Why Singlet Oxygen Is Important in the Degradation of Organic Molecules in the Environment - Essay Example

Section A a) Explain, with examples, why singlet oxygen is important in the degradation of organic molecules in the environment. Unlike the oxidative properties of the ground-state oxygen, singlet oxygen which by nature works in its excited state is capable of degrading molecules through photosensitization. A singlet oxygen is typically produced by a photosensitizer pigment with which such element facilitates the sun’s detrimental impact upon organic matters in significant number. Concrete instances would be the study conducted on the selective oxidative degradation of organic pollutants by singlet oxygen-mediated photosensitization as well as the eradication of cancer cells by singlet oxygen during photodynamic therapy. (b) Give an account of the preparation, biological effects and environmental degradation of (polychlorobiphenyls) PCBs. PCBs are organohalogens prepared by having chlorine react with biphenyl, an aromatic hydrocarbon used as a heat-transfer fluid. These compounds are found to have high resistance toward decomposition and due to low levels of environmental degradation, PCBs proceed to dwell in bodies of water, soil, and in an organism’s fatty tissue. They are biologically toxic to aquatic creatures, both vertebrates and invertebrates. As a grave hazard to human health, PCBs may lead to dysfunction of liver, carcinogenic effect, dizziness, and dermatitis. (c) Give a brief account of the factors that affect the interaction of exogenous organic compounds with soil. Several factors may affect the soil’s interaction with exogenous organic compounds including soil aggregation by waste products of bacteria. Besides aggregate stability, the interactive potential or resilience of soil to exogenous organic matters may be determined further by factors as land technology used, farming system, and agricultural management employed. 2. (a) Show how hydrolysis is important in the environmental degradation of endogenous chemicals. In the field of biotechnology, this can be demonstrated in terms of the water’s reaction to break polymeric bonds such as the ester links that are often present between two amino acids in a protein. After such bonds have been broken, a hydroxyl group (-OH) in water is yielded to one of the products while carboxylic acid becomes of the other upon acquisition of H+. Moreover, the environmental degradation of endogenous substances may be hydrolytically carried out with the aid of catalyzing enzymes in the process of decomposing other polymers like nucleotides, complex sugars, starch, and even fats. (b) For THREE of the following molecules or classes of molecule outline the key type of reaction which is responsible for its toxicity. Atrazine --- derived from the synthesis reaction involving cyanuric chloride, the chief precursor to the toxicity of atrazine, and isopropyl amine; this reaction is performed in xylene or toluene in the presence of water and addition of NaOH; Benzpyrene --- the new key reaction for the production of the environmentally carcinogenic benzo[a]pyrene (BaP) includes the method of coupling of naphthalene 2-boronic acid with 2-bromobenzene-1,3-dialdehyde which requires three steps, all in all; Section B 3. Answer BOTH parts. (a) pH electrodes are an example of potentiometric sensors. Describe in detail the term “potentiometric sensor”, ensuring that your answer includes a comprehensive account of the principles that control the mode of operation of such devices. A potentiometric sensor is a type of electrochemical sensor that measures voltage and in basic principle, it is through voltage measurement in cell potentials that the magnitude of electromotive force (EMF) causing electron flow may be determined. Since there is no way that the interfacial potential can be measured directly, potentiometric sensor is installed in an electrochemical cell whereby potentiometry, cell potential is estimated under zero current. Essentially, a potentiometric sensor depends upon the evolution of electrical potential it can detect at the surface of a solid material placed in a solution that can facilitate free ion-exchange with such surface. (b) pH electrodes can detect protons over a wide range of concentrations. Explain how this is possible. Your explanation should include a detailed description of the terms: surface and bulk interactions, dynamic range and saturation. It is common knowledge that a pH electrode is a hydrogen-ion sensor of most pH meters that make use of membrane-type glass electrode. It can detect protons over a wide concentration range due to surface and bulk interactions in which species are adsorbed on the surface and may partition by bulk between the sensor and the sample phase. This way, species in varying activities (concentration measure) interact with the pH electrode/sensor which bears the capacity of detecting the total number of active sites following the sensor’s dynamic range. The upper detection limit of the sensor in this dynamic (activity or concentration) range would indicate the level of saturation that the pH electrode can reach whereas the lower limit of detection refers to the minimum activity below which the sensor would not respond. 4. A serious leak of Hg vapour and methane gas is suspected at a plant. The leak is supposedly due to negligence by an operator suspected of consuming large amounts of alcohol. Describe what chemical sensors you would use, in situ, to confirm these suspicions. Your description should include details (chemical and physical) of how these sensors operate, in addition to information about their response times. To confirm leak suspicions, I would particularly use ‘chemiresistor’ for sensing Hg vapour and a ‘catalytic gas sensor’ for sensing methane gas. Since mercury (Hg) is a liquid metal at ordinary temperatures, a conductimetric sensor such as the chemiresistor would be suitable to detect it via conductivity and chemiresistor would operate as a bipolar device to measure conductance of mercury at either terminal once a thin film of gold adsorbs Hg ions for which it is both selective and sensitive. On the other hand, I would opt for ‘catalytic gas sensor’ in detecting methane gas leak since heat is evolved as a consequence of catalytic reaction with a flammable gas like methane which undergoes combustion upon contact with heat and air where oxygen is available. This sensor consists of a ThO2/Al2O3 pellet with a catalytic metal coil inside which is expected to measure resistance after physical acquisition of heat from the methane gas within a couple of seconds reaction time. Section A 1. Dissolved oxygen and pH are two parameters that are used in the analysis of water quality. The former can be measured using an amperometric sensor (Clark electrode) while the latter is assessed using a potentiometric sensor (pH glass electrode). Discuss in detail the physicochemical properties that govern the mode of operation of such electrodes. Your answer should include a plot of current-potential curves and the relevant equations that are used to determine the output of each sensor. Hence, describe the advantages and disadvantages of both types of sensors. In Clark electrode, a thin Teflon membrane divides the electrode compartment and the reaction chamber and this membrane is permeable to molecular oxygen which is allowed to reach the cathode for an electrolytic reduction in order to generate potential difference as oxygen receives electron flow out of oxidative phosphorylation. So basically, an amperometric sensor like the ‘Clark Oxygen Electrode’, governed by mass transport, can measure dissolved oxygen concentration on a catalytic platinum surface where the net reaction established is given by O2 + 2H2O + 2e- --- H2O2 + 2OH- / H2O2 + 2e- --- 2OH- under current that is limited by the diffusion of the electroactive species to the surface of the electrode. The amperometry of the Clark electrode follows the linear equation I = nFkC in which current (I) varies directly as the concentration (C). Potentiometric sensor via the pH glass electrode, on the other hand, evaluates the pH based on the EMF quantity found in an electrochemical cell which is related to the measured voltage by the Nernst equation E = E’ + [(RT) / (nF)] ln (Reactance / Product) and unlike the Clark electrode, no current is involved in this setting and the device’s wide dynamic range can detect protons at various activities or concentration degrees due to surface and bulk interactions occurring across binding sites. Furthermore, potentiometric sensors have selective permeability and are commonly available instrumentation. However, these sensors exhibit slow logarithmic response to change in dose and their selectivity is limited whereas amperometric sensors are quite sensitive and behave with linear response. Sensors that are amperometric, nevertheless, are subject to interferences and protein fouling. 2. Answer ALL parts (a) – (b). (a) Explain how calorimetry can be used in designing chemical sensors. Calorimetry deals with the science that involves determination of heat or enthalpy of reaction based on the energy derived from changes in the state or physical attributes of matter. With this primary idea, chemical sensors may be designed to become capable of detecting a chemical substance with an associated value of energy as estimated through sensible change in temperature or phase transition which the substance has undergone. (b) Explain in detail the physico-chemical differences between thermistors, pyroelectric devices, and catalytic gas sensors. Among the three thermal sensors, thermistors possess high-temperature sintered metal oxides covered with glass shields. These sensors are small and chemically inert yet are sensitive, stable, and affordable. In thermistors, the change in resistance is inversely proportional to the square of temperature whereas in pyroelectric devices, no amount of resistance is assessed, instead the modified position or polarization of atoms within a crystal structure is attributed to temperature change. Pyroelectric devices function by the concept of ‘pyroelectricity’ or the migration of positive and negative charges from one end to the opposite end of a polar crystal so that when an electric field is applied to such structure, the crystal is deformed due to the relocation of ions made by electrostatic forces. Not like the pyroelectric devices which are able to produce electrical potential when cooled or heated, catalytic gas sensors are sensitive to the liberated heat rather than potential in a controlled combustion setting. The sensing device does not go through physical deformation when used as it mainly utilizes platinum coil within ThO2/Al2O3 capsule in detecting any presence of combustible gases that tend to build up some measurable resistance. Section B 3. Answer ALL parts (a) – (d): (a) Ozone represents one of the most important natural oxidants. Describe its structure and generation in the environment and its common reactions. Ozone is a bluish gas with a boiling point of about -112°C and the following structure: Ozone is generated in the stratosphere through a process called “photolysis” which takes place when O2 molecules struck by solar radiation (with light rays < 240 nm), splitting atoms after which the free oxygen atoms combine to form O3 this time. Common reactions include reaction with both organic and inorganic species, especially with PAHs on surfaces. At troposphere, ozone reacts with the alkenes as well. (b) How is ozone generated for use in water purification? For use in water treatment, ozone is generated by passing electric discharge through air or O2. (c) Explain the structure and common reactions of ground-state oxygen. The structure of ground-state oxygen, or triplet oxygen, appears as a diradical and its electronic configuration has two unpaired electrons that occupy two degenerate molecular orbitals. Typical reactions include chains of free radical mechanisms, redox, and combustion. (d) Explain the structure and generation of singlet oxygen and describe its typical reactions. Singlet oxygen is reportedly developed on soil surfaces that are sensitized by soil organics wherein a photosensitizer pigment principally aids in its synthesis from the excitation of ground-state oxygen molecules. Absorption of light in the visible region and energy by the sensitizer proceeds to excite the singlet oxygen toward formation of its intended structure. By convention, singlet oxygen participates in oxidation of reactive organics in water and weathering of wood surfaces. It is also noted for its ability to influence reactions via Diels-Alder cycloaddition, -ene, heteroatom, and organometallic complex oxidation. 4. Answer ALL parts (a) – (c): (a) Describe the main components of a normal soil. A normal soil is composed of four major components namely – (i) mineral (inorganic) materials, (ii) organic materials, (iii) water, and (iv) air. These components may vary in relative proportions depending on climate and type of soil. (b) Give an account of the factors that control the ways in which exogenous organic molecules interact with soil. Several factors may affect the soil’s interaction with exogenous organic compounds including pH of soil and soil aggregation by waste products of bacteria. Besides aggregate stability, the interactive potential or resilience of soil to exogenous organic matters may be determined further by factors as land technology used, farming system, and agricultural management employed. Other ways include soil adsorption, chemical alteration, microbial, and photochemical degradations. (c) With examples, show how the pH of a soil might affect the degradation of exogenous organics. For one, the degradation of exogenous organics may be facilitated by the growth of bacteria the rate of which may prove to increase steadily under low pH conditions of soil since certain decomposers find it more conducive to thrive in an acidic environment. If, however, in another instance the soil pH is adjusted by adding lime to raise the pH, then this would necessarily increase the availability of phosphorus and nitrogen as it hastens the decomposition of exogenous organic matter. Section A 1. Piezoelectric and pyroelectric crystals can be used as mass and thermal chemical sensors respectively. Discuss in detail the piezoelectric and pyroelectric effects and explain, using examples, how such devices are incorporated into chemical sensors. By piezoelectric effect, the governing principle of piezoelectricity works such that the formation of electric dipoles is made possible by subjecting a piezoelectric crystal to a mechanical stress that would correspondingly change its mass. Consequently, the mechanical deformation that shifts the crystal’s mass herein further leads to change in the crystal’s resonant frequency. Similarly, electric potential and polarity are induced via pyroelectric effect only that instead of mechanical exertion, pyroelectric crystals experience displacement of electrical charges due to the effect of heating, cooing, or application of an electric force field. Piezoelectric crystals, traditionally α-quartz, are initially cut to the advantage of low temperature dependence and are incorporated in chemical sensors as membranes so that eventually, piezoelectric chemical sensors are mostly selective surface film-coated piezoelectric crystal oscillators by which wave property as resonant frequency may be assessed during the sensing stage. 2. Dissolved oxygen and pH are two parameters that are used in the analysis of water quality. The former can be measured using an amperometric sensor (Clark electrode) while the latter is assessed using a potentiometric sensor (pH glass electrode). Discuss in detail the physicochemical properties that govern the mode of operation of such electrodes. Your answer should include a plot of current-potential curves and the relevant equations that are used to determine the output of each sensor. Hence, describe the advantages and disadvantages of both types of sensors. In Clark electrode, a thin Teflon membrane divides the electrode compartment and the reaction chamber and this membrane is permeable to molecular oxygen which is allowed to reach the cathode for an electrolytic reduction in order to generate potential difference as oxygen receives electron flow out of oxidative phosphorylation. So basically, an amperometric sensor like the ‘Clark Oxygen Electrode’, governed by mass transport, can measure dissolved oxygen concentration on a catalytic platinum surface where the net reaction established is given by O2 + 2H2O + 2e- --- H2O2 + 2OH- / H2O2 + 2e- --- 2OH- under current that is limited by the diffusion of the electroactive species to the surface of the electrode. The amperometry of the Clark electrode follows the linear equation I = nFkC in which current (I) varies directly as the concentration (C). Potentiometric sensor via the pH glass electrode, on the other hand, evaluates the pH based on the EMF quantity found in an electrochemical cell which is related to the measured voltage by the Nernst equation E = E’ + [(RT) / (nF)] ln (Reactance / Product) and unlike the Clark electrode, no current is involved in this setting and the device’s wide dynamic range can detect protons at various activities or concentration degrees due to surface and bulk interactions occurring across binding sites. Furthermore, potentiometric sensors have selective permeability and are commonly available instrumentation. However, these sensors exhibit slow logarithmic response to change in dose and their selectivity is limited whereas amperometric sensors are quite sensitive and behave with linear response. Sensors that are amperometric, nevertheless, are subject to interferences and protein fouling. 3. Answer ALL parts (a) – (d). Describe the basic structural features (with diagrams where appropriate), origin in the environment and relevance of each of the following: (a) Humic substances are most abundant polymeric components of the natural organic matter (NOM) in soil and water as well as in geologic organic deposits such as lake sediments, peats, brown coals, and shales. They are derived through oxidation and condensation of polyphenols, polysaccharides, poly-amino acids, and poly-electrolytes. As they largely contribute to the browning or blackening of surface soils, they also improve soil fertility. (b) Polycyclic aromatic hydrocarbons (PAHs) pertain to a group of naturally occurring chemicals found in coal, crude oil, and gasoline. PAHs are recovered out of conversion of coal to natural gas or incomplete combustion of oil, gas, and organic wastes. As potent atmospheric pollutants, significant exposure to PAHs happens on eating grilled foods and breathing air that has been contaminated with coal tar or wild fires. (c) Dibenzofurans are heterocyclic organic compounds that are white crystal-like solid by nature and are derived from the production of coal tar. As an aromatic compound normally found in coke dust, grate ash, fly ash, and flame soot, a dibenzofuran is commonly used as an insecticide and an agent for manufacturing specific chemicals. (d) Organic nitrate esters are widely known for their explosive properties, such as nitroglycerin. They may be yielded through reaction of an alcohol with acid (as HNO3) to produce water and alkyl nitrate which embodies the ester portion. Recent studies indicate that organic nitrate esters, especially glyceryl trinitrate and isosorbide dinitrate are a class of compounds that bear huge potential in treating different vascular ailments upon bioactivation to nitric oxide. 4. Answer ALL parts (a) – (b). (a) With examples in each case, describe the following general routes by which organic substances in the environment are degraded: Oxidation enables organic matter decomposition by aerobic respiration wherein microbes and plants utilize oxygen to establish metabolic activity for organic compounds. During this process, carbon compounds are enzymatically oxidized to generate carbon dioxide, water, and decomposed biomass. Hydrolysis is the water’s reaction to break polymeric bonds in organic degradation wherein, after such bonds have been broken, a hydroxyl group (-OH) in water is yielded to one of the products while carboxylic acid becomes of the other upon acquisition of H+. This may be specifically demonstrated by the manner simple haloalkenes are degraded by hydrolysis and organic substitution to produce alcohols or related products. Moreover, the environmental degradation of endogenous substances may be hydrolytically carried out with the aid of catalyzing enzymes in the process of decomposing other polymers like nucleotides, complex sugars, starch, and even fats. Reduction normally couples the oxidation process in order to facilitate the biological degradation of complex hydrocarbon contaminants. While there occurs loss of electrons in oxidation, electrons are gained when substances are reduced during decomposition. (b) Comment briefly on the general mechanisms of microbial degradation. Microbial degradation is brought about by the decomposing action of microbes or biological agents like bacteria and fungi rather than chemicals. In general, factors such as the salinity, pressure, moisture, and pH in soils may affect the rate by which microbial degradation proceeds with its traditional mechanism. 2010/2011 1. Answer ALL parts A – B A. Compounds that block acetylcholinesterase have been widely used as insecticides. (i) Explain why blocking this enzyme causes death. One must note that acetylcholinesterase is an enzyme responsible for relaxing the activity of the neurotransmitter acetylcholine so that when this enzyme is blocked, it would cause nerve agents to interfere nerve signals to the brain. Eventually, blocking the action of acetylcholinesterase by nerve agents impedes on the acetylcholine’s level of transmission that this could possibly cause breathing problems, confusion, convulsions, and death at worst. (ii) Describe the types of compound that are commonly used in this way and comment briefly on their mode of action. Acetylcholinesterase inhibitors such as VX nerve agents are compounds that block acetylcholinesterase. While reversible inhibitors are of the therapeutic type, quasi-irreversible inhibitors like organophosphates and carbamates have been used as chemical weapons or pesticides. The blocking mechanism of these compounds basically suppresses the enzyme from breaking down acetylcholine, thereby increasing the degree and span of time for acetylcholine to continue neurotransmission. (iii) Why are such compounds generally relatively non-toxic to humans? Such compounds are generally relatively non-toxic to humans and even animals because majority of them, as insecticides (organophosphates), are developed to be highly toxic to target species only. B. pH electrodes can detect protons over a range of 30 decades. Explain how this is possible. Your explanation should include a detailed description of the terms: surface and bulk interactions, dynamic range and saturation. It is common knowledge that a pH electrode is a hydrogen-ion sensor of most pH meters that make use of membrane-type glass electrode. It can detect protons over a range of 30 decades due to surface and bulk interactions in which species are adsorbed on the surface and may partition by bulk between the sensor and the sample phase. This way, species in varying activities (concentration measure) interact with the pH electrode/sensor which bears the capacity of detecting the total number of active sites following the sensor’s dynamic range. The upper detection limit of the sensor in this dynamic (activity or concentration) range would indicate the level of saturation that the pH electrode can reach whereas the lower limit of detection refers to the minimum activity below which the sensor would not respond. 2. A serious leak A serious leak of Hg vapour and methane gas is suspected at a plant. The leak is supposedly due to negligence by an operator suspected of consuming large amounts of alcohol. Describe what chemical sensors you would use, in situ, to confirm these suspicions. Your description should include details (chemical and physical) of how these alcohol, Hg Vapour, and methane sensors operate, in addition to information about their response times. To confirm leak suspicions, I would particularly use ‘chemiresistor’ for sensing Hg vapour and a ‘catalytic gas sensor’ for sensing methane gas. Since mercury (Hg) is a liquid metal at ordinary temperatures, a conductimetric sensor such as the chemiresistor would be suitable to detect it via conductivity and chemiresistor would operate as a bipolar device to measure conductance of mercury at either terminal once a thin film of gold adsorbs Hg ions for which it is both selective and sensitive. On the other hand, I would opt for ‘catalytic gas sensor’ in detecting methane gas leak since heat is evolved as a consequence of catalytic reaction with a flammable gas like methane which undergoes combustion upon contact with heat and air where oxygen is available. This sensor consists of a ThO2/Al2O3 pellet with a catalytic metal coil inside which is expected to measure resistance after physical acquisition of heat from the methane gas within a couple of seconds reaction time. 3. Answer ALL parts A – B. A. Draw the chemical structure of ozone. Discuss its origin in the environment, and the nature of its reactions with organic molecules. Illustrate your answer with suitable chemical reaction schemes. Ozone is a bluish gas with a boiling point of about -112°C and the following structure: Ozone is generated in the stratosphere through a process called “photolysis” which takes place when O2 molecules struck by solar radiation (with light rays < 240 nm), splitting atoms after which the free oxygen atoms combine to form O3 this time. Common reactions include reaction with both organic and inorganic species, especially with PAHs on surfaces. At troposphere, ozone reacts with the alkenes as well. B. Describe the basic structural features (with chemical structures wherever appropriate), environment stability and toxicity of: References http://www.ncbi.nlm.nih.gov/pubmed/22852818 http://www.britannica.com/EBchecked/topic/468382/polychlorinated-biphenyl-PCB http://www.fao.org/docrep/009/a0100e/a0100e09.htm http://biotech.about.com/od/glossary/g/hydrolysis.htm http://www.ncbi.nlm.nih.gov/pubmed/14961695 http://www.lenntech.com/ozone/ozone-properties.htm http://www.idph.state.il.us/envhealth/factsheets/polycyclicaromatichydrocarbons.htm http://www.epa.gov/osw/hazard/wastemin/minimize/factshts/dibenzof.pdf http://dmd.aspetjournals.org/content/27/4/502.full.pdf http://www.landfood.ubc.ca/soil200/interaction/orgmatter_air.htm Read More