An innovative hybrid biosorbent composed of nano ZnO and marine macro algae Jania rubens embedded in an alginate/PVA matrix: insights into Pb²⁺ removal in water

Abstract

Nanoparticles of zinc oxide (ZnO) combined with a Jania rubens (JR) biosorbent have been embedded in a sodium alginate (SA)–polyvinyl alcohol (PVA) matrix. This hybrid biosorbent was characterized by FTIR, and the presence of functional groups involved in the adsorption of Pb²⁺ was revealed. SEM/EDX analyses have shown that the hybrid biosorbent exhibited porous microstructures which are decorated with ZnO nanoparticles (hydrodynamic size of 68 ± 2 nm). The removal of Pb²⁺ from aqueous medium was thoroughly investigated. The adsorption capacity has been measured at q_e = 39.1 mg g⁻¹ at pH = 5 and T = 303 K with the concentration of biosorbent and Pb²⁺ at 2.0 g L⁻¹ and 100 mg L⁻¹, respectively. The Freundlich and Langmuir isotherms have been used to model the adsorption process, from which the maximum adsorption capacity (q_m) of the hybrid biosorbent was calculated to be 111 mg g⁻¹. The adsorption kinetics are represented by a pseudo-second order model. In addition, the hybrid biosordent was regenerated and reused for four cycles for the removal of Pb²⁺.

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1. Introduction

Non-biodegradable and hazardous heavy metal ions including lead (Pb²⁺), cadmium (Cd²⁺), chromium (Cr⁶⁺), mercury (Hg²⁺), and arsenic (As³⁺) are still released into the natural environment via inefficiently treated wastewaters from various industries, thus being a serious threat to the ecosystem and human health.¹ Therefore, wastewater treatment should be improved by additional techniques like biological, chemical, or physical ones. In this work, particular attention is paid to removing Pb²⁺ using a physico-chemical method i.e. an adsorption process using an innovative hybrid biosorbent. Even in small concentrations, Pb²⁺ can cause carcinogenicity/genotoxicity, thus resulting in severe damage to brain, kidneys, and blood cells.¹ The World Health Organization has set the permissible limit of lead in potable water at 0.01 mg L⁻¹.² On the other hand, effective and eco-friendly approaches are required to remove such low concentrations of lead. Adsorption using a biomass-based sorbent (biosorption) is a technologically proven method for the sequestration of heavy metals, especially for biosorbents composed of algae,^3–7 fungi,⁸ bacteria,⁹ plants^10,11 and chitin.¹² In addition, hybrid biosorbents, i.e., biosorbents combined with synthetic adsorbents, are even more promising regarding their stability, reproducibility, cost-effectiveness, thus their efficiency. In particular, the design of hybrid biosorbents by their entrapment with metal oxide nanoparticles in a polymeric matrix has appeared as an attractive technology since such nanoparticles can provide specific adsorption of heavy metal ions.¹³ For example, a TiO₂-yeast nanocomposite exhibited a higher metal removal of Cr(VI) compared to conventional biosorbents.¹⁴

In addition, the immobilization of adsorbents into biological matrices appears promising for practical reasons like the ease of complete post-separation after treatment along with a limitation of nanoparticle release in the treated system. Therefore, natural polymers are excellent candidates because of their biodegradability and eco-friendly nature.¹⁵ Among natural polymers, alginate is an excellent candidate due to its non-toxicity and availability (from marine algae). The chemical structure of alginates is composed of guluronic and mannuronic acids that can interact and entrap hybrid biosorbents. The whole system can be enclosed into a film like a flexible polymer such as polyvinyl alcohol (PVA) which is cheap, stable and non-toxic.^16,17 Several hybrid biosorbents have already exhibited good efficiency in the removal of aqueous Pb²⁺. For instance, iron oxide nanoparticles combined with a fungal biosorbent (Phanerochaete chrysosporium) and immobilized in an alginate matrix have exhibited an adsorption efficiency of 96% for Pb²⁺ removal at pH = 5.¹⁸ Furthermore, Fe₃O₄ nanoparticles embedded in an alginate/PVP/calcium gel have been reported to remove Cd²⁺ with a maximum adsorption efficiency of 98 mg g⁻¹.¹⁹ Similarly, a magnetic alginate composite has been synthesized in the form of beads for the removal of Pb²⁺ from aqueous solutions.²⁰ Another work reported the preparation of alginate-based Fe₃O₄−MnO₂ xerogels for the removal of Cr⁶⁺ and Cd²⁺.²¹ However, no report has been found on the use of a metal oxide adsorbent combined with marine algae and embedded in an alginate and PVA matrix which is the focus of this present work.

Indeed, a new type of hybrid biosorbent is prepared based on ZnO nanoparticles and Jania rubens (JR) biomass. ZnO is biocompatible, non-toxic and has already been used in environmental applications, while nanosized ZnO (particle size ranging from 60 to 200 nm) does not show mutagenic activities.^22–26 Subsequently, the ZnO–JR hybrid biosorbent is embedded in the alginate/PVA biomatrix. The efficiency of this hybrid biosorbent is investigated toward the removal of aqueous Pb²⁺. The effects of different parameters (adsorbent and pollutant concentrations, pH, and temperature) are assessed while the adsorption mechanism is extensively discussed based on isotherm models, kinetics and thermodynamics. Therefore, the present work could be considered as a promising approach for the development of innovative and efficient hybrid biosorbents.

2. Materials and methods

2.1. Preparation of materials

2.1.1. ZnO nanoparticles. ZnO nanoparticles were prepared using a method adapted from Meghana et al.²⁷ In brief, 0.3 M pyridine solution was added to 0.2 M zinc acetate solution, and subsequently 0.4 M sodium hydroxide solution was added dropwise to the mixture under slow magnetic stirring.²⁷ The resulting precipitate was washed with deionized (DI) water and with ethanol before being dried in air at 333 K for 4 h.

2.1.2. JR biosorbent. JR was collected from the East Indian Ocean nearby in the Bay of Bengal. The preparation of the biosorbent was explained in our previous work.⁴ In brief, JR was washed with DI water and then placed in an oven at 343 K for 24 h. The dried JR was mashed using a domestic mixer and then sieved. The resulting powder was treated with 1M HNO₃ for 30 min. After washing several times with DI water, vacuum filtration followed by drying at 343 K for 24 h in air was performed. The biosorbent was milled, sieved (through #200 sieve) and stored in a dry atmosphere.

2.1.3. Hybrid biosorbent. 2% w/v sodium alginate (SA) and 5% w/v of polyvinyl alcohol (PVA) solutions were prepared separately in Milli-Q water and then mixed with stirring for 30 min at 313 K. On the other hand, 100 mg of ZnO nanoparticles were added slowly to 8 g of JR biosorbent suspension (in 200 mL) and stirred for 1 h before being sonicated for 30 min. Subsequently, the ZnO–JR mixture was added to the PVA–SA solution with stirring at 313 K for 1 h. Then, the PVA–SA–ZnO–JR mixture was added dropwise using a peristaltic pump to a solution containing 0.05M CaCl₂·2H₂O and 10% w/v H₃BO₃ and then cured for 24 h to obtain beads of the hybrid biosorbent. It is worth noting that calcium ions were used as crosslinking agents between PVA and SA that subsequently formed an “egg-box” model.^41,42 The excess calcium ions were removed via several washing with Milli-Q water.

2.2. Characterization methods

The hybrid biosorbent as well as the separated components i.e. the JR, ZnO and ZnO–JR adsorbents, and the PVA–SA biomatrix were ground into fine powders. The powders were analyzed using a Fourier transform infrared spectrophotometer (FTIR, Tensor 37, Bruker) using a KBr pellet to determine the functional groups of the materials. The morphology of the particle surface and the cross section of the materials was studied using scanning electron microscopy (SEM, JXA-8100, JEOL) and the hydrodynamic size of the ZnO nanoparticles in the hybrid biosorbent was measured using a dynamic light scattering device (DLS, Zetasizer, Malvern). The surface area of the ZnO nanoparticles was estimated by N₂ adsorption using a BET (Brunauer–Emmett–Teller) isotherm technique (NOVA 2200e). The EDX spectra were also monitored. In addition, atomic absorption spectrophotometry (AAS, AA400-PerkinElmer) was used to analyze the Pb²⁺ concentration in the aqueous samples. The point of zero charge (PZC) of ZnO–JR was measured by the pH drift method in 0.1 M NaCl electrolyte.²⁸ Briefly, six flasks of 50 mL NaCl solution were prepared, and the pH was adjusted with 0.1 N HCl and NaOH at 2, 4, 6, 8, 10 and 12, respectively. Then, 100 mg ZnO–JR was added to each flask that was kept in a shaker at 160 rpm for 24 h. After the shaking, the content was filtered, and the pH of each solution was measured. This procedure was repeated but without the addition of ZnO–JR. Finally, the final pH vs. initial pH was plotted, and the point of intersection was considered as the PZC of ZnO–JR nanoparticles.²⁹

2.3. Calculations and measurements

The Pb²⁺ metal uptake q_e was calculated using eqn (1) and the percentage of Pb²⁺ removal (R%) by the adsorbents was calculated using eqn (2):where V is the volume of Pb²⁺ solution (L); C and C_e are the initial and equilibrium concentration of Pb⁺² (mg L⁻¹), respectively; and M is the mass of the adsorbent (g).where C_t is the concentration of Pb²⁺ at time t (mg L⁻¹).

The batch studies of Pb²⁺ removal were performed in Erlenmeyer flasks under constant stirring (160 rpm) of 50 mL Pb²⁺ solution at different concentrations (25, 50, 75, and 100 mg L⁻¹) and containing adsorbent at different concentrations i.e. ZnO particles 25, 50 and 100 mg L⁻¹ loaded with JR i.e. 2, 4, and 8 g L⁻¹, respectively. The pH of was adjusted using 0.1N NaOH and HNO₃ solutions and the effect of different temperatures (293, 303, 313, and 323 K) was assessed. The adsorption experiments were performed for a duration of 120 min and repeated thrice. Also, the reusability of the hybrid biosorbent was evaluated by repeating the adsorption–desorption cycles four times. The desorption of Pb²⁺ from the hybrid biosorbent was performed using 2N HCl. It is worth noting that Ca²⁺ (crosslinking agent in the PVA–SA matrix) was released at low pH while at high pH, Pb²⁺ started to precipitate, thus a pH range between 3 and 6 was used in the present work.

3. Results and discussion

3.1. Characterization of adsorbents

The FTIR spectra of native JR biosorbent, the JR biosorbent embedded in PVA–SA matrix, the hybrid biosorbent (i.e. ZnO–JR–PVA–SA) and the hybrid biosorbent loaded with Pb²⁺ are depicted in Fig. 1. The peaks at 3470.94 and 2518.74 cm⁻¹ correspond to –OH and –CH (Fig. 1a), respectively, and were shifted to 3448.77 and 2521.29 cm⁻¹ (Fig. 1b), thus suggesting the successful immobilization of JR biosorbent in the PVA–SA matrix.³⁰ In addition, the peaks of the JR biosorbent at 1635.16 cm⁻¹, 1473.37 cm⁻¹ and 856.18 cm⁻¹ (Fig. 1a) were assigned to the vibrations of amide I (i.e., –CO–NH), amide II (i.e., –CO–NH and –COO⁻ NH⁺) and –NH₂, respectively, and were slightly shifted after their entrapment in the PVA–SA matrix (Fig. 1b, 1c and 1d). Similarly, the peak at 1077.85 cm⁻¹ which was assigned to –C–OH of PVA (Fig. 1b) was shifted to 1082.84 cm⁻¹ in the hybrid biosorbent (Fig. 1c), thus suggesting this chemical function was involved in the entrapment of JR–ZnO in the PVA–SA matrix. The peak assigned to PVA at 1257.45 cm⁻¹ (Fig. 1b) due to the presence of –C–O vanished in the hybrid biosorbent that was probably caused by the interaction with ZnO.⁴ Furthermore, a new peak at 2930.67 cm⁻¹ (Fig. 1b) was assigned to the PVA–SA matrix, especially to the –OH stretching in the sugar ring of SA. By comparison with the FTIR spectra of hybrid biosorbent (Fig. 1c) and Pb²⁺ loaded hybrid biosorbent (Fig. 1d), this peak vanished since it was implicated in the Pb²⁺ surface complexation during the adsorption process. The sharp peak in JR biosorbent at 856.18 cm⁻¹ (Fig. 1a) was due to –NH₂, and this peak was shifted to 859.30, 859.39 and 858.71 cm⁻¹ in the JR biosorbent embedded in the PVA–SA matrix, the hybrid biosorbent and the Pb²⁺ loaded hybrid biosorbent, respectively. A similar trend in the shift of the peak 603.85 cm⁻¹ (Fig. 1a), which was assigned to the –SH group, was observed, but it disappeared in the Pb²⁺ loaded hybrid biosorbent (Fig. 1d). This might be due to the involvement of the –SH group in the adsorption process of Pb²⁺. It could also be assumed that the –OH, –C–N, –C–O, –C–N–C, –CO–NH, –NH₂ and –SH functional groups of the hybrid biosorbent were involved in the adsorption process of Pb²⁺.³¹

Fig. 1 FTIR spectra of the (a) JR biosorbent, (b) JR biosorbent embedded in the PVA–SA matrix, (c) hybrid biosorbent and (d) Pb²⁺ loaded hybrid biosorbent.

SEM images of the hybrid biosorbent before and after loading of Pb²⁺ (Fig. 2a and b) revealed that the surface was rough and the Pb²⁺ adsorption did not affect the surface morphology of the hybrid biosorbent. The white dots were ZnO nanoparticles (marked with arrows). Their presence was confirmed by EDX and DLS which determined the hydrodynamic size to be 68 ± 2 nm (Fig. S1, ESI†). The BET analysis of ZnO nanoparticles showed that the specific surface area was 14.6 m² g⁻¹ (Fig. S2, ESI†). Similar results were also observed in the literature.^34,35Fig. 2c and d show the surface and cross-section images of the hybrid biosorbent. The presence of micropores (Fig. 2d) was clear, which was a significant benefit for the removal of Pb²⁺. Similar conclusions were highlighted in other works on immobilized biosorbents.^32,33 It is worth noting that the hybrid biosorbent was also characterized by SEM and EDX after 4 cycles of Pb²⁺ adsorption (Fig. S3, ESI†). The SEM images showed that the surface morphology of the hybrid biosorbent remained almost intact (Fig. S3b, ESI†) although the porous structure was slightly smoothed, probably due to Pb²⁺ adsorption but also release of Ca²⁺ (ion exchange). The EDX analysis (Fig. S3a, ESI†) still confirmed the presence of ZnO after 4 cycles of reuse while the quantity of Pb²⁺ increased, thus confirming the efficiency of the hybrid biosorbent.

Fig. 2 SEM images of the hybrid biosorbent (a) before and (b) after Pb²⁺ loading, and of the (c) surface and (d) cross-section of the hybrid biosorbent.

3.2. Assessment of the effect of experimental parameters

The effect of different concentrations of the hybrid biosorbent (2, 4, and 8 g L⁻¹) was assessed on the metal uptake (q_e) and the removal efficiency (R) of Pb²⁺solution (100 mg L⁻¹) at pH = 5 and T = 303 K (Fig. 3a and b). The metal uptake occurred relatively quickly at the early stages because of the availability of binding sites, before reaching a plateau when approaching the equilibrium. Concerning the removal efficiency of Pb²⁺, it was more than 80% after 90 min and even reached 94% using 8g L⁻¹ biosorbent, thus suggesting an optimal contact time of about 120 min. Similar trends were observed for Pb²⁺ removal using C. fastigiata and Ulva lactuca biosorbents.^36,37

Fig. 3 Effect of hybrid biosorbent concentration on (a) metal uptake and (b) removal efficiency of Pb²⁺ (100 mg L⁻¹ at pH = 5 and T = 303 K). Effect of pH on the metal uptake vs. (c) the equilibrium Pb²⁺ concentration and (d) the initial Pb²⁺ concentration.

The effect of pH was significant due to the (de)protonation of the functional groups involved in the Pb²⁺ adsorption.³⁸ An increase of pH from 3 to 5 (for C_e = 15 mg L⁻¹) led to an increase of metal uptake from about 5 to 22 mg g⁻¹ and a further increase to pH = 6 lead to a decrease to q_e = 17 mg g⁻¹ (Fig. 3c). A similar trend was observed at different initial Pb²⁺ concentrations (from 25 to 100 mg L⁻¹) where the highest metal uptake was observed at pH = 5 (Fig. 3d). At low pH (pH <5), the protonation of the functional groups at the surface of a hybrid biosorbent might lead to electrostatic repulsion of positively charged lead and the biosorbent, while at higher pH (pH >5), a decrease of metal uptake might be caused by precipitation of lead into Pb(OH)₂.³⁹ At the optimal pH i.e. at pH = 5, the surface hydroxyl group in ZnO and JR can chemically adsorb Pb²⁺ according to the following reaction:

Zn/JR–OH + Pb⁺² ⇋ Zn–O–Pb⁺ + H⁺ (3)

In addition, at this pH, it is assumed that surface carboxyl groups could also improve the adsorption of Pb²⁺ by the hybrid biosorbent via surface complexation.⁴⁰ To further explain the effect of pH, the pH of PZC for the hybrid biosorbent was measured as pH_PZC = 4.32 (Fig. S4, ESI†). Therefore, below pH_PZC, the Pb²⁺ adsorption was low due to the electrostatic repulsion effect. However, the adsorption increased from pH_PZC to pH = 5 since the adsorbent surface became negatively charged, thus allowing the adsorption of Pb²⁺ (electrostatic attraction). In addition, Ca²⁺ ions which were used as crosslinking agents were released during the Pb²⁺ adsorption process via ion exchange, thus enhancing the adsorption process at pH = 5. Furthermore, a previous report revealed that with hybrid magnetic alginate adsorbent, the calcium concentration is almost constant and the maximum adsorption capacity of Pb²⁺ was found in the pH range 4.0–6.5.²⁰ Therefore, the present results are in accordance with this report since the optimal pH for Pb²⁺ adsorption is 5.

Concerning the effect of the initial Pb²⁺ concentration (25, 50, 75, and 100 mg L⁻¹), the metal uptake and removal efficiency of Pb²⁺ were investigated (Fig. 4a and b). The q_e increased from 10.1 to 39.1 mg g⁻¹ with increasing initial Pb²⁺concentration from 25 to 100 mg L⁻¹ using 2 g L⁻¹ hybrid biosorbent at T = 303 K and pH = 5 (Fig. 4a), since higher concentrations were required to overcome the mass transfer resistance of cations in between the solid and aqueous phases.⁴³ However, at pH = 5, the removal efficiency of Pb²⁺ decreased from 86 to 75% as the initial Pb²⁺concentration increased from 25 to 100 mg L⁻¹ (Fig. 4b), due to the congestion of adsorption sites.⁴⁴

Fig. 4 Effect of the initial Pb⁺² concentration on (a) metal uptake and (b) removal efficiency of Pb²⁺ (using 2 g L⁻¹ hybrid biosorbent at pH = 5 and T = 303 K). The effect of hybrid biosorbent concentration (i.e. biomass weight) on the (c) metal uptake and (d) removal efficiency of Pb²⁺ (for 100 mg L⁻¹ solution at pH = 5 and T = 303 K).

The effect of biomass dosage i.e. the biosorbent concentration, was also investigated on the efficiency of Pb²⁺ adsorption (Fig. 4c and 4d). For 100 mg L⁻¹ Pb²⁺ solution at pH = 5 and T = 303 K, the metal uptake decreased from 39.1 to 11.3 mg g⁻¹ as the biosorbent concentration increased from 2 to 8 g L⁻¹ (Fig. 4c), while the removal efficiency of Pb²⁺ continuously increased from 75 to 90% (Fig. 4d). Although the removal efficiency logically increased with increasing biomass dosage (by increasing the available surface area),⁴⁵ the metal uptake is the highest for 2 g L⁻¹ hybrid biosorbent, thus it is considered to be the optimal concentration. Also, as the hybrid biosorbent quantity increased, particle collision took place, thus leading to a decrease of the adsorption efficiency. Indeed, further increase of concentration led to a decrease of q_e due to negative interactions between the adsorbent particles. Also, lower hybrid biosorbent concentrations would lead to a reduction in process costs at an industrial level.

Finally, the effect of temperature was investigated (Fig. 5a and b) which is probably one of the most crucial parameters for the efficiency of an adsorption process. The metal uptake was enhanced from about 15 to 38 mg g⁻¹ (for C_e = 15 mg L⁻¹) as the temperature increased from 293 to 323 K (Fig. 5a), while the removal efficiency increased continuously from 70 to 90% for the 25 mg L⁻¹ Pb²⁺ solution using 2 g L⁻¹ hybrid biosorbent at pH = 5 (Fig. 5b). It worth noting that the observed differences in the temperature range 303–323 K was slightly significant. Therefore, due to energy consumption and the potential scale-up to an industrial process, 303 K was considered as the optimal temperature. The enhancement of Pb²⁺ removal with increasing temperature was explained by a decrease of viscosity, thus improving the diffusion rate through the polymeric SA–PVA matrix until the adsorption sites onto the ZnO–JR adsorbent. This trend was reported for Pb²⁺ using the Phanerochaete chrysosporium biosorbent.^46,47

Fig. 5 Effect of temperature on the (a) metal uptake and (b) removal efficiency of Pb⁺² (25 mg L⁻¹ using 2 g L⁻¹ of hybrid biosorbent at pH = 5). The plots of (c) Langmuir and (d) Freundlich models according to the temperature.

3.3. Adsorption isotherms

To discuss the adsorption isotherms (Fig. 5c and d), two models including Langmuir (eqn (4)) and Freundlich (eqn (5)) ones were considered.where q_m is the maximum adsorption capacity of Pb²⁺ (mg g⁻¹); q_e is the equilibrium adsorption capacity (mg g⁻¹); C_e is the equilibrium Pb²⁺concentration (mg L⁻¹); b is a constant for the affinity of binding sites (L mg⁻¹); K_f and n_f are constants related to the adsorption capacity and intensity. Table 1 summarizes the important parameters for each model at different temperatures.

Table 1 Langmuir and Freundlich models as adsorption isotherms at different temperatures

Langmuir model				Freundlich model
T (K)	q m (mg g^−1)	b (L mg^−1)	R ^2	K f	n f	R ^2
293	111.11	0.01278	0.945	0.5662	1.2033	0.980
303	111.11	0.02222	0.962	0.3273	1.4104	0.991
313	100	0.03424	0.991	0.2415	1.2970	0.993
323	111.11	0.03896	0.965	0.2032	1.2804	0.986

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Although the R² of the Langmuir model is lower than the Freundlich one, the data were relatively consistent (Fig. 5c). Since there were different binding sites on the hybrid biosorbent (JR, ZnO, SA and PVA), the Langmuir model is difficult to apply for such heterogeneous surface properties. The values of b constants (Table 1) indicated that the adsorption capacity of the hybrid biosorbent for Pb²⁺ is high. Indeed, the calculated maximum metal uptake (q_m) was about 111 mg g⁻¹ which was 43% greater than the value of native JR alone (71 mg g⁻¹) under similar experimental conditions.⁴ Concerning the Freundlich model (Fig. 5d), the values of R² were much higher (Table 1), thus indicating that this model fits the adsorption of Pb²⁺ on the hybrid biosorbent. For comparison, the maximum metal uptake (q_m) for Pb²⁺ was reported on hybrid magnetic alginate beads as 100 mg g⁻¹²⁰ and further q_m values of various adsorbents based on biomass algae are summarized in Table S1 (ESI†).^{4,32,36,37,44,46,48,49}

To highlight the adsorption process of Pb²⁺ by the hybrid biosorbent, a schematic representation is shown in Fig. 6. One of the advantages of this hybrid biosorbent is related to the post-separation technique since it is easy to separate and regenerate the beads after potential treatment of wastewater contaminated by heavy metals like Pb²⁺, thus reducing the risk of metal oxide nanoparticle leaching.⁵⁰

Fig. 6 Schematic representation of the hybrid biosorbent along with the adsorption of Pb²⁺.

3.4. Adsorption kinetics and thermodynamics

As mentioned above, there were several functional groups (e.g., –COOH, –NH₂,–SH, and –OH) from the SA, PVA, ZnO and JR components of the hybrid bisorbent that could interact with Pb²⁺. To propose the elucidation of the adsorption kinetics, the pseudo–first (eqn (6)) and pseudo-second (eqn (7)) order kinetic models were applied (Fig. 7a and b).where q_e and q_t are the Pb²⁺ concentrations (mg g⁻¹) at the equilibrium and time t, respectively; and k₁ and k₂ are the rate constants of the pseudo-first and pseudo-second order reaction, respectively. Table 2 summarizes the kinetic parameters for each model at different concentrations of the hybrid biosorbent.

Fig. 7 Kinetic models using different concentrations of the hybrid biosorbent and assuming (a) pseudo-first order reaction and (b) pseudo-second order reactions. Determination of enthalpy and entropy changes for different initial Pb²⁺ concentrations using 2 g L⁻¹ of hybrid biosorbent at pH = 5 and T = 303 K from (c) Vant Hoff plot, and (d) adsorption–desorption cycles of Pb²⁺ (100 mg L⁻¹) on the hybrid biosorbent (2 g L⁻¹) at pH = 5 and T = 303 K.

Table 2 Kinetic parameters of pseudo-first and pseudo-second order kinetics models at different hybrid biosorbent concentrations and pH = 5 and T = 303 K

Hybrid biosorbent conc.	Pseudo-first order model	Pseudo-second order model
2 g L^−1	q ecal = 40.4575 mg g^−1	q ecal = 58.823 mg g^−1
	k 1 = 0.0345 h^−1	k 2 = 4.129 × 10^−5 g mg^−1h^−1
	R ^2 = 0.914	R ^2 = 0.977
4 g L^−1	q ecal = 21.6271 mg g^−1	q ecal = 27.027 mg g^−1
	k 1 = 0.0368 h^−1	k 2 = 1.527 × 10^−4 g mg^−1h^−1
	R ^2 = 0.923	R ^2 = 0.991
8 g L^−1	q ecal = 11.9949 mg g^−1	q ecal = 13.515 mg g^−1
	k 1 = 0.0391 h^−1	k 2 = 4.064 × 10^−4 g mg^−1h^−1
	R ^2 = 0.934	R ^2 = 0.993

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The pseudo-first order kinetic model (Fig. 7a) did not exhibit a linear behavior while the pseudo-second order one (Fig. 7b) is clearly linear. In addition, the R² coefficients for the pseudo-second order kinetic model were higher and close to 0.99 (Table 2), thus were appropriate to describe the adsorption kinetics of Pb²⁺ on the hybrid biosorbent. Similar results were reported for hybrid magnetic alginate beads for the removal of Pb²⁺.⁴⁹

Concerning the thermodynamic parameters, the changes in standard free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) from the adsorption process can be calculated from the following equations:

where ΔG° is the standard free energy change (J mol⁻¹); K_c is the apparent equilibrium constant; C_s is the concentration of Pb²⁺ in solid adsorbent (mg g⁻¹); C_e is the equilibrium Pb²⁺concentration (mg L⁻¹). From the Vant Hoff plot (Fig. 7c), ΔH° and ΔS° were estimated. The thermodynamic parameters are summarized in Table 3.

Table 3 Thermodynamic parameters of Pb²⁺ removal (100 mg L⁻¹) by the hybrid biosorbent at different temperatures at pH = 5

T (K)	ΔH° (kJ mol^−1)	ΔS° (kJ mol^−1 K^−1)	ΔG° (kJ mol^−1)
293	84.840	316.6937	−92.7064
303	44.574	177.8004	−53.8289
313	31.592	131.8855	−41.2486
323	24.5083	106.3432	−34.3244

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The free energy values (ΔG°) were negative, thus revealing the spontaneity of Pb²⁺ adsorption on the hybrid biosorbent. In addition, the adsorption is endothermic due to positive values of ΔH° while the decrease of ΔS° suggested an improvement in the randomness at the solution/adsorbent interlayer.^51,52

3.5. Reuse of the hybrid biosorbent

To highlight the potential application of the hybrid biosorbent, four cycles of adsorption–desorption of 100 mg L⁻¹ Pb²⁺ were investigated using 2 g L⁻¹ hybrid biosorbent at pH = 5 and T = 303 K (Fig. 7d). The desorption process was performed with 2 N HCl.^53,54 After the first cycle, about 40 mg g⁻¹ of Pb²⁺ metal uptake was measured from which 91% was desorbed (37.6 mg g⁻¹). As the hybrid biosorbent was reused, the adsorption of Pb²⁺ decreased to about 25 mg g⁻¹ from which 85% was desorbed after the fourth cycle (Fig. 7d). The weak reusability of the hybrid biosorbent can be explained by several reasons: (i) the Pb²⁺ is chemically adsorbed at the surface of the hybrid biosorbent, e.g. non-electrostatic interactions between the adsorbent and the cations. Similar observations were reported in works that also focused on the removal of Pb²⁺ by other hybrid biosorbents.⁵⁵ It is worth noting that the regeneration of the hybrid biosorbent was performed using 2N HCl,⁵⁶ thus resulting in low pH that partially damaged the structure of the hybrid biosorbent. Therefore, after 4 cycles of reuse, the efficiency of the present hybrid biosorbent decreased. However, the adsorption efficiency remained high for the removal of Pb²⁺ and its eco-friendly nature is a significant benefit for potential use in environmental remediation on a higher scale.

4. Conclusions

The hybrid biosorbent composed of ZnO–JR–PVA–SA is a promising material for the treatment of waste waters contaminated with Pb²⁺. The functional groups at the surface of the hybrid biosorbent were involved in the chemical adsorption of Pb²⁺ including surface complexation. The microporous structure was undoubtedly a benefit due to the higher surface area for Pb²⁺ adsorption. The metal uptake was about 39.1 mg g⁻¹ for 100 mg L⁻¹ Pb²⁺ solution using 2 g L⁻¹ hybrid biosorbent at pH = 5 and T = 303 K. The adsorption isotherm was fitted by the Freundlich model, and the maximum metal uptake was calculated to be about 111 mg g⁻¹. The adsorption kinetics followed a pseudo-second order reaction since the functional groups of the components of the hybrid biosorbent were involved in the adsorption of Pb²⁺. The adsorption process using the hybrid biosorbent was spontaneous and endothermic. Finally, its eco-friendly nature and relatively high efficiency upon reuse are beneficial for the development of such adsorbents in future environmental applications, especially the treatment of wastewaters contaminated with heavy metals.

Author contributions

Kadimpati Kishore Kumar: conceptualization, methodology, investigation, and writing: original draft. Sanneboina Sujatha: methodology, investigation, analysis, data curation, and writing: original draft. Wojciech Skarka: methodology, data curation, and writing: original draft. Olivier Monfort: conceptualization, methodology, and writing: review & editing.

Conflicts of interest

The authors declare that there is no conflict of interest.

Acknowledgements

This work was partially financed by the Slovak Research and Development Agency under contract No. APVV-21-0039 and it was also supported by the Operation Program of Integrated Infrastructure for the project “UpScale of Comenius University Capacities and Competence in Research, Development and Innovation” (ITMS 2014+: 313021BUZ3) co-financed by the European Regional Development Fund. Kadimpati Kishore Kumar acknowledges the Department of Science and Technology (SERB) for the research grant (Grant No. SB/EMEQ103/2013) and also the Indian Institute of Chemical Technology for SEM/EDX analyses.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nj04896e

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