DETERMINATION, DEGRADATION AND REMOVAL OF POLYCYCLIC AROMATIC HYDROCARBONS

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are among the few toxic materials present ubiquitously in the environment. Pollution is growing explosively as a result of rapid industrialization in the developed and developing countries. The degradation of the environment, mainly in urban areas, is a global issue. The problem is acute in the developing countries where the implementation of the legislation is poor. A variety of inorganic and organic pollutants get access to the terrestrial and aquatic bodies from various sources. Out of these the organic pollutants are larger in number as compared to inorganic pollutants. The organic pollutants are of particular concern because of their toxicity and bioaccumulative nature mainly due to their tendency to bind with protein molecules. A majority of the organic pollutants easily degrade in the environment but some of them persist for a longer time and are known as persistent organic pollutants (POPs). POPs include insecticides, pesticides, dioxins, polychlorinated biphenyls (PCBs) and PAHs etc. Among the POPs, polyaromatic hydrocarbon (PAHs) is a major category of pollutants. PAHs are of prime concern because of their ubiquity, undesirable concentrations encountered in the industrial, domestic and agricultural wastes, potential for magnification in the food chain and harmful biological effects. Large volumes of PAHs are released into the environment from various sources such as bitumen and asphalt industries, petrochemical industries, rubber tire manufacturing, waste incineration and thermal power plants. After emission from various sources and due to persistent characteristics, PAHs can disperse into the environment through atmospheric transport and deposit on the soil surface. The wastewater containing PAHs is released directly to the fresh water systems by the various industries resulting into the contamination of fresh water. The relentless release of PAHs results in irrevocable hazard to the human health and environment. Agency for Toxic Substance and Disease Registry (ATSDR, Georgia) governed by U.S. Department of Health and Human Services has reported a large number of ecotoxicolgical effects of PAH exposure such as leukaemia, leukoplakia, hematuria, buccal-pharyngeal cancer, bladder cancer and respiratory disorders. The PAHs released into the environment, are partitioned into the air, water and soil on the basis of their properties. An understanding of PAH distribution is essential to design remedial measures for PAH contamination. Determination of PAHs in various segments of ecosystem is important as all of us are affected in some way or the other by their presence in the environment. Soil ecosystem is the ultimate repository for most of the hydrophobic organic IV contaminants such as PAHs. The top layer of the soil is relatively more polluted as a result of atmospheric deposition. The contaminated soil is not only harmful for humans, but also for biota - plants and microorganisms. In the last two decades, a number of studies have been carried out globally to monitor PAH contamination and designate major emission sources of PAHs at various sites. Different research groups have monitored the levels of PAHs in ambient air and soils of various cities of India. Delhi, the capital of India is one of the top ten polluted cities of the world. A few research groups have monitored PAH contamination in different compartments of the ecosystem such as air, water and soil of Delhi. In the present study a large area of Delhi covering interstate bus terminals, busy traffic sites, thermal power plants and crematorium has been monitored. A total of seven PAHs including both low molecular weight PAH namely naphthalene, fluorene, phenanthrene, anthracene and high molecular weight PAH namely fluoranthene, pyrene and benzo[a]pyrene were selected for the study. The low molecular weight congeners show long range atmospheric transport and disperse to a wider extent and are abundant as compared to high molecular weight PAHs. Benzo[a]pyrene is generally considered as an environmental indicator to assess the toxicity due to high molecular weight PAHs. Once PAHs are released in the environment, they impair the quality of air, water and soil depending on their persistence. Due to high prevalence, persistence and harmful effects on human health the studies on polycyclic aromatic hydrocarbons have been lately focused on developing efficient remediation methods for PAH contamination. In the last two decades, thermal desorption and biodegradation methods have been studied for the treatment of polluted soils. These methods have some limitations such as high cost and slow biodegradation of high molecular weight PAHs. Photocatalytic degradation of PAHs is an efficient and cheap process for the remediation of contaminated soil. The degradation of PAHs and the type of metabolites formed depend upon the chemical nature of PAH, type of medium and environmental conditions. Sometimes the metabolites are more toxic than the parent compound. In order to keep a proper track of the cycle of these materials a careful study on their persistence in soil under different environmental conditions is required. In view of the above premise photocatalytic degradation of pyrene and benzo[a]pyrene has been studied in soils of different pH in the presence of iron oxides. The metabolites have been identified and probable pathways proposed. V Widespread concern over toxicity and environmental impact of pollutants have led the scientists and environmentalists to develop effective and cost effective technologies for the removal of toxic pollutants from water and wastewater systems. Adsorption is a simple, versatile and powerful technique for the removal of pollutants from water and wastewater systems. An adsorbent is considered as low cost if it is abundant, requires a little processing, by product or waste product of any industry. Therefore, agricultural, municipal and industrial wastes have been used as low cost adsorbents in their natural form or after physical or chemical modifications. In the case of aquatic ecosystem, generally, the high molecular weight PAHs settle down on the sediments, whereas, low molecular weight PAHs are highly available in the overflowing water systems based on their solubility. In view of this, studies on the adsorption of low molecular weight PAHs from water were planned. Activated carbon is the most widely used adsorbent for the removal of pollutants. Several raw materials such as rice husk, coconut husk, plant bark, vegetable waste and chicken waste have been used for the synthesis of activated carbon. In the present study, two different waste materials viz banana peels and waste vehicular tires have been used to develop activated carbons. Banana peels are easily available and cheap raw materials for the development of efficient adsorbents. India is the largest producer of banana and hence the largest producer of banana peels waste. On the other hand, automobiles have become an essential means of transportation in our daily life and a huge number of vehicular tires are discarded every year. Vehicular tires do not decompose easily due to their cross-linked structure, presence of stabilizers and additives. Therefore, these are disposed by incineration and landfilling, which pose serious environmental pollution. In the present study, banana peels and waste vehicular tires have been converted to activated carbons which have been utilized for the removal of polycyclic aromatic hydrocarbons from aqueous systems. The adsorption studies were focused on the removal of three low molecular weight PAHs namely, naphthalene (Naph), fluorene (Flu) and phenanthrene (Phen) from aqueous systems. For convenience and clarity of presentation, the subject matter of the thesis has been divided into following six chapters: I. General Introduction II. Materials, Methods and Techniques III. Distribution, sources and toxic potential of PAHs in urban soils of Delhi, India IV. Photocatalytic degradation of benzo[a]pyrene and pyrene and identification of VI degradation products V. Adsorption of polycyclic aromatic hydrocarbon on banana peel activated carbon VI. Adsorption of polycyclic aromatic hydrocarbon on vehicular tire activated carbon Chapter I gives a brief idea about the terrestrial and aquatic pollution by PAHs. The sources of polycyclic aromatic hydrocarbons are highlighted. The objective of the present study and need to carry out determination and removal of PAHs is discussed. The relevant literature on different aspects has been included in the respective chapters. Chapter II deals with the details of sampling and preservation of the samples. The methodology used for the extraction, filtration and clean up of the samples is also discussed. Soil samples were analysed using RP-HPLC procedure and the water samples were analysed using UV spectrophotometer. The results obtained for the determination of PAHs in urban soils of Delhi were subjected to statistical analysis such as principle component analysis, hierarchical cluster analysis. The degradation products formed after photocatalytic degradation of PAHs were analysed using LC-MS. The synthesis of five iron oxide photocatalysts and activated carbons from waste materials is also described. Various instrumental techniques employed for the characterization of photocatalysts and adsorbents have been discussed in brief. Chapter III embodies a systematic study on PAHs distribution in soils of Delhi, in pre-winter and post-winter seasons. Investigations were focused on seven PAHs namely Naphthalene, Fluorene, Phenanthrene, Anthracene, Fluoranthene, Pyrene and Benzo[a]pyrene. In order to draw some latent and relevant information the data were subjected to multivariate statistical analysis. The isomer pair ratios, principle component analysis were employed to ascertain the major sources of PAHs in soils of Delhi. Hierarchical cluster analysis was performed for the assessment of risk associated with the studied sites. The work will provide an overview of PAH levels in the soils of Delhi region due to different sources. Moreover the data will act as a benchmark for further studies. Chapter IV describes the decay profiles and metabolic pathways of benzo[a]pyrene and pyrene on laboratory synthesized pure iron oxides as well as in soils of varying pH in presence of goethite under UV irradiation. Among the investigated iron oxides, goethite has fastest photocatalytic activity. The effect of various parameters such as photocatalyst dose, soil pH, wavelength of irradiation, presence of oxalic acid on the degradation of B[a]P and pyrene is discussed. The results suggest fast and efficient degradation of B[a]P and pyrene in the VII presence of goethite and oxalic acid. The photodegradation of B[a]P and pyrene occurs through the oxidative metabolism. The degradation pathways of the said PAHs in three different types of soil are illustrated. Identification of metabolites suggest that in acidic soil in presence of goethite, B[a]P is converted to smaller PAH, pyrene in 120 h. The metabolite study of pyrene suggests that, pyrene is degraded to smaller hydrocarbons, naphthalene, phenanthrene and their derivatives. Some of the metabolites such as diones, diols and epoxides, reported to be toxic, disappear after 120 h in all the three soils. The results of the study thus provide an efficient method for the remediation of PAHs and also provide a data bank for toxicological analysis. Chapter V includes studies on the adsorption of polycyclic aromatic hydrocarbons on activated carbon developed using banana peel waste. Three PAHs with less than four rings, namely, naphthalene, fluorene, phenanthrene were chosen for the study due to their high availability in different water and wastewater systems. The adsorption data was subjected to Freundlich and Langmuir isotherms. The effect of various parameters such as contact time, adsorbent dose, pH and temperature on the adsorption was evaluated. Different thermodynamic parameters were calculated to ascertain the nature and spontaneity of the adsorption. The negative values of Gibb’s free energy change, ΔG°, and the positive value of enthalpy change, ΔH°, reveal the spontaneity and endothermic nature of the adsorption process, respectively. The desorption studies suggest that the adsorbent can be readily regenerated using NaOH solution in 50 % ethanol which is cheap and easily available. Chapter VI discusses adsorptive removal of PAHs from aqueous system using activated carbon developed from waste vehicular tires. The adsorption of PAHs was achieved with varying contact time, adsorbent dose, solution pH, adsorbate concentration and temperature. Langmuir and Freundlich isotherm models were applied and thermodynamic parameters were evaluated. The regeneration of activated carbon is achieved using 1 M NaOH in 50 % ethanol. The work not only provides a solution for the tire disposal problem but also provides a cheap and highly efficient adsorbent for the treatment of wastewater generated by various industries. The thesis concludes with a brief discussion on the findings of the present investigations. Based on the results an attempt has been made to offer remedial measures to conserve and avoid further deterioration of the environment. The results suggest that photodegradation of PAHs can be used for the treatment of soils of polluted sites. A major VIII fraction of the domestic and industrial waste getting access to surface water bodies is untreated. More treatment plants have to be installed and the plants operating under efficiency are to be upgraded. The small scale industries located in various areas should be connected to the common treatment plants to ensure that no effluent enters freshwater systems. The levels of pollutants in soil and water should be regularly monitored.