Synthesis of a Novel Adsorbing Agent by Coupling Chitosan, β-Cyclodextrin, and Cerium Dioxide: Evaluation of Hexavalent Chromium Removal Efficacy from Aqueous Solutions

Abstract

The present study aimed at synthesizing a novel adsorbing agent by coupling chitosan, β-cyclodextrin, and cerium dioxide (Chit/β-CyD/Ce). Its efficiency towards the removal of hexavalent chromium from aqueous solutions was studied and compared to an adsorbent comprising of only chitosan and cerium dioxide. Batch water purification experiments in varying experimental conditions (initial adsorbent concentration 5–100 mg/L, adsorbate concentration 0.1–2 g/L, pH 2–11, and temperature 15–50 °C) were carried out to evaluate the effectiveness of both adsorbents. In all the experimental cases, the Chit/β-CyD/Ce adsorbent exhibited the higher efficacy. The optimum operating conditions were found to be at an initial adsorbent concentration of 2 g/L, pH = 3, and temperature of 50 °C, with the Chit/β-CyD/Ce adsorbent being able to fully remove Cr(VI) from solutions with up to 50 mg/L Cr(VI) at these conditions. The adsorption of hexavalent chromium onto both adsorbents occurs in a multilayer pattern of a heterogeneous surface following the Freundlich isotherm model. Furthermore, the adsorption process was exothermic and obeyed the pseudo-second-order kinetic model, thus indicating the occurrence of chemisorption. Finally, FTIR, XRD, and SEM analyses were performed to characterize the synthesized adsorbents and verify the adsorption process.

1. Introduction

Chromium is a heavy metal of important significance during the treatment of water and wastewater [1], and hexavalent chromium (Cr(VI)), in particular, is of major concern due to its high toxicity [2]. Hexavalent chromium is an abundantly used element by numerous industries, including metallurgical, chemical, and mining [3], with its main applications being in electroplating, pigments production, welding, and leather tannery [4]. Due to the rapid growth in industrial operations during the past decades, heavy metals have been widely used, resulting in increased concentrations in industrial effluents [5]. The aforementioned extensive use of hexavalent chromium has led to reported cases of water pollution in numerous countries, such as China, Brazil, and Italy [6]. Hexavalent chromium as a strong oxidizing agent which, during its reduction to the trivalent form, leads to the formation of free radicals inside the cells of the human body [7]. The latter can cause severe health effects, including neurotoxicity, carcinogenesis, renal impairment, liver dysfunction, and dermal necrosis [7,8]. The toxicity of hexavalent chromium has resulted in the adoption of strict legislation limits by the World Health Organization (WHO), the European Union (EU), and the United States (US). The WHO, EU, and US Environmental Protection Agency (USEPA) have set the maximum regulatory standard of hexavalent chromium in drinking water at 0.05 mg/L [9,10,11]. Additionally, a maximum level of 2 mg/L has been adopted by USEPA for discharges containing any form of chromium, including hexavalent chromium [11]. Another crucial parameter for measuring the toxicity of inorganic metals is the median lethal dose (MLD50), which for the case of Cr(VI) varies between 50 and 150 mg/kg [12]. Consequently, the adverse health effects caused by hexavalent chromium combined with its presence in the aquatic environment necessitate the development of efficient treatment processes for contaminated waters.

Various methods are proposed for the decontamination of hexavalent chromium-contaminated waters, with the most prominent being chemical precipitation, ion exchange, membrane filtration, adsorption, and photocatalysis [13,14]. Although the reported established methods have been proven effective towards hexavalent chromium removal, they present several drawbacks. For instance, chemical precipitation generates a large amount of toxic sludge; ion exchange and membrane filtration demand high operating costs; and photocatalysis often requires the use of catalysts which may be produced in a non-environmentally friendly manner [15]. Adsorption, in contrast to the other methods, is considered to be eco-friendly, low-cost, and efficient in a broad spectrum of alternative operating conditions for water decontamination [16]. Materials used as adsorbents for the purification of water contaminated by heavy metals are mainly carbonaceous and polymer-based ones [17,18,19]. In recent times, interest has shifted in the synthesis of novel adsorbents comprising polysaccharides, e.g., chitosan, or oligosaccharides, e.g., β-cyclodextrin [20]. Moreover, metal oxides, e.g., cerium dioxide, have presented promising results in heavy metals removal and decontamination of the aquatic environment [21]. Therefore, coupling metal oxides with chitosan or β-cyclodextrin exhibits a strong potential towards the decontamination of hexavalent chromium-contaminated waters.

Chitosan (Chit) is a well-known linear polysaccharide, which consists of β-(1-4)-linked d-glucosamine and N-acetyl-d-glucosamine [22]. Chit is classified as a biocompatible and biodegradable compound that does not exhibit toxicity, which makes it a suitable material for environmental applications [23,24]. Moreover, due to its unique structure, namely the presence of functional hydroxyl and amino groups, it possesses exceptional adsorption abilities which, combined with its elevated mechanical strength, renders it an attractive adsorbent material [25]. β-Cyclodextrin (β-CyD) is an oligosaccharide with a cyclic structure comprising seven glucose units, which are joined as α-(1-4) isomers [26]. β-CyD, like Chit, is also a non-toxic and biocompatible material that possesses satisfactory adsorption capacities due to the existence of hydroxyl functional groups in its structure [27]. Cerium dioxide (Ce) is a biocompatible metal oxide that has attained the interest of the scientific community as it has interesting properties, i.e., thermal stability and redox abilities [28]. In recent years, Ce has been studied as an alternative adsorbing agent for the purification of contaminated waters exhibiting promising results [29,30,31]. Although various studies have indicated the efficient removal of heavy metals using β-cyclodextrin coupled with chitosan [13,32,33,34,35], and cerium dioxide coupled with chitosan adsorbents [36], the synthesis of adsorbents by coupling chitosan, β-cyclodextrin, and cerium dioxide has yet to be performed.

The main scope of the current study was the synthesis of an innovative adsorbing agent by coupling cerium dioxide with chitosan and β-cyclodextrin (Chit/β-CyD/Ce). The adsorptive efficiency of the resulting adsorbent towards hexavalent chromium was in turn investigated. To thoroughly assess the efficacy of the synthesized adsorbent, an additional adsorbing agent, which consisted only of chitosan and cerium dioxide (Chit/Ce), as previously reported by Li et al. [35], was synthesized and tested. Hexavalent chromium was adsorbed onto both adsorbents in varying experimental conditions (initial concentrations of hexavalent chromium and adsorbent, pH, and temperature) to evaluate their effect and determine the optimum operating conditions. Furthermore, experiments with varying initial concentrations of hexavalent chromium in the optimum operating conditions (pH, adsorbent dosage, and temperature) were performed to evaluate the efficacy of the process in achieving the regulatory standards set by the WHO, EU, and USEPA regarding the content of hexavalent chromium in drinking water. Additionally, the isotherms, as well as kinetics and thermodynamics, of the process were examined to supply a comprehensive analysis on the adsorptive removal of hexavalent chromium. Finally, the functional groups present in the adsorbing agents synthesized along with the structure were further characterized by FTIR, XRD, and SEM analyses.

2. Materials and Methods

2.1. Materials

Cerium dioxide (purity > 99%), β-cyclodextrin (purity > 98%), and glutaraldehyde (50% solution) were obtained from Acros Organics (Newark, NJ, USA). Glacial acetic acid, 1,5-diphenylcarbazide, and chitosan (high molecular weight with deacetylation degree > 75%) were purchased from Sigma Aldrich Chemical Co. (St. Louis, MO, USA). Potassium dichromate was provided by Mallinckrodt (Dublin, Ireland). All other chemical reagents used in the experimental procedure were of analytical grade.

2.2. Synthesis of the Adsorbents

For the synthesis of the Chit/β-CyD/Ce adsorbent (Figure 1), 4 g of Chit was dissolved in 400 mL of an acetic acid (1% v/v) aqueous solution, and 4 g of β-CyD was dissolved in 400 mL of distilled water. Subsequently, the two solutions were mixed and 4 g of Ce was added to the mix. The resulting solution was stirred for 2 h to obtain a homogeneous solution, followed by a dropwise addition of 80 mL glutaraldehyde (2.5% v/v) aqueous solution, which resulted in the precipitation of the adsorbent. The stirring of the resultant solution was continued for 24 h in order to ensure the completion of the crosslinking procedure. The precipitate was washed three times with distilled water to remove any impurities, freeze-dried, and ground to achieve a constant particle size. For the synthesis of the Chit/Ce adsorbent, the same procedure was followed without the addition of β-CyD.

2.3. Cr(VI) Adsorption Experiments

An appropriate amount of potassium dichromate was dissolved in distilled water to achieve the desired initial concentration of Cr(VI) for each experimental run. Batch experiments were conducted to investigate the effect of the operating conditions (pH, initial concentration of adsorbent and Cr(VI), and temperature) on the adsorption process. Nitric acid and sodium hydroxide solutions (0.5 M) were used to adjust the pH value of the studied solutions. The effect of each parameter was evaluated by changing the value of the studied parameter, while maintaining all the other studied parameters constant. The value range for each parameter studied is presented in

Table 1. List of the experimental conditions examined, their respective value range, and the constant values of the other operating conditions.

Experimental Condition	Value	Constant Operating Conditions
pH	2	C0,Cr(VI) = 50 mg/L, Cadsorbent = 1 g/L, and T = 25 °C
	3
	4
	5
	6
	7
	8
	9
	10
	11
C0,Cr(VI) (mg/L)	5	Cadsorbent = 1 g/L, pH = 3, and T = 25 °C
	10
	25
	50
	75
	100
Cadsorbent (g/L)	0.10	C0,Cr(VI) = 50 mg/L, pH = 3, and T = 25 °C
	0.25
	0.50
	0.75
	1.00
	1.25
	1.50
	1.75
	2.00
T (°C)	15	C0,Cr(VI) = 50 mg/L, Cadsorbent = 1 g/L, and pH = 3
	25
	35
	50

. Samples from the studied solutions were collected and analyzed at predetermined time intercepts (0, 10, 20, 30, 40, 50, 60, and 70 min) to evaluate the Cr(VI) concentration; equilibrium was reached within 70 min in all experimental runs. All experiments were conducted twice.

2.4. Determination of Cr(VI) Concentration

Cr(VI) concentration was evaluated by measuring the absorbance of the samples at 540 nm, following the 1,5-diphenylcarbazide method, using a Hitachi U-2900 UV–Vis spectrophotometer (Tokyo, Japan) [35]. Measurements for the determination of Cr(VI) concentration were conducted in triplicate.

2.5. Analysis of the % Cr(VI) Removal and of the Adsorbed Amount of Cr(VI)

The % Cr(VI) removal from aqueous solutions and the amount of adsorbed Cr(VI) onto the synthesized adsorbents were estimated using Equations (1) and (2), respectively.

$$\% Removal=\frac{C_{0,Cr\left(VI\right)}-C_{e,Cr\left(VI\right)}}{C_{0,Cr\left(VI\right)}}\ast 100$$

(1)

$$q_{e,t}=\frac{(C_{0,Cr\left(VI\right)}-C_{e,t,Cr\left(VI\right)})\ast V}{m}$$

(2)

2.6. Analysis of the Adsorption Isotherms

Isotherm models describing the adsorption process are a useful tool to assess the correlation between the adsorbed amount of the studied contaminant onto the adsorbent and the remaining concentration of the contaminant in the solution at equilibrium. Adsorption isotherm models provide meaningful information regarding the mechanisms involved in the adsorption process, as well as the interactions between the adsorbent and the adsorbate [37]. In the present study, the adsorption data obtained experimentally were fitted in three different models, namely Langmuir, Freundlich, and Temkin models, which are widely used in the evaluation of adsorption processes. According to the Langmuir model (Equation (3)) the adsorption proceeds in a monolayer pattern of a homogeneous surface; the model considers a dimensionless separation factor (R_L), provided by Equation (4), that estimates the favorability of the process: 0 < R_L < 1 favorable, R_L > 1 unfavorable, R_L = 1 linear, and R_L = 0 irreversible. According to the Freundlich model (Equation (5)), adsorption proceeds in a multilayer pattern of a heterogeneous surface. Finally, the Temkin model (Equation (6)) considers that an increase in the coverage of the adsorbent’s surface by the adsorbate results in a decrease in the adsorption heat [38].

$$q_e=\frac{q_m\ast K_L\ast C_e}{1+K_L\ast C_e}$$

(3)

$$R_L=\frac{1}{1+K_L\ast C_0}$$

(4)

$$q_e=K_F\ast C_e^{1/n}$$

(5)

$$q_e=\frac{R\ast T}{b_T}\ast ln{A}_T+\frac{R\ast T}{b_T}\ast ln{C}_e$$

(6)

2.7. Analysis of the Adsorption Kinetics

A kinetic analysis of the adsorption can be used to describe the uptake rate of the adsorbate onto the adsorbent and correlates the adsorbed amount of the studied contaminant with the time of the adsorption process [39]. In the present research, three different kinetic models were assessed: pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. The pseudo-first-order model (Equation (7)) assumes that the adsorption rate is directly proportional to the difference in saturation concentration and the amount of solid uptake with time. The pseudo-second-order model (Equation (8)) assumes that the adsorption process depends on the adsorption capacity of the used adsorbent and that chemisorption is the rate-limiting step of the process [40]. The intraparticle diffusion model (Equation (9)) is based on the assumption that the rate-limiting step of the adsorption is the diffusion of the adsorbate in the pores of the adsorbent [41].

$$log\left(q_e-q_t\right)=log{q}_e-\frac{k_1}{2.303}\ast t$$

(7)

$$\frac{t}{q_t}=\frac{1}{k_2\ast q_e^2}+\frac{t}{q_e}$$

(8)

$$q_t=K_{id}\ast \sqrt{t}+C$$

(9)

2.8. Analysis of the Adsorption Thermodynamics

A thermodynamic analysis of the adsorption process is a vital step to examine the feasibility and spontaneity of the process, as the estimated thermodynamic parameters can provide useful information concerning the nature of the process [42]. In the present study the activation parameters, i.e., the activation energy of the adsorption (E_a), the Gibbs free energy of activation (ΔG°), the enthalpy change (ΔH°), and the entropy change (ΔS°), were calculated by plotting the Arrhenius and van’t Hoff’s diagrams, using Equations (10)–(12).

$$k=-\frac{E_a}{R\ast T}+ln\left(A\right)$$

(10)

$$\Delta G^o=-R\ast T\ast ln \left(\frac{q_e}{C_e}\right)$$

(11)

$$ln \left(\frac{q_e}{C_e}\right)=\frac{\Delta S^o}{R}-\frac{\Delta H^o}{R\ast T}$$

(12)

2.9. Instrumental Characterization of the Synthesized Adsorbents

FTIR analysis was performed in the synthesized adsorbents to identify their functional groups and to verify the adsorption of Cr(VI) onto them using a Jasco 4200 (Jasco Inc., Cremella, LC, Italy) FTIR-spectrometer. The analysis was performed in the range of 400–4000 cm⁻¹ using the KBr pellet method.

XRD analysis was carried out to evaluate the crystallographic nature of the synthesized adsorbents. The analysis was performed using a Bruker D5000 X-ray Powder Diffraction System (Siemens, Germany) in the range of 2θ from 10 to 80° (operating conditions: 40 kV, 30 mA, and λ = 0.154 nm).

SEM analysis of the synthesized adsorbents prior to and after the adsorption process was performed using a QUANTA200 SEM microscope (Thermo Fisher Scientific, Waltham, MA, USA). SEM was used to characterize the beads’ surface structure and evaluate the impact of the adsorption process onto them.

The aforementioned methods were selected in order to validate the efficient synthesis of the novel adsorbing agents and the adsorption of Cr(VI) onto them, and to evaluate critical aspects of the adsorbents, such as crystallinity and surface structure.

2.10. Statistical Analysis

The experimental results were subjected to statistical analysis of variance to investigate the effect of the independent factors on the dependent variables (% Cr(VI) removal, q_e). Further, Duncan’s multiple-range test was used to highlight significant differences among the independent factors (p < 0.05). Statistica 6.0 (Starsoft, Tulsa, OK, USA) was used for all statistical analyses conducted in this work.

3. Results and Discussion

3.1. Effect of Operating Conditions in the Adsorsption of Cr(VI)

Cr(VI) removal percentage and the amount of Cr(VI) adsorbed onto the synthesized beads in all experimental scenarios are presented in

Table 2. Removal percentage and amount of Cr(VI) adsorbed onto the Chit/Ce and Chit/β-CyD/Ce adsorbents.

Studied Parameter		Chit/Ce		Chit/β-CyD/Ce
		% Removal	qe (mgCr(VI)/gadsorbent)	% Removal	qe (mgCr(VI)/gadsorbent)
pH (C0,Cr(VI) = 50 mg/L, Cadsorbent = 1 g/L, and T = 25 °C)	2	77.04 ± 2.76 Ab	38.52 ± 1.38 Ab	79.91 ± 2.32 Bb	39.95 ± 1.14 Bb
	3	87.06 ± 1.18 Aa	43.53 ± 0.59 Aa	93.57 ± 2.19 Ba	46.78 ± 0.68 Ba
	4	75.94 ± 2.38 Ab	37.97 ± 1.20 Ab	78.24 ± 2.18 Bb	39.12 ± 1.36 Bb
	5	69.06 ± 1.57 Ac	34.53 ± 0.79 Ac	69.35 ± 1.86 Bc	34.68 ± 0.95 Bc
	6	68.72 ± 2.09 Ac	34.36 ± 1.04 Ac	69.18 ± 1.78 Bc	34.59 ± 0.83 Bc
	7	64.86 ± 3.21 Acd	32.43 ± 1.60 Acd	66.65 ± 1.39 Bcd	33.33 ± 1.22 Bcd
	8	61.32 ± 2.10 Ad	30.66 ± 1.05 Ad	64.81 ± 2.03 Bd	32.41 ± 1.01 Bd
	9	54.94 ± 1.28 Ae	27.47 ± 0.64 Ae	61.94 ± 1.36 Be	30.97 ± 1.11 Be
	10	54.20 ± 0.66 Aef	27.10 ± 0.33 Aef	60.05 ± 0.75 Bef	30.02 ± 0.56 Bef
	11	50.76 ± 1.23 Af	25.38 ± 0.61 Af	57.14 ± 0.98 Bf	28.57 ± 0.82 Bf
C0,Cr(VI) (mg/L) (Cadsorbent = 1 g/L, pH = 3, and T = 25 °C)	5	100.00 ± 0.00 Aa	5.00 ± 0.00 Aa	100.00 ± 0.00 Ba	5.00 ± 0.00 Ba
	10	92.10 ± 2.97 Aab	9.21 ± 0.30 Aab	100.00 ± 0.00 Bab	10.00 ± 0.00 Bab
	25	90.44 ±1.20 Aab	22.61 ± 0.40 Aab	95.35 ±1.69 Bab	23.84 ± 0.81 Bab
	50	87.06 ± 1.18 Ab	43.53 ± 0.59 Ab	93.57 ± 2.19 Bb	46.78 ± 0.68 Bb
	75	76.39 ± 3.29 Ac	57.29 ± 1.89 Ac	83.16 ± 2.81 Bc	62.37 ± 1.95 Bc
	100	65.40 ± 2.28 Ac	65.40 ± 2.28 Ac	79.06 ± 3.02 Bc	79.06 ± 2.14 Bc
Cadsorbent (g/L) (C0,Cr(VI) = 50 mg/L, pH = 3, and T = 25 °C)	0.1	35.80 ± 1.66 Ag	179.00 ± 4.59 Ag	41.65 ± 1.82 Bg	208.24 ± 3.19 Bg
	0.25	59.62 ± 2.55 Af	119.24 ± 3.97 Af	63.40 ± 2.41 Bf	126.80 ± 2.58 Bf
	0.5	75.78 ± 0.92 Ae	75.78 ± 0.92 Ae	82.86 ± 1.04 Be	82.86 ± 1.04 Be
	0.75	81.74 ± 1.13 Ad	54.49 ± 1.03 Ad	88.68 ± 1.25 Bd	59.12 ± 0.97 Bd
	1	87.06 ± 1.18 Ac	43.53 ± 0.59 Ac	93.57 ± 2.19 Bc	46.78 ± 0.68 Bc
	1.25	89.92 ± 1.42 Abc	35.97 ± 0.87 Abc	94.95 ± 1.18 Bbc	37.98 ± 0.48 Bbc
	1.5	91.70 ± 1.39 Aab	30.57 ± 0.61 Aab	95.91 ± 0.86 Bab	31.97 ± 0.52 Bab
	1.75	93.62 ± 1.47 Aa	26.75 ± 0.49 Aa	96.86 ± 1.21 Ba	27.67 ± 0.44 Ba
	2	93.98 ± 0.87 Aa	23.50 ± 0.34 Aa	97.68 ± 0.73 Ba	24.42 ± 0.49 Ba
T (°C) (C0,Cr(VI) = 50 mg/L, Cadsorbent = 1 g/L, and pH = 3)	15	83.24 ± 1.69 Ac	41.62 ± 0.84 Ac	87.36 ± 1.78 Bc	43.68 ± 0.81 Bc
	25	87.06 ± 1.18 Ab	43.53 ± 0.59 Ab	93.57 ± 2.19 Bb	46.78 ± 0.68 Bb
	35	89.22 ± 1.37 Aab	44.61 ± 0.68 Aab	95.71 ± 1.74 Bab	47.85 ± 0.59 Bab
	50	91.86 ± 0.93 Aa	45.93 ± 0.46 Aa	97.89 ± 1.69 Ba	48.94 ± 0.72 Ba

Superscript letters A, B represent significant differences (p < 0.05) between the two adsorbents. Superscript letters a–g represent significant differences (p < 0.05) between the values of every operating condition.

. The effect of each operating condition on the adsorption process is discussed in detail separately in the following subsections depending on each studied parameter. From the results depicted in Table 2, the Chit/β-CyD/Ce adsorbent exhibited higher efficacy in the adsorption of Cr(VI), when compared to the Chit/Ce adsorbent.

3.1.1. pH

The influence of pH on the efficiency of the synthesized adsorbents is presented in Figure 2. In all cases, the Chit/β-CyD/Ce-synthesized adsorbent exhibited significantly better performance compared to the Chit/Ce adsorbent (p < 0.05): higher Cr(VI) removal efficiency and higher amount of adsorbed Cr(VI) were observed using the Chit/β-CyD/Ce adsorbent. Optimum pH conditions for the adsorption of Cr(VI) were observed at pH = 3 for both adsorbents, with Cr(VI) removal percentage being 87.06 ± 1.18% and 93.57 ± 2.19% for Chit/Ce and Chit/β-CyD/Ce, respectively. The decline in the removal efficiency for both adsorbents at pH = 2 is attributed to the limited stability of the adsorbents consisting of chitosan in extreme acidic conditions [43,44] and to the repulsion between β-cyclodextrin and Cr(VI) ions [45]. The decrease in the removal efficiency for both adsorbents with an increase in pH can be associated with the pK_a value of chitosan (6.20); high pH values do not favor the protonation of the functional amine groups present in the adsorbents, which interact with the Cr(VI) ions ($HCr{O}_4^{-}$ and $Cr{O}_4^{2-}$) [46]. Moreover, the competition for the active sites of the adsorbents between the Cr(VI) species and the hydroxides present in solution may be responsible for the sharp decrease in the efficiency of both adsorbents at neutral and alkaline pH [47]. The obtained results regarding the influence of pH on the efficacy of the process can be characterized as rational, since extreme acidic conditions hinder adsorption, and the increase in pH leads to a gradual decrease in the removal of hexavalent chromium in agreement with literature [43,44,45,46,47].

3.1.2. Initial Cr(VI) Concentration

Figure 3 illustrates the influence of the initial Cr(VI) concentration on the adsorption of Cr(VI) onto the Chit/Ce and Chit/β-CyD/Ce adsorbents. Generally, highest removal efficiency of Cr(VI) was observed at the experiments where Chit/β-CyD/Ce was used as the adsorbing agent (p < 0.05). Chit/β-CyD/Ce adsorbent was able to achieve complete removal of Cr(VI) at initial Cr(VI) concentrations of 5 and 10 mg/L. On the contrary, in the case of Chit/Ce, an almost complete removal of Cr(VI) was attained only with a 5 mg/L initial Cr(VI) concentration in the solution. Furthermore, an increase in the initial Cr(VI) concentration resulted in a decrease in the adsorption efficiency of both adsorbents; however, the amount of Cr(VI) adsorbed onto the adsorbents increased. The highest adsorbed amount of Cr(VI) was reported at an initial Cr(VI) concentration equal to 100 mg/L: 65.40 ± 2.28 and 79.06 ± 2.14 mg or Cr(VI) per g of Chit/Ce and Chit/β-CyD/Ce were adsorbed, respectively. The results of this subsection were also applied in the analysis of the adsorption kinetics (Section 3.3).

3.1.3. Initial Adsorbent Concentration

The influence of the adsorbent concentration on the efficiency of the synthesized adsorbents is depicted in Figure 4. In all experimental runs, Chit/β-CyD/Ce adsorbent exhibited significantly highest removal efficiency and adsorption capacity than the Chit/Ce adsorbent (p < 0.05). The highest amount of Cr(VI) adsorbed was reported at an adsorbent concentration of 0.1 g/L (179.00 ± 4.59 mg/g and 208.24 ± 3.19 mg/g for Chit/Ce and Chit/β-CyD/Ce adsorbent, respectively), while the highest Cr(VI) removal percentage was achieved at 2 g/L of adsorbent (93.98 ± 0.87% and 97.68 ± 0.73% for Chit/Ce and Chit/β-CyD/Ce adsorbent, respectively). Further, based on Figure 4, the Cr(VI) removal percentage along with the amount of Cr(VI) adsorbed onto the synthesized adsorbents reached a plateau at an adsorbent concentration of 1.25 g/L for the Chit/β-CyD/Ce adsorbent (94.95 ± 1.18% and 37.98 ± 0.48 mg/g) and 1.5 g/L for the Chit/Ce adsorbent (91.70 ± 1.39% and 30.57 ± 0.61 mg/g). The aforementioned observations indicate that purification of Cr(VI)-contaminated aqueous streams can be achieved without the need of an excessive adsorbent dosage. The results obtained from the study of the adsorption process versus the adsorbent dosage were also used in the analysis of the adsorption isotherms (Section 3.2).

3.1.4. Temperature

Figure 5 illustrates the influence of the initial Cr(VI) concentration on the adsorption of Cr(VI) onto the Chit/Ce and Chit/β-CyD/Ce adsorbents. In all of the studied temperatures, Chit/β-CyD/Ce adsorbent exhibited significantly higher removal efficiency compared to the Chit/Ce adsorbent (p < 0.05). Moreover, an increase in the temperature resulted in an increase in the Cr(VI) removal percentage and the adsorbed amount of Cr(VI) for both adsorbents. The highest removal percentage (91.86 ± 0.93% and 97.89 ± 1.69% for Chit/Ce and Chit/β-CyD/Ce adsorbents, respectively) and adsorbed amount of Cr(VI) (45.93 ± 0.46 mg/g and 48.94 ± 0.72 mg/g for Chit/Ce and Chit/β-CyD/Ce adsorbents, respectively) were attained at 50 °C. The thermodynamic analysis (Section 3.4) of the adsorption process was based on these results.

3.1.5. Adsorption in the Optimum Conditions

According to the results of the previous subsections, optimum operating conditions were observed at pH = 3, adsorbent concentration of 2 g/L, and temperature of 50 °C. Adsorption experiments were also performed using different initial concentrations of Cr(VI) at these optimum operating conditions and the results are presented in

Table 3. Removal percentage and amount adsorbed of Cr(VI) onto the Chit/Ce and Chit/β-CyD/Ce adsorbents at optimum operating conditions.

Initial Cr(VI) Concentration (mg/L)	Chit/Ce		Chit/β-CyD/Ce
	% Removal	qe (mgCr(VI)/gadsorbent)	% Removal	qe (mgCr(VI)/gadsorbent)
5	100.00 ± 0.00 A	2.50 ± 0.00 A	100.00 ± 0.00 B	2.50 ± 0.00 B
10	100.00 ± 0.00 A	5.00 ± 0.00 A	100.00 ± 0.00 B	5.00 ± 0.00 B
25	95. 34 ± 0.82 A	11.92 ± 0.41 A	100.00 ± 0.00 B	12.5 ± 0.00 B
50	93.73 ± 1.02 A	23.43 ± 0.53 A	100.00 ± 0.00 B	25 ± 0.00 B
75	90.57 ± 1.27 A	33.96 ± 0.62 A	96.16 ± 1.14 B	36.06 ± 0.57 B
100	86.57 ± 1.53 A	43.28 ± 0.55 A	95.79 ± 0.72 B	47.89 ± 0.36 B

Superscript letters A, B represent significant differences (p < 0.05) between the two adsorbents.

.

According to Table 3, the Chit/β-CyD/Ce-synthesized adsorbent presented greater efficiency in the removal of Cr(VI) compared to the Chit/Ce (p < 0.05). Specifically, a complete removal of Cr(VI) from solutions was achieved when initial concentrations were 5 and 10 mg/L using the Chit/Ce-synthesized adsorbent and from solutions with 5, 10, 25, and 50 mg/L using the Chit/β-CyD/Ce-synthesized adsorbent. Thus, the treatment of Cr(VI)-contaminated water with Chit/β-CyD/Ce at the optimum operating conditions results in total detoxification even in solutions with high content of Cr(VI). Moreover, although the Chit/Ce adsorbent was unable to totally adsorb the Cr(VI) from solutions of 25 mg/L, the remaining concentration of Cr(VI) was lower than 2 mg/L, which is the standard set by USEPA for industrial discharges. Therefore, both synthesized materials can be characterized as effective adsorbing agents with high potential in the heavy metals adsorption from both the aquatic environment and industrial effluents.

3.2. Isotherm Models

The data presented in the Section 3.1.3 were fitted to three different adsorption isotherm models, namely the Langmuir, Freundlich, and Temkin models (Figure 6), to assess the nature of the involved process; the constants of the models are presented in

Table 4. Calculated constants of the adsorption isotherm models.

Isotherm Constants
Langmuir	Chit/Ce	Chit/β-CyD/Ce
qm (mgCr(VI)/gadsorbent)	270.27	144.93
KL (L/mgCr(VI))	0.032	0.157
RL	0.386	0.113
R2	0.9856	0.9613
Freundlich	Chit/Ce	Chit/β-CyD/Ce
KF ((mgCr(VI)/gadsorbent) ∗ (L/mgCr(VI))1/n)	9.294	20.831
1/n	0.842	0.650
R2	0.9954	0.9920
Temkin	Chit/Ce	Chit/β-CyD/Ce
AT (L/g)	0.386	0.906
bT (kJ/mol)	0.042	0.049
R2	0.9044	0.8752

.

According to the correlation coefficients derived from the fitting of the three studied models, the Freundlich isotherm model yielded the best fit for both adsorbents. Therefore, the adsorption of Cr(VI) more likely occurs in a multilayer pattern of a heterogeneous surface [48]. The 1/n and K_F values from the Freundlich model for both adsorbents suggest a strong affinity between the adsorbents and the contaminant, which indicates that the adsorption of Cr(VI) occurs spontaneously [49]. However, the higher K_F and n values for the Chit/β-CyD/Ce indicate that this adsorbent is more efficient towards the adsorption of Cr(VI) compared to the Chit/Ce adsorbent [50]. Although Langmuir and Temkin isotherm models do not obtain an adequate fit for the data of the adsorption process, their calculated constants highlight the spontaneity of the process and the involvement of physicochemical interactions between the contaminant and the adsorbents, respectively [51,52,53]. The results from the adsorption isotherm analysis are in good agreement with other studies that have associated the adsorption of Cr(VI) onto various adsorbents with the Freundlich isotherm model [54,55,56,57,58]. Moreover, the Langmuir isotherm model has also been reported to describe the adsorption of Indigo Carmine onto the Chit/Ce adsorbent [59].

3.3. Kinetics

A kinetic analysis of the adsorption process was carried out by fitting the data of Section 3.1.2 to the pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. The calculated constants of each model are presented in

Table 5. Calculated constants of the kinetic models.

C0,Cr(VI) (mg/L)	qexp	Kinetic Models and Parameters
		Pseudo-First-Order			Pseudo-Second-Order			Intraparticle Diffusion
		qcal	k1	R2	qcal	k2	R2	Kid	C	R2
Chit/Ce
5	5.00	3.95	0.0601	0.944	5.66	0.0233	0.9899	0.6849	0.5127	0.9326
10	9.21	6.44	0.0778	0.9497	9.91	0.0218	0.9993	1.2749	1.3442	0.8756
25	22.61	13.50	0.0933	0.9164	23.64	0.0172	0.9998	3.1293	3.9568	0.8289
50	43.53	23.64	0.0534	0.8570	45.45	0.0055	0.9965	5.1430	8.7681	0.8095
75	57.29	33.97	0.0677	0.8961	60.98	0.0043	0.9999	7.1198	10.173	0.8451
100	65.40	38.34	0.0783	0.912	68.97	0.0039	0.9986	8.1092	12.585	0.8251
Chit/β-CyD/Ce
5	5.00	5.05	0.4670	0.9857	5.29	0.0498	0.9860	0.7686	0.5995	0.8760
10	10.00	10.84	0.2432	0.9193	10.80	0.0165	0.9929	1.3529	1.3230	0.8910
25	23.84	26.56	0.1352	0.8635	24.88	0.0162	0.9998	3.2736	4.2483	0.8234
50	46.78	53.85	0.0601	0.8378	50.51	0.0062	0.9977	6.4320	6.7477	0.8776
75	62.37	66.97	0.0690	0.8371	63.29	0.0020	0.9999	8.4827	13.475	0.7574
100	79.06	95.59	0.0444	0.7934	83.33	0.0012	0.9999	11.0150	13.251	0.8427

.

As shown in Table 5, the adsorption of Cr(VI) onto the synthesized adsorbents is better described by the pseudo-second-order kinetic model, which indicates that chemisorption is the driving force of the process [60]. In all experiments, the Chit/β-CyD/Ce adsorbent was found to be able to adsorb higher amounts of Cr(VI) compared to the Chit/Ce one, both experimentally and theoretically by the pseudo-first and pseudo-second-order models. Moreover, the calculated values of q_e, based on the pseudo-second-order model were similar to those obtained experimentally, which further strengthens the claim that chemisorption occurs between the adsorbents and the adsorbates. Additionally, for the case of the pseudo-second-order kinetic model, an increase in the initial concentration of Cr(VI) resulted in a decrease in the kinetic rate constants; hence the adsorption occurred at a slower rate due to the decrease in the ratio of the active sites of the adsorbent media and the Cr(VI) molecules [35]. The high values of the C constant, especially at high Cr(VI) concentrations, indicate that the intraparticle diffusion does not govern the process. Thus, it can be stated that bulk mass transfer onto the Chit/Ce and Chit/β-CyD/Ce adsorbents is important during the course of the process [61]. The kinetic analysis of the present study agrees with previously reported studies, whereby chemisorption has been reported as the driving force for the removal of Cr(VI) from the aquatic environment via adsorption [62,63,64,65].

3.4. Thermodynamics

The results from the experiments on the adsorption versus temperature (Section 3.4) were used to generate the Arrhenius and van’t Hoff’s diagrams (Figure 7) and to calculate the thermodynamic parameters (

Table 6. Calculated activation energy of the adsorption.

Adsorbent	Ea (kJ/mol)	R2
Chit/Ce	68.88	0.9958
Chit/β-CyD/Ce	67.14	0.9971

and

Table 7. Calculated thermodynamic parameters of the adsorption process.

T (K)	ΔG° (kJ/mol)	ΔH° (kJ/mol)	ΔS° (J/mol∗K)	R2
Chit/Ce
288.15	−3.84	−17.91	75.67	0.9956
298.15	−4.73
308.15	−5.41
323.15	−6.51
Chit/β-CyD/Ce
288.15	−4.63	−41.19	159.55	0.9919
298.15	−6.64
308.15	−7.95
323.15	−10.31

).

The activation energy for the Chit/β-CyD/Ce adsorbent was estimated at 67.14 kJ/mol, and that for the Chit/Ce at 68.88 kJ/mol. The calculated E_a values are within the typical range of values for chemisorption, namely from 8.4 to 83.7 kJ/mol and, hence, the adsorption can be characterized as chemisorption [66]. Moreover, the calculated activation energy values are relatively low, which suggests that the adsorption onto the adsorbent surface occurs rapidly with the rate-limiting step of the process being the binding of Cr(VI) with the adsorbents’ surface active sites [67,68]. The negative values in the Gibbs free energy of activation indicate that the adsorption of Cr(VI) onto the synthesized adsorbents is feasible. Moreover, an increase in temperature resulted in a decrease in ΔG°. Therefore, increasing the temperature favors the adsorption and results in an enhanced efficacy of the process. The exothermic nature of the adsorption onto the Chit/Ce and Chit/β-CyD/Ce adsorbents is highlighted by the negative values in the enthalpy change for both adsorbents. Moreover, the positive calculated values in the entropy change indicate an increase in the randomness of the system and the stable attachment of Cr(VI) onto the adsorbents [69]. The negative values of ΔH° combined with the positive values of ΔS° verify the spontaneous and feasible nature of the process [70].

3.5. Adsorbent Characterization

The FTIR spectra of the synthesized adsorbents prior to and after the adsorption, as well as the FTIR spectra of potassium dichromate, are depicted in Figure 8.

Prior to the adsorption, FTIR analysis revealed major peaks at 3430 and 3441 cm⁻¹ for Chit/Ce and Chit/β-CyD/Ce adsorbents, respectively, that are attributed to the existence of primary amines in Chit structure. Moreover, the peaks at 2905 and 2880 cm⁻¹ for Chit/Ce and Chit/β-CyD/Ce adsorbents, respectively, are a result of the bonding between carbon and hydrogen occurring in Chit and β-CyD. The bond between carbon and nitrogen, present in the structure of Chit, is depicted by the peak at the wavenumber of 1080 and 1075 cm⁻¹ for Chit/Ce and Chit/β-CyD/Ce adsorbents, respectively [71]. Additionally, the peak at 464 cm⁻¹ for both adsorbents indicates the presence of cerium dioxide in their structure. The reported slight differences in the major peaks at the synthesized adsorbents are a result of the coupling between Chit and β-CyD in the Chit/β-CyD/Ce adsorbent. Once the adsorption of Cr(VI) takes place, new peaks at 555, 755, and 890 cm⁻¹ for both adsorbents are presented, with Cr-O-Cr stretching vibrations being responsible for the former two and Cr-O₃ for the latter, thus verifying the adsorption process [72]. Furthermore, the shift in some major peaks for both adsorbents (i.e., 3430 and 2905 to 3445 and 2916 cm⁻¹ for Chit/Ce, and 3441 and 2880 to 3458 and 2895 cm⁻¹ for Chit/β-CyD/Ce) indicates the successful adsorption of Cr(VI) onto Chit/Ce and Chit/β-CyD/Ce, while hindering the role of the amine and hydrocarbons in the process [59].

The XRD spectra of plain chitosan, β-cyclodextrin, cerium dioxide, and the Chit/Ce and Chit/β-CyD/Ce-synthesized adsorbents are presented in Figure 9.

The XRD analysis of plain chitosan revealed a major peak at 20.58°, which is typical of the semi-crystalline peaks of chitosan [73]. β-Cyclodextrin exhibited several peaks, especially between 10 and 30°, which indicates a semi-crystalline structure obtained through hydrogen bindings in its structure [74]. Cerium dioxide presented characteristic peaks at 28.76, 33.26, 47.64, and 56.51°, in accordance with the International Centre of Diffraction Data (ICDD: 02-8709). On both synthesized adsorbents, the intensity of the peaks was reduced, especially in the case of the Chit/β-CyD/Ce adsorbent, indicating a change in the crystalline nature of each individual compound due to their coupling [74]. Moreover, the shift in the major peaks at the synthesized adsorbents, when compared to the plain compounds, indicates the successful coupling of chitosan, β-cyclodextrin, and cerium dioxide [73].

Figure 10 shows the results from the SEM analysis of the synthesized adsorbents prior and after the adsorption of Cr(VI). According to the SEM analysis, both adsorbents prior to the adsorption presented a rugged surface with cavities, in which Cr(VI) ions can attach. The surface of the Chit/Ce and Chit/β-CyD/Ce-synthesized adsorbents after the adsorption was found to be uniform, due to the linkage between the adsorbent and the adsorbate, thus validating the effectiveness of the adsorption process.

4. Conclusions

In the present study, an innovative adsorbing agent was synthesized by coupling chitosan, β-cyclodextrin, and cerium dioxide (Chit/β-CyD/Ce), and its efficacy towards the removal of Cr(VI) from aqueous solutions was studied. Moreover, an adsorbent consisting only of chitosan and cerium dioxide (Chit/Ce) was fabricated for comparison reasons. In all experimental conditions, the novel Chit/β-CyD/Ce exhibited higher Cr(VI) removal efficiency compared to Chit/Ce. Conditions found to be optimum for the adsorption process were: pH = 3, temperature = 50 °C, and initial adsorbent concentration ranging between 1.25 and 2.00 g/L for both adsorbents. Although high concentrations of Cr(VI) resulted in a decreased adsorptive efficacy, at optimum operating conditions, the Chit/β-CyD/Ce adsorbent proved to be able to totally detoxify Cr(VI)-contaminated waters with initial concentrations of Cr(VI) up to 50 mg/L. The process was better described by the Freundlich isotherm model; thus, adsorption occurs in a multilayer pattern of a heterogeneous surface. Additionally, adsorption proved to obey the pseudo-second-order kinetic model and to be exothermic, spontaneous, and more favorable at high temperatures. The adsorbents were characterized by means of FTIR, XRD, and SEM analyses, and the adsorption process was verified based on the obtained results. The overall efficiency of both adsorbents, especially for Chit/β-CyD/Ce, indicates that the synthesized adsorbents can in fact be used in the field of water purification and provide a viable solution towards detoxification of water from high-impact contaminants.