Kinetic, Isothermal and Thermodynamic Study on the Removal of Hexavalent Chromium with Biocomposites (Cellulose–PLA)

Abstract

Currently, water is being polluted via various anthropogenic activities, resulting in wastewater contaminated with multiple pollutants, including heavy metals. Hexavalent chromium is a toxic heavy metal that poses significant health risks upon exposure. Biocomposites are materials that are partially composed of organic substances that enhance different properties of a composite. The aim of this study was to evaluate the kinetic, isothermal, and thermodynamic behaviour of a cellulose-based biocomposite with polylactic acid (PLA) for the removal of Cr (VI) from synthetic water. The results indicated that the Freundlich and Elovich models provided the best fit for the isothermal and kinetic data, with R² values of 0.671 and 0.973, respectively, suggesting that the adsorption process was chemical in nature and occurred on a heterogeneous, multilayer surface. Additionally, the thermodynamic analysis revealed that the adsorption process was exothermic, irreversible, and non-spontaneous. This study presents an innovative approach to the removal of metal ions using a cellulose–PLA biocomposite for wastewater treatment, offering kinetic, isothermal, and thermodynamic data applicable to the adsorption of other heavy metals.

1. Introduction

One of the most essential components for all living organisms is water, particularly for humans, as approximately 60% of the human body weight consists of this substance. The contamination of this resource is caused by various anthropogenic activities, which results in wastewater containing a variety of substances [1]. Among the pollutants present in wastewater are heavy metals, which accumulate in the tissues of living organisms upon exposure and lead to the development of various health conditions, such as cancer, respiratory system disorders, and even death [2].

Chromium (Cr) is a heavy metal that, in aqueous media, is found in trivalent and hexavalent states, the latter being the most dangerous and toxic by comparison. When it enters the body, it can cause multiple negative health effects, such as digestive, reproductive, and immunologic system dysfunction, as well as cancer and death [3]. Chromium is widely used by different industries, such as metal processing, the dyeing of materials, leather tanning, and paint manufacturing, among others [4]. The World Health Organization allows a maximum limit in drinking water of 0.05 mg/L for Cr (VI) [5].

Different techniques have been developed for the removal of contaminants in water bodies, such as flocculation [6], electrocoagulation [7], and adsorption, among others. Adsorption is a surface phenomenon where adsorbates are transferred to adsorbents. This phenomenon has been shown to be an excellent method for the removal of pollutants present in water bodies, due to its scalability, feasibility, and chemical removal efficiency; this is a simple and affordable strategy due to its remarkable ability to remove different heavy metals [6,7]. In adsorption processes, it is very important to determine the interaction between the adsorbate and the adsorbent, and mathematical models have been developed to describe these interactions, known as isothermal models [8]. On the other hand, in adsorption processes, kinetics is of the utmost importance because it reflects the rate at which a sorbate is retained or released from an aqueous solution to the interface present in the solid phase [9]. In the adsorption process, the interactions between the adsorbent and adsorbate can include physical adsorption (physisorption), where adsorption is based on Van der Waals forces, presenting weak interactions between the adsorbed contaminant molecules and the surface of the adsorbent material, or chemical adsorption (chemisorption), where chemical bonds are formed between the adsorbed molecules and the adsorbent surface, and these bonds can be ionic or covalent [10].

Various studies have employed the adsorption method for the removal of contaminants in solutions, using biomass derived from agricultural waste. The study conducted by Abdul Rahim et al. [11] developed an adsorbent based on coconut waste modified through a carbonisation method for the removal of Pb (II). The results showed that the adsorbent followed the Langmuir isotherm and the pseudo-second-order kinetic model, indicating that the adsorption process occurred on a monolayer surface and was of a chemical nature. The maximum adsorption capacity of 3.51 mmol/g for the removal of Pb (II) was also evidenced. Furthermore, after conducting an adsorption–desorption process, it was demonstrated that the biomaterial was regenerable for up to five adsorption–desorption cycles of Pb (II), with efficiency above 85% for Pb (II) removal. On the other hand, a study conducted by Tejada-Tovar et al. [12] evaluated water hyacinth as an adsorbent material for Cr (VI) and Hg (II) in solution. The results showed that the adsorbent contained components such as cellulose, hemicellulose, and lignin, which explained its adsorptive properties. This biomass presented a removal percentage of 73.4% for Cr (VI) and 79.3% for Hg (II). The adsorbent best fitted the Elovich model, confirming that the adsorption process occurred both on the surface and within the pores, with heterogeneous active sites.

Materials that are composed of two or more substances to generate a new material with superior characteristics to those of the separate components used are known as composites. The materials used for the generation of composites are usually metallic, polymeric, inorganic, or organic [13]. Among the different existing composites are biocomposites, which are materials that are mixed with one or more biological raw materials. These compounds are usually combined with plant fibres, such as oil palm fibres, bamboo [14], coconut shells [15], or sugar palm [16], among others.

Polylactic acid (PLA) is a biobased linear aliphatic polyester derived from renewable resources such as corn starch and sugar beet. Compared to other polyesters, it possesses excellent physical and mechanical properties, as well as the characteristics of biodegradability and biocompatibility. Furthermore, it requires up to 50% less energy consumption for its production, making it a low-cost product and a good alternative to the petroleum-based plastics used in products such as food packaging, non-woven fabrics, and electronics [17]. PLA has been used as a reinforcement to improve the properties of materials of organic origin, including cellulose—a biopolymer derived from organic material that, depending on the extraction process, can be found in fibre, micro-fibrillated, or nanoscale forms. Combining cellulose with PLA results in a biocomposite with unique characteristics, such as a hydrophobic surface due to the properties of PLA, as well as antimicrobial and crystalline capacities. Compared to common plastics, these biocomposites are also biodegradable [18]. Some applications involving the implementation of cellulose with PLA include the production of sustainable and environmentally friendly packaging, improving the resistance of materials, and serving as a barrier for water vapor and gases, among others [19,20].

Different bio-compounds have been used for the removal of pollutants present in water bodies. Some of the compounds generated are activated carbon/cellulose biocomposites based on sisal cellulose for the removal of methylene blue [21]; biocomposites of polypyrrole, polyaniline, and sodium alginate with cellulosic biomass based on barley husk for the removal of 2,4-dichlorophenol [22]; and biocomposites based on chitosan derived from shrimp shells and modified with kaolinite-rich clay for the removal of Cr (VI), Pb (II), and methylene blue [23]. Biocomposites of birefringent cellulosic materials have also been developed for environmental treatment [24].

Consequently, this study aimed to evaluate the kinetic, isothermal, and thermodynamic behaviour of a biocomposite from cellulose extracted from agroindustrial waste with polylactic acid for the removal of Cr (VI) in synthetic waters. This biocomposite (cellulose–PLA) is presented as an alternative for the treatment of wastewater and the use of agroindustrial waste, thus contributing to a reduction in the existing gap in the development of innovative products based on organic materials. Furthermore, there is limited scientific literature reporting studies using residual biomass from banana pseudo-stems modified with PLA for the removal of Cr (VI) in solution.

2. Materials and Methods

2.1. Preparation and Characterisation of the Biocomposite

For the preparation and characterisation of the biocomposite, the methodology proposed by Villabona-Ortíz et al. [25] was used, where cellulose acetate was initially obtained by subjecting banana pseudo-stem fibre to a pretreatment. Subsequently, cellulose was extracted through double alkaline extraction using sodium hydroxide (NaOH 2 wt.%) to remove some impurities, such as lignin, hemicellulose, waxes, and pectins. The obtained fibres were then washed with distilled water, and the cellulose was subsequently dried. After this, a bleaching process was performed using NaClO₂ and CH₃COOH with distilled water under magnetic stirring to remove the remaining impurities, such as residual hemicellulose and lignin from the previous stage, as well as pigments that gave colour to the cellulose. The bleached cellulose was washed with water and then dried. The cellulose was then added to CH₃COOH and subjected to a stirring process. After this process, a solution of CH₃COOH and 95% H₂SO₄ was added, and stirring was carried out again. A mixture of acetic anhydride and H2SO₄ was then added, followed by further stirring. Finally, distilled water with CH₃COOH was added, and the solution was left under stirring before being vacuum-filtered. After obtaining the cellulose acetate, a solution of dichloromethane with PLA and another solution of acetone with the obtained acetate were prepared. Each solution was then poured into a glass container with a lid and exposed to an ultrasound process. Subsequently, 20 mL of the acetate solution was mixed with 5 mL of the PLA solution in a Petri dish and allowed to stand for a few minutes, obtaining a biocomposite with proportions of 85% cellulose and 15% PLA. The synthesised biocomposite was subjected to scanning electron microscopy (SEM) analysis to study the morphology and surface composition before and after the adsorption process on a TESCAN model MIRA 3, and Fourier transform infrared spectroscopy (FTIR) was used to identify the functional groups present on the biocomposite’s surface and to identify possible chemical interactions after the chromium adsorption process. The samples were examined between 4000 and 400 cm⁻¹. The bands corresponding to the hydroxyl, carbonyl, and aromatic groups were evaluated; these bands show the presence of components such as cellulose and PLA. This characterisation was carried out in an IRAffinity-1 FTIR instrument, SHIMADZU, Series A213749. Figure 1 shows a flowchart of the biocomposite preparation process.

2.2. Experimental Study and Adsorption Isotherm Models

To perform the analysis of the adsorption isotherm models, batch experiments were conducted in plastic containers using a Cr (VI) solution at different concentrations, with varying adsorbent doses and at an adjusted pH, with a contact time of 24 h, to select the best operating conditions for the study.

2.2.1. Langmuir Isothermal Model

The Langmuir model is used to describe the equilibrium that is generated between the adsorbate and the adsorbent; this model presents the adsorption process in a monolayer, being homogeneous and without interaction between the adsorbed molecules [26]. This model is described by Equation (1):

$$q_e={\displaystyle \frac{q_{max}b{C}_e}{1+b{C}_e}}$$

(1)

2.2.2. Freundlich Isothermal Model

The Freundlich model indicates that the adsorption process occurs on the heterogeneous surface of the adsorbent, forming a multilayered adsorbent shell in it [27], where the distribution of the components is influenced by the time and energy of the adsorbed sites present [28]. This model is described by Equation (2):

$$q_e=k_f{C}_e^{1/n}$$

(2)

2.2.3. Dubinin–Radushkevich Isothermal Model

The Dubinin–Radushkevich model relates to the energy of the adsorption process. This model describes adsorption in micropores, where it is assumed that the pore size distribution is heterogeneous and can be described by a Gaussian function [29,30]. This model is described by Equation (3):

$$q_e=q_{mDR}\ast e^{-A{\left(\ln \left(1+{\displaystyle \frac{1}{C_e}}\right)\right)}^2}$$

(3)

The parameter A is defined by Equation (4):

$$A=b_{RD}{R}^2{T}^2$$

(4)

2.3. Kinetic Models of Adsorption

In order to corroborate the yield of an adsorbent in an adsorption process, it is necessary to carry out a study of the adsorption kinetics, because this describes the chemical and physical mechanisms that occur in the adsorption process [31].

2.3.1. Pseudo-First-Order Kinetic Model

The pseudo-first-order kinetic equation was proposed by the Swedish chemical physicist Sten Lagergren in 1898 [32]. This model is presented in its linear and non-linear forms in Equations (5) and (6), respectively.

Linear equation:

$$ln\left(q_e-q_t\right)=ln{q}_e-k_{1}t$$

(5)

Non-linear equation:

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

(6)

2.3.2. Pseudo-Second-Order Kinetic Model

The pseudo-second-order kinetic equation was presented by Ho, Y.S. and McKay, G. in 1996. This model is often used to predict adsorption experimental data and calculate adsorption rate constants [32]. This model is presented in its linear and non-linear forms in Equations (7) and (8), respectively.

Linear equation:

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{1}{q_e}}$$

(7)

Non-linear equation:

$$q_t={\displaystyle \frac{q_e^2{k}_{2}t}{1+q_e{k}_{2}t}}$$

(8)

2.3.3. Elovich Kinetic Model

The Elovich model makes two assumptions: that the activation energy increases with the adsorption time and that the adsorbent surface is heterogeneous. This model is an empirical model that has no physical meaning. It is generally used to model the chemisorption of gases on solids [33]. This model is presented in its linear and non-linear forms in Equations (9) and (10), respectively.

Linear equation:

$$q_t={\displaystyle \frac{1}{\beta }}\ln \left(\alpha \beta \right)+{\displaystyle \frac{1}{\beta }}\ln t$$

(9)

Non-linear equation:

$$q_t={\displaystyle \frac{1}{\beta }}\ln \left(1+\alpha \beta t\right)$$

(10)

2.3.4. Intraparticle Diffusion Kinetics Model

The intraparticle diffusion kinetics model was presented by Weber and Morris in 1967, modified from the model proposed by Boyd et al. in 1948 [34]. This model is presented in its linear and non-linear forms, respectively.

Linear equation:

$$q_t=k_{int}\sqrt{t}$$

(11)

Non-linear equation:

$$q_t=k_{int}\sqrt{t}+c$$

(12)

2.4. Thermodynamic Study

The effect of temperature variations on the adsorption process was evaluated through a thermodynamic study. For this, 30 mg of the bio-adsorbent was placed in contact with 3 mL of the Cr (VI) contaminant at 65 mg/L and then mounted on a rotary device and stirred at 140 rpm for 24 h. The temperatures used were 295.15 K, 299.15 K, and 301.15 K. The values of enthalpy, entropy, and Gibbs free energy were determined using the Van’t Hoff graphical model and Equations (13)–(16) [35].

$$k_c={\displaystyle \frac{q_T}{C_e}}$$

(13)

$$∆\mathrm{G}°=-\mathrm{R}\mathrm{T}\times \mathrm{l}\mathrm{n}{k}_c$$

(14)

$$∆\mathrm{G}°=∆\mathrm{H}°-\mathrm{T}\times ∆\mathrm{S}°$$

(15)

$$\mathrm{l}\mathrm{n}{k}_c={\displaystyle \frac{-∆\mathrm{H}°}{\mathrm{R}\mathrm{T}}}+{\displaystyle \frac{∆\mathrm{S}°}{\mathrm{R}}}$$

(16)

3. Results and Discussion

3.1. SEM Analysis

SEM analysis is an analytical technique used to observe the surface of an object by utilising a beam of electrons to generate a magnified image. In Figure 2, the results of the SEM analysis are shown, with magnification of 200 (Figure 2a) and 3000 (Figure 2b). In Figure 2a, the surface of the biocomposite is observed, which has a fibrous appearance. Upon closer inspection, more details of the biocomposite’s surface can be seen, where a homogeneous mixture between the banana pseudo-stem fibre and PLA is evident, resulting in a material with a rough and fibrous appearance, hydrophobic characteristics, and greater tensile strength. These results are similar to those presented in the study conducted by Kamaludin et al. [36], who developed a cellulose–PLA and cellulose–PLA–chitosan biocomposite, achieving the homogenisation of the mixture of components used. In Figure 2b, the state of the biocomposite’s surface after the adsorption process is observed, showing that the Cr (VI) adhered to the surface of the biocomposite, demonstrating its capacity to remove the contaminant in the solution [37].

3.2. FTIR Analysis

An FTIR analysis was conducted to determine the functional groups present in the synthesised biocomposite. In Figure 3, two graphs are presented, showing the state of the functional groups before (Figure 3a) and after (Figure 3b) the adsorption process. In Figure 3a, the presence of amine (N-H) with strong interactions is observed at the peak of 3491.06 cm⁻¹. Alkanes (C-H) are also present near the peak of 2950.40 cm⁻¹, extending to 1450.38 cm⁻¹, indicating that the molecules of the biocomposite are stable. Additionally, the presence of aromatic rings (C-H) between 901.37 and 589.80 cm⁻¹ is evidenced, demonstrating that the banana pseudo-stem and PLA are bonded homogeneously. On the other hand, Figure 3b shows the presence of other functional groups. Hydroxyl groups (O-H) are found at the peak of 3442.71 cm⁻¹, followed by alkane molecules (C-H) at the peak of 2898.92 cm⁻¹. Amines (C-N) are also observed between the peaks of 1230.29 and 1041.70 cm⁻¹. Finally, the presence of aromatic rings (C-H) at the peak of 899.51 cm⁻¹ indicates that the biocomposite has stability [38].

3.3. Adsorption Kinetics Analysis

An analysis of the kinetics for the removal of Cr (VI) ions by the synthesised biocomposite was carried out in order to understand the adsorbent yield, the adsorption rate, and the mass transfer mechanisms involved in the adsorption process [31]. The kinetic models used for this study were the pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models. The kinetics analysis of the adsorption of Cr (VI) metal ions using the biocomposite was performed using solutions with an initial Cr (VI) concentration of 35 mg/L. Figure 4 shows different graphs of $q_t vs t$ for the adsorption of Cr (VI) using the synthesised biocomposite.

Table 1. Fitting parameters obtained for the kinetic adsorption models.

Model	Parameter	Non-Linear Fitting
Pseudo-first-order	qe	8.483
	k1	0.056
	R2	0.885
	Squared error	7.449
Pseudo-second-order	qe	9.031
	k2	0.009
	R2	0.961
	Squared error	2.541
Elovich	$\mathsf{\beta }$	0.899
	$\mathsf{\alpha }$	7.241
	R2	0.973
	Squared error	1.751
Intraparticle diffusion	k3	0.375
	R2	0.566
	Squared error	114.85

presents the various results regarding the fitting parameters obtained for the kinetic adsorption models of Cr (VI).

From the R² values obtained for the fitting of the different kinetic models used, it can be observed that the Elovich model best describes the adsorption behaviour of Cr (VI) on the synthesised biocomposite. This suggests that the adsorption process is chemical in nature and that the surface of the biocomposite consists of heterogeneous active sites, influenced by the activation energy required for chemisorption processes. This can be explained by the interaction of the functional groups found on PLA and cellulose, such as carboxyl and hydroxyl groups, with Cr (VI). These groups provide active sites on the surface of the biocomposite that aid in the adsorption of Cr (VI) through electrostatic interactions and coordination bonds. The results regarding the Elovich model coefficients, α and β, where α represents the initial adsorption rate and β the desorption coefficient, show that α is greater than β (α > β), indicating that the adsorption process predominates over the desorption process [27,28].

3.4. Analysis of the Adsorption Isotherms

An isothermal adsorption study was carried out, because this provides useful information on the adsorption capacity and the characteristics of the adsorbent mechanism for the removal of the selected pollutant using an experimental analysis of the equilibrium results. It also helps to explain how the sorbate is distributed between the liquid and solid phases at equilibrium [27,29]. The analysis of the adsorption isotherm models for the removal of Cr (VI) in solution using the biocomposite was conducted using solutions with an initial concentration of 35 mg/L. In this study, the Langmuir, Freundlich, and Dubinin–Radushkevich isotherm models were used for the equilibrium removal of Cr (VI) on the synthesised biocomposite. Figure 5 shows different plots of $q_e vs Ce$ for Cr (VI) adsorption using the synthesised biocomposite. The calculated parameters of the isothermal models presented different results for the adsorption of Cr (VI), where, for the Langmuir isothermal model, values of $q_{max}=2.970$, $k_L=0.075$, and ${\mathrm{R}}^2=0.563$ were obtained; for the Freundlich isothermal model, values of $k_F=0.21$, $\mathrm{n}=2.663$, and ${\mathrm{R}}^2=0.671$ were obtained; and for the Dubinin–Radushkevich isothermal model, values of $q_{BR}=2.318$, $k_{DR}=7.50e^{-2}$, and ${\mathrm{R}}^2=0.420$ were obtained. Analysing the correlation coefficient, R², obtained from the different models used, it can be deduced that the Cr (VI) adsorption process had a tendency to better fit the Freundlich isothermal model, implying that the process presents a multilayer adsorption tendency on a heterogeneous surface, i.e., as it adsorbs, the pollutant is successively arranged in layers. Moreover, the value of the Freundlich exponential factor, n, is greater than 1, so the adsorption that occurs involves chemical interactions [30,31].

3.5. Effect of Temperature Variations According to Thermodynamic Study

The effect of temperature variations on the adsorption process of Cr (VI) in solution was analysed in relation to the parameters of enthalpy, entropy, and Gibbs free energy. For the enthalpy parameter, a negative value was obtained, indicating that, in this study, the Cr (VI) removal process with the biocomposite used is exothermic, i.e., there was a release of energy during the adsorption process. On the other hand, for the entropy parameter, it was observed that the value was also negative, suggesting that the bonds that existed between the biomass and the metal were strong and their active centres had affinity, so the process was not reversible, indicating that a stable adsorption process took place. Finally, the Gibbs free energy parameter was positive, showing that the process was not spontaneous, i.e., for the adsorption process to occur, it is necessary to supply energy to the system [32,33]. The adsorption process described in this study can be improved by lowering the temperature, as working with lower temperatures in exothermic processes favours the attractive forces between the adsorbent and adsorbate, thereby enhancing the adsorption capacity [39]. The thermodynamic results obtained are shown in

Table 2. Thermodynamic parameters of Cr (VI) adsorption.

Calculated Thermodynamic Parameter	Temperature (K)
	295.15	299.15	301.15
$∆\mathrm{H}°$ (kJ/mol)	−9.007	−9.007	−9.007
$∆\mathrm{S}°$ (kJ/mol·K)	−0.080	−0.080	−0.080
$∆\mathrm{G}°$ (kJ/mol)	14.578	14.897	15.057

.

To understand the performance of the synthesised biocomposite, this study conducted an evaluation of adsorption–desorption cycles, using 0.2 M HCl and 0.2 M NaOH as recovery agents. The results showed that the synthesised biocomposite had a regeneration capacity of three adsorption–desorption cycles of Cr (VI) in both cases [40].

3.6. Overview of Multi-Application Biocomposites

The obtained results show that the studied biomaterial presents good characteristics as an adsorbent material for the treatment of wastewater polluted with heavy metals. In order to obtain an overall overview of the efficacy and versatility of the synthesised biomaterial, as can be seen in

Table 3. Overview of biocomposites with multiple applications for the removal of pollutants from water bodies.

Biocomposite	Removed Pollutant	Adsorbent Dose (g)	Pollutant Concentration (mg/L)	Adsorption Capacity (mg/g)	Source
Polyhydroxy butyrate (PHB)-modified coconut husk	Methylene blue	0.01	40	35.98	[25]
Polyethyleneimine-modified magnetic peanut shells	Cr (VI)	0.03	60	58.4	[41]
	Congo Red		20	71.3
	Phosphate		100	13.5
Sn (IV)-modified cellulose	As (III)	0.4	5	16.64	[42]
Chitosan-modified sugarcane bagasse bio-coal	Phosphate	0.05	20	37.2	[43]
Polyaniline-modified almond biocomposite	Orange G	0.075	10	8.92	[44]
Polyaniline-modified rice husk biocomposite				17.25
Banana pseudo-stem modified with polylactic acid (PLA)	Cr (VI)	0.03	30, 65, 100	3.275	Present study

, results from different studies are presented for the removal of different pollutants present in water using different biocomposite materials based on lignocellulosic materials. These suggest greater potential for the development of adsorbent materials from agricultural waste, suggesting that these biocomposites can potentially be used as adsorbent materials for the treatment of contaminated water. This is of great interest to the scientific community because biocomposites aim to improve the efficiency and the adsorption capacity.

4. Conclusions

The thermodynamic parameters obtained for ∆H°, ∆S°, and ∆G° indicated that the process studied was exothermic, suggesting that no external energy is required for adsorption. Additionally, the biocomposite demonstrated a strong affinity for Cr (VI) due to the presence of strong bonds between the biomass and the pollutant. Regarding the adsorption kinetics, the Elovich model provided the best fit for the experimental data, indicating that the adsorption process was chemical in nature, with the surface of the adsorbent biocomposite comprising heterogeneous active sites. In this study, since α > β, the adsorption process predominated over desorption. Conversely, in the isothermal study, the Freundlich model best fitted the experimental data, demonstrating that the adsorption process occurred on a heterogeneous, multilayer surface. These results highlight the potential of developing biocomposites from lignocellulosic agroindustrial waste materials as adsorbents for the treatment of water contaminated with heavy metal ions.