Tetracycline Removal from Water by Adsorption on Hydrochar and Hydrochar-Derived Activated Carbon: Performance, Mechanism, and Cost Calculation

Abstract

The objective of this study was to investigate the adsorption performance and mechanisms of tetracycline (TC) on hydrochar and hydrochar-derived activated carbon. We also assessed the influence of the solution pH and ionic strength on the adsorption of these compounds and studied their removal by synthetic adsorbents. The maximum adsorption capacities of TC estimated by the Langmuir model in pH 5.5 solution at 25 °C were found to follow the order: ACZ1175 (257.28 mg/g) > MGH (207.11 mg/g) > WAC (197.52 mg/g) > MOPH (168.50 mg/g) > OPH (85.79 mg/g) > GH (75.47 mg/g). The pH value and ionic strength affected TC’s adsorption on the adsorbents. These results indicate that the electrostatic interaction plays a critical role in these adsorption processes. Moreover, adsorption kinetic curves and adsorption isotherms demonstrated that electrostatic interactions were not the only adsorption driving force. Except for electrostatic interactions, the main adsorption mechanisms involved hydrogen bonding and π-π interaction. In addition, the cost of oxidized hydrochar (USD 4.71/kg) is slightly higher than that of hydrochar-derived activated carbon (USD 3.47/kg). This production cost would be lower when it can be produced on a large scale. The outcomes of this study show that the modified-hydrochar and hydrochar-derived activated carbon had the potential for TC removal in wastewater.

1. Introduction

During the 21st century, antibiotics in the environment have received increasing attention, because they might cause antibiotic resistance in microorganisms [1]. Antibiotics are considered to be persistent micro-pollutants in the water environment; most of them cannot be absorbed and metabolized completely by the bodies of humans and animals, which causes a large amount of them to be released into the environment through urine and feces [1,2].

Tetracycline hydrochloride (TC) is one of the most commonly used antibiotics and is also classified as an emerging contaminant. It could be found in soils, surface water, and groundwater, even in drinking water [2]. In general, TC is used in human infection medicine, veterinary medicine, husbandry, and aquaculture growth promoters [3]. Moreover, it might be transferred to the environment via human activities and animal wastes [4]. When TC is widespread in the environment, it can cause a diversity of harmful influences such as chronic toxicity, reduced human immunity, and diffusion in antibiotic-resistant genes [1,5]. Hence, the removal of TC is of great interest and attention in recent years.

A number of methods have been developed to remove TC from an aqueous solution. These technologies include oxidative degradation [6,7], photo-electro catalytic degradation [8,9], biodegradation [10], ozonation [11], and membrane filter [12]. Although these technologies could effectively remove TCs, the technologies frequently need a relatively higher cost and complex operation process.

Adsorption technique is a practical method for the removal of TC from an aqueous solution [13,14,15]. Moreover, adsorption is regarded as a more efficient and cost-effective method. Many investigators have developed new adsorbents for the adsorption of TC, such as graphene oxide [16], aluminum oxide [17], Fe-Mn binary oxide [18], montmorillonite [19], and carbonaceous materials [20,21]. Among these adsorbents, activated carbon (AC) and biochar (BC) have received high interest because they showed the benefit in adsorption amount, cost effectiveness, and operation [5,22,23,24].

A number of investigators have presented the potential adsorption mechanisms between TC and carbonaceous materials including metal bridging, H-bonding, π-π interaction, and van der Waals force [25,26,27]. In a previous work, the electrostatic force was regarded as the primary mechanism controlling the adsorption of organic pollutants (methylene blue) onto the hydrochars [28] and hydrochar-derived activated carbon [29], whereas hydrogen bonding and other mechanisms played minor roles. Hydrochar (HC) has been developed for adsorption in recent years. Only a few studies regarding hydrochar and hydrochar-derived AC to remove TC in wastewater have been presented. The adsorption mechanisms have not been widely discussed in the literature. Thus, the adsorption amount and adsorption mechanisms of TC on the given hydrochar and hydrochar-derived AC need to be further evaluated.

With this background, it is clear that the adsorption of antibiotics on carbon materials has yet to be fully elucidated. Hence, in this study, orange peel and teak sawdust were used as the feedstock of HCs and ACs due to their abundance in agricultural wastes. To improve the adsorption capacities of adsorbents, HNO₃ and ZnCl₂ were used to activate HCs and enhance the specific surface area. The purposes of this study were (i) to examine the adsorption efficiency and mechanisms of hydrochar and ACs for TC removal; and (ii) to confirm electrostatic interaction between adsorbent and TC. Moreover, the effect of pH on the adsorption capacity of TC was studied in detail and adsorption mechanisms of TC on each adsorbent were elucidated. Finally, this research can address the key relevant challenges for the future applications of hydrochar and hydrochar-derived AC.

2. Materials and Methods

2.1. Tetracycline Characterization

In this study, tetracycline hydrochloride (TC) with purity ≥ 95% was purchased from Sigma-Aldrich, Merck KGaA, Darmstadt, Germany. Nitric acid (HNO₃), ZnCl₂, and other analytical grade chemicals were obtained from J. T. Baker. All chemicals were used without further purification.

2.2. Adsorbent Preparation

The procedure to prepare the compound-synthetic adsorbents was similar to our recent work [28,29,30]. Orange peel (OP) was collected from a local market. After collection, the OP was washed with tap water and deionized water to remove any water-soluble impurities and adhering dirt. Then, the OP was dried at 80 °C for 24 h and crushed to 0.074–0.105 mm particles using an electric grinder. Commercial D-glucose was purchased from Sigma-Aldrich (Darmstadt, Germany). Hydrochars were prepared following a typical hydrolysis process. In brief, approximately 15 g of the precursor was added in a 150 mL Teflon-lined autoclave containing 110 mL of DI water. After a 24 h HTC process at 190 °C with a heating rate of 8 °C/min, the brown precipitate was separated by filtration. In previous studies, we have shown the results exhibited from: (i) GH vs. OPH: glucose–orange peel–hydrochar without HNO₃ reflux, (ii) MGH vs. MOPH: glucose–orange peel–hydrochar with HNO₃ reflux [28]. Moreover, the sawdust hydrochar samples were activated using ZnCl₂ in various weight ratios (0 and 1.75) of the activating agent to the sample. The hydrochar samples were impregnated in 100 mL solutions of the activating agents in the above-mentioned weight ratios at 50 °C for 30 min. The solutions were dried in an oven at 105 °C. The dried samples were placed in the oven at 800 °C for 4 h to finish the process of activation. The products were allowed to cool to room temperature and then were washed with deionized distilled water until their pH values were approximately 7.0. The AC samples were thus synthesized: (iii) WAC: activated carbon derived from teak-sawdust hydrochars (only high-temperature calcination), and (iv) ACZ1175: AC derived from ZnCl₂-activated teak-sawdust hydrochar (high-temperature, weight ratio of ZnCl₂ to hydrochar = 1.75:1.0) [29,30]. Figure 1 illustrates that the preparation procedure for the hydrochar, modified-hydrochar and hydrochar-derived activated carbon by different chemical activation methods.

2.3. Adsorption Kinetics Experiment

Adsorption kinetics experiments were performed to evaluate the equilibrium time for the subsequent adsorption isotherm experiments. Approximately 0.05 g of each sample (GH, MGH, OPH, MOPH, WAC, and ACZ1175) was added to 50 mL glass tubes containing 25 mL TC solution with a concentration of 500 mg/L. All tubes were put in a table concentrator shaker and shaken at 150 rpm at 25 °C from 0.5 h to 24 h. After reaching adsorption equilibrium, the suspensions were filtered by a 0.45 µm polytetrafluoroethylene membrane filter, and aliquots of the filtrate were analyzed by UV–Vis spectroscopy (Genesys 10S UV-VIS, Thermo Scientific) at a wavelength of 355 nm (Figure 2). All experiments were conducted in duplicate, and the tubes containing only 25 mL TC solution of the same concentration were used for observing the loss of TC during this procedure. The buffer (pH = 5.5) containing acetate was used to maintain the pH of the solution during the experimental process. The amount of TC uptake at equilibrium, q_e (mg/g), was calculated by the mass balance equation (Equation (1)):

$$q_e=\frac{(C_o-C_e)}{m_1}{V}_1$$

(1)

where C_o (mg/L) and C_e (mg/L) are the TC concentrations at the beginning and equilibrium, respectively; m₁ (g) is the mass of used hydrochar or AC samples; and V₁ (L) is the volume of the TC solution. All batch adsorption experiments were carried out at the constant solid/liquid ratio of approximately 1.0 g/L.

2.4. Adsorption Equilibrium Experiment

Adsorption processes were performed using batch experiments. Approximately 25 mL TC aqueous solution with initial concentrations of 50–400 mg/L was placed in contact with 0.05 g adsorbent and was then shaken at 150 rpm for 24 h at room temperature. Afterwards, the following procedures, such as centrifugation, filtration, and measurement, were the same as in Section 2.3. Citrate buffer (pH 3.0), acetate buffer (pH 5.5), potassium phosphate (pH 7.0), and 2-(cyclohexyl amino) ethane-sulfonic acid (CHES-pH 9.5) buffer were used to maintain the solution pH during the test experiment.

2.5. Influence of pH Solution on TC Adsorption

The pH values of TC solution were controlled to pH 3, pH 5.5, pH 7, and pH 9.5 by using the buffer solution. After that, adsorption isotherms were conducted at room temperature (near 25 °C) in the same way as designated in Section 2.4.

2.6. Adsorption Data Analysis

In this study, the adsorption mechanisms might be evaluated based on the pseudo-first-order and pseudo-second-order models. The experimental data of the adsorption kinetics evaluated based on the linear and non-linear forms of the pseudo-first-order model [31] and pseudo-second-order model [32] are mathematically expressed in Equations (2) and (3), respectively.

$$q_t=q_e(1-e^{-k_{1}t})$$

(2)

$$q_t=\frac{q_e^2{k}_{2}t}{1+k_2{q}_{e}t}$$

(3)

where k₁ (1/min) and k₂ (g/mg × min) are the rate constants of the pseudo-first-order and pseudo-second-order models, respectively; and q_e and q_t are the amounts of TC uptake per mass of the hydrochar and AC samples at equilibrium and at any time t (min), respectively.

To investigate the characterization between adsorbents and TC, the data of adsorption isotherms were fitted by Freundlich [33] and Langmuir [34] models as follows:

$$\mathrm{Freundlich} \mathrm{model}: q_e=K_F{C}_e{}^{1/n_F}$$

(4)

$$\mathrm{Langmuir} \mathrm{model}: q_e=\frac{Q_{max}^o{K}_L{C}_e}{1+K_L{C}_e}$$

(5)

where q_e is the adsorbed amount of TC in equilibrium; C_e is the equilibrium TC concentrations; Q^o_max (mg/g⁻¹) is the maximum adsorption capacity of adsorbent, K_L (L/mg⁻¹) is the equilibrium constant in Langmuir equation, and K_F (mg/g⁻¹ (mg/L⁻¹)^−1/n) is the Freundlich constant. All experiments were carried out in duplicate. The average values are indicated in this study.

3. Results

3.1. Effect of pH of the TC Solution on λmax Values

The absorbance spectra of TC in various pH solutions (range of 2.11∼11.02, without adsorbent) are indicated in Figure 2. The optimal wavelength for TC in pH 2.11~9.07 solutions is found at 355 nm, while at pH > 10 the absorbance shifts to ∼385 nm. For consistency, TC aliquots of the filtrate were analyzed by UV–Vis spectroscopy at a wavelength of 355 nm (shown in Figure 2).

3.2. Morphological and Textural Properties of Synthetic Adsorbents

The partial SEM images for the synthetic adsorbents are illustrated in Figure 3. Other SEM images of adsorbent can be found in the literature adapted from Nguyen et al., 2019 and Hai et al., 2019 [28,29].

The SEM images shown in Figure 3 reveal that the sizes of these hydrochar particles range from several hundreds of nanometers to several micrometers. Interestingly, ACZ1175 showed a rugged surface with a rather large pore. The carbonaceous materials exhibited their relatively porous characteristics [28,29,35].

3.3. Characteristics of the Synthetic Adsorbents

The sizes of these hydrochar particles ranged from several hundreds of nanometers to several micrometers. In a previous study, the GH and MGH were spherical particles with a relatively more uniform size and smooth surface, whereas OPH, MOPH, and WAC displayed fragment shapes. In particular, ACZ1175 possesses an obvious large pore. The carbonaceous materials have exhibited their relatively porous characteristics in previous studies [35,36]. The detailed descriptions of characterizing the hydrochar and hydrochar-derived activated carbonaceous materials have been used in other studies [28,29].

Table 1. Textural properties of the synthetic adsorbents.

Name of Samples	SBET (m2/g)	Pore Volume (cm3/g)
GH	7.33	0.015
MGH	4.49	0.012
OPH	34.06	0.047
MOPH	20.25	0.042
WAC	792	0.340
ACZ1175	1757	1.020

shows that the BET specific surface area (S_BET) values of synthetic adsorbents are ranged from 4.49 to 34.06 m²/g following the increasing order MGH < GH < MOPH < OPH. The obtained result was attributed to the blocking of the narrow pores by HNO₃ treatment [37]. Moreover, the increases in pore size after the oxidation process seemed to reflect this blocking effect. WAC was synthesized through high-temperature calcination without activation. Thus, WAC possessed a relatively lower S_BET than the ACZ1175. Activation using ZnCl₂ significantly enhanced the S_BET values of the AC samples [29,38].

3.4. Adsorption Kinetic

To deeply understand the interactions of TC on hydrochar and AC samples, Figure 4 shows the results of adsorption kinetics. The q_t values reached the adsorption equilibrium in around 4–5 h at room temperature. Most curves can fit the PSO model well. Furthermore, it was found that the increase in adsorption performance of MGH, MOPH, and ACZ1175 were relative to non-activated hydrochar samples.

In this study, the PFO and PSO kinetic models were used to fit the adsorption data for understanding possible mechanisms associated with the adsorption of TC on these hydrochar and hydrochar-derived ACs.

Table 2. Adsorption kinetics parameters of TC on the hydrochar and AC samples.

Kinetic Models		Pseudo-First-Order			Pseudo-Second-Order
Sample	qt,exp	qt,cal = qt(1 − e−k1t)			qt,cal = (k2 qt2t)/(1 + k2 qt2t)
		qt,cal	k1	R2	qt,cal	k2	R2
GH	57.42	50.64	0.43	0.899	57.49	0.009	0.957
MGH	194.83	184.19	0.64	0.977	198.78	0.030	0.997
OPH	67.96	53.89	0.54	0.918	60.95	0.011	0.974
MOPH	154.74	135.21	0.88	0.912	144.14	0.021	0.972
WAC	180.19	167.17	0.91	0.965	176.63	0.035	0.997
ACZ1175	243.08	205.89	0.94	0.807	234.29	0.037	0.992

shows that the adsorption kinetic results fitted better with the PSO model than the PFO model. The R² of PSO (0.957–0.997) were higher than the PFO model (0.807–0.977). Thus, PSO was believed to fit experimental data more accurately, similar to TC adsorption results [5,23]. Moreover, the theoretical q_t,cal values calculated by the PSO kinetic model were closer to the experimental q_t,exp values than fitting by the PFO kinetic model (Table 2).

Therefore, the PSO kinetic model is more appropriate to describe the kinetic adsorption of TC onto hydrochar and hydrochar-derived ACs. The adsorption behavior of TC was followed by the PSO kinetic model, which demonstrates chemical adsorption interaction between TC and these hydrochar and AC samples including forces via sharing or exchange of electrons between TC and tested adsorbents [23,24,29].

3.5. Adsorption Isotherms

Using the HNO₃ modified-hydrochar and ZnCl₂-hydrochar-derived AC [28,29,30,38] showed higher adsorption capacity than untreated hydrochar (

Table 3. The isotherm parameters for the adsorption of TC onto the hydrochar, modified-hydrochar and hydrochar-derived AC samples.

Temperature/Samples		Langmuir Parameters			Freundlich Parameters
		qe = (QomaxKLCe)/(1 + KLCe)			qe = KFCe1/nF
	Qe,exp (mg/g)	Qo(max) (mg/g)	KL (L/mg)	R2	KF (mg/g)	nF	R2
Initial pH = 3
GH	42.40	58.46	0.005	0.990	1.95	0.51	0.967
MGH	137.25	134.98	0.015	0.984	9.92	0.44	0.939
OPH	57.63	65.09	0.006	0.991	2.23	0.52	0.969
MOPH	85.57	81.08	0.011	0.993	5.42	0.43	0.962
WAC	97.97	101.09	0.014	0.994	8.34	0.41	0.965
ACZ1175	160.74	178.32	0.060	0.993	28.29	0.37	0.972
Initial pH = 5.5
GH	58.37	75.47	0.006	0.987	2.94	0.52	0.963
MGH	195.22	207.11	0.094	0.975	43.35	0.33	0.983
OPH	67.96	85.79	0.010	0.998	5.38	0.42	0.974
MOPH	154.27	168.50	0.011	0.996	7.21	0.53	0.979
WAC	180.19	197.52	0.026	0.996	15.01	0.49	0.995
ACZ1175	243.08	257.28	0.299	0.999	64.24	0.50	0.981
Initial pH = 7
GH	55.02	69.42	0.005	0.984	1.97	0.56	0.956
MGH	157.98	172.77	0.014	0.985	10.56	0.48	0.943
OPH	65.54	81.68	0.009	0.994	4.47	0.43	0.964
MOPH	126.54	135.54	0.011	0.976	7.64	0.48	0.934
WAC	139.65	149.04	0.017	0.986	11.11	0.45	0.946
ACZ1175	181.11	190.86	0.116	0.982	39.98	0.36	0.972
Initial pH = 9.5
GH	40.65	51.21	0.002	0.968	0.33	0.73	0.951
MGH	115.51	126.23	0.010	0.995	5.31	0.51	0.979
OPH	51.97	55.66	0.005	0.995	1.27	0.55	0.978
MOPH	58.69	61.11	0.011	0.993	4.66	0.41	0.997
WAC	70.21	73.20	0.013	0.993	9.19	0.34	0.956
ACZ1175	123.62	157.94	0.017	0.998	8.51	0.49	0.979

); therefore, modified hydrochar and ACs were employed to explore the potential difference in adsorption between these adsorbents and TC. Adsorption data of TC onto hydrochar and AC samples at 25 °C, with different pHs, are illustrated in Figure 5.

As presented in Figure 5b, the adsorption capacities of modified hydrochar and ACs improved with increasing initial TC concentration (50–400 mg/L) at pH 5.5. To investigate the adsorption behavior between TC and all adsorbents, two kinds of isotherm models (Langmuir and Freundlich model) were used to fit the experimental data of several pH values (3.0, 5.5, 7.0, and 9.5). The parameters of TC on hydrochar and AC fitted by Langmuir and Freundlich models are listed in Table 3. Both the Langmuir and Freundlich models can fit adsorption isotherms well, as confirmed by the high R² values (0.934–0.999), and the q_e,cal is closer to q_e,exp in the case of both models. The result of Langmuir model fitting indicates that the adsorption of TC on adsorbents takes place as monolayer adsorption on a surface that is homogenous in adsorption affinity. The maximum adsorption capacity of TC at 25 °C with pH 5.5 estimated by the Langmuir model was found to follow the order: ACZ1175 (257.28 mg/g) > MGH (207.11 mg/g) > WAC (197.52 mg/g) > MOPH (168.50 mg/g) > OPH (85.79 mg/g) > GH (75.47 mg/g).

Notably, both Langmuir and Freundlich models could fit the adsorption data of TC on hydrochar, modified-hydrochar and hydrochar-derived AC adsorbents well, showing the adsorption of TC onto adsorbents may be affected by multiple mechanisms. The adsorption mechanisms of TC on carbonaceous materials have been well documented in the literature, such as cation exchange, π-π interaction, H-bonding, electrostatic interaction, surface complexation, pore-filling effect, and nonspecific van der Waals force [24,25,39].

As with previous results, the HNO₃ modification process enhanced oxygen- and nitrogen-containing functional groups and unsaturated bonds on the surface of modified hydrochars. The MGH has more oxygen-containing functional groups which could serve as H-bonding acceptor, and hence the adsorption of TC on MGH was higher than GH and OPH [5,28]. On the other hand, physical adsorption may contribute to the higher adsorption capacity. The ACZ1175 possessed the highest S_BET (1757 m²/g⁻¹) and pore volume (1.02 cm³/g⁻¹) to generate high adsorption capacity although chemical adsorption dominated in this study owing to oxygen functional groups [29,38]. In this case, these results suggested that the ACZ1175 had stronger adsorption ability and higher adsorption capacity than other samples, which might be mainly explained by the higher surface area and rich oxygen-containing functional groups.

3.6. Influence of Solution pH and Adsorption Mechanism

In this study, the effects of pH on the adsorption of TC varied with the properties of TC and adsorbents. As shown in

Table 4. Examples of the chemical formula and characteristics of tetracycline.

Property Name	Chemical Formula	Molecular Weight	Molecular Structure	pKa1	pKa2	pKa3
Tetracycline (TC)	C22H25ClN2O8	480.9 g/mol		3.30	7.68	9.68

, TC is an amphoteric molecule with three pKa values (3.3; 7.68; and 9.68).

TC might alter its functional groups in different pH ranges. The predominant species are cation (H4TC⁺) at pH < 3.4, zwitterion (H3TC^o) at 3.4 < pH < 7.6, anion (H₂TC⁻) at 7.6 < pH < 9.7, and other anion (HTC²⁻) at pH > 9.7 [14,40,41]. To investigate the interaction between TC and adsorbents, we used citrate buffer (pH 3.0), acetate buffer (pH 5.5), potassium phosphate (pH 7.0), and two-(cyclohexyl amino) ethanesulfonic acid (CHES—pH 9.5) buffer to maintain the solution pH during the experimental process. As with the previous results, the pH_PZC values of GH, MGH, OPH, MOPH, WAC, and ACZ1175 were 6.51, 4.09, 6.25, 5.12, 8.03, and 8.11, respectively [28,29,38]. Thus, the surface charge of each adsorbent was positive charge when the solution pH was lower than their pH_PZC, while it was negative at pH > pH_PZC [39].

Interestingly, TC molecules (H₃TC^o—zwitterion species) in the pH range (3.4–7.6) carry no net electrical charge. When the hydrochar and AC have negative or positive charges on the surface, the electrostatic attraction or repulsion is minimized. Due to the MGH and ACZ1175 being rich in oxygen-containing functional groups (e.g., hydroxyl and carboxyl groups), the adsorption mechanisms of TC on the adsorbents may be ascribed to van der Waals force, surface complexation, or hydrogen bonding. The interaction of hydrogen bonding has been seldom discussed in the literature. However, the functional groups such as –CH₃, –OH, –NH₂, and N–H in the TC could form hydrogen bonding with –OH and –COOH on hydrochar and AC sample surfaces [23,42,43].

At higher pH > 7.0, a progressive decline in the adsorption capacities of TC on the hydrochar and AC samples was observed. This can be attributed to a highly strong electrostatic repulsion between the negatively charged TC and a negative charge on the surface of all samples in this study. The interaction here is a weak π−π electron-donor−acceptor (EDA) interaction and physical force between adsorbate and adsorbents [41]. At pH above 9.5, a decrease in the adsorption of TC is due to the higher pH solution; the surface of the sample is negatively charged. Thus, a repulsive electrostatic interaction occurred between the surface of the adsorbent and TC species (HTC²⁻), which decreases the TC adsorption. Moreover, another reason for the reduction is many of the reactive oxygen-containing functional groups sited on the surface of the MGH and ACZ1175, such as –OH and –COOH, were passivated or blocked when pH > 9.5.

All samples showed the highest adsorption of TC at pH 5.5 followed by pH 3 and 7, although the adsorption capacity of the raw hydrochar was lower than those of the modified-hydrochar and hydrochar-derived AC samples. In this case, the lowest adsorption capacity of TC onto the sample is at pH 9.5 (Table 3 and Figure 6).

Results of the solution pH effect revealed repulsive electrostatic interaction exists between TC and hydrochar, modified-hydrochar or hydrochar-derived AC samples. Based on the solution pH, electrostatic interaction also plays a critical role in these adsorption processes. However, electrostatic interactions were not the unique driving force for adsorption. Hence, the adsorption mechanism is affected by van der Waals force, pore filling, π-π interaction, and hydrogen bonding. The potentially different adsorbents and different mechanisms to bind contaminants onto hydrochar and hydrochar-derived ACs are shown in Figure 7.

3.7. Estimation of Adsorbent Production Cost

The adsorbents of hydrochar, modified-hydrochar, and hydrochar-derived activated carbon sourced from agricultural wastes can be used for adsorptive removal of various contaminants. The cost analysis is very important to determine whether the whole production process of the HC/AC samples is feasible or not. The production cost of adsorbent consists of various steps such as a collection of samples, size reduction, and preparation of adsorbent, carbonization, activation, and reusability [44]. The availability, treatment conditions, process requirement, and reuse are the influencing factors for cost analysis of adsorbent materials [45]. In this study, the cost estimation of preparing 1 kg adsorbent has been tabulated in

Table 5. Cost estimation of compound-synthetic adsorbent production.

(A) Cost Estimation of Modified Hydrochar
Particulars	Sub-Sections	Cost Break-Up	Total Cost (USD)
Processing of raw material	Collection of raw material	Orange peel was collected free of cost from the local market	0.0
	Hand sorting and washing	DI water gained from laboratory set-up	0.0
	Drying cost	It was done under the sun	0.0
Preparation of oxidized hydrochar	Hydrothermal carbonization cost	Hour × unit × cost per unit = 24 × 0.5 × 0.16	1.92
	Acid reflux with HNO3	Unit consumed × cost per unit = 13.9 mL × 78.7/500 mL	2.18
	Cost of drying	Hour × unit × cost per unit = 6 × 0.3 × 0.16	0.28
Net cost			4.38
10% to overhead charge			0.43
Total cost			4.71
(B) Cost Estimation of Hydrochar-Derived Activated Carbon
Processing of raw material	Collection of raw material	The teak (T. grandis) sawdust was obtained from a furniture factory	0.0
	Washing cost	DI water gained from laboratory set-up	0.0
	Drying cost	It was done under the sun	0.0
Preparation of AC	Hydrothermal carbonization cost	Hour × unit × cost per unit = 24 × 0.5 × 0.16	1.92
	Impregnation with ZnCl2	Unit consumed × cost per unit = 1.75 g × 94/500 g	0.32
	Pyrolysis at 800 °C	Hour × unit × cost per unit = 4 × 1 × 0.16	0.64
	Washing cost	DI water gained from laboratory set-up	0.0
	Cost of drying	Hour × unit × cost per unit = 6 × 0.3 × 0.16	0.28
Net cost			3.16
10% to overhead charge			0.31
Total cost			3.47

.

As shown in Table 5, the cost of oxidized hydrochar (USD 4.71/kg) is slightly higher than that of hydrochar-derived activated carbon (USD 3.47/kg) due to the higher price of nitric acid which compares with ZnCl₂. However, this production cost will be lower when it is produced on a large scale. The cost estimation of HC/AC evidently suggests that the activated-hydrochar preparation process using agricultural waste as a raw input is quite cost-effective. Both oxidized hydrochar and AC can be used as promising adsorbents for the removal of TC from aqueous solution.

3.8. Comparison of the Estimation of Adsorbent Production Cost

In recent years, various indigenous biomasses such as agricultural wastes have been enormously used as a sustainable precursor to produce hydrochar, biochar, and activated carbon [46]. Successful implementation of a technique for the removal of contaminants in wastewater treatment highly depends on the cost of adsorbent production [45,47].

According to the data, the costs of adsorbents derived from different biomasses have been calculated and summarized as being in a range of USD 2–6/kg (

Table 6. Comparative analysis with other biomass-based chars reported in the literature.

Raw Materials	Pre–Post Treatment	Post-Treatment	SBET Surface Area (m2/g)	Cost/kg (USD)	References
Sugarcane bagasse	Pyrolysis	H3PO4	557	3.81	[47]
		Super-heated steam		3.49
Orange peel	HTC	HNO3	20	4.71	This study
Teak sawdust	HTC	ZnCl2	1757	3.47	This study
Teff straw	Pyrolysis	H3PO4	627	3.73	[48]
	HTC	H3PO4	43	2.93
Rice husk	Pyrolysis	Zeolite Z-RHA	76	5.42	[45]
Parthenium hysterophorus	Pyrolysis	NaOH	308	2.88	[49]
Ion-exchange resins	-	-	-	150	[50]

), excepting commercial products. From this comparative analysis, modified-hydrochar and hydrochar-derived AC as adsorbents are inexpensive or even have negligible cost. The production technique is also very inexpensive and does not consume costly reagents, in comparison to AC production and activation, which require high temperature and expensive reagents.

4. Conclusions

The objective of this study was to analyze and compare the behavior of carbonaceous materials, both hydrochar and hydrochar-derived activated carbon, with different chemical and textural natures in the adsorption of TC. The following conclusions can be presented in this study:

In order to improve the adsorption capacity of TC, HNO₃ and ZnCl₂ were applied under different conditions to activate hydrochars sourced from agricultural waste. The maximum adsorption capacities of TC estimated by the Langmuir model were found to follow the order: ACZ1175 (257.28 mg/g) > MGH (207.11 mg/g) > WAC (197.52 mg/g) > MOPH (168.50 mg/g) > OPH (85.79 mg/g) > GH (75.47 mg/g) in pH 5.5 solution at 25 °C. Modified-hydrochar and hydrochar-derived AC adsorbents sourced from agricultural waste showed high efficiency for removal of TC in wastewater.

The adsorption kinetic curves could fit the PSO model well. Chemical adsorption is regarded as the primary adsorption mechanism. The solution pH and ionic strength had a major influence on TC adsorption on the carbons, indicating that electrostatic adsorbent–adsorbate interactions also play an important role in TC adsorption at pH values that produce TC deprotonation. In addition, the adsorption mechanisms of TC on the test adsorbents also included hydrogen bonding, π-π interaction, pore-filling, and electrostatic interactions.

Most significantly, the total production cost spent on the preparation of hydrochar-derived activated carbon is a little bit cheaper as compared with oxidized hydrochar. The good cost made it a feasible adsorbent instead of commercial-activated carbon for potentially wide application in wastewater treatment.