April 2014, Volume 5, No.2 International Journal of Chemical and Environmental Engineering
Removal of lead from wastewater by adsorption Using Saudi Arabian Clay Farooq Ahmad *, Elsayed Fouad , and Naveed Ahmad Department of Chemical and Materials Engineering, Northern Border University , Kingdom of Saudi Arabia * Corresponding Author E-mail:
[email protected]
Abstract: Heavy metal ions such as cadmium, zinc, copper, lead and nickel are hazardous to both human life and the environment. In the present work study on the feasibility of the thermally treated Saudi Arabian clay for the removal of lead was carried out. The removal of lead (Pb2+) using thermally treated Saudi Arabian clay as a low-cost alternative natural adsorbent from aqueous solutions is investigated. Batch adsorption kinetic studies show the adsorption of lead metal ions (Pb2+) is strongly affected by initial pH of the solution, initial metal ion concentration and adsorbent doses. It has been found that the amount of adsorption of lead increases with initial metal ion concentration and solution pH but decreases with the amount of adsorbent. The adsorbent exhibited good sorption potential for lead at pH 6. Freundlich isotherm adsorption equations reasonably describe the adsorption isotherm within the metal ion concentration range. It has been established that thermally treated Saudi Arabian clay has the potential to be used for large scale separation of lead from wastewater. Keywords: Adsorption, Freudilich isotherm, Adsorption kinetics, Saudi Arabian clay, Lead removal
1. Introduction Heavy metal discharge into environment causes a serious problems to the human and plant by causing soil and water pollution. Lead, Chromium (VI), Zinc, Arsenic and Copper (II) are among the most hazardous as they tend to accumulate in the human body causing numerous problems [1]. As a result of increased health awareness, their removal from solution has received detailed attention. Heavy metals are a very real danger in our modern world. An estimated 65% of North Americans have high levels of heavy metals in their bodies. With scarce public recognition for the harm they cause, these heavy metals do untold damage to people’s lives in terms of heart disease, cancer, skin disorder, neurological disease, brain fog and depression [2,3]. Many methods such as Chemical precipitation, ion exchange, membrane filtration, and electrochemical treatment and adsorption technologies were employed for the removal of heavy metals from aqueous stream. All these methods have their inherent advantages and limitations. Chemical precipitation is relatively simple and inexpensive. However, the chemical precipitation is ineffective when the heavy metal ion concentration is very high. Further, chemical precipitation produces large amount of sludge to be treated and thus it becomes less economic. Mirbagheri and Hosseni in 2005 used hydroxide precipitation process for the removal of chromium from waste water [4]. Maximum precipitation
of Chromium occurred at pH 8.7 and the concentration of chromate was reduced from 30 mg/l to 0.01mg/l. Ion exchange has been applied widely for the removal of heavy metal from the aqueous stream. The most common cation exchangers are strongly acidic resins with sulfonic acid groups and weekly acid groups with carboxylic groups. In 2006 Gode and Pehlivian found that update of heavy metals ions by ion exchange resin is strongly affected by pH, temperature, initial metal concentration and contact time[5]. Although it is widely applied but it still has some short comings. Ion exchange resins needs periodic regeneration treatments and thus it causes a secondary pollution. In this way it is expensive and less economic when treating a large amount of waste water. Adsorption is well established method for the removal of heavy metal from waste water. Activated carbon adsorbents are widely used for the removal of heavy metal from industrial waste water. Its usefulness has been derived from the fact that it has large micro pore and meso pore volumes and the resulting high surface area. However, the high cost of activated carbon has restricted its use [6]. Searching a low cost activated carbon has become a main focus of research. To date, hundreds of studies on the low cost adsorbents have been published. Studies have shown that natural clay is an effective adsorbent for heavy metal removal due to its efficiency, low cost and its availability. The adsorption capabilities of the natural clay are attributable due to their high surface area and exchange capacities [7, 8]. Clay
Removal of lead from wastewater by adsorption Using Saudi Arabian Clay
possesses negative charge which can attract positive metal ions. The high specific area and the ability of holding water in the inter layer sites has given clay higher adsorbent capacities which can be further increased by acid and thermal treatments [9]. Jiang et al. investigated the kaolin clay obtained from longyan china to remove heavy metal ions such as lead, cadmium, nickel and copper from waste water. The uptake was maximum and the maximum adsorption being observed within 30 minutes. They reduced the lead concentration from 160mg/l to 8.00 mg/l [10]. Natural and modified clay have been used extensively by many researchers for the removal of heavy metals from industrial waste water. Kaolin and montmorllonite have been successfully employed as adsorbents for the removal of heavy metals from industrial waste water. In the present study the potential of Saudi Arabian clay as an adsorbent for the removal of lead from aqueous solution is explored. The effect of pH, adsorbent dosage and initial metal ion concentration on adsorption are examined. Furthermore the adsorption data are analyzed using Langmuir and Freudlich isotherms. Freundlich isotherm adsorption equation is found to describe reasonably the adsorption isotherm within the metal ion concentration range.
concentration retained in the adsorbent phase was calculated according to
qt
Co Ct V m
where Co (mg/L) and Ct are the concentration in the solution at time t=0 and at time, t, V is the volume of solution (L) and m is the amount of adsorbent (g) added
3.Results and Discussion 3.1 Effect of Initial Metal Ion Concentration Batch adsorption experiments were performed by contacting 0.2 g of the thermally activated clay with 100ml of the aqueous solution of different initial concentration (5ppm, 10ppm and 20ppm). The mixture was shaken in a rotary shaker and 5 ml samples of solution were withdrawn from the bottle sample at known time intervals. Preliminary experiment was used to determine the contact time for the batch tests. The sample was filtered to remove any fine particles and analyzed for the metal ions. The effect of metal ion concentration on adsorption is shown in Fig.1.
2.Materials and Method
Time vs qt at 5 ppm Time vs qt at 10 ppm Time vs qt at 20 ppm
10
(qt) Amount of adsorption
2.1Material Stock solution of 1000 ppm of lead was prepared using lead nitrate (Pb(NO3)2) salt because of its good solubility in water. The stock solution containing 1000 mg/L of standard Pb(II) were prepared by dissolving 1.60 g of Pb(NO3)2 [11]. Standard solution of particular Pb(II) concentration was prepared by proper dilution with 1000 mL of distilled water. The clay used as an adsorbent was first thermally treated.
8 6 4 2 0
0
2.2Activation of Clay
50
100
150
200
250
Time (min)
Locally obtained clay samples were first activated using thermal method. In the thermal activation, screened clay was heated in oven at 100oC for four hours without any chemical treatment. The dried activated clay was pulverized and stored in air-tight containers. The same technique was applied by many researchers [12].
Figure.1: Effect of Initial Metal Ion Concentration.
From these plots, it is found that the amount of adsorption increases with increasing contact time at each initial metal ions concentration and equilibrium is attained within about 165 min for the systems. Further, it was observed that the amount of metal ion uptake, qt is increased with increase in initial metal ion concentration. At lower concentration, lead ion in the solution would interact with the binding sites and thus facilitated almost 100% adsorption while at higher concentration, more lead are left un-adsorbed in the solution due to saturation of the binding sites.
2.3 Adsorption Experiment A measured quantity of the adsorbent and 25 ml of the metal ion solution was shaken in a shaker at 120 rpm at constant temperature for a given time and then the su pensions was filtered through a 0.45 μm syringe filter [11]. The filtrate was analyzed using flame atomic absorption spectrophotometer with air-acetylene flame. The pH of the liquid was adjusted with dilute HCl and NaOH solution. All the experiments were carried out by varying the initial metal ion concentration, amount of adsorbent and the pH. Adsorption rate is measured according to predefined procedure with both metal ion concentration ranging from 5.0 to 50 mg/L [11]. The metal ion
3.2 Effect of pH Effect of initial pH of solution on adsorption was determined by mixing 0.2 g of adsorbent with 100 ml of solution containing metal concentration of 20 ppm at various pH values ranging from 3.0 to 6.0. Solution pH
66
Removal of lead from wastewater by adsorption Using Saudi Arabian Clay
was adjusted with 0.5M HCl and NaOH solutions. The mixture was shaken for 1 hour and the solution was filtered and analyzed. The pH of aqueous solution is an important controlling parameter in the adsorption process. 10.0
(qt) Amount of adsorption
pH vs qt 9.5 9.0 8.5
Figure 3: Effect of Adsorbent Dose 8.0
It shows that Pb+2 adsorbed per unit weight of adsorbent decrease as the adsorbent mass increases. This is due to the fact that at higher adsorbent dose the solution ion concentration drops to lower value and the system reaches equilibrium at lower value of qt indicating the adsorption sites remain unsaturated. Similar observations are also reported by various researchers [13-14]. The adsorption isotherm study was carried out for the removal of metal ion using initial metal ion concentration of between 5 and 20 ppm at an adsorbent dosage level of 0.2g for Pb+2 250C. The measured adsorption equilibrium data are fitted with Langmuir and Freundlich adsorption isotherm. These models were used to describe the adsorption of lead ions within the initial metal ion concentration range. The Freundlich adsorption isotherm, which assumes that adsorption takes place on heterogeneous surfaces, can be expressed as
7.5 7.0 6.5 2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
pH
Figure 2: Effect of Initial pH of the Solution.
Fig.2 shows the effect of pH on amount of metal ion adsorbed, qt (mg/g), where qt was found from the mass balance equation which is given by eq(1). Fig.2 also shows that Pb+2 adsorption reach the highest point at pH of 6. It means that the removal of metal ions was found to increase when the solution pH was increased from 3.0 to 6.0 for the system. This phenomenon can be explained by the surface charge of the adsorbent and the H+ ions present in the solution. At low pH, the cations compete with the H+ ions in the solution for the active sites and therefore lower adsorption. The pH range was chosen as 3-6 in order to avoid metal hydroxides, which has been estimated to occur at pH> 6.5 for Pb(OH)2 (Tarun Kumar Naiya, Ashim Kumar Bhattacharya, Sailendranath Mandal and Sudip Kumar Das, 2009). In addition when pH increases, there is a decrease in positive surface charge (since the deprotonation of the sorbent functional groups could be occur), which results in a lower electrostatic repulsion between the positively charged metal ion and the surface of activated carbon thus favoring adsorption.
1 ln qe ln K f (ln Ce ) n
(2)
Where qe is the amount of lead ions adsorbed at equilibrium time, Ce is equilibrium concentration of lead metal ions in solution. Kf and n are isotherm constants which indicate the capacity and the intensity of the adsorption, respectively and can be calculated from this plot (refer to Fig.4.) are 3.294 and -0.445 respectively. Langmuir isotherm can also be used for the analysis of our data but for our research work we will fit our adsorption data with the Freundlich isotherm since this type of isotherm is used for solid-liquid systems.
3.3 Effect of Adsorbent Dose The effect of adsorbent dosage was studied by batch experiment of 0.2 g adsorbent in 100 ml of 20 ppm Pb(II) solutions. The solution was shaken continuously over rotary shaker at constant speed of 120 rpm. 5ml samples of solution were withdrawn from the sample bottle at known time intervals. The sample was filtered and analyzed. The experiment was repeated by using different amount of adsorbent of 0.3 g and 0.5 g. The results of the kinetic experiments with varying adsorbent concentrations are presented in Fig.3.
2.4
ln(Ce) vs ln(qe)
2.2
Plot 1 Regr
2.0
ln(qe)
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 -0.6
-0.4
-0.2
0.0
0.2
0.4
ln(Ce)
Figure 4: Freundlich isotherm plot for thermally activated clay as adsorbent (initial Pb2+= 5,10 and 20 ppm; amount of Clay= 0.2g; shaker speed= 120 rpm)
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Removal of lead from wastewater by adsorption Using Saudi Arabian Clay [11] Tarun, K. N, Ashim,K.B, Sailendranath, M and Sudip, K.D.. The sorption of lead (II) ions on rice husk ash. J.Hazard.Mater.14 (2009),pp.1254-1264.
4.Conclusion
[12] Igbokwe, P.K,Olebunne, F.L, Nwakaudu, M.S. Effect of activation parameters on conversion in clay catalyzed esterification of acetic aicd. IJBAS-IJENS. 11(2011),pp.1-8.
Saudi Arabian clay used as an adsorbent has the potential to remove Pb+2 in polluted water. The results indicate that Saudi Arabian clay is a low-cost good adsorbent for the removal of lead (Pb+2) ions form aqueous solution. Batch adsorption kinetic studies show that the adsorption of lead metal ions (Pb+2) is strongly affected by initial pH of the solution, initial metal ion concentration and adsorbent doses. The amount of metal ion (Pb+2) adsorption on thermally activated clay increases with initial metal ion concentration and pH of solution but decrease with the amount of adsorbent. pH is one of the important parameters for metal ion adsorption on clay and it has found that lead adsorption increases with increasing pH of the solution.
[13] M. Sekar, V. Sakthi, S. Rengaraj. Kinetics and equilibrium adsorption study of lead(II) onto activated carbon prepared from coconut shell. J. Colloid Interface Sci., 15(2004),pp.307-313. [14] Tushar, K. S and Meimoan, V.S. Removal of Cadmium metal ion from its aqueous solution by Aluminium oxide: A kinetic and equilibrium study. Chem. Eng. J., 16(2008), pp. 256-262
Acknowledgment The financial support from Northern Boarder University is gratefully acknowledged
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