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The present study reports the adsorptive potential of an alternative and regenerative adsorbent, lead sulphide (PbS) modified with calix[4]arene towards a methyl orange (MO) dye solution. The chemical and morphological aspects of synthesised PbS nanoparticles and calix[4]arene-modified PbS were analysed by FESEM, EDX and FTIR, which indicated successful immobilisation of calix[4]arene on the PbS surface. The response surface methodology (RSM), based on central composite design (CCD), was conducted to evaluate the significant factors and to optimise the influence of various factors (adsorbent dosage, contact time and pH) on the adsorption of MO. Under optimal conditions (pH of 4, a dosage of 89.70 mg and a contact time of 129.76 min), the maximum adsorption of MO by calix[4]arene-modified PbS was observed. The MO uptake behaviour was well described by the Freundlich model and the pseudo-second-order kinetic model, indicating a multilayer chemical adsorption with a maximum adsorption capacity (qmax) of 3.268 mg/g. The adsorption mechanism of MO dye on the surface of calix[4]arene-modified PbS can be attributed to various interactions such as dipole–dipole hydrogen bonding interactions, Yoshida hydrogen bonding, π–π interactions and electrostatic interaction. The results strongly demonstrated the use of novel adsorbents, calix[4]arene-modified PbS and a reusability strategy as an alternative adsorbent in MO dye removal.

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  • The use of a regenerative adsorbent called calix[4]arene-PbS for the adsorption of methyl orange (MO) dye.

  • The response surface methodology (RSM), based on central composite design (CCD), to determine the significant factors and optimise the adsorption process.

  • The MO uptake behaviour was well described by the Freundlich model and the pseudo-second-order kinetic model.

calixarene, kinetic modelling, methyl orange adsorption, PbS, response surface methodology
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In recent decades, wastewater effluents, containing toxic synthetic dyes, have become a critical issue due to the rapid development of industries and economies worldwide, posing a great threat to human health and safety (Yaseen & Scholz 2019). Among the various dye classifications, azo dyes are a particular concern, due to their high consumption, accounting for up to 70% of the total dyes used in most common industries including the production of paper, textiles and plastics (Jaafarzadeh et al. 2018). Azo dyes are available in different colours and are resistant to oxidising agents, sunlight and heat. Methyl orange (MO) is a type of azo dye that is carcinogenic in nature; it is made up of a complex aromatic molecular structure known as benzidine that is hard to degrade, so it has low biodegradability (Hir et al. 2017; Znad et al. 2018).

Numerous approaches have been employed to determine the best method to remove dyes, including coagulation (Gao et al. 2017; Dotto et al. 2019) and electrocoagulation (Bashir et al. 2019), membrane separation (Chen et al. 2020), chemical and electrochemical oxidation (Baddouh et al. 2018) and adsorption (Shen et al. 2018; Baig et al. 2020; Pelalak et al. 2021). Among these techniques, adsorption is a promising way to remove dyes because of its multiple benefits such as simplicity in design and operation, high removal efficiency and low cost (Bhatti et al. 2017; Lei et al. 2017a, 2017b; Saleh et al. 2017; Zheng et al. 2017). In a recent study, the physical and chemical properties of nanomaterial adsorbents, such as their easy availability and high surface area, have resulted in higher efficiencies and faster adsorption rates in wastewater treatment, making them the best candidates. Some commonly used nanomaterials are activated carbon, iron oxide (Fe3O4), diatomite, manganese oxide (MnO), magnesium oxide (MgO) and zinc oxide (ZnO) (Wang et al. 2015; Lei et al. 2017a, 2017b; Xu et al. 2018; Hassan et al. 2019; Nworie et al. 2022), all of which have specific advantages and limitations. Metal sulphides are nanomaterials known for their interesting electronic properties and several technological applications (Zagorac et al. 2017). The efficiency of metal sulphide, especially when loaded onto carbon-based materials (e.g. activated carbon) for the removal of dyes from wastewater, has been established in past studies (Ghaedi et al. 2013; Roosta et al. 2014). Only a few studies have investigated the modification of metal sulphide alone as an alternative adsorbent for wastewater treatment. The majority of recent findings have focused on metal sulphide for the removal of heavy metal ions. Aside from that, metal sulphides like CdS and ZnS have been identified as excellent photocatalysts for the degradation of dye (Jeong et al. 2007; Sun et al. 2017). However, there has never been any effort towards investigating PbS as an adsorbent for the adsorption of dye.

Calix[n]arenes are multi-functionalisable macromolecules that consist of aromatic subunits connected by methylene linkers. Typically, these macromolecules possess a hydrophobic upper rim and a hydrophilic lower rim surrounding a hollow cavity, with dimensions depending on the specific phenolic units incorporated (Zainal et al. 2019). The availability and readiness of calix[n]arene, to undergo chemical alterations, make it a highly suitable platform for the assembly of various functional groups (Nabeel et al. 2019). The rigid structures of calix[4]arene enable molecular recognition in four different conformations including cone, partial cone, 1,2-alternate and 1,3-alternate. In addition, its spatial molecular structure has also attracted great interest in the supramolecular and water treatment industry (Zhou et al. 2018; Kamboh et al. 2019).

Generally, the stabilisation properties of functionalised nanomaterials indicate the ability of the material to prevent aggregation, thus affecting the efficiencies of the dye removal. One of the well-known optimisation strategies is a statistical method known as the response surface methodology (RSM), which combines factorial experimental design, model development, the main variables and response under an optimised condition. A recent study reported the RSM application in the optimisation of sulphated carboxymethyl cellulose nanofilters for the removal of Cr (VI) ions from tannery wastewater. The maximum removal was achieved using SO3/Pyridine:CMC ratio (1:1), pressure at 3 bar and pH 4 (Gasemloo et al. 2019). Uddin and coworkers employed the RSM technique to assess the influence of various independent factors (such as pH, concentration, time and temperature) for the removal of MO dye by cobalt oxide (Co3O4) nanoparticles. They found the maximum adsorption capacity at optimum conditions was 46.08 mg/g (Uddin & Baig 2019).

We have recently prepared new adsorbents by modifying calix[8]arene and calix[6]arene for the adsorption of dye in wastewater (Rosly et al. 2021, 2022). It can be concluded that the adsorbents can be successfully applied in the dye removal from wastewater. To the best of our knowledge, there are no studied reports on modifying PbS with calix[4]arene for the removal of dye. Therefore, we innovatively proposed to develop an alternative nanoadsorbent, calix[4]arene-modified PbS that could exhibit more unique properties and potential for the removal of the MO dye from an aqueous solution. The most striking advantage of the proposed adsorbent lies in its advanced RSM optimisation method, which was employed to enhance the efficiency of MO removal. Investigated experimental variables were the adsorbent dosage, contact time and pH of the dye solution. The adsorption data were then fitted with isotherm and kinetic models to evaluate the adsorption characteristics and mechanism. This work offers a new guide for developing adsorbents and a promising prospect in wastewater treatment.

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Chemicals and reagents

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Lead (II) acetate (Pb(CH₃COO)₂, purity, 99%), sodium sulphide (Na₂S, purity, 99%), sodium hydroxide (NaOH), potassium hydroxide (KOH), ethanol (C₂H₅OH, purity, 95%), triethylamine N(CH₂CH₃)₃, hydrogen chloride (HCl, purity, 37%), toluene (C₆H₅CH₃), hydrogen nitrate (HNO₃, purity, 69%) and sodium bicarbonate (Na₂CO₃) were procured from R & M Chemicals (Semenyih, Malaysia). MO (C14H14N3NaO3S, purity, 99%), potassium nitrate (KNO₃) and p-tert-butylcalix[4]arene (C44H56O4, purity, 99%) were purchased from Merck (Bandar Sunway, Selangor), HmbG Chemicals (Hamburg, Germany) and Alfa Aesar (Tewksbury, MA, USA). Dithioglycerol (DTG, purity, 97%), thioglycerol (TGL, purity, 98%) and 3-glyxidoxypropyltrimethoxy silane (C9H20O5Si, purity, 98%) were obtained from Sigma-Aldrich (Gillingham, UK).

Preparation of the dye solution

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In this experiment, MO without any purification was used. The MO solutions (1,000 mg/L) were prepared, and the pH solutions were adjusted with a small amount of 0.1 mol/L NaOH or 0.1 mol/L HCl. The required concentrations of dye solutions were prepared by diluting the stock solution appropriately with dH₂O.

Synthesis of lead sulphide (PbS) nanoparticles

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PbS nanoparticles were synthesised based on a previous method with slight modifications (Zaini et al. 2020). The PbS nanoparticles were prepared by mixing Pb(CH₃COO)₂, dH₂O, TGL and DTG. The pH of the solution was adjusted using sodium hydroxide until it reached a pH of 10. The sulphide precursor was prepared by mixing Na₂S with dH₂O and poured into the solution until it turned black. The solution was stirred and degassed with nitrogen (N₂) gas. The precipitant of PbS was collected by mixing the solution in ethanol at a 3:1 ratio and then dried in an oven for 2 h at 80 °C.

Modification of lead sulphide (PbS) with p-tert-butylcalix[4]arene

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The calix[4]arene-modified PbS was synthesised as follows: 3-glycidoxypropyltrimethoxy silane (4.18 mmol), p-tert-butylcalix[4]arene (3.03 mmol) and 0.2 g PbS were mixed and stirred with three drops of triethylamine as the catalyst in 10 mL dry toluene (dried before use using molecular sieves) at room temperature for 6 h in an inert atmosphere of N₂ gas. The calix[4]arene-modified PbS was centrifuged and washed with toluene, acetone and dH₂O, respectively. The calix[4]arene-modified PbS was dried at 80 °C for 3 h.

Optimisation of the adsorption of the MO dye solution by calix[4]arene-modified PbS using RSM

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The adsorption of MO dye by calix[4]arene-modified PbS was evaluated under various conditions, such as adsorbent dosage, contact time and pH. A central composite design (CCD), RSM (Design-Expert Software version 6.0.6), was applied to determine optimum conditions for calix[4]arene-modified PbS to adsorb the MO dye. The design evaluates the individual factors and their corresponding interactions and identifies the optimum response in a minimum number of runs. The adsorbent dosage (A), contact time (B) and pH (C) were set as the independent factors in this study. A 2³ factorial design, i.e. three factors for two levels with a total of 20 experiments (8 at the factorial point, 6 at the axial point and 6 at the central point) was suggested by the RSM/CCD, as displayed in Table 1.

Table 1

The three factors’ central composite design (CCD) matrix and the value of the response function MO (%)

RunFactors
Response adsorption (%)
A: Adsorbent dosage (mg)B: Contact time (min)C: pHExperimentalPredicted
1 50 80 4 50.30 50.44 
2 50 130 4 53.42 53.78 
3 70 105 4 58.55 57.66 
4 50 130 8 61.22 61.04 
5 70 105 6 53.76 53.34 
6 70 105 6 52.43 53.34 
7 50 80 8 61.96 61.49 
8 90 130 4 70.22 70.56 
9 70 105 6 53.78 53.34 
10 90 80 8 61.03 60.54 
11 70 105 6 53.56 53.34 
12 90 80 4 63.98 64.03 
13 70 105 8 58.15 59.55 
14 70 105 6 53.54 53.34 
15 70 80 6 51.63 52.39 
16 90 105 6 58.40 58.76 
17 70 105 6 53.98 53.34 
18 70 130 6 55.68 55.43 
19 50 105 6 50.68 50.84 
20 90 130 8 63.54 63.28 
RunFactors
Response adsorption (%)
A: Adsorbent dosage (mg)B: Contact time (min)C: pHExperimentalPredicted
1 50 80 4 50.30 50.44 
2 50 130 4 53.42 53.78 
3 70 105 4 58.55 57.66 
4 50 130 8 61.22 61.04 
5 70 105 6 53.76 53.34 
6 70 105 6 52.43 53.34 
7 50 80 8 61.96 61.49 
8 90 130 4 70.22 70.56 
9 70 105 6 53.78 53.34 
10 90 80 8 61.03 60.54 
11 70 105 6 53.56 53.34 
12 90 80 4 63.98 64.03 
13 70 105 8 58.15 59.55 
14 70 105 6 53.54 53.34 
15 70 80 6 51.63 52.39 
16 90 105 6 58.40 58.76 
17 70 105 6 53.98 53.34 
18 70 130 6 55.68 55.43 
19 50 105 6 50.68 50.84 
20 90 130 8 63.54 63.28 
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The reproducibility and experimental error of the data were assessed from the centre points. All runs were demonstrated at a fixed MO concentration (25 mg/L) and constant temperature (27 ± 1 °C). The adsorption of the MO response data from the suggested experiments was generated using the second-order polynomial of Equation (1):
(1)

The significance of the model and the prediction of the individual and interaction of process variables were evaluated by using analysis of variance (ANOVA), which is equivalent to the 95% confidence level. The goodness-of-fit model was evaluated by the values of the coefficient of determination (R²), adjusted coefficient (adjusted R²) and a lack-of-fit test. The optimum conditions were predicted and three-dimensional response surface graphs were plotted based on the obtained p- and F-values.

Characterisations of calix[4]arene-modified PbS

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Morphological analysis was evaluated by field emission scanning electron microscopy (FESEM; Quanta 400F) operated in a low vacuum mode. Energy-dispersive X-ray (EDX) spectroscopy coupled with FESEM was used to analyse the elements present in calix[4]arene-modified PbS. The chemical functional groups of calix[4]arene-modified PbS were analysed using Fourier-transform infrared spectroscopy (FTIR; PerkinElmer). The samples were scanned within a wave number region of 4,000–400 ⁠.

Determination of the point of zero charge (PZC) and surface chemistry

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The point of zero charge (PZC) is the point where the density of electrical charge is zero. In this study, the solid addition method was used to evaluate the PZC of calix[4]arene-modified PbS. Briefly, 0.10 g of calix[4]arene-modified PbS was added to a conical flask containing 50.0 mL of the 0.01 mol/L KNO₃ solution. The initial pH values were adjusted between pH 3 and pH 11. The pH adjustment was done using 0.10 mol/L KOH and 0.10 mol/L HNO₃ solutions. All conical flasks were shaken for 48 h under atmospheric conditions. After centrifugation of the samples, the final pH was recorded using a pH meter (Sedolis Sartorius, Germany). The graph of ΔpH versus initial pH was plotted and the initial pH value at which ΔpH became zero was considered as the PZC for calix[4]arene-modified PbS.

Adsorption experiments

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Adsorption experiments were done with 10.0 mL MO dye solution in a conical flask at a specific concentration. The experiments were performed in a laboratory shaker at 150 rpm and room temperature (27 ± 1 °C). The operational conditions of contact time (80–130 min), initial pH (pH 4–8) and adsorbent dosage (50–90 mg) on dye removal were examined as shown in Table 1. The solution was then separated from the adsorbates after the sample was centrifuged for 20 min at 2,500 rpm. The final concentration of residual dye in the sample was measured by a UV–Vis spectrophotometer (PerkinElmer Lambda 35, USA) at 464 nm (the λmax value for MO). The percentage removal of MO was calculated using Equation (2):
(2)
where (mg/L) is the initial concentration of MO solution before adsorption and (mg/L) is the final concentration after the adsorption of MO dye.

Desorption study

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To investigate the efficiency of the adsorbent, a desorption study was performed. Different solutions [NaOH (0.1 M), Na₂CO₃ (0.1 M) and distilled water] were used to select the best eluent to elute the MO solutions. The adsorbent was first equilibrated with 0.025 g/L MO solution in a shaker under ambient temperature conditions. At equilibrium, the adsorbent was washed with distilled water several times to remove unadsorbed traces of MO. The adsorbent was then oven-dried at 80 °C for 2 h. Various concentrations of selected eluent were studied and further tested with MO. All residual concentrations of MO were measured via UV absorbance.

Leaching test

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The MO solution of each cycle was collected and analysed to examine the amount of lead (Pb) and total suspended solids (TSS) of calix[4]arene-modified PbS. Pb and TSS were determined based on the Standard Methods (2017), 2540D. The volume of the adsorbed MO solution was measured and then the solution was filtered through a pre-weighed filter. The MO supernatant was analysed for Pb element by inductively coupled plasma mass spectrometry (ICP-MS, PerkinElmer, DRC-e). Meanwhile, the residue on the filter was heated at 104 °C until the weight obtained remained constant. The concentration of suspended calix[4]arene-modified PbS was calculated based on Equation (3):
(3)
The overall procedure for this study is shown in Figure 1.
Figure 1
Schematic illustration of calix[4]arene-modified lead sulphide (PbS) in the dye solution.
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Schematic illustration of calix[4]arene-modified lead sulphide (PbS) in the dye solution.

Figure 1
Schematic illustration of calix[4]arene-modified lead sulphide (PbS) in the dye solution.
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Schematic illustration of calix[4]arene-modified lead sulphide (PbS) in the dye solution.

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Characterisation

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Field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray (EDX)

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The surface morphologies and chemical compositions of the surface of unmodified PbS and calix[4]arene-modified PbS were examined by performing FESEM and EDX, respectively. The results of the FESEM and EDX analysis for unmodified PbS and calix[4]arene-modified PbS are depicted in Figure 2. There are obvious morphological differences in the FESEM micrographs of unmodified PbS and the calix[4]arene-modified PbS. At a magnification of 100,000 × , the unmodified PbS (Figure 2(a)) is composed of compact and agglomerated nanoparticles with particle size of below 30 nm. The agglomeration of nanoparticles was governed by attractive van der Waals forces between the particles (Son et al. 2015). For calix[4]arene-modified PbS (Figure 2(b)), the micrograph shows nanoparticles covered by flaky material, presumably p-tert-butylcalix[4]arene. Based on the EDX results, the elements in the unmodified PbS (Figure 2(c)) mainly contain oxygen (O), sulphur (S) and lead (Pb). Meanwhile, the presence of carbon (C) elements (Figure 2(d)) was discovered, attributed to the successful attachment of p-tert-butylcalix[4]arene on the surface of PbS.
Figure 2
FESEM-EDX analysis of (a,c) unmodified PbS and (b,d) calix[4]arene-modified PbS.
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FESEM-EDX analysis of (a,c) unmodified PbS and (b,d) calix[4]arene-modified PbS.

Figure 2
FESEM-EDX analysis of (a,c) unmodified PbS and (b,d) calix[4]arene-modified PbS.
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FESEM-EDX analysis of (a,c) unmodified PbS and (b,d) calix[4]arene-modified PbS.

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FTIR analysis

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The modification of calix[4]arene onto lead sulphide (PbS) nanoparticles was confirmed from the FTIR spectroscopy. As shown in the FTIR spectrum in Figure 3(a) the unmodified lead sulphide (PbS) showed a broad peak at 3,244 cm−1, assigned to the OH stretching vibration. In Figure 3(b), the characteristic vibration of p-tert-butylcalix[4]arene peaks was clearly observed at 3,143 cm−1 for OH, the C–CH₃ stretching vibration was represented by the peak at 2,956 cm−1 and the C = C aromatic vibrations correlated to the peaks at 1,603 and 1,477 cm−1. In Figure 3(c), the bands observed at 1,608 and 2,952 cm−1 corresponded to the C = C aromatic and C–CH₃ stretching vibration, respectively, indicating that p-tert-butylcalix[4]arene was immobilised on the surface of PbS. However, the band around 1,477 cm−1 that was attributed to C = C aromatic vibration does not appear in Figure 3(c) might be due to the overlapping band with the PbS nanoparticles spectrum. Similar findings were reported by some studies on the immobilisation of calixarene derivatives (Temel et al. 2020; Kutluay & Temel 2021).
Figure 3
FTIR spectra of (a) unmodified lead sulphide (PbS), (b) p-tert-butylcalix[4]arene and (c) calix[4]arene-modified PbS.
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FTIR spectra of (a) unmodified lead sulphide (PbS), (b) p-tert-butylcalix[4]arene and (c) calix[4]arene-modified PbS.

Figure 3
FTIR spectra of (a) unmodified lead sulphide (PbS), (b) p-tert-butylcalix[4]arene and (c) calix[4]arene-modified PbS.
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FTIR spectra of (a) unmodified lead sulphide (PbS), (b) p-tert-butylcalix[4]arene and (c) calix[4]arene-modified PbS.

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PZC of calix[4]arene-modified PbS

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The pH of the point of zero charge (⁠⁠) of the calix[4]arene-modified PbS is the pH value where the net charge density on the surface is zero. Thus, the pHPZC of calix[4]arene-modified PbS was investigated to provide information about the ionisation of the surface functional groups and their interaction with the species in the solution. These values are significant for examining the effect of the solution pH on the adsorption process of the adsorbent material. The intersection of the plot of ΔpH against initial pH with the x-axis corresponds to the value of the adsorbent measured at pH 6.67, as shown in Figure 4. This finding indicates that at pH less than pH 6.67, the surface of the adsorbent was positively charged while at pH above pH 6.67, the surface of the adsorbent was negatively charged. The suspension of adsorbent in the KNO₃ solution developed an electrical charge due to the dissociation of surface hydroxyl groups and the complexation of the background electrolyte ions. At pH less than ⁠, the adsorption of protons increased and the adsorbent behaved as an anion exchanger bearing positive charges. Meanwhile, at pH higher than that of ⁠, the desorption of protons occurred and the surface behaved as a cation excharger with a net negative charge (Zbair et al. 2018).
Figure 4
Point of zero charge () of calix[4]arene-modified PbS using KNO3 (0.01 M).
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Point of zero charge (⁠⁠) of calix[4]arene-modified PbS using KNO3 (0.01 M).

Figure 4
Point of zero charge () of calix[4]arene-modified PbS using KNO3 (0.01 M).
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Point of zero charge (⁠⁠) of calix[4]arene-modified PbS using KNO3 (0.01 M).

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Optimisation of the adsorption of the MO dye solution by calix[4]arene-modified PbS using RSM

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The modelling and optimisation of adsorbent dosage, contact time and pH were performed using RSM/CCD to maximise the percentage adsorption of MO. The quadratic regression equation was created by the CCD design based on the experimental data, as shown in Equation (4):
(4)
where A is the adsorbent dosage, B is the contact time and C is the pH of MO solution.

To assess the compatibility and suitability of the quadratic model, indicators, such as coefficient of determination (R²), p-value and F-value, were analysed. As shown in Table 2, the coefficient of determination was R² = 0.9931, adjusted R² was adj. R² = 0.9869 and predicted R² (0.9663) was close to 1.0. This finding indicates that the suggested model was statistically significant with a confidence level of more than 95%. The statistical significance of the model for the response was determined using the F-value and p-value Prob >F. The F-value quantifies the variance of the mean data and is calculated using the error of the ratio of the mean square of group variance. A higher F-value indicates that the model is highly significant. For the p-value Prob >F, a value less than 0.1000 indicates the model term is significant, whereas a value greater than 0.1000 indicates that the model term is insignificant (Kakoi et al. 2017; Gasemloo et al. 2019). According to the results, the p-value Prob >F of the presented model is less than 0.0001, implying that the model can be used to predict the experimental values. Meanwhile, the F-value of the presented model was 160.60, which shows that the model was suitable and efficient for data analysis. Other than that, the F-value and p-value Prob >F of the lack-of-fit tests were 1.42 and 0.3555, respectively, revealing that the null hypotheses could be rejected because the values were greater than 0.1000.

Table 2

ANOVA of the adsorption of the response surface quadratic model

SourceCoefficientsSum of squaresDegree of freedomMean squareF-valuep-value Prob > F
Intercept 53.410      
Linear       
A 3.959 156.700 1 156.700 424.800 0.0001 
B 1.518 23.040 1 23.040 62.450 0.0001 
C 1.048 10.980 1 10.980 29.770 0.00028 
Quadratic       
A2 1.270 4.435 1 4.435 12.020 0.0001 
B2 0.385 0.408 1 0.408 1.105 0.0001 
C2 5.605 86.390 1 86.390 234.200 0.0001 
Interaction       
AB 0.794 5.072 1 5.072 13.750 0.0001 
AC −3.636 105.800 1 105.800 286.700 0.0001 
BC −0.949 7.201 1 7.201 19.520 0.0001 
Model  533.300 9 59.250 160.600 0.0001 
Residual  3.690 10 0.326   
Lack-of-fit  2.164 5 0.433 1.418 0.3555 
R² = 0.9931       
Adjusted R² = 0.9869       
Predicted R² = 0.9663       
SourceCoefficientsSum of squaresDegree of freedomMean squareF-valuep-value Prob > F
Intercept 53.410      
Linear       
A 3.959 156.700 1 156.700 424.800 0.0001 
B 1.518 23.040 1 23.040 62.450 0.0001 
C 1.048 10.980 1 10.980 29.770 0.00028 
Quadratic       
A2 1.270 4.435 1 4.435 12.020 0.0001 
B2 0.385 0.408 1 0.408 1.105 0.0001 
C2 5.605 86.390 1 86.390 234.200 0.0001 
Interaction       
AB 0.794 5.072 1 5.072 13.750 0.0001 
AC −3.636 105.800 1 105.800 286.700 0.0001 
BC −0.949 7.201 1 7.201 19.520 0.0001 
Model  533.300 9 59.250 160.600 0.0001 
Residual  3.690 10 0.326   
Lack-of-fit  2.164 5 0.433 1.418 0.3555 
R² = 0.9931       
Adjusted R² = 0.9869       
Predicted R² = 0.9663       
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The ANOVA of the quadratic model regression results for the adsorption of MO is given in Table 2. As can be seen, p-values for all linear terms (A, B, C) were highly significant (p-value less than 0.05). This indicates that all factors (A, B and C) contribute significantly towards the adsorption of MO by calix[4]arene-modified PbS. Based on the result, it was found that the order of influencing factors for adsorption of MO was A B C. The per cent contribution of individual factors depicted in Figure 5(a) shows that factor A influenced the adsorption of MO primarily. Although factor C affected the adsorption process, its influence is much lower, as proven by the p-value (0.00028). On the other hand, Figure 5(b) shows the per cent contributions of the interacting factors for the optimum adsorption of MO. Among the interacting factors, the interaction between factor A and factor C was the most significant with an 89.60% contribution.
Figure 5
The percentage contributions of (a) individual factors and (b) interacting factors on the adsorption of MO.
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The percentage contributions of (a) individual factors and (b) interacting factors on the adsorption of MO.

Figure 5
The percentage contributions of (a) individual factors and (b) interacting factors on the adsorption of MO.
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The percentage contributions of (a) individual factors and (b) interacting factors on the adsorption of MO.

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A plot of studentised residual against the predicted response was generated to control and evaluate the adequacy of the developed model. In a plot of residuals against predicted, the graph (Figure 6(a)) displays the points distributed randomly around a straight line at zero, which suggests that the quadratic model successfully established the adsorption of MO. At the same time, the proposed quadratic model for MO removal seems adequate as all the points lie in between −3.0 and +3.0 of the red lines and no unusual pattern is detected. The plot in Figure 6(b) displays all run number data was distributed within the limits, indicating that all the data in this study are acceptable. A plot of predicted against actual (Figure 6(c)) shows that the data points lying around the diagonal line pass the origin with a slope of 1. This behaviour of the graph indicates that the quadratic model could predict response appropriately. Therefore, based on the acceptable adequate correlation model, it is understandable that the developed model can be used to represent the experimental data.
Figure 6
Plot of (a) residual versus predicted, (b) outlier versus run number and (c) predicted versus actual.
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Plot of (a) residual versus predicted, (b) outlier versus run number and (c) predicted versus actual.

Figure 6
Plot of (a) residual versus predicted, (b) outlier versus run number and (c) predicted versus actual.
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Plot of (a) residual versus predicted, (b) outlier versus run number and (c) predicted versus actual.

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The graphical representations of the response surfaces are illustrated based on the quadratic regression in Equation (3). The 3D response surface plots are illustrated in Figure 7(a)–7(c) based on the interaction of adsorbent dosage (A)–contact time (B), adsorbent dosage (A) –pH (C) and contact time (B) –pH (C). These plots imply that the percentage adsorption increased with increasing adsorbent dosage (A) and contact time (B). This increase corresponds to the more active binding sites available on the surface of the adsorbent for interaction with the MO solution. The percentage adsorption decreased with increasing pH (C) from pH 4 to pH 6. At lower pH, surface association occurred between the acidic phenolic hydroxyl (⁠⁠) groups of calix[4]arene moiety in calix[4]arene-modified PbS and the MO dye (anionic dye).
where A denotes the surface of calix[4]arene-modified PbS.
Figure 7
3D surface plots of process variables for the adsorption of MO dye (a) adsorbent dosage (A) and contact time (B), (b) adsorbent dosage (A) and pH (C) and (c) contact time (B) and pH (C).
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3D surface plots of process variables for the adsorption of MO dye (a) adsorbent dosage (A) and contact time (B), (b) adsorbent dosage (A) and pH (C) and (c) contact time (B) and pH (C).

Figure 7
3D surface plots of process variables for the adsorption of MO dye (a) adsorbent dosage (A) and contact time (B), (b) adsorbent dosage (A) and pH (C) and (c) contact time (B) and pH (C).
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3D surface plots of process variables for the adsorption of MO dye (a) adsorbent dosage (A) and contact time (B), (b) adsorbent dosage (A) and pH (C) and (c) contact time (B) and pH (C).

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The attraction of the positively charged surface of calix[4]arene-modified PbS and MO dye may be expressed as

where MO− denotes the anionic MO dye.

The adsorption was lower at high pH due to the repulsion between the negatively charged surface of calix[4]arene-modified PbS and the anionic dye (Zbair et al. 2018).
The repulsion of the negatively charged surface of calix[4]arene-modified PbS and MO dye may be expressed as

The model was validated by performing three sets of experiments generated from CCD-RSM and then the predicted and experimental values were compared, as shown in Table 3. The predicted and the experimental values of percentage adsorption of MO indicate that all the adsorption experiments outputted a response of less than 2% residual standard error (RSE), thus validating the model. Based on the experiments performed, the prediction of this model was 98% accurate. Meanwhile, for the optimisation step, the adsorption response was set as the main goal, while the other factors were maintained in the studied range. The optimised values for adsorbent dosage, contact time and pH were 89.70 mg of calix[4]arene-modified PbS, a contact time of 129.76 min and a pH of 4.

Table 3

The predicted and experimental value of validation sets

Run numberAdsorbent dosage (mg)Contact time (min)pHAdsorption (%)
PredictedExperimentRSE (%)
1 80 100 4 61.53 60.47 1.72 
2 60 105 4 54.49 53.60 1.63 
3 55 120 4 53.38 52.41 1.82 
Run numberAdsorbent dosage (mg)Contact time (min)pHAdsorption (%)
PredictedExperimentRSE (%)
1 80 100 4 61.53 60.47 1.72 
2 60 105 4 54.49 53.60 1.63 
3 55 120 4 53.38 52.41 1.82 
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Adsorption isotherms

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Adsorption isotherms were generated to study the relationship between the adsorbed dye and the adsorbent at the equilibrium concentration of the dye solution. Optimised data of the adsorption process design can be described by isotherm studies with the most common isotherms being Freundlich and Langmuir (LeVan & Vermeulen 1981). These isotherm models were used to evaluate the adsorption capacities of calix[4]arene-modified PbS in MO dye. The Langmuir and Freundlich isotherm models were applied using Equations (5) and (6), respectively.

The Langmuir model describes that the adsorption of molecules onto the adsorbent surface has an equal activation energy with no transmigration of the adsorbate in the plane of the surfaces. The Freundlich model assumes heterogeneous adsorption occurring on the surface with multilayer adsorption.

Langmuir equation:
(5)
Freundlich equation:
(6)
where (mg/L) and (mg/g) are the concentration and the adsorption capacity at equilibrium conditions, respectively. The constants (L/mg) and (mg/g) are derived from the intercept and slope of the linear plot of Equation (5), respectively. Meanwhile, (L/mg) and 1/n which are the Freundlich constant and the heterogeneity factor, respectively, are calculated by plotting log against log from Equation (6).
Figure 8(a) and 8(b) illustrates the Langmuir and Freundlich adsorption isotherms, respectively. In this study, the Langmuir model did not fit (R² = 0.553) the experimental data. Although the Langmuir isotherm was ruled out for MO adsorption, the parameter values are significant for explaining the adsorption mechanism. All parameter values calculated for the Langmuir isotherm are shown in Table 4. The equilibrium dimensionless parameter (RL) value represents the ratio of the remaining adsorption capacity over the total adsorption capacity. Based on RL values, it showed that as the concentration increases, the RL value decreases. The decrease in RL, as the MO initial concentration was increased, indicated that adsorption is more favourable at higher concentrations. The value of ⁠, for all MO concentrations, fell between 0 and 1, suggesting that the MO adsorption onto calix[4]arene-modified PbS was favourable in this study. The was determined using the formula, ⁠.
Table 4

Isotherm constant and regression data for the adsorption of MO on calix[4]arene-modified PbS at an adsorbent dosage of 89.70 mg, pH = 4, and a contact time of 129.76 min at room temperature

IsothermParameterValue
Langmuir qmax 3.268 mg/g 
 0.2383 L/mg 
 0.553 
Freundlich  1.000 L/mg 
n 1.678 
 0.951 
IsothermParameterValue
Langmuir qmax 3.268 mg/g 
 0.2383 L/mg 
 0.553 
Freundlich  1.000 L/mg 
n 1.678 
 0.951 
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Figure 8
Adsorption isotherms for MO by calix[4]arene-modified PbS at room temperature (a) Langmuir isotherm and (b) Freundlich isotherm at an adsorbent dosage of 89.70 mg, pH = 4 and a contact time of 129.76 min.
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Adsorption isotherms for MO by calix[4]arene-modified PbS at room temperature (a) Langmuir isotherm and (b) Freundlich isotherm at an adsorbent dosage of 89.70 mg, pH = 4 and a contact time of 129.76 min.

Figure 8
Adsorption isotherms for MO by calix[4]arene-modified PbS at room temperature (a) Langmuir isotherm and (b) Freundlich isotherm at an adsorbent dosage of 89.70 mg, pH = 4 and a contact time of 129.76 min.
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Adsorption isotherms for MO by calix[4]arene-modified PbS at room temperature (a) Langmuir isotherm and (b) Freundlich isotherm at an adsorbent dosage of 89.70 mg, pH = 4 and a contact time of 129.76 min.

Close modal

As shown in Table 4, the good fit of the Freundlich model represents an R² of 0.951, indicating that the adsorption of MO was multilayer adsorption occurring on a heterogeneous surface. The KF and n values of Freundlich isotherm are 1.000 L/mg and 1.678, respectively, as shown in Table 4. Accordingly, the higher KF value suggests a higher affinity, whereas n values revealed the favourability of MO adsorption onto calix[4]arene-modified surfaces (Ahmed et al. 2022). The calculated Freundlich constant ‘n’ was greater than 1, indicating that the multilayer adsorption of MO was favourable and occurred on the heterogeneous surface of the adsorbent. However, most studies on MO adsorption onto adsorbents (e.g. mesoporous carbon, chitosan) were best fit to the Langmuir isotherm (Singh et al. 2003; Ayar et al. 2007; Mittal et al. 2007).

Table 5 compares the maximum adsorption capacities of different types of adsorbents previously reported for MO removal with calix[4]arene-modified PbS. As can be seen, the maximum adsorption capacity of calix[4]arene-modified PbS (3.268 mg/g) exhibits a better choice for MO dye removal than other adsorbents. Although the value is lower than some of the adsorbents this study used the RSM technique which evaluates the interactive effects among the variables studied and thus gives the complete effects between variables in the process.

Table 5

Comparison of the maximum adsorption capacity of MO with various adsorbents

Adsorbent for MOMaximum adsorption capacity (mg/g)Type of optimisation methodReference
Bottom ash 3.618 One-factor-at-time Mittal et al. (2007)  
Modified sporopollenin 5.200 One-factor-at-time Ayar et al. (2007)  
Activated carbon 9.500 One-factor-at-time Singh et al. (2003)  
Kaolinite 1.247 One-factor-at-time Fumba et al. (2014)  
Metakaolinite 3.076 One-factor-at-time Fumba et al. (2014)  
Magnetic cellulose beads 1.470 One-factor-at-time Luo & Zhang (2009)  
Chitosan/organic rectorite composite 5.560 One-factor-at-time Zeng et al. (2015)  
Magnetic halloysite nanotubes/iron oxide composites 0.650 One-factor-at-time Xie et al. (2011)  
Rectorite/iron oxide nanocomposites 0.360 One-factor-at-time Wu et al. (2011)  
Calix[4]arene-modified PbS 3.268 RSM This work 
Adsorbent for MOMaximum adsorption capacity (mg/g)Type of optimisation methodReference
Bottom ash 3.618 One-factor-at-time Mittal et al. (2007)  
Modified sporopollenin 5.200 One-factor-at-time Ayar et al. (2007)  
Activated carbon 9.500 One-factor-at-time Singh et al. (2003)  
Kaolinite 1.247 One-factor-at-time Fumba et al. (2014)  
Metakaolinite 3.076 One-factor-at-time Fumba et al. (2014)  
Magnetic cellulose beads 1.470 One-factor-at-time Luo & Zhang (2009)  
Chitosan/organic rectorite composite 5.560 One-factor-at-time Zeng et al. (2015)  
Magnetic halloysite nanotubes/iron oxide composites 0.650 One-factor-at-time Xie et al. (2011)  
Rectorite/iron oxide nanocomposites 0.360 One-factor-at-time Wu et al. (2011)  
Calix[4]arene-modified PbS 3.268 RSM This work 
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Kinetic studies

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The adsorption behaviour, such as the adsorption type and the rate of the solute uptake, is an important criterion to express the adsorption efficiency. The pseudo-first-order and pseudo-second-order kinetic models were employed in this work. The best kinetic model was selected based on the determination of and the adsorption capacity values and ⁠.

The pseudo-first-order model is given by Equation (7):
(7)

The values of and were determined by calculating the slope and the intercept of the linear graph of ln (⁠⁠) versus t, respectively.

The pseudo-second-order equation is expressed as Equation (8):
(8)

To obtain the values of and ⁠, a linear graph of versus t was plotted and its slope and intercept were calculated.

A comparison of the values of the two models is presented in Table 6. The table also summarises of the two kinetic models with MO concentrations ranging from 5 to 40 ppm. value of the pseudo-second-order kinetic model was higher than 0.990 at different initial concentrations, compared to the of the pseudo-first-order kinetic model (0.864). The values also showed a good agreement with the experimental values. Therefore, the calculated data suggest that the MO adsorption process onto calix[4]arene-modified PbS follows a second-order kinetic model and does not obey the first-order kinetic model.

Table 6

Kinetic study of methyl orange (MO) adsorption onto calix[4]arene-modified PbS at an adsorbent dosage of 89.70 mg, and pH = 4 at room temperature

(mg/L) (mg/g)Pseudo-first-order
Pseudo-second-order
(mg/g) (min−¹) (mg/g) (min−¹)
5 0.470 0.3289 0.031 0.864 0.4355 0.2967 0.990 
10 0.900 0.5278 0.027 0.983 0.8606 0.1676 0.992 
15 0.940 1.014 0.014 0.919 0.9320 0.3654 0.997 
25 1.810 0.7269 0.025 0.922 1.741 0.1278 0.999 
40 3.710 0.8685 0.022 0.894 3.759 0.0600 0.999 
(mg/L) (mg/g)Pseudo-first-order
Pseudo-second-order
(mg/g) (min−¹) (mg/g) (min−¹)
5 0.470 0.3289 0.031 0.864 0.4355 0.2967 0.990 
10 0.900 0.5278 0.027 0.983 0.8606 0.1676 0.992 
15 0.940 1.014 0.014 0.919 0.9320 0.3654 0.997 
25 1.810 0.7269 0.025 0.922 1.741 0.1278 0.999 
40 3.710 0.8685 0.022 0.894 3.759 0.0600 0.999 
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The curve-fitting plots of MO adsorption by calix[4]arene-modified PbS for both kinetic models are presented in Figure 9(a) and 9(b). The result shows that the curve-fitted plots of versus t of the second-order kinetic model display acceptable fitting while the first-order kinetic model plots of ln (⁠⁠) against t did not have a good fit with the experimental value. These observations suggest that the MO adsorption by calix[4]arene-modified PbS followed a chemical sorption process that controlled the rate of MO adsorption.
Figure 9
(a) Pseudo-first-order kinetic model and (b) pseudo-second-order kinetic model methyl orange adsorption onto calix[4]arene-modified PbS at an adsorbent dosage of 89.70 mg and pH = 4 at room temperature.
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(a) Pseudo-first-order kinetic model and (b) pseudo-second-order kinetic model methyl orange adsorption onto calix[4]arene-modified PbS at an adsorbent dosage of 89.70 mg and pH = 4 at room temperature.

Figure 9
(a) Pseudo-first-order kinetic model and (b) pseudo-second-order kinetic model methyl orange adsorption onto calix[4]arene-modified PbS at an adsorbent dosage of 89.70 mg and pH = 4 at room temperature.
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(a) Pseudo-first-order kinetic model and (b) pseudo-second-order kinetic model methyl orange adsorption onto calix[4]arene-modified PbS at an adsorbent dosage of 89.70 mg and pH = 4 at room temperature.

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Adsorption mechanism

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The proposed adsorption mechanism of MO dye by calix[4]arene-modified PbS is displayed in Figure 10. The adsorption mechanism of MO dye can be explained by various interactions including hydrogen bondings, π–π interactions and electrostatic interactions. For hydrogen bonding, two types of interactions can take place between calix[4]arene-modified PbS and the molecular structure of the MO dye. The first type is dipole–dipole hydrogen bonding interaction that occurs between the hydrogen of the hydroxyl groups on the surface of the adsorbent with oxygen and nitrogen atoms of the MO dye. The second type is a Yoshida hydrogen bonding (Abdulhameed et al. 2019) that occurs between hydroxyl groups on the adsorbent with aromatic rings in MO molecules. Meanwhile, the π–π mechanism occurs due to the π–π interaction between the delocalised π-electrons on the surface of the adsorbent and the π-electrons of the MO dye molecule present in the aromatic rings. According to the previously discussed results, the positively charged functional groups available on the surface of calix[4]arene-modified PbS at a pH environment below pHPZC (6.67). At a pH lower than that of the pHPZC value, the electrostatic interaction occurred between the positively charged adsorbent and negatively charged sulphonate (⁠⁠).
Figure 10
Illustration of the possible interaction between the calix[4]arene-modified PbS and MO dye solution.
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Illustration of the possible interaction between the calix[4]arene-modified PbS and MO dye solution.

Figure 10
Illustration of the possible interaction between the calix[4]arene-modified PbS and MO dye solution.
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Illustration of the possible interaction between the calix[4]arene-modified PbS and MO dye solution.

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Desorption study

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Desorption study is an important criterion for obtaining an ideal adsorbent. It also helps increase the efficiency of the adsorbent and evaluate its reliability for long-term use. The adsorption of MO on calix[4]arene-modified PbS was very low at higher pH values, indicating that an alkaline condition should be used for the desorption process. Thus, a desorption study was performed to select the best eluent (NaOH, Na₂CO₃ or dH₂O). As shown in Figure 11(a), 0.10 M NaOH was the optimum desorption agent for the desorption of MO. The maximum percentage of adsorption of MO obtained before the desorption process was 70.26% (control). It was observed that the NaOH solution was the best eluent for the desorption of MO. It is, therefore, suggested that under alkaline conditions, the calix[4]arene-modified PbS presents a negatively charged adsorbent that eventually desorbs the MO (anionic dye) and increases desorption efficiency.
Figure 11
Adsorption of MO by calix[4]arene-modified PbS (a) after treatment with different types of desorption eluents, (b) under various NaOH concentrations and (c) reusability tests.
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Adsorption of MO by calix[4]arene-modified PbS (a) after treatment with different types of desorption eluents, (b) under various NaOH concentrations and (c) reusability tests.

Figure 11
Adsorption of MO by calix[4]arene-modified PbS (a) after treatment with different types of desorption eluents, (b) under various NaOH concentrations and (c) reusability tests.
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Adsorption of MO by calix[4]arene-modified PbS (a) after treatment with different types of desorption eluents, (b) under various NaOH concentrations and (c) reusability tests.

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Next, the effect of the desorption process under various concentrations of NaOH was further investigated. Figure 11(b) illustrates the effects of NaOH concentration on MO desorption. It can be observed that the MO desorption decreased with an increased NaOH concentration. The highest amount of desorption (64%) was observed when 0.10 M NaOH was used as an eluent. Meanwhile, Figure 11(c) demonstrates the cyclic adsorption–regeneration test of the calix[4]arene-modified PbS. It can be observed that after four cycles, the adsorbed percentage of MO decreased markedly from 70 to 50%.

The surface morphology and chemical compositions of the surface of calix[4]arene-modified PbS after the treatment were further confirmed by performing FESEM and EDX, respectively. The results of the FESEM and EDX analysis for calix[4]arene-modified PbS after the treatment are depicted in Figure 12. As shown in Figure 12(a), the micrograph shows nanoparticles covered by flaky material due to the presence of p-tert-butylcalix[4]arene. The morphology in the FESEM micrograph remained after the treatment compared to before the treatment of adsorbent (Figure 2(b)). In addition, the EDX result in Figure 12(b) shows the same elements as before the treatment of calix[4]arene-modified PbS (Figure 2(d)) that mainly contain carbon (C), oxygen (O), sulphur (S) and lead (Pb).
Figure 12
FESEM-EDX analysis of (a,b) calix[4]arene-modified PbS after treatment using 0.1 M NaOH.
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FESEM-EDX analysis of (a,b) calix[4]arene-modified PbS after treatment using 0.1 M NaOH.

Figure 12
FESEM-EDX analysis of (a,b) calix[4]arene-modified PbS after treatment using 0.1 M NaOH.
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FESEM-EDX analysis of (a,b) calix[4]arene-modified PbS after treatment using 0.1 M NaOH.

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The calix[4]arene-modified PbS by the treatment of 0.1 M NaOH was further analysed using FTIR. The comparison of Figure 13(a) and 13(b) shows that all important peaks for calix[4]arene-modified PbS are still observed after the treatment, including peaks for O–H stretching (3,143 cm−1), the C–CH₃ stretching vibration (2,941 cm−1) and the C = C aromatic vibrations (1,597 and 1,390 cm−1).
Figure 13
FTIR spectra of calix[4]arene-modified PbS (a) after treatment by 0.1 M NaOH and (b) before treatment.
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FTIR spectra of calix[4]arene-modified PbS (a) after treatment by 0.1 M NaOH and (b) before treatment.

Figure 13
FTIR spectra of calix[4]arene-modified PbS (a) after treatment by 0.1 M NaOH and (b) before treatment.
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FTIR spectra of calix[4]arene-modified PbS (a) after treatment by 0.1 M NaOH and (b) before treatment.

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Leaching test

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The leaching test is an important indicator for quality control of wastewater processing. TSS test is a measurement of all particles in the water which did not pass through the filter after undergoing the adsorption process. Based on the compound listed in the Malaysian Environmental Quality (Industrial Effluent) Regulations 2009 (DOE 2010), the acceptable values for the discharge of Pb and suspended solids are 0.50 and 50 mg/L, respectively. Table 7 shows Pb and TSS values of four different cycle numbers of calix[4]arene-modified PbS. The values are acceptable since they are lower than the limit values.

Table 7

Pb element and TSS test of calix[4]arene-modified PbS at different cycle numbers

Cycle number1234
Pb element (mg/L) 0.093 0.01 0.1318 0.02 0.1834 0.01 0.2447 0.04 
TSS test (mg/L) 11.11 2.13 14.67 2.53 14.78 2.05 16.32 2.35 
Cycle number1234
Pb element (mg/L) 0.093 0.01 0.1318 0.02 0.1834 0.01 0.2447 0.04 
TSS test (mg/L) 11.11 2.13 14.67 2.53 14.78 2.05 16.32 2.35 
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This study determines the adsorption behaviour of MO dye based on calix[4]arene-modified PbS nanoadsorbents under optimal conditions. The calix[4]arene-modified PbS was characterised by FTIR, FESEM and EDX. The presence of functional groups, such as O–H stretching vibration, C–CH3 stretching vibration, C = C aromatic vibrations and C–O stretching in the FTIR result, shows that the immobilisation of p-tert-butylcalix[4]arene on the PbS surface was successful. The appearance of flake-like structures on the surface of unmodified PbS in the FESEM image indicated the presence of p-tert-butylcalix[4]arene. RSM/CCD was employed to optimise the process variables and their interactions in order to improve dye adsorption performance. The findings revealed that the optimised values for adsorbent dosage, contact time and pH were 89.70 mg of calix[4]arene-modified PbS, a contact time of 129.76 min and a pH of 4. Equilibrium isotherm and kinetic studies were also done at optimised conditions. The adsorption of MO was best fit the Freundlich isotherm, indicating heterogeneous adsorption occurring on the surface of the adsorbent by multilayer adsorption. The maximum capacity of calix[4]arene-modified PbS was 3.268 mg/g in optimised conditions. The kinetic data on the adsorption of MO by calix[4]arene-modified PbS closely follow the pseudo-second-order kinetic model, indicating chemisorptions as the mechanism of adsorption. The adsorption mechanism included mainly dipole–dipole hydrogen bonding interactions, Yoshida hydrogen bondings, π–π interactions and electrostatic interactions. The reusability and leaching tests were performed to reduce the adsorbent disposal issues. The findings suggested that calix[4]arene-modified PbS could be a promising nanoadsorbent applied in wastewater treatment in the future. However, further investigations are needed, such as improving the quality and effectiveness, ensuring long-term usefulness and monitoring the selectivity adsorption containing multiple pollutants.

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This research was funded by the Ministry of Higher Education (FRGS/1/2017/STG01/UPM/02/4) and Universiti Putra Malaysia (Grant number GP-9647600).

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Conceptualisation: S.A.A., A.H.A.; Methodology: N.Z.R.; Formal analysis and Investigation: N.Z.R.; Writing – orginal draft preparation: N.Z.R.; Writing – review and editing: A.H.A., M.A.K., S.E.A. and S.A.A.; Supervision: S.A.A.

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

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The authors declare there is no conflict.

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