E12 Water and Soil

E.12.1 Solve problems relating to the removal of heavy-metal ions, phosphates and nitrates from water by chemical precipitation.

For any salt (even 'insoluble salts') formed from a metal M with a non-metal X,

MX(s) ↔ M+ (aq) + X- (aq)

Ksp (solubility product) = [M+][X-]

Many metal sulphides have low solubility products so to effectively precipitate heavy metal ions, hydrogen sulphide is bubbled through polluted water. The solubility product can then be used to calculate the amount of a metal ion that will remain in solution after it has been precipitated.

MS(s) ↔ M2+ (aq) + S2- (aq) [metal M has valency 2]

Ksp = [M2+][S2-] but [M2+]=[S2-]

Therefore Ksp = [M2+]^2

[M2+] = Ksp^0.5

By doing this, the precipitation of ions reduces the amount of ions left remaining in the polluted water. However, this can be further reduced through the common ion effect by adding more sulphide ions to the solution so that the concentration of lead ions is not the same as the concentration of the sulphide ions. For example, if the concentration of sulphide ions is one, then

Ksp = [M2+][1]

[M2+]= Ksp

And since metal sulphides have low solubility products, the concentration of metal ions will be equally low. However, when reducing using the common ion effect, care must be taken to ensure that soluble complex ions are not formed (e.g. zinc hydroxide is soluble in excess).

Phosphate can be precipitated as aluminium phosphate. For many heavy metal ions found in waste water, the process is slightly more complex as the salts formed are not binary but the principal is fundamentally the same.

M2+(aq) + 2OH-(aq) ↔M(OH)2(s)

Ksp= [M2+][OH-]^2

[OH-]=2[M2+]

Ksp= [M2+][2M2+]^2

=4[M2+]^3

[M2+]= (Ksp/4)^1/3

E.12.2 State what is meant by the term cation-exchange capacity (CEC) and outline its importance.

Cation Exchange Capacity is the amount of positively-charged cations that a soil can hold. Both the SOM and clay particles in the soil are negatively charged and hence attract and bond to cations. These ions are classified either as basic (e.g. Ca, Mg, Na, K) or acidic ions such as H or Al. These ions are exchanged with cations such as hydrogen ions on the root hairs of plants and thus provide nutrients to the plant.

E.12.3 Discuss the effects of soil pH on cation-exchange capacity and availability of nutrients.

Soil pH is determined by whether more acidic or more basic cations are present, and hence affects CEC. Natural rain water is itself acidic (because of how carbon dioxide dissolves to form carbonic acid) and has a pH of 5.6. This slight acidity makes it possible for more metal ions to dissolve. However, as pH decreases, nutrient cations in the soil may be replaced.

e.g. ZnS(s) + 2H+(aq) --> Zn2+(aq) + H2S(aq)

e.g. Zn(OH)2(s) + 2H+(aq) --> Zn2+(aq) +2H2O(l)

H+ from acid deposition can replace nutrient cations in the soil such as Zn2+ and Mg2+.

Alumina (aluminium oxide) is insoluble in water and reacts as a base with H+ ions to form soluble Al3+(aq) ions. In basic conditions, Fe3+ and Al3+ react with OH- to fom insoluble hydroxides. Hydrated ions with high charge densities attract the negative part of the water dipole to produce weakly acidic solutions.

[Al(H2O)6]3+ ↔ [Al(H2O)5OH]2+ + H+

Al3+ and Mn2+ are toxic to plants. Al3+ in pHs below 5 results in the displacement of important plant nutrient cations such as Ca2+ and Mg2+ from the soil.

3Mg2+(soil) + 2Al3+(aq) --> 2Al3+(soil) + 3Mg2+(aq)

(Mg2+(aq) etc. can be washed from the soil i.e. by irrigation, rainwater etc.).

In acidic soil, carbonates are converted into soluble ions and these can be lost to water drainage.

Adding lime/other carbonates to the soil increases pH and also replenishes the concentration of basic cations held by the clay and SOM.

Acidic soil below pH 5.5. affects microbes ability to fix NH4+ to N2, and hence these weakly acidic ions accumulate in the soil. Soil below pH 4 is too acidic and results in poor plant growth.

Soil pH that is too high causes the precipitation of ions such as Fe3+ to give insoluble hydroxides, leading to a lack of important nutrients.

Soil pH also affects the amount of phosphate ion present in solution. Phosphoric acid (triprotic acid) dissociates first at low pH (<4),>10), and hence plants absorb inorganic phosphorus from the soil as H2PO4- (first dissociate) and HPO4 2- (second dissociate).

Micronutrients such as iron, manganese and zinc are available in acidic soil but decrease as pH increases as they precipitate out as hydroxides. Ammonium sulphate can be added to soil that is too basic as it is weakly acidic:

NH4+(aq) ↔ NH3(aq) + H+(aq)

SOM contains humic acids that exhibit buffering capacity over the pH 6.5 to 7.5. e.g. weakly carboxylic acids

RCOOH(aq) ↔ H+(aq) + RCOO-(aq)

E.12.4 Describe the chemical functions of soil organic matter (SOM).

SOM has the ability to increase CEC, behaves as a buffer (controls soil pH), and removes heavy metals and pesticides through its chelating abilities. The presence of anions in SOM from carboxylic acids and phenolic functional groups (RCOO- and ArO-) prevent nutrient cations from precipitating out as insoluble compounds. SOM hence acts as a nutrient reservior for plants but is unable to bind to nitrates, sulphates and phosphates and therefore these must be made available to plants through microbial activity in the SOM (enhanced by large surface area of SOM).

Phosphorus: required for early root development and growth, if not yellow leaves & stunted growth. Main source is from calcium phosphate. Organic phosphate in the soil is relatively immobile and is slowly converted into inorganic phosphate (mineralization), and SOM enhances phosphorus availability through its chelating ability.

SOM also chelates to toxic cations such as Al3+ and heavy metal ions. Pesticides are deactivated as they are broken down by soil organisms. By binding to contaminants, less pollution reaches and affects the water supply. SOM also binds to minerals, sometimes forming stable complexes with cations, to make them available to plants rather than preipitating out as insoluble salts/being leached.

E11 Acid deposition (HL)

E.11.1 Describe the mechanism of acid deposition caused by the oxides of nitrogen and oxides of sulfur.

Two primary pollutants that cause acid deposition are sulphur dioxide and nitrogen dioxide. They are converted into acids by a free radical mechanism involving hydroxyl free radicals, formed either by the reaction of water vapour with ozone or by the reaction of water vapour with oxygen free radicals formed when ozone decomposes.

H2O(g) + O3 (g) --> 2HO∙(g) + O2(g)
H2O(g) + O∙(g) --> 2HO∙(g)

Reacts with sulphur dioxide and nitrous oxides in the presence of water to give the dissolved acids.

HO∙(g) + NO2(g) --> HNO3(aq)
HO∙(g) + NO(g) --> HNO2(aq)
HO∙(g) + SO2(g) --> HOSO2∙(g)
then HOSO2∙(g) + O2(g) --> HO2∙(g) + SO3(g)
followed by SO3(g) +H2O(l) --> H2SO4(aq)

E. 11.2 Explain the role of ammonia in acid deposition.
The atmosphere contains trace amounts of ammonia and also in the soil from the action of rhizobia (bacteria) in the root nodules of leguminous plants. The ammonia in the atmosphere neutralises partially the acids to form ammonium sulphate, which is a slightly acidic as it is a product of a weak base and strong acids. As they are washed out by precipitation or sink to the ground the ammonium ion is deposited and enters the soil where acidification and nitrification an occur.

NH4+(aq) + 2O2(g) --> 2H+(aq) + NO3-(aq) + H2O(l)

E10 Smog

E.10.1 State the source of primary pollutants and the conditions necessary for the formation of photochemical smog.

Smog is a poisonous mixture of smoke, fog, air, and other chemicals, is formed in large cities and is favoured by lack of wind and bowl-shape cities (land that is sounded by higher ground in all directions). Smog is likely to occur when there is a thermal inversion, whereby a layer of still warm air to blanket a layer of cooler air, so that convection currents cannot form). As a result, trapped pollutants cannot rise and can rise to dangerous levels if the stillness persists.

‘Pea smog’ (classically English) occurred before the introduction of clean air controls in the mid 1950s, whereby the combustion of coal and oil produced sulphur dioxide mixed with soot, fly ash and partially oxidising organic material (reducing smog). Photochemical smog is the more common smog now (e.g. Los Angeles) that is converted into secondary pollutants in sunlight, consisting of oxides of nitrogen and VOCs (brown/yellow colour).

E.10.2 Outline the formation of secondary pollutants in photochemical smog.

NO2(g) --> NO(g) + O ·(g)

Nitrogen dioxide is broken down to give an oxygen free radical and nitrous oxide. The oxygen radicals can react with oxygen to form ozone and with water to form hydroxyl radicals. These photochemical oxides can react with a variety of molecules including nitrous oxide to form nitric acid and VOCs to form peroxides (ROOR), aldehydes (RCHO), and ketones (RCOR). Peroxides are extremely reactive and aldehydes and ketones reduce visibility by condensing to form aerosols.

O·(g) + H2O (l) --> 2OH·(g) [hydroxyl radical]

OH·(g) + NO2(g) --> HNO3(aq) [nitric acid]

OH·(g) + RH(g) --> R·(g) + H2O [radical]

R·(g) + O2 --> ROO· [peroxide radical]

E9 Ozone depletion (HL)

E.9.1 Explain the dependence of O2 andO3 dissociation on the wavelength of light.

The amount of energy required to break an O=O bond is 496kJ/mol. Using the formulas E=hf and C=λf gives λ=hc/E, where h is Planck's constant and c is the velocity of light. To achieve this amount of energy, the wavelength of light required would be 242nm, as calculated from the formula, and is in the high energy region of the ultraviolet spectrum.

For ozone dissociation, ozone has two resonance hybrids, each with one double bond and one single bond, and as only the single bond is broken less energy is required (light of a lower wavelength) i.e. Ultraviolet light with wavelength 330nm. Working backwards, this means the strength of the O-O bond in ozone is 362kJ/mol.

E.9.2 Describe the mechanism in the catalysis of O3 depletion by CFCs and NOx.

CFCs catalyse the destruction of ozone because the high energy ultraviolet light in the stratosphere causes the homolytic fission of the C-Cl bond that produces chlorine free radicals, that then go one to break down ozone molecules and generate more radicals in a continuous process until the radicals eventually escape/terminate. One molecule of CFC can break down 100000 molecules of ozone.

CCI2F2 (g)-->CCIF•(g) + CI•(g)
CI•(g) + O3(g) --> ClO•(g) + O2(g)
CI O•(g) + O•(g) --> O2(g) + Cl•(g)

Evidence for this mechanism can be seen in how an increase in chlorine monoxide in the antarctic mirrors the decrease in ozone concentrations.

Nitrogen oxides also catalytically decompose ozone by a radical mechanism, whereby oxygen radicals are generated by the breakdown of NO2 in ultraviolet light.

NO2 (g) --> NO(g) + O•(g)
O•(g)+ O3(g) --> 2O2 (g)
NO(g) + O3(g) -->NO2 (g)+ O2(g)

Overall: 2O 3(g) --> 3O 2 (g) [NO2- homogenous catalyst]

E.9.3 Outline the reasons for greater ozone depletion in polar regions.

During winter, low temperatures cause small amounts of water vapour in the air to freeze and form ice crystals, which contain small amounts of chlorine compounds (e.g. HCl, ClONO2) that react in the presence of catalysts to form hypochlorous acid (HClO) and chlorine (Cl2). In spring, when the sun shines, these molecules break down to produce chlorine radicals which catalyse the destruction of ozone, with the largest holes in the ozone occuring during early spring. As the temperature increases and warmer winds blow into the region, the ice crystals disperse and the ozone concentration gradually increases.

OTHERS:

Sun-screening compounds --> Sunlight can be harmful; causes genetic mutations and damage to proteins. Ozone decreases more harmful ultraviolet light in reaching the Earth's surface. Other sun-screens include glass, melanin (pigment in body that is increased by the action of the sunlight on the skin), commercial substances (e.g. PABA [para-aminobenzoic acid) that absorb light of a particular frequency (e.g. for PABA, 265nm) as they contain delocalised π electrons with conjugated double bonds (alternate between C-C and C=C) which are excited to higher energy levels when they absorb UV light. Sunblocks include white pigments, like titanium dioxide or zinc oxide, which reflect and scatter sunlight.

E8 Waste

E. 8.1 Outline and compare the various methods for waste disposal.

Landfills: efficient method to deal with large volumes, filled land can be used for other purposes. However local residents may object to new sites, filled land needs time to settle and requires maintenance as methane is released.

Open dumping: Convenient and inexpensive. However causes air, ground and water pollution; health hazard and attracts pests, and unsightly.

Ocean dumping: Source of nutrients, convenient and inexpensive. However dangerous to marine animals, pollutes the sea.

Incineration:Reduce volume, requires minimal space, produces stable odourless residue, can be source of energy. However expensive to build/operate, cause pollutants if inefficiently burned, requires energy.

Recycling: provides a sustainable environment. However expensive, difficult to separate different materials (sometimes impossible).

E.8.2 Describe the recycling of metal, glass, plastic and paper products, and outline its benefits.

Metal: Mainly aluminium and steel. Sorted and melted, or re-used directly or added to purification stage of metals formed from their ores. Important for metals like aluminium (large amounts of energy needed to produce direct from ore).

Glass: Sorted by colour, washed, crushed and then melted and moulded into new products. Not degraded during the recycling process so can be recycled many times.

Plastic: Sorted (though this may be problematic), degraded to monomers by pyrolysis, hydrogenation, gasification and thermal cracking, then repolymerised. Recycling causes less pollutants and uses less energy than producing new plastics from crude oil.

Paper: Sorted into grades, washed to remove inks, made into a slurry to form new types of paper. High transportation costs- may be more efficient to compost.

E.8.3 Describe the characteristics and sources of different types of radioactive waste.

Two types of radioactive waste: High level waste and low level waste.

Low level waste is where activity is low and the half-lives of the radioactive isotopes are generally short lived. Such items include rubber gloves, paper towels and protective clothing that have been used to handle radioactive materials.

High level waste is where activity is high and the half-lives of the radioactive isotopes are generally long and so the waste remains active for a long period. Most high level waste comes from spent fuel rods or the reprocessing of spent nuclear rods.

E.8.4 Compare the storage and disposal methods for different types of radioactive waste.

Low level waste:

-is sometimes discharged straight into the sea, but this is now banned by many governments and so the following methods are used:

-produces heat during decay and so is stored in 'ponds' of cooled water where it loses much of its activity. Before discharge, it is filtered through an ion exchange resin which removes stronium and caesium (elements responsible for radioactivity).

-Storage of waste in steel containers inside concrete-lined vaults.

High level waste:

-96% uranium is recovered during reprocessing for reuse.

- 1% is plutonium (valuable fuel)

-3% is level liquid waste that is vitrified. The liquid waste is dried in a furnace and then fed into a melting pot together with glass making materials. This molten material is then poured into stainless steel tubes where it solidifies, with air flowing around the containers to keep them cool. High activity and long half-lives means that the waste will remain active for hundreds of years, and hence the solid matter should be buried in a geologically stable place such as disused mines or in granite rock. However there is concern that the radioactive material may eventually leach into the water table and then into drinking water.

E7 Soil

Soil is composed of inorganic and organic material including living organisms, and its composition varies considerably. Soil degradation is when the quality of soil has been affected in some way so that crop production is lowered, and can be caused by changes in weather pattern or manmade factors (i.e. acidification, desertification, contamination, erosion and salinization, timber cutting, overgrazing, industrailization).

http://geodata.grid.unep.ch/mod_map/map.php


E.7.1 Discuss salinization, nutrient depletion and soil pollution as causes of soil degradation.

Salinization: Constant or excess irrigation using water that contains salts, which upon evaporation, remain in the soil and accumulate in the fertile topsoil. These salts can either build up to toxic levels in the plant or dehydrate plants by preventing the uptake of water, both ways resulting in the death of crops.

Nutrient depletion: Intensive farming, without proper management practises results in nutrients being depleted without replacement. Plants require minerals and nutrients for healthy growth and by harvesting these plants, the normal cycling of nutrients through the soil food web is unable to occur. As a result, there is a amelioration of nutrient depletion. Soil should be left to fallow for some periods and organic material like manure and compost should be added to maintain the levels of nutrients.

Soil pollution: Caused by a variety of factors- industrial discharge and improper dumping of toxic waste material ( long term soil pollution), organic soil pollution from the transport and illegal dumping of spent engine oil (short term effect), use of pesticides and fertilizers disrupt the food chain and reduces soil's biodiversity, pollutants can run into surface waters and move through the soil through throughflow and percolation, polluting groundwater.

E. 7.2 Describe the relevance of the soil organic matter (SOM) in preventing soil degradation, and outline its physical and biological functions.

Soil requires not only minerals to allow healthy plant growth, but also organic matter (e.g. in compost). SOM represents the organic constituents of the soil including undecayed plant and animal tissues, their partial decomposition products (i.e. polysaccharides, proteins, sugers, amino acids) and the soil biomass. Humus is a complex mixture of simple and more complex organic chemicals of plant, animal or microbial origin.

SOM can be used in three main ways:

-biological

-chemical

-physical

BIOLOGICAL: Humus provides a source of energy and essential nutrient elements (N, P, S); contributes to resilience of soil/plant system.

PHYSICAL: Humus is involved in structural stability and water-retention and thermal properties. It helps soil to retain moisture and therefore increases capacity to withstand drought, and encourages formation of good soil structure. Its dark colour causes it to absorb more heat and this hence helps warm the soil in colder seasons.

CHEMICAL: Cation exchange capacity helps it act like clay. Contains active sites that bind to nutrient cations to prevent them from being washed away and making them more available to plants. Toxic cations also bind to humus and this stops them from entering the wider ecosystem. Humus acts as an acid-base buffer and enhances the ability of the soil to maintain a constant pH.

E.7.3 List common organic soil pollutants and their sources.

1) Hydrocarbons, VOCs, SVOC (semi-volatile organic compounds): from transport, solvents and industrial processes.

2) Agrichemicals: pesticides, herbicides, fungicides

3) Polyaromatic hydrocarbons: incomplete combustion of coal, oil, gas, wood and garbage.

4) Polychlorinated biphenyls (PCBs): coolant and insulater in electrical equipment (e.g. transformers, generators)

5) Organotin compounds: Bactericides and fungicides used in paper, wood, textile and anti-fouling paints.

E6 Water treatment

75% of the Earth's surface is covered by water. 97.3% of this is ocean, 2.1% glaciers and ice caps, 0.6% ground water, 0.015% lakes and rivers, and 0.001% in the atmosphere.

E.6.1 List the primary pollutants found in waste water and identify their sources.

Water is capable of hydrogen bonding and is highly polar- this allows is to dissolve many chemicals and as a result some toxic substances, bacteria and viruses can be carried by water. Pollutants in water arise from human usage of water for personal use and industry, and the subsequent release of water into the environment. Water unsuitable for drinking, irrigation, industrial use or washing is considered polluted water.

Pollutants include:

- Heavy metals. i.e. Cadmium from rechargeable batteries, metal plating and pigments. Copper from household plumbing, copper mining and smelting. Mercury from batteries, mercury salts (fungicides), mercury cells in chlor-alkali industry, discharge from pulp and paper mills. Lead, Zinc.

- Pesticides from agricultural practices, like DDT, fungicides and herbicides.

-Nitrates: from acid rain and artificial fertilizers. Causes infantile methaemoglobinaemia (oxygen starvation) because of less acid in babies' stomachs that are converted to nitrite by bacteria- this oxidises the iron in haemoglobin irreversibly preventing it from binding with oxygen. In adults, nitrites are converted into nitrosamines (carcinogenic).

-Dioxin: Waste materials containing organochlorine compounds form dioxins when they are not incinerated at high enough temperatures. It accumulates in fat and liver cells and causes malformation of fetuses.

-Polychlorinated biphenyls (PBCs): One to ten chlorine molecules attached to a biphenyl molecule, chemically stable and high electrical resistance. Accumulate in fatty tissues and remain in the environment; affect reproductive efficiency, impair learning in children, carcinogenic.

E.6.2 Outline the primary, secondary and tertiary stages of waste water treatment, and state the substance that is removed during each stage.

Raw sewage can either be discharged untreated into water bodies, or, in rural areas, deposited into cesspits/septic tanks where they are broken down before they leach into the ground. However, the treatment of waste enables fresh water to be recycled from it.

Primary treatment: Removes 60% of solid material and 1/3 of oxygen demanding wastes. Coarse mechanical filters are used to remove large objects and then a sedimentation tank allows for suspended solids to settle out as sludge. Calcium hydroxide and aluminium sulphate are added to form aluminium hydroxide, which precipitates together with suspended dirt particles in flocculation. Grease is removed by skimming, and the effluent is discharged into a waterway or for secondary treatment.

Secondary treatment: removes 90% of oxygen-demanding wastes, where waste are aerobically degraded using oxygen and bacteria. Two methods- a) waste water is left to trickle through a bed of stones with bacteria; b) sewage is aerated with pure oxygen in a sedimentation tank. Sludge contains active microorganisms that digest organic waste (activated sludge) and some of it is recycled. Water is discharged into a water way where chlorine is added for disinfection or ozone.

Tertiary treatment: expensive; removes heavy metal ions, nitrates, phosphates and residual organic compounds.

- Precipitation- Aluminium sulphate and calcium oxide can be used to precipitate phosphates. Heavy metal ions can be precipitated as insoluble hydroxides/basic salts by the addition of calcium hydroxide or sodium carbonate, or insoluble sulphides by the bubbling of hydrogen sulphide. However some metal hydroxides redissolve as the complexes are soluble (Zn, Hg, Cd).

- Activated carbon bed- carbon is activated by heating periodically to high temperatures and this removes dissolved organic material by oxidising the adsorbed organic compounds to carbon dioxide and water and regenerates the carbon surface.

- Removal of nitrates- ion exchange zeolites can be used to exchange hydroxide ions for nitrates but this is very expensive. The other method would be to use denitrifying bacteria to reduce them to nitrogen or to pass the water through algae pools as algae utilize nitrates as nutrients.

E.6.3 Evaluate the process to obtain fresh water from sea water using multi-stage distillation and reverse osmosis.

Desalination removes salts from sea water (3.5%) that make the water unfit for consumption and for industrial or agricultural purposes. Seawater is heated in a series of coiled pipes before being introduced to a partially evacuated chamber where the water boils instantaneously under the reduced pressure and is condensed and piped off as freshwater. A multi-stage distillation system is often used so that heat released from the condensation of steam can be reused to heat more seawater. This distillation system separates more volatile water from the less volatile salts, and is quite a costly system to maintain.

Reverse osmosis is when pressure is applied to a solution across a semi-permeable membrane that is greater than the osmotic pressure (of water diffusing across the membrane down the concentration gradient) which in turn causes osmosis to occur in the reverse direction, leaving the salts behind as water is forced out of the salt solution. In this case, the membrane used is cellulose ethanoate. It does not require a phase change and hence needs less energy, but still requires energy to produce the necessary pressure (70atm) for reverse osmosis.