Lecture on Environmental Chemistry

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Published Date:21-07-2017
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Narrative for a Lecture on Environmental Chemistry Slide 1: Environmental chemistry is that branch of chemical science that deals with the production, transport, reactions, effects, and fates of chemical species in the water, air, terrestrial, and biological environment and the effects of human activities thereon. ENVIRONMENTAL CHEMISTRY Environmental chemistry is that branch of chemical science that deals with the production, transport, reactions, effects, and fates of chemical species in the water, air, terrestrial, and biological environments and the effects of human activities thereon. Reference: Stanley E. Manahan, Fundamentals of Environmental Chemistry, 3rd ed., Taylor & Francis/CRC Press, 2009 (manahansmissouri.edu) For additional information about environmental chemistry and to download this and other presentations see: http://sites.google.com/site/manahan1937/Home http://manahans1.googlepages.com/Slide 2: The definition of environmental chemistry is illustrated with a typical pollutant species. In this case sulfur in coal is oxidized to sulfur dioxide gas that is emitted to the atmosphere. The sulfur dioxide gas can be oxidized to sulfuric acid by atmospheric chemical processes, fall back to Earth as acid rain, affect a receptor such as plants, and end up in a "sink" such as a body of water or soil. ILLUSTRATION OF THE DEFINITION OF ENVIRONMENTAL CHEMISTRY SO + 1/ O + H O 2 2 2 2 H SO 2 4 SO 2 H SO 2 4 S(coal) + O SO 2 2 H SO , sulfates 2 4Slide 3: In the past many environmental problems were caused by practices that now seem to be totally unacceptable as expressed from the quote in this slide from what was regarded as a reputable book on the American chemical industry in 1954. The result was polluted air, polluted water, dangerous hazardous waste sites, and harm to living organisms. The Old Attitude: “By sensible definition any by-product of a chemical operation for which there is no profitable use is a waste. The most convenient, least expensive way of disposing of said waste—up the chimney or down the river—is best.” (From American Chemical Industry —a History, W. Haynes, Van Nostrand Publishers, 1954) Up the stack Into the Out the door river Slide 4: Dating from around 1970, laws and regulations were implemented to control air and water pollution and to clean up hazardous waste sites. These measures have relied largely upon “end of pipe” controls in which pollutants were generated but were removed before release to the environment. Although costly and requiring constant vigilance to make sure that standards have been met, these measures have been successful in reducing pollution and preventing increases in pollutant releases. Currently: A “command-and-control” approach using “end- of-pipe” treatment measures and remediation of waste sites has reduced major environmental problems Stack controls Wastewater treatment Remediation Slide 5: As an alternative to the regulatory approach, the tendency now is toward sustainability with systems that emphasize recycling of materials, exchange of materials between concerns, and zero discharge of wastes. Properly designed, such systems are inherently friendly to the environment and reduce the need for measures that emphasize purely pollution control. Now the goal must be to close the loop, recycle as much as possible, avoid discharge of pollutants or wastes, and apply the principles of industrial ecology, green chemistry, and green engineering Zero discharge Recycle Material exchange Biosphere Anthrosphere Slide 6: Traditionally, environmental science has considered four environmental spheres: water, air, living organisms, and earth. But it is important to consider a fifth environmental sphere, the anthrosphere, which consists of the things that humans make and do. The remainder of this lecture considers environmental chemistry within a framework of these five environmental spheres. Atmosphere Geosphere Hydrospheren o i t Slide 7: A variety of chemical and biochemical phenomena occur in the hydrosphere. Some of these are shown in this slide for a body of water stratified with a warmer oxygenated upper layer floating on top of a cooler oxygen-deficient lower layer, a common phenomenon during summer months. Gases are exchanged with the atmosphere at the surface and solutes are exchanged between water and sediment. Photosynthesis produces biomass, represented here as CH O. In the oxygen-deficient lower layer 2 biomass undergoes biodegradation by the action of anoxic bacteria using oxidizing agents other than O . When sulfate functions as the oxidizing agent odorous hydrogen sulfide 2 gas may be evolved. CHEMICAL AND BIOCHEMICAL PHENOMENA IN A STRATIFIED BODY OF WATER Gas exchange with the atmosphere O 2 CO 2 - Photosynthesis NO 2HCO + h 3 3 2- CH O + O (g) + CO 2 2 3 Acid-base 2- HCO + OH CO + H O 3 3 2 2+ Cd 2+ 2- Precipitation CaCO (s) Ca + CO 3 3 2 Microbial - + 2CH O + SO + 2H 2 4 action H S(g) + 2H O + 2CO (g) 2 2 2 + NH Uptake 4 Leaching Sediment Ground- water a l e h C Reduction OxidationSlide 8: An important aspect of water chemistry is water treatment. This slide shows the activated sludge process used to treat municipal wastewater. A suspension of microorganisms in an aerated tank biodegrades organic wastes represented as CH O. 2 This removes oxygen demand from the water so that it will not deplete oxygen when discharged to a stream or body of water. The microorganisms settle in a settling basin and are pumped back to the aeration tank, greatly speeding the biodegradation process. Excess microorganisms, sewage sludge or biosolids also represented as CH O, are 2 taken to an anaerobic digester where they produce combustible methane, CH , and carbon 4 dioxide. The methane may provide fuel sufficient to run the engines that produce all the power needed by the plant. ACTIVATED SLUDGE WASTEWATER TREATMENT Wastewater containing BOD, CH O 2 Aeration tank CH O + O CO + H O + Biomass 2 2 2 2 Sludge + - Organic N NH , NO 4 3 settling - 2- Organic P H PO , HPO 2 4 4 2- Organic S SO 4 Purified water Air with reduced BOD Settled sludge with viable microorganisms Byproduct methane fuel gas Anaerobic digester Excess sludge to 2CH O CH + CO anaerobic digester 2 4 2 Slide 9: As water supplies become more limited around the world, renovation and re-use of wastewater become more important. This slide shows a system for purifying wastewater treated by the activated sludge process so that it can be used for applications such as groundwater recharge and irrigation. Advanced treatment processes can even bring the wastewater up to drinking water standards. A feature of the system shown is a wetlands area where algae and plants growing profusely in the fertile wastewater remove excess nutrients and produce biomass that can be converted to biofuels. The largest purification and water reuse project of its kind worldwide, the Orange County California Groundwater Replenishment System, utilizes a three-step process of microfiltration, reverse osmosis and ultraviolet light to purify highly-treated sewer water to state and federal drinking water standards (http://www.gwrsystem.com/). Secondary wastewater Plants and algae harvested treatment effluent for energy production Wetlands Water purified in wetlands Groundwater recharge Non-potable use, irrigation Activated carbon filtration for organics removal Membrane filtration, may include reverse osmosis Potable water to municipal water system Retentate with impurities not passed through membrane to disposal, treatment Slide 10: The atmosphere is essential for life on Earth as a source of oxygen for organisms, carbon dioxide for plants, stabilization of surface temperatures (the good greenhouse effect), and protection from damaging ultraviolet radiation from ozone (O ) in the stratosphere. Although the atmosphere extends far above Earthís 3 surface, over 99% of it is within a few kilometers of the surface. In fact, if Earth were a classroom globe, virtually all of the atmosphereís air would be in a layer the thickness of the varnish on the globe A distinctive feature of atmospheric chemistry is the ability of energetic photons of ultraviolet radiation (hînuî) to put large amounts of energy into a single molecule as shown here for the photodissociation of stratospheric O leading 2 to ozone formation. Thinning air High-altitude ozone, O , protection from ultraviolet radiation 3 Temperature stabilization (greenhouse effect) N , O , argon, raw material gases 2 2 to the anthrosphere Chemical and photochemical processes Exchange of O and CO 2 2 Water vapor from with the biosphere, release the hydrosphere of H O from plants Gases and particles 2 Gases and particles from the anthrosphere from the geosphere Water (rainfall) to Temperature the hydrosphere inversions and geosphereSlide 11: Atmospheric chemistry is complex involving literally hundreds of reactions. Photochemical reactions occur when photons of ultraviolet radiation split molecules apart producing reactive fragments with unpaired electrons (represented as dots in the slide) known as free radicals. These species undergo chain reactions such as those responsible for the ozone, organic oxidants, aldehydes, and solids characteristic of the unpleasant ingredients of photochemical smog that afflicts Los Angeles, Mexico City, and other urban areas around the world. Reactions such as the formation of sulfuric acid from sulfur dioxide may occur inside water droplets and on the surfaces of particles suspended in the atmosphere. "%&'()+,+'(%")-, Slide 12: There are many different kinds of air pollutants. Particles obscure visibility and can be detrimental to respiration. Primary particles are those that enter the atmosphere as particles whereas secondary particles are produced by reactions of gases in the atmosphere. Several inorganic gases, mostly oxides of nitrogen, sulfur, and carbon are air pollutants. Photochemical smog results when nitrogen oxides and hydrocarbons in stagnant air masses subjected to solar radiation undergo photochemical reactions to form ozone, organic oxidants such as peroxyacetyl nitrate (PAN), ozone, aldehydes, and other species that tend to be detrimental to visibility, respiratory systems, and eyes. AIR POLLUTANTS Primary Particles • Pollen • Dust • Fly ash • Polycyclic aromatic hydrocarbons Secondary Particles (formed from gas reactions) • Smog particles • Sulfuric acid droplets • Salts such as (NH ) SO 4 2 4 Inorganic Gases • O • SO • NO • NO • CO • H S • HCl • NH 3 2 2 2 3 Organics • Hydrocarbons including those that form photochemical smog • Odorous organic sulfur compounds • Organohalides • Amines and other organonitrogen compounds • Organo-oxygen compounds including aldehydes and ketones Photochemical Smog • Smog particles • Ozone • Organic oxidants (PAN) • Aldehydes Slide 13: Although it is a normal constituent of the atmosphere essential to supply the carbon required for plant photosynthesis, carbon dioxide may turn out to be a harmful air pollutant at higher levels. This is because of its role in re-absorbing the infrared radiation by which Earth radiates back into space the solar energy received from the sun. This greenhouse effect is essential to life on Earth because it keeps the surface warm enough for living organisms to thrive. However, if too much carbon dioxide is present in the atmosphere, Earthís surface will become too warmóthe bad greenhouse effectóleading to melting of polar ice caps and glaciers, elevated sea levels, severe droughts, and other detrimental effects. As shown in the plot, on this slide, atmospheric carbon dioxide levels from the burning of fossil fuels and other sources have been increasing by about 1 part per million per year, now closer to 2 parts per million per year. The resulting doubling of atmospheric carbon dioxide levels within the next century could have substantial, perhaps catastrophic, effects upon the global climate. LEVELS OF ATMOSPHERIC CARBON DIOXIDE 385 380 390 380 2010 2005 370 360 350 345 340 335 330 325 320 315 2000 2010 1960 1970 1980 1990 Year CO level, ppm by volume 2Slide 14: The geosphere has a very close relationship with the hydrosphere, the atmosphere, and the biosphere and is strongly affected by human activities in the anthrosphere. On Earth’s surface, the geosphere is composed of rocks in turn made of various minerals and usually a thin layer of soil. Igneous rocks thrust upward by tectonic forces are broken down (weathered) by physical, chemical, and biological processes and material from them is deposited as sedimentary rock. Heat and pressure convert sedimentary rock to metamorphic rock. Weathering, erosion Transport, sedimentation Igneous Heat, pressure rock Sedimentary rock Metamorphic rock Upwelling, solidifying magma Hot, molten magma Slide 15: Geochemistry is the chemistry of rocks and minerals in the geosphere as it relates to the hydrosphere, atmosphere, biosphere, and, very recently in the geological timetable, the anthrosphere. The geochemical reaction shown in the slide is important in putting dissolved calcium and bicarbonate ion (water alkalinity) into water. The dissolution of calcium carbonate (limestone) by this process causes the formation of caves and cavities in limestone rock formations. Environmental geochemistry is the environmental branch of geochemistry and is important in considering the effects and fates of pollutants in the geosphere. GEOCHEMISTRY Geochemistry"%&'()+,(+%&-+(&./+012&.2"-/1+&&&(2) 31&-/2")(/(2)/+012&'())),1&-/2"4"/1&-/5 For example, carbon dioxide from the atmosphere dissolves in water in the hydrosphere, then reacts with limestone in the geosphere: 2+ - • CaCO (s) + H O + CO (aq) Ca (aq) + 2HCO (aq) 3 2 2 3 ENVIRONMENTAL GEOCHEMISTRY 627(/12,2)%31+,(&)/4(&)8/2+1931+,(&)/4)):-%1/&) +1,-%:(2)/+012&,123)/1+;')/(/%(9&4&),&))")/,(2) +,(+%+/+)/(&0+&19)&=/9+27(/12,2)5 Slide 16: The geosphere is a crucial source of natural capital including essential metals, fuels, and plant nutrients. The geosphere is an important repository of wastes. One such waste for which the geosphere is likely to become a very important repository in the future is carbon dioxide from fossil fuel combustion that is currently released to the atmosphere where it causes global warming. THE GEOSPHERE AS A SOURCE OF NATURAL CAPITAL Natural capital is a term that describes the resources or value of the environment such as in providing essential materials or absorbing wastes The geosphere is a crucial source of natural capital • Fuels, such as natural gas • Metals, such as iron or copper • Nonmetals, such as clay, sand, gravel • Plant nutrients, such as phosphorus • Absorption of wastes, such as carbon dioxide from fuel combustion pumped underground (carbon sequestration) The most important aspect of natural capital in the geosphere is the ability of soil to support plant growth for food production (next slide) Slide 17: Soil is a crucial component of natural capital because all terrestrial organisms including humans depend upon it for their existence. The layer of productive soil on Earth’s surface is extremely thin and subject to damage and erosion. Much productive soil has been lost or ruined and soil conservation is a top priority for sustainability. SOIL: THE MOST IMPORTANT GEOSPHERIC RESOURCE Soil is obviously of crucial importance as a support for plant growth Soil is composed of finely divided, highly weathered mineral matter and organic matter from the partial biodegradation of plant material Earth’s soil is a very thin layer; if Earth were a classroom globe, the average thickness of the total soil resource would be about the same as the diameter of a human cell Soil is divided into layers called soil horizons (next slide) Slide 18: Soil is typically divided into layers called horizons. Many important chemical and biochemical reactions occur in soil. Soil is subject to water and wind erosion, and one of the earliest environmental movements, dating back to around 1900 in the U.S., has been soil conservation. Large areas of formerly productive agricultural land have been turned to desert by poor agricultural and animal grazing practices, a large problem for sustainability. SOIL HORIZONS Vegetation O horizon from decayed and decaying plant biomass A horizon, topsoil, layer of root growth, biodegradation, high organic content E horizon, layer that is weathered and leached, often by organic acids from the A horizon B horizon, or subsoil, reposi- itory of organic matter, salts, and clay particles C horizon, weathered parent rock Bedrock Regolith: unconsolidated rock debris, organic matterSlide 19: Living organisms constitute the biosphere. Through photosynthesis of plants and algae it is the basis of the food chain. In addition to food, the biosphere is a crucial source of biomass for raw material and, in the future, for synthetic fuel. This slide shows a heavy growth of switchgrass, a plant that is remarkably productive of biomass on poor soil and that has significant potential as a renewable fuel resource. "%&'()" (+&,"-.((/.)'0&.112&3)'.0 ('4,'5%&'3.(( .67898:;678=7?:;8A978=BAC9D7:;BBCBB78AE7; CE97)8A;)8CE96;BE9F8E Slide 20: A major consideration with respect to the biosphere is the effect of toxic substances on organisms. Toxicological chemistry addresses the relationship between the chemical nature of substances and their toxic effects. A MAJOR CONCERN REGARDING THE BIOSPHERE IS THE EFFECT OF TOXIC SUBSTANCES ON ORGANISMS: TOXICOLOGICAL CHEMISTRY . . . . . . . . . . Toxic Organism Toxicant + effect Toxicological chemistry Toxicology

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