Chemical pretreatment of Lagenaria breviflora seeds used as biosorbents for the removal of aqueous-bound Ni²⁺

Anthropogenic activities such as paper making, smelting, indiscriminate discharge of industrial effluents, and over-dependence on pesticides, herbicides, and fertilizers for agricultural activities have led to an increase in potential toxic elements contamination of the environment (Azimi et al. 2017). Contamination of the aqueous environment by potentially toxic elements has posed a serious concern for scientists and environmentalists with increasing interest in Pb^2+, Cd²⁺, and Ni²⁺ (Cui et al. 2020). These potential toxic elements are non-biodegradable leading to a possible accumulation and human exposure through food and water (Kulkarni et al. 2014). They tend to cause severe disorders and diseases when they accumulate in the human body through successive food chains (Binet et al. 2018).

Nickel, which is the 28th element of the periodic table, is a hard, ductile silvery-white transition metal. It is mainly distributed in the environment as a result of natural and anthropogenic activities (Genchi et al. 2020). Ni exists in more than one oxidation state; however, the +2 oxidation state is the most abundant in the environment (Muñoz & Costa 2012). It exists in water in the form of nitrates, sulfides, and oxides (Elkhaleefa et al. 2020). Ni²⁺ is highly toxic and carcinogenic even at a minimal concentration (Monier et al. 2010; Tran et al. 2010). In addition, Ni²⁺ contaminates the aqueous environment majorly through metal cleaning industries, plating and tanning industries, and electronic gadgets and it poses serious ecological threats to the environment (Vijaya et al. 2008; Liakos et al. 2021). Several methods normally used in the removal of Ni²⁺ from an aqueous solution such as cation exchange and precipitation are expensive and produce toxic sludge (Alomá et al. 2012; Putra et al. 2014).

Thus, to address the present challenges of increasing deterioration of clean water resources and depletion of available freshwater supplies due to contamination emerging from the discharge of effluents into water bodies and indiscriminate disposal of waste associated with potentially toxic elements, there has been a growing interest by the scientist in developing new technologies for the removal of threatening pollutants from aqueous medium (Ibrahim 2011; Ugwoke et al. 2020; Eze et al. 2021, 2022; Shafiq et al. 2021). Numerous methods such as chemical precipitation, membrane separation, ion exchange are used for metal ion uptake from an aqueous solution (Aji et al. 2012; Orooji et al. 2019). Biosorption is known to be among the most effective techniques due to its efficiency, availability, cheapness, and environmental friendliness (Liu et al. 2020). In addition, the use of agricultural materials which are low in cost, effective, and universal, as well as showing the capability of adapting to numerous experimental conditions are also some of the advantages of using biosorbents for the sorption of metal ions from an aqueous solution (Park et al. 2010; Anastopoulos & Kyzas 2015; Vijayaraghavan & Balasubramanian 2015).

Presently, the focus has been on the use of modified biosorbents for the uptake of potentially toxic elements. Base and acid solutions, organic compounds, and other modifying agents have been used to functionalize biosorbents which can eliminate the coloring of effluents by extracting soluble organic components and thereby increasing the rate of potentially toxic elements uptake (Shukla & Pai 2005; Li et al. 2006; Sćiban et al. 2006). Thus, the adsorption of Cd²⁺ was reported to be almost doubled when the precursor (rice husk) was treated with NaOH (Tarley et al. 2004). More so, Alfalfa biomass showed maximum adsorption capacity for Pb²⁺ when treated with NaOH (Tiemann et al. 2002).

In literature, previous studies on the adsorption of Ni²⁺ onto bentonite/grapheme oxide showed that it correlates with the Langmuir isotherm with excellent adsorption capacity (Chang et al. 2020). Khan et al. (2019) investigated the removal of Ni²⁺ from wastewater by natural clay, the result revealed that the metal uptake was rapid while the maximum adsorption was attained in 120 min. The system followed a pseudo-second-order reaction. The sorption of Ni²⁺ by functionalized Henna powder had previously been studied (Mehrmad et al. 2020). The result showed that the process was defined by Freundlich and Langmuir isotherm models while pseudo-second-order reaction gave the best fit for the kinetic model.

In the present study, the sorption of Ni²⁺ by chemically treated biomass (Lagenaria breviflora) was investigated. The potential of L. breviflora (which has no economic value in Nigeria) as an adsorbent for Ni²⁺ has never been reported and as the race to discover the best biosorbent for the removal of potentially toxic elements is on top gear, reporting the potential of base and acid-modified L. breviflora in removing Ni²⁺ is of utmost importance. The sorption isotherm and the kinetics of the sorption processes were characterized using Fourier transformation infrared spectroscopy (FTIR), scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), and thermogravimetric analyzer (TGA). The influence of pH, initial concentration of the metal ion, and contact time were also investigated. Therefore, this report investigated the feasibility of using chemically treated L. breviflora for the uptake of Ni²⁺ from an aqueous solution.

L. breviflora is a species of climbing vine flowering plant found across West, East, and Central Africa (Okoli 1984). It has approximately 7–20 cm large ovate-triangular leaves with hairy undersides and partly dense hairs on the leaf petioles. Vine branches of L. breviflora grow up to 6 m in height. It forms approximately 9 × 7 cm oblong, green fruits with whitish spots across the surface. The fruits are similar to those of other members of the Lagenaria genus (Okoli 1984). It belongs to the family of Cucurbitaceae, also called cucurbits or the gourd family consisting of about 965 species in around 95 genera. Those that are most important to humans are the Cucurbita – squash, pumpkin, zucchini or courgette,and some gourds (Okoli 1984).

L. breviflora was collected from a farm in Nsukka, Enugu State, Nigeria. The identification of the sample was done in the Plant Science and Biotechnology Department, University of Nigeria, Nsukka by Dr N.E. Abu. The seed was initially washed with water to remove dirt and sand and it was later washed again and rinsed with de-ionized water to ensure the total removal of impurities. The L. breviflora seed was then sun-dried for 16 days and then pulverized to a powdery form using a locally made grinder. The pulverized samples were then sieved through a 125–250 μm seive to obtain the prepared L. breviflora adsorbent for the characterization, modification, and then adsorption studies.

350 g of 125–250 μm of the ground and the sieved sample was soaked in 1 M H₂PO₄ for 24 h at room temperature. This was followed by washing the sample repeatedly with de-ionized water (until a pH of about neutral was obtained). The residues from the sample filtration were dried in an oven at 150 °C, labeled (AMLB), and stored in an airtight container.

For the base-modified (BMLB), ammonia was used to functionalize the precursor. 350 g of the dried biomass was modified using ammonia where 1 L of 30% NH₃ was mixed with 350 g of the powdered L. breviflora and it was stirred thoroughly and allowed to stand for 24 h. Thereafter, the activated sample was washed with de-ionized water until the pH reached neutral.

The FTIR was obtained with Agilant FTIR G8043AA, Malaysia. The spectral analysis was carried out at the scanning frequency of 4,000 to 400 cm⁻¹.

TGA (TGA4000) was used to assess the thermal stability of the modified precursor while XRD (Thermo-scientific ARL XTRA, Switzerland) was used to determine the crystalline structure of the prepared adsorbents. The calibration of the XRD was checked with a silicon (Si) standard which was mounted in the XRD. The pore structure of the modified adsorbents was determined with a Scanning Electron Microscope SEM (Phenom ProX, MVE01570775, Netherlands). The surface area of the adsorbents was determined with Branauer–Emmeth–Teller (BET) using Quantachrome NovaWin Version 11.03. The samples were placed on a specific surface area and pore size analyzer and were analyzed by the nitrogen adsorption at 77 K. Specific surface area was estimated by Brunauer–Emmett–Teller (BET) algorithm, and pore volume and average pore size were estimated by the Barrett–Joyner–Halenda (BJH) method (Manawi et al. 2018).

In this study, all the chemicals were of analytical grade and were used without further purification. A stock solution of Ni²⁺ was prepared by dissolving the appropriate amount of nickel(II) chloride (NiCl₂·6H₂O) in 50 mL of de-ionized water. To ensure proper dissolution, the solution was properly stirred with a glass rod. 50 mL of the solution was then placed in a 1-L volumetric flask and the volume was made up to the meniscus mark with de-ionized water to obtain a stock solution of concentration 1,000 mg/L of the metal ions. Other concentrations of Ni²⁺ (10, 20, 30, 40, 50, and 60 mg/L) were then prepared from the stock solution by serial dilution.

Several solutions of Ni²⁺ concentrations such as 10, 20, 30, 40, 50, and 60 mg/L were prepared as described above. 0.5 g of the modified L. breviflora seed was then placed in six 100-mL glass bottles followed by the addition of 20 mL of each solution; the glass containing the solutions was corked and agitated for 10 min at room temperature. After the filtration, the filtrate was taken to an atomic absorption spectrophotometer (AAS) for residue metal ion concentration.

0.5 g of L. breviflora seed husk was placed in six 100-mL glass bottles, followed by the addition of 20 mL of metal ion solution to the containers at 40 mg/L concentration. The container was corked and agitated for 10 min after which it was allowed to stand for the following contact time: 10, 20, 30, 40, 50, and 60 mins. After filtering the solution at each contact time, the concentration of metal ions in the filtrate was estimated using an atomic absorption spectrophotometer (AAS).

pH is one of the main factors that influence the adsorption of adsorbate from aqueous solutions (Wahab et al. 2021). 0.5 g of modified L. breviflora seed was placed in six 100-mL glass bottles, followed by the addition of 20 mL of metal ion solution to the containers at 40 mg/L concentration, and a different pH of 2, 4, 6, 8, 9, and 10 was maintained in each bottle, respectively, and agitated for 10 min after being corked. After filtering the solution, the concentration of metal ions in the filtrate was analyzed using AAS.

The non-linear plot of pseudo-first-order (PFO), pseudo-second-order (PSO), and intra-particle diffusion kinetic models was used to explain the adsorption mechanism of Ni²⁺ onto AMLB and BMLB as shown in Table 1a. Kinetic studies on the adsorption of Ni²⁺ were carried out at room temperature (25 °C). Since the information provided by kinetics studies can give knowledge on the mechanism and the adsorption rate, conducting the experiments at a particular temperature is important. The adsorption process was also subjected to the Weber and Morris intra-particle diffusion model (Maruthapandi et al. 2018). This model states that the uptake of the adsorbate varies with the square root of adsorption time if intra-particle diffusion is the rate-controlling factor (Maruthapandi et al. 2018; Umar et al. 2021). It predicts the rate-limiting step in the adsorption of Ni²⁺. For a solid–liquid sorption process of this nature, the solute transfer is usually characterized by external mass transfer (boundary layer diffusion), intra-particle diffusion, or both.

Table 1a

Kinetic models used for the sorption study

Model .	Non-linear and linear equation .	Parameters .
Pseudo-first-order		(mg/g): quantity of Ni2+ uptake at time t
		(mg/g): quantity of Ni2+ uptake at time t (min−1): rate constants of the pseudo-first-order
Pseudo-second-order		(min−1): rate constants of the pseudo-second-order
Intra-particle diffusion		(mg_g−1_min−1/2): rate constant of intra-particle diffusion C: intercept

Model .	Non-linear and linear equation .	Parameters .
Pseudo-first-order		(mg/g): quantity of Ni2+ uptake at time t
		(mg/g): quantity of Ni2+ uptake at time t (min−1): rate constants of the pseudo-first-order
Pseudo-second-order		(min−1): rate constants of the pseudo-second-order
Intra-particle diffusion		(mg_g−1_min−1/2): rate constant of intra-particle diffusion C: intercept

Jabeen et al. (2021).

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One among many important means of calculating, predicting, and analyzing the various possible mechanisms that occur in the adsorption process is the use of adsorption isotherm (Eze et al. 2021, 2022; Ragadhita & Nandiyanto 2021). The models of the adsorption isotherms were examined to reveal the particular facts concerning the surface adsorbent material properties of the LB and the adsorption nature. The dynamic concept of adsorption equilibrium is found as soon as the rate of the Ni²⁺ adsorption process is equal to the desorption rate. The recorded data of Ni²⁺ adsorption onto LB are fitted to the isotherm models (Table 1b). The Langmuir isotherm model stresses the adsorption process occurring in a monolayer manner in all adsorption sites since the adsorbent surface is homogeneous (Maruthapandi et al. 2018). In contrast to the proposition of Langmuir, the Freundlich isotherm model assumes that the adsorption is a multilayer process that is localized to a heterogeneous surface (Maruthapandi et al. 2018). The Temkin isotherm model suggests that the heat of adsorption of all molecules decreases linearly with the increase in coverage of the adsorbent surface, and that adsorption is characterized by a homogenous distribution of binding energies, up to a maximum binding energy (Piccin et al. 2011). Flory–Huggins isotherm takes into account the degree of surface coverage of the adsorbate on the adsorbent. This isotherm also assumes that the adsorption process occurs spontaneously (Saadi et al. 2015; Ragadhita & Nandiyanto 2021). Dubinin–Radushkevich (D–R) isotherm expresses the adsorption process on the adsorbent which has a pore structure or adsorbent which has a heterogeneous surface and expresses the adsorption free energy. Its adsorption process is based on micropore volume filling (Ragadhita & Nandiyanto 2021).

Table 1b

Adsorption isotherm models used for the sorption study

Model .	Non-linear and linear equation .	Parameters .
Langmuir		(mg/g): quantity of Ni2+ uptake at the equilibrium. (mg/L): equilibrium concentration of the Ni2+ KL (L/mg): Langmuir constant (mg/g): theoretical maximum adsorption capacity of adsorbent
Freundlich		KF: Freundlich constant nF: intensity of the adsorbents
Temkin		T is the temperature (K) R is the universal gas constant BT is Temkin constant A is a constant related to adsorption capacity.
D–R		B is a constant associated with the adsorption free energy is the theoretical saturation capacity based on D–R isotherm (mg/g)
Flory–Huggins		θ is the degree of surface coverage, nF quantity of metal ions covering sorption sites KFH is the Flory–Huggins equilibrium constant

Model .	Non-linear and linear equation .	Parameters .
Langmuir		(mg/g): quantity of Ni2+ uptake at the equilibrium. (mg/L): equilibrium concentration of the Ni2+ KL (L/mg): Langmuir constant (mg/g): theoretical maximum adsorption capacity of adsorbent
Freundlich		KF: Freundlich constant nF: intensity of the adsorbents
Temkin		T is the temperature (K) R is the universal gas constant BT is Temkin constant A is a constant related to adsorption capacity.
D–R		B is a constant associated with the adsorption free energy is the theoretical saturation capacity based on D–R isotherm (mg/g)
Flory–Huggins		θ is the degree of surface coverage, nF quantity of metal ions covering sorption sites KFH is the Flory–Huggins equilibrium constant

Ayawei et al. (2017), Manawi et al. (2018), Ribeiro et al. (2019), Mahmoud et al. (2020) and Elkhaleefa et al. (2020).

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The spectrum of AMLB presents the characteristic stretching vibration absorption band at (2,980–2,840 cm⁻¹) which denotes the presence of C–H of developed aliphatic alkanes. The broadband at 3,600–3,100 cm⁻¹ shows the presence of the stretching vibration of OH and NH₂ bonds (Khan et al. 2022). The peak at 1,748 cm⁻¹ can be ascribed to the stretching vibration of C = O. Other notable spectra include 2,080 cm⁻¹ (C triple bond carbon or CN), 1,485 cm⁻¹ (C–H of aromatic carbon) and peak 1,033 cm⁻¹ denotes the C–O of ester, ether, and carboxylic acid. On the other hand, BMLB recorded OH stretching vibration at 3,301 cm⁻¹ and C–H of aliphatic alkane at 2,921. The peak at 1,655 cm⁻¹ is ascribed to a C = C double bond. The peak at 2,136 cm⁻¹ (CN or CC triple bond) 1,462 cm⁻¹ (CH of aromatic carbon) and 1,033 cm⁻¹ (C–O of ester).

In Figure 2(c) and 2(d), there were some changes and shifts in the broad bands and the formation of new ones after adsorption. The peak before the process of adsorption at 3,324 and 3,301 cm⁻¹ for AMLB and BMLB shifted to 3,342 and 3,327 cm⁻¹, respectively, after metal ion adsorption, showing the interactions between the OH group and metal ion contaminant. In addition, the band at 2,921 CH₂ for AMLB increased to 2,924 cm⁻¹ while CH₂ decreased from 2,921 to 2,919 cm⁻¹ in BMLB. The FTIR of BMLB presented in Figure 2(d) shows the formation of a new one (C = C at 1,734 cm⁻¹). This shows that the acid and base treatment had a significant effect on the functional groups and the above-mentioned functional groups on the surface of AMLB and BMLB show their likelihood to be promising adsorbents for the removal of Ni²⁺ (Bartczak et al. 2018).

From the analysis, the Langmuir model (Figure 9(a) and 9(b)), due to its high value of correlation coefficients R² (0.937–0.995) for both the AMLB and BMLB, gave a perfect fit in describing the monolayer adsorption of Ni²⁺ onto AMLB and BMLB adsorbents (Wei et al. 2018). The Langmuir model had a perfect fit for Ni²⁺ adsorption onto BMLB among other isotherm models examined. The q_m value was 37.853 mg/g for AMLB and 13.737 for BMLB. The AMLB and BMLB showed low values of K_L (Langmuir constant). The AMLB value for K_L (0.092 L/g) was much greater than the BMLB value (0.0089), showing a higher affinity for Ni²⁺ when compared with BMLB. More so, the values of separation factor (R_L) ranged from 0.00325 to 3.733 for AMLB and BMLB, respectively. This, however, indicates favorable adsorption of the metal ions on chemically treated L. breviflora seed husk.

Table 3 shows the comparison of the kinetic model equations on the adsorption of Ni²⁺ solution onto AMLB and BMLB. From the table, it can be noted from the plot that the non-linearized PFO kinetic model does not correlate with the experimental data of Ni²⁺ adsorption by AMLB and BMLB. This is as a result of the relatively smaller value of the correlation coefficients (0.4336–0.57322) and the rate constant K₁. In addition, the calculated adsorption capacities for the PFO were relatively smaller than the experimental values when compared with the PSO. For the PSO kinetic model, the adsorption data of Ni²⁺ concentration correlates better with the PSO kinetic model. The R² values were relatively higher (0.60362–0.91532) and the estimated adsorption capacities for this kinetic model (q_ecal, mg/g) for both AMLB and BMLB are much closer to the experimental ones (q_eexp, mg/g). This shows that the PSO model best fits into the experimental data obtained from the uptake of Ni²⁺ onto AMLB and BMLB. Therefore, the suitability of the PSO kinetics model confirms the chemisorptions of the Ni²⁺ onto the AMLB and BMLB. A similar result was obtained by Shafiq et al. (2021) and Eze et al. (2021) on the use of modified biosorbents for adsorption.

The intra-particle diffusion model (Figure 16) predicted the rate-limiting step in the adsorption of Ni²⁺ to be characterized by external mass transfer (boundary layer diffusion) and intra-particle diffusion since the plot did not start from the origin even after extra-polation. Though, the interpretation of this kinetic model lacks a theoretical basis (if the line passes through the origin point (0, 0), the adsorption is dominated by the intra-particle diffusion; if not, it is a multiple adsorption process) according to Wang & Guo (2022). It does not account explicitly for the effect of adsorption (except in the limit of very low adsorbate concentration) as noted by Simonin & Boute (2016).

Table 4 illustrates the estimated thermodynamic parameters. The negative sign of ΔG^o for both AMLB and BMLB reveals that the thermodynamic process is spontaneous at all temperatures. It can be observed from the table that as the temperature increases, the ΔG^o decreases. This observation is because, at a very high temperature, additional positions on the surface of chemically pretreated L. breviflora seeds are destroyed. This observation is in line with the reports of other researchers on Ni²⁺ adsorption (Elkhaleefa et al. 2020). More so, the physical nature of the metal uptake by the AMLB and BMLB was suggested based on the fact that the values of the change in Gibbs free energy ΔG^o are within the range of –20 to 0 kJ/mol (Khan et al. 2019). This observation is consistent with the results obtained from Freundlich, D–R, and Flory–Huggins isotherm models. The obtained positive value of entropy ΔS^o for both AMLB and BMLB can be attributed to an increase in random interaction during the sorption process between the pretreated biosorbent and Ni(II) ion (Dehmani et al. 2020). More so, the negative value of ΔH^o obtained for both AMLB and BMLB depicts an exothermic sorption process.

The result of this study was compared with other sorbents previously used for the adsorption of Ni²⁺ under the same experimental conditions such as pH, contact time, and initial ion concentration as presented in Table 5. Chemically modified L. breviflora has shown to be a relatively good adsorbent when compared with other biosorbents that have been previously reported. While Priyantha & Kotabewatta (2019), Pandey et al. (2007), and Malkoc & Nuhoglu (2005) recorded lower adsorption capacities in the use of different biomass as adsorbent, Feng et al. (2011) observed higher adsorption capacity.

In this study, chemically pretreated L. breviflora was employed as a potential adsorbent for the uptake of Ni²⁺ from an aqueous solution. Experimental conditions such as contact time, pH, and initial metal ion concentration indicated that the maximum adsorption was attained at 30 min at pH 6 while the adsorption efficiency increases as the concentration of the AMLB and BMLB increases. The FTIR spectrum shows the functional groups present in the chemically pretreated L. breviflora seed which acted as a binding site for Ni²⁺ adsorption. The shift and formation of new broadband after metal ion adsorption implies that the metal ion was successfully adsorbed by the interaction between the OH groups on the biosorbent surface. Further experiments conducted to examine the surface area of the pretreated samples revealed that both AMLB and BMLB show possible good adsorption capacity by recording a high value of the surface area. SEM analysis under different magnifications shows the external surface of L. breviflora exhibited several cracked surfaces, different pore structures, and cavities. The presence of broadband on the XRD analysis revealed that the biosorbent has an amorphous structure.

The linear plot of kinetic and isotherm models was employed to investigate the sorption mechanism of Ni²⁺. The uptake of Ni²⁺ onto BMLB was well correlated with the Lagmuir isotherm model suggesting monolayer adsorption of the Ni²⁺ while the AMLB composite was adequately explained with the Freundlich model. Pseudo-second-order models best describe the sorption mechanism associated with the removal efficiency of the adsorbent. The sorption thermodynamics shows that the sorption process was exothermic and spontaneous. This result shows that chemically enhanced L. breviflora seeds can be an effective adsorbent for Ni(II) ion.

The authors did not receive any funding.

All authors contributed equally.

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

The authors declare there is no conflict.