1.                Introduction

There are no specific definitions of heavy metals however they are commonly known as those that have a specific density greater than 5g/cm³[1]. They are elements found naturally on the crust of the Earth and are non-degradable[2].At higher concentrations they may pose a threat to human health. Heavy metal poisoning could be possible due to instances such as from drinking water that was polluted with lead or inhaling mercury.In the past heavy metals were used on a regular basis as a means of construction and even in food and beverages [1].

Even in recent times heavy metals are still incorporated into our daily lives for instance, in gold mining, mercury is still being used.Heavy metal pollution is more dominant around industrial areas such as mining. When mining takes place, the soils get exposed and thus lead to the release of these heavy metals. These metals will then be transported via rainwater through rivers and streams whereby the metals may be deposited on the beds or may even dissolve in the water disrupting the ecosystem surrounding it[2]. Accumulation of these toxic heavy metals in the rivers or ocean poses a threat to multiple aspects of the environment. Hence, handling of heavy metals should be notably taken as a serious matter in which proper management of disposal and usage should be implied to reduce adverse effects towards the environment.Therefore, the need to counter this issue becomes increasingly popular in order to sustain the quality of living of not just individuals but the environment as well.

Various techniques of heavy metals removal from wastewater have been developed and tested to assist in saving the environment. These methods such as reverse osmosis[3], coagulation and flocculation[4], activated carbon adsorption[5], nanofiltration[6] and many more[7-11]. However, some of these are costly for the developingcountries and incapable to perform effectively in treating a wastewater. Amongst all the methods, adsorption seems to provide a more promising approach in removing heavy metals from wastewater treatment processes.As time progresses, there are numerous scientific discoveries and researches towards various techniques in treating wastewater and adsorption seems to form a plausible impact towards becoming a sufficiently effective technique amongst others. It has several enhanced attractions as a technique for wastewater treatment. For instance the cost of said experiment being relatively cheaper has proven to be efficient in helping not only developed countries but also may contribute in helping developing countries[12].The actual technique is very simple and does not require very skilled or highly trained workers.

For an adsorption Process to be of high quality, expenditure as well as removal Capability of the said adsorbent plays a vital role. The adsorbents should be cheap and are readily available in abundance. Hence, over the past decades, multiple potential adsorbents were tested ranging from fruit wastes[13-16], remainders from industries[17-19]as well as cultivates wastes [20-23] in removing of dyes and heavy metals from aqueous solutions. In this study, Averrhoabilimbi, commonly known asbilimbi or ‘Belimbing’,was being used and investigated. Bilimbiis a remedial plant belonging to the Oxalidaceae family[24]. A. bilimbi is closely related to A. Carrabolla generally known as ‘star fruit’. It originated in the Southeast Asia and is easily available here in Brunei Darussalam. Mostly used in cooking, the fruits are sour and are extremely acidic.It is believed that A. bilimbi and its leaves possess a very high value in medicine through various studies and research[24].It is alleged that the fruits have been used in traditional medicine for the treatment of a variety of ailments. Brewed BilimbiLeaves (BL) is used as an alternative method to modern medication and as an antibacterial, postpartum protective medicine and in the treatment of fever. The paste of leaves is used in the treatment of multiple skin disorders such as acne, rashes, itch and many more[25, 26]. These techniques are commonly used amongst the natives, however are not regularly used on a daily basis.To date, literature search indicated aBilimbihas never been utilised for the removal of dyes or heavy metals. In this study Averrhoabilimbi leaves were chosen as the adsorbent instead of Averrhoabilimbiitself.

The study focused on whether BilimbiLeaves (BL) could be a possible candidate as a low-cost adsorbent for removal of PB (II) from aqueous solutions. This is due to the usage of lead throughout the years has transitioned immensely, from being a coating material to be a constituent in paint for the vast infrastructures that exist nowadays. Although it may serve as a multifunctional material in various areas of needs, however this metal with the molecular weight of 207.2 g/mol causes a greater amount of damage to the environment. Upon entering the human body, it attacks the normal metabolic processes as well as vital organsespecially the brain[27, 28].Therefore, the need to suitably dispose of this particular heavy metal is highly required.

2.                  Materials and Methods
2.1.              Preparation of Bilimbi Leaves and Chemical Reagents: The BilimbiLeaves (BL), used as the adsorbent in this study, were freshly obtained from the bilimbi tree homegrown at the back of a house in Kampong Telanai. The BL were plucked from the tree and cleansed with distilled water followed by oven dried at 80°C for about a week a constant mass is obtained.The dried sample wassubsequently blended and sieved using laboratory metal sieves. Samples of particle size <355μmwere used throughout this study. To avoid contamination and reduce moisture build up, the samples were stored in zip-lock bags that were tightly sealed.

Acquired from Sigma Aldrich Corporation, Lead (II) Chloride (PbCl₂) was used directly without supplementary refining steps. Pb(II) stock solution (1000 mg/l) was prepared via calculation, i.e.0.14g of Pb(II) were dissolved in distilled water a 1 L volumetric flask. Solutions of different Pb(II) concentrations (500mg/L and 100mg/L) were prepared by diluting this stock solution.

2.2.              Instrumentation:The Analytic Jena novAA300 Atomic absorption spectrophotometer was used to measure the absorbance of PB(II) solutions. For the calibration curves for Pb(II), standard solutions in the range of 2 to 10 mg/L concentrations were Prepared.Both the ThermoscientificMaxQ 3000 shaker and the Stuart Scientific Flask Shaker SF1 were used to shake the mixture of adsorbent withadsorbate solution at 250 rpm. The Shimadzu IRPrestige-21 spectrophotometer (FTIR) was used to identify the functional groups present in BL. Furthermore, to determine the surface morphology of the adsorbent, the Tuscan Vega XMU Scanning Electron Microscope (SEM) was used.

2.3.              Characterization
of BL:To determine the point of zero charge (PH_PZC) of BL, BL (0.05 g) was weighed into six labeled 250 ml conical flasks then 25.0 ml of different pH values of 0.1M of KNO₃ were added. Adjustment of the pH values of the 0.1 mol/L KNO₃ (values of 2 to 10) solutions by done by adding either 0.01mol/LHCl or 0.01mol/LNAOH solution. After 24 h of shaking at 250 rpm, the mixtures were filtered, and the final pH of the solutions was measured. The graphs of pHfinal versus pHinitial were plotted.The pH_PZCof BL is the point of intersection on the x-axis.

For the morphological analysis using Scanning electron microscope (SEM), the solid residues of lead–loaded adsorbent were left to dry in an oven at around 50°C. The original adsorbent was used as well in the analysis, as it provides the comparison of surface structure before and after being treated withPb(II).

The original adsorbent and lead-treated adsorbent were also characterized using Fourier Transform Infrared Spectroscopy (FTIR). These samples were thenanalyzed by FTIR to determine the presence of functional groups. Potassium bromide (KBr) crystals (0.15 g) were weighed and dried in an oven. After 2 h of drying, KBr crystals were grinded into powder in order to make into pellet by using a presser. This KBr pellet was then run in FTIR as background. Each sample (0.0015 g) was then mixed and grinded with the dried powder of KBr to make into pellet using presser. The pellet was run in FTIR under the wavelength ranges from 600 cm^-1 to ~3800 cm^-1.

2.4.              Batch adsorption Experiment: The effect of contact time was investigated where BL (0.05 g) was weighed into eight labeled 250 ml conical flasks. Lead solutions (25.0 ml) of known concentrations were then pipette respectively into each flask containing the sample. The conical flasks containing mixtures of adsorbent and adsorbate solutions were then shaken at room temperature at the speed of 250 rpm for 4 h. The samples were withdrawn from the shaker at a 30 min interval. They were filtered using filter paper into plastic bottles. The filtrates were then diluted to 10 mg/L for the analysis of absorbance using Atomic Absorption Spectrophotometer (AAS). This experiment was carried out in duplicates.

For isotherm studies, the adsorbent, BL (0.05 g) was weighed into twenty labeled 250 ml conical flasks and treated with a series of concentrations of lead solutions (0-1000 mg/L). These series of concentrations were then shaken at 250 rpm according to its optimum shaking time.

In investigation of kinetics studies, roughly 0.05 g of sample was used and mixed with 100 and 500 mg/Loflead solution in 250 ml conical flask. The solutions were stirred using a shaker at 250 rpm up to the optimum contact time. For every 3 min interval up to 30 min, each of the samples was withdrawn and filtered. For the following 30 min, the solutions were shaken at 10 min interval and the remainder of the solutions was shaken at 30 min interval. Gravity filtration was carried out for each solution and the filtrate was analyzed using AAS instrument. This experiment was carried out in duplicate.

While for the removal of Pb(II) in different medium pH, the effect of pH was studied by adding the sample (0.05 g) into six labeled 250 mL conical flasks containing adjusted pH values of 100 mg/Lof lead solution. The untreated pH of lead solution, known as ambient pH, was measured and recorded. A total of seven different pH ranges from 2 to 10 including the ambient pH was used. The pH values of these solutions were adjusted by adding 0.1mol/LNaOH or 0.1 mol/LHCl solution. Once the desired pHs were obtained, 25.0 ml of this solution were then pipette into 250 ml conical flasks containing the adsorbent. The solutions were then shaken at 250 rpm under its optimum shaking time. After filtration of the solutions, the filtrates were diluted to 10 mg/Lfor the analysis of absorbance.

Thermodynamic studies involved two different concentrations of lead solutions (100 and 500 mg/L) were used. The adsorbent (0.05 g) was mixed with individual concentration of lead solution into separate 250 ml conical flasks and shaken using a water-bath shaker at 250 rpm with temperature altered to 298 K, 313 K, 323 K, 333 K and 343 K under optimum shaking time. After gravity filtration, the diluted filtrate (10 mg/L) was analyzed. The Gibbs free energy (∆G°), entropy (∆S°) and enthalpy (∆H°) were used to investigate the nature of adsorption process.

Ionic strength effect on the removal of Pb(II) by BLwas investigated using potassium nitrate (KNO₃), sodium nitrate (NaNO₃), potassium chloride (KCl) and sodium chloride (NaCl) salts. These salts were prepared in 2mol/L and diluted into a range of 0.01mol/L to 0.1mol/L using a serial dilution. The solutions were mixed with 10 ml of 1000 mg/Lof the lead solution into 100 ml volumetric flask. The adsorbent (0.05 g) was then weighed into 250 ml conical flask containing 25.0 ml of the mixture of different concentrations of salt and lead solution. The solutions were shaken at room temperature to the optimum shaking time on a shaker set at 250 rpm. After respective allocated shaking times, the solutions were filtered and the filtrates were diluted to 10 mg/Lfor the analysis.

2.5.              Regeneration Studies: The adsorbent (1.0g) was weighed into a 1000 ml conical flask with 100 mg/Lof respective adsorb ate solution (500 ml) added onto it. The volume of adsorb ate to mass of adsorbent was in 1:500 ratios in which 1.0g of sample was combined with 500 ml of Pb(II). The flask was then shaken using a shaker at 250 rpm at room temperature under optimum shaking time. During the filtration, the solid residues were collected and dried in an oven at ~70 °C overnight while the filtrate was diluted to 10 mg/Lfor the analysis of absorbance using mentioned instrument.

The dried solid residues were weighed and divided into four portions into 250 ml of labeled conical flasks for allocated treatment purposes. The solids were then further treated with either 0.1mol/LNAOH, 0.1mol/LHCL or distilled water and a control experiment was also carried out. In the treatment with acid, base and distilled water, the volume used was based on the 1:50 ratio. The washed adsorbents were then dried in an oven. Then, another cycle of adsorption was continued and the regeneration studies were carried for five consecutive cycles. 

3.                  Results and Discussions 

BilimbiLeaves (BL), the adsorbent used in this study to removePb(II) from aqueous solutions, was characterized by Fourier Transform Infrared (FTIR) spectroscopy and scanning electron microscope (SEM). Batch adsorption experiments were performed to investigate how parameters such as contact time, pH_PZC, pH and ionic strength could affect the adsorption process. The Lagergren pseudo-first order and pseudo-second order models were applied to study the adsorption kinetics mechanism. Followed by, the adsorption isotherm data were investigated using six different isotherm models. The changes in enthalpy (∆

Figure 1:Plot to determine the point of zero charge of BL.

Figures2(a-b):Surface morphology of BL before (left) and after (right) after treatment of Pb(II).

Figure 3: FTIR spectra for functional group characterization of BL (top-black) and Pb-BL (bottom-blue).

Figure 4:Contact time required for BL-Pb system to reach equilibrium.

Figure 5: Linear plot of (a) pseudo first order and (b) pseudo second order model of the adsorption of Pb(II) onto BL at 100 mg/L.

Figure 6:Comparison between the simulation plots of different isotherm models with the experimental data of the adsorption of Pb onto BL.

Figure 7: The Van’t Hoff plot for the adsorption of Pb(II) onto BL at 100 mg/L.

Figure8: Adsorption of Pb(II) onto BL under various pH medium.

Figure 9:Effect of ionic strength using different salts of KNO₃, KCl, NaNO₃and NaClon the adsorption of Pb(II) onto BL.

Figure 10: Regeneration of BL with Pb(II) showing five consecutive cycles using different treatment methods of HCl (blue rhombus), NaOH (red square), water (purple circle) and control (green triangle).

Table1: Linear equations of kinetics models.

Table 2: Kinetics parameters and error values for adsorption of Pb(II) onto BL at 100 mg/LPb(II) concentrations.

Table 3: Adsorption isotherm parameters of adsorption of Pb(II) onto BL for different models.

Table 4: Comparison of maximum adsorption capacities between BL and other selected adsorbents. 

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