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Concept 02 / 05Environmental Chemistry

Water and Soil Pollution, Industrial Waste and Green Chemistry

Causes of water pollution (pathogens, organic waste, chemicals), dissolved oxygen, biochemical and chemical oxygen demand, eutrophication, international drinking water standards for fluoride, lead, sulfate and nitrate, soil pollution by pesticides and herbicides with biomagnification of DDT, industrial waste, and the principles and examples of green chemistry.

Figure 1
Water pollution. Sources: point sources (a factory outfall, a sewage pipe) and non-point sources (runoff from agricultural land, construction, urban areas). Pathogens: bacteria and other organisms from domestic sewage and animal excreta (Escherichia coli, Streptococcus faecalis) cause gastrointestinal diseases. Organic wastes: leaves, grass, food waste and sewage are decomposed by aerobic bacteria, which consume dissolved oxygen. Clean cold water holds about 10 ppm10\,\mathrm{ppm} of dissolved oxygen; below about 6 ppm6\,\mathrm{ppm} the growth of fish is inhibited. When oxygen runs out, anaerobic bacteria take over, producing foul-smelling and harmful products. Chemical pollutants: heavy metals (Cd, Hg, Ni, Pb) that the body cannot excrete, which damage kidneys, the nervous system and the liver; petroleum products (oil spills); pesticides; detergents and fertilisers containing phosphates and nitrates. Oxygen demand. Biochemical oxygen demand (BOD) is the amount of oxygen needed by bacteria to break down the organic matter in a certain volume of water (measured over 55 days at 20∘C20^{\circ}\mathrm{C}). Clean water has BOD below 5 ppm5\,\mathrm{ppm}; highly polluted water 17 ppm17\,\mathrm{ppm} or more (see figure). Chemical oxygen demand (COD) is the oxygen equivalent of all oxidisable matter, measured by oxidation with acidified potassium dichromate; it is always at least as large as BOD, since it includes non-biodegradable organics. Eutrophication. Phosphates and nitrates from fertilisers and detergents promote excessive growth of algae (algal bloom), which covers the water surface. When the algae die, their decomposition consumes dissolved oxygen, killing fish and other aquatic life. The water body gradually fills in and becomes a swamp. Drinking water standards (WHO and national guidelines). Fluoride: about 1 ppm1\,\mathrm{ppm} is beneficial (converts hydroxyapatite in teeth to harder fluorapatite, [3Ca3(PO4)2⋅CaF2]\mathrm{[3Ca_{3}(PO_{4})_{2} \cdot CaF_{2}]}); above 2 ppm2\,\mathrm{ppm} causes brown mottling of teeth; above 10 ppm10\,\mathrm{ppm} harms bones and teeth. Lead: above 50 ppb50\,\mathrm{ppb} damages kidneys, liver and the reproductive system. Sulfate: above 500 ppm500\,\mathrm{ppm} has a laxative effect. Nitrate: maximum 50 ppm50\,\mathrm{ppm}; excess causes methaemoglobinaemia (blue baby syndrome). Other metals have their own limits (Fe 0.2 ppm0.2\,\mathrm{ppm}, Mn 0.050.05, Al 0.20.2, Cu 3.03.0, Zn 5.05.0, Cd 0.005 ppm0.005\,\mathrm{ppm}). Soil pollution. Pesticides: insecticides such as DDT are persistent, fat-soluble and non-biodegradable, so they accumulate in fatty tissues and become more concentrated at each step of a food chain (biomagnification), for example thinning the eggshells of birds of prey. Many insects have developed resistance. Organophosphates and carbamates are less persistent but more acutely toxic. Herbicides such as sodium chlorate and sodium arsenite are toxic to mammals; some organic herbicides are linked to birth defects. Industrial waste: fly ash, slag, metal waste, and non-biodegradable plastics; proper collection, recycling, incineration and composting are the remedies. Green chemistry. Designing chemical processes to reduce or eliminate hazardous substances, using minimum energy and producing minimum waste: prefer catalytic over stoichiometric reagents, safer solvents (water, supercritical CO2\mathrm{CO_{2}}), renewable feedstocks and atom-economical reactions. Examples: supercritical liquid CO2\mathrm{CO_{2}} replacing tetrachloroethene in dry cleaning; hydrogen peroxide with catalysts replacing chlorine for bleaching paper and clothes (no chlorinated by-products); ethanal made by one-step oxidation of ethene in water with an ionic catalyst (Pd(II)/Cu(II)\mathrm{Pd(II)/Cu(II)}), with about 90%90\% yield. Key Takeaways: - BOD below 5 ppm5\,\mathrm{ppm} is clean; 17 ppm17\,\mathrm{ppm} or more is highly polluted; COD uses K2Cr2O7\mathrm{K_{2}Cr_{2}O_{7}}. - Eutrophication: nutrients, algal bloom, oxygen depletion. - Fluoride 1 ppm1\,\mathrm{ppm} good, over 2 ppm2\,\mathrm{ppm} mottling; nitrate limit 50 ppm50\,\mathrm{ppm}. - DDT is persistent and biomagnifies. - Green chemistry: fewer hazards, less waste, safer solvents and catalysts. - Pitfall: a high BOD means more organic pollution, not more oxygen in the water. Brain-Teaser 1: 10 mL10\,\mathrm{mL} of a sewage sample is diluted to 300 mL300\,\mathrm{mL} with oxygen-saturated water. The dissolved oxygen falls from 8.08.0 to 4.0 mg L−14.0\,\mathrm{mg\,L^{-1}} in 55 days. Find the BOD of the sewage. Solution: The diluted sample consumed 4.0 mg L−14.0\,\mathrm{mg\,L^{-1}}. The sewage made up 10300=130\frac{10}{300} = \frac{1}{30} of the volume, so BOD =4.0×30=120 mg L−1= 4.0 \times 30 = 120\,\mathrm{mg\,L^{-1}} (120 ppm120\,\mathrm{ppm}): highly polluted. Brain-Teaser 2: A 50 mL50\,\mathrm{mL} water sample consumes 10 mL10\,\mathrm{mL} of 0.01 M0.01\,\mathrm{M} K2Cr2O7\mathrm{K_{2}Cr_{2}O_{7}} in a COD test. Find the COD in mg L−1\mathrm{mg\,L^{-1}}. Solution: Dichromate used =1.0×10−4 mol= 1.0 \times 10^{-4}\,\mathrm{mol}, accepting 6×10−4 mol6 \times 10^{-4}\,\mathrm{mol} electrons. O2\mathrm{O_{2}} accepts 44 electrons per molecule, so the oxygen equivalent is 1.5×10−4 mol=4.8 mg1.5 \times 10^{-4}\,\mathrm{mol} = 4.8\,\mathrm{mg} in 50 mL50\,\mathrm{mL}, that is 96 mg L−196\,\mathrm{mg\,L^{-1}}. Brain-Teaser 3: Why does the concentration of DDT increase along a food chain from plankton to fish-eating birds? Solution: DDT is fat-soluble and non-biodegradable, so it is stored in fatty tissue rather than excreted. Each organism eats many organisms from the level below and retains nearly all their DDT, so the concentration multiplies at each step (biomagnification), reaching toxic levels in top predators.

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Water and Soil Pollution, Industrial Waste and Green Chemistry — DPP

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