The Adsorption of Divalent Metal Ions from Aqueous Media Using Unmodified Orange Peel (Musa sapientum) as Adsorbent

Water is one of the world’s most valuable resources, which is under constant threat due to climate change, drought, explosive population growth and waste. A wide variety of heavy metal species enters the aquatic compartment through atmospheric deposition, lixiviation of mining areas and cultivated fields, and industrial discharges. These activities follow an upward curve in response to the world’s ever growing population and its needs. Therefore, there is need to discover non-toxic adsorbent from natural source to combat the menace of water pollution. Orange peel is a non-toxic and environmentally friendly fruit, adsorbent potentials. The present study investigates the adsorption of divalent metal ions from aqueous media using unmodified orange peel as adsorbent. The orange peels for the adsorption experiment was cut, washed, dried and ground into fine powder. The powdered material was characterized using Fourier Transform Infrared (FT-IR) spectrophotometer and Scanning Electron Microscope (SEM). Determination of heavy metals adsorbed was performed on Perkin Elmer model 214 Atomic Absorption Spectrometry (AAS). The effect of pH, contact time, initial metal ion concentration and adsorbent dosage in the removal of heavy metals from aqueous media were evaluated. The Orange peel showed high metal uptake for the tested metal ions Co2+ (97.8%), Cu2+ (95.5%) and Zn2+ (97.0%). The optimum pH for Orange peel was 3, the optimum contact time for the dehydration method of the orange peel was 150 min, and the optimum adsorbent dosage for the Orange peel was 2 g while the orange peel biosorbent optimum initial metal ion concentration was 40 ppm. Adsorption data fitted well with Freundlich and Langmuir equilibrium adsorption isotherms. The maximum adsorption capacity Q value for Langmuir model for the initial metal ion concentration were in the order Cu2+>Co2+>Zn2+. Conclusively, the effective adsorbent properties displayed by the orange peels in the removal of divalent ion from aqueous media studies indicate their potentials in the treatment of water pollution.


Introduction
Divalent metals ions are group of undefined metals that include the transition metals, metalloids, the actinides and lanthanides. So many definitions have been propounded and put forward as regard to definition of heavy metals [1]. Heavy metals are toxics pollutants, released into the various water body through different activities such as industries, mining and agriculture. The rapid pace of industrialization has led to severe problem of water pollution. Awareness about the menace of water pollution gives rise to the treatment of wastewater by industries and municipal authorities before discharging to the natural water bodies [2].
The accumulation of heavy metals in human body leads to several complication and organ impairments. Heavy metals enter the human system through different means, which include contaminated food, water and air, industrial and domestic Industrial exposure is common in adult while injection is the most common root in children. The toxicity of metals by some researchers has been stimulated by the increased concern on how to reduce environmental pollution that has plagued the existence of man for past centuries. Virtually all metals can produce toxicity when ingested in sufficient quantities but there are several metals which are especially important because they are either so pervasive or produce toxicity at even very low concentrations [1,2].
Many countries have regulatory guidelines for heavy metals presence and exposure as well as remediation and treatment options. The concentration limits to which these methods are economical and become ineffective or too expensive to treat wastes having metal ions in concentrations of 100 mg/L or below [3].
Water pollution from heavy metals such as cobalt, zinc, copper, nickel and lead emanated from industrial sources constituted major environmental problem. The treatment of large volumes of waste streams from industrial activities like mining operations required expensive processes. The discharging of heavy metals into the environment in the last three decades has been a case of major concern to the world. The adverse effects of metals on the population and existence of life have been a major point of study in the developing nations of the world [4]. Nigeria is not an exception to these trends. The release of industrial wastewaters to the environment causes several side effects. These waste water commonly contain heavy metals such as Cd, Pb, Cu, Co, Cr, and Zn and are not biodegradable and their presence in streams and lakes lead to bioaccumulation in living organisms, causing health problems in animals, plants and human beings. It is a known fact that most industries discharge their waste in liquid form which is the easiest way of waste disposal. Major industries involve in the discharge of these heavy metals containing waste include textile, paint, metallurgical, mining, electric appliances, fertilizers, pesticides, metal surface treating industries [5].
Many methods and sorbent materials have been introduced in the treatment of waste containing heavy metals. Among these are reverse osmosis, electro-dialysis, ion-exchange, precipitation, solvent extraction, phytoremediation and ultrafiltration. Chemical precipitation and electrochemical method are ineffective especially when the concentration of metal in aqueous solution is less than 50 mg/L. Ion exchange, electrodialysis, ultra-filtration are reliable method of waste treatment [6]. However, these methods suffer several drawbacks such as high capital and operational cost, high reagent requirement, requirement of expensive monitoring system, unpredictable and incomplete metal ion removal and generation of toxic sludge. Conventional adsorbents such as activated carbon have been used widely in the treatment of wastewater [7]. However, the high cost manufacturing activated carbon limits its usage. Agricultural waste like are one of the rich sources of low-cost and environmental friendly adsorbent, due to its abundant availability. Agricultural waste such as peanut husk, rice husk, banana waste, wheat bran and sawdust offer little economic value, they also create serious disposal problems. Activated carbons derived from peanut husk and rice husk have been successfully employed for the removal of heavy metals from aqueous solutions [8].
Adsorption methods are based on physical and chemical phenomena that occur when the adsorbate molecules accumulate on the adsorbent surfaces. Adsorption presents some advantages over others. Such as, low investment cost, as well as the possibility of using environmentally friendly natural adsorbents. Biosorption results from electrostatic interactions and also from the formation of complexes between the metallic ions and functionalities present in the cell surface which exhibits some chemical affinity for metallic ion [9][10][11][12].
In this work the utilization of orange residue was studied for metal adsorption (Cu 2+ , Zn 2+ and Co 2+ ) from aqueous solution due to the great renewability production and low cost for these adsorbent.

Experimental
The orange were bought from iyana-ipaja market of Lagos State. Synthetic stock solutions at different concentrations were prepared from analytical grade CuSO 4 , ZnSO 4 and COCl 2 were all supplied by sigma Aldrich (Johannesburg South Africa). All chemical used were analytical grade.

Sample preparation
The orange peels for the experiment where repeatedly washed with distilled water and oven-dried for 24 h at 105°C to reduce the moisture content. The dried materials were then pulverized and sieved at 25 µm.

Method of adsorption
The batch adsorption studies were carried out in 100 mL flask, each containing 100 mL solutions of Cu 2+ , Zn 2+ and Co 2+ standards solutions. A mass of 2 g of the orange peel as shown in Figure  1.0(b) was introduced into the flask and agitated at 250 rpm in a thermostat shaker at 25°C for 150 min. After the addition of the adsorbent, the solution pH was adjusted to 4.0 in a minimum amount of dilute hydrochloric acid (HCl) at 60°C, room temperature. The solution was stored at 4°C for one month.
Working solutions of the desired Cu 2+ , Zn 2+ and Co 2+ concentrations were prepared from the stock solution daily. The following concentrations of (10, 20, 30 and 40) ppm was each prepared from the 1000 ppm stock [13][14][15][16][17][18][19][20][21][22][23][24][25]. After the sorption, the orange peel were separated from the solution, using 0.45 µm membrane filter briefly rinsed with distilled water to remove the residual solutions trapped among the orange peel and then prepared for other analysis. The initial and final Cu 2+ , Zn 2+ and Co 2+ concentrations in the solutions in each of the flasks were quantified using atomic absorption spectroscopy (AAS) Perkin Elmer.
The percent removal can be calculated using the formula below:

FT-IR spectroscopy
The Fourier transform infrared (FT-IR) spectra of orange peels before and after application were obtained using a Perkin Elmer Spectrum 100 FT-IR spectrometer with an AutoIMAGE System.

Scanning electron microscopy (SEM)
Scanning Electron Microscopy (SEM) was used to determine the surface morphology of the orange peel. The orange peel was dusted onto a carbon sticker, then coated with gold using a sputter coater (Balzers Union, FL-9496) for 30 min. Images were recorded using INCAPentaFETx3 (Vega Tescan) SEM fitted with an Oxford ISIS EDS.  In order to get more structural information and understanding on the adsorption mechanism of orange peel after adsorption, an energy dispersive x-ray spectrometer (EDS) was used to determine the mineral composition of the adsorbents as shown in Figure 1.4. The EDS detects X-rays from the sample when excited by the highly focused, high-energy, primary electron beam, penetrating into the sample. Comparing EDS images of orange peel after adsorption.

Characterization of orange peel
The Effect of Solution pH on the Removal of heavy metals from aqueous solution onto the orange peel. Operation Performed Thrice, n=3.

Effect of pH
The pH is a controlling factor for any kind of metal adsorption process from aqueous solution. Effect of pH is a controlling factor for any kind of metal adsorption process from aqueous solution [27]. The pH dependent experiments were conducted and the result are shown in Figure 1.5. The copper, zinc and cobalt attained a maximum value (97.8%, 95.5% and 97%) at pH 4 (which was acidic).

Effect of contact time
The percentage metal ions removal approached equilibrium within 150 minutes for all metals (Co 2+ , Cu 2+ and Zn 2+ ) with Co 2+ recording 97.8%, Cu 2+ 95.5% and Zn 2+ 97% removal, having a trend of Co 2+ >Zn 2+ >Cu 2+ . The decrease in percentage removal is due to presence of some of the metal ions on the adsorbent surface. This experiment shows that the different metal ions attained equilibrium at different times.   Wavenumber (cm -1 )

Adsorption isotherm
Adsorption isotherms describe the equilibrium relationships between adsorbent and adsorbate. Two adsorption isotherms (Figures 1.7 and 1.8) were used to fit the equilibrium data namely Freundlich.
The Freundlich sorbtion isotherm describes the equilibrium on heterogeneous surfaces and the linear form of the isotherm, which can be represented by     Figures 1.9-2.2).

Conclusion
This study shows that it is feasible to use Orange peel as an adsorbent for Co 2+ , Cu 2+ and Zn 2+ . The Orange peel bio sorbent showed high metal uptake making them more suitable for the adsorption process. The extraction percentage was controlled Shows the spectra analysis of the orange peel after adsorption.