findings of our study.

Table 1. Proximate and ultimate analysis of JFS, JFS-HC, and JFS-PC and comparison with the other activated carbon available in the literature.

Proximate Analysis (%)	Moisture Content	Ash Content	Volatile Matter		Reference
JFS	68.32	6.56	2.89		This study
JFS-HC	58.11	6.23	2.43		This study
JFS-PC	-	0.51	–-		This study
Activated carbon from apricot stones	-	2.10	-		[6]
Activated carbon from almond shell	-	0.5	-		[8]
Activated carbon from sugarcane baggase	-	1.65	-		[53]
Activated carbon from thin walnut shell	-	5.01	-		[54]
Ultimate Analysis (%)
	C	H	N	S	Reference
JFS	44.12	7.12	0.92	0.87	This study
JFS-HC	62.34	4.32	0.04	0.02	This study
JFS-PC	84.21	1.20	0.02	0.03	This study
Activated carbon from almond shell	58.78	3.26	1.25	0.07	[8]
Activated carbon from apricot stones	82.76	-	-	-	[6]
Activated carbon from Phyllanthus emblica	70.23	2.98	0.64	0.41	[12]
Activated carbon from orange peel	83.9	1.9	0.4	-	[33]
Activated carbon from cupuassu	74.16	2.32	0.66	-	[50]
Activated carbon from Brazilian nut shell	71.30	2.65	1.03	-	[50]

Table 1. Proximate and ultimate analysis of JFS, JFS-HC, and JFS-PC and comparison with the other activated carbon available in the literature.

Proximate Analysis (%)	Moisture Content	Ash Content	Volatile Matter		Reference
JFS	68.32	6.56	2.89		This study
JFS-HC	58.11	6.23	2.43		This study
JFS-PC	-	0.51	–-		This study
Activated carbon from apricot stones	-	2.10	-		[6]
Activated carbon from almond shell	-	0.5	-		[8]
Activated carbon from sugarcane baggase	-	1.65	-		[53]
Activated carbon from thin walnut shell	-	5.01	-		[54]
Ultimate Analysis (%)
	C	H	N	S	Reference
JFS	44.12	7.12	0.92	0.87	This study
JFS-HC	62.34	4.32	0.04	0.02	This study
JFS-PC	84.21	1.20	0.02	0.03	This study
Activated carbon from almond shell	58.78	3.26	1.25	0.07	[8]
Activated carbon from apricot stones	82.76	-	-	-	[6]
Activated carbon from Phyllanthus emblica	70.23	2.98	0.64	0.41	[12]
Activated carbon from orange peel	83.9	1.9	0.4	-	[33]
Activated carbon from cupuassu	74.16	2.32	0.66	-	[50]
Activated carbon from Brazilian nut shell	71.30	2.65	1.03	-	[50]

Table 2. Textural properties of JFS, JFS-HC, JFS-PC.

Materials	Surface Area	Average Pore Size	Total Pore Volume
	(m2 g−1)	(nm)	(cm3 g−1)
JFS	2.40	6.05	0.004
JFS-HC	10	3.40	0.027
JFS-PC	440.8	3.97	0.437

Table 2. Textural properties of JFS, JFS-HC, JFS-PC.

Materials	Surface Area	Average Pore Size	Total Pore Volume
	(m2 g−1)	(nm)	(cm3 g−1)
JFS	2.40	6.05	0.004
JFS-HC	10	3.40	0.027
JFS-PC	440.8	3.97	0.437

Table 3. Isotherm models, kinetics, and thermodynamics equations.

Models	Mathematical Expression	Plot Axis	Evaluated Parameters	Description	Reference
Isotherm Models
Langmuir	1/qe = 1/qmax+1/qmaxbCe   RL = 1/1+bCo	1/qe vs.1/Ce	qmax and b	Ce; adsorbate concentration at equilibrium qe (mmol g−1); amount of pollutant adsorbed at Ce qmax (mmol g−1); Monolayer adsorption capacity of JFS-PC. b (L mol−1); Langmuir adsorption coefficient	[55]
Freundlich	log qe = logKf + (1/n) log Ce	logqe vs.logCe	Kf and n	Ce; adsorbate concentration at equilibrium qe(mmol g−1); amount adsorbed at Ce KF (mmol g−1) and n; Freundlich constant affiliated to the adsorption capacity and heterogeneity, respectively.	[56]
Temkin	qe = BT ln KT + BT lnCe BT = RT/ bT	qe vs. lnCe	KT and BT	BT; heat of adsorption bT (J mol−1); constant related to the heat of adsorption KT (L mg−1); Temkin constant	[57]
D-R	ln qe = lnqm− βDRε2 ε = RT ln(1+1/Ce)	ln qe vs. ε2	Qm and βDR	qm (mg g−1); D-R monolayer capacity βDR (mol2 kJ−2); constant related to the adsorption energy ε; Polanyi potential	[58]
Kinetic Models
PFO	log ( qe − qt ) = log qe − ${\displaystyle \frac{\mathrm{K}1}{2.303}}$ t	log (qe − qt) vs. t	Qe and k1	qe and qt (mmol g−1); adsorbed amount of CB at equilibrium and at time t, respectively. k1; PFO rate constant of adsorption	[59]
PSO	${\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}}$ = ${\displaystyle \frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}}$ + ${\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{e}}}}$ t	${\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}} \mathrm{v}\mathrm{s}.\mathrm{t}$	Qe and k2	qe and qt (mmol g−1); adsorbed amount of CB at equilibrium and at time t, respectively. k2; PSO rate constant of adsorption	[60]
Elovich	qt = 1/β ln(αβ) + 1/β ln(t)	qt vs. ln(t)	α and β	β (g mmol−1); desorption rate constant α (mmol g−1 min−1); initial adsorption rate constant qt (mmol g−1); adsorption capacity at time t	[61]
Intraparticle Diffusion Model	qt = Kid. √t + C	qt vs. √t	Kid and C	qt (mmol g−1); adsorption capacity at time t Kid; rate constant of intraparticle diffusion C; intercept of intra particle diffusion	[62]
Thermodynamics Equations
Gibb’s Free Energy	Δ G° = −R T ln (b)		ΔG°	T; temperature, b (L mol−1) and R are the Langmuir adsorption and universal gas constants, respectively, ΔG°; change in free energy of the adsorption system
Van’t Hoff	ln b = −ΔH°/RT + ΔS°/R	lnb vs.1/T	ΔH° and ΔS°	T; temperature, b (L mol−1), and R are the Langmuir adsorption and universal gas constants, respectively, ΔH°; change in enthalpy, ΔS°; change in entropy	[63]

Table 3. Isotherm models, kinetics, and thermodynamics equations.

Models	Mathematical Expression	Plot Axis	Evaluated Parameters	Description	Reference
Isotherm Models
Langmuir	1/qe = 1/qmax+1/qmaxbCe   RL = 1/1+bCo	1/qe vs.1/Ce	qmax and b	Ce; adsorbate concentration at equilibrium qe (mmol g−1); amount of pollutant adsorbed at Ce qmax (mmol g−1); Monolayer adsorption capacity of JFS-PC. b (L mol−1); Langmuir adsorption coefficient	[55]
Freundlich	log qe = logKf + (1/n) log Ce	logqe vs.logCe	Kf and n	Ce; adsorbate concentration at equilibrium qe(mmol g−1); amount adsorbed at Ce KF (mmol g−1) and n; Freundlich constant affiliated to the adsorption capacity and heterogeneity, respectively.	[56]
Temkin	qe = BT ln KT + BT lnCe BT = RT/ bT	qe vs. lnCe	KT and BT	BT; heat of adsorption bT (J mol−1); constant related to the heat of adsorption KT (L mg−1); Temkin constant	[57]
D-R	ln qe = lnqm− βDRε2 ε = RT ln(1+1/Ce)	ln qe vs. ε2	Qm and βDR	qm (mg g−1); D-R monolayer capacity βDR (mol2 kJ−2); constant related to the adsorption energy ε; Polanyi potential	[58]
Kinetic Models
PFO	log ( qe − qt ) = log qe − ${\displaystyle \frac{\mathrm{K}1}{2.303}}$ t	log (qe − qt) vs. t	Qe and k1	qe and qt (mmol g−1); adsorbed amount of CB at equilibrium and at time t, respectively. k1; PFO rate constant of adsorption	[59]
PSO	${\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}}$ = ${\displaystyle \frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}}$ + ${\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{e}}}}$ t	${\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}} \mathrm{v}\mathrm{s}.\mathrm{t}$	Qe and k2	qe and qt (mmol g−1); adsorbed amount of CB at equilibrium and at time t, respectively. k2; PSO rate constant of adsorption	[60]
Elovich	qt = 1/β ln(αβ) + 1/β ln(t)	qt vs. ln(t)	α and β	β (g mmol−1); desorption rate constant α (mmol g−1 min−1); initial adsorption rate constant qt (mmol g−1); adsorption capacity at time t	[61]
Intraparticle Diffusion Model	qt = Kid. √t + C	qt vs. √t	Kid and C	qt (mmol g−1); adsorption capacity at time t Kid; rate constant of intraparticle diffusion C; intercept of intra particle diffusion	[62]
Thermodynamics Equations
Gibb’s Free Energy	Δ G° = −R T ln (b)		ΔG°	T; temperature, b (L mol−1) and R are the Langmuir adsorption and universal gas constants, respectively, ΔG°; change in free energy of the adsorption system
Van’t Hoff	ln b = −ΔH°/RT + ΔS°/R	lnb vs.1/T	ΔH° and ΔS°	T; temperature, b (L mol−1), and R are the Langmuir adsorption and universal gas constants, respectively, ΔH°; change in enthalpy, ΔS°; change in entropy	[63]

Table 4. Parameters of Langmuir, Freundlich, D-R, and Temkin isotherm models at different temperatures for CB dye adsorption onto JFS-PC surface.

Adsorbate/ Temperature	Experimental			Langmuir			Freundlich			D-R			Temkin
	qexp (mg g−1)	qexp (mmolg−1)	qmax (mmol g−1)	qmax (mg g−1)	b (L mol−1)	R2	Kf (mmol g−1)	n	R2	qm (mg g−1)	E (kJ mol−1)	R2	At (L mg−1)	b (kJ mol−1)	R2
CB/25 °C	153	0.291	0.310	156	2.21 × 10−4	0.993	30.7	1.82	0.953	74.8	0.492	0.814	0.662	0.091	0.990
CB/35 °C	162	0.305	0.324	163	2.40 × 10−4	0.995	51.9	1.69	0.972	78.7	0.538	0.787	0.646	0.079	0.975
CB/45 °C	169	0.329	0.334	168	2.62 × 10−4	0.998	63.1	1.66	0.975	81.1	0.584	0.777	0.689	0.076	0.973

Table 4. Parameters of Langmuir, Freundlich, D-R, and Temkin isotherm models at different temperatures for CB dye adsorption onto JFS-PC surface.

Adsorbate/ Temperature	Experimental			Langmuir			Freundlich			D-R			Temkin
	qexp (mg g−1)	qexp (mmolg−1)	qmax (mmol g−1)	qmax (mg g−1)	b (L mol−1)	R2	Kf (mmol g−1)	n	R2	qm (mg g−1)	E (kJ mol−1)	R2	At (L mg−1)	b (kJ mol−1)	R2
CB/25 °C	153	0.291	0.310	156	2.21 × 10−4	0.993	30.7	1.82	0.953	74.8	0.492	0.814	0.662	0.091	0.990
CB/35 °C	162	0.305	0.324	163	2.40 × 10−4	0.995	51.9	1.69	0.972	78.7	0.538	0.787	0.646	0.079	0.975
CB/45 °C	169	0.329	0.334	168	2.62 × 10−4	0.998	63.1	1.66	0.975	81.1	0.584	0.777	0.689	0.076	0.973

Table 5. Comparison of JFS-PC with other ACs available in the literature for the removal of dyes.

Precursor	Activation Temperature (°C)	Surface Area (m2 g−1)	Target Adsorbate	Adsorption Capacity (mg g−1)	Isotherm Model Applied	Kinetic Model Applied with Rate Constant and Mechanism	Reference
Factory-rejected tea waste	800	368.92	Methylene Blue	487.4	Langmuir	Pseudo-second-order K2—0.214 g. mg−1min−1 Contact time—0.5–25 h Mechanism–film and intraparticle diffusion	[64]
Pine needle	-	-	Malachite Green	97.08	Langmuir	Pseudo-second-order K2—2.21 × 10−3 g. mg−1min−1 Contact time—180 min Mechanism–intraparticle Diffusion, film diffusion, and boundary layer control	[65]
Chickpea stem	600	455	Methylene Blue	887	Langmuir	-	[66]
Carnauba leaves	500	402.43	Rhodamine B	35.06	Freundlich	Pseudo-second-order K2—0.236 g. mg−1min−1 Contact time—120 min	[67]
Pine nut shells	500	296.01	Rhodamine B	29.62	Freundlich	Pseudo-second-order K2—0.299 g. mg−1min−1 Contact time—120 min	[67]
Apricot stone	700	359.40	Methylene Blue	36.68	Langmuir	Pseudo-second-order K2—0.721 g. mg−1min−1 Contact time—130 min Mechanism–chemisorption and intraparticle diffusion	[6]
Almond shell	450	733	2,4,6- trinitrophenol	74.03	Sips	Pseudo-second-order K2—0.016 g. mg−1h−1 Mechanism–chemical sorption	[8]
Jamoya fruit seeds	850	440.8	Carmoisine B	153	Langmuir	Pseudo-second-order	This study

Table 5. Comparison of JFS-PC with other ACs available in the literature for the removal of dyes.

Precursor	Activation Temperature (°C)	Surface Area (m2 g−1)	Target Adsorbate	Adsorption Capacity (mg g−1)	Isotherm Model Applied	Kinetic Model Applied with Rate Constant and Mechanism	Reference
Factory-rejected tea waste	800	368.92	Methylene Blue	487.4	Langmuir	Pseudo-second-order K2—0.214 g. mg−1min−1 Contact time—0.5–25 h Mechanism–film and intraparticle diffusion	[64]
Pine needle	-	-	Malachite Green	97.08	Langmuir	Pseudo-second-order K2—2.21 × 10−3 g. mg−1min−1 Contact time—180 min Mechanism–intraparticle Diffusion, film diffusion, and boundary layer control	[65]
Chickpea stem	600	455	Methylene Blue	887	Langmuir	-	[66]
Carnauba leaves	500	402.43	Rhodamine B	35.06	Freundlich	Pseudo-second-order K2—0.236 g. mg−1min−1 Contact time—120 min	[67]
Pine nut shells	500	296.01	Rhodamine B	29.62	Freundlich	Pseudo-second-order K2—0.299 g. mg−1min−1 Contact time—120 min	[67]
Apricot stone	700	359.40	Methylene Blue	36.68	Langmuir	Pseudo-second-order K2—0.721 g. mg−1min−1 Contact time—130 min Mechanism–chemisorption and intraparticle diffusion	[6]
Almond shell	450	733	2,4,6- trinitrophenol	74.03	Sips	Pseudo-second-order K2—0.016 g. mg−1h−1 Mechanism–chemical sorption	[8]
Jamoya fruit seeds	850	440.8	Carmoisine B	153	Langmuir	Pseudo-second-order	This study

Table 6. Thermodynamic parameters for CB dye adsorption onto JFS-PC surface.

Dye	Temperature (°C)	ΔG° (kJ mol−1)	ΔS° (J mol−1 K−1)	ΔH° (kJ mol−1)
CB	25 35 45	−24.8 −25.8 −26.9	105.6	6.69

Table 6. Thermodynamic parameters for CB dye adsorption onto JFS-PC surface.

Dye	Temperature (°C)	ΔG° (kJ mol−1)	ΔS° (J mol−1 K−1)	ΔH° (kJ mol−1)
CB	25 35 45	−24.8 −25.8 −26.9	105.6	6.69

Table 7. Parameters of PFO, PSO, Elovich, and Intraparticle diffusion model for CB dye adsorption onto JFS-PC surface.

Kinetic Model	Parameters	Conc. (2 × 10−4)
Experimental	qe (mmol g−1)	0.147
PFO	qe (cal) (mmol g−1)	0.0592
	K1 (min−1)	0.0128
	R2	0.855
PSO	qe (cal) (mmol g−1)	0.1526
	K2 (g mmol−1 min−1)	0.477
	R2	0.998
Elovich	α (mmol g−1min−1)	0.0273
	β (g mmol−1)	35.09
	R2	0.923
IPD	Kp1	0.022
	C1	0.013
	R2	0.975
	Kp2	0.0022
	C2	0.112
	R2	0.794

Table 7. Parameters of PFO, PSO, Elovich, and Intraparticle diffusion model for CB dye adsorption onto JFS-PC surface.

Kinetic Model	Parameters	Conc. (2 × 10−4)
Experimental	qe (mmol g−1)	0.147
PFO	qe (cal) (mmol g−1)	0.0592
	K1 (min−1)	0.0128
	R2	0.855
PSO	qe (cal) (mmol g−1)	0.1526
	K2 (g mmol−1 min−1)	0.477
	R2	0.998
Elovich	α (mmol g−1min−1)	0.0273
	β (g mmol−1)	35.09
	R2	0.923
IPD	Kp1	0.022
	C1	0.013
	R2	0.975
	Kp2	0.0022
	C2	0.112
	R2	0.794

4. Conclusions

In summary, an effective adsorbent from an inexpensive and sustainable precursor, Jamoya fruit seeds, using hydrothermal carbonization and physical activation has been successfully developed for efficiency and tested for the removal of CB dye from the aqueous phase. Characterization of the prepared adsorbent along with the precursor and hydrochar was performed. Elemental composition showed that the prepared adsorbent was rich in carbon. The textural and chemical characteristics determined by means of FTIR, XRD, SEM, and BET analysis indicate that JFS-PC can be efficiently used for adsorption applications. pH analysis showed that the adsorption was enhanced in an acidic pH range as the surface of the adsorbent holds positive charge below pH 5.2 (pHpzc—5.2). It achieves a maximum Langmuir adsorption capacity of 168 mg g⁻¹ for CB at 45 °C. Mechanism analysis revealed that pore diffusion, electrostatic interactions, H-bonding, and $\pi$-$\pi$ interactions governed the adsorption process. Thermodynamics findings revealed the endothermic and physisorption nature of the process. Kinetic adsorption of Carmoisine B followed a pseudo-second-order model. A hydrothermal route for the development of porous carbons was found to be energy efficient as mild temperature was used during the process. The fact that no hazardous gases are emitted during HTC makes it environmentally friendly compared to the conventional methods. This work not only provides an alternative for environmental remediation, through the treatment of dye-contaminated water, but it paves the way for the management of organic solid waste too. The potential of the developed material for the removal of different pollutants, such as emerging contaminants, pharmaceutical contaminants, and fertilizers, can be explored in future works.