chemistry and the environment
Introducing the Treatment of Waste and Wastewater in the General Chemistry Course Applying Physical and Chemical Principles to the Problems of Waste Management S. W. Dhawale Division of Natural Science and Math, Indiana UniversRy East, Richmond. IN 47374
Today's industrially advanced complex societies face the great problem of increased industrial and household wastes. Although nature usually has the ability to clean the air and water, the rate of pollution often exceeds the rate of natural cleaning due to increased population, new life-styles, and the growth of process industries. Waste management problems can be handled more effectively using strategies of waste control, conservation, recycling, and cost-effective waste-treatment technologies. No single strategy is enough to reduce the volume of waste. Extremists have overemphasized stopping the use of all chemicals, thus creating a bias against chemistry. It appears to many environmentalists that chemists are playing an adversarial role with the environment. Thus, chemistry instructors should address the problem of waste and wastewater treatment in their general chemistry classrooms. The following discussion presents a few applications of simple physical and chemical principles that are commonly used in industrial waste or wastewater (effluent) treatment technology (1, 2).Examples from the following discussion might be useful in teaching some principles of general chemistry. Ph sical Separation as Applied to Waste or hastewater Treatment In the introductory chemistry laboratory the students learn simple techniques, such as
decantation gravity and sudion filtration centrifugation extraction distillation dialysis and osmosis Once the students have been exposed to these principles and techniques, they can discuss applications, such as the cleanup of a small-scale oil spill and the processes used to obtain drinkable pure water from seawater. Industrial effluents contain soluble and insoluble solids and liquids. The physical principles applied in separation and purificati. of the components of a mixture can be used to remove the suspended impurities from the wastewater. Removalof Suspended Solids
The separation of suspended solids by settlement (decantation) from industrial effluents is a widely used physical treatment. Sand and gravel heds, polymeric filters, fiberglass screens, and combinations of these media are used in large-scale fdtration. Charcoal filters cause certain kinds of materials to adhere to the surface (adsorption) and thus can be used to remove organic, inorganic, and biological substances that impart color, odor, and taste.
Removalof DissolvedSolids by Reverse Osmosis
While teaching the principle of osmosis and dialysis, instructors can introduce the concept of reverse osmosis. Reverse osmosis is useful in desalination or purification of water removing heavy metal salts and alkali and alkaline earth metals from solution 'removing acids from pickling liquors (scaleremoval) Industrial waste from chemical. metal-ulatine. -, and uharmaceutical industries and waste of biological origins can also be treated satisfactorily by reverse osmosis. Chemical Principles and Waste Treatment Acid-base, oxidation-reduction, and siwle- and doubledisplacement rcactionp,involving precipitalion and gas formatwn are usually discussed in thegeneral chemistry lecture and laborat&. The applications of such reactions to waste treatment can be discussed in their appropriate context. Waste may include many types of substances ( I , 2):
acids or bases metals and their ionic compounds non-ionic substances (compounds): gasoline phenols organic solvents pesticides plastics Detoxifying Industrial Waste
Industrial wastes and effluents may contain contaminants that are lethal to humans and animals a t certain concentrations or harmful to the ecolo~calbalance. Dissolved toxic solids can be converted to ~nsolublcderivaiivcs and then removed by physical processes, or they can be converted to soluble nontoxic substances. Although each kind of industrial waste creates its own special problems, there are some general principles that are applicable to many wastes and industrial effluent treatments. Acid Neutralization
During the discussion of acids and bases, one can ask the following types of questions. How can we take'care of an acid spill? How will you know that an acid has been neutralized? If the spilled acid is hydrochloric acid, which bases can be used? Can you think of any problems that could arise when sodium hydroxide or potassium hydmxide is used? Can you use acidic or basic salts for neutralization? Limestone and dolomite are abundant in nature. Thus, it is more economical to use these compounds (weak bases) Volume 70 Number 5 May 1993
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for acid neutralization. Industries with low acid discharges use slump containing marble chips or limestone (CaO and Ca(OH)2).Dolomite (CaMg(COs)z)treatment is usually preferred for sulfuric acid discharges (3.4). Neutralization processes require sound controls to avoid an excess of acid or base. The following reactions represent neutralization of a monoprotic acid by lime and dolomite, respectively. CaO + 2HA + ca2++ 2A- + Hz0 CaMg(CO& + 4HA+ ca2++ M$ + 2Hz0 + 2C02+ 4A-
Redox Conversion of Cyanide to Harmless Gases One of the most toxic and lethal inorganic contaminants is cyanide, and most metal-plating and metal-finishing treatments require cyanide baths. There are strict regulations on releasing cyanides to the enviromnent. The following equations show the use of the redox principle to change cyanide to cyanate and then to harmless carbon dioxide and nitrogen. 2NaCN + 4NaOH + 2CI2+ 2NaCNO + 4NaCI+ 2Hz0 (pH = 10)
Limiting Reagent and Acid Waste in Abandoned Mines The following reaction introduces an application for the concept of limiting reagent. 2FeSz + 2Hz0+ 702 + 2FeS04+ 2HzS04 Ever since coal mines were first opened for operation, the combined release of air, water, and iron pyrites has increased acid waste. Although the release of waste ceases when an industrial plant is not operating, such release does not end when mining operations cease and the mines are abandoned. The above reaction continues whether or not a mine is operating. As a result, curtailing the problem of acid wastes in abandoned unsealed mines is very expensive. However, some improvement occurs if the mine is sealed to reduce the amount of oxygen (limiting reagent) that contacts the ore. Oxidation-Reduction Redox principles are u s e l l in treating waste that contains organic compounds, such as phenols and amines, and inorganic compounds that wntain Commonly used oxidizing agents include oxygen or air, ozone, chlorine, chlorine dioxide, and hydrogen peroxide. Incineration: Oxidation of Compounds to a Stable Form Incineration oxidizes organic and inorganic compounds to their most stable oxide form. After separation of recyclable and noncombustible material, the concentrated waste is burned. A general unbalanced incineration reaction is shown below.
A tem~eratureof about 1800 O F is needed for this reaction. well-designed incineration technology reduces the bulk of waste and destroys certain toxic chemicals. Such thermal processes can be Lseful in the recovery of both energy and material. Practical use of incineration technology (5)involves application of several principles, such as incomplete and complete oxidation the rate of recombination of product the physical and chemical stabilities of the materials spe& heats Thermal waste incinerators are also useful in eliminating the problem of liquid waste disposal. Because no new incinerators have been licensed due to social pressure, it is worthwhile to make our students aware of the positive and negative sides of this technology (6).Most objections to incineration technology are based on the fear that incomplete incineration and technological flaws can lead to the emission of toxic flue gases and the generation of toxic residual products. 396
Journal of Chemical Education
Instructon can discuss the mole ratios of chlorine to cyanide for the effective complete conversion of cyanides to harmless products. Precipitation The influence of heavy metals on our health and envimnment is very well-documenkd by Fergussen (71. Removal of toxic heavy metals allows microorganismn to degrade oreanic com~oundsefficiently. This is esoecially important if ;he conce&ation of heavymetal is over 50 &/L: When a high value for total dissolved solids (TDS)is due to heavy metals or their ions, a reduction can be obtained by chemical precipitation. The selection of the precipitation method depends on factors such as the economy and the toxicity of the compounds involved in the reaction. Temperature, the concentration of reactants, and other factors that affect precipitation reactions are usually controlled during the process. The followingnet ionic reactions can be used to illustrate the use of precipitation in removing toxic heavy metals from the waste. ~ d + ~ +( 20K(aq) a ~ ) + Cd(OH)z(s) ) PbC03(s) ~ h ' + ( a+~~) 0 r ( a q+ ~ $ ( a q ) + &aq) + HgS(s) Reduction and Precipitation com~oundsare usua11~ found in Soluble chromium (CI+~) effluents of metal-finishing plaks. First Cr'vs reduced to a soluble chromium Cr~.'comwund.Then it is precipitated - . as a hydroxide.
Insoluble chromium hydroxide is then removed from the wastewater and converted to a useful compound. Chromium is an expensive metal, and thus its recovery is important. Question Concerning Metal-Bearing Waste Metal-finishingindustries are involved in improving the surface of less-noble metals and alloys to improve cormsion resistance, durability, and attractiveness. The effluents from these industries may consist of cyanides of toxic heavy metals, such as chromium, copper, and cadmium. When a high TDS results fmm alkali metals, methods other than precipitation are needed for their removal. Ask students &expiain the reason for this. Ion exchange and reverse osmosis are methods of choice in cases of high alkali metal waste.
Summary
The common principles of removing inorganic contaminants from was& and wastewater c& be easily discussed in the introductory general chemistry class. Inorganic waste treatment involves chemical treatments such as
thk bulk of waste or wastewater. The major obstacles in applying many of these are not the principles and techniques used, but the cost of achievinggood process control. Along with reduction of waste and
increased pollution control, problems of toxic waste can be using Literature Cited Bssael~wre, E. B.;s ~ , , M~ T~ ,,.~%ofofnt ~~,
wante; M~D~~~-H~II:
of~ndustriol
5. Pearson, C.V.CSUI-ANL conf.nmon I ~ W in * wmte~ . ~ g p m n t~rpm , NStionsl ~abaratow,oct. 25-26, 1530.
: ~ ~~ ,m~f,: " , " ~ ~ ; , " , " ~ ~ & ~ ~ ~ i ~ ~ & > ~ ~ ~ ~ ~ , " i P ~ W ~m Om ~d :,1530.
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