INTRODUCTION
Hazardous dyes, pigments and metals which are created from dye manufacturing, textile and pulp and paper industries are released into wastewaters (Jain et al., 2003; Mittal et al., 2009). This typically makes treating of water pollution problematic, because the color tends not to eliminate even after the conventional removal processes has been completed (Jain et al., 2003; Mittal et al., 2010; Visa et al., 2010; Gupta et al., 2012).
Dye pollutions in water usually prevent light dispersion in water, this therefore, disturb photosynthesis (Banerjee and Chattopadhyaya 2013; Hameed et al., 2013; Hajati et al., 2014). The cleaning of wastewater from color dyestuff is naturally important (Gupta et al., 2012; Ghaedi et al., 2013), since the extensive application of dyes and the many hazards and toxic byproducts cannot be stopped (Mittal et al. 2010), Artificial origins of aromatic dyes are even biologically non-degradable and their treatment by other effective conventional procedure is impossible (Mittal et al., 2009; Saleh and Gupta 2012).
Numerous methods are available for color removal from waters and wastewaters, this comprise: membrane separation, aerobic and anaerobic degradation using various microorganisms, chemical oxidation, coagulation and flocculation, and reverse osmosis (Mittal et al., 2009, 2010; Gupta et al., 2012; Karthikeyan et al., 2012). They is however limits in some of these techniques in as much as they have been operative. These limits include; excess amount of chemical usage, accumulation of concentrated sludge that has serious disposal problems and lack of effective color reduction (Jain et al., 2003; Mittal et al., 2009; Saleh and Gupta 2012).
1.1 Adsorption technique
The adsorption technique, is centered on the transfer of pollutants from the solution to the solid phase. Adsorption is one of the effective and general wastewater treatment method (Gupta et al., 2011; Ghaedi et al., 2012; Saleh and Gupta 2012). This process is better than other dye removal techniques in terms of initial cost, simplicity of design, ease of operation, and non-toxicity of the utilized adsorbents compared to other conventional wastewater treatment methods (Kismir and Aroguz 2011).
1.2 Activated carbon
Activated carbon, is a generally used adsorbent in industrial processes, it is made of a microporous, homogenous structure with high surface area and shows radiation stability (Iqbal and Ashiq 2007; Mittal et al., 2010). There is a great importance in finding inexpensive and effective alternatives to the existing commercial activated carbon (CAC) (Al-Othman et al., 2013). Discovering effective and low cost activated carbon may contribute to environmental sustainability and offer benefits for future commercial applications (Gupta et al., 2011).
Activated carbon is a form of carbon that has been treated to make it very porous and thus to have a very large surface area accessible for adsorption or chemical reactions. Sufficient activation for useful applications may come solely from the high surface area, though further chemical treatment often enhances the adsorbing properties of the material. (Eddleston et al.,2008).
Activated carbon is formed from carbonaceous source materials like nutshells, wood and coal. Activated carbon is used in gas purification, gold purification, metal extraction, water purification, medicine, sewage treatment, air filters in gas masks and filter masks, filters in compressed air and many other applications. One major industrial application involves the use of activated carbon in metal finishing field. It is very widely employed for the purification of electroplating solutions. For example, it is a main purification technique for removing organic impurities from bright nickel plating solutions.
Carbon as adsorbent has many applications in eliminating pollutants from air or water streams both in the field and in industrial processes such as: Spill cleanup, Groundwater remediation, Drinking water filtration, Air purification, Volatile organic compounds capture from painting, dry cleaning, gasoline dispensing operations, and other processes. Activated charcoal is also used for the measurement of radon concentration in air. Filters with activated carbon are usually used in compressed air and gas purification to remove oil vapours, odours, and other hydrocarbons from the air. Activated carbon filters can be used to filter vodka and whisky of organic impurities which can affect colour, taste, and odour (Eddleston et al.,2008).
Activated carbon can be produced in one of the following processes: Physical reactivation: The precursor is developed into activated carbons using one or a combination of the following processes:
i. Carbonization: Material with carbon content is pyrolyzed at temperatures in the range 600-900 °C, in absence of air (usually in inert atmosphere with gases like argon or nitrogen).
ii. Activation/Oxidation: Raw material or carbonised material is exposed to carbon dioxide, oxygen, or steam at temperatures above 250 °C, usually in the temperature range of 600-1200 °C (Eddleston et al.,2008).
Chemical activation: Before carbonization, the raw material is saturated with certain chemicals. The chemical is typically a strong acid, strong base, or a salt (phosphoric acid, potassium hydroxide, sodium hydroxide, zinc chloride, respectively). Before, the raw material is carbonized at lower temperatures (450-900 °C). It is believed that the carbonization / activation step proceeds concurrently with the chemical activation. This technique can be challenging in some cases, because, for example, zinc trace residues may remain in the end product. However, chemical activation is preferred over physical activation owing to the lower temperatures and shorter time needed for activating material (Eddleston et al., 2008).
1.3 Moringa oleifera
M. oleifera tree also identified as a drumstick tree is a rapid growing deciduous shrub or small tree of about 13 m tall and 35 cm in diameter with an umbrella-shaped open cap (Anjorin et al. 2010). M. oleifera is the most widely distributed species of the Moringa ceae family throughout the World (Kansal and Kumari 2014; Reddy et al., 2010). When matured, the fruit becomes brown and has 10–50 seeds inside (Anjorin et al., 2010). The tree is native to India but has been planted around the world and is naturalized in many locales. It has also been reported (Hsu et al., 2006) that M. oleifera oil and micronutrients contain antitumor, antiepileptic, antidiuretic, anti-inflammatory and venomous bite characters. M. oleifera contains specific plant pigments with demonstrated powerful anti-oxidative ability such as vitamins C, E, A, caffe-oylquinic acids, carotenoids-lutein, alpha-carotene and beta carotene, kaempferol, quercetin, rutin (Ho 1994; Siddhuraju and Becker 2003; Aslam et al. 2005).
In Nigeria, the worth of M. oleifera plant has been so recognized that the Federal Government of Nigeria has set up M. oleifera farm and it is found everywhere due to its medicinal values and ability to cure various ailments.
The use of M. oleifera seed shell as an absorbent has not been recorded. There was no study on the characterization and the use of treated M. oleifera seed shell as adsorbent.
1.4 Theoretical Framework
Adsorption is a surface occurrence, which arises due to interactions between the individual atoms, ions or molecules of an adsorbate and those present in the adsorbent surface. Adsorption can also be described as a process that occurs when a gas or liquid solute gathers on the surface of a solid or a liquid (adsorbent), forming a molecular or atomic film (the adsorbate). The process involves in the selection of occurrences that can alter the distribution of the solute among the constituent phases and in the interfaces (Weber et al., 1991). The process is accompanied by separation of the solute from one phase to another following its accumulation at the surface of the latter.
Adsorption involves different types of attractive forces between adsorbate, adsorbent and solvent molecules. Such forces usually act in concert, but one particular type may be more predominant than the others in any particular situation (Ho and McKay, 1999). Adsorption interactions generally lead to the accumulation of concentration at a surface and may be essentially represented as arising from attraction of adsorbate molecules (gaseous or liquid components) to an adsorbent surface (a solid). It is one of the fascinating phenomena connected with the behaviour of fluids in a force field extorted by the solid surface (Borowko, 2002). The adsorption of a substance from one phase to the surface of another in a particular system leads to a thermodynamically defined distribution of that substance between the phases when the system reaches equilibrium. A variety of different isotherm equations have been proposed, some with firm theoretical foundation and others of empirical nature. Experimental isotherms are useful for describing adsorption capacity of a variety of solids and to find out the viability for the process for a given application such that the most appropriate adsorbent may be chosen. Adsorption equilibrium is established when the concentration of adsorbate in the bulk solution is in dynamic balance with that of the interface. The two most well-known isotherms are described below:
The key-point of every adsorbent material is its adsorption capacity. Three isotherm models are given they are; the Langmuir Equation (1) (Fishman and Mallorm 1968) the Freundlich Equation (2) (Mitchenko and Strelko 1983) and the combinational Langmuir-Freundlich (L-F) Equation (3) isotherm model (Henni and Righetti 1981):
(1)
(2)
(3)
Where Qe (mg/g) is the equilibrium concentration of pollutant in the solid phase; Qm (mg/g) is the maximum amount of adsorption; KL (L/mg) is the Langmuir adsorption equilibrium constant; KF (mg1−1/n∙L1/n/g) is the Freundlich constant representing the adsorption capacity; n (dimensionless) is the constant depicting the adsorption intensity; KLF (L/mg)1/b is the Langmuir-Freundlich constant; and b (dimensionless) is the Langmuir-Freundlich heterogeneity constant. The amount of total uptake of pollutant in equilibrium (Qe) will be calculated using the mass balance equation Equation (4):
(4)
Where M (g) is the mass of adsorbent; V (L) the volume of adsorbate; C0 and Ce (mg/L) are the initial and equilibrium concentrations of pollutant in the liquid phase, respectively. In this study, in order to avoid repeating the up-to-now literature, it will not be discussed how appropriate a particular isotherm model is for an adsorbent-adsorbate system. These reports (and especially review articles) have already been extensively discussed in other works (Raymond 1999; Rajaguru et al., 2002). However, a discussion will be had regarding the adsorption capacity of each green adsorbent. In this review, the most appropriate classification is based on the pollutants removed. However, another classification could be realized based on adsorbents and insights on structural relevancy or their main structural conversion strategies. However, in that case, the scope of this study would be changed.
1.5 Purpose of the study
Contamination of water with organics and inorganics is a matter of concern, because of their harmful effects on living organisms. Several methods are available for effective removal of metal ions and dyes. Adsorption process is superior to any other method by virtue of its low initial cost, low energy requirement, simplicity of design and possibility of reusing the spent carbon via regeneration. Designing of an adsorption system with suitable adsorbent and well defined operational parameters for each category of industry would help to solve the problems of economic feasibility and acceptability.
Adsorbent such as activated carbon available to remove organic compounds from wastewaters are costly and hardly available (Demirbas et al., 2008; Ghaedi et al., 2012) base on this, there is a need to study the effectiveness of uptake of heavy metals and dyes from aqueous solution by cost effectiveness, availability and adsorptive properties. Agricultural waste are readily available for this purpose. On this note a study on the adsorptive properties on Moringa Oleifera seed shell charcoal is needed.
1.6 Significance of the Research
The findings of this research will be useful in developing the most efficient means of water remediation. With this knowledge apparatus that are relatively cheap and affordable may be developed to purify water of heavy metals that plague most if not all of the water sources in our environment.
1.7 Aims and Objectives
i. To investigate and compare the uptake of dyes from aqueous solutions by Moringa seeds Shell Raw, (MSSR) Moringa Seeds Shell Acid (MSSA) and Moringa Oliefera Seeds Shell Base (MSSB) by batch adsorption experiment.
ii. To examine the adsorption characteristics at different contact times and concentrations of dyes discharged to the environment by human activities.
iii. To examine the adsorption characteristics with varying amounts of adsorbents.
iv. To fit the data obtained to the Langmuir and Freundlich isotherms.
v. To determine the better adsorbent between Moringa Seeds Shell Raw, (MSSR) Moringa seeds Shell Acid (MSSA) and Moringa oliefera Seeds Shell Base (MSSB).
vi. To report the potential adsorptive capacity of raw and activated carbon produced,
vii. To analyze the moisture content, volatile matter, fixed carbon and ash contents in the precursor.
viii. To compare with commercially available expensive activated carbon.
1.8 The Scope of the work
i. To develop activated carbons from the Moringa Oleifera seed shell biomass by
sulphuric acid and Sodium hydroxide treatment followed by physical activation.
ii. To characterise the prepared activated carbons for various physicochemical parameters such as particle size, density, moisture content, loss on ignition, Acid soluble matter, water soluble matter, pH of aqueous solution (pH), approximate shape, functional groups present and surface texture.
iii. To study the effect of process parameters like initial pH, adsorbent dosage, contact time, initial adsorbate concentration, co-ions and temperature on the adsorption process by systematic batch- mode sorption studies.
iv. To study the effect of time (Kinetic study) to know the reversibility of the adsorption process and intra particle diffusion using Natarajan and Khalaf equation and Webber and Morris plot respectively.
v. To evaluate the effect of temperature (thermodynamic study) parameters like Free energy change Entropy change and Heat of the adsorption process to understand the mechanism.
vi. To investigate the mode of transportation (Diffusion study)
vii. To investigate desorption studies in order to elucidate the nature of adsorption and recycling of spent carbon and adsorbates.
viii. To model the equlibrium data obtained with Freundlich and Langmuir isotherm equations to find out which model best fits the experimental data and derive adsorption constants.
ix. To confirm the proposed mechanism using analytical techniques using UV-vis spectroscopy, FTIR, and SEM-TEM.