Study of modified biomass of Gossypium hirsutum as heavy metal biosorbent

2.1 Collection and analysis of real water sample

The collection of real water samples was executed by collecting industrial effluents from discharge points of different industries (including textile, electric cable manufacturers, tanneries, pesticides, pharmaceuticals, and fertilizer) located in Multan, Pakistan. A total of 12 water samples were collected in 250 mL high-grade prewashed plastic bottles, labelled and kept in room conditions for 24 h to settle down organic matter. From all collected water samples, a representative sample was prepared by mixing 25 mL from each sample and was filtered first through Whatmann filter paper 42, then through 0.45 µm membrane filter paper and stored at ambient conditions until further analysis.

2.2 Stock solutions

For the preparation of stock solution for each investigated heavy metal, dried salts of 3.929 g of CuSO₄·5H₂O, 1.598 g of Pb (NO₃)₂, 4.549 g of Zn(NO₃)₂·6H₂O, and 3.107 g of NiSO₄·6H₂O were dissolved in double distilled water (DDW) to prepare 1000 mgL^-1 stock solution separately.

2.3 Batch adsorption studies of heavy metal ions

For the adsorption experiments in all sets of conical flasks labelled for different heavy metals, 50 mL of each metal solution ranging from 25 to 150 mg/L was mixed with 0.5 g of biosorbent at 30℃ and homogenized by stirring at 100 rpm for 2 h. 0.1 M HNO₃ was used in all biosorption experiments to keep the initial solution pH between 4 and 6 because preliminary tests showed that this value was the best. The kinetics tests were carried out for time intervals of 2 h, 0.5 g of biosorbent in 50 mL of M(II) ions solutions. Following filtration, the concentration of M(II) ions in each solution was quantified spectrophotometrically (Shimadzu UV-1800 ENG 240 V, SOFT P/N 206–25400-58) using a calibration curve for each metal ion. Each heavy metal was analyzed at its specific λ_max after developing colored complexes by following the published methods (Zook et al., 1970, Moghadam et al., 2016).

2.4 Surface adsorption and biosorption isotherm models

There are four presumptions about surface adsorption: equivalent adsorption sites are available due to uniform adsorbent surface, molecules that are adsorbed on neighboring sites do not interact, all adsorptions take place using the same mechanisms, and adsorbed molecules are on specific sites on the surface of adsorbent (Gul et al., 2023). Isotherm models are frequently used to explain the relationship between the concentration of pollutants in a solution and the amount of pollutant adsorbed onto an adsorbent material at equilibrium, at a constant temperature. These models are used to ascertain an adsorbent material's adsorption capacity and efficiency, as well as to aid in the optimization of the adsorption process design. The Langmuir and Freundlich isotherms are two mathematical models that are frequently used to describe the adsorption of a solute onto a solid surface. The Langmuir isotherm indicates that adsorption takes place on a homogeneous surface with a fixed number of adsorption sites. It depends on evaluating the solute particle maximum adsorption in a saturated monolayer with constant adsorption energy and no adsorbate transformation on a plane surface (Greydanus et al., 2022). It also implies that the adsorption process follows the law of mass action, which states that the rate of adsorption is proportional to the concentration of the solute in the solution.

In the linearized form C_f is the metal ion equilibrium concentration (mg/L) in solution, q is sorbed metal ion (mg/g), b is Langmuir constants and q_max is maximum uptaking capacity.

While, the Freundlich isotherm, on the other hand, assumes that the adsorption occurs on a heterogeneous surface with varying adsorption energies. It also assumes that the rate of adsorption is proportional to the concentration of the solute raised to a power. The Freundlich isotherm is expressed as:

2.5 Characterization

The spectrophotometric studies were performed using Shimadzu UV1800 ENG 240 V, SOFT P/N 206–25400-58. This instrument was used for the determination of lambda max of each metal complex and their subsequent quantification. The functionalities present in modified biomass was determined by using FT-IR (CHNO/S 2400 from PerkinElmer, USA) and morphological characteristics of acid and base treated biomass were examined using scanning electron microscopy (Thermal Field Emission SEM LEO 1560, Zeiss, Oberkochen, Germany). In synthetic air, a thermal gravimetric analysis (TGA Q5000 V3.17 Build 265) was employed to thermal stability featured of treated biomass samples. The quantitative analysis of heavy metals was executed using atomic absorption spectrometer (AAS; Hitachi model A-1800 equipped with standard burner and air acetylene flame and standard hollow cathode lamps as a radiation source for Pb²⁺, Cu²⁺, Zn²⁺ and Ni²⁺) in measurement mode with integration of absorbance signals.

3.1 Fourier-transform infrared spectroscopy (FT-IR)

In order to confirm the identification of surface functional groups and the nature of bonds between functional groups of biomass FT-IR spectra were obtained as given in Fig. 3a. The broad peak appeared around 3600 cm⁻¹ indicated the presence of OH group which are responsible for adsorption of target metal ions. The weak peaks near 3000 cm⁻¹ corresponds to the stretching vibrations of C–H bond of CH₂ groups. The absorption band at 1733 cm⁻¹ was attributed to the absorption of ester functional groups of hemicellulose. On the other hand, the strongest absorption peak observed between 1680 and 1620 cm⁻¹ was assigned to C = O stretching vibrations. The intense peak at 1052 cm⁻¹ along with the weak peak at 1252 cm⁻¹ and the shoulder at 1162 cm⁻¹ were assigned to C-O stretching vibrations of ethers and alcohols. It was observed that the intensity of OH peak increased in base modified biomass due to presence of OH groups provided from base while the acid modified biomass, the peak near 800 cm⁻¹ indicated the presence of Cl and didn’t show such type of broadening as in base modified (Amir et al., 2023). The OH group in base modified and Cl group present in acid modified biomass confirmed the chemical pretreatment of biomass. FTIR obtained after the removal of heavy metals on acid and base modified biomass is given in Fig. 3b.

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Fig. 3a

FTIR spectra of biomass: (a) acid modified and (b) base modified.

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Fig. 3b

FTIR spectra of (a) acid modified and (b) base modified biomass after adsorption of heavy metals.

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3.2 Scanning electron microscopy analysis (SEM)

The particle size and morphology of acid and base modified biosorbent was determined using a scanning electron microscope. The magnification of images was taken at 2500X and scale bar was 10 µm. SEM image of acid and base modified biomass is given in Fig. 4a and 4b, respectively. Additionally, both modified biomass surfaces have tiny pores that can be used to attach Cl and OH to their surfaces from acids and bases, respectively. The particle size of acid and base modified biomass was calculated by the software (Image-J software). Due to crushing and grinding of biomass different sizes of particles were obtained in the sample ranging from 250 to 500 µm. However, most of the biomass particles have a size of less than 350 µm. Due to modification with acid and base, the biosorbent material shows agglomeration and exhibits a film like appearance. Moreover, rough surfaces and small pores were also found on the surface of acid and base modified biomass which is suitable for the affinity of heavy metals to its surface.

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Fig. 4

SEM images of biomass at X2500 (a) acid modified and (b) base modified.

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3.3 Thermogravimetric analysis (TGA)

The TGA curves demonstrate the thermal stability of acid and base-modified Gossypium hirsutum. The initial and final decomposition temperatures of acid and base modified biomass were 25 −390℃, respectively, as shown in Fig. 5. It shows that approximately 10 % weight loss of acid and base-modified biomasses occurred between 25 and 110 °C as a result of the evaporation of water molecules. As the temperature was raised from 310 to 390℃, the degradation of organic material caused the weight loss of acid and base-modified biomass to about 75 %. Both acid and base-modified biomass may thus be utilized effectively at temperatures below 390℃ without any prominent degradation. Both biomasses presented similar degradation patterns owing to identical chemical characteristics.

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Fig. 5

TGA and DSC of (a) acid modified biomass (b) base modified biomass (10 °C/min heating rate in nitrogen environment).

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The differential scanning calorimetry (DSC) analysis of the biomass (represented by the blue line in Fig. 5) affirmed the findings from the TG analysis. Within the temperature range where a reduction in weight due to water loss was observed, the DSC curve exhibited an endothermic feature linked to the evaporation of water in the case of acid-modified biomass. Furthermore, at higher temperatures, there was an endothermic peak with an initiation point at approximately 310 °C and a peak around 390 °C, indicative of the decomposition process. Similarly, a corresponding peak with an onset of around 30 °C slightly exothermic and then broad endothermic pattern from 30 to 100 °C was due to the evaporation of water in the case of base-modified biomass. Furthermore, at higher temperatures, there was an endothermic peak from 300 to 320 °C associated with the decomposition process, which was also evident for base-modified biomass.

3.4 Adsorption isotherm of acid modified biomass

Langmuir and Freundlich adsorption isotherm plots for the adsorption of target heavy metal ions are represented in Fig. 6. The Langmuir and Freundlich constants for the fitting of both equations for heavy metal ions adsorption on acidic cellulosic adsorbent are summarized in Table 1. The Langmuir maximum adsorption capacity of acidic cellulosic biomass for Pb²⁺, Cu²⁺, Zn²⁺, Ni²⁺ metal ions was 96.40, 93.45, 117.80 and 113.02 mg g⁻¹, respectively. For the sake of optimization, the quantification was performed using UV–Visible spectroscopic technique. In case of Cu²⁺ and Ni², colored complexes were formed by reacting Cu²⁺ with 2 mmol of methionine in 5 mL warm distilled water and 30 % NaOH (added for deprotonation of amino acid) and Ni² with 1 mmol/L imine ligands and refluxed for 5 h at 70 °C. The R_L (separation factor) values were calculated according to Equation (3) for known initial metal ion concentrations at 25 °C from Langmuir constants as given in Table 2. Since all R_L values were between 0 and 1, it can be stated that the metal adsorption on Gossypium hirsutum stem biomass is favorable for investigated heavy metal ions. The Freundlich maximum adsorption capacity of acidic cellulosic biomass for Pb²⁺, Cu²⁺, Zn²⁺, Ni²⁺ metal ions were 91.34, 99.30, 116.86, and 111.12 mg g⁻¹, respectively.

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Fig. 6

Langmuir isotherm (a) Freundlich isotherm (b) for removal of Pb²⁺, Cu²⁺, Zn²⁺, and Ni²⁺ by acid modified biomass.

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3.5 Adsorption isotherm of base modified biomass

Adsorption isotherm knowledge is necessary to comprehend the mechanism of adsorption. The adsorption isotherm, which offers crucial information, can be used to infer the dispersion of the adsorbate molecules between the solid phase and liquid phase. The adsorption equilibria in the adsorption of heavy metals on various adsorbents may be correlated using a number of adsorption isotherms. The Langmuir and Freundlich plots of the adsorption isotherms for all metal ions are shown in Fig. 7. In Table 3, the Langmuir and Freundlich constants for the fitting of both equations for basic cellulosic adsorbent systems for heavy metals are summarized. Basic cellulosic biomass had maximal Langmuir adsorption capacities of 121.24, 117.09, 130.65, and 111.09 mg g⁻¹ for Pb²⁺, Cu²⁺, Zn²⁺ and Ni²⁺ metal ions respectively. In other words, the adsorption of metal ions onto basic functional groups on the surface of modified biomass is referred to as monolayer adsorption. The separation factor “ R_L ” which is represented as the following equation, can be used to estimate the affinity between the adsorbate and adsorbent utilizing the fundamental properties of the Langmuir isotherm parameters (Ho and Wang 2004).

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Fig. 7

Langmuir isotherm (a) Freundlich isotherm (b) for removal of Pb²⁺, Cu²⁺, Zn²⁺, and Ni²⁺ using base modified biomass.

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3.6 Kinetic modeling

Kinetic-based models such as pseudo-first and pseudo-second order were applied to explore adsorption mechanisms and the steps for limiting adsorption rates. Various kinetic models, such as linear and non-linear, of differing degrees of complexity are commonly used. Non-linear modelling is considered better contrasting to linear modelling as it provides more realistic kinetic parameters. Also, it assesses all models, allowing for a more realistic comparison to determine which model best reflects a specific kinetic dataset. The discontinuity of models and the determination of any parameter before time can be minimized by using this technique. The low value of χ2, the high value of R², and the correlation of qe experimental values to the calculated qe values can explain the best fitting of a kinetic model for adsorption studies. We applied a non-linear approach of Pseudo first order (PFO) and pseudo-second-order (PSO) models to analyze the kinetic data Fig. 8. The minimum values of χ2 (0.154, 0.760, 0.166 and 0.889) and maximum R² (0.99) values close to 1 for the Pb²⁺, Cu²⁺, Zn²⁺, Ni²⁺ ions on the acid-modified Gossypium hirsutum stem biomass, indicated the best fitting of pseudo-second-order (PSO) as given in Table 5. Similarly, base-modified Gossypium hirsutum stem biomass with low values of χ2 (0.107, 0.121, 0.165 and 0.667) for Pb²⁺, Cu²⁺, Zn²⁺, Ni²⁺ ions and maximum R² (0.99) values also follow PSO in Table 6. These results indicate that Pb²⁺, Cu²⁺, Zn²⁺, Ni²⁺ metal ions are adsorbed via a chemosorption mechanism involving chemical interaction, like ionic bonding between the adsorbent and the metal ions.

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Fig. 8

Kinetic modeling of Pb²⁺, Cu²⁺, Zn²⁺and Ni²⁺ in acid and base modified biomass.

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3.7 Surface coverage (θ) by modified biomass

Surface coverage (θ) is a frequently used term when discussing isotherms and is defined as “A specific adsorbate's maximal (saturation) surface coverage on a given surface must always be one i.e., θ_max = 1″. The method of determining surface coverage is different from that often used in surface science, where the more typical approach is to equate θ with the ratio of adsorbate species to surface substrate atoms. From the collected data of base modification biomass, the surface coverage (θ) value of every sample was less than 1 which indicated that the method was favorable for adsorption. As the initial concentration (Ci) of the sample increased the surface coverage (θ) value also increased, showing the direct relation between them. The uptaking capacity of Pb²⁺ was increased from 0.61 to 0.90, the value of Cu²⁺ increased from 0.59 to 0.89, the value of Zn²⁺ increased from 0.53 to 0.87 and the value of Ni²⁺ increased from 0.52 to 0.87 when the initial concentration of the sample was changed from 25 to 150 mg.

Surface coverage data for the heavy metals by using base modification biomass and metal ions surface coverage related equation of Langmuir was used.

From the collected data of acid modification biomass, the surface coverage (θ) value of every sample in this case was also less than 1. In this method, the direct relationship between concentration (Ci) and surface coverage (θ) was observed. The uptaking capacity of Pb²⁺ increased from 0.36 to 0.77, the value of Cu²⁺ increased from 0.41 to 0.80, the value of Zn²⁺ increased from 0.47 to 0.84 and the value of Ni²⁺ increased from 0.49 to 0.85 when initial concentration of sample was changed from 25 mg to 150 mg. Surface coverage data for the heavy metals by using acid modification biomass and metal ions surface coverage related equation of Langmuir was used. Base modified biomass gave better up-taking capacity relative to acid modified biomass.

3.8 Regeneration studies and effect of pH on modified biomass

In order to make the adsorption process efficient and economical, researchers are interested in the regeneration of adsorbents through desorption. This procedure facilitates the recovery of adsorbate, reduces the need for virgin adsorbents, stabilizes adsorbents, and provides knowledge on the reversibility of an adsorption process. In order to regenerate modified biomass, dilute solutions of HNO₃ or HCl are generally used. As a result of their competition for active sites, the hydronium ions from these acids displace metals and vacate active sites on the surface of the adsorbent which can be used for another adsorption cycle (Ding et al., 2016). In the current study, 0.1 M HNO₃ was employed for the desorption of adsorbed heavy metal ions from Gossypium hirsutum base modified biomass. To 50 mL of metal ion solution (150 mgL^-1), 0.5 g of dried Gossypium hirsutum biomass was added and the mixture was stirred at 100 rpm for 1 h to attain the equilibrium. Afterwards, the metal ion-adsorbed Gossypium hirsutum biomass was submerged in a solution of 0.1 M HNO₃ for 2 h while being stirred to ensure the complete removal of adsorbed metal ions from the modified biomass. The results of desorption efficiency for five cycles are given in Table 7.

The impact of pH on Pb²⁺, Cu²⁺, Zn²⁺ and Ni²⁺ adsorption on the surface of Gossypium hirsutum base modified biomass was tested at pH 3.0 – 7.0 for the initial metal concentration of 150 mg/L and time duration of 35 min for all heavy metal ions. The ionic state of the functional groups at the adsorbent surface and metal ions present in the solution are both influenced by the pH of the aqueous medium (Bayu et al., 2022). It is evident from Fig. 9 that in base modified biomass, the adsorption rate is poor at low pH (pH < 3.0) and increases with pH increase. This situation can be ascribed to the fact that the adsorbent surface becomes more negatively charged and the functional groups of the biomass becomes more available for electrostatic interactions as the pH of the solution increases above pH > 4.0, thus leading to an enhanced adsorption of heavy metal ions. However, at lower pH of the aqueous medium, the presence of enormous protons may compete with heavy metal ions for active sites on biosorbent resulting in lower adsorption capacity in strongly acidic media. From the Fig. 9 it is obvious that adsorption of heavy metals on base modified biomass is relatively high at pH 5.5. For all metal ions investigated, the removal of metal ions rises from 20 to 95.0 % in the pH range of 4.0 – 7.0. The results are in agreement with the similar studies carried out for the removal of heavy metal ions (Nizam et al., 2022). Almost similar effect of pH was observed for heavy metal ions adsorption on acid modified biomass. Therefore, Fig. 9 only shows the effect of pH on the adsorption capacity of base modified biomass.

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Fig. 9

Effect of pH on the uptake of Pb²⁺, Cu²⁺, Zn²⁺ and Ni²⁺ metal ions by base modified Gossypium hirsutum cellulosic biomass (initial concentration 150 mg/L; contact time 35 min; stirring rate 100 rpm).

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3.9 Heavy metal ions removal from industrial effluent using modified biomass

The efficacy of base modified biomass in wastewater treatment was tested using effluent water collected from drains of different industries as mentioned above. To find the initial concentration of each target heavy metal, the filtered representative real water sample was analyzed through AAS by making calibration curve for each metal ion by preparing standard solutions (Iqbal et al., 2006). From the triplicate analysis of each target metal ion using AAS, the concentrations for Pb²⁺, Cu²⁺, Zn²⁺ and Ni²⁺ were 3.7 ± 0.12, 3.9 ± 0.06, 6.3 ± 0.02 and 2.5 ± 0.17 mg/L given in Fig. 10 which were much higher than the permissible limits of heavy metals set by WHO. Furthermore, the existence of high concentrations of heavy metal ions in real water samples were assumed to be due to the fallout of industrial emissions and discharged effluents. Almost similar results for the levels of heavy metals in varied water samples have been reported (Waseem et al., 2014). In order to determine the efficacy of biosorbent for the removal of heavy metal ions, 0.5 g of the base modified biomass was equilibrated with 50 mL of real water sample in a conical flask and shaken in a mechanical shaker for 2 h. The percentage of metal ions that were adsorbed on base modified biomass in the real water sample for Pb²⁺, Cu²⁺, Zn²⁺ and Ni²⁺ was 78.5, 80.3, 81.4, and 82.6 %, respectively. The results demonstrate that Gossypium hirsutum modified biomass may adsorb significant amounts of Pb²⁺, Cu²⁺, Zn²⁺ and Ni²⁺ ions however at a lesser rate than with synthetic solutions. This might be explained by the abundance of competing cations and ligands in natural fluids.

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Fig. 10

Calibration curve of heavy metals using AAS.

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In a comparative study based on the assessment of adsorption capacities of different adsorbents for the removal of heavy metal ions (Pb⁺², Cu⁺², Zn⁺², Ni⁺²) from water, distinct performance trends were observed (Table 8). Moringa oleifera seeds displayed moderate adsorption capacities, with 23.3 mg g⁻¹ for Pb⁺², 42.3 mg g⁻¹ for Cu⁺², and 16.1 mg g⁻¹ for Zn⁺² (Tokay and Akpınar 2021). Conversely, magnetite nanorods exhibited significantly higher adsorption capacities, recording impressive values of 112.8 mg g⁻¹ for Pb⁺², 79.10 mg g⁻¹ for Cu⁺², 107.3 mg g⁻¹ for Zn⁺², and 95.4 mg g⁻¹ for Ni⁺² (Karami 2013). Chondrus crispus, a type of seaweed, demonstrated moderate adsorption capabilities with values of 63.7 mg g⁻¹ for Pb⁺², 21.3 mg g⁻¹ for Cu⁺², 21.6 mg g⁻¹ for Zn⁺² and 17.1 mg g⁻¹ for Ni⁺² (Romera et al., 2007). Granulated activated carbon displayed relatively low adsorption capacities for heavy metal ions (Minceva et al., 2008), while Clinoptilolite (Zeolite) exhibited moderate adsorption for Pb⁺² (30 mg g⁻¹) but low capacities for Cu⁺², Zn⁺², and Ni⁺² (Zendelska et al., 2018). However, the standout performer Gossypium hirsutum, the cotton plant, which displayed exceptional adsorption capacities across the board, with values of 121.24 mg g⁻¹ for Pb⁺², 117.09 mg g⁻¹ for Cu⁺², 130.65 mg g⁻¹ for Zn⁺², and 111.09 mg g⁻¹ for Ni⁺². All these results show that Gossypium hirsutum can successfully remove heavy metal ions from water, making it a promising candidate for environmental rehabilitation.