Removal of Cu (II) ion from Ezana (Meli) Gold Mining Wastewater Using Eucalyptus Bark as Adsorbent

This study investigated the potential use of Eucalyptus Bark (EB) powder as an adsorbent in batch mode experiments for removal of Cu from Ezana (Meli) wastewater. The discharge of untreated gold mining wastewater contaminated by Cu (II), which is threatening ecosystems and carcinogenic to the human. Since the removal by using adsorption method is cost effective and environmentally friendly, it has been widely studied by many researchers. Characterizations of Eucalyptus Bark were analyzed using proximate analysis, Fourier transform infrared (FTIR) and X-ray diffractometer (XRD). Various characterization techniques showed that the effluent discharged from the factory contains: total suspended solid (TSS), turbidity, Electrical conductivity (EC), Total dissolved solid (TDS), COD, Temperature, pH, cyanide WAD with <11°C (ppm). Atomic absorption spectroscopy study indicated that heavy metals found in the wastewater were in the order Fe> Cu (II) >Pb (II) >Mn> Cr (VI) >Zn > Co > Ni > Cd in ppm. The selected parameters were pH, adsorbent dosage and contacting time. The highest percentage of Cu (II) removal achieved was 92%. In this study, the adsorption data were well-fitted to the Langmuir isotherm model.


Introduction
Ezana is the second largest gold mining company in Ethiopia with a capacity of producing 4.5kg of gold per day. The project area is located close to the rural northwestern lowlands where basic data on the physical and biological environment is lacking. The gold mining operations in Ezana (Meli) where the waste water was discharged as slurry through the pipelines into only one tailings pond, which is covered by geotextile membrane on the bottom and plastic geo membrane on the top. The wastewater is recycled using the overflow return water pump from tailing pond to prevent seepage, but that is not sustainable due to creation of a small perforation plastic geo membrane wastewater and is leaching into the ground water. Moreover, the dam due to smaller in size frequently overflows during summer and is discharged to the nearby river. There are a lot of merits of adsorption including low cost, locally abundant, require, less maintenance, simple alkali and acid treatment etc. The presence of different heavy metals in wastewater due to toxicity and non-biodegradability tends to accumulate in down streams [1]. In Ethiopia, the application of advanced technology is technically complex, expensive, and non-feasible. Having the above mentioned problems in mind, the factory need to develop on-site facilities to treat their own effluents using low-cost technologies based on locally available, cheap, environmental friendly sorption materials like eucalyptus bark in order to minimize the contaminant concentration. Adsorption is a natural process by which molecules of a dissolved compound i.e.
adhere to the surface of an adsorbent solid [2,3]. The present study focused on adsorption of copper using eucalyptus bark. The purpose of treating the adsorbent with acid was to remove lignin part of the material prior to adsorption thereby increasing their percent removal [2].
Copper was selected as potential contaminant due to its maximum concentration than other element i.e. Cu (8360 to 59280 ppm) and Pb (6.4 to 1652 ppm) of Ezana (Meli) [4]. Sulfuric acid was used to treat eucalyptus bark for removal of lignin which packs the cellulose and it increases the carboxyl active groups which are used for increasing the adsorption efficiency [5]. The bio adsorbents have The affinity for heavy metal ions to form metal complexes or chelates through their functional groups like carboxyl, hydroxyl, phosphate, sulfate, thioether, phenol and carbonyl amide have the behavior of the bio adsorbent [4]. The effect of physical parameters such as pH, adsorbent dosage and contacting time was investigated to examine the efficiency of eucalyptus bark. The efficiency of eucalyptus bark was examined by using the effect of physical parameters such as pH, adsorbent dosage and contacting time. Batch study was conducted using heavy metal containing real wastewater sample laboratory from five selected sites of tailing storage dam. The heavy metal were analyzed by digesting with acid to prevent precipitation due to high acidity of the sample then diluted with deionized water.

Eucalyptus bark (EB) as adsorbent
Eucalyptus trees are evergreen in all season, fast growing, locally available, cheap and huge available in almost everywhere. Due to cost consideration, it is used as an adsorbent which is an important parameter for selection and design of for this adsorption process. R. Saliba et al., (2001) studied the benefits of eucalyptus barks for the adsorption of heavy metal ions and dyes.
Copper adsorption by activated carbon from EB have the optimum pH 5 and reached equilibrium within 45 min for whole range of concentration (0.1-10 mM) [6]. Further, Akmal et al., (2014) the biosorption system of Cu(II) ions from aqueous solution using eucalyptus bark powder at pH values (pH>5), and dramatically decreased the biosorption yield. The deviation of pH can be explained by considering the surface charge on the biosorbent. The highest biosorption efficiency for Cu(II) ions on Eucalyptus bark powder was observed at pH 5.0 [7].
At higher pH values (>6) due to promote higher adsorption capacity, it decreases the electrostatic repulsion between cations and the positively charged surface. Therefore, highest adsorption often occurs at high pH level. However, at this pH range, the formation of insoluble hydrolyzed species (CuOH) + , (PbOH) + , Cu(OH)2 and Pb(OH)2, this condition is often not clear as the metal precipitation could lead to a misunderstanding for the adsorption capacity [1].

Chemicals
Most of the chemicals were analytical grade and the chemicals would be collected from Ezana gold mining development Plc laboratory and chemical shops in Addis Ababa and most of them were analytically grade. NaOH and H2SO4 were used to adjust the solution pH and pretreatment of Eucalyptus Bark by H2SO4. Throughout the experimental studies distilled water main benefits to perform [1]. Al2(SO3)3, FeCl3, Potassium dichromate were used for COD and KI, AgNO3 for determination of cyanide using titrimetric in the outlet waste water; and HNO3, HClO4 and HF were also used for digestion of the wastewater sample.

Equipment
The major equipment's like batch reactor was used for removal of heavy metal using adsorbent, shaker, FAAS (gases: acetylene + air) obtained from Ezana (Mekelle, Meli) model NovAA400P, Germany for estimation of metal ion concentration. The equipment like air tight container, oven drier, heating bath, micropipette, mechanical stirrers, common laboratory glassware (flasks, beaker), COD (digester, sucker), analytical instrument used for the preparation sorbent.
Analytical instruments like digital balance for weight measurement and pH-meter for reading pH were also used. The titrimetric by using standard solution to titrate cyanide, turbidity meter used to determine turbidity of the sample and digester for COD.

Preparation and Characterization of Eucalyptus Bark
The proximate analysis was characterized by the following: Where W1= weight of the dish, W2 = weight of the sample, W3 =weight of residue after drying.
Where W1 =weight of the dish, W2 = weight of the sample, W3 =weight of residue after igniting To determine the specific surface area of the adsorbent by Sear's method and the titrations were carried out with standard 0.1M NaOH in a thermostatic bath at 25 o C from pH 4.0 to pH 9.0 [8]: Where V is the volume of 0.1M NaOH solution in ml required to achieve pH of 9 from pH 4 with 0.5g of sorbent and 10g of NaCl in 50mL of distilled water.
The X-ray diffraction analysis was determined by crystalline index (CI) and calculated by Segal's formula using intensity measurement at 22.5º and 18.5º (amorphous background) 2θ [9]: Where, I2 denotes the maximum intensity of the 2 peak at about 2θ reading and Iam is the lowest intensity of the amorphous corresponding to 2θ value near to the peak 1.
The preparation of Bark of Eucalyptus tree species which were collected from the local area Addis Ababa and washed dried using oven and ground into small particles size [4], in these studies the sample was crushed manually into a small pieces and dried in sun light for 3days. It was then washed by distilled water to remove some foreign materials (dusts). Finally, the bark powder would dried in oven at 105°C for 24hrs [10]. The laboratory procedure was briefly described in Figure 1.

Preparation of sample wastewater
The wastewater sample was collected from Ezana gold mining factory discharged in to the dam by selecting from four sites (south, west, north, east inlet into the dam). For transporting and taking sample, plastic bottles were used and the bottles were cleaned using HNO3 acid and distilled water. The concentration of waste water from four sites and maximum value obtained for Cu (71ppm) at east outlet slurry in to the dam. The analysis was undergone using flame atmospheric spectroscopy (FAAS).

Determination of heavy metals (Cu, Fe, Mn, Pb, Cd, Cr, Ni, Co and Zn)
The blank sample was determined by transferring 100ml of distilled water in to beaker so as to allow a blank correction. First, the water sample was digested with 100cm 3 then, put in to beaker and finally five ml concentrated HNO3 was added. After that, the beaker placed on a hot plate and evaporated to about 20min then beakers were cooled, successively another five ml of concentrated HNO3 was added. The beakers were closed by watch glass and also put to the hot plate. The heating was continued, and until the solutions appear light colored and clear add small content of HNO3. The distilled water was used for washing of beaker wall and watch glass, and finally to remove some insoluble materials that could clog the atomizer the samples were filtered [9]. The elements that were accredited in the Ezana laboratory institution was Au, Ag, Cu, Zn, Ni, Pb and Co. However, the others non-accredited (Mn, Fe, Cr and Cd) were possible to read by changing the flame. Cu (II) and Pb (II) samples (40ml) were added into conical flasks of 250ml and 10ml HNO3, respectively with time 6hr, temperature 90-100 o C in the digester then dilution with 100ml flask and filled into test tubes then read using atomic adsorption spectroscopy (AAS).

Batch adsorption experiments
Adsorption efficiency in percent and milligram of copper adsorbed per gram of adsorbent was calculated: Where % R is percent removal, Co is initial concentration and Cf concentration after adsorption.

9)
Where q is metal ion removal in mg/g, Co is initial concentration, Cf is concentration after adsorption, M is adsorbent mass in gram and V is volume of wastewater used during the experiment [11].

pH
To determine the effect of pH on the adsorption process, to find the optimum pH and the experiments were carried out at three levels (2,5,8).

Contact time
The adsorption efficiency affected by contacting time and the optimum selected value for the process studied by conducting the experiment under 30, 90 and 150 min contact of the adsorbent.
The wastewater sample for various pH by keeping the amount of adsorbent dosage constant and for various adsorbent doses while keeping constant pH. Nevertheless, agitation speed in rpm, room temperature, particle size and initial concentration constant throughout the experiments.
These durations were selected because many studies in the literature show that too much contact time will not have a significant change on the adsorption [9], [6] and [12].

Adsorbent dose
The selected doses for the removal of Cu are 0.6, 1.8 and 3g, and the effect of adsorbent dosage on percentage removal efficiency of bark powder was studied [12] and [13]. The experiments were conducted under different pH, while keeping contact time constant and under different contact time while keeping constant pH.

Adsorption isotherm
This isotherm describes the equilibrium interaction between adsorbate concentration in the liquid phase and adsorbent surface at a given condition. The description how pollutants relate with the adsorbent materials, expression of the surface properties, abilities (capacity) of adsorbents, effective design of the adsorption systems. Thus, are critical for optimization of the adsorption mechanism pathways [4]. In this study, the experimental result was analyzed against two isotherm equations (Langmuir and Freundlich). The adsorption data for the removal of copper was correlated with Langmuir (Ce/qe) vs. (Ce) and Freundlich models of logCe vs. logqe adsorption isotherm. Its applicability was estimated with the correlation coefficient (R 2 ) which was found from the graph.

Characterization of adsorbent
This section describes physio-chemical and surface characteristics of eucalyptus bark produced at previously predicted optimum production parameters using equation 2.1 to 2.6.

Fourier transform infrared spectroscopy (FTIR) analysis
Some important changes in some bands were observed as presented in Figure 3.  [14].
The raw bark loaded with metal ion shows that the peaks are five as seen in Table 2 (before metal adsorption) and after binding with metals had shifted slightly from FTIR spectrum of table 2. This was due to the participation of these functional groups in the binding of metal ions. The shifting of wave number depends on the concentration of metal present in the given sample agreed with the literature [7] and [11]. The FTIR absorption frequencies (cm -1 ) were almost similar with many papers reported [19] and [8].

X-ray diffraction analysis
Eucalyptus bark showed two peaks ( Figure 4) [15], due to absence of great part of poly cellulose and lignin due to an increase of the cellulose amorphous fraction made possible for the hydroxyl ions directly act on cellulose fibers promoting hydrolysis of the cellulose chains. A crystalline index increasing directly related to rigidity of the cellulose structure and leads to higher tensile strength of the powder [19].

Experimental results of wastewater from tailing dam effluent
The present studies of laboratory analysis of Ezana (Meli) wastewater samples are presented in Table 3. The results were carried out with digesting and without digesting with acid to prevent precipitations due to high acidity of the samples and then diluted with deionized water. Table 3 The laboratory result and effluent standard to discharge in the comprehensive environmental response, compensation, and liability act (CERCLA) list of priority chemicals (2005) and for industrial waste water and water for drinking [20] and [3] Heavy The cyanide concentration in the process effluent measured during the night to get low temperature below 11°C in the site of out let effluent (discharging from the pipe) one-hour difference 0.025mg/l and in the afternoon were zero.

Parameters Laboratory result of present study
The pH of waste water in the dam gradually decreased over time and might be due to neutralization of the alkaline environment by rain water and possibly also as a result of CO2 uptake and other compounds which may contribute to acidity.

Effect of pH
As shown in figure 5 the effect of pH on adsorption showed the maximum removal of Cu occurred at pH 5 (90min, 1.8g) >pH 8 (90min, 3g)>pH 2 (150min, 1.8g). From pH 8 to pH 5 an increase in adsorption was observed and at low pH i.e. below pH 2 very less Cu was removed. At pH 2, the adsorption capacity was 58.9% at constant Cu (71ppm) which is not far from the result of almost 65% of Cu removal was observed at this pH at 100 mg/l Cu concentration [25]. The pH of sample wastewater decreased from 8 to 5 and the percentage removal rapidly increased (79.75% to 92%) and further decreasing pH to 2 leads the removal efficiency to rapidly decrease from 92% to 58.9%. At pH 5 with concentration of 50mg/l removal efficiency of copper by algae as adsorbent from mining waste water was 87% [25]. For adsorption this heavy metal the efficiency of the adsorbent at pH 8 was found to be 79.75%. This value agreed with the explanation to optimum pH values (3)(4)(5)(6) and linked H + was released from the active sites, and adsorbed amount of metal ions was increased [14,23].
The results in this factor variation of removal from pH 8 to 2 have varied almost simultaneously and this might be due to further increase of the pH leads to the precipitation rather than adsorption. Generally, after pH 5 there was a slight decrease in adsorption and at low pH i.e. below and above pH 5 very less copper was removed. The metal uptake capacity also increased due to the presence of negatively charged functional groups on the adsorbent surface, resulting in increased binding sites with increasing the pH [26].

Figure 5
Effect of pH on copper ions removal efficiency on to Eyuclapltus bark (EB)

Effect of adsorbent dosage
As seen from Figure 6, the removal capacity increased with increasing the amount of adsorbent dosage for most of the result with small variation. The removal efficiency of Cu (II) by EB based on the dose 0.6g, 150min, pH 5, 1.8g, 90min, pH 5 and 3g, 30min, pH 5 were 77.25%, 92% and 79.75%, respectively. Adsorption of metals often refuse the formation of link between metal ion and the active site due to excessive protonation at low pH <3 of the active site at the carbon surface . Usage of Eucalyptus bark (EB) as a dosage increased in percentage of removal increased may be due to the availability of more active sites in adsorbent surface [27].

Adsorption isotherm
The isotherms i.e. Langmuir and Freundlich are the most commonly used models to describe the experimental result of adsorption. In the current study, Langmuir and Freundlich were applied to examine the adsorption process of Cu (II) at different conditions of factors. Form these two the best model is Langmuir adsorption. As result, it is successfully applied in many adsorption processes of other reported.

Desorption experiments
Even if, the desorption efficiency increased with an increase in the concentration from 0.1M of the agent (HCL, H2SO4 and HNO3) after some time the bio sorbent deteriorating at higher concentration of the same and it was noticed that HCl gave maximum desorption (99%) as compared to H2SO4 (83%), HNO3 (74%) and H2O2 [4].