ABSTRACT
In this work, ZrO2 nanoparticles were synthesized by green method using Plumeria acuminata leaf extract via the variation of pH. Subsequently, carbon and sulphur were incorporated onto the lattice layer of the prepared ZrO2 nanoparticles by wet impregnation method. The prepared undoped, doped and co-doped zirconium oxide based nanomaterials were characterized for their morphology, phase structure, surface area, microstructure, absorption band, crystallinity and elemental composition using HRSEM, XRD, BET, HRTEM, UV-visible and EDS. The photocatalytic and adorptive properties of ZrO2 based materials for the treatment of local dyeing wastewater were investigated. The photocatalytic and adsorptive performance of ZrO2, C-ZrO2, S-ZrO2 and C-S-ZrO2 nanocomposites were determined by measuring the reduction in the level of .the following indicator parameters in local dyeing wastewater (TOC, COD, BOD, 2- SO42-, CO3 , Cl , NO3 and pH) in the presence and absence of natural sunlight irradiation. HRSEM analysis revealed the formation of agglomerated spherical particles for ZrO2 alone and mixture of spherical and hexagonal shaped particles for C-S co- doped ZrO2 nanocomposites. The EDS confirmed the presence of Zr and O as the dominant elements for ZrO2 alone and also C and S in C-S-ZrO2 nanocomposites. The XRD pattern of pure ZrO2 and carbon doped ZrO2 revealed the formation of tetragonal phase and doping with sulphur revealed orthorhombic, while the co-doped carbon- sulphur ZrO2 revealed a mixture of tetragonal and orthorhombic phases. The crystalline size decreases with addition of the dopants in the order of ZrO2(20.03 nm), S-ZrO2 (17.51 nm), C-ZrO2(16.03 nm) and C-S-ZrO2 (12.40 nm). Optical characterization of the nanomaterial revealed reduction of band gap from 5.2 eV to 3.4 eV for ZrO2 and C- S-ZrO2 respectively. The BET analysis revealed increase in the surface area upon the addition of the dopants in the order of C-S-ZrO2 (80.165 m2/g) >.C-ZrO2 (52.637 m2/g)>.S-ZrO2(24.824 m2/g)> ZrO2 (10.682 m2/g). It was found that the photocatalytic and adsorptive behaviour of the four materials differ with respect to the target pollutants. Of all the four samples studies, C-S-co-doped ZrO2 nanocomposites with highest mesoporosity exhibited excellent photocatalytic activity under natural sunlight than carbon doped ZrO2 , sulphur doped ZrO2 and ZrO2 alone with TOC, COD, BOD, Cl-, NO3 and pH removaal efficiency of 97.7%, 77.3%, 87.6%, 63.9%, 84.4%, 70.3%, 83.3% and 19.4% within 120 minutes respectively. The order of adsorptive behaviour of the four samples for the removal of TOC, COD, BOD, Cl-, NO3- and pH were C-S- ZrO2 (82.9%) >.C-ZrO2 (82.7%)>.S-ZrO2(81.7%)> ZrO2 (80.3%). This study revealed carbon-sulphur co-doped ZrO2 have excellent adsorption and photocatalytic properties for the removal of target pollutants from local dyeing wastewater except for COD, BOD, Cl-, SO42- and CO3 that were above the permissible limits.
CHAPTER ONE
1.0 INTRODUCTION
1.1 Background to the Study
Local dyeing industry is one of the oldest and most complex due to consumption and generation of high amount of wastewater (Kaliraj et al., 2019). It is a known fact that most textile industries in Nigeria have folded up due to enabling environment and lack of managerial skills (Mondal et al., 2017). As a consequence, there has been increased number of indigenous dyeing industries across the country due to the increasing demand of the citizens for the dye fabrics (Yaghoubi et al., 2020). This cottage industry accounts for two third of the total dyestuff market, and during dyeing process approximately 10-15% of the dye used are released into the wastewater (Aneyo et al.,2016). These activities are widely recognized as the root cause of environmental pollution because the wastewater contains complex organic aromatic compounds and colours that can cause several health problems (Kaliraj et al., 2019). The effect of exposure to such wastewater includes skin irritation, diarrhea, vomiting, bladder cancer depending on the length of time of exposure and concentration (Zhang et al., 2019).
The treatment of local dyeing wastewater using membrane separation process, equalization and sedimentation, coagulation and sedimentation have proved to be insufficient in removing colour, toxic organic dyes and inorganic constituents (Yaghoubi et al., 2020). Conventional treatment techniques suffer from high application cost, low treatment efficiency and severe secondary pollution. Therefore, it is critical and urgent to develop cost effective and sustainable technology to treat local dyeing wastewater. Recently, advance oxidation processes (AOPS) have attracted attention due to their strong ability to treat complex industrial wastewater containing organic pollutants (Ameta et al., 2018). Among the AOPS, photocatalytic technology is considered as a primary wastewater technique due to its high efficiency, environmental protection and eco-friendliness. In fact some dyes are difficult to biodegrade, particularly, the azo types, reactive and certain acidic dyes which are not readily absorbed by active sludge (Zhang et al., 2019).
In the recent times, metal oxides nanoparticles such as TiO2, ZrO2, WO3, ZnO have been widely used in the area of environmental remediation particularly ZrO2 semiconductor due to its low cost, physical and chemical stability, excellent photocatalytic properties under an excitation by UV or visible light through absorption, scattering and reflection of light (Mondal et al., 2017). The optical, mechanical, electrical, chemical and photocatalytic properties of this material (ZrO2) made it suitable for different applications (Zarei et al., 2019). However, the band gap of ZrO2 in the range of 5-7eV is high and hence limits its photocatalytic activity in the visible region. To reduce this band gap, several strategies has been adopted including the incorporation of metallic and non-metallic elements as dopants (Yadav et al., 2020). Doping using metal or non-metal alone has not been able to show any significant band gap reduction, while metal doping in most cases produced controversial photocatalytic activity results at both above UV and visible wavelengths (Mondal et al., 2017). In addition, metal doping with ZrO2 also cause thermal instability, an increase of carrier- recombination centres (Kianfar et al., 2021). Other effects include, dopant insolubility, the formation of secondary phase or surface aggregation rather than substitution, phase transformation among the polymorphs (Kianfar et al., 2021). There is also problem of alteration of charged carrier diffusion length and narrow band bending, amongst others (Melchor-Lagar et al., 2020). In view of these, non-metal doping has been identified as a more viable strategy to reduce the band gap and hence improve the visible light driven photocatalytic activity of ZrO2 (Danilenko et al., 2018). Co-doping of ZrO2 nanoparticle with non-metals is expected to bring about a significant reduction in band gap and enhancement of photocatalytic properties of ZrO2 (Zhang et al., 2019). Non- metal elements such as C, N, P , S, and B has been doped with metal oxides and enhanced visible light driven photocatlytic activity was observed (Renuka et al., 2021). Carbon and sulphur have strong oxidizing ability and ionic radius of 180 pm and 170 pm respectively (Sun et al., 2018). These values are greater than the ionic radius of oxygen (152 pm) which allows the two (non-metal) to displace oxygen from ZrO2 and form zirconium carbide and zirconium sulphide respectively. Moreso, researches have shown that effective band gap narrowing take place as a result of the mixing of the 2p and 3p state of carbon and sulphur with the 2p state of oxygen in ZrO2 (Farag et al., 2020). Till date, satisfactory results in terms of appropriate band gap and photocatalytic performance of ZrO2 under visible light have not been reported on the degradation of organic pollutants. Efforts are still ongoing towards developing highly visible light driven photo-catalyst with enhanced efficiency.
In this work, ZrO2 nanoparticle was synthesized and co-doped with carbon and sulphur via combination green synthesis and wet impregnation method. Subsequently, the synthesized materials were analyzed using several characterization tools to ascertain its morphology, microstructures, elemental composition, crystallinity, phase structure, surface area. The photocatalytic/adsorptive properties of the synthesized carbon-sulphur co-doped ZrO2 nanoparticles on local dyeing wastewater in the presence or absence of sunlight was investigated.
1.2 Statement of the Research Problem
Local dyeing wastewater constitute a serious threat to the environment, especially in the aquatic environment because it contains considerable amount of heavy metals, oil, grease and organic aromatic compounds (Aneyo et al., 2016). Exposure to local dyeing wastewater could lead to several health problems such as skin irritation or bleaching of skin depending on the dose and time (Mondal et al., 2017).
The local dyeing wastewater unlawfully discharged into aquatic eco-system depleted the dissolve oxygen content, chemical oxygen demand, biochemical oxygen thereby making life difficult for aquatic species (Aghabeygi and Khademi-Shamami, 2018). In addition, more than 80% of the local dyeing wastewater is coloured due to reactive dyes and the presence of coloured wastewater is an evidence of high biological oxygen demand (BOD) values and low dissolved oxygen (Aghabeygi and Khademi-Shamami, 2018).The treatment of wastewater from local dyeing industry involving the use of membrane separation process, equalization and sedimentation, coagulation and sedimentation have proved to be insufficient in removing colour, toxic organic dyes and inorganic constituents (Danilenko et al., 2018). Some of these organic dyes are difficult to degrade, particularly the reactive and certain acidic dyes are not readily absorbed by active sludge. Physical and chemical methods of preparing ZrO2 nanoparticles namely vapour phase, pyrolysis, hydrolysis, precipitation, hydrothermal and microwave plasma has shown problems of complicated procedure, high reaction temperature, long reaction time, toxic reagent and high cost of production which makes it difficult to produce ZrO2 nanoparticles in a large scale. Mono doping of ZrO2 nanoparticle with metal did not cause a significant band gap reduction, while the co-doping with two metals shows controversial photo-catalytic properties in thin film and bulk sample (Zhang et al., 2019).
1.3 Justification of the Study
Adsorption and photocatalytic technology in lieu of other conventional methods is expected to reduce the COD, BOD, TOC, DO level in the local dyeing wastewater. Adsorption and photocatalytic treatment of local dyeing wastewater by carbon-sulphur doped ZrO2 nanocomposite is expected to remove toxic dyes and inorganic constituents from wastewater (Aneyo et al., 2016). Green synthesis method of producing nanoparticles is simple and less complicated and operates at a very low temperature compared to other physical and chemical synthesis methods (Shinde et al., 2018). Co- doping of ZrO2 nanoparticles with non-metals with 2p electrons such as carbon and sulphur is expected to show effective band gap reduction, increased surface area and enhance photo activity under visible light (Aneyo et al., 2016). Carbon and sulphur have the ionic radius of 180pm and 170pm respectively. This values is greater than the ionic radius of oxygen (152pm) which allows the two to displace oxygen from ZrO2 and form zirconium sulphide and zirconium carbide respectively (Kianfar et al., 2021).
1.4 Aim and Objectives of the Study
The aim of this study is to synthesize, characterize and examine the catalytic and adsorptive properties of carbon-sulphur co-doped zirconium oxide nanocomposite for the purification of local dyeing wastewater.
The aim was achieved through the following objectives;
(i) Synthesis of zirconium oxide nanoparticles based on variation of solution pH.
(ii) Incorporation carbon and sulphur onto the prepared ZrO2 nanoparticles.
(iii) Characterization of zirconium oxide (ZrO2), carbon doped ZrO2, sulphur doped ZrO2 and carbon-sulphur co-doped ZrO2 nanocomposite using different analytical tools.
(iv) Physico-chemical analysis of the local dyeing wastewater for (pH, COD, BOD, 2-TOC, DO, Cl-, NO3-, SO42-, PO4) before and after adsorptive and photocatalytic experiment.
(v) Evaluation of the adsorptive and photocatalytic behaviour of ZrO2, carbon doped ZrO2, sulphur doped ZrO2 and carbon-sulphur co-doped ZrO2.
(vi) Evaluation of the adsorptive/photocatalytic data using different kinetic models (parabolic, pseudo-first, modified freundlich and zero order)
This material content is developed to serve as a GUIDE for students to conduct academic research
STUDY OF PHOTOCATALYTIC AND ADSORTIVE BEHAVIOUR OF PURE ZIRCONIUM OXIDE AND CARBON-SULPHUR CO-DOPED ZIRCONIUM OXIDE NANOCOMPOSITES FOR PURIFICATION OF DYE WASTEWATER
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