ABSTRACT
In this study, the adsorption effect on selected physicochemical parameters from tannery wastewater onto the beneficiated kaolin, kaolin/TiO2, kaolin/ZnO and kaolin/TiO2/ZnO nanocomposites was investigated by employing the batch adsorption technique. The effects of pH value on crystal size of TiO2 and ZnO nanoparticles prepared by sol-gel method were also examined. The kaolin/TiO2 (KT), kaolin/ZnO (KZ) and kaolin/TiO2/ZnO (KTZ11, KT21 and KTZ12) nanocomposites were prepared bywet impregnation methods.  The prepared adsorbents were characterized using X-ray diffraction (XRD), nitrogen gas adsorption-desorption Brunauer-Emmett-Teller (BET), Fourier transform infrared (FTIR), High Resolution Scanning Electron Microscopy (HRSEM), Energy Dispersive X-ray spectroscopy (EDX), High  Resolution  Transmission Electron Microscopy  (HRTEM)  and  X-ray Photoelectron Spectroscopy (XPS). The results of the characterization showed the morphology, phase identification, surface area, functional groups as well as the adsorption process involved of the nanoadsorbents. From XRD patterns, it was found that the crystallite size of the synthesized TiO2 at pH 4-12 and ZnO at pH 6-12 calcined at 450 oC were from 5.67 to 15.02 nm and 11.84 to 24.82 nm respectively. The kaolinite (plate-like), anatase (tetragonal) and zincite (hexagonal) shapes of kaolin, TiO2 and ZnO were further confirmed by the HRSEM and HRTEM. The BET results showed that the surface areas of kaolin/TiO2 and kaolin/ZnO nanocomposites increased compared to the surface area of the beneficiated kaolin. The XRD and HRTEM results demonstrated that the TiO2 and ZnO nanoparticles were distributed within the kaolin framework. The EDX analysis also established that the loading correlated to the anchored different ratios of the nanoparticles on the beneficiated kaolin by wet impregnation method. The adsorption method was established to investigate the influence of some experimental parameters such as contact time, adsorbent dosage and temperature. The adsorption isotherm studies were investigated at different temperatures (30-70 oC). The adsorption data were fitted to Jovanovic, Halsey, Flory-Huggins and Redlich-Peterson isotherm models. The results indicated that Jovanovic isotherm model best fitted the data in the temperature range studied with correlation coefficient, R2 > 0.999. However, the Flory-Huggins adsorption isotherm explained the spontaneous adsorption system. The experimental results at different contact times were subjected to kinetic adsorption models (fractional power, Bangham and Avrami) and adsorption mechanism determinations (Boyd and intra-particle diffusion). The results showed that the kinetic adsorption process followed Banghham kinetic model. The mechanism for the adsorption of tannery wastewater onto the adsorbents indicates that external mass transfer is the rate-determining step since the plots of Boyd and intra-particle diffusion did not pass through the origin. Thermodynamic studies showed that the adsorption system is temperature dependent since increase in temperature showed increase in the adsorption of the pollutants, indicating that the adsorptivity was endothermic and spontaneous. The values of the thermodynamic parameters; change in enthalpy (∆H) and change in entropy (∆S), calculated from Vant’s Hoff plots confirmed the spontaneity of the adsorption process. A mechanistic pathway for the synthesized nanocomposites was presented and the nanocomposite filter produced was resistant to temperature above 900 oC. The flow rate optimized by changing the proportion of kaolin and saw dust for the filter pot formulation was observed and this served as the controller of the flow rate and percentage porosity of the filter pot. The bactericidal effects of TiO2 and ZnO nanoparticles in the composites explained the nanoscale and physicochemical properties of the nanoparticles along with surface area of the kaolin. The filtrates using the nanocomposites filter pots gave concentrations of some investigated physicochemical parameters less than the WHO and NESREA standards. This illustrates that the filter pots nanocomposites could be a potentially safe and viable adsorbent in water filters for water purification systems.
CHAPTER ONE
1.0 INTRODUCTION
1.1 Background to the Study
In the last couple of years, population growth, migration, increasing urbanization, industrialization and climate change have influenced the supply and demand chain for freshwater resources. According to United Nations Department of Economic and Social Affairs (UNDESA, 2013), the world population is projected to reach 9.1 billion by 2050, with more than 2.4 billion living in Sub-Saharan Africa (SSA) without access to sustainable water resources. World Health Organization (WHO, 2015) also reported that more than 1.1 billion people in Africa do not have access to clean drinking water and sanitary facilities due to growing populations and increasing economic activities with deprivation of environmental services.
Industrial production, intensive agriculture, mining and untreated urban run-off have led to increase in water use, having potential impacts on the quality of water. The availability of potable drinking water to households in underdeveloped and developing countries still remains a challenge due to several natural and human factors (Samans et al., 2017). For instance, the continuous discharge of domestic wastewater either black or grey and industrial effluents containing a variety of organic and inorganic pollutants into the ecosystem has become a daily occurrence. Until now, about 20 % of the world wastewater is effectively treated and UN (2016) revealed that almost 70 % of untreated industrial effluents were discharged into the environment. The absence of wastewater management process coupled with anthropogenic activities poses momentous threats to economic activities and well-being of the human race. In Nigeria, many people do not have access to safe water, leading to a looming water crisis (Naik, 2017). This is principally because of water pollution placing the country on the danger of greater outbreak of diseases such as typhoid, polio, cholera and dysentery (Atuyambe et al., 2017). Among various organic and inorganic contaminants discharged into the environment, potential toxic metals (PTMs) also known as heavy metals have been identified as possible causes of human diseases (Vasistha and Ganguly, 2020). These toxic metal ion pollutants in the water bodies have become topic issues to the human race and other living organism due to their detrimental effects. These metals are mobile, non-biodegradable, recalcitrant and toxic to aquatic species even at low concentrations. The sources of heavy metals in water bodies include electroplating, mining, smelting, pharmaceuticals, battery manufacturing, textile and tannery industries. Specifically, tanning activities generate almost forty million litres of wastewater yearly, containing constituents such as chromium, chloride, lime with high dissolved and suspended salts and other contaminants (Ahmed et al., 2016). In most developing countries like Nigeria, tanning industries discharged wastewater without proper treatment into the sewage system causing a serious detrimental effect on the environment. Therefore, the removal of toxic pollutants such as chromium and other related toxic metals in tannery wastewater is considered imperative.
Several conventional techniques such as reverse osmosis, membrane filtration, electro- coagulation, electro-dialysis, chemical precipitation, adsorption, membrane bioreactor, sludge reactor, amongst others have been employed for the treatment of wastewater. Of all these methods, adsorption is widely used due to its unique nature such as distinct energy saving, cost-effectiveness, simplicity, wide operational range of factors such as pH, concentration, dosage and temperature, environmental friendliness, fast reclamation of organic and inorganic pollutants and easy recycling of the sorbents (Sani et al., 2017; Vahidhabanu et al., 2017). On the other hand, adsorption technology with respect to material used, sometimes does not readily degrade and typically remove pollutants from the wastewater treatment system. In the viewpoint of the shortcomings of adsorption process via the material used, nanotechnology involving the use of nanomaterials as nanoadsorbents has been incorporated to achieve greater efficiency.
Several natural adsorbents such as chitosan (Preethi et al., 2017), carboxymethyl cellulose (Zahedi et al., 2017), clay minerals (Motshekga et al., 2016), zeolite and carbon nanotubes (Bhattachajee et al., 2016) have been used for the sequestration of contaminants from wastewater. A wide documentations on clay minerals such as kaolinite, bentonite, montmorillonites (smectite), illites, vermiculites and chlorites composed mainly of silica, alumina, water and weathered rock which could serve as alternative cheap materials for remediation of wastewater, free of toxic chemicals have been studied (Uddin, 2016). They possess some unique characteristics compared to other natural adsorbents, used for adsorption of heavy metals and also serve as remedies for ailments and can be used as excellent adsorbents for environmental bioremediation of wastewater (Unuabonah et al., 2017).
In addition, nanoparticles such as metal oxides with dimensions less than 100 nm exhibit exceptional physical and chemical properties and have been utilized for wastewater treatment (Zhang et al., 2016). Crystal morphology and particle size play vital roles in the application, which have led scientific researchers to focus on the synthesis of different nanoparticles like zirconium oxide (ZrO2) (Zinatloo-Ajabshir and Salavati-Niasari, 2016), zinc oxide (ZnO) (Chang et al., 2020), titanium oxide (TiO2) (Boudjemaa and Gómez-Ruiz, 2020), magnesium oxide (MgO) (Sagadevan et al., 2020), palladium oxide (PdO) (Muniz- Miranda et al., 2020) and lots more. Both physical and chemical methods have been employed for the synthesis of ZnO nanoparticles which include sol-gel, thermal plasma synthesis, hydrothermal synthesis, spray pyrolysis, chemical precipitation, combustion synthesis and electrochemical routes. Extensive literature evaluation concludes that sol-gel is the commonly selected method for the synthesis of nanoparticles. This is because this method produces excellent crystalline structure and narrow size distribution. It also requires low-temperature, easy composition control and low cost-effectiveness (Mourdikoudis et al., 2018).
The nanosized metal oxides such as zinc oxide and titanium oxide exhibit favourable sorption towards organic and inorganic pollutants and have higher adsorption capacity than conventional adsorbent (Chouchene et al., 2017; Syngouna et al., 2017). TiO2 nanoparticles (T-NPs) have received much attention due to their extensive characteristics such as low cost, non-toxicity, high stability (Nasirian and Mehrvar, 2016; Dariani et al., 2016; Lin et al., 2018). In the same vein, zinc oxide is mostly considered as nanoadsorbent due to its non-toxic profile, high adsorptive properties, effective antibacterial activity, chemical, mechanical and thermal stability (Ibrahim and Asal, 2017). ZnO nanoparticles have a higher adsorption capacity than titania nanoparticles for heavy metals removal (Rafiq et al., 2014). However, these nanoparticles are difficult to separate from wastewater after treatment and steps have been adopted to overcome these shortcomings. These include doping and co- doping of metal oxide nanomaterials and immobilization of nanomaterials on suitable matrices (Soltani et al., 2016; Belver et al., 2017). These substrates could function as supports in order to overcome the difficulties involved in post-separation and recovery of the nanoparticles.
More so, the support of nanosized semiconductor materials on matrices help in enhancing their binding activities towards contaminants compared to ordinary bulk nanomaterials (Zhao et al., 2020). Different clay matrices such as kaolinite, montmorillonite and bentonite have been employed as supports. Kaolinite, for instance, has exceptional crystallochemical features and thus could act as a suitable matrix for anchoring TiO2 and ZnO nanoparticles (Dědková et al., 2015; Hadjltaief et al., 2017). Immobilization and anchoring of nanosized TiO2 and ZnO nanoparticles on the surface of clay minerals provide more active surface sites, reduce the agglomeration of the nanoparticles and prevents the leaching of nanoparticles to the environment (Ruiz-Hitzky et al., 2019).
Taking into account the fact that rural dwellers are mostly affected by these contaminants from wastewater; a cheaper and more environmentally friendly method for wastewater treatment needs to be adopted before the discharge of industrial wastewaters. Therefore, in addressing these difficulties, this study focused on the preparation, characterization and application of clay/TiO2/ZnO filters for the treatment of tannery wastewater.
1.2 Statement of the Research Problem
The contamination of water bodies caused by the discharge of industrial effluents has become alarming. This has led to serious environmental problems because of the high toxicity of the discharged contaminants. Parameter indicators such as temperature, total suspended solids, odour, colour, taste, pH, biological oxygen demand, chemical oxygen demand dissolved oxygen, heavy metals, nitrates and coliform bacteria affects level of contaminants in wastewater. Waterborne diseases such as typhoid fever and dysentery are caused by bacteria, viruses and parasitic worms via water pollution. These diseases have seriously affected man. Sequel to their threat to human and aquatic organism, various attempts have been employed towards removing them from wastewater using techniques such as chemical precipitation, coagulation, electro-dialysis, ion exchange, adsorption, photocatalysis and membrane filtration (Adebayo et al., 2021). Most of these have however been found to be costly and ineffective to completely remove the contaminants from wastewater (Asses and Ayed, 2021). For instance, photocatalysis produces hazardous by- products and are thus difficult to separate from solution after the treatment of effluents (Wang et al., 2020). Several investigations have been submitted on the application of TiO2 and ZnO nanoparticles for the removal of contaminants from wastewater. However, studies have revealed that these nanoparticles do not possess high specific areas and have strong colloidal stability or low settling velocities in water, making recycling expensive and time- consuming. Thus, immobilization of these nanoparticles on suitable porous carriers is necessary. This will help to enhance filtration and reusability of the nanoparticles during wastewater treatment.
Leather tanning industries have been identified as a major cause of environmental pollution releasing a large quantity of untreated effluent into the atmosphere and water bodies (Jahan et al., 2014). The public concern over tanneries has been pollutants such as chlorides, chromium, sulphide, lead and cadmium. These substances are toxic and persistent causing serious health problem. The common health issues are asthma, diarrhoea, jaundice/typhoid, blood pressure, urticaria, militia, folliculitis, sores, pruritus, eczema, gastrointestinal and eye problems (Mahamudul et al., 2016). Thus, the removal of these substances from tannery wastewater is a key requirement for the safety of public health and the environment. Various methods have been used to treat tannery wastewater such as flotation, electrochemical treatment, sedimentation, coagulation, filtration, ultra-filtration and reverse osmosis process. However, these methods faced limitations of being highly expensive and energy consuming (Hashem et al., 2020). It is also characterized by high operation and maintenance cost for tannery wastewater and sludge production. On the contrary, nanotechnology offers vast potentials for wastewater treatment and could substitute conventional wastewater treatment technologies. The removal of harmful pollutants in wastewater by TiO2 and ZnO nanoparticles could be popular materials due to low toxicity of the materials. However, some limitations present in the use of pure TiO2 and ZnO nanoparticles are low adsorption capacity of pollutants related to its low surface area affecting the removal efficiency of pollutants. Other practical problems are catalyst agglomeration and formation of uniform suspension in water which makes it difficult to recover. This implies that the removal of these nanoparticles could require costly phase separation methods.
1.3 Justification of the Study
Pollutants have globally gained increasing attention due to their detection in the environment and their detrimental effects on human and other life. Over the years, standard purification techniques have been adopted to remove these pollutants. Amongst these, adsorption has proved to be a simple and environmentally friendly alternative method. This method relies on the use of low-cost adsorbents, which can significantly reduce process costs. Because of its simplicity and cost-effectiveness, the adsorption process has been studied for decades for the treatment of pollutants in tannery wastewater. Thus, adsorption treatment of tannery wastewater will aid in reducing the release of organic and inorganic substances that are hazardous to the environment, human and animal health (Rafiq et al., 2014).
TiO2 and ZnO nanoparticles are effective nanoadsorbents with low cost and toxicity, a long life span, high activity, and exceptional stability. Because of their decreased mobility in aqueous media, these nanoparticles may be less toxic if properly anchored via a chemical bond to suitable matrices such as kaolin. Kaolin, a cheap, locally available and effective adsorbent as reported by Adeyemo et al. (2015), if used for this purpose will certainly solve a lot of water purification problems due to the interaction involved between adsorption and the catalytic activity of the exchangeable cations in wastewater. As a result of the aforementioned features, clay/TiO2/ZnO nanocomposites will be of interest in this research work considering its enormous surface area and enhanced porosity which is expected to ensure availability of enhanced surface active sites. This in effects will be helpful in improving adsorption and is expected to have enhanced adsorption capacity compared to the mono-metallic nanocomposites.
1.4 Aim and Objectives of the Study
The aim of this research work was to synthesize and characterize titanium oxide/ zinc oxide nanocomposites immobilized on kaolin for the treatment of tannery wastewater. The aim of the study was achieved through the following objectives:
i) Beneficiation and characterization of kaolin
ii) Synthesis of TiO2 and ZnO nanoparticles by sol-gel method and kaolin/TiO2, kaolin/ZnO and kaolin/TiO2/ZnO nanocomposites via wet impregnation.
iii) Characterization of TiO2 and ZnO nanoparticles, kaolin/TiO2, kaolin/ZnO and kaolin/TiO2/ZnO nanocomposites for their surface areas, elemental composition, morphologies, phase structures as well as functional groups.
iv) Determination of physicochemical parameters and bacterial analysis of the tannery wastewater before and after batch adsorption processes.
v) Fabrication of filter pots developed from the beneficiated kaolin, kaolin/TiO2. kaolin/ZnO and kaolin/TiO2/ZnO for tannery wastewater.
vi) Performance evaluation of the fabricated filters on the removal of some physicochemical parameters in tannery wastewater.
This material content is developed to serve as a GUIDE for students to conduct academic research
SYNTHESIS AND CHARACTERISATION OF TiO2/ZnO NANOCOMPOSITES IMMOBILIZED ON KAOLIN FOR THE TREATMENT OF TANNERY WASTEWATER
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