STUDIES ON ADSORPTION AND SOLAR PHOTO-FENTON PROCESSES BY MINERAL AND SULPHUR RICH COALS

Abstract

Wastewater has drawn significant scientific attention. Composition of wastewater can vary depending on its source for example; wastewater from industrial effluents may contain high concentration of metals and/or organic compounds which are often hazardous to human as well as aquatic flora and fauna. On the other hand domestic wastewater comprises of traces of metal ions of Al, Cu, Zn, Fe, Pb, Hg, polyaromatic hydrocarbons and also phthalates According to the United Nations World Water Development Report, around 2,212 km3 of wastewater is released into the environment per year globally. It includes agricultural wastes (e.g., pesticides, fertilizers), industrial and municipal effluents. It is related to detrimental impact on humans, animals, on the quality of ambient fresh water resources and also on ecosystems. Moreover the seriousness of this problem can be gauged from a World Health Organization (WHO) report which pointed out that water pollution is major reason for high mortality rate, which accounted for an approximate 2.2 million people die every year in developing and under developing countries. Thus water pollution is a challenging problem and its remediation requires immediate and serious attention. Among several conventional treatment methods, adsorption is the most widely explored for the removal of dissolved contaminants form aqueous phase onto the surface of solid substrate called adsorbent. As adsorption is not an ultimate remediation of removing hazardous substances. It helps in transferring the pollutants from one domain to another. Alternately degradation is also another remediation choice. Degradation of pollutants through Advanced Oxidation Processes (AOPs) has received increasing attention in the research and development of waste water treatment technologies. AOPs involve the simultaneous use of more than one oxidation process that cause the generation of highly reactive hydroxyl free radical species. Due to its high standard potentials of 2.8 V versus normal hydrogen electrode (NHE) in acidic media and 1.55 V versus NHE in basic media, the hydroxyl radical is highly reactive. It can oxidise and decompose numerous hazardous compounds non-selectivity into CO2 and water. In this thesis work, we explored utilization of low grade coal for removing hazardous metals and organic compounds. Mineral rich or pyrite rich coals are not suitable as fuel for ii thermal power plants as they tend to generate large volume of fly ash and also release toxic flue gases such SOx into air due to burning of coal. Though there are ample studies on adsorption of toxic heavy metals on low grade coals, but no studies are available on use of pyrite rich coal for removal of hazardous compounds. The idea behind this thesis work is to alternately utilize these low grade coals as adsorbents and also explore the option for photo-Fenton process for removal of hazardous constituents. The major advantages of using coal as an adsorbent arises from its low cost, easy availability and scope for chemical functionalization of the surface of the coals. The functional groups in coal, e.g., carboxylic acid, ethers, esters and phenolic groups could enable removal of heavy metals and phthalates from water medium. In addition, it was realized during formulating this research work that pyrites in coal could potentially generate hydrogen peroxide via Haber – Weiss reaction and can be a good material for degradation of organic pollutants via photocatalytic or photo-Fenton process. Such concept is very novel as there are no scientific reports on using coals for degrading organic compounds. This thesis consists of seven chapters. Chapter 1 consists of a general introduction about the environmental impact of these pollutants and their removal techniques. Chapter 2 presents a brief but comprehensive literature review on the removal studies of selected contaminants which are of our interest. For example adsorption of uranium, organic dyes and phthalates by carbon rich adsorbents are discussed. The status quo of use of coal as adsorbent is briefly outlined. A brief description about degradation of dye molecules via photocatalytic process and by photo-Fenton process is discussed. It includes aim and scope of the present research work on the basis of well identified research gaps from literature. Chapter 3 comprises of coal sample collection from five different coal fields in India. The composition of the coal samples was determined by energy dispersive X-ray analysis (ED-XRF) and the elemental analysis were performed by carbon, hydrogen, nitrogen sulphur (CHNS) analysis. Chapter 4 consists of two parts i.e., (i) characterization of one representative coal sample from each coal field as adsorbent; (ii) adsorption studies using basic blue 41 dye as a model dye. The XRD results revealed mineral phases, e.g., silicon dioxide (quartz) and aluminium silicate (kaolinite). These minerals are mostly found as inclusions in coals, which is corroborated by 2-D iii elemental maps obtained from scanning electron microscopy coupled with energy dispersive X-ray analyzer ((SEM-EDAX) measurements. The FT-IR results supported the occurrences of clay minerals and confirmed various functional groups. The coal samples with low carbon content revealed higher basic blue dye adsorption capacity (qe ~ 8.0-9.3 mg/g) at optimized adsorbent dose (2 mg/mL), pH 9 and contact time (120 min). The negative zeta potentials of the coal samples dispersed in aqueous medium were favourable for adsorbing more than 90% cationic azo dye (Basic Blue-41). The adsorption kinetic studies satisfied pseudo second order model and the intra-particle diffusion of the dye was evident. All adsorption followed Freundlich isotherm. The kinetic and thermodynamic studies Basic Blue-41 dye is performed to understand the adsorption mechanism. Chapter 5 describes synthesizing coal modified by chitosan composite (referred as coal-chitosan composite) where the surface of a mineral rich low grade coal was modified with chitosan biopolymer. The idea was to increase the amine groups on the surface of coal for achieving enhanced adsorption of di-ethyl phthalates. The synthesis of coal-chitosan composite has been ascertained from XRD, zeta potential, thermogravimetric analysis (TGA), FE-SEM and CHNS. The surface area of the modified coal was determined from Brunauer Emmett Teller (BET) measurements. An optimized 9:1 weight ratio of coal-chitosan composite exhibited positive zeta potential in aqueous medium and offered 91.1 % adsorption of DEP owing to electrostatic interaction and hydrogen bonding between the DEP and the composite, at an optimized pH 5.8, dose of 4 mg/mL and contact time of 4 h. The adsorption of DEP on coal-chitosan composite was thermodynamically favorable and followed pseudo-second order kinetics, attributable to chemisorption process. The adsorption isotherm data was best fitted non-linearly by Sips isotherm model, which is consistent with Freundlich adsorption isotherm for lower concentration of DEP (5 to 100 mg/L), while Langmuir adsorption was favored for adsorption of higher DEP concentrations (100 to 400 mg/L). The qmax was 42.67 mg/g, which was a significant improvement in DEP sorption from aqueous medium among the readily degradable adsorbents reported so far. The practical application of coal-chitosan composite as adsorbent has been demonstrated by its re-usability and ability for sorption of DEP from spiked municipal wastewater, supported by decrease in chemical oxygen demand value of treated wastewater. iv Chapter 6 comprises of a simple strategy of synthesizing coal modified with Adenosine 5/ monophosphate (referred to as Coal-AMP composite). The idea for this scheme was to incorporate amines and phosphate groups at the surface of mineral rich waste coals for achieving enhanced uranium removal from aqueous medium. The synthesis of coal-AMP has been ascertained from FT-IR, zeta potential, 31P NMR, BET. The adsorption of uranium has been confirmed by 2D uranium mapping from FE-SEM coupled with EDAX studies and from X-ray photoelectron spectroscopy (XPS) studies. The temperature dependent uranium adsorption data was best fitted with Langmuir isotherm model, with very high adsorption capacity (qmax = 624.62 mg/g) at pH 6. Such high uranium adsorption capacity is attributable to favourable conditions e.g., higher amide and phosphate binding sites and negative zeta potential of Coal-AMP at optimized condition. A systematic study of uranium adsorption revealed pseudo second order kinetic model and the thermodynamic parameters confirmed physisorption mechanism. Adsorption studies were also conducted in presence of common interfering metal ions present in drinking water to simulate uranium adsorption efficiency from drinking water. Looking at the scope for enhancing uranium adsorption by functionalizing with AMP, we have explored modification of graphene oxide by AMP as uranium adsorbent. The synthesis of graphene oxide reduced by AMP (referred to as RGO-AMP) and qualitative uranium adsorption has been ascertained from FT-IR, zeta potential, 31P NMR, XPS, BET, FE-SEM, Raman spectroscopy. The adsorption isotherm was well fitted with Langmuir isotherm with extremely enhanced uranium adsorption capacity (qmax = 3024.5 mg/g) at pH 6. Similarly, systematic study of uranium adsorption revealed pseudo second order kinetic model and the thermodynamic parameters confirmed physisorption mechanism. Adsorption studies were also conducted in presence of common interfering metal ions present in drinking water to simulate uranium adsorption efficiency from drinking water. In Chapter 7 we have explored the scope of generation of reactive oxygen species on surface of pyrites made us to explore an alternate use of sulphur rich pyritic coals as solar photo-Fenton agent for degradation of toxic azo dyes. The textural, compositional and morphological features of a sulphur rich pyritic coal are studied by an array of characterization techniques, e.g., X-ray diffraction, elemental analyzer, X-ray fluorescence spectrometry, X-ray photoelectron spectroscopy and by field emission scanning electron microscopy. The spatial distribution of pyrite phase in coal was obtained from 2D elemental mapping by scanning electron microscopy coupled with energy dispersive X-ray analyzer. The v dye degradation capacity by the pyrite rich coal has been studied using model dye solutions e.g., Basic Blue 41 (single azo goup) and Trypan Blue (two azo groups). The rate of degradation (k) is faster for Basic Blue-41 (0.0192 min-1, ~99% degradation) for optimized solar radiation exposure time, pH 6 and dose of 1 g/L pyritic coal. The degradation of dye is given as a measure of solar radiation accumulation energy per unit volume, attributed to in-situ generation of hydrogen peroxide and hydroxyl radicals, confirmed by terepthalic acid assay iodometry titration, and was re-confirmed by their scavenging studies. The degradation of dye is evident from qualitative identification of trace quantities of degradation products by gas chromatography-mass spectrometry technique. Leaching of 2700 mg/L iron in the reaction medium, measured by inductive coupled plasma optical emission spectrometry and requirement of sunlight exposure suggested solar photo-Fenton mechanism for the degradation of azo dye by pyrite rich coal. Finally, the thesis work has been summarized to present an overview of our ideas of alternate utilization of mineral rich coals as adsorbent and photo-Fenton agents for potential wastewater treatment. Still there are enough opportunities to convert such waste coals to valuables. These are outlined as future scope for further pursue.