Hydrothermally Produced Activated Carbon Impregnated with ZnO for the Adsorptive Removal of Toxic Pharmaceutical Contaminants from Aqueous Solution

Vardhan, Kilaru Harsha; Sree, Hrishitha; Kumar, P. Senthil; Rathi, B. Senthil

doi:https://doi.org/10.1155/2024/8434050

International Journal of Chemical Engineering

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Abstract Introduction Discussion Conclusion Data Availability Ethical Approval Conflicts of Interest Authors’ Contributions References Copyright Related Articles

Research Article | Open Access

Volume 2024 | Article ID 8434050 | https://doi.org/10.1155/2024/8434050

Hydrothermally Produced Activated Carbon Impregnated with ZnO for the Adsorptive Removal of Toxic Pharmaceutical Contaminants from Aqueous Solution

Kilaru Harsha Vardhan,^1,2Hrishitha Sree,^1,2P. Senthil Kumar ,³and B. Senthil Rathi^1,2

Academic Editor: Li Qingchao

Received07 Feb 2024

Revised07 Mar 2024

Accepted29 Apr 2024

Published15 May 2024

Abstract

This research explores the adsorption (AD) of diclofenac sodium (DS) onto a Hydrothermally produced activated carbon impregnated with ZnO (HTC-AC/ZnO) surface, considering various factors such as initial concentration (IC), adsorbent dose, contact time, and pH. The characterization of HTC-AC/ZnO was performed using X-ray diffractometer (XRD), scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FTIR), and nitrogen physisorption spectroscopy (BET). Tests were conducted with different adsorbent doses (0.5–4 g/L) at 303 K and various initial diclofenac concentrations (ranging from 50 mg/L to 250 mg/L) to observe their effects. Additionally, pH values were altered from 2 to 12 to study their influence on AD. Kinetic studies, thermodynamic studies, and AD isotherm models were examined. The Temkin isotherm model (TIM) was found to be the most accurate for DS-AD on HTC-AC/ZnO. For DS-AD on HTC-AC/ZnO, pseudo-first-order models (PFOM), intraparticle diffusion model (IPDM), and pseudo-second-order models (PSOM) were applied, with a correlation coefficient of 0.945, indicating a good fit for PFOM. The kinetics suggested rapid adsorption. Notably, the HTC-AC/ZnO composite exhibited consistent AD characteristics across four consecutive cycles, with a removal efficiency exceeding 99.38%. This suggests that HTC-AC/ZnO is an appropriate and economically viable adsorbent for the elimination of DS from water-based solutions. The investigation provides compelling evidence that HTC-AC/ZnO is a viable adsorbent for the effective elimination of DS from water sources.

1. Introduction

In even the most developed nations, emerging contaminants (ECs) represent uncontrolled substances that pose risks to human well-being and ecology [1]. La Farré et al. [2] define these contaminants as substances that encompass chemical pesticides, flame retardants, plasticizers, food additives, medications, personal care products, wood preservatives, laundry detergents, surfactants, disinfectants, and hormonal substances, both organic and artificial. Among these substances, pharmaceutical contaminants (PCs) are the most frequently encountered heterocyclic aromatic products [3]. These PC substances are typically found in small quantities (ranging from parts per billion to parts per million) in wastewater treatment (WWT) facilities [4]. Primary sources of PC pollutants include wastewater from healthcare facilities, communities, and pharmaceutical companies [5]. Wang et al. [6] highlight the potential for pharmaceutical contaminant (PC) pollution to disrupt aquatic ecology and pose risks to human health. Consequently, the development of efficient techniques to control their levels in the atmosphere is imperative [7].

One of the most frequently encountered pharmaceutical contaminants is NSAID or nonsteroidal anti-inflammatory drugs. These medications are widely employed in treating conditions such as gout, arthritis, migraines, and other inflammatory disorders by mitigating discomfort and inflammation [8]. Among NSAIDs, DS stands out as the most commonly used one—an analgesic employed in the treatment of joint, muscle, and tendon disorders, primarily reducing pain, swelling, and inflammation [9]. However, DS is not entirely metabolized following ingestion. Due to limitations in current sewage treatment facilities, approximately 15% of its active ingredient remains unremoved, ultimately being expelled into the atmosphere [10]. Consequently, DS has been detected in both ground and drinking waters. Its presence in water systems has deleterious effects: it disrupts the endocrine glands of marine and terrestrial organisms, interferes with the metabolic pathways of microbes in soil and water, and impedes sunlight penetration into the water [11]. Lonappan et al. [12] note that the overuse of DS causes host organisms to become resistant.

Several techniques such as chemical oxidation [13], biodegradation [14], photo-degradation [15], nano-filtration [16], reverse osmosis [17], and anaerobic digestion (AD) [18] are available for the elimination of DS from WW. However, most of these methods come with certain drawbacks, such as energy consumption and the generation of toxic byproducts. Among these, AD stands out as a technique that is user-friendly, cost-effective, swift, efficient, and devoid of harmful byproducts [19, 20]. Alessandretti et al. [21] have extensively documented the use of various adsorbents, including carbon nanotubes, activated carbon (AC) [22], metal-organic frameworks [23], metals, metal oxides, nanocomposites [24], polymeric nanocomposites, microalgae strains, pyrite, clay minerals, and resins.

In the elimination of DS, carbon-based adsorbents such as AC derived from waste biomass have been utilized. AC stands out as one of the most successful adsorbents due to its widespread application, attributed to its high pore volume, significant permeability, developed internal porous structure, and abundance of surface functional groups [25]. The AD capacity of AC can be notably enhanced when impregnated with metals or metal oxides, owing to its extensive surface area. Gusain et al. [26] note its easy renewability and cost-effectiveness. Additionally, hydrothermal carbonization (HTC)—a thermochemical conversion method involving biowaste at room temperature and moderate pressure—has been highlighted by Zhang et al. [27]. This process produces biochar enriched with numerous oxygenated functional groups that aid in retaining incoming contaminants [28]. Moreover, the advantage of hydrothermal carbonization (HTC) lies in its minimal energy consumption, as it eliminates the necessity for pretreatment of biomass [29]. Despite the potential for increased energy usage in treating biomass over extended durations through HTC, the resultant yield is notably higher compared to other methods [30, 31].

In the DS-AD process on the activated carbon (AC) surface of sycamore balls, various factors were investigated, including pH, temperature, and adsorbent dosage. The adsorbent dose ranged from 2.5 mg/50 mL to 30 mg/50 mL at 25°C, while the IC of DS ranges from 10 to 50 mg/L. To assess the impact of AD temperature while maintaining a constant adsorbent dose (10 mg/50 mL), temperatures ranging from 25°C to 45°C were examined. The study by Avcu et al. [32] concluded that AC from sycamore balls serves as the best and cheapest adsorbent for DS-AD. Another approach involves saturating the surface of palm coconut AC with iron compound nanoparticles, utilizing pomegranate and leaves of Moringa oleifera extracts in an environmentally friendly preparation method to assess its AD capacity for DS [33].

Moreover, coconut shell-derived AC demonstrated a remarkable elimination rate of over 98% for DS under specific conditions, with an adsorbent dosage of Rads = 0.10 and a contact duration of 15 minutes [34], lasting up to 90 minutes. Additionally, AC derived from citrus industrial processing waste, through phosphoric acid activation, has been utilized for decontaminating water tainted with synthetic DS. Under ideal working conditions, the maximum DS adsorption capacity of AC was found to be 185.19 mg/g [35]. The primary aim of this research is to employ hydrothermal carbonization (HTC) as a cost-effective method to produce carbon from bamboo waste. The carbon obtained from HTC was then combined with ZnO synthesized via a sonochemical method. In laboratory settings, the resulting HTC-derived AC/ZnO composite was evaluated as a successful adsorbent for removing DS from water under ideal conditions. The incorporation of ZnO into the AC enhanced the microstructure, thereby increasing active sites and augmenting the adsorptive removal of harmful substances.

2. Experimental Protocol

2.1. Substances and Preparations

Analytical-grade materials were procured for all compounds, including concentrated H₂SO₄, Zn (NO₃)₂, and NaOH. A stock solution of 100 mg/L was prepared by dissolving 0.010 g of DS in 0.1 L of double-distilled water. Table 1 provides the characteristics of diclofenac sodium, and Figure 1 illustrates the structure of DS. The bamboo debris used as the carbon source in this study was sourced from discarded bamboo furniture obtained from a nearby warehouse in Chennai, India. Subsequently, the required solutions with the desired intensity were obtained from the base solution using the dilution law.

2.2. Preparation of Bamboo-Derived AC/ZnO

The bamboo underwent a thorough cleaning process, being rinsed three to four times with deionized water to ensure cleanliness. To eliminate moisture content, the bamboo was subjected to shade drying for three days. Subsequently, a mixer grinder was employed to pulverize the dry bamboo debris. The resulting mixture was then sieved using a shaking sieve with a 160 μm mesh size range to achieve various particle sizes. The dried bamboo waste was then subjected to heat treatment in an HTC autoclave maintained at 250°C for approximately three hours. After cooling, the liquid portion was drained, leaving behind the char resulting from the HTC process. This char underwent alkali activation by immersion in a NaOH solution for 8-9 consecutive hours. Neutralization was achieved through water washing or rinsing. The activated carbon (AC) was dried in a hot air oven until all moisture was completely removed.

For the creation of a 0.1N NaOH solution, 14.875 g of Zn (NO₃)₂ was mixed in 0.05 L of water to achieve a 1 M solution. This solution was kept in a 250 mL beaker and left at room temperature. While continuously stirring at 120 rpm using a magnetic stirrer, the prepared NaOH solution was slowly added drop by drop using a burette. This gradual addition resulted in a creamy white solution. Upon settling, a milky white precipitate of Zn(OH)₂ formed. The resulting precipitate was found to be alkaline. To neutralize it, the precipitate was rinsed with water three or four times. The pH was measured and recorded as 7.2. The collected Zn (OH)₂ was kept in a crucible and subjected to heating in a furnace at 500°C for approximately three hours, transforming it into ZnO. Subsequently, the resulting sludge was dried for 12 hr in a hot air oven.

During sonication, ZnO settled within the cracks of the activated carbon (AC) structure, causing surface fractures that enhanced its adsorption capabilities. This increases the removal percentage of DS in the AD process. In the experimental setup, a mixture comprising 50 mL of ethyl alcohol, 0.05 g of HTC-AC, and ZnO was subjected to sonication for 30 minutes. After settling, the dried material obtained from this process was identified as HTC-AC/ZnO (Figure 2).

2.3. Characterization of HTC-AC/ZnO

FTIR spectroscopy (Nicolet iS50, Thermo Fisher Scientific) was employed to determine the existence of surface functional groups on the produced materials and the mechanistic interaction of HTC-AC. The HTC-AC/ZnO surface morphology was analyzed using an SEM operating at 10 kV (JEOL/EO—JCM 6000 plus) before the materials were gold flashed. The PAN-ALYTICAL X’Pert PRO X-ray diffractometer was used to capture XRD patterns. At a scanning rate of 10 min⁻¹, Cu Kα (λ = 1.540 Å) radiation was used in the 2ϴ range of 10–80°. Using a Micromeritics 2000 device, N₂ adsorption at −196°C was measured to calculate the BET surface area. Then, using the cumulative nitrogen adsorption-desorption as a basis, the pore volumes were determined utilizing the BJH (Barrett–Joyner–Halenda) technique.

2.4. Adsorption Studies for the Removal of DS by AC/ZnO

In a typical AD procedure, measured quantities of HTC-AC/ZnO (0.5–4 g/L) were mixed with successive 100 mL glass vials containing 50 mL of DS solutions with IC ranging from 50 mg/L to 250 mg/L. These mixtures were agitated at ambient temperature in a thermostatic shaker incubator at 120 rpm. The filtrate was filtered using a 0.22 μm nylon membrane syringe filter and collected in a syringe for subsequent quantitative analysis. Quantitative testing of DS was conducted utilizing a UV double-beam spectrophotometer set at its largest wavelength of 275 nm. Each study was run three times, and analysis was done using the mean results. The following formulas were employed to calculate the percentage of DS removed and the quantities of DS adsorbed per unit mass of HTC-AC/ZnO:where is the IC of DS (in mg/L) and is the concentration of DS at any time “t” (in mg/L).

The quantities of DS adsorbed per unit mass of HTC-AC/ZnO at any time “t” ( in mg/g) and at equilibrium ( in mg/g) were determined by utilizing the expression:where V is the volume of the solution (in L), m is the mass of the HTC-AC/ZnO (in g), and is the equilibrium concentration of DS in the solution (in mg/L).

2.5. Thermodynamic Studies

To characterize the thermodynamic nature of the AD process of DS, thermodynamic activation parameters involving ΔH° (enthalpy), ΔS° (entropy), and ΔG° (Gibbs free energy) were computed. ΔG° was determined using equation (5).

2.6. Adsorption Isotherm Studies

The distribution of the adsorbate among the HTC-AC/ZnO and DS solution upon reaching adsorption equilibrium can be elucidated using AD isotherms, which play a crucial role. In this research, three adsorption isotherm models—the Freundlich isotherm model (FIM), Temkin isotherm model (TIM), and Langmuir isotherm model (LIM)—were employed to characterize the DS-AD process and delineate the distribution of DS in aqueous solution and on the surface of HTC-AC/ZnO during AD equilibrium. Table 2 presents the nonlinear mathematical formulas used to determine the adsorption isotherm parameters.

2.7. Adsorption Kinetic Studies

A high AD rate and substantial AD capacity are essential characteristics for the practical industrial uses of an adsorbent in WWT facilities. The kinetic study of DS-AD on HTC-AC/ZnO was conducted over a duration ranging from 0 to 210 minutes. Models including pseudo-first order model (PFOM), pseudo-second order model (PSOM), and intraparticle diffusion model (IPDM) were studied to fit the data in AD kinetics from DS-AD tests. The formulas for these models are presented in Table 3.

3. Outcome and Discussion

3.1. Adsorbent Characterization

XRD analysis was conducted to find the crystalline structures of HTC-AC, ZnO, and HTC-AC/ZnO, as illustrated in Figures 3(a)–3(c), respectively. The AC derived from HTC exhibits a broad peak between 20 and 30°, exactly around θ = 23.53°, without a distinct graphitic structure at its summit. This XRD pattern at 23.53° signifies the amorphous state of carbon [42]. Figure 3(a) indicates that the peak intensity at 23.53° is higher than that at 42.59°. The heightened peak intensity at 23.53° is suggestive of a stack of parallel aromatic layers, while the lower intensity at 42.59° corresponds to the regular arrangement of a single-layer segment. ZnO displayed distinct diffraction peaks at various angles, notably at 2θ = 33.23°, 34.52°, 36.43°, 47.62°, 56.71°, 63.03°, and 68.12°, with the most prominent peak observed at 36.12° as shown in Figure 3(b). This pattern confirms the absence of any visible impurities or multiple phases in ZnO, indicating its exceptional purity [43]. In contrast, the XRD pattern of AC-ZnO exhibited more peaks in comparison to ZnO and AC. Notable peaks were observed at 2θ = 24.03°, 31.70°, 34.51°, 36.38°, 47.49°, 56.72°, 62.98°, 67.99°, 72.85°, and 77.37°. Two more prominent peaks formed at 2θ = 24.03° and 2θ = 36.98° (seen in Figure 3(c)), with the peak at 2θ = 24.03° indicating the presence of AC and the peak at 2θ = 36.98° indicating the presence of ZnO [44]. This suggests the incorporation of ZnO into the outer layer of AC, leading to these distinctive peaks. The hexagonal wurtzite structure is evident in the HTC-AC/ZnO composite.

(a)

(b)

(c)

A nitrogen AD isotherm at 77 K was conducted to comprehensively assess the porous characteristics of the synthesized HTC-AC/ZnO material. The resulting isotherm, depicted in Figure 4(a), includes the BJH pore-size distribution plot (Figure 4(b)). HTC-AC/ZnO exhibited a combined type II and IV AD-desorption isotherm, indicating its integrated nature [45]. The hysteresis loop observed in the isotherm, characterized by a broad condensation step in the medium-to-high region and a flatter section at the center (relative pressure, p/P° = 0.1–1.0), strongly shows the prevalence of macropores and mesopores in HTC-AC/ZnO. The BJH pore-size distribution plot (Figure 4(b)) further corroborated this observation, revealing the presence of mesopores ranging in size from 2 to 50 nm and micropores with a mean size of roughly 2.848 nm. The BET surface area of HTC-AC/ZnO was measured at 89.124 m²/g. Analysis revealed a micropore volume of 0.008 cm³/g and a larger mesopore volume of 0.145 cm³/g, resulting in a total pore volume of 0.153 cm³/g. This indicates a prevalence of mesoporous structure over microporous structure, supported by the pore volume and average pore diameter data. The presence of a higher proportion of mesopores compared to micropores suggests favorable conditions for high diffusion rates and effective capture of DS molecules upon interaction with the active AD sites on the outer layer of HTC-AC/ZnO [46].

(a)

(b)

Figure 5 displays the resulting FTIR spectrum obtained from the HTC-AC sample, aiming to identify the surface’s functional groups. The carbon framework of bamboo comprises intricate blends of polysaccharides, notably rich in O-H and C=O groups found in hemicellulose, cellulose, and lignin. After heat treatment, the resulting biochar underwent infrared spectrum analysis spanning 400 to 4000 cm⁻¹ to discern structural changes. The spectra exhibit a widened intensity at 3564 cm⁻¹, corresponding to OH stretching vibrations, indicative of decreased carbonization [47]. Additionally, a brief peak at 2788 cm⁻¹ is attributed to the stretching vibrations of the carbonyl aliphatic C-H group. The identification of AC formation through aromatization suggests the dehydrogenation of existing cyclic compounds, resulting in the existence of aliphatic C groups in the material. The synthesis of aromatic rings during the hydrothermal process is evidenced by the strength of aromatic ring stretching vibrations, notably observed at around 1521 cm⁻¹ [48]. Additionally, the FTIR spectrum exhibited a characteristic fingerprint region with a broad and shallow intensity at 1069 cm⁻¹. This intensity corresponds to carbonyl groups, which are attributed to the formation of C=O groups originating from the dehydrogenation of one or more saccharide molecules.

(a)

(b)

(c)

Figure 6 exhibits the structure and morphology of HTC-AC impregnated with ZnO. Hydrochars produced through hydrothermal carbonization (HTC) are known to display sphere-shaped formations, suggesting the carbon’s spherical structure or the presence of smooth-surfaced sphere aggregates. These spheres were observed to have diameters ranging from a few microns. Furthermore, the aggregation of these spheres into clusters suggests the accumulation of hydrolysis products derived from bamboo polymers [49]. These products are likely composed of polyaromatic structures, featuring hydrophobic (less reactive) groups concentrated in the core and hydrophilic (more reactive) groups on the surface. Key processes such as polymer aromatization and dehydration contribute significantly to the creation of polyaromatic structures within the HTC-AC/ZnO. The notable characteristics of this material, including its substantial pore volume (0.153 cm³/g) and extensive surface area (89.124 m²/g), are attributed to the existence of active sites. These features play an important role in facilitating the effective elimination of DS from contaminated effluents.

3.2. Adsorption Studies

The AD capacity of the obtained HTC-AC/ZnO material was systematically assessed in a batch process for the elimination of DS from contaminated effluents. DS was selected as the model PC due to its widespread use as an NSAID, its ubiquitous presence in global wastewater systems, its significant toxicity towards various organisms, high water solubility, and prolonged persistence in aquatic environments [12].

3.2.1. Effect of HTC-AC/ZnO Dosage on DS Removal

Several studies were conducted using different quantities of HTC-AC/ZnO, ranging from 0.5 to 4 g/L, under specific conditions: pH 2, 50 mg/L DS initial concentration, and a 30-minute adsorption duration at 303 K. The variation in the adsorption capacity and efficiency of HTC-AC/ZnO as a function of its dose is depicted in Figure 7. The figure illustrates a notable increase in adsorption efficiency as the adsorbent concentration reaches 4 g/L. This rise in efficiency is attributed to the higher availability of accessible binding active sites, indicating an improved adsorption capacity at the higher adsorbent concentration. The observed increase in adsorption efficiency is likely a result of the rise in surface active sites of HTC-AC/ZnO. These findings suggest that the optimal adsorbent concentration for our experimental conditions was determined to be 4 g/L of HTC-AC/ZnO, which will be utilized in subsequent research [50].

3.2.2. Effect of DS-IC on DS Removal

Multiple tests were conducted using different initial concentrations (IC) of DS, ranging from 50 to 250 mg/L, under specific conditions: pH 2, HTC-AC/ZnO dosage of 4 g/L, and a 30-minute adsorption duration at 303 K. Figure 8 illustrates the variation in the adsorption capacity and efficiency of HTC-AC/ZnO in relation to DS’s initial concentration (IC). The observed decrease in adsorption efficiency with increasing DS concentration can be attributed to competitive adsorption among sorbate molecules. As DS concentration in the solution increases, more DS ions can access the available surface sites of HTC-AC/ZnO. The maximum removal percentage was 99.38%, obtained at 50 mg/L. Consequently, the higher concentration drives a stronger force to overcome the mass transfer barrier for DS between the DS solution and the HTC-AC/ZnO, resulting in a decrease in the elimination of DS [51].

3.2.3. Impact of Contact Time on DS Removal

The removal of DS from a water-based solution was assessed at varying contact times and initial concentrations (ICs) of DS, ranging from 50 to 250 mg/L, at pH 2 and 303 K, as depicted in Figure 9. The results obtained from the adsorption process of DS on HTC-AC/ZnO indicate that prolonged contact durations enhance the adsorption mechanism. The adsorption capacity (AD) of DS onto HTC-AC/ZnO demonstrated an accelerated phase during the initial half hour, reaching optimal values for five distinct ICs of DS. Subsequent increases in contact time marginally affected the proportion of DS removal [52, 53].

The larger surface area of HTC-AC/ZnO initially contributed to its enhanced capacity for absorbing DS ions. The rate of movement of adsorbed DS ions from the outermost layer to the inner part of HTC-AC/ZnO governs the speed of adsorption once DS forms a monolayer and saturates the potential of HTC-AC/ZnO. This process is crucial as only a small number of efficient adsorption sites are available, allowing each binding site to accommodate a single ion within this monolayer. As the surface reaches saturation, there is competition for a finite number of binding sites. This competition leads to an initial rapid adsorption of DS onto the HTC-AC/ZnO surface, followed by a slower rate. The existing DS in the sample enhances the competition for these binding sites, contributing to the slower adsorption rate later in the process.

3.2.4. Impact of pH on DS Removal

Figure 10 illustrates the impact of the starting solution’s pH on DS removal. To adjust the initial pH levels, 0.1 M solutions of HCl or NaOH were added. The percentage clearance of DS decreased from 99.38% at pH 2 to 18.39% at pH 12, as evident from the figure. The fluctuation in adsorption efficiency with pH can be attributed to the electrostatic attraction between HTC-AC/ZnO and DS. As pH values increase, the outer layers of HTC-AC/ZnO gradually lose positive charges, transitioning to a more negative state.

At low pH, the positive charge on the HTC-AC/ZnO surface was high, leading to a stronger interaction between the HTC-AC/ZnO surface and the π-electron cloud of the DS molecule. At higher pH, the positive charge on the HTC-AC/ZnO surface decreases, and OH- ions also compete with the DS molecules for adsorption on the active sites, resulting in a reduction in the adsorption efficiency of HTC-AC/ZnO. At neutral pH, only a slight influence of H+ or OH- ions was observed on the HTC-AC/ZnO surface; therefore, almost constant adsorption of DS was observed [54].

The significant electrostatic interaction between anions and positively charged sites (H+) on HTC-AC/ZnO primarily accounts for its substantial adsorption capacity at low pH levels. However, at alkaline pH, the abundance of OH- ions introduces competition for the same adsorption sites, potentially reducing the adsorption of DS ions [55].

3.2.5. Effect of Temperature on DS Removal

Figure 11 illustrates the correlation between temperature variations and the efficiency of DS removal by HTC-AC/ZnO. The removal efficiency of DS notably decreases with increasing temperatures, showing reductions from 97.38% to 92.41% at 50 mg/L, 97.35% to 92.36% at 100 mg/L, 91.73% to 85.33% at 150 mg/L, 85.27% to 79.22% at 200 mg/L, and 73.31% to 67.21% at 250 mg/L. This observation supports the conclusion of an exothermic nature in the DS adsorption process, indicating that adsorbing DS at lower temperatures is advantageous. Additionally, this characteristic is corroborated by the negative enthalpy energy (ΔH°) values [56].

3.3. Thermodynamic Studies

The plotting of log Kc against 1/T resulted in a linear relationship (Figure 12). Enthalpy change and entropy change parameters were derived from the gradient and intercept of the graph, respectively, and subsequently added to Table 4 alongside the remaining parameters. The adsorption-desorption process is evidently exothermic, indicated by ΔH° <0 in the table. Moreover, ΔS° <0 suggests a decrease in unpredictability at the HTC-AC/ZnO/DS solution interface. The adsorption process appears both spontaneous and feasible, as evidenced by the negative values of ΔG°. Additionally, a decrease in ΔG° with an increase in temperature suggests a strong preference for the AD of DS. Moreover, the ΔG° values being less than −40 kJ/mol indicate that a physisorption process might be responsible for the interactions occurring between the surface of HTC-AC/ZnO and DS.

3.4. Adsorption Isotherms

While maintaining constant values for pH, adsorbent dose (2 and 4 g/L), and contact duration (30 minutes) and varying IC-DS, we examined its impact on the sorption process. Figure 13 presents a schematic representation illustrating the change in adsorption (AD) capacity and efficiency concerning IC-DS. To analyze DS equilibrium AD onto HTC-AC/ZnO, we employed LIM, FIM, and TIM. The computed parameters for these models using Origin pro 8.5 are shown in Table 5. The correlation coefficients () obtained from the utilized isotherm models generally fall within the range of 0.88 to 0.99. Notably, TIM exhibits the highest value, suggesting a consistent interaction among adsorbate molecules on the surface. Furthermore, this model indicates a linear decrease in the heat of AD corresponding to surface coverage, highlighting the presence of heterogeneous sites with varying AD energies.

3.5. Adsorption Kinetics

Adsorption studies were conducted at various time intervals while keeping the other process variables constant at optimal levels (pH around 2, adsorbent dose maintained at 4 g/L, and initial concentrations of DS between 50 and 250 mg/L) to investigate the influence of contact duration on the adsorption (AD) capacity. The analysis of Figures 14 and 15 using the PFOM, PSOM, and IPDM was plotted using Origin pro 8.5, aimed to determine the kinetic order and AD mechanism of DS on HTC-AC/ZnO in this research. The calculated rate constants, maximum quantity adsorbed, and R² from the relevant models are listed in Tables 6 and 7. Notably, the PFOM exhibited an exceptionally high R² value, nearly reaching unity. Moreover, its calculated q_e value of 25.69 mg/g showed close agreement with the experimental value of 25.21 mg/g. These findings strongly suggest that the PFOM accurately represents the kinetics of DS adsorption by HTC-AC/ZnO and stands out as the most suitable kinetic model for depicting this process. Table 8 gives the comparison of DS removal by various adsorbents.

4. Cost Analysis

For the treatment of commercial water and wastewater, the adsorbent’s cost-effectiveness is a crucial component. Material, labour, and operating costs were added to additional overhead fees to get the total cost of the synthesized HTC-AC/ZnO. The primary expenses for raw materials included bamboo, H₂SO₄, Zn (NO₃)₂, NaOH, and DS. On the other hand, magnetic stirring, autoclave synthesis, centrifugal washing, and drying incurred power expenditures. As shown in Table 9, the cost analysis of synthesized HTC-AC/ZnO powder was computed. According to the cost analysis results, the cost of HTC-AC/ZnO manufactured in a laboratory was Rs. 193.3 g⁻¹.

5. Conclusion

In this study, we synthesized low-cost porous activated carbon (HTC-AC/ZnO) using bamboo as a carbon source via hydrothermal carbonization (HTC) and subsequently impregnated it with ZnO. Our aim was to determine the adsorptive efficiency of this resulting material in eliminating DS from polluted water. HTC-AC/ZnO exhibited a high specific surface area and total pore volume, measuring 89.124 m²/g and 0.153 cm³/g, respectively, indicating an organized micro/mesoporous morphology. The adsorption isotherm analyses revealed a substantial binding capacity of 88.15 mg/g for the adsorption of DS onto HTC-AC/ZnO, suggesting that the adsorption behavior can be accurately interpreted using the TIM model. Moreover, the adsorption kinetics exhibited rapid adsorption rates, aligning well with the PFOM. Notably, the substantial removal efficiency of 99.38% of DS from the aqueous solution using HTC-AC/ZnO is remarkable. With its straightforward production process, exceptional adsorption capacity, and swift adsorption rates, HTC-AC/ZnO emerges as a praiseworthy adsorbent for efficiently purifying DS-contaminated wastewater.

Data Availability

Data are available on request.

Ethical Approval

This article does not contain any studies involving animal or human participants performed by any of the authors.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Authors’ Contributions

Kilaru Harsha Vardhan was responsible for investigation, methodology, supervision, and original draft preparation. P. Senthil Kumar was responsible for conceptualization, validation, and supervision. Hrishitha Sree and B. Senthil Rathi were responsible for investigation, methodology, data curation, and original draft preparation.

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