Efficient adsorption of nickel and chromium(VI) from aqueous solutions using lignocellulose nanofibers: Kinetics, isotherms, and thermodynamic studies

Nowadays, with increasing environmental problems and concerns, caused by heavy metal pollution, various industries such as metallurgical plants, plating, alloys, and battery production, technologies to treat polluted water should be developed rapidly (Rodiguez et al. 2018). One of the most important environmental concerns is the pollution caused by the toxicity of heavy metal ions, which accumulates in the food chain and has a high shelf life in nature (Xavier et al. 2018). The use of heavy metals in various industries is increasing which leads to environmental pollution (Maleki et al. 2019) Heavy metal ions are found naturally in volcanic activity, and weathering of rocks, as well as in many industries. The discharge of metal-polluted wastewater (e.g., brine) degrades water quality and thus water cannot be directly used for potable water (via desalination) and industrial applications (Panagopoulos 2022a, 2022b; Panagopoulos & Giannika 2022).

There are various methods for the purification of heavy metal ions in an aqueous solution, including chemical deposition, ion exchange, membrane filtration, precipitation, electrolytic method, reverse osmosis, solvent extraction, and adsorption (Manjuladevi et al. 2018; Anand et al. 2019; Kumar et al. 2019a, 2019b). Among all these methods, the adsorption method is preferred to other methods due to proper filtration, easy efficiency, and availability of various adsorbents (Zhang et al. 2018). The type of adsorbent and its method is essential in the adsorption method. In recent decades, the study of adsorbents has been expanded because they are highly effective, cheap, and economical (Kong et al. 2018).

The wide spread of nickel as a toxic heavy metal in the environment is harmful to human health. Ni(II) pollution in the aquatic environment is mainly caused by mines, melting, coal combustion, and plating (Yu et al. 2019). Once nickel enters the body, it penetrates all organs and accumulates in various tissues, causing damage to them, which may cause nerve, liver, kidney, and reproductive damage, and increase the risk of cancer (Shi et al. 2018). The high amount of nickel in the human body is dangerous to the skin (Zhou et al. 2018). Chromium is a natural element that is found mostly in nature in the form of trivalent and hexavalent chromium. Hexavalent chromium is more toxic than trivalent chromium (Arim et al. 2018). Chromium enters the human body through the skin, inhalation, or oral consumption and is transmitted by blood to the kidneys and liver. Hexavalent chromium is carcinogenic and damages DNA (Shi et al. 2018). Due to the high solubility of chromium in water, its accumulation is common in aqueous environments (Rizzo et al. 2019). Hexavalent chromium(VI) is a highly toxic heavy metal that is used in various industries such as leather, catalysts, fungicides, pigment production, ceramics, art, glass, photography, plating, etc. (Peng et al. 2019). Chromium causes problems such as bronchitis, cancer, wound formation, liver damage, weakened immune system, respiratory problems, and kidney damage (Gebru & Das 2018). The chromium(VI) concentration in drinking water used for human consumption may not be higher than 0.05 mg L⁻¹, and the maximum allowable concentration in water and wastewater for the final discharge in the environment is 0.1 mg L⁻¹ (Vilela et al. 2019).

Many inexpensive biopolymers such as chitosan, chitin, pectin, starch, cellulose, bacterial cellulose, lignin, and hemicellulose have been grafted with a variety of vinyl monomers for their use in separation technologies (Kumar & Sharma 2019) Cellulose is one of the most natural and plentiful renewable polymers (Kumar et al. 2019a, 2019b). Lingo cellulose is an ideal biological adsorbent because it is renewable, cheap, and has special structural properties. The main components of lignocelluloses are cellulose, hemicellulose, and lignin and it includes different kinds of functional groups such as hydroxyl, acetyl, phenolic, carboxyl, and methyl (An et al. 2018).

Removal of heavy metals through the application of various adsorbents has been of interest to many researchers because removal of this substance from wastewater before discharge into aquatic systems is necessary. Various technologies have been used in the past including chemical deposition, adsorption, ion exchange, membrane separation, solvent extraction, and biological processes to remove heavy metals from water and wastewater. Among these technologies, adsorption is the most appropriate method due to its high efficiency, low cost, convenient preparation, and operation. In the current study, the adsorption process was used to remove nickel and chromium(VI) ions. Adsorption is favored over other techniques due to ease of operation, low energy input, and removal of pollutants even at trace concentrations. The removal of nickel and chromium(VI) ions can be achieved by using various adsorbents like pomelo peel (Wu et al. 2017), Nano Kaolinite (Alasadi et al. 2019), Eichhornia crassipes and Lemna minor (Balasubramanian et al. 2019), microwave-functionalized cellulose (Qu et al. 2020), and coconut shell (Bal and Bhasarkar).

In this study, the application of lignocellulose nanofiber was investigated for the adsorption of nickel and hexavalent chromium from aqueous solutions in a batch system. The main objectives of this study were to investigate the effect of pH, initial concentration, contact time, temperature, and adsorbent dose on the adsorption of nickel and hexavalent chromium from aqueous solutions using lignocellulose nanofiber. For determining the mechanism of the adsorption in this study kinetic, thermodynamic, and adsorption isotherm models were investigated.

Cellulose as a natural and renewable material has valuable physical properties which can be improved with detailed knowledge of the structure (Peter 2020). In addition, cellulose is the most abundant biomaterial and natural regenerating polymer (Ji et al. 2020). Lignocellulose nanofiber is composed of cellulose, hemicellulose, and lignin, and its raw materials include wood and other lignocellulose residues (e.g., wheat straw, rice straw, and sugarcane bagasse). This study used lignocellulose nanofiber for the adsorption of nickel and chromium(VI) ions from aqueous solutions. The advantages of this adsorbent over other adsorbents are specific surface area and ease of separation from the soluble phase (Zhang et al. 2020).

Cellulose nanofiber is typically produced using various mechanical methods such as homogenization, micro-fluidization, micro-grinding, refining, cryocrushing, or in combination with chemical and enzymatic pretreatments (Ferrer et al. 2012; Brodin et al. 2014). Many studies have been carried out on the use of cellulose nanofiber produced from virgin fiber as a strengthening agent for improving the physical and mechanical properties of paper, while the use of cellulose nanofiber isolated from bleached virgin fiber is not necessary or reasonable for many recycled/impure products (Yousefhashemi et al. 2019).

Lignocellulose nanofiber has been produced from cheap, recycled, old, corrugated container pulp by the use of an ultra-fine grinding technique. The diameter of the lignocellulose nanofiber is in the range of 10–80 nm, and the cellulose crystallinity index and crystallite size is 49% and 4 nm, respectively, during the process (Yousefhashemi et al. 2019).

This study was conducted on lignocellulose nanofiber as an adsorbent of heavy metal chromium(VI) and nickel ions from aqueous solutions on a laboratory scale in the Science and Research Branch of Azad University, Tehran, with two batch replications. Lignocellulose nanofiber was purchased from the Nano-Novin Polymer Company (Gorgan, Iran) with the physiochemical properties of lignocellulose nanofiber as shown in Table 1. The other chemicals were purchased from Merck Company (Germany).

Batch adsorption studies were carried out to evaluate the efficiency of lignocellulose nanofiber adsorbent on the removal of nickel and chromium(VI) from the aqueous solutions. The stock solution of 1,000 mg L⁻¹ was prepared by adding a certain amount of K₂Cr₂O₇ and deionized water. The pH of each solution was adjusted using 0.1 M NaOH and 0.1 M HCl. Adsorption experiments were performed in the batch system. The pH of the solutions was measured using Crison pH Meter Basic 20. For agitation of the adsorbent and the heavy metal ions, the Heimolph Unimax 2010 shaker was used. For all treatments, the agitation speed was constant and equal to 120 rpm. After the reaction time, the lignocellulose nanofiber adsorbent was separated using a Kokusan H-108 centrifuge at 10 min and 4,000 rpm. Chromium(VI) and nickel ions concentration were measured using the Atomic Adsorption Device (AAS) model (Unicam-919). The effects of pH, adsorbent dose, contact time, initial concentration, and temperature were examined on the efficiency and adsorption capacity of nickel and chromium(VI) ions by the adsorbents. SPSS and Excel software were used for data analysis.

The experimental results were collected to identify the efficiency of adsorption criteria such as adsorption percentage, adsorption capacity, and breakthrough points at different operating conditions.

The study of biological adsorption at the temperature range produces thermodynamic constants such as the change of free energy of Gibbs (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°).

The solution pH has a considerable effect on the uptake of metal ions because it indicates the surface charge of the adsorbent and the degree of ionization and speciation of the adsorbate (Balasubramanian et al. 2019). In the adsorption process, pH has an important role in the removal of heavy metals (Barsbay et al. 2018; Esmaeili & Beni 2018). Adsorption experiments were carried out in the pH range of 2–7, keeping all other parameters constant (initial concentration = 10 mg/L, adsorbent dose = 0.1 g, contact time = 60 min, and T = 25 °C). For all treatments, the agitation speed was constant and equal to 120 rpm. When the pH rises above 7, the nickel and chromium hydroxides become insoluble, the adsorption capacity of nickel and chromium decreases, so the pH above 7 has not been investigated (Guo et al. 2018). The results of nickel and chromium(VI) are given in Figure 2(a) and 2(b), respectively. When the pH increases, the adsorption efficiency of nickel and chromium increases, so the adsorption percentage of nickel and chromium adsorption at pH = 5 are 91.2 and 83.4, respectively. Then at pH 6 and 7, the adsorption efficiency and adsorption capacity decrease. The results showed that the adsorption of nickel and chromium is pH-dependent, which affects the adsorbent surface properties and adsorption efficiency. Other researchers reported similar results for the adsorption of nickel and chromium by different adsorbents (An et al. 2019; Dehghani et al. 2019; Sethy & Sahoo 2019).

Statistical analysis of data by one-way ANOVA test indicated the directional effect of pH on the adsorption rate, which was significant (p < 0.05). Duncan's statistical test also showed that the optimum pH was 5. Duncan's test results also indicated significant differences in all cases, except the average pH of nickel at 5–7 and 2 and 3, the average pH of chromium at 4 and 6 and 4 and 7.

The initial concentration is one of the important driving forces that affect the sorption process. The effect of changing the initial concentration of nickel and hexavalent chromium in the range of 5–50 mg/L while keeping the adsorbent dose (0.1 g), pH (6), contact time (60 min) and temperature (25 °C) constant are illustrated in Figure 5. For all treatments, the agitation speed was constant and equal to 120 rpm. According to Figure 3, the maximum adsorption percentages of nickel and chromium at concentration of 5 mg/L are 91.9 and 84.5%, respectively. However, at higher concentrations, the metal cations are far beyond the capacity of the receptors. As a result, competition for access to the contact area increases, and all connection sites are exposed to ions and are activated. The maximum adsorption capacity of nickel and chromium at concentration of 50 mg/L is 44.97 and 41.46, respectively. As the initial concentration increases, the adsorption capacity also increases (Aslan et al. 2018; Zhou et al. 2018). At low concentrations, nickel and chromium have more receptors to adsorb metal cations (Farokhi et al. 2018). As a result, nickel and chromium ions are simply able to bind to receptors on the surface, so the adsorption percentage increases. As can be seen in Figure 3, lignocellulose nanofibers have been investigated at different concentrations. The difference in the percentage of adsorption at the highest and lowest concentrations of nickel and chromium is small, which is 2 and 3% for nickel and chromium, respectively, which indicates good and satisfactory adsorption of adsorbent in different concentrations. Other researchers reported similar results for the adsorption of nickel and chromium by different adsorbents (Sivakumar et al. 2018; Mahmoud et al. 2020).

Statistical analysis of data by one-way ANOVA test indicated the directional effect of initial concentration on adsorption rate, which was significant (p < 0.05). Duncan test results of both nickel and chromium also indicated significant differences in all cases, except the average initial concentration of nickel at 5 and 30 and 10, 20, and 40 mg/g, and the average initial concentration of chromium at 30 and 40 mg/g.

Statistical analysis of data by one-way ANOVA test indicated the directional effect of adsorbent dose on adsorption rate, which was significant (p < 0.05). Duncan's statistical test also showed that the optimum adsorbent dose of nickel and chromium were 0.4 and 0.6 g, respectively. Duncan test results of both nickel and chromium also indicated significant differences in all cases, except the average adsorbent dose of nickel at 0.3, 0.4, and 0.5 and 0.2 and 0.3 g, and the average adsorbent dose of chromium at 0.1 and 0.2 g.

The effect of contact time was investigated within the range of 15–120 min, pH = 5, initial concentration of 5 mg·L⁻¹ nickel and chromium, adsorbent dose of 0.1 g and temperature of 25 °C. As demonstrated in Figure 5, the adsorption increased with the test time. For all treatments, the agitation speed was constant and equal to 120 rpm. The highest adsorption rate of both nickel and chromium(VI) given as 96.4% was reached at 120 min. Figure 5 shows the effect of contact time on the adsorption percentage and adsorption capacity of nickel and chromium by lignocellulose nanofiber. As seen in Figure 5, the absorption rate increased from 15 to 120 min. Contact time must play an important role in the adsorption of heavy metals (Huang et al. 2018). Each substance must have a specific contact time to complete the adsorption process. Therefore, the adsorption property is highly dependent on the mixing time between the adsorbed substance and the adsorbent (Jiang et al. 2019). From the time of contact, it can be concluded that the amount of adsorption first increased and then reached equilibrium. Because of a larger number of active sites, the structure and area of the adsorbent are faster in the early times and allow the heavy metal to penetrate to a large extent (Bhowmik et al. 2018; Dim et al. 2021). Other researchers reported similar results for the adsorption of nickel and chromium by different adsorbents (Dim et al. 2021; Siuki et al. 2021).

In addition, statistical analysis of data by one-way ANOVA test indicated the directional effect of contact time on adsorption rate, which was significant (p < 0.05). Duncan's statistical test also showed that the optimum contact time of nickel and chromium were 60 and 90 min, respectively. Duncan test results of both nickel and chromium also indicated significant differences in all cases, except the average contact time of nickel at 60, 90, and 120 min, and the average contact time of chromium at 30 and 45 and 45 and 60 min.

Statistical analysis of data by one-way ANOVA test indicated the directional effect of temperature on the adsorption rate, which was significant (p < 0.05). Duncan's statistical test also showed that the optimum temperature of nickel and chromium were 25 and 30 °C, respectively. Duncan test results of both nickel and chromium also indicated significant differences in all cases, except the average temperature of nickel at 25, 30, 35, and 40 °C, the average temperature of chromium at 20, 25, and 30, 30 and 35 °C, and 35 and 40 °C.

There are many models that can express the relationship between adsorption and residual concentration in solutions. Langmuir and Freundlich are more common and acceptable adsorption isotherms for the measurement of the adsorption of heavy metals in aqueous solutions (Hosseini et al. 2016). Therefore, in this study, Langmuir and Freundlich models have been applied to investigate and analyze experimental data and descriptions of the adsorption equilibrium between solid and liquid phases. The Langmuir model assumes uniform adsorption on the surface and is valid for monolayer adsorption with a homogeneous distribution of the adsorption sites and adsorption energies, while the Freundlich model shows multi-layer adsorption (Khelifi et al. 2018; Sun et al. 2019).

The calculated values of the Langmuir constant (K_L) show a proper affinity of adsorption sites for nickel and chromium ions with lignocellulose nanofiber. Based on the calculated value of separation factor R_L, its magnitude indicates the shape of the isotherm to be either favorable (0 < R_L < 1), unfavorable (R_L > 1), linear (R_L = 1), or irreversible (R_L = 0) (Al-Abbad et al. 2020).

As can be seen in Table 2, K_f is the Freundlich constant related to the bonding energy (L/g), and n is an empirical constant characterizing the heterogeneity of the process (g/L). According to the Freundlich model assumptions, if n < 1, the adsorption process is unfavorable, while if 1 < n > 10, the adsorption process is favorable. Results show that n in nickel and chromium were equal to 1.07 and 1.073, respectively, which indicated the favorable process for this adsorbent. Table 2 shows the constant coefficients and correlation coefficients of the Langmuir and Freundlich isotherms in chromium adsorption using lignocellulose nanofiber.

Since R² is larger in Freundlich, the adsorption isotherm follows the Freundlich model. Freundlich isotherm is applicable to adsorption processes that occur on heterogonous surfaces. This isotherm gives an expression, which defines the surface heterogeneity and the exponential distribution of active sites and their energies.

The study of adsorption kinetics is essential to study the adsorption contact time (Mousavi et al. 2019). Adsorption kinetics is an important factor in the evaluation of adsorbents because it shows the amount of ions adsorbed by the adsorbent (Qureshi et al. 2017; Siddiqui et al. 2020). Adsorption kinetics models can be divided into two groups: reaction-based models and diffusion-based models. Reaction-based models include pseudo-first-order, pseudo-second-order, and Elovich models, and diffusion-based models include intraparticle diffusion, external-film diffusion, and internal-pore diffusion (Sarici-Ozdemir 2012). The models used in this study included reaction-based models of pseudo-first-order and pseudo-second-order.

The pseudo-first-order kinetic equation is based on adsorbent capacity and is used when adsorption occurs through the diffusion process inside a boundary layer (physical adsorption), while for the pseudo-second-order kinetic model, the model assumes that a metal ion is adsorbed to the available (two polar) sorption sites of the sorbent. The rate of pseudo-second-order sorption depends on the mass of the heavy metal ion on the sorbent surface and the number of heavy metal ions adsorbed at equilibrium (Aigbe & Osibote 2020).

The analysis of the adsorption kinetics data according to the model Equations (10) and (11) resulted in the parameters listed in Table 3. The plot of ln(q_e − q_t) vs. t should yield a straight line if the experimental data conform to this pseudo-first-order kinetic model. The plot of t/q_t vs. t will give a straight line if the experimental data conform to this pseudo-second-order kinetic model, and the values of q_e and K₂ are obtained, respectively, from the slope and intercept of such a plot.

The results indicated that both the pseudo-first-order equation kinetic model and the pseudo-second-order kinetic model were followed, but it fits more with the second-order equation. In this model, the rate-limiting step is the surface adsorption that involves chemisorption, where the removal from a solution is because of physicochemical interactions between the two phases. The model is usually represented by its linear form.

Thermodynamic parameters were obtained by varying temperature conditions over the range of 10–40 °C by keeping other variables constant. The values of the thermodynamic parameters such as ΔG°, ΔH°, and ΔS°, describing nickel and chromium(VI) ions uptake by lignocellulose nanofibers, were calculated using the thermodynamic equations. The thermodynamic parameter indicates whether the adsorption reaction is spontaneous, random, exothermic, or endothermic.

Thermodynamic studies showed that the Gibbs free energy change (ΔG°) indicates whether a chemical reaction would take place spontaneously, negative ΔG° confirms spontaneous adsorption, and positive ΔG° indicates its prohibition (ΔG° < 0, ΔG° > 0), ΔH° (enthalpy change) is the amount of energy released (exothermic process) or consumed (endothermic process) by adsorption (ΔH° < 0, ΔH° > 0). A positive ΔS° (change in entropy) specifies that the adsorption comprises a dissociative mechanism, while a negative ΔS° corresponds to an associative mechanism (Kaewsichan & Tohdee 2019).

The thermodynamics of nickel and chromium adsorption at various temperatures were studied according to Tables 4 and 5 where Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) were calculated.

As Tables 4 and 5 show, thermodynamic parameters of nickel and chromium(VI) adsorption using lignocellulose nanofiber ΔG°, ΔH°, and ΔS° are indicators of the possible nature of adsorption, spontaneous, endothermic, and random, respectively. Other researchers reported similar results for the adsorption of nickel and chromium by different adsorbents (Adebayo et al. 2020; Mondal & Chakraborty 2020).

In this study, the application of lignocellulose nanofiber was investigated for the adsorption of hexavalent chromium and nickel from aqueous solutions in a batch system. This study evaluated the effect of pH, temperature, contact time, initial Cr and Ni concentration, and adsorbent dose parameters on the percentage of adsorption and absorption capacity. The results showed that a pH of 6, an adsorbent dose of 0.1 g at 25 °C and 60 min of contact time offered the optimum percentages of adsorption in this research. The lignocellulose nanofiber proved to be excellent for the removal of nickel and chromium metal ions from aqueous solutions not only at low concentrations but at high concentrations too. The results of the study of two-parameter isotherms (Langmuir and Freundlich) revealed that in general, the studied isotherms well predicted the equilibrium of the system. The adsorption data fitted well the Freundlich isotherm model, showing that the adsorption of nickel and chromium on the adsorbent was multi-layered. Kinetic studies also showed that the data corresponded to the pseudo-second-order model and the intraparticle. The calculation of thermodynamic parameters of chromium and nickel showed a spontaneous, endothermic, and random process. Maximum adsorption capacity (q_max) of nickel and chromium(VI) was equal to 270.271 and 344.827 mg·g⁻¹, respectively, which indicates that the adsorbent has a relatively high adsorption capacity and cost-effectiveness of the adsorbent. Lignocellulose nanofiber, as the most abundant molecular biomass polymer in nature, has been characterized as possessing high biocompatibility, low toxicity, and high biodegradability properties. Thus, researchers are interested in such substances. According to the results of this study, lignocellulose nanofiber has a high potential for the adsorption of chromium and nickel from aqueous solutions.

The authors thank The Islamic Azad University Science and Research Branch of Iran for providing laboratory materials and facilities. I would like to acknowledge the help and support of a dear friend, Arash Azizi, who provided me with certain resources while writing this paper.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.