Biosorption Potentials of Modified Groundnut Shells adsorbents for Lead and Cadmium ion removal from Industrial Wastewater.

Biosorption Potentials of Modified Groundnut Shells adsorbents for Lead and Cadmium ion removal from Industrial Wastewater.

Pamela N. O.¹ , Ibifuro Altraide² , Akuma Oji¹ , Amodu Da-Silva³

¹Chemical Engineering Department, University of Port Harcourt, Nigeria

²Chemical/Petrochemical Engineering Department, Rivers State University, Port Harcourt. Petroleum Training Institute, Effurun, Delta State, Nigeria.

Corresponding Author Email: Ibifuro.altraide@ust.edu.ng

DOI : http:// dx.doi.org /10.5281/zenodo.8378346

Abstract

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1. Introduction

In Nigeria, water availability and sustainable clean water solutions have been exacerbated by pollution from municipal, industrial, and anthropogenic activities [1] [2]. An increase in urbanization and industrialization has led to the contamination of ground and surface water, which in turn has reduced the water quality due to excessive waste generation [3] [4]. Hence, water sources are contaminated through the discharge of untreated or inadequately treated industrial, commercial, pharmaceutical, agricultural and domestic effluents [3] [5] with loads of heavy metals. The most common heavy metals are Lead (Pb), Zinc (Zn), Mercury (Hg), Nickel (Ni), Cadmium (Cd), Copper (Cu), Chromium (Cr), and Arsenic (As). Although these heavy metals can be detected in traces; however, they are still hazardous. These metals and others such as silver (Ag), iron (Fe), manganese (Mn), Molybdenum (Mo), Boron (B), Antimony (Sb), Cobalt (Co), etc. are commonly available in wastewater and needs to be removed. Excessive level of toxic heavy metals at concentrations above drinking water guidelines is of great danger to human and the environment and requires immediate attention [6]. 

The accumulation of heavy metals in human beings, plants, and animals, even in low concentrations, is a serious health concern because these metals do not biodegrade. Exposure to high levels of heavy metals can cause various illnesses, including gastrointestinal disorders and  nervous, hematopoietic, and kidney diseases [7] [8]. In order to mitigate the negative effects of water contaminants on the environment and human health, discharge regulations and limits are established. However, these measures are often inadequate, so it becomes necessary to treat domestic and industrial wastewater [9] [10]. A range of treatment techniques are available for eliminating water pollutants, but some of them are quite costly and have limited effectiveness [11]. Conventional techniques utilized to remove heavy metals from water include chemical precipitation, flotation, flocculation, sedimentation, solvent extraction, oxidation/reduction, dialysis/electro-dialysis, reverse osmosis, ultra-filtration, electrochemical deposition, ion exchange, and adsorption [12] [11].

The selection of an appropriate technique is determined by its effectiveness and cost efficiency [12]. The adsorption method has been found to be an effective means of removing heavy metals, and this is due to its many advantages. These advantages include cost-effectiveness and availability of adsorbents, less production of sludge, ease of operation, and the ability to reuse the adsorbent after passing through subsequent treatment steps. [13]. Adsorption technology is a surface-based process in which contaminants in solution are removed by coming into contact with a solid material known as an adsorbent. The porous surface structure of the adsorbent allows the solute molecules in the solution, or adsorbate, to adhere to the adsorbent surface [14]. During the adsorption process, several mechanisms take place, including complexation, chelation, reduction, precipitation, and ion exchange. More than one mechanism may occur, and different functional groups such as –COOH–, –NH₂–, –OH–, and –SH– may be involved [15].

Now, complexation and chelation refer to chemical processes where a metal atom is bound to other molecules. During this process, active groups surround the molecules of metal atoms on the cell surface to form a complex. This leads to the formation of a polyatomic molecule [16] [17]. Alternatively, chelation occurs when more than two functional groups of organic molecules give away available electron pairs to metal atoms, resulting in a formation of a ring structure also known as a chelate compound [18] [19]. On the other hand, the Physisorption process has a weak interaction force that is non-specific, which makes it different from the chemical bonding process. In this process, the metal is trapped on the surface due to Van der Waals forces. Heavy metal removal by precipitation occurs through the interaction between the cell surface and the metal, leading to the formation of insoluble precipitates of organic – metal [15]. The reduction of contaminants occurs in a discrete point mainly due to a redox reaction, where a metal is bonded to acts as nucleation before being reduced [16].

Numerous researchers have used the adsorption technique to remove contaminants from water by using affordable adsorbents prepared locally from plant and animal waste, such as palm fibre [20], bamboo [21], soursop [22], rice husk [23], garlic and ginger [5]. Locally formulated adsorbents can be modified by means of physical or chemical activation. The activation of these wastes increases the carbon content to over 70% when pyrolysed into charcoal with high porosity and large surface area [4]. Activation of adsorbent also reduces energy consumption, toxicity, and global warming [24]. Therefore, this study utilized the adsorption method to eliminate heavy metals from industrial wastewater. One of the locally available under-utilized agricultural wastes is Bambara groundnuts shells. It is a type of legume that is native to Africa and considered to be one of the most significant crops on the continent. The utilization of Bambara shells as a bio-adsorbent can lead to cost reduction since they are readily available in many communities in Eastern Nigeria.

Hence, the purpose of this study is to modify Bambara groundnut shells and compare the performance of the modified Raw Bambara Groundnut Shells (RBGS) and Carbonized Bambara Groundnut Shells (CBGS) as bio-adsorbents for the removal of Lead and Cadmium ions from industrial wastewater. The relevance of this research will be vital to Nigeria and other highly industrialized countries with a history of consistent water pollution. More specifically, in the rain forest zones, where Bambara groundnuts are easily available, the use of Bambara groundnuts as bio – adsorbent will serve as a low-cost source of raw material for wastewater treatment. Again, the use of Bambara groundnut wastes as adsorbents will serve as a relief to the ecosystem from its major invasion emanating from their mode of disposal. The knowledge gained via this research will have both social and economic value, as it will enhance job creation for the teaming youths while curbing the escalating crime rate. Finally, the applicability of this research will be critical in the rehabilitation of water bodies, now and in generations to come as it will help in achieving goal number 6 (Clean water and Sanitation) of the Sustainable Development Goals of the United Nations.

2. Literature Review

The adsorption method using natural materials has been found to be an effective means of removing heavy metals, and this is due to its many advantages. These advantages include cost-effectiveness and availability of adsorbents, less production of sludge, ease of operation, and the ability to reuse the adsorbent after passing through subsequent treatment steps [16]. Hence, numerous researchers have used the adsorption technique to remove contaminants from water by using affordable adsorbents prepared locally from plant and animal wastes. Locally formulated adsorbents can be modified by means of physical or chemical activation. The activation of these wastes increases the carbon content to over 70% when pyrolysed into charcoal with high porosity and large surface area. Activation of adsorbent also reduces energy consumption, toxicity and global warming [24].

Consequently, [27] explored the use of Bambara groundnut hulls powder as a bioadsorbent for removing atrazine from aqueous solutions through batch adsorption. They investigated the effect of pH, dosage, contact time, initial concentration, and temperature on the effectiveness of the Bambara adsorbent. The results showed that the pH of the solution played a significant role in the adsorption process, with an optimum adsorption capacity achieved at a pH of 7.0, 0.9g dosage, and 120 minutes contact time. The desorption percentage was between 45% and 70%. According to [28], heavy metals such as nickel, cadmium, copper, and lead were removed using activated carbon derived from African palm fruit. The study revealed that the maximum removal efficiency for nickel, cadmium, and copper occurred after 60 minutes of contact time, beyond which the percentage removal decreased. The highest removal efficiency for lead was observed at the highest optimal contact time of 90 minutes, after which the percentage removal also decreased.

The research paper [29] conducted a study to examine the efficacy of rice husks (RHA) as an inexpensive adsorbent for removing of heavy metals from aqueous solution in both single and multiple-component systems. The study showed that there was competition between the binary and mono-component systems and that the heavy metals were adsorbed in the order of Zn (II) > Cd (II) > Hg (II) in a single system. The authors in [5] conducted a comparison between unmodified garlic and ginger nanoparticles regarding their adsorption capacity for the removal of lead, cadmium, and chromium ions from crude oil-polluted well water. Various factors such as the initial concentration of heavy metals, adsorbent dosage, and contact time, in addition to analyzing the adsorption kinetics, were investigated. The authors observed that equilibrium was attained within 60 minutes, but the garlic nano-particles had higher adsorption capacity than the ginger nano-particles at all experimental conditions.

[30] investigated carbonized bambara groundnut shell as an adsorbent for removal of paraquat dichloride from aqueous solution. It was observed that, water pH, adsorbent dosage, contact time, temperature, and initial concentration are crucial in batch adsorption of paraquat dichloride. The best removal of paraquat dichloride (98%) was achieved at a solution pH of 5, 0.05g dosage, and a contact time of 60 minutes. The researchers concluded that carbonized bambara shell is a potential adsorbent for water treatment. Another batch adsorption study conducted by [31] using untreated Bambara groundnut shells to remove herbicide Pendimethalin (PE) from an aqueous solution also varied the contact time, initial PE concentration, adsorbent dosage, solution pH, and temperature. Their findings equally revealed that the adsorption process is chemisorption, and the raw Bambara groundnut shell was effective in removing PE from the aqueous solution.

Also, [32] conducted research on the use of biosorbents derived from the leaves of Ziziphus spina-christi for removing cadmium (Cd²⁺) ions from aqueous solutions through adsorption. Their findings revealed that the biosorbent was successful in eliminating Cd²⁺ from the solutions, achieving a maximum removal efficiency of 99.57% at an initial concentration of 50mg/L and a solution pH of 6.5. Again, the study conducted by [33] explored the efficacy of biosorbents derived from the leaves and stems of Calotropis procera for eliminating chromium ion from aqueous solutions. The results indicated that the removal of chromium ion was more effective at higher biosorbent dosage, pH levels, and contact time. Conversely, it was less effective at higher initial concentrations of chromium ion and temperatures.

The authors in [30] used chemically activated carbon from Bambara groundnut shell to remove pendimethalin (PE) (herbicides) and paraquat dichloride (PQ) from aqueous solution. Trioxonitrate (V) acid was used as a chemical for the activation. They also varied pH, contaminant concentration, contact time, adsorbent dosage, and temperature in order to optimized the adsorption process. It was observed that the extent of PE and PQ removal depended on time of contact it makes with the adsorbent, dosage of activated carbon Bambara groundnut shell, pH, and initial concentration of PE and PQ. [34] examined how chitosan-modified bamboo charcoal adsorbent could be used to simultaneously adsorb Pb(II) and Cd(II) ions from an aqueous solution. They investigated the effects of pH (ranging from 2 to 10) and temperature (ranging from 10 to 50°C). The researchers found that the maximum removal efficiency for Pb(II) and Cd(II) was 97.6% and 94.9%, respectively. The study determined that the adsorption capacity for Pb(II) and Cd(II) was 141.84 mg/g and 139.86 mg/g, respectively.

Again [35] conducted a study to improve the removal of Cd(II) ions from aqueous solutions using date palm waste in a batch adsorption process. The experiment involved testing different pH levels (ranging from 3 to 9), initial Cd(II) ion concentrations (ranging from 50 to 200mg/L), adsorbent dosages (ranging from 1 to 5g/L), and contact times (ranging from 15 to 120 minutes). The results indicated that the highest removal efficiency of 96.53% was achieved at optimal conditions of pH 6.5, an adsorbent dosage of 2.8g/L, and a contact time of 120 minutes. Also, [36] investigated the adsorption capacity and optimum condition for adsorption of Arsenic ion (As (V)) onto grapefruit peel biosorbent. They varied the solution pH, temperature, adsorbent dosage, and initial concentration of As(V) to obtain the optimal condition for best removal. The findings revealed that grapefruit peel biosorbent has a strong adsorption capacity for removing As(V), and a maximum adsorption capacity of 11.49mg/g was achieved at pH 6.0, 25°C, and an adsorbent dosage of 1.5 g/L.

However, in this study, different modifications of Bambara groundnut shells which include the Raw Bambara Bambara Shell (RBGS) and Carbonized Bambara Groundnut shells (CBGS) were investigated as bio – adsorbents for the removal of Lead and Cadmium ions from industrial wastewater effluent. This investigation was conducted to ascertain the performance of modified Bambara groundnut shells and the potential for application as adsorbents in the adsorption process for the treatment of industrial waste effluent and domestic wastewater.

3. Methodology

• Collection and preparation of samples

Wastewater samples were collected from the effluent unit of a Petrochemical plant. The water samples collected were preserved with nitric acid (HNO₃) to a pH of 2.0 and placed in an ice chest. The Bambara Groundnut Shells were collected from Enugu State, Nigeria while the chemical reagents and apparatus used were provided in the laboratory. All materials collected for the experimental investigations were transported to the laboratory.

The Bambara Groundnut Shells was prepared for experimental analysis according to the method of processing biosorbents for application in the adsorption process as described in the work of [30] [31]. Firstly, the shells were washed and cleaned thoroughly using fresh tap water to remove any dirt, and afterwards spread out on a clean floor to dry off the water content for 3 days under the natural sun at average ambient temperature of 29ºC. The moisture content in the shells was further dried off using an oven, with temperature controlled at 70-100 °C for 24 hours. The dried Bambara groundnut shells were subsequently divided into two portions and further processed to obtain the raw sample and carbonized adsorbent for ndustrial wastewater treatment.

3.1.1 RawBambara Groundnut Shells (RBGS) Adsorbent

The dried shells were cooled and crushed to fine powdered particles to obtain the Raw Bambara Groundnut Shell (RBGS) adsorbent.  The particles were soaked in a solution of 0.1M NaOH for 9 hours. After this, they were washed with distilled water and dried again. The dried particles were soaked in a solution of 0.1M H₂SO₄ for 9 hours to remove any trace of alkalinity. This was followed by thorough washing using distilled water and then dried in a desiccator [45]. The dried samples were sieved to a maximum of 1.18mm particle sizes and stored in an airtight container ready to be used for the adsorption process.

• Carbonized Bambara Groundnut Shell (CBGS) Adsorbent

Quantities of 1.5kg of the dried Bambara Groundnut Shells were weighed into Pyrolysizer using analytical weighing balance. The samples were pyrolyzed at temperatures between 500ºC and 800 ºC. The process of pyrolysis took place at 800 ºC, and the first condensate in the form of an oil droplet was observed at about 32 minutes. Following the completion of the pyrolysis process, the resulting carbonaceous materials were crushed into fine particles using a ball mill crusher and then sieved to attain a maximum particle size of 1.18mm [45]. The particles were then stored in containers, ready to be utilized for the adsorption process.

• Analysis of Samples
  • AAS, SEM and FTIR

Preliminary analysis of the wastewater was carried out using Atomic Adsorption Spectrophotometer (AAS). This revealed the concentrations of the heavy metals of interest (Pb and Cd). The structural arrangement and pore coverage of the particles before and after adsorption were analyzed using Scanning Electron Microscopy (SEM), FEI ESEM Quanta 200 to investigate the morphology of RBGS and CBGS. Additionally, the band range of the particles was determined through Fourier Transform Infrared (FTIR) spectra analysis, which was conducted with a Nicolet iS10 FT-IR Spectrometer.

• Batch Adsorption studies

The formula used to calculate the percentage of heavy metal adsorbed by RBGS and CBGS adsorbents is given as:

                                                                      (1)

Theformula used to calculate the adsorption capacity of RBGS and CBGS at any time is expressed as:

                                                                                                        (2)

The formula used to calculate the adsorption capacity of RBGS and CBGS at equilibrium is expressed as:

                                                                                                        (3)

Where:

 Concentration of heavy metal analysed in wastewater before treatment (mg/l)

Concentration of heavy metal analysed in wastewater after treatment (mg/l)

 Concentration of heavy metal analysed in wastewater after time, t (mg/l)

 Concentration of heavy metal analysed in wastewater at equilibrium (mg/l)

 Concentration of heavy metal on the surface of adsorbent at time, t (mg/g)

 Concentration of heavy metal on the surface of adsorbent at equilibrium (mg/g)

Volume of the wastewater

Weight of the modified Bambara formulations (g) [36] [37] [38].

• Experimental Studies

3.4.1 Effect of dosage

The effect of varying dosages of different Bambara groundnut formulations was investigated at 0.2g, 0.4g, 0.6g and 0.8g weight. 150ml of the wastewater was measured and poured into conical flask. A 0.2g powdered particles of the raw Bambara groundnut shell was added into the conical flask. The solution pH was initially 8.6, was adjusted to 7.0 using sulphuric acid (H₂SO₄).  The flask was placed on rotary disc. The rotary disc was switched on and magnetically stirred the content of the flask at rotational speed of 100 rpm for 2 hours [27].

The purpose of stirring was to ensure that the particles and contaminants in the solution were well in contact. After stirring for 2 hours, the mixture was filtered using filter paper. The resulting filtrate was analysed using an Atomic Absorption Spectrophotometer to determine the concentration of lead and cadmium ions left in the solution [30]. These procedures were repeated using dosages of 0.4g, 0.6g, and 0.8g, while maintaining the solution pH and contact time at 7 and 2 hours, respectively. Similarly, the same procedures were carried out for using carbonized Bambara groundnut shells for comparative studies.

• Effect of pH

The effect of wastewater pH on the adsorption was investigated at 3, 7, 9, and 12. To adjust the wastewater pH level to the desired pH, H₂SO4 and NaOH were used. The same procedures were followed as in the investigation of the dosage effect, but the adsorbent dosage was maintained at a 0.8g while varying the water pH. Contact time was also held constant at 120 minutes for each experimental run. After 120 minutes, the solution was filtered to analyze the concentration of lead and cadmium ions using Atomic Absorption Spectrophotometer. This experiment was carried out on the raw and carbonized Bambara groundnut shells.

• Effect of Contact Time

This group of experiments was aimed to examine the impact of contact time on the adsorption of lead and cadmium from wastewater using raw and carbonized Bambara groundnut shell in batches at different times of 30, 60, 90, and 120 minutes. The procedures stated for the investigation of dosage and pH effects were repeated for all the experimental runs. The pH of wastewater was consistently maintained at 7.0, while the adsorbent dosage was kept constant at 0.8g for all the experimental runs. After a 30-minute contact time, the solution was filtered, and the filtrate was subjected to analysis for the concentration of lead and cadmium ions via Atomic Absorption Spectrophotometer (AAS) analysis. The above experimental steps were repeated for raw and carbonized Bambara groundnut shells at 60 and 90-minute intervals.

The various batch experiments for each formulated adsorbent are specified in Table 1.

Table 1: Batch adsorption design for the formulated adsorbents

Parameters	RBGS / CBGS
	m (g)	pH	t (mins)
Effect of Dosage	0.2	7	120
0.4	7	120
0.6	7	120
0.8	7	120
Effect of pH	0.8	3	120
0.8	7	120
0.8	9	120
0.8	12	120
Effect of Contact time	0.8	7	30
0.8	7	60
0.8	7	90
0.8	7	120

*RBGS = Raw Bambara Groundnut Shell; CBGS = Carbonized Bambara Groundnut Shell

4. Results and Discussions

4.1 Industrial wastewater analysis

The results of preliminary analysis carried out on the industrial wastewater sample are given in Table 2

Table 2: Concentrations of ions in the wastewater

Parameter	Concentration (mg/l)	WHO Limit (mg/l)
Lead (Pb)	1.27	0.01
Cadmium (Cd)	1.06	0.003

An initial test of the samples of wastewater indicated the presence of heavy metals and other water contaminants. Atomic Adsorption Spectrophotometer (AAS) analysis revealed that all the metals of interest (Pb and Cd) had levels exceeding the permissible limits specified by the World Health Organization (WHO) for water and wastewater before discharge to the environment. These heavy metals in the wastewater are from the manufacturing or production process of any materials [6].

• Morphology and FTIR spectroscopy of Bambara Groundnut Shells

The SEM images in Figures 1a and 1c revealed that the surfaces of the raw and carbonized Bambara groundnut shells were rough before the adsorption process which was agreed with that reported by [39]. The surface roughness was irregularly patterned with microspores of different sizes. These characteristics enhanced the adsorption of the metals. However, Figures 1b and 1d revealed that the pores on the surfaces, which were visible prior to the adsorption process, had been filled up. This suggested that the molecules of lead (Pb) and cadmium (Cd) ions had bonded to the vacant sites on the surface of the various modified Bambara groundnut shells after the adsorption. A careful observation of the images before and after adsorption showed the images have smother surfaces after the adsorption [39].

Figure 1a: Morphology of Raw Bambara Groundnut Shells before adsorption

Figure 1b: Morphology of Raw Bambara Groundnut Shells after adsorption

Figure 1c: Morphology of Carbonized Bambara Groundnut shells before adsorption

Figure 1d: Morphology of Carbonized Bambara Groundnut shells after adsorption

Similarly, Figures 2a and 2b show the FTIR spectra of the different modified adsorbents from Bambara groundnut shells. The figures displayed several absorption bands indicating the presence of multiple functional groups on the surface of the modified Bambara groundnut adsorbents. From the spectra analysis, bands with medium and sharp absorption were freely bonded with the molecules of the contaminant in the solution. There were also bands that had strong and broad absorption, some of which were inter-molecularly bonded, while others were intra-molecularly bonded. These bands belong to hydroxyl (OH) groups (range of 3445 – 3450 cm^-1) that were present due to adsorbed moisture. Additionally, there were other adsorption bands of lower energy that represented functional groups of C-H (2900 -2930 cm^-1), C=O (1730 -1735cm^-1), C=C (1620 -1634 cm^-1), others include N – H and C-O groups [34]. The significant adsorption capacity recorded in the different Bambara groundnut formulations is likely due to the existence of these functional groups on their surface. Various studies on adsorption of contaminants in water and wastewater have shown that the presence of these functional groups is the reason why bio adsorbents perform well in removing contaminants [40] [41].

Figure 2a: FTIR of Raw Bambara Groundnut Shells

Figure 2b: FTIR of Carbonized Bambara Groundnut Shells

4.3 Results of Batch Adsorption Studies

4.3.1 Effect of Adsorbent Dosage

Figure 3a shows the effect of Raw Bambara Groundnut Shell (RBGS) and Carbonized Bambara Groundnut Shell (CBGS) dosages on the removal of lead (Pb) ion from industrial wastewater. The profiles indicated that the percentage of lead ion removal increased as the dosage of various Bambara groundnut shell formulations increased. Thus, as dosage was increased from 0.2 to 0.8g, the percentage of Pb ion removed from the wastewater increased as well. Specifically, the percentage increased from 71.04% to 84.66% and 73.98% to 87.60% for RBGS and CBGS bioadsorbent respectively, this agreed to that reported by [26] [28] [36]. Comparatively, the Carbonized Bambara Groundnut Shell (CBGS) removed lead ions more from the industrial wastewater than the Raw Bambara Groundnut Shell (RBGS).This is as a result of carbonization which increases the pore size of biochar, expands the pore volume, increases the specific surface area, and form a preliminary carbon skeleton [44].

The increase in Pb ions removal with increase in dosage of the adsorbent can be mainly attributed to the availability of more exchangeable binding sites for sorption of the metal ions. A similar study by [37] using 0.1 – 1.0g dosage of Bambara groundnut husk, also reported an increase in the adsorption of lead ion as the dosage in solution was increased. According to a study, the increase in adsorption of lead or other metals at higher adsorbent dosage occurred because there are more active sites for sorption, so that the sorption sites remained unsaturated [45].Bambara groundnut shell has not gain wide attention in comparison to other adsorbents, however, some researchers had reported the efficacy of untreated and treated Bambara groundnut shell in adsorption of atrazine [44], paraquat dichloride [31] [32],and pendimethalin herbicide [32]. These studies reported high removal percentages of the desired contaminants from water.

The efficacy of the modified Bambara groundnut shell adsorbents, with respect to dosage effect for the removal of Pb ion, is comparable to some bioadsorbents used in the adsorption of Pb ion from water and wastewater. Similar results have been reported for the effect of dosage on lead ion removal from polluted water using adsorbent modified from sawdust[49]maize tassels [50], coconut shell and palm kernel shell[49] garlic and ginger [5], activated palm kernel husk, coconut and groundnut shell [39] plantain pseudo stem [50] and activated coconut shell or coir [51].

Figure 3b shows the percentage of cadmium (Cd) ion removed from industrial wastewater by the modified Bambara groundnut shell adsorbents (RBGS and CBGS) at varying dosages. By increasing dosage from 0.2 to 0.8g, the percentage of Cd ion removed increased from 83.38 – 92.39% and 86.32 – 95.32% for RBGS and CBGSbioadsorbentrespectively. In the study by [37], using Bambara groundnut husk, just about 60% of cadmium was removed from an aqueous solution. Comparatively, CBGS adsorbent removed the cadmium ion from the wastewater more than the raw RBGS adsorbents. The higher performance recorded in CBGS adsorbent may be due to its finer microstructures compared to those of RBGS, as this specific property can lead to an increase in more available vacant site for binding of Cd ion, particularly as the surface area of adsorbent is increased. This also agreed with observations of [37] for Bambara groundnut husk adsorbent.

Based on the results, an increase in dosage of RBGS and CBGS enhanced their removal ability of Pb and Cd ions from the industrial wastewater. This suggests that dosage played a significant role in the level of heavy metals removed from contaminated wastewater solution. However, research has shown that beyond an optimal dosage, a further increase in adsorbent dosage will not lead to a significant removal of contaminants because the total surface area available for more uptake of contaminants has fully been utilized [37].

Comparatively, the two modified Bambara groundnut adsorbents have high affinity to cadmium more than lead. Therefore, more concentration of cadmium ion was removed from the wastewater compared to lead. On the contrary, the Bambara groundnut husk used by [37] removed more lead than cadmium from an aqueous solution between 0.1 to 1.0g.  However, some authors have observed that the preference of heavy metal removal may be higher at increased dosage for some metals and lower for other metals. Therefore, the removal efficiency of heavy metals for competitive adsorption may depend on ionic nature of the adsorbent [27] [50]. Conclusively, the optimal removal of Pb and Cd ions occurred at 0.8g dosage for the two modified adsorbents.

Fig 3a: Effect of dosage on Pb removal

Fig 3b: Effect of dosage on Cd removal

• Effect of Solution pH

Figure 4a shows the removal of lead (Pb) ion from industrial wastewater at varying solution pH. The Pb ions removed was found to increase from 72.91% – 84.66 and 75.85 – 87.60% at a pH of 3 – 7 for RBGS and CBGSbioadsorbentsrespectively and then decreased with increasing pH from 9 – 12. The enhanced adsorption of the metals with increasing pH from 3 to 7 suggests that the surface of the adsorbents becomes more negatively charged, which resulted in a favourable electrostatic force of attraction to the positive metal ions [38]. [37] obtained a maximum adsorption of Pb ion at a solution pH of 6 using Bambara groundnut husk.According to [37], pH of a solution influences the state of chemical changes of the binding sites. Thus, at a low pH, there is high tendency of increased concentration and mobility of hydrogen ion, which will favour the adsorption of H⁺ than metal ions. This scenario creates competition between protons and metal ions on the binding sites of adsorbent.

Similarly, Figure 4b shows the effect of solution pH on the removal of Cadmium ion from industrial wastewater. The percentage of Cd ion removed increased from 79.80 – 92.39% and 82.72 – 95.32% at pH of 3 to 7 for RBGS and CBGS bioadsorbents respectively and then decreased with increasing pH from 8 – 12. Similar to Pb on effect of pH, the removal of Cd ionswas higher with the CBGS compared to RBGS adsorbents. However, [39] reported a maximum adsorption of Cd ion onto Bambara groundnut husk at pH of 6. Again, the increase in removal of Cd ion at pH from 3 to 7 suggests that the surface of the Carbonized Bambara Groundnut shell was negatively charged, and hence, attracted the positive Cd ion to the vacant sites [27] [51].

Previous studies on adsorption using Bambara groundnut shell reported optimum of organic substances at different pH values. For example, pH of 6 was reported for adsorption of atrazine [36] while a pH of 5 was reported for the adsorption of paraquat dichloride [43]. Meanwhile, findings of some researchers shows that as pH level exceeds 6 – 6.5, there may be no significant improvement in heavy metal removal [41] [51].

Fig 4a: Lead ion removal at various pH of solution

Fig 4b: Cadmium ion removal at various pH of solution

• Effect of Contact Time

Contact time is another important adsorption parameter that influences the removal of impurities from wastewater. Figure 5a shows the removal of Lead ions from industrial wastewater at varying contact time. Increase in contact time from 30 – 120 mins increased the percentage of Pb ion removed from 64.37 to 84.66% for RBGS and 67.31 to 87.60% for CBGS adsorbent. The removal of Pb ion with effect of contact time was higher with CBGS adsorbent than that of RBGS adsorbent.

The percentage removal of Pb increased rapidly as contact time was extended, particularly from 30 to 60 minutes, and then became relatively stable as the time was increased further to 120 minutes. Equilibrium adsorption of Pb ion was achieved around 90 min for RBGS and CBGS adsorbents. After reaching the equilibrium state, an increase in contact time did not lead to significant adsorption. The results indicate that maximum adsorption of Pb ion onto the modified adsorbents was achieved at 120 min. [37] also reported a maximum adsorption of Pb ion onto Bambara groundnut husk at contact of 120 min.

Some studies have equally reported equilibrium at time below the 120 minutes obtained in this study. Thus,20 minutes was reported for Pbadsorption onto garlic particlesbut 60 minutes was reported for same adsorption of Pbonto garlic and ginger nano-particles [6] while 10 minutes was recorded for adsorption of Pbiononto coconut shell adsorbent [50]. Several factors may be responsible for the differences in equilibrium time, such as nature, particle size and modification of adsorbents; differences in sorption processes such as ion-exchange, physisorption/chemisorptions and complexion ions from hydroxyl, carboxylic and phenolic functional groups present in the adsorbents [41].

Furthermore, Figure 5b shows the removal of Cadmium ion from industrial wastewater at varying contact time. Thus, from 30 to 120 minutes contact time, the percentage of cadmium removed increased from 69.07 to 92.39% for RBGS and 72.01 to 95.32% for CBGS adsorbents. Similar to the adsorption of Pb ions, there was rapid increase in the percentage removal of Cadmium within 60 minutes, and thereafter, it became slow until 120 minutes. Although the equilibrium adsorption of Cd ion was achieved around 90 min for RBGS and CBGS bioadsorbents, but the maximum adsorption was recorded at 120 min. The maximum adsorption time of 120 min was also observed for the adsorption of cadmium onto Bambara groundnut husk [37]. Effect of contact time on the adsorption capacities of the modified Bambara groundnut adsorbents is similar to other previous works on cadmium adsorption using biosorbent [42] [49].

Again, previous studies had reported different times for attainment of equilibrium for adsorption of heavy metals using agricultural based adsorbents. [27] achieved equilibrium at 30 minutes for adsorption of cadmium by modified garlic waste. Other studies also reported equilibrium at 30 minutes for adsorption of cadmium and other heavy metals using modified wastes from plants [26]. In other studies, higher times were reported for establishment of equilibrium: 180 minutes for manganese adsorption [21] and 175 minutes for chromium adsorption [33]. [5] achieved equilibrium at just 20 minutes for adsorption of lead and other heavy metals onto garlic adsorbent.

However, the faster removal rate of cadmium, compared to lead was attributed to a smaller ionic radius that resulted to rapid diffusion into the adsorbent surface, while the rapid adsorption during the initial stages were due to the abundant of active sites on the adsorbent surface [37]. Studies had proved that at initial stages of adsorption, the take up of contaminants are primarily controlled by diffusion from the bulk of solution to the adsorbent surface, but as adsorption progresses, the contaminants attach themselves to the fewer active sites available on the adsorbent surface, leading to the slow rate of adsorption [5] [26].

Figure 5a: Lead ion removal at various contact time

Figure 5b: Cadmium ion removal at various contact time

4.4 Analysis of treated wastewater

The results of analysis carried out on the industrial wastewater sample after adsorption process are given in Table 3, including the concentration and percentage of lead and cadmium removed from the wastewater after the adsorption.

Table 3: Industrial wastewater contamination level after treatment

Parameters	Initial value	Bioadsorbents	WHO Limits	Removal (%)
		RBGS	CBGS		RBGS	CBGS
Pb (mg/l)	1.27	0.1948	0.1205	0.01	84.66	87.60
Cd (mg/l)	1.06	0.0868	0.0201	0.003	92.39	95.32

The concentration of metals removed from the wastewater after treatment by adsorption method, particularly for Pb ions at optimum conditions reached 84.66% and 87.60% for adsorption by RBGS and CBGS respectively. For Cd ions, the percentages removed were 92.39% for RBGS and 95.32% for CBGS adsorbent.

However, within the contact time used in the study, the values for Pb and Cd ions recorded are still fractions above the recommended limit [6] for drinking water quality. Although, these values are above the specified limits, the level of heavy metal ions removed after 120 minutes of adsorption implied that the formulated adsorbents have the potential for removing heavy metals effectively from industrial water effluent. Therefore, to further reduce the concentration of heavy metals below the acceptable limit, it is necessary to either increase the contact time or the dosage of the adsorbent. This is because contact time and dosage favored the removal of water contaminants [42] [43].

5. Conclusion and Future Work

The performance of modified Raw Bambara Groundnut Shells (RBGS) and Carbonized Bambara Groundnut Shells (CBGS) as bioadsorbents for the treatment effluent wastewater from a Petrochemical Plant was studied andbased on the results; the following conclusions have been drawn:         

1. .
2. The smoother surfaces of the RBGS and CBGS after the adsorption process, as revealed by the scanned images, is an indication that the pore spaces or vacant site were occupied by the Lead and Cadmium ions, thereby reducing their concentration in the wastewater.
3. Generally, considering the parameters (dosage, pHand contact time) studied, the performance of CBGS was higher in for Lead and Cadmium ion removal from the industrial wastewater compared to RBGS. This might be due to the process of the carbonization of the groundnut shells which increased the pore size of biochar, expand the pore volume, increase the specific surface area, and form a preliminary carbon skeleton.
4. The amount of lead and cadmium ions in the wastewater after treatment by the RBGS and CBGS adsorbents for 120 minutes contact time was still above the permissiblelimits for good water quality. Therefore, increasing the adsorbent dosage might completely remove or reduce the concentration of lead and cadmium ions to level below the permissible limits.
5. It is therefore recommended that contact time with RBGS and CBGS bioadsorbents should be increased to enhance the removal of these heavy metal ions from contaminated industrial effluent to value below the permissible limits recommended by World Health Organization.                

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