AN EVALUATION OF AN ADSORPTION AND KINETIC MODELLING OF HEAVY METAL UPTAKE FROM WASTE WATER EFFLUENTS
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Adsorption and kinetic modelling of heavy metals uptake from wastewater effluents using indigenous cellulose based waste biomass, such as nipa palm nut (NPN), palmyra palm nut (PPN), oil palm empty fruit bunch (EFB), oil palm fibre (OPF), and oil palm shell (OPS), as potential raw materials for the preparation of activated carbons was studied. Full factorial design of experiments was used to correlate the preparation variables (activation temperature, activation time and acid impregnation ratio) to the lead and copper uptake from aqueous solution. Minitab Release 11.21 was used for statistical analysis.
The optimum conditions for preparing activated carbon from NPN for Pb2+ adsorption were as follows: activation temperature of 5000C, activation time of 1hr and acid impregnation ratio of 2:1, which resulted in 99.82% uptake of Pb2+ and 30.20% of activated carbon yield. Different results were obtained for other adsorbents. The experimental results obtained agreed satisfactorily with the model predictions. Proximate analysis was carried out in order to determine the ash content, fixed carbon, volatile matter and moisture content of the raw materials and activated carbons produced under optimum conditions. Bulk density, pH, iodine number and surface area of the activated carbons were also determined.
The Fourier Transform Infrared Spectra of the activated carbons indicated the presence of hydroxyl, carboxyl, lactones, phenols and amino groups which are important sorption sites. The results of adsorption studies showed that activated carbons produced from OPS and NPN are the most efficient adsorbents for the removal of Pb2+ from aqueous solutions while NPN and PPN are the most efficient for Cu2+ removal. The amount of Pb2+ and Cu2+ adsorbed was found to be dependent on adsorbent dosage, pH, initial ion concentration, particle size, contact time and temperature. Equilibrium data fitted well to the Freundlich, Langmuir and Tempkin models. The equilibrium data was best described by Freundlich model. The kinetic data obeyed the pseudo first-order, pseudo second-order, Elovich and Bhattachanya-Venkobachor models. Pseudo second-order best described the kinetics of the adsorption process. The determined negative free energy changes (∆G) and positive entropy changes (∆S) indicate the feasibility and spontaneous nature of the adsorption process. The positive values of enthalpy change (∆H) suggest that the adsorption process is endothermic. A mini packed bed adsorption column was fabricated and used for continuous adsorption study. The column experiments showed that adsorption efficiency increased with increase in the influent concentration and bed depth and decreased with increasing flow rate. Column adsorption kinetics was described with Thomas and Yoon and Nelson models and the kinetic constants. The comparison of the experimental breakthrough curves to the breakthrough profiles calculated by Yoon and Nelson method showed a satisfactory fit for all the adsorbents.
1.1 BACKGROUND OF THE STUDY
Industrial wastewater represents the main source of environmental pollution with heavy metals, e.g. Cu, Pb, Fe, Cd, Mn, etc. Such metal may be discharged into the wastewater from various industries, including metal plating, storage batteries, alloy industries, dying, textile, fertilizers and other chemical industries. The progressive increase of industrial technology result in continuous increase of pollution, so that a great effort has been devoted for minimizing these hazardous pollutants and therefore avoiding their dangerous effects on animals, plants and humans (Al-Omair and El- Sharkawy, 2001)
Heavy metal ions are reported as priority pollutants, due to their mobility in natural water ecosystems and due to their toxicity (Volesky and Holan, 1995). These heavy metals are not biodegradable and their presence in streams and lakes leads to bioaccumulation in living organisms causing health problems in animals, plants, and human beings (Ong et al, 2007)
Lead is a pollutant that is present in drinking water and in air. In air, it is derived from lead emissions from automobiles because it is used as an anti knocking agent in the form of lead tetraethyl in gasoline. In water, lead is released in effluent from lead treatment and recovery industries, especially from lead battery manufacturing units. Lead is toxic to living organisms and if released into the environment can both accumulate and enter the food chain. Lead is known to cause mental retardations, reduces haemoglobin production necessary for oxygen transport and it interferes with normal cellular metabolism (Qaiser et al, 2007). Lead has damaging effects on body nervous system. It reduces 1.Q Level in children. Lead is used as industrial raw materials in the manufacture of storage batteries, pigments, leaded glass, fuel, photographic materials, matches and explosives (Raji and Anirudhan, 1997). For drinking water, the maximum permissible limit of lead is 0.1mg/l (WHO, 1971). The maximum concentration allowed for discharge into inland water is less than 1mg/l (FEPA, 1991).
Copper is one of the few metallic elements found in the earth’s crust. It constitutes 70mg/kg of the earth’s crust, occurring as a constituent of several ores like chalcopyrite (CuFeS2 ), which is about 50% of total world copper deposits ( Maheswari et al, 2008 ). Copper is an essential nutrient to all higher plants and animals. In humans, it is found primarily in the blood stream as a cofactor in various enzymes and in Cubased pigments. Copper is metal that has a wide range of applications due to its good properties. It is used in electronics, for production of wires, sheets, tubes, and also to form alloys (Antonijevic and Petrovic, 2008). Since copper is a widely used material, there are many actual or potential sources of copper pollution. Copper may be found as a contaminant in food, especially shellfish, liver, mushrooms, nuts, and chocolate. Copper is essential to life and health but, like all heavy metals, is potentially toxic as well. For example, continued inhalation of copper-containing spray is linked with an increase in lung cancer among exposed workers. For drinking water, the maximum permissible limit of copper is1.5mg/l (WHO, 1971).
The maximum concentration allowed for discharge into inland water is less than 1mg/l (FEPA, 1991). The main physiological processes in which copper participates in the formation of blood and utilization of iron in haemoglobin synthesis, the synthesis and cross linking of clastin and collagen in the aorta and major blood vessels, etc. Various disorders such as nephritic syndrome (Mason, 1979), copper intoxication and burning injuries (Kaur, et al, 2008), hematemesis, melena, coma and jaundice (Maheswari et al, 2008 ), have been associated with higher concentration of copper.
Several methods such as ion exchange, solvent extraction, reverse osmosis, precipitation, and adsorption have been proposed for the treatment of wastewater contaminated with heavy metals (Gupta, 2003) Among several chemical and physical methods, the adsorption onto activated carbon has been found to be superior to other techniques in water-re-use methodology because of its capability for adsorbing a broad range of different types of adsorbates efficiently, and its simplicity of design (Ahmad et al, 2006). The major advantages of an adsorption system for water pollution control are less investment in terms of initial cost and land, simple design and easy operation, and no effect of toxic substances compared to conventional biological treatment processes (Markovska et al, 2006).
However, commercially available activated carbons are still considered expensive (Chakraborty et al, 2005). Consequently, many researchers have studied cheaper substitutes, which are relatively inexpensive, and are at the same time endowed with reasonable adsorptive capacity. These studies include the use of coal (Mohan et al, 2002), fly ash (Mohan et al, 2002; Nollet et al, 2003; Gupta, 2003; Ricou et al, 2001, Gupta and Ali, 2004), activated clay (Wu et al, 2001), palm–fruit bunch (Nassar, 1997),
Bagasse pith (Mckay;1998), bentonite, slag and fly ash (Ramakrishna and Viraraghavan, 1997; Bereket et al, 1997), rice husk (Low and Lee, 1997), wood charcoal (Keerthinarayana, and Bandyopadhyay, 1997), hazelnut shell (Kobya,2004), coconut shell (Goel, et al 2004); peat (Brown et al, 2000; Ho and Mckay, 2000), etc Activated carbons are versatile adsorbents (Castro et al, 2000). Activated carbons are becoming more and more interesting on account of their excellent properties as adsorbents, which make it possible to use them in purification and pollutant removal from both liquid and gaseous media (Sanchez et al, 2006).
Their adsorptive properties are due to their surface area, a micro porous structure, and a high degree of surface reactivity. Activated carbons are usually obtained from materials with high carbon content and possess a great adsorption capacity, which is mainly determined by their porous structure (Otero et al, 2003). The inherent nature of the precursor or starting material, as well as the method and conditions employed for carbon synthesis, strongly affects the final pore size distribution and the adsorption properties of the activated carbons (Shopova et al, 1997, Biota et al, 1997). In recent years, special emphasis on the preparation of activated carbons from several agricultural by products has been given due to the growing interest in low cost activated carbons from renewable, safe, copious supplies, especially for applications concerning treatment of drinking water and wastewater (Castro et al, 2000).
The selection of solid wastes as precursor for activated carbon depends on the potential for obtaining high quality activated carbon, presence of minimum inorganics, volume and cost of raw materials and storage life of raw materials (Al-Omair and EL-
Active carbons are unique and versatile adsorbents, and they are used extensively for the removal of undesirable colour, taste, and other organic and inorganic impurities from domestic and industrial waste water, solvent recovery, air purification in inhabited places, restaurants, food processing, and chemical industries; in the removal of colour from various syrups and pharmaceutical products; in air pollution control from industrial and automobile exhausts; in the purification of many chemical, pharmaceutical and food products; and in a variety of gas-phase applications(Bansal and Goyal, 2005)
There are two methods of preparing activated carbons: physical and chemical activation. The advantage of chemical activation over physical activation is that it is performed in one step and at relatively low temperatures. The most important and commonly used activating agents are phosphoric acid, zinc chloride and alkaline metal compounds, such as KOH (Serrano-Gomez et al, 2005; Srinivasakannan and Zailani, 2004). There have been a number of works describing phosphoric acid activation of different precursors, and some advantages of this process, in comparison with the more studied physical activation, have been printed out (Castro et al, 2000; Girgis et al, 1994; Philip et al, 1996). Phosphorous acid activation only involves a single heat treatment step and activation is achieved at lower temperatures. Higher yields are obtained and most of the phosphoric acid can be recovered after the process is completed. In addition, the use of chemical reagents allows another degree of freedom in the choice of process conditions (Solum et al, 1995). Johns et al (1999) have used the physical activation procedure for the production of activated carbon using steam and carbon dioxide. The characteristics of activated carbon depend on the physical and chemical properties of the precursor as well as on the activation method.Apart from the starting material and the oxidizing agent; activation time and temperature affect the structural properties of the resulting activated carbon. Many researchers observed that BET surface area and pore volume increased with activation and temperature (Guo and
Chong, 2002; Villegas-Pastor and Valle-Duran, 2001; Yang, and Chong, 2003)
Adsorption equilibrium is the most fundamental data on an adsorption system (Lee et al, 2006). It is also very important in model prediction for analyzing and designing an adsorption process. Adsorption is usually described through isotherms, that is, the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (if liquid) at constant temperature (Metcalf and Eddy, 2003). Adsorption isotherms are developed by exposing a given amount of adsorbate in a fixed volume of
liquid to varying amounts of activated carbon.
The study of sorption kinetics in wastewater treatment is important since it provides valuable insights into the reaction pathways and mechanism of adsorption process (Mincera et al, 2008). A study of adsorption kinetics provides information about the mechanism of adsorption, which is important for the efficiency of the process (Maximova and Koumanova, 2008). Also the kinetics describes the solute uptake rate and mass transfer resistance at the solid-solution interface.
In order to examine the mechanism of adsorption process such as mass transfer and chemical reaction, a suitable kinetic model is needed to analyse the rate data (Ozacar,
2003). Many models such as homogeneous surface diffusion model, pore diffusion model, and heterogeneous diffusion model (also known as pore and diffusion model) have been extensively applied in batch reactors to describe the transport of adsorbates inside the adsorbent particles (Wu et al, 2001a-c; Raven et al, 1998). These kinetic models are useful for the design and optimisation of effluent – treatment process (Sivakumar and Palanisamy, 2009).
The reaction rate can be calculated from the knowledge of kinetic data. However, the changes in reaction that can be expected during the sorption process require the knowledge of thermodynamic parameters. The three main thermodynamic parameters include enthalpy of adsorption (∆H), Gibbs free energy charge (∆G) and entropy of adsorption (∆S).
The main feature of the dynamic behaviour of fixed-bed adsorption is the history of effluent concentration (Tien, 1994). These concentration-time curves (or their equivalents) are commonly referred to as the breakthrough curves, and the time at which the effluent concentration reaches the threshold value is called the breakthrough time. It is obvious that rational design of adsorption systems should be based on accurate predictions of breakthrough curves for specific conditions.
Although the fixed-bed mode is highly useful, its analysis is unexpectedly complex. Fixed-bed operation is influenced by equilibrium (isotherm and capacity), kinetic (diffusion and convention coefficients), and hydraulic (liquid hold-up, geometric analysis, and mal-distribution) factors (Inglezakis and Poulopoulos, 2006).
1.2 MOTIVATION FOR THE WORK
The contamination of water by toxic metals through the discharge of industrial wastewater is becoming a serious environment problem in Cameroon. Heavy metals ions are reported as priority pollutants, due to their mobility in natural water ecosystems and due to their toxicity. The presence of these heavy metals in streams and surface waters has been responsible for several health problems with animals, plants and human beings.
Cameroon imports large quantities of AC annually and at very high cost.
Studies show that there are more than enough of the agricultural waste raw materials available for activated carbon production to meet local demand.
`Activated carbons have excellent properties as adsorbents, which make it possible to use them in purification and pollutant removal from both liquid and gaseous media.
1.3 AIMS AND OBJECTIVES
In this study, the use of waste biomass of oil palm shell(OPS), palmyra palm nut(PPN), oil palm empty fruit bunch(EFB), oil palm fibre(OPF), and nipa palm nut (NPN) as low-cost adsorbents for the removal of toxic metals (lead and copper) from aqueous solution is investigated. In addition, adsorption and kinetic modeling of the uptake of these metals from aqueous solution is done.
This research will achieve the following objectives:
To prepare activated carbons from agricultural raw materials using full factorial design of experiment.
To characterize the prepared active carbons with respect to their bulk density, ash content, moisture content, pH, surface area, fixed carbon, iodine number, surface characteristics, etc.
To investigate the effect of heat-treatment temperature, weight ratio of phosphoric acid to precursor in the impregnation step, and carbonization time on adsorption of heavy metal from aqueous solutions.
To determine the adsorptive capacity of prepared active carbons.
To study the influence of batch sorption specific parameters, such as initial metal concentration, adsorbent dosage, particle size, pH, contact time and temperature.
To study adsorption isotherms using four model equations – Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich equations.
To investigate the kinetics of heavy metal adsorption on the activated carbons using first- order, second- order, Pseudo first- order, Pseudo second-order, Bhattacharya-Venkobachor model, Elovich equation, and Weber-Morris intra- particle diffusion models to test the kinetic data.
To determinate thermodynamics parameters such as ΔG, ΔH and ∆S as well as energy of activation.
To fabricate mini packed bed adsorption column.
To study breakthrough curves using locally fabricated packed bed adsorption column.
To investigate the efficiency of lead and copper removal in a packed column with respect to experimental parameters, such as bed height, initial concentration and flow rate.
To study the kinetics of lead and copper adsorption in a packed bed.
To model dynamic adsorption behaviour of lead and copper in packed bed column
1.4 SIGNIFICANCE OF THE STUDY
With the rapid development of chemical and petroleum processing industries in Cameroon, there is a rapid increase in the amount and the variety of chemicals that are discharged into waters. Wastewater from various industries and municipal corporations are discharged into ground and surface water, making it unfit for human and animal consumption. Some of the organic and inorganic compounds, when present in water, are toxic, carcinogenic, and mutagenic, and cause several ailments in humans.