Sequestration of Pb²⁺ from aqueous solution using bio-based-alkaline modified sorbent from waste Irvingia gabonensis seed husk

Potable water is vital for sustaining life and is also a basic material required for industrial, agricultural, and urban activities to thrive (Sun et al. 2018; Shahid et al. 2021). Unfortunately, as we experience an increase in these (agricultural, urban, and industrial) activities, there is a decrease in the availability of clean water and an increase in the level of contaminants in the environment (Li et al. 2019; Hoang et al. 2020). Different organic and inorganic pollutants are usually emitted into water bodies, thereby causing several damages to the ecosystem (Mustafa et al. 2022). Some of the contaminants that have been previously reported to be the major contributors of water pollution are organic matter, nutrients, fertilizers, pesticides, dyes, pharmaceuticals, oils, and heavy metals (Häder et al. 2020).

Among these pollutants, heavy metal contamination of the water bodies is of the utmost concern, largely due to their persistent and non-degradable nature (Xu et al. 2021; Zhang et al. 2021). They tend to accumulate in organisms, thereby causing several health issues when their concentration reaches a certain level (Pujari et al. 2021). They are very soluble and enter the water bodies mainly through natural and human activities such as flooding, weathering of rocks, tanning, oil exploration, combustion of coal, and the use of pesticides and herbicides (Kumar et al. 2019; Mustafa et al. 2022).

Lead (II) ion as a contaminant is currently among the major global concerns due to their toxic properties and negative influence on the environment. They are non-degradable and bio-accumulate in living organisms, causing different health challenges (Baniamerian et al. 2009; Futalan & Wan 2022). The Agency for Toxic Substances and Disease Registry (ATSDR 2007) reported that, of all the known toxic substances, Pb²⁺ was ranked 2nd. Its acceptable limit in drinking water set by the World Health Organization (WHO) and the United States Environmental Protection Agency (USEPA) is 0.05 and 0.015 mg/L, respectively (Arbabi et al. 2015; Rusmin et al. 2022). Frequently reported sources of Pb²⁺ in wastewater are often from effluents of manufacturing industries, which include fuels, pigments, mining, ceramic industries, painting manufacturing, and battery production (Hayes 2012; Elias et al. 2021). More so, when water contaminated with Pb²⁺ enters the ecosystem, the Pb²⁺ accumulates in organisms through the consumption of food or water contaminated with lead, causing several diseases (Abdel-Salam 2018). Some diseases frequently linked with a high accumulation of Pb²⁺ are anemia, cardiovascular diseases, infertility in men, cognitive impairment, and many others (Anantha & Kota 2016). In plants, diseases such as low production of leaves, stunted growth, and a decrease in CO₂ fixation are also associated with high exposure to Pb²⁺ (Chibuike & Obiora 2014).

For decades, several physicochemical techniques such as ion-exchange (Bashir et al. 2019), oxidation (Zhang et al. 2020), activated carbon (Shahrokhi-Shahraki et al. 2021), membrane separation (Sunil et al. 2018), and among others, have been employed in the sequestration of toxic Pb²⁺ from aqueous solutions with solvent extraction, and adsorption (Imran et al. 2020; Iqbal et al. 2021) having been widely used. Among all these methods of sequestration of lead, adsorption techniques have been shown to be cost-effective, user friendly, and produce low or even no secondary pollutants (Amaku et al. 2021). While high efficiency for pollutant elimination, ease of use, environmental friendliness, and lower cost effectiveness make the adsorption method most suited for Pb(II) from the aqueous phase (Athman et al. 2020). Recently, different adsorbents such as orange peels (Adesanmi et al. 2020), chitosan-coated bentonite (Futalan & Wan 2022), Cassia sieberiana seed (Eze et al. 2021), African Star Apple seed shells (Adebisi et al. 2022), saponified melon peels (Chaudhary & Ijaz 2014), eggshell (Hussain & Sheriff 2014), peanut shells and compost (Mustafa et al. 2022), and others, have been successfully used for the elimination of Pb(II) from the aqueous solution.

However, the efficiency of an adsorbent is dependent on pore structures, adsorption capacity, and adsorbent reusability (Akpomie & Dawodu 2015). Unfortunately, most of the biosorbents employed for sorption of Pb²⁺ show demerits in one of these characteristics. Hence, there is a need for modification of an adsorbent for an improved adsorption capacity (Fenti 2020; Yang et al. 2020). Previous reports showed that different functional groups associated with different biosorbents are responsible for the sorption of the adsorbates (Akpomie & Conradie 2020). These compounds can be extracted and modified in various ways to improve their sorption capacity for Pb²⁺.

Despite several works on the use of biomaterials for adsorption, there is not sufficient literature on the use of modified Irvingia gabonensis seed husk for the adsorption of contaminants. In this study, I. gabonensis seed husk was modified using NaOH and its adsorption performance was evaluated. The sorption isotherm and the kinetics of the sorption were characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), and thermogravimetric analyzer (TGA). The impacts of pH and contact time were also studied. This research is aimed at studying the possibility of using base (NaOH)-modified I. gabonensis for the sequestration of Pb²⁺ from aqueous solutions.

I. gabonensis seed husk was obtained from Enugu-Ezike in Enugu state, Nigeria. It was identified by Dr E.N. Abu of the Plant Science Department at the University of Nigeria, Nsukka. It was washed to remove dirt, sand, and other impurities and subsequently sun-dried for several days, after which it was milled to fine particles using a locally made grinder. The samples were sieved through a <1.0–250 μm automatic sieve shaker to obtain the adsorbent for the unmodified I. gabonensis. Around 350 g of I. gabonensis seed husk from the sample was modified with 700 mL of 28% NaOH for 24 h; afterwards, the activated sample was washed with de-ionized water until the pH of the filtrate was about pH of 7. The activated residue was dried in an oven at 105 °C, allowed to cool and then stored in an airtight container.

The entire set of reagents used in this study was of analytical grade and thus was used without further purification. The standard method was used to prepare the laboratory stock solution of Pb²⁺ by dissolving 0.02 g of Pb(NO₃)₂ in a 1 L volumetric flask. Several concentrations of Pb²⁺, which included 10, 20, 30, 40, 50, and 60 mg/L were prepared from the stock solution by serial dilution.

SEM (Phenonm World, MVE01570775, 800-07334, the Netherlands) was employed to investigate the surface morphology, while FTIR (Agilent G8043AA, Cary 630 FTIR, Agilent Technologies, USA) was used to study the functional groups that were present in the binding process between the sorbent and Pb²⁺. During the analysis, potassium bromide pellets were used as the reference material and calibrated in the range 650–4,000 cm⁻¹. The surface crystallinity of the biosorbents was determined using an XRD (XRL X'TRA X-ray, Thermoscientific, model 197492086, Switzerland). BET (Qauntachrome Novawin Instrument, Version 11.03) surface area analyzer was employed to examine the surface area and pore structures of the sorbent. TGA of Unmodified I. gabonensis (UMIG) and Base modified I. gabonensis (BMIG) was done on a PerkinElmer simultaneous thermal analyzer (TGA4000, USA).

A batch adsorption process was adopted to examine the adsorption efficiency (Abugu et al. 2014, 2015). The effect of the initial Pb²⁺ concentration on adsorption was determined using 10, 20, 30, 40, 50, and 60 mg/L concentrations of the target metal. One gram of the NaOH-modified I. gabonensis (IG) and 20 mL of different initial Pb²⁺ concentrations were agitated at 100 ppm for 10 min at room temperature. The solutions were then filtered and taken to an atomic absorption spectrometer to measure the residual Pb(II) concentration of each sample. To determine the effect of pH on adsorption, 30 mL of 30 mg/L adsorbate was measured and the pH range varied from 4 to 9 using NaOH and HCl. The pH value was determined with a pH meter. Each pH solution was then poured into six flat-bottom flasks containing 1 g of the modified I. gabonensis and a magnet stirrer. The solutions were then agitated for 10 min and after which they were filtered and taken to an atomic absorption spectrometer to determine the residual Pb²⁺ concentration.

The sorption isotherm models were applied by varying the initial metal ion concentrations from 10 to 60 mg/L to ascertain the mechanism of the sorption process. Langmuir, Freundlich, Temkin, Dubinin–Radushkevich (D-R), and Flory–Huggins isotherm models were examined. The isotherm model equations and parameters of the employed isotherm models are presented in Table 1.

Table 1

Adsorption isotherm models used for sorption studies

Model .	Linear equation .	Parameters .
Langmuir		qe – quantity of Pb2+ uptake at the equilibrium Ce – equilibrium concentration of the Ni2+ KL – Langmuir constant qm – theoretical maximum adsorption capacity of the adsorbent
Freundlich		KF – Freundlich constant 1/n – Freundlich exponent Ce – Equilibrium concentration of adsorbate after adsorption
Temkin		B – Temkin constant AT – constant associated with adsorption capacity
D-R		B – constant-related free energy qm – theoretical maximum adsorption capacity based on D-R isotherm Ε – the Polanyi potential
Flory–Huggins		Θ – the degree of surface coverage nFH – quantity of metal ions covering sorption sites KFH – Flory–Huggins equilibrium constant

Model .	Linear equation .	Parameters .
Langmuir		qe – quantity of Pb2+ uptake at the equilibrium Ce – equilibrium concentration of the Ni2+ KL – Langmuir constant qm – theoretical maximum adsorption capacity of the adsorbent
Freundlich		KF – Freundlich constant 1/n – Freundlich exponent Ce – Equilibrium concentration of adsorbate after adsorption
Temkin		B – Temkin constant AT – constant associated with adsorption capacity
D-R		B – constant-related free energy qm – theoretical maximum adsorption capacity based on D-R isotherm Ε – the Polanyi potential
Flory–Huggins		Θ – the degree of surface coverage nFH – quantity of metal ions covering sorption sites KFH – Flory–Huggins equilibrium constant

Radha et al. (2021), Tsai et al. (2016), Desta (2013), and Cui et al. (2020).

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The parameter q_t (mg/g) is the quantity of Pb²⁺ uptake at time t, k₁ (min⁻¹) is the rate constant of the PFO, k₂ (min⁻¹) denotes the rate constant of the PSO, k_p (mg g⁻¹ min^−1/2) is the rate constant of intra-particle diffusion, and C is the intercept of the intra-particle diffusion.

On the NaOH-activated biosorbent, the FTIR spectra showed an observable shift in peaks attributed to pretreatment with NaOH, which necessitated the breakdown of the organic compounds and the transformation of the methyl ester group hydrolysis and ester groups generated into carboxylate ions (Feng et al. 2009; Castro et al. 2021). This modification ensured that there were no interferences with undesirable compounds during the adsorption process and increased the pore structure and surface area of the adsorbent (Li et al. 2008; Gupta & Gogate 2016). The spectra of the spent base-modified adsorbent confirmed the participation of (–OH, –NH, C–O, and C = O) functional groups in the removal process of Pb²⁺ by the changes in the broadband of those peaks. This reveals that the removal process of the target metal binds to the sorbent surface (Sari et al. 2007; Azam et al. 2022).

The electrostatic interactions existing between the negatively charged functional groups present in the biomass and the cationic species bring about the binding of Pb²⁺ onto the adsorbent (Shaker 2007). The solution chemistry of the Pb²⁺ in aqueous solution (which is pH dependent) and the nature of the adsorbent binding sites determine the mechanism of Pb²⁺ binding to the adsorbent (Shaker 2007). At lower pH, the functional groups on the surface of the adsorbent are closely associated with the hydronium ions H₃O⁺ and restriction of Pb²⁺ is experienced resulting from the repulsive force existing between two positively charged ions (Shaker 2007). As the aqueous solution pH increased, more functional groups would be exposed that possess negative charges, with subsequent attraction of Pb²⁺ and biosorption onto the adsorbent (Brad & Duncan 1994). The uptake decrease at a much higher pH could be a result of the occurrence of metal precipitation according to Shaker (2007). The adsorbent surface contains mostly carboxyl functional groups (Brad & Duncan 1994), so that the adsorbent surfaces are negatively charged under acidic pH conditions with a high affinity for metal ions in solution (Brad & Duncan 1994; Shaker 2007).

From the data in Table 4, the removal of Pb²⁺ by BMIG best correlates with the Temkin isotherm model. This is established by the higher correlation value (R² = 0.989) which was greater than the other isotherm models, suggesting heterogeneous uptake of Pb²⁺ onto the base-modified adsorbent (Shafiq et al. 2021). This result is similar to the previous report of Pb²⁺ removal using Raphia-microorganism composite biosorbent (Staroń & Chwastowski 2021) with an R² value of 0.9884 and 0.9984, respectively. However, Freundlich and D-R isotherms equally gave a good match with the same correlation values (R² = 0.954), respectively. The estimated binding energy was below 8 kJ mol⁻¹, indicating a physical adsorption mechanism. In physisorption, the process is described with relatively low binding energies due to the weak Van der Waals interaction between the adsorbate and adsorbent (Araújo et al. 2018). The maximum adsorption capacity of the BMIG was compared with some reported biosorbents for the uptake of the same metal ion (Table 5).

Two error functions HYBRID and MPSD were used to study the influence of error functions on the predicted isotherm model. The error functions used to minimize the error distribution between the experimental equilibrium and calculated isotherm were consistent with the PSO kinetic model, which gave the lowest values indicating a chemical (chemosorption) adsorption process (Sreńscek-Nazzal et al. 2016). This result is in line with the result of Eze et al. (2021) in their study of adsorption of Pb²⁺ onto C. siberiana seed.

This study investigated the potential of BMIG for the sequestration of Pb²⁺ from aqueous solutions. The characterization results confirmed the presence of oxygen-rich functional groups on the surface of the adsorbents, while on activation of the precursor with NaOH, the FTIR spectra showed an observable shift in peaks attributed to pretreatment with NaOH, which necessitated the breakdown of the organic compounds. SEM analysis under different magnifications revealed a heterogeneous, rough, and uneven display with sufficient cavities on the biosorbent surface. The large pore structures were further observed using BET analysis. XRD revealed a sharp peak on the activated biosorbent layer, indicating a distinct alignment of the layers of a typical crystalline structure, while the TGA studies revealed that the samples were thermally stable up to 500 °C. The experimental conditions investigated showed that the maximum adsorption capacity was attained at 30 min at pH 6. The highest correlation value (R²) was employed to determine the model that best correlates with the sorption process; it was shown that the sorption process best correlates with the Temkin isotherm model, while the kinetic studies proved that the PSO best describes the kinetic data.

The authors did not receive any funding from any funding agency.

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

The authors declare there is no conflict.