SOLAR PHOTOCATALYTIC DEGRADATION OF ORGANIC POLLUTANTS IN LOCAL DYEING WASTEWATER USING NEEM- MODIFIED GRAPHITIC CARBON NITRIDE (g-C3N4)

ABSTRACT

The discharge of dyeing wastewater containing high concentration of contaminants from the local dyeing industry has caused a serious environmental concern. The present study reports the synthesis of graphitic carbon nitride and itsutilization in degradation of dyeing effluent pollutants under solar irradiation. The synthesized g-C3N4 was modified using the aqueous leaves extract of AzadiratchaIndica (Neem plant): a clean,   non-hazardous   and   environmentally   friendly   procedure   (green   modification   route).   The morphology,  elemental  composition,  crystallography,  Surface  area  and  adsorption  bands  of  the synthesized modified and unmodified g-C3N4 samples were comprehensively examined using SEM, EDS, XRD, FT-IR and BET characterization techniques. The photo-oxidation of organic pollutants in the local dyeing wastewater by the modified and unmodified g-C3N4nanomaterials under sunlight irradiation in a batch reactor system was tested. The photocatalytic degradation efficiency of the local dyeing wastewater by the photocatalysts was assessed using chemical oxygen demand (COD) as the indicator parameter. Different experimental conditions such as the effects of solution pH, time, and Photocatalyst dosage on the efficiency of COD degradation process were investigated and then optimum conditions were established.  The  SEM  and  XRD  analysis  of  the  modified  and  unmodified  g-C3N4demontrated  the formation of  crystalline tri-s-triazine  unit  and  agglomerated  morphology.  The BET  surface  analysis displayed that the surface area of the g-C3N4nanopaticles increased from 58.88 cm3/g to 141.0 cm3/g upon modification, while the EDS analysis demonstrated the existence of C and N as the major constituents of the as-synthesized nanomaterials. The study found that the modified g-C3N4 nanoparticle exhibited significant higher catalytic activity of 61% COD removal compared to the unmodified g-C3N4nanoprticle (40% COD removal). It was established from the optimization study that pH of 9.8 and photocatalyst dosage of 0.7 g/200ml were the optimum conditions. The experimental data obtained were evaluated using three different kinetic models: first order, second order and Behnajady-Modirshahla-Ghanbery (BMG). It was found that the Behnajady-Modirshahla-Ghanbery model best described the data based on the correlation coefficient (R2). This study demonstrated that the modified g-C3N4  nanoparticle showed better photocatalytic activity in comparison to the unmodified g-C3N4. The  enhancement  in  performance is  because  of its  high  surface  area  which signifies  more  active  sites  for  reaction, small  bulk diffusion length  and  strong redox  ability of charge  carriers.

CHAPTER ONE

1.0  INTRODUCTION

1.1 Background to the Study

Water is life and remains an essential resource required for human survival, socio-economic and political stability (Cantrella et al., 2015). However, 0.01% out of 0.7% total available water is only available for human consumption (Gupta et al., 2015). Today, the most discussed issues around the globe by both government and non-governmental organizations at different forums are sanitation, land, air and water pollution (Khalid et al., 2017). The quality and quantity of water has continued to deteriorate due to natural and anthropogenic activities such as flooding, rapid population increase, poor agricultural practices, industrial expansion and climate change coupled with urbanization (Das, 2014). Specifically, human activities are responsible for huge generation and disposal of toxic wastes into the environment, with a consequential impact on surface and  groundwater quality (Gupta et al.,  2013). Almost all of the natural sources of drinking water such as surface water, groundwater, lakes and reservoirs, rivers and canals, and rainwater have been reported to be contaminated with toxic materials and pathogenic microorganisms (Baruah et al., 2015).  For  instance, the dyeing industries in Nigeria use and dispose complex organic dyes and inorganic constituents which are toxic, persistent and non- biodegradable into the water ways, thereby causing devastating effects on both human and aquatic life (Lee et al., 2016). The local dyeing industry utilizes large amount of water and at the same time generate large volume of wastewater containing complex organic dyes, thus require proper treatment prior to discharge into the environment (Ghaly, et al., 2014). These organic dyes at low and high concentrations are considered as environmental nuisance and continuous exposure to such wastewater can cause disruption of endocrine glands and respiratory system

depending on the exposure time and dosage (Hadi and Wahab, 2015). The organic dyes in the dyeing waste water are responsible for the intense coloration notice in Rivers which inhibits sunlight, increase the level of chemical oxygen  demand (COD), biological oxygen demand (BOD), and also obstruct photosynthesis and reoxygenation processes (Ghaly et al., 2014). Other chemicals in the wastewaters include dispersants, leveling agents, acids, alkalis, carriers and various dyes and the pH of water bodies (Gupta and Suhas. 2013). With an increase in the use of different varieties of dyes, pollution by dye wastewater is becoming increasingly alarming and thus become imperative to treat such wastewater prior to discharge in order to reduce associated risks in the environment (Das, 2014).

Several conventional wastewater treatment methods such as flocculation (Ghaly et al., 2014), coagulation (Sadi et al., 2015) ozonation (Khaki et al., 2017), biological treatments (Hu, 2013), adsorption  (Harikumar  et  al.,  2013),  electrodialysis  (Baruah  et  al.,  2015),  reverse  osmosis (Dariani et al., 2016), ion exchange (Ghaly et al., 2014), precipitation (Ying, 2015) among others have been applied to treat local dyeing wastewater. However, these methods have certain shortcomings such as high cost of chemical/ reagents, complex treatment procedure, generation of toxic sludge, low efficiency as regards degradation of organic pollutants (Hu, 2013).

Recently, advanced oxidation processes (AOPs) involving photocatalysis has been identified as an efficient technique for the decomposition of organic dyes in local dyeing wastewaters into harmless substances such as CO2  and H2O based on radical mechanism (Sarnali and Bhagchandani, 2016). More so, semi-conductor metal oxides and non metals such as vanadium, chromium, titanium, zinc , tin and carbon nitride exhibit excellent photocatalytic activity in the presence of light source and induce a charge separation process with the formation of positive holes which oxidize organic substrates (Hisatomi et al., 2014). In this process, a photocatalyst is activated with either UV light, visible light  or a combination of both, and photo exited electrons are elevated from the valence band to the conduction band, forming an electron/hole pair (e-/h+) (Khaki  et  al.,  2017).  The  photo  generated  pair  (e-/h+)  is  able  to  reduce  and  /or  oxidize  a compound absorbed on the photocatalyst surface (Das, 2014). Among the non metal photocatalysts,  graphitic  carbon  nitride  (g-C3N4)  nanoparticles  is  considered  due  to  its exceptional features such as non-toxicity, stable chemical properties, mild band gap (2.7 eV), absorption of visible light (Niu et al., 2012). Despite these unique features, graphitic carbon nitride still has limitations for practical applications due to low efficiency of visible light utilization, high recombination rate of the photo generated charge carriers, low electrical conductivity and small specific surface area (<10 m2g−1) (Niu et al., 2012). Furthermore, researchers have employed different strategies to solve problems associated with g-C3N4  based photocatalyst, which is through modification. This modification strategy involves the incorporation of foreign impurities such as metals (Ag, Pd, Pt, and Au) and non metals (C, S, F, and N) into the crystal lattice layer of g-C3N4  nanoparticles (Khaki et al., 2017). These foreign impurities mostly serve as electron trappers and prevent high electron-hole recombination rate.

Green synthesis involving the use of plant extracts or microorganisms as reducing agents has been identified as an alternative route to prepare nanoparticles of desired shapes and sizes due to its relative abundance, cost effectiveness and environmental friendliness (Deepa et al., 2016). In this study, g-C3N4  nanoparticles will be synthesized and then modified with the leaf extract of Azadirachta Indica (Neem).The neem leaf extract, when integrated into the crystal lattice of g- C3N4  nanoparticles, will create a synergy thereby reducing the band gap energy and enhancing the surface area. In addition, the photocatalytic activity of the prepared g-C3N4  nanoparticles under sunlight using local dyeing wastewater will be investigated.

1.2 Statement of the Research Problem

Environmental pollution and inadequate disposal of local dyeing wastewater containing organic dyes have become an increasing menace in Nigeria. Exposure to low or high amounts of organic dyes in such wastewater causes detrimental effects on human and aquatic life. There is also the challenge of discoloration factor from dyes when expelled into land or into water bodies which eventually distorts the ecosystem. Similarly, local dyeing wastewaters have offensive odour and continuous exposure can cause skin hemorrhage and lesions (Nese et al., 2007).

Current conventional wastewater treatment methods such as flocculation, ion exchange, coagulation, distillation, filtration, reverse osmosis and adsorption are not suitable for the eradication of toxic organic dyes (Pokharna and Shivastava, 2013). In addition, adsorption technology produces toxic sludge, and transforms pollutants from one phase to another (Maletz et al., 2013).

Despite graphitic carbon nitride having some advantages such as mild band gap (2.7 eV), absorption of visible light and flexibility, it still has limitations for practical applications due to low efficiency of visible light utilization, high recombination rate of the photo generated charge carriers, low electrical conductivity and small specific surface area (<10 m2g−1) (Niu et al., 2012). Thus, modification of the photocatalysts with the aim of ensuring high efficiency, fast separation of electron/hole pair and increased surface area is required.

1.3 Justification of the Research

Photocatalysis is identified as a logical technique to treat local dyeing wastewater and achieve mineralization  of  toxic  organic  dyes  molecules  into  CO2    and  H2O  compared  to  other conventional methods (Baruah et al., 2015). Synthesis and modification of g-C3N4  via green route, where the plant extract is used as a reducing/capping and stabilizing agent will help to solve the problem of contamination and formation of hazardous bye products. This method is relatively cheap, eco-friendly, and highly efficient method due to non usage of toxic precursors as compared to other chemical and physical synthesis methods. Modification of the g-C3N4 with the neem leaf extract solution helps to increase the surface area, suppress electron/hole recombination   and   subsequently   enhance   the   photocatalytic   efficiency   of   the   g-C3N4 nanoparticle.

1.4 Aim and Objectives of the Study

The aim of this work is to study the solar photocatalytic degradation of organic pollutants in local dyeing wastewater using neem-modified graphitic-carbon nitride (g-C3N4) photocatalyst.

The aim of this study is to be achieved through the following objectives;

i.      Synthesis of g-C3N4 nanoparticles using melamine as the precursor.

ii.       Modification of  the as-synthesized g-C3N4 using nem leaf extract

iii.     Characterization of  the modified g-C3N4 photocatalyst

iv.      Evaluations of the phytochemicals present in the neem leaf extract solution.

v.      Performance evaluation of the modified g-C3N4   using chemical oxygen demand (COD) reduction as the response, with respect to the effects of time, solution pH and catalyst dosage.

vi.      Evaluate the kinetics of the photocatalytic reaction process.

1.5 Scope of the Study

The scope of this research work is limited to the synthesis of graphitic-carbon nitride (g-C3N4) photocatalyst, modification of the as-synthesized photocatalyst and its application in the degradation of organic pollutants in local dyeing wastewater.

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