1.1 Background of the Study
Chlorophenols are of greater environmental concern because of their higher toxicity and carcinogenic with strong odour emission, not readily biodegradable and persistent in the environment and thus poses a serious ecological problem and public health risk to human and marine life. These compounds are widely distributed due to the anthropogenic contributions from the industrial wastes generated from bleaching, iron steel, paper and cellulose, pesticides and biocides, petrochemical, pharmaceutical, plastic, rubber proofing, textile, and wood preserving industries (Fattahi et al., 2007; Hamad et al., 2010). This makes it necessary to develop methods that allow one to detect, quantify and remove chlorophenols from aqueous solution as an important prior to discharging wastewater into the environment (Mahvi, 2008).
Consequently, numerous conventional methods are existing in the treatment of chlorophenols wastewater which includes anaerobic processes, biodegradation, biosorption, distillation, combined applications of flotation and coagulation processes, ion exchange, the electro Fenton method, membrane separation, pervaporation, precipitation, reverse osmosis, solvent extraction, stripping and oxidation, etc. (Busca et al., 2008). Adsorption process is considered better among a variety of methods used in chlorophenols wastewater treatment because it is easy to operate and convenient.
Adsorption is a phenomenon in which a substance (adsorbate) either in gas or liquid phase accumulates on a solid surface (adsorbent), which rely on the capability of porous materials with large surfaces to selectively keep compounds on the surface of the solid (adsorbent). The adsorption process of the adsorbate molecules from the bulk liquid phase into the adsorbent surface is supposed to involve the following stages:
a. Mass transfer of the adsorbate molecules across the external boundary layer towards the solid particle.
b. Adsorbate molecules transport from the particle surface into the active sites by diffusion within the pore–filled liquid and migrate along the solid surface of the pore.
c. Solute molecules adsorption on the active sites on the interior surfaces of the pores.
d. Once the molecule adsorbed, it may migrate on the pore surface trough surface diffusion (Mohamed, 2011).
Activated carbons are materials with large specific surface areas, high porosity; adequate pore size distributions and high mechanical strength which are extensively used as an adsorbent in the removal of heavy metals, hydrocarbons, and other hazardous chemicals that can be found in wastewaters (Bohli et al., 2013). Granule or powder form of activated carbons have good adsorptive capacity to attract soluble organic molecule materials from solution to its surface, but due to its high cost and difficulty in regeneration which limits its commercial application in large scale treatment of wastewater (Popuri et al., 2007). This has led to research for cheaper substitutes such as agricultural waste materials obtained as the by-products from the forestry and agricultural industries which is a ubiquitous green waste generally inexpensive, renewable source of activated carbons and often cause serious environmental pollution problem. Agricultural waste is a rich source for activated carbon production due to its low ash content and reasonable hardness (Bhatnagar & Sillanpaa, 2010). These are organic compounds comprised of cellulose, hemicelluloses, lignin, lipids, proteins, simple sugars, water, hydrocarbons, starch, and containing a variety of functional groups having potential sorption capacity for various organic pollutants. Therefore, conversion of agricultural wastes into low-cost adsorbents is a promising alternative to solve environmental problems such as disposal of waste and also to reduce the preparation costs (Ahmedna et al., 2000).
Different kinds of activated carbon have been achieved from different agriculture wastes and used as low-cost adsorbents with varying success for the removal of organic compounds from aqueous solution. Almond (Terminalia catappa) nut shells is one of the important agricultural materials belong to the family Combrataceae, it is a large spreading tree distributed throughout the tropics and coastal environment (Species profiles, 2006). The fruit is a sessile, laterally compressed, ovoid to ovate and smooth skinned drupe. The oil containing seeds are encased in a tough fibrous husk with a fleshy pericarp. The shells of almond are abundant, inexpensive and readily available lignocellulosic substance generally discarded as a waste material, and can be collected on community basis for reuse as adsorbent. The cell walls of almond shell consist of cellulose, silica, lignin and carbohydrates which have hydroxyl groups in their structures. Others agricultural waste products include: banana peels (Achak et al., 2009), banana stalk (Ogunleye et al., 2014), orange peels (Owabor & Audu, 2010; Agarry & Aremu, 2012), peanut husk (Hu et al., 2011), pineapple peels (Agarry & Aremu, 2012; Solidum, 2013), spent tea leaves (Hameed, 2009; Agarry et al., 2013a), etc. More and more interests are focused on developing these agricultural wastes as adsorbent for wastewater treatment due to their relative high sorption affinity, ubiquitous presence in the environment, and the ease of being modified to materials with higher efficiency (Chen et al., 2011; Agarry & Aremu, 2012).
1.2 Statement of the Problem
One of the main problems causing environmental pollution of watercourses is industrial effluents that have high concentrations of dissolved organic compounds, with disagreements existing on the maximum values allowed by current legislation. The adsorption by absorbent is one of the methods used for the removal of pollutant (i.e. 2,6-Dichlorophenol) from aqueous waste stream using activated carbon obtained from agricultural waste material (Almond nut shells). The search for practical, efficient and low cost alternatives has been a constant to circumvent these problems.
1.3 Aim and Objectives of the Study
The aim of this research work is to investigate the potentiality of using cellulose based wastes, almond nut shells (Terminalia catappa) as a non-conventional low cost adsorbent for 2,6-Dichlorophenol removal from aqueous solution.
In order to achieve the broad goal of this study, the specific objectives are to;
? Prepare adsorbate solution (2,6-Dichlorophenol) and the adsorbent (Almond nut shells).
? Characterize the modified almond nut shells by Fourier Transform Infrared (FTIR) spectroscopy studies and other physicochemical parameters such as pH and Conductivity, Moisture Content, Ash Content, Bulk Density, Specific Density, Porosity, and Pore Volume.
? Examine the effect of various factors such as time of contact, adsorbent dosage, pH and initial adsorbate concentrations on this adsorption process under batch equilibrium technique.
? Analyse the experimental data by Langmuir and Freundlich models in order to describe the equilibrium isotherms.
? Modelling of adsorption kinetic using Lagergren pseudo-first order, pseudo-second order and intra-particle diffusion (Weber-moris Model).
1.4 Significance of the Study
The purpose of the study is of high importance to test the possibility, and provide summary information concerning the use of locally available materials as adsorbents for the removal of phenolic compound. Until this present work, little information is available on the suitability of using this selected cellulose-based agricultural waste (Almond nut shells) in the removal of toxic 2,6-Dichlorophenol and seldom work has been reported in literature so far. Finally, the study will be an important source of reference to the researchers and students of natural and applied sciences who might want to embark on scholarly investigation in future.
1.5 Scope of the Study
For the aim and objectives of the study to be achieved, the scope of the study is specifically limited to the removal of 2,6-Dichlorophenol (adsorbate) from an aqueous solution employing cellulose-based almond nut shells (adsorbent) because of its availability.