Enhancement of Trihalomethane Adsorption Capacity Using Chitosan-Modified Coconut Shell Activated Carbon: Adsorption Characteristics and Mechanisms

Abstract

There is a rising concern about the safety risk that trihalomethanes (THMs) in drinking water pose. In this work, to adsorb THMs such as chloroform (TCM), dibromochloromethane (DBCM), bromodichloromethane (BDCM), and bromoform (TBM), we coated chitosan (CS) on coconut shell activated carbon (CAC). The adsorbents were characterized using BET, XRD, FTIR, and SEM techniques. The impact of various variables was examined, including contact time, quantity of adsorbent, initial pH, and initial THM concentrations. Under the same conditions, TCM was adsorbed most efficiently, followed by BDCM, DBCM, and TBM. When the pH was between 4 and 8, the adsorption of THMs onto the coconut shell activated carbon supported chitosan (CS/CAC) varied relatively little; however, when the pH increased above 8, the adsorption of THMs decreased. For THMs, CS/CAC adsorption was a chemical reaction and monolayer adsorption that fit better with the pseudo-second-order kinetic model and the Langmuir isotherm model. According to the thermodynamic study, THMs were adsorbed endothermically and spontaneously on CS/CAC. For column experiments, the adsorption of THMs was influenced by bed height and flow rate. After up to four cycles of adsorption and desorption, it was found that the adsorbent was reusable. The maximum adsorption capacities for Langmuir were 187.27, 114.29, 93.28, and 89.61 µg/g for TCM, BDCM, DBCM, and TBM, respectively. CS/CAC has a high adsorption capacity, especially for TCM, which is responsible for a major portion of THMs in drinking water. This indicates that CS/CAC has a lot of potential uses when it comes to removing THMs from water.

1. Introduction

It is critical for society to have a consistent and secure source of drinking water. Water quality is the sixth goal of the UN’s 2015 adoption of the 2030 Agenda for Sustainable Development. One of society’s greatest contributions to public health is drinking water chlorination in order to prevent the spread of waterborne diseases. On the other hand, halogenated disinfection byproducts (DBPs), some of which are harmful, are formed by the chlorination of organic matter [1,2,3].

Trihalomethanes (THMs), haloacetic acids (HAAs), and haloacetonitriles (HANs) are examples of the halogenated organic compounds that are included in DBPs. The THMs are the most prevalent of the previously stated DBPs, followed by the HAAs, which may be detected in finished water at levels up to µg/L. As for HANs, their concentrations range from ng/L to µg/L [4,5]. The European Union (EU) and the U.S. Environmental Protection Agency (U.S. EPA) established the maximum allowable limits for total trihalomethanes in treated water systems at 100 and 80 μg/L, respectively [4]. Epidemiologic research has linked DBPs to harmful human health conditions such as colon cancer, bladder cancer, miscarriage, male reproductive health, and birth malformations. These findings pose serious concerns [6,7]. It is necessary to develop an effective technique for eliminating THMs from drinking water. Removing THMs can occur via one of two methods: either remove them once they are produced or decrease their precursors to prevent their production in the future [8].

Adsorption produces no degradation byproducts and requires less energy and operating costs. One of the most effective treatments for eliminating DBPs is activated carbon (AC), which is also the most widespread and durable sorbent [9]. The main factors controlling the adsorption of DBPs are anion exchange and electrostatic phase interactions rather than hydrophobic interactions [10]. The protonation sites in unmodified GAC are limited, and it is difficult to produce strong electrostatic interactions [11]. As a result, numerous researchers have enhanced the adsorption capacity of THMs by modifying activated carbon [12,13,14,15,16,17]. Xiao et al. [15] prepared NZVI/AC via nanoscale zero-valent iron (NZVI) modification of activated carbon (AC) using liquid-phase reduction to remove THMs from actual wastewater and observed that NZVI/AC had a higher affinity for all THM molecules than either AC or NZVI. Ndagijimana et al. [13] synthesized nanoscale zero-valent iron/silver@activated carbon-reduced graphene oxide (nZVI/Ag@AC-RGO). When compared to pristine AC, the removal capabilities of TCM, BDCM, DBCM, and TBM by nZVI/Ag@AC-RGOC improved by 30.37, 30.14, 30.56, and 20.43%, respectively. Chitosan (CS) is a naturally occurring polymer that is abundant, non-toxic, and rich in amine and hydroxyl functional groups. These properties make CS a promising candidate for THM adsorption by electrostatic interactions. Nevertheless, the low pore volume, surface area, and weak mechanical strength and chemical stability have restricted its adsorption applications. As a result of these drawbacks, chitosan must be modified in order to produce adsorptive materials that are more effective at eliminating particular pollutants from aqueous solutions [18]. Because GAC lacks protonation sites, loading CS on it can compensate for this and increase the material’s strength [19]. Am activated carbon/chitosan biocomposite has been successfully employed to adsorb nitrate ions [20], Cd (II) [21], perfluoroalkyl acids [19], pharmaceutical pollutants [22], and Cr (VI) [23]. The preparation of the activated carbon/chitosan biocomposite and its applications have been documented; however, research on the material’s potential as an adsorbent for the elimination of THMs from fresh water has received less attention.

Few studies have looked at the performance of sorbents with modified surfaces in a continuous run, such as in a column study, a pilot-scale experiment, or a full-scale experiment. Despite the fact that surface modification of sorbents has been the subject of numerous investigations, which include AC in batch studies with positive outcomes, further research is needed to determine how surface-modified AC is actually used in the real world [9].

As far as we know, to date, the coconut shell activated carbon supported chitosan (CS/CAC) method has not been used for the removal of THMs. Therefore, the objectives of this work are to (i) examine how each THM molecule is adsorbed by the synthetic composites in batch and fixed-bed column systems and (ii) explain the interactions and mechanisms. This study indicates that enhancing the CAC pre-enrichment leads to increased THM removal efficiency.

2. Materials and Methods

2.1. Chemicals and Materials

Without any additional purification, all chemical reagents were purchased from commercial sources. THM standards, including chloroform (TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM), and bromoform (TBM), were obtained from CNW (Shanghai, China). Commercial granular coconut shell activated carbon (CAC) was purchased from PingDingshan Lvlin Activated Carbon Co., Ltd. (PingDingshan, China). Table S1 contained information on the CACs’ features. Chitosan was purchased from Aladdin Company (Shanghai, China). It was in the form of a white powder with a deacetylation degree of 95.0% and a viscosity of 100–200 MPa. The initial pH values were adjusted using solutions of sodium hydroxide and hydrochloric acid. The Millipore Milli-Q Gradient water purification system (Billerica, MA, USA) was used to prepare all reagent solutions. THMs were detected using GC-ECD (7890B, Agilent Technologies Inc., Tianjin, China) based on EPA 510 and our previously published method [24,25,26]. A detailed GC-ECD method for THMs was provided in Table S2.

2.2. Preparation and Characterization of (CS/CAC)

Hydari et al. [27] used the following procedure to prepare the chitosan/activated carbon composite (CS/CAC): Activated carbon was initially immersed in 0.2 M oxalic acid for two hours. After filtering using a 1.0 μm glass fiber filter, it was rinsed with deionized water and allowed to dry for twelve hours at 70 °C in an oven. Then, 3 g of chitosan (CS) was added to 3% acetic acid (v/v) and stirred continuously. After that, the mixture was allowed to stand for two hours at 45–50 °C to facilitate gel formation. Next, approximately 30 g of cleaned activated carbon (CAC) was gradually added to the CS gel, maintaining a CS to CAC ratio of 1:10 (as shown in Figure S1 and Text S1), and stirred for twelve hours at 45–50 °C. After filtering the CS/CAC composite from the solution, it was washed with deionized water until a neutral pH was achieved. Finally, it was dried in a drying oven at 60 °C.

The Brunauer–Emmett–Teller method with N₂ adsorption–desorption (BET, V-Sorb 2800TP, Tianjin, China) was used to determine the specific surface area, pore volume, and pore size of adsorbents. Elemental analysis and surface morphology were observed by scanning electron microscopy (SEM). Using a Fourier-transform infrared (FTIR) spectrophotometer, we analyzed CAC both before and after loading CS in order to identify any changes in functional groups. The following technique was used to determine the pH_PZC (point of zero charge) of CS/CAC: In various glass flasks with a 0.1 M NaCl solution and a predetermined initial pH (1–12), 0.1 g of dry material was added. The solutions were shaken for 48 h, and the pH of the filtrates was then determined.

2.3. Batch Adsorption Experiments

Stock solutions (1000 μg/L) of each THM (TCM, BDCM, DBCM, and TBM) were prepared using methanol (MeOH) as a solvent. In all batch adsorption experiments, we used 50 mL sealable polypropylene bottles containing 20 mL of THMs solution and 5 mg of adsorbents. After the addition of the adsorbents, the sealed polypropylene bottles were placed in an orbital shaker at a constant temperature of 25 ± 0.2 °C. A 0.45-μm membrane was used to extract and filter the water sample at certain intervals. All experiments were repeated twice. Controlled samples followed the same procedures without the addition of adsorbents to check on the loss of THMs other than by adsorption on the adsorbents.

The isotherm experiments were carried out by mixing a predetermined amount of the adsorbents (250 mg/L) with 20 mL of different concentrations of THM solution (total initial concentration from 14 to 287 μg/L). The kinetic experiments were carried out by mixing a predetermined amount of the adsorbents (250 mg/L) with 20 mL of a predetermined concentration of THM solution at different time values (up to 1320 min).

Equation (1) yields the adsorption capacity at time t, or the quantity of THMs adsorbed per unit of mass qt (µg/g).

$$q\mathrm{t}={\displaystyle \frac{(Ci-Cf)}{m}} V$$

(1)

where Ci and Cf (µg/L) refer to the initial and final concentrations of THMs in solution at time t, respectively. V (L) is the solution volume, and m (g) is the amount of CS/CAC used.

Equation (2) yields the removal efficiency of THMs (R).

$$R={\displaystyle \frac{(Ci-Cf)}{Ci}} 100\mathrm{\%}$$

(2)

Equation (3) yields the quantity of THMs adsorbed at the equilibrium qe (µg/g) per unit of mass.

$$q\mathrm{e}={\displaystyle \frac{(Ci-Ce)}{m}} V$$

(3)

where Ce (µg/L) is the equilibrium concentration of THMs.

2.4. Fixed-Bed Study

The efficacy of CS/CAC in a fixed-bed reactor was investigated in a continuous operation mode. Two glass columns (P1-H and P2-h) were used. Detailing was conducted in a laboratory glass column (inner diameter = 2.0 cm, length = 24 cm) with bed heights of 4 and 2 cm and flow rates of 2 and 4 mL·min⁻¹ to assess the impact of the empty bed contact time (EBCT) on treatment. Aliquots were collected in amber flasks at various time intervals (up to 60 h) in order to draw breakthrough curves (C/Co vs. time, where C and Co represent the exiting and initial concentration, respectively). Each column was uniformly packed with adsorbents as the media by gradually adding two grams of the media, then pouring Milli-Q water until the media was settled, up until the total depth of the media. Glass wool was utilized at the adsorbent bed’s inlet and exit to stop adsorbent loss. The THM solution was fed into the column in a down-flow mode at a predetermined concentration. Periodically, samples in the effluent were withdrawn and analyzed. The backwash was performed using ultrapure water in the up-flow direction at 20 mL·min⁻¹ to confirm the columns could withstand the pressure of the backwash. The experimental setup for the column study is presented in Figure 1.

2.5. Recycling Experiments

In order to examine the CS/CAC regeneration performance, the adsorbent was immersed in THM solution and shaken at 150 rpm for 30 min. The mixture was filtered, and the adsorbed CS/CAC was collected after the reaction. NaOH solution (0.1 M) was utilized as the eluent in the desorption studies. The adsorbent and NaOH solution (0.1 M) were shaken for 30 min at 150 rpm. Regenerated adsorbent was washed with deionized water, dried to a constant weight, and then reused. The regenerated adsorbent was utilized in four consecutive adsorption–desorption recycling experiments to assess the adsorbents’ reusability.

3. Results and Discussion

3.1. Characterization

The BET-specific surface area, pore volume, and average pore diameter of CAC, CS, and CS/CAC are presented in

Table 1. Textural properties of CAC, CS, and CS/CAC.

Adsorbents	BET Specific Surface Area/ (m2/g)	Pore Volume/ (cm3/g)	The Average Pore Diameter/ (nm)
CAC	1254	0.782	2.495
CS	1.752	0.007	15.768
CS/CAC	968	0.589	2.017

. The largest pore volume (0.782 cm³/g) and surface area (1254 m²/g) were found in CAC, while CS’s values were determined to be 0.007 cm³/g and 1.752 m²/g, respectively. It was found that the CS/CAC material, which was produced by coating CS on the surface of CAC, had a larger surface area (968 m²/g) and pore volume (0.589 cm³/g) than pure chitosan and less than pure activated carbon. Notably, CS had the largest pore size (15.768 nm), followed by CAC (2.495 nm) and CS/CAC (2.017 nm). These differences are due to the fact that chitosan polymeric chains block some CAC channels or pores [20]. The XRD patterns of CAC and CS/CAC were depicted in Figure 2. We observed that the XRD spectra of CS/CAC had a sharp peak of CS at 2θ = 19.8°, which corresponds to the miller indices (110) for chitosan. A significant low intensity peak appeared for chitosan, which might be due to strong interaction occurred between activated carbon and chitosan molecule in the formation of CS/CAC, consistent with the literature [20,28].

The FTIR spectrometer was used to measure the FTIR spectra of CS, CAC, and CS/CAC, as shown in Figure 3. The adsorption band around 3434 cm⁻¹ in the chitosan spectra could be attributed to O-H stretching vibration and/or N-H bond extension vibration [27]; N-H stretching vibrations of the NH₂ group could be explained by the adsorption band at 1627 cm⁻¹; and -NH deformation vibration in -NH₂ was confirmed by the adsorption band at 1390 cm⁻¹ [29]. In the FTIR spectra of CAC, the peak at 3425 cm⁻¹ was referred to as the vibration of the -OH bond. The peaks between 1600 and 1750 cm⁻¹ were presented by the C=O bond in the carboxylic acid, aldehydes, and ketones. The strong band at 1561 cm⁻¹ was indicative of the aromatic (C=C) ring’s vibration in CAC [20]. The FTIR spectra of CS/CAC showed that the peaks at 1532–1602 cm⁻¹ were the particular peak for N-H from the amines and amides [27]. It was also noted that the peak in CAC at 1705 cm⁻¹ was displaced to 1697 cm⁻¹. SEM images showing the morphology of CAC and CS/CAC are presented in Figure 4. As indicated in Figure 4a,b, the surface of CAC had a rough, massive structure with numerous tiny pores. Nevertheless, the CS/CAC surface was comparatively smooth, with a modest number of macrospores (Figure 4c,d). This phenomenon could potentially be attributed to the tiny pores on the CAC surface that were coated in CS [21]. Based on the aforementioned findings, CS and CAC were effectively combined to form CSCC, which had numerous functional groups.

3.2. Adsorption Studies

3.2.1. Adsorption of THMs over Time

Figure 5 illustrates how contact time affects THM adsorption onto Cs/CAC at an initial concentration of 66 μg/L. The removal happened quickly in the first few minutes, and equilibrium was reached after around 360 min. One possible explanation for the quick rise in the amount adsorbed in the first five minutes of treatment could be the abundance of adsorption sites found in the first phase of the treatment. The repelling forces between the THMs already adsorbed on the surface of the adsorbents and those still in the liquid phase complicate access to the unoccupied sites that remain on the surface over time [20,30]. Diffusion mechanisms controlling THM adsorption onto CS/CAC could be responsible for the long contact time needed to reach equilibrium for THM adsorption. According to [31], in very dilute solutions, mass diffusivity decreases with decreasing concentration, resulting in a decrease in the amount of adsorbate that diffuses onto the adsorbent’s surface. The percentage removals of chloroform, bromodichloromethane, dibromochloromethane, and bromoform after a 6 h equilibration time were 64.53 ± 0.9%, 58.74 ± 0.6%, 54.80 ± 0.9%, and 51.80 ± 0.6%, respectively. The adsorption selectivity order was TCM > BDCM > DBCM > TBM. It is shown that TCM, the smallest molecule, preferentially adsorbed onto CS/CAC, followed by BDCM, DBCM, and TBM. Two potential explanations could be given for this. First, molecules tiny enough to pass through the pores and into the inner cavities were adsorbed onto surfaces. Consequently, as the size of the adsorbed molecules decreases, the adsorption rate rises. Second, the CAC became more hydrophilic and appropriate for the adsorption of comparatively polar molecules due to the chitosan coating. The magnitudes of the dipole moments for the C-Cl and C-Br bonds in THM molecules were 1.56 and 1.48 Debye, respectively. As a result, when the bromine ions in the THM molecule increased, the polarity of the covalent decreased [32]. Consequently, TCM had the strongest polarity, followed by BDCM, DBCM, and TBM. This polarity difference made it simpler for TCM to be adsorbed onto CS/CAC.

3.2.2. Effect of Adsorbent Amount

The adsorbent amount is another essential factor that influences the adsorption process. As indicated in Figure 6, the adsorbent amount was varied from 0.1 to 3.0 g/L to investigate the impact of mass. THM removal efficiency increased with increasing CS/CAC dosages because of an increase in surface area and adsorption sites. As adsorbent concentrations increased, the removal efficiency rate decreased and eventually stagnated. This phenomenon could be attributed to the overcrowding of adsorbent particles, which reduces the adsorption surface area and makes it more difficult to access the adsorption sites. Simultaneously, there was also an increase in the diffusional path length [30,33].

3.2.3. Effect of pH

This study investigated the impact of pH on THM adsorption by CS/CAC (Figure 7). The pH and THM properties had a significant impact on THM removal efficiency. Each THM molecule had a halogen group (-Br or -Cl), which could be replaced by a hydrogen atom [16]. When the initial pH increased from 3.0 to 4.0, the adsorption capabilities of THMs increased significantly, remained unchanged until pH 8.0, and then decreased significantly as pH increased further, consistent with the literature results [16,32]. In this work, the pH_pzc of CS/CAC was determined to be 7.78, as shown in Figure S2. At 4.0 ˂ pH ˂ pH_pzc, the adsorbent surface could be partially positively charged because of the protonation of the amino groups on CS. The adsorption process is mainly controlled by the electrostatic attraction between that weak positive surface and the electron-rich chlorine atoms in THMs. The significant decrease in adsorption at pH ˃ pH_pzc could potentially be attributed to OH^- competing with THMs for adsorption sites on the adsorbent surface. Furthermore, in higher pH solutions, the abundance of OH^- would result in a higher number of repulsive electrostatic interactions [20].

However, when the pH is <4.0, the capacity decreases as a result of the function of protonation caused by the high concentration of H⁺ in an acidic condition [34]. Also, there are more protons available in the solution and on the surface, leading to the conversion illustrated by the following Equation (4). THM adsorption was decreased as a result of the positive water cluster formation on these groups, which prevented THM molecules from accessing adsorption sites [35].

R – OH + H⁺ → R – OH₂⁺

(4)

The highest uptake was attained at pH 7, where the adsorbed amounts of TCM, BDCM, DBCM, and TBM were 51.22, 37.81, 28.38, and 24.43 µg/g, respectively. According to the obtained results, the optimal pH value was 4–8. As a result, in practical applications, there would be no need to adjust the pH values of the contaminated solutions.

3.2.4. Effect of Initial Concentration

THM concentrations varied from 14 to 287 µg/L at a contact time of 30 min, as indicated in Figure 8. It could be noted that the adsorbed capacity increases as the initial THM concentration increases. Two facts could be responsible for the results obtained: (i) a higher adsorption rate and the use of all activated sites available for adsorption at higher concentrations could be the cause of the increase in the adsorbed capacity with increasing initial THM concentration, and (ii) the motive force for mass transfer between the phases increases as the initial concentration increases because the difference in the concentrations of the substances in the solid and liquid phases increases [20].

3.3. Adsorption Isotherm Study

Data on adsorption equilibrium are important for characterizing and comprehending the behavior of an adsorbent–adsorbate interaction. In this work, we examined the equilibrium data for the four THMs using linear regression and fitted them to three different isotherm models, namely the Freundlich, Langmuir, and Temkin isotherm models, as presented in

Table 2. Parameters of the isotherm models for THM adsorption onto CS/CAC.

Isotherm Models	Parameters	Adsorbates
		TCM	BDCM	DBCM	TBM
Langmuir	qmax (µg/g)	187.27	114.29	93.28	89.61
	KL (L/µg)	0.0291	0.0554	0.0687	0.0644
	RL	0.29	0.20	0.18	0.20
	R2	0.992	0.995	0.994	0.994
Freundlich	Kf (µg/g)	6.01	6.65	6.49	5.91
	n	1.24	1.34	1.36	1.35
	R2	0.988	0.988	0.992	0.992
Temkin	Bt (J/mol)	38.50	30.17	25.73	24.23
	Kt (L/µg)	0.46	0.59	0.67	0.64
	R2	0.761	0.763	0.784	0.786

and Figures S3–S5. The data were collected at a constant temperature of 25 °C. The monolayer adsorption of THMs to a surface that has a finite number of homogeneously distributed identical sites over the adsorbent surface is the basis for the Langmuir adsorption isotherm. On the other hand, the Freundlich isotherm makes the assumption that adsorption occurs over a heterogeneous surface with reversible adsorption and uniform adsorption energy. According to the Temkin isotherm, as a result of adsorbate–adsorbate interactions, the adsorption energy of every molecule decreases linearly with layers, and adsorption can be described by a consistent and even spread of binding energies [36]. The high R² value of the adsorption data exhibited a good fit with Langmuir and Freundlich, indicating that both chemical and physical processes on the heterogeneous surface could have an impact on the adsorption mechanism. However, the Langmuir isotherm exhibited superior overall conformity to the data. The adsorption process as a function of RL may be described as follows: irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1), and unfavorable (RL ˃ 1) [37]. In this study, the adsorption process was found to be favorable since the RL value lies between 0 and 1. The Freundlich constant n was found to be higher than 1, which indicated that the adsorption of THMs on CS/CAC was favorable. The order of the CS/CAC selectivity for the THMs was indicated by the qmax value as follows: TCM (187.27 µg/g) > BDCM (114.29 µg/g) > DBCM (93.28 µg/g) > TBM (89.61 µg/g). The experimental values also supported this behavior.

3.4. Adsorption Kinetics Study

To comprehend the adsorption reaction dynamics in terms of the rate constant’s order, a kinetic study was carried out using the adsorption data at various contact times. The behavior of the four THMs during the adsorption process on the adsorbents was studied by applying four kinetic models to the adsorption kinetic data. These models include the pseudo-first-order kinetics (PFO), the pseudo-second-order (PSO), the Elovich models, and the intraparticle diffusion models (IPD) [38].

Table 3. Parameters of the kinetic models for THM adsorption onto CS/CAC.

Kinetics Models	Parameters	Adsorbates
		TCM	BDCM	DBCM	TBM
Qe,(exp)		40.32	38.76	37.94	38.73
PFO	Qe,(cal)	5.45	7.41	12.73	9.96
	K1 × 10−5	0.39	0.45	0.58	0.44
	R2	0.894	0.851	0.759	0.843
PSO	Qe,(cal)	40.44	38.87	37.95	38.94
	K2 × 10−3	4.65	3.54	2.18	2.19
	R2	0.999	0.999	0.999	0.999
Elovich	β	0.513	0.471	0.396	0.330
	α	2,227,479.21	192,671.23	7067.54	1106.09
	R2	0.971	0.970	0.973	0.972
IPD	Kid	0.611	0.613	0.648	0.711
	I	24.211	22.229	19.664	19.008

and Figures S6–S8 present the constant parameter values and R² values for these models. The experimental data could be effectively explained by the pseudo-second-order model, as indicated by the R², Qe (cal), and Qe (exp) constant parameters. The real THM adsorption capacity (Qe,exp) of the adsorbent was comparable to the theoretical adsorption capacity (Qe,cal) obtained from the pseudo-second-order kinetic model. The calculated values of the adsorbed quantity (Qe (cal), 40.44, 38.87, 37.95, and 38.94 µg/g for TCM, BDCM, DBCM, and TBM, respectively) from this model were very close to the experimental values (Qe (exp)) given in Table 3. Thus, the adsorption of THMs on CS/CAC was controlled by a chemical reaction, which could be explained by a second-order kinetic model. In the Elovich kinetic model, the rate constant α was greater than the desorption constant β. Based on these findings, it was suggested that the adsorption mechanism of THMs on CS/CAC is controlled by both chemisorption and physisorption, which take into account the electrostatic contact between the adsorbent and adsorbate. According to the intra-particle diffusion model, it could be noted that the plots of qt against t^0.5 fitted curves did not pass through the origin point, suggesting that the adsorption of THMs onto CS/CAC could be controlled by intraparticle diffusion and film diffusion. The significance of the film diffusion was indicated by the value of the (I) parameter in the intra-particle diffusion model.

3.5. Thermodynamic Analysis

The thermodynamics for the removal of THMs by CS/CAC were studied at different temperatures (5 °C, 25 °C, and 45 °C). Equations (5) and (6) were used to compute the thermodynamic parameters for the removal process, and

Table 4. Thermodynamic parameters of THM adsorption by CS/CAC.

THMs	Temp (K)	∆G° (KJ/mol)	∆H° (KJ/mol)	∆S° (KJ/mol)
TCM	278	−2.85	13.80	58.97
	298	−3.17
	318	−5.29
BDCM	278	−2.91	13.97	59.75
	298	−3.22
	318	−5.38
DBCM	278	−2.79	11.61	50.97
	298	−3.05
	318	−4.90
TBM	278	−2.48	8.99	40.63
	298	−2.72
	318	−4.16

contained a list of the relevant parameters.

$$\ln \mathrm{K}°={\displaystyle \frac{\mathsf{\Delta }\mathrm{S}°}{R}}-{\displaystyle \frac{\mathsf{\Delta }\mathrm{H}°}{RT}}$$

(5)

$$\mathsf{\Delta }\mathrm{G}°=-\mathrm{R}\mathrm{T}\mathrm{l}\mathrm{n}\mathrm{K}°$$

(6)

K° represents the equilibrium constant, R represents the universal gas constant (8.314 kJ·mol⁻¹), and T represents the temperature in Kelvin (K). The symbols ΔS°, ΔH°, and ΔG° represent entropy (kJ·mol⁻¹), enthalpy (kJ·mol⁻¹), and the change in Gibbs free energy (kJ·mol⁻¹), respectively. The lnK° vs. 1/T linear regression curve was used to determine the values of ΔS° and ΔH°. The positive change in enthalpy (ΔH°) indicates that the process of removing THMs was an endothermic reaction. A possible explanation for the positive change in entropy (ΔS°) is the release of water molecules as a result of the exchange of molecules between THM molecules and the functional groups attached to the adsorbent surface, which indicated the adsorbent’s affinity for THMs and the increase in disorderliness at the boundary between the solid and liquid phases. The enthalpy change (ΔH°) values for TCM, BDCM, DBCM, and TBM were 13.80, 13.97, 11.61, and 8.99 kJ·mol⁻¹, respectively, and the entropy change (ΔS°) values for these compounds were 58.97, 59.75, 50.97, and 40.63 kJ·mol⁻¹, respectively. Furthermore, the negative ΔG° value indicated that the adsorption process was spontaneous, showing a strong affinity of THM molecules for the adsorbent. Consequently, the thermodynamic study demonstrated that the removal of THMs was spontaneous and endothermic [21,29,39].

3.6. Test Using Fixed-Bed Column

Adsorption in dynamic systems was observed to provide a more accurate representation of the behavior of the process used in large-scale water treatment, which included complex mass transfer and liquid flow. Breakthrough curves were used to show the THM adsorption results in the continuous system. The ratio of the initial (Co) and final (C) concentrations as a function of time (C/Co vs. t) was used to describe adsorption in an aqueous solution [40]. It would be necessary to observe the adsorption for long periods to determine the total saturation time (C/Co = 1) for every THM molecule as a result of the high capacity of CS/CAC to adsorb THMs (particularly the chlorinated compounds). Consequentially, the breakthrough experiments were performed until the concentration of TBM exiting the column reached approximately 80% of the input concentration. The variables controlling adsorption (time, adsorbent dosage, pH, and concentration) were theoretically evaluated before the fixed-bed column experiments were carried out.

Effect of Bed Height and Flow Rate

Two different heights (2 and 4 cm) were used to investigate the impact of bed heights on THM adsorption by CS/CAC at a steady flow rate of 2 mL·min⁻¹. According to the findings shown in Figure 9A,B, higher bed heights improved THM adsorption because they had a larger contact surface. As a result, more active sites were available for adsorption [40]. The breakthrough curves for various flows (2 and 4 mL·min⁻¹) are presented in Figure 9C,D. The CS/CAC’s capacity to adsorb THMs was significantly decreased with increased flow. This was due to the fact that higher flow decreased solute residence time in the adsorbent bed, which in turn decreased the solute diffusion into the adsorbent’s pores [41]. Testing performed using a 2 mL·min⁻¹ flow rate showed a 20% greater removal efficiency compared to a 4 mL·min⁻¹ flow rate and a 15% greater removal efficiency when using a 4 cm bed instead of a 2 cm bed. For each THM, the selectivity of the CS/CAC varied and was in the following order: TCM > BDCM > DBCM > TBM. These findings were consistent with [40], who used the fixed-bed column technique to study the adsorption of THMs by humin, and [32], who studied the adsorption of THMs by powdered activated charcoal in batch experiments. The experimental results demonstrate that CS/CAC is a promising, effective adsorbent for THM removal. Similar results for the removal of metal ions have been previously documented [27].

3.7. Desorption Studies and Reusability

The examination of the reusability and desorption efficiency of an adsorbent is critical for economic reasons. Desorption investigations were carried out in this study to determine the degree of CS/CAC reusability. The experiment was carried out four times, and Figure S9 shows the changes in its adsorption capability. After four desorptions with 0.1 M NaOH, the adsorption performance only decreased by approximately 10%. This result suggests that the adsorption process might include physical forces and electrostatic interaction. Thus, the saturated adsorbent might not completely regenerate in strongly alkaline conditions. Further, it is possible that a few adsorption sites were deactivated during the regeneration process, which contributed to the decrease in CS/CAC’s adsorption performance [42]. According to the reusability investigation, the prepared CS/CAC adsorbent had a high reusable performance for removing THMs from aqueous solutions.

3.8. Comparison of the Result Obtained in This Study with Other Published Works

Table 5. Adsorption capacity of different adsorbent for THM adsorption.

Adsorbents	THMs	Initial Concentrations (µg/L)	M/V (g/l)	SBET (m2/g)	Adsorption Capacity (µg/g)	Ref.
Activated carbon/nanoscale zero-valent iron (NZVI/AC)	TCM	42.19	1.80	87.4	20.86	[16]
	BDCM	43.93			22.45
	DBCM	45.71			24.58
	TBM	10.18			5.57
Humin	TCM	250	10	84.75	18.65	[43]
	BDCM				19.25
	DBCM				19.5
	TBM				20.58
Carbon nanotubes (CNTs)	TCM	200	0.40	225 to 295	110	[32]
	BDCM				60
	DBCM				50
	TBM				50
	TCM	3200	0.40		1510
	BDCM				780
	DBCM				720
	TBM				640
Powdered activated carbon	TCM	200	0.40	900	63
	BDCM				89
	DBCM				108
	TBM				119
	TCM	3200	0.40		980
	BDCM				1200
	DBCM				1290
	TBM				1520
Nanoscale zero-valent iron/silver@activated carbon-reduced graphene oxide ((nZVI/Ag@AC-RGO)	TCM	800	0.20	770.54	3814.7	[13]
	BDCM				3973.4
	DBCM				4000
	TBM				4000
GAC A (acid washing carbon)	TCM	1000	0.50	671	1710	[14]
	TBM				1910
GAC B (coconut shell carbon)	TCM			1063	1750
	TBM				1910
GAC C (briquetting carbon)	TCM			927	1610
	TBM				1820
GAC D (coal carbon)	TCM			1097	1580
	TBM				1860
GAC E (fibred carbon)	TCM			1059	1720
	TBM				1870
Multiwalled carbon nanotubes (MWCNTs)	TCM	1600	0.40	225 to 295	820	[39]
	BDCM				470
	DBCM				440
	TBM				390
Chitosan-modified coconut shell activated carbon (CS/CAC)	TCM	15.46	0.25	968	39.91	This article
	BDCM	16.2			38.07
	DBCM	16.47			36.1
	TBM	18.16			37.63

indicates the THM adsorption capacity of the coconut shell activated carbon modified with chitosan in comparison to other adsorbent materials obtained from the previous literature. According to the tabulated data, the CS/CAC had a higher adsorption capacity than several other adsorbents. Furthermore, in this study, an infinitesimal concentration of THMs was used.

4. Conclusions

Research aiming at the development of advanced techniques to remove THMs from drinking water is essential. The outcomes of this investigation can be summarized in the following points:

• The optimum value of pH was in the range of 4–8, so there would be no need to adjust the pH values of the contaminated solutions. The maximum adsorption was reached at pH 7, where the adsorbed removals of TCM, BDCM, DBCM, and TBM were 50.95%, 43.09%, 36.66%, and 32.23%, respectively.
• The adsorption isotherm experiment demonstrated that the adsorption of THMs could be accurately characterized by both Freundlich and Langmuir models. Furthermore, THM adsorption was best characterized by the pseudo-second-order kinetic model and was an endothermic reaction. In addition, after four regeneration cycles, the adsorption performance only decreased by approximately 10%, meaning that CS/CAC could be recovered and reused for THM removal.
• According to the results of the continuous adsorption experiments, the testing performed using a 2 mL·min⁻¹ flow rate showed a 20% greater removal efficiency compared to a 4 mL·min⁻¹ flow rate and a 15% greater removal efficiency when using a 4 cm bed instead of a 2 cm bed.

This work demonstrated the considerable potential of CS/CAC as a safe, economical, and promising THM adsorbent.