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  EVALUATIVE ASSESSMENT OF THE FATE AND TRANSPORT OF OXYTETRACYCLINE FERNANDEZ, JEFFREY JOSEPH P. School of Chemical Engineering and Chemistry, Mapua Institute of Technology Intramuros, Manila Philippines  ABSTRACT The use and abuse of oxytetracycline (OTC) both in human medicine, veterinary medicine, and livestock promotion are potential human and environmental exposure routes for low concentrations of this antibiotic. While the mechanisms of such exposure scenarios are unknown, continuous emission of OTC to the environment may facilitate the development or  proliferation of antibiotic resistance. Due to the health and economic impacts of antibiotic resistance, it is imperative to improve our understanding of the fate of OTC in the environment. This paper provides an example of an evaluation procedure to predict the fate of OTC in the environment that can also be applied to other antibiotics by using the EQuilibrium Criterion (EQC) model. Three emission scenarios were investigated: 100% emission to soil, 100% emission to water, and 50% emission to soil with 50% emission to water. It has been found that when OTC is released 100% to water, more than 99% will remain in the water compartment until it is degraded and advected. When 100% released to soil, only 12.4% leaves the soil compartment and the residence time of OTC in the environment increases by a magnitude of 15. In the mixed emission scenario, about 75% of the OTC remains in the soil compartment due to low reaction and advection rates in soil. The results of the study also suggest that the future risk assessments should focus on the soil compartment and the terrestrial ecosystems. Keywords: oxytetracycline, chemical fate, EQC model, antibiotic resistance INTRODUCTION Antibiotics are invaluable to humans in the fight against infectious diseases. However, the emergence of antibiotic-resistant strains has challenged the effectiveness of antibiotics and has already become a major threat to global public health. Oxytetracycline (OTC) is one of the most widely used pharmaceuticals today not only to humans but also in veterinary medicine [1]. It is also commonly used in livestock animals, including cattle, swine, poultry, and fish for therapeutic treatment and as a growth promoter due to its broad spectrum and low cost [2].   In the US, Streptomycin and OTC are the only antibiotics allowed for plant agriculture while OTC is the only antibiotic that can be used internally in plants [3]. OTC, together with other antibiotics, is widely found in the environmental compartments [4][5][6][7][8][9]. This is due to the fact that 75% of the administered OTC dose is eliminated in the urine and feces of animals, still in its biologically active form [10]. This is one of the dominating pathways by which OTC can enter the environment [11]. Also, studies show that elimination in municipal wastewater treatment plants is often incomplete [12] and although the concentration is low, the presence of OTC in the environment could facilitate the exchange of antibiotic-resistant genes among microorganisms [13][14][15]. Antibiotic resistance is a major threat to global public health and jeopardizes even advanced medical treatments [16]. It has been noted that antibiotic-resistant infections double the duration of hospital stay, mortality, and probably morbidity and the costs as compared with  drug-susceptible infections [17]. Along with the adverse effects of antibiotic resistance, other negative effects of OTC and other antibiotics in the environment may include, but are not limited to allergic reactions, nephrotoxicity (kidney toxicity), negative interaction with the gut, sensitivity to light, and teeth damage among children [18].   Most of the studies regarding the occurrence of OTC in the environmental compartments are conducted in the USA and Europe. The data on environmental levels of OTC are also insufficient and needs to be expanded geographically, particularly in developing countries. Lack of data can be attributed to the high cost of modern equipment and techniques being used to detect trace amounts of antibiotics in the environmental compartments that developing countries cannot afford. Moreover, the behaviour of OTC in the environment is different from that of the human body [19]. In this respect, very little is known about the fate and risks of OTC found in environments like wastewater, sludge, surface water, and soil as compared to its medical use and effectiveness [20]. Without fundamental data on the fate and risks associated with OTC entering the environment, a full environmental risk assessment cannot be performed. The main objective of this study is to provide an example of an evaluation procedure on the fate of OTC which can also be applied to other antibiotics. The study also aims to determine the possible distribution and residence time of OTC in the environment and to identify the major processes that govern the fate of OTC by using the EQC multimedia fugacity model. Minimizing exposure to antibiotics is still the best way to combat antibiotic resistance as development of new antibiotics takes time and money. If data are available regarding the fate and transport of antibiotics, environmental “hotspots” can be identified and a better course of action can be planned. In that way, exposure to antibiotics is minimized and the antibiotic becomes effective for a longer period of time. For example, regulations in developing countries mostly emphasize on limiting the biological oxygen demand (BOD) and chemical oxygen demand (COD) but not the concentration of antibiotics present in wastewater effluents. In addition, antibiotics as growth promoters for animals are often not regulated. The results of this study can be directly used to perform a hazard assessment and may guide assessors in performing an environmental risk assessment. It can also help decision-makers whether or not additional treatments or new policies are needed. The focus of this paper is limited to the parent molecule, OTC. Metabolites of OTC may merit further investigation regarding their fate and transport in the environmental compartments. Furthermore, hydrolysis, photolysis, and biodegradation are the only reactive  processes covered in this study. The EQC model is an evaluative fate and transport model for  predicting and quantifying migration and partitioning within a standard environment [21]. Therefore, the environmental parameters (i.e., air volume, water depth) are fixed.  Nevertheless, even if the model used in this study is evaluative, as opposed to a real model, the EQC model is sufficient in meeting the objectives of this study. MATERIALS AND METHODS The main objective of this study is to provide an example of an evaluation procedure of OTC fate that can be applied to other antibiotics and establish the general features of OTC  behaviour namely, percentage distribution and residence times. This process also aims to identify the dominating processes that govern OTC fate. Figure 1 shows the process of how the evaluation procedure of OTC fate will take place. The data gathering involves an extensive literature research to look for the experimental values of the physico-chemical  properties needed as model input. When data from the literature are not sufficient, the EPI Suite TM  will be used to estimate the missing data. The input data will then be processed in the EQC model program and the results obtained.   Figure 1:   Conceptual Framework Data gathering. Data gathering is the first step in developing an evaluation procedure of OTC fate using the EQC model. The data to be gathered consist of physico-chemical and environmental properties needed as model inputs. Under the literature review, the available data for OTC is summarized in Table 1. Table 1: Properties of OTC Property Value Reference Molecular weight 460.4 g/mol *calculated from molecular formula Melting point 184.5 ºC Lewis (1997) Water solubility 313 mg/L (25ºC) Yalkowsky and Dannenfelser (1992) log Octanol-water partition coefficient Hydrolysis reaction half-life Photodecomposition half-life in water -1.12 14.04 d (25ºC) 12.4 d (27ºC) Loke et al. (2002) Doi and Stoskopf (2000) Choo (1994) Using the EPI Suite TM . In case of insufficient experimental data, the EPI Suite TM  uses the molecular structure of the substance in predicting physico-chemical and environmental  properties. There are three ways by which the molecular structure can be interpreted by the software. Any one of the CAS number, the SMILES string, or the chemical name of the substance can be used as input. After processing the input, one may click on one of the many tabs above the results window according to the property being sought. EQC Model Simulation . Once the input data is complete, they will be encoded in the EQC computer program. However, some properties are temperature dependent so one must make sure that the reference temperature is the same for all the properties being encoded. The  following equations will help the assessor in bringing the parameter values to the same reference temperature To estimate a chemical property at a different temperature using the enthalpy of phase change,  ΔH  , use Eq. (1).      [(    )]   (1) To estimate a reaction half-life at a different temperature using the activation energy,  E  a   (always positive), use Eq. (2).     [  (    )]  (2) Reaction half-lives are more common in the literature than the overall environmental half-lives in each of the compartments. For first order kinetics, use Eq. (3) to determine the overall half-life in each compartment,          (3) where n  is the number of degradation reactions considered. In the context of this study, hydrolysis, photolysis, and biodegradation all occur in the water phase however, only the effects of biodegradation were considered in the soil and sediment compartment. Note that all the reactions follow first order kinetics. In the simulations, only Level III calculations will be performed as it is the most realistic of the three levels and is the most useful. Three emission scenarios will be considered as outlined in Table 3. Table 3: Emission Scenarios for EQC Simulation Scenario 1 Scenario 2 Scenario 3 Air 0 0 0 Water 1000 kg/h 0 500 kg/h Soil 0 1000 kg/h 500 kg/h Sediment 0 0 0 The percentage distribution and the residence time will be directly obtained from the output diagram. The dominating processes will be determined by comparing the process flow rates in each process. RESULTS AND DISCUSSION Scenario 1. When OTC is emitted 100% to water, as depicted in Figure 5, most of the OTC will be present there. The major disappearance phenomena is reaction at 830 kg/h. This only means that when OTC is initially released to the water compartment, most of it (about 99.80%) will remain in the waters and only 0.20% will move out to the other compartments. Then, in the water, the OTC will stay until 83% of it is lost by degradation reactions  primarily photolysis and hydrolysis and some (17%) is advected or transported into other systems.
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