ACTIVATED CARBON DERIVED FROM GROUNDNUT SHELL AND RICE HUSK FOR THE ADSORPTION OF HEAVY METALS IN TANNERY WASTEWATER

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INTRODUCTION
The tanning industry is widely acknowledged as a significant contributor that causes environmental pollution, with particular concern on tannery wastewater (Hussein et al.,2020).This wastewater is mostly associated with potential risk due to its composition, which includes ammonia, sulfides, heavy metal ions and various organic compounds including different azo dyes, which result from hides and skins, as well as from the addition of reagents during the various transformation made on these materials (lsarain-Chavez et al., 2014).The effluents generated by tanning operations frequently find their way into surface water, leading to the downstream contamination of water used for domestic purposes.This contamination poses a significant risk as toxins are carried along, compromising the quality and safety of the water supply (Ashutosh and Manju, 2015).The damage cause as a result of hazardous tannery effluents is becoming an acute problem to the environment.Therefore, it is important to reduce these pollutants to their permissible limits especially toxic heavy metals in the aquatic environment.The exposure of heavy metals can be detrimental to human health and other living organisms (Bayuo et al.,2019).Heavy metals are extremely toxic, nonbiodegradable, and do not undergo thermal degradation, and once they enter the food chain, they can accumulate at low concentrations in the living organisms (Zafar et al., 2020).The conventional methods of treatment of heavy metal contamination includes chemical precipitation, chemical oxidation, ion exchange, membrane separation, reverse osmosis, electro dialysis, adsorption etc.Most of these techniques are very expensive for large scale implementation, dangerous for constant monitoring and control due to their incomplete removal (Abate et al., 2021).Besides, it was revealed that these techniques when applied, some of them are usually incapable of meeting the discharged standards limits for heavy metals concentrations ranging between 0.1 and 3 mg/L (Sanchez et al., 2002).Among many these techniques, adsorption emerges as a notably option owing to its proven efficacy and adaptability in effectively removing heavy metals and other compounds.Groundnut shell is an agricultural by-product from an oilseed leguminous crop with high amount of lignin, potassium and zinc content (Kothai et al., 2019).It is also a source of solid waste that requires proper disposal to prevent environmental pollution.Moreover, Rice husk (RH) is one principal agricultural waste generated during rice processing worldwide (Okoro et al., 2022) which in turn is a major source of concern interms of solid waste management in the environment.Due to contamination of heavy metals from tannery wastewater, there is need to utilize solid waste agricultural waste to a useful product by producing a composite material with high removal abilities most especially for heavy metals.This study prepared a composite adsorbent from groundnut shell and rice husk for the removal of heavy metals from Tannery wastewater in a batch process.Additionally, the impact of kinetic parameters on the adsorption process were also investigated.

Precursor preparation and adsorption studies
The Groundnut shell and Rice Husk were weighed, sorted and washed severally with distilled water, dried in an oven at 105⁰C for 24 hours, then sieved in No. 30 mesh according to Tyler series (0.30 mm) and stored in an airtight container.It was then carbonized at temperature of 400 ͦ C for 1 hour to obtain bio char.The obtained char was impregnated with NaOH (≥99%, BDH limited England) and H3PO4 (≥99%, BDH limited England) for 24 hours thereafter, filtered and washed several times until a neutral pH was obtained.A measured amount of the adsorbent, 0.5g was introduced into 250mL vessel with 50mL wastewater, it was stirred at 250rpm for 60minutes and later subjected to centrifugation and the concentrations of residual Lead, Chromium and Cadmium was determined using Flame Atomic Absorption Spectrometry (Agilent Technologies).Multiple operational parameters such as temperature, initial concentration, contact time, pH, and adsorbent dose were studied.Furthermore, the adsorption studies were conducted to study the effect of initial concentration of the heavy metals (Cr, Cd, and Pb) from 50-250 mg/L; the effect of dosage (0.1-1.2g); temperature (30,40,50 ͦ C); the effect of pH (6.8-7.5);effect of contact time (20-120 min).These operating parameters were kept constant except the one being studied.The surface morphology of the hybrid adsorbent (GR-AC) was determined by Phenom Prox, manufactured by phenom World Eindhoven, Netherlands.The surface morphology the adsorbent is revealed when the electron beam on the samples interacts with the atoms of the sample showing signals relating to the surface topography and composition of the sample.In addition, the elemental composition of the GR-AC were also determined from energy dispersive spectroscopy (EDXRF).Fourier Transform Infrared Spectroscopy (FT-IR) was used to determine the presence of certain functional groups by passing an IR signal through the organic compound causes the functional groups to vibrate at specific frequencies.The FTIR spectra were recorded in the wavenumber range 400-4000 cm −1 on the FTIR.
At equilibrium, the adsorption qe (mg/g), was calculated by equation 1:   = .  .The peak positions on the spectra illustrates the presence of certain functional groups in the forms of vibration.

Operating Parameters studies on Adsorption of Pb, Cd and Cr 4.1
Effect of Adsorbent dosage The availability and accessibility of adsorption site is controlled by adsorbent dosage.Adsorbent dosage was varied from 0 -1.2 g, under the specific conditions (contact time of 60 min, 250 rpm shaking speed and at room temperature of 40 o C. The effect of dosage on percentage removal of heavy metals is shown in Figure 4.The result indicate that Lead exhibits highest percentage removal of heavy metal, with a remarkable rate of 82.4%.It is closely followed by Chromium, which demonstrates a similarly impressive removal percentage of 82.4%.Cadmium on the other hand, shows slightly lower removal efficiency but still achieves a notable percentage of 75.2%.The trend might be due to the availability of more active's sites on the surface of the adsorbent.The enhanced adsorption capacity at low dose can be attributed to higher surface area and availability of more binding sites (Idris et al., 2012).Considering the removal efficiency of Pb, Cr, Cd it has been realized that none of the adsorbent dosage goes beyond 0.5g.Therefore, 0.5g of GR-AC was considered as optimum dosage.The increase in adsorption with the addition of adsorbent might be attributed to the increased number of binding sites available for heavy metals uptake (Abdel salam et al , 2013).This result also suggests that after a certain dose of adsorbent, equilibrium was reached and hence percentage removal of number of ions bound to the adsorbent and the number of free ions in the solution remains constant even with further addition of the dose of adsorbent, hence the adsorbent becomes saturated.

Effect of Contact Time
Contact time is one of the most effective factors in batch adsorption process.The contact time was optimized for the maximum removal of heavy metals with the initial concentration of 50mg/l, adsorbent dosage 0.5g and pH 7-8 by varying the agitating time of (20-120 min) respectively.It can be seen from Figure 5, that the removal was very fast at the beginning and it gradually decreased with time till it reached equilibrium.Since the contact time of the ions and binding sites increase as contact time is made longer, the adsorption process will also be more effective.
The experimental data showed during the first 20 minutes of adsorbent -adsorbate contact adsorption slowly decreased with time due to the saturation of the adsorption sites and the optimal removal efficiency was reached within about 40min for Cd, 60 min for Pd and Cr.The reason for the differences of pattern of adsorption of Pb, Cr by the absorbents based on size could be attributed to the fact that as the particle size increase, the binding site increased as well resulting in enhanced metal adsorption by the biosorbents (Ugya et al., 2019).As the contact time changed from 40 minutes to 60 minutes, the percent removal showed slight increase and started to show a bit decline after 120 minutes.

Effect of pH
Estimating the optimum pH for metal removal is vital since the pH of a solution affects the surface charge of the adsorbents, degree of ionization and solution composition (metal speciation).This behavior of GR-AC can be attributed with different nature of adsorbents because immobilization offers new sites for the binding of ions by inducing cross linking between polymeric matrix and Agricultural waste .Also, the level of dissociation of functional groups on the adsorbent surface, solubility of metal ions and concentration of the counter ions in solution are affected by pH (Venkatesan & Narayanan, 2017).In this study, the initial pH was varied from 6 to 8. The percentage removal efficiency of Chromium, Cadmium and Lead as related to pH is presented in Figure 6.It can be observed that, Chromium, Cadmium and Lead are all better adsorbed at pH of 7, with percentage removal efficiency of 99.98, 80.71 and 78.48 for Pb, Cr AND Cd respectively.This clearly signifies that adsorption of chromium, Cadmium and lead is decreasing with increase in pH.This result is in agreement with (Onwu , 2010) which state that the decrease in removal efficiency at higher pH may be attributed to the formation of precipitates of the metal hydroxides at higher pH which might compete with the metal ions for active sites on the adsorbent surface.The trend is in agreement with the finding of (Purna et al., 2016), where it was observed that the maximum uptake of lead occurred under the conditions where the pH ranged from 7 to 8.This pH ranged was identified as the optimal condition for the simultaneous adsorption of Lead, Cadmium and Chromium in a co-current manner.The behavior of Rice husk and Groundnut shell adsorbent were similar, however, rice husk showed higher adsorption capacity at all pH values than Groundnut shell (Guna et al., 2019).The variable behavior of Pb adsorption at different pH values was due to different surface chargers at different pH values.It is well known that the HPbO 4− ion is the dominant specie of Pb in the pH range of 6-8 and by increasing the pH, this dominant specie is converted in to PbO4 2− ions (Manzoor et al., 2017).

Effect of Temperature Variation
The temperature variation dependence on the adsorption process is related to several thermodynamic parameters.The effect of temperature on removal of Chromium, Cadmium and Lead using GR-AC were studied at 30, 40, and 50 0 C and is shown in Figure 7(a, b and c).The removal of Cr, Cd, and Pb from tannery waste was not significantly affected as the temperature increased.The percentage removal was higher at (99.91%) for Cr, Pb and lower for Cd (89.65%).These shows that the adsorption capacities improved as a spontaneous endothermic process.It can be seen that both parameters increase with increasing temperature (Oumani et al., 2019).The effect of temperature on adsorption of Cr, Cd, Pb using GR-AC is similar to the influence of contact time.However, the percent removal was observed to slightly decline at 40-50 0 C.

Effect of Metal Ions Initial Concentration
The effect of Cr, Pb and Cd ions initial concentration was studied at the range of 50-250 mg/L using adsorbent dose (0.5 g), pH (7.2), temperature (40°C) and 60 min contact time.The effect of varying concentrations on adsorption with time for the adsorption of Chromium, Lead and Cadmium is shown in Figure 8 (a, b and c).As it can be seen from results, by increasing the Cr, Pb, Cd ions initial concentration, the adsorption capacities increased, whereas percentage removal decreased as the initial concentration increased.The removal efficiency of GR-AC was highly dependent on the initial concentrations in the solutions.The maximum adsorption took place at 250 mg/L, which decreased up to 150mg/L from 99.65, 99.40 and 87.77 to 90.60, 89.43, and 84.23 % for Lead, Chromium and Cadmium respectively with increasing adsorbate concentration from 50 to 250 mg/L.The decrease in adsorption with increase in metal ion concentration could be explained on the basis that heavy metals removal depended on the availability of the binding sites on GR-AC surface.The initial concentration of heavy metals has been reported by many authors to affect their uptake.(Padmavathy et al., 2016).For an amount of GR-AC, total available adsorption sites were used at 150 mg/L concentration.This means that the higher adsorption affinity occurred at at higher concentration of the heavy metals and this was agreed with the results obtained (Shirzadeh et al., 2020).from tannery wastewater.The tannery effluents were treated with GR-AC modified with H3PO4 and NaOH.The produced adsorbent was characterized using Fourier transform infrared (FTIR) spectroscopy, Energy-dispersive X-ray fluorescence (EDXRF) and scanning electron microscopy (SEM).The effects of adsorbent dosage and contact time on the adsorption process were studied in a batch system.From the research conducted, the contact time of the ions and binding sites increase as contact time is made longer, the adsorption process will also be more effective.Lead exhibited the highest percentage removal among the heavy metal, with a remarkable removal efficiency of 82.4%.In addition, it can be seen that adsorption capacity increase with increasing temperature indicating a spontaneous endothermic process.Increasing the Cr, Pb, Cd ions initial concentration, the adsorption capacities increased, whereas percentage removal decreased as the initial concentration increased.The removal efficiency of GR-AC was highly dependent on the initial concentrations in the solutions.

(
−   ) m …………..(1) The calculation of the adsorption efficiency is done by the following equation 2 Heavy metal removal efficiency = (  −   ) Where Co and Ce (mg/l) are the concentrations of the heavy metals at initial and at equilibrium, V is the volume of the solution (L) and m is the mass of the adsorbent used (g).
The surface morphology and the surface texture of the Rice Husk (RRH), raw Groundnut shell (RGS) and hybrid adsorbent (GR-AC) before and after adsorption at different magnification (200,100 and 80µm) are presented in Figure1 (a, b, c, d, e, f).The SEM image of the raw adsorbents (RRH, RGS) is rough with lots of big particles with poor surface topography and pores.Figure1 (g, h, i) image revealed various pores on the surface of the GR-AC which indicated a good possibility of metal ions to be adsorbed due to enhanced chemical activation with uniform microporous structure.In addition, Figure1 (j, k, l) clearly revealed that the pores earlier seen on the (GR-AC) adsorbent are no longer visible due blockage of the pores during the adsorption process.Similar finding has been reported by Ahiduzzaman et al., 2016.In addition, Figure2(m and n) shows the Energy dispersive X-ray spectroscopy (EDXRF) analysis of GR-AC which indicated the various elemental composition of elements present.However, the dominance of Zn reduces while Cd, Pb and Cr elements are not visisble due to their presence in trace amount compared to the amount of the adsorbent used and hence were totally adsorbed.

Figure 4 :
Figure 4: Effect of Adsorbent Dosage on heavy metals adsorption on GR-AC

Figure 5 :
Figure 5: Effect of contact Time on heavy metals adsorption on GR-AC.

Figure 6 :
Figure 6: Effect of contact Time on heavy metals adsorption on GR-AC Figure 7: (a) Effect of Temperature Variation for Lead on GR-AC, (b) Effect of Temperature Variation for Chromium on GR-AC, (c) Effect of Temperature Variation for Cadmium on GR-AC Figure 8: (a) Effect of lead initial Concentration on GR-AC, (b) Effect of Chromium Initial Concentration on GR-AC, (c) Effect of Cadmium Initial Concentration on GR-AC

Table 1 :
Functional groups present in RRH, RGS, GR-AC-B, GR-AC-A a FT-IR peak of RGS, b FT-IR spectrum of RRH, c FT-IR spectrum of GR-AC-B before absorption, d FT-IR spectrum of GR-AC-A after adsorption.