Hydrosphere Environmental Chemistry PDF

CHEMISTRY OF THE HYDROSPHERE Dr. András Székács [email protected] The subject of environmental chemistry Definition  Hydrosphere Water is the most commonly occuring molecule on Earth (0.027% of the mass of Earth, 1.38×1021 kg, 7.66×1019 mol) Water...

CHEMISTRY OF THE HYDROSPHERE Dr. András Székács [email protected] The subject of environmental chemistry Definition  Hydrosphere Water is the most commonly occuring molecule on Earth (0.027% of the mass of Earth, 1.38×1021 kg, 7.66×1019 mol) Water represents 8% of the top layer of Earth. There is no life without water Its roles: liquid medium, solvent physico-chemical properties (dielectric constant) light (radiation) transmission and absorbance processes related to life activities are determined by water clean drinking water is of strategic importance. Water and air content on Earth - The Earth is seemingly rich in water (70% of the surface covered with water at a depth of ~4267 m). - A entire quantity of water would separately fill a globe with a diameter of 1400 km. - The amount of air is not much more (5,14 x109 tons) (the sphere show the amount of air at its density at the sea level).  Natural water assets More than 2/3 of the surface of Earth is water body. Glaciers, ice and snow contain most surface fresh water  Average residence time of water in various matrices Atmosphere 1 day Lakes, rivers 7 years Surface layers of oceans 80 years Deep layers of oceans 1600 years Polar ice, glaciers 500 years The greatest material turnover, 423 000 km3 per year, is originated from evaporation and precipitate condensation. Nearly 9% (37 000 km3/year) of this water cycling takes place between the oceans and terrestrial regions.  Water use and management The United Nations warns that water supplies are likely to become one of the most pressing environmental issues of the twenty-first century. By 2025 two-thirds of all humans could be living in places where water resources are inadequate. Estimation of the demand for drinking water Drinking water is of strategic importance currently. 1 billion people have access only polluted water to drink, 5 million children die yearly due to water-born infections. The hydrological cycle The hydrological cycle constantly redistributes water Physico-chemical properties of water Latent heat of vaporisation: 40.8 kJ/mol Latent heat of melting: 5.985 kJ/mol  The chemical structure of water Water molecules are of strong dipole character Hydrogen bonds are formed among water molecules, therefore, the boiling point of water is much higher than apolar molecules with similar molecular weights Water molecules form temporary associations/clusters (2-150) Phase diagram of water Water density by the temperature  The solvent features of water Water molecules have high dipole moment and dielectric constant (ε = 78.54 at 25 °C). They can stabilize particles with electrostatic charge and high dipole moment, and separate charged molecules. Ions and polar molecules have solvent spheres of water. Although water itself is of poor conductivity (R=18 MΩ), solvated ions can show high conductivity features.  Water associations with polar and apolar molecules Solvate spheres Micelles  Measures of particles in water nanoparticles: 10-9-10-7 m (1-100 nm) [Aitken particles: airborne (aerosol) particles < 0.1 µm (100 nm) but larger than molecules, small ions]  The solubility of gases in water The solubility of gases decreases with increasing temperature. The concentrations of different gases are crucial for aquatic living organisms. Low oxygen content is lethal to fish. High nutrient content can cause low oxygen levels (eutrophication).  Oxygen in water The greatest part of the oxygen content of water comes from the surface with convection and diffusion. The solubility of oxygen in water depends on the temperature, partial pressure of the oxygen in the atmosphere, and salt concentration in water. The equilibrium concentration of oxygen in water is 8.32 mg/l (1 atm, 25°C). The oxygen concentration is regulary less than the equilibrium concetration. The solubility of oxygen in water decreases with increasing temperature.  Wind determines the chemistry of shallow lakes Generally the concentration of dissolved oxygen is less than its saturation level. Startification of a deep lake in summer Thermocline is a a steep temperature gradient in a body of water such as a lake, marked by a layer above and below which the water is at different temperatures.  Stratificacion of lake by seasons water expands under 4°c ice melts under pressurre (migrations of glaciers) phase trasition of water requires high energy ice epilimnion layer hypolimnion winter summer startifications change by seasons - complete mixing, - nearly identical temperatures autumn/spring  Autoprotolysis of water, pH H2O↔ H+(aq) + OH–(aq) aH+, a OH–, illetve aH2O are chemical activities of ions and molecules in the dissociation process Dilute solvents have aH2O = 1 pH< 7 = in acidic conditions Acidity of the oceans by depth  Strength of acids and bases Stronger acids substitute weaker acids in their salts. CO2–bicarbonate–carbonate equilibrium The CO2/HCO3–/CO32– buffering effect is important in surface waters. When water is in equilibrium with CO2 content from the atmosphere and carbonate minerals, its pH is buffered to 8.3, close to the pKa of the weak acid bicarbonate (pKa=8.4). The buffer factor air can compensate effects of other water acids or bases. Interactions of Ca2+-minerals with dissolved CO2- content are of rock, soil or importance. sediment  Carbon dioxide in water The partial pressure of CO2 (0.038 v/v% in dry air) is 3.68×10–4 atm, therefore the pH of the rain is 5.63. CO2 content in water is generally higher than in the atmosphere, as CO2 is in an exilbrium with its reaction products. Clean water is slighly acidic due to dissolved CO2. Alkalinity of natural waters is caused by dissolved cations (e.g. Ca2+, Na+). Salt concetrations in different water bodies Average salt concentration of the oceans and seas: 3.5%. Salts are leachates of rivers.  Dissolved calcium, water hardness Permanent hardness Temporary hardness Water softening (lime and ion exchanger) Effects of pH on the solubility of heavy metal hydroxides and sulfides  Redox conditions of the hydrosphere Redox reactions often occur not only in chemical, but also in microbiological processes. Redox reactions have close relationship to acid-base reactions. The solubility of ions depends on their oxidation states. Fe2+ is solube in water, but Fe3+ is not. Concentration of nitrogen compounds as a function of the redox conditions. pE – proportional to the redox potential Species of iron ions as a function of pH and redox conditions  Redox conditions of a lake Solubility of certain substances (e.g. metal compounds) is usually higher under the reducing conditions formed  facilitated mobilization from sediments, minerals  Characteristic features of hydrosphere reactions liquid state reactions, bulk (solved state) and surface reactions, ionic reactions, mostly second order reactions, acid-base and redox reactions  Ground water Upper soil layers that hold both air and water make up the zone of aeration. Lower soil layers where all spaces are filled with water make up the zone of saturation. The top of this zone is the water table. An aquifer is an underground layer of water-bearing permeable rock, rock fractures or unconsolidated materials (gravel, sand or silt) from which ground water can be extracted using a water well. Aquatic life  Major aquatic chemical processes - Dissolved oxygen is frequently the key substance in determining the extent and kind of life in a water body. - Salinity also determines the kinds of life forms present. - The role of carbohydrate sulfate reduction.  Characteristic pollution features of different water bodies seas: immense buffer capacity, but slow recovery surface water: quick response, easy remediation, high self cleaning capacity subsurface water: late recognition of pollution, weak self cleaning capacity rain water: quick changes (precipitation), intensive reactions with atmospheric components  Pollutants in water infections (bacilli, viruses, amoebas/protozoans) oxygen depleting substances nutriens (eutrophication) poisonous (toxic) substances mineral oil suspended particles (heavy metals) radioactive materials thermal pollution  Acid rain in the USA Acid rain results in deforestation by promoting fungal growth. Acid rain in Europe  Temporal effects of pollutants there are essential differences between surface and sub-surface waters in pollution diffusion and recovery potential surface water: pollution is transient (a few days to weeks) sub-surface water: pollution is persistent (decades, even centuries)  DDT concentration (µg/g fat) in aquatic animals in the Baltic sea Persistent contaminants escape decomposition for long, Their solubility in water and bioavailability are generally low.  Definitions Trophity (ability to produce organic matter) is a key indicator of organic matter production in an aquatic ecosystem. Its rate is determined by the chlorophyll- based vegetation (alga, seaweed), inorganic nutrient content in the water (phosphorous and nitrogen), and solar radiation. Increasing primary production of vegetal biomass indicates the oligotrophic, mezotrophic, eutrophic, polytrophic and hypertrophic states of the freshwater ecosystem. Increases in trophity leads to proliferation of the vegetation (mostly alge), causing increased eutrophication.  Definitions Saprobity is the organic matter decomposition ability of an aquatic ecosystem. Unlike trophity, this process leads to energy loss. If organic matter content is high, decomposing microorganisms populate the ecosystem, using up the dissolved oxygen content in the water body. It is characterized by the biological/biochemical oxygen demand (BOD) and the chemical oxygen demand (COD), total organic carbon (TOC) and organic nitrogen content, as well as by saprobiological analysis (saprobity rate, saprobity index).  Pollution by nutrients eutrophication (influx of nutrients of high N, P content) accelerated assimilation (e.g. algal bloom) oxygen depletion, anaerobic degradation (swamp, dry-out, fish poisoning) C : N : P mass proportions in an average biomass are approximately 100:17:1 origins of nutrients: fertilizers, municipal waste water  Definitions Toxicity is caused by the presence (exposure to) substances killing aquatic organisms or disturbing their life funcions. The presence of known and unknown toxic substances is determined by chemical analysis and biological tests, respectively. The toxicity of water polluting substances is chracterized by their LD50 or LC50 values (dosage or concentration, respectively, causing mortality of 50% of the population tested).  The effects of phosphorous Phosphates may precipitate as poorly soluble Fe(III)- or aluminum-phosphate. Former is of particularly high significance in remobilization of phosphates from sediments. In surface waters of high nutrient and contaminant levels phosphates may become remobilized in water layers in close proximity to the sediment under acidic and anoxic (reducing) conditions: FePO4(s) + H+ + e– Fe2+(aq) + HPO42– (aq) As sudden increases in biomasss production (eutrophication) in the aqueous environment are controlled by the concentration of phosphates, the above process definitely leads to negative effects. The greatest difficulty is that phosphate cannot be removed (or can be removed only at great expenses) from the sediments. Contaminants using up dissolved oxygen content in water organic compounds food for bacteria: C (in org. compounds) + O2 (dissolved in water) CO2 4 H (in org. compounds) + O2 (dissolved in water) 2 H2O  The concentration of dissolved O2 may drop exceedingly e.g., 3 ppm C uses up 9 ppm O2 C content of 1 droplet of oil uses up O2 content of 5 liter of water origin: human/animal waste, industrial (food, paper, leather industry) sewage  algal toxicity further contamination  Biochemical oxygen demand (BOD) oxygen quantity required for biological oxidation of organic materals mostly requires 20-30 days, but determined in 5 days (BOI5) e.g. BOI5 tap water: 1 ppm municipal waste water: 100 – 400 ppm animal farm: 100 –1000 ppm food industry waste water: 100 – 10000 ppm  Chemical oxygen demand (COD) COD values show the dissolved organic substance content in water high COD causes oxygen depletion – inhibits oxygen uptake and CO2 excretion in living organisms mitigation: air bubbling, lime addition, sedimentation measurement: CODMn, CODCr (two types) the organic substance content of water can also be characterized by total organic carbon (TOC) content – this is often high due to humin content  Mineralization of elments in water Aerobic or anaerobic mineralization depends on the oxygen content of water. etc. organicsubstances etc. The self cleaning potentials of natural waters are limited.  Different sedimentation patterns in summer and winter - alternating CaCO3 and FeS layers in lake sediments - sedimentation is more oxidative in the summer due to more intensive assimilation  Water contamination by Hg Hg is a nerve poison, accumulative, bioaccumulated in fish Acceptable daily intake (ADI): 0.1 μg/b.w.kg/day Hg is methylated under anaerobic conditions in the sediment, and becomes orders of magnitude more toxic biological methylation – methylcobalamin (MeCbl) it reaches exceedingly high concentrations in marine fish used to be applied as seed coating in agriculture wide industrial uses (gold enrichment, electric connectors, dentistry) Lead deposition in sediments Transport of pollution in ground water Mineral oils consist of mostly water insolube (hardly soluble) compounds.  Mineral oil spill in surface water. Impoundment by floating barrier (perlite, cellulose, wool)  Dead birds The components of mineral oil are poisonous and dissolve the insulation materials (fat) from the feathers of the birds.  Temporal spread of contamination a substantial portion of an oil pollution in the Winter may get absorbed on the sediment in the river basin the absorbed pollutant may become remobilized in the Summer and reach water surface, where it may substantially block oxygen diffusion into the water body the level of dissolved oxygen is low anyway due to higher Summer temperatures, and further decreases may lead to mortality in aquatic populations Purification of drinking water natural biofilters play an important role in bank-filtered wells  Adverse side-products are formed during chlorination chloroform: carcinogenic poison chlorinated alkylphenols: hormonal disruptors oxidated PAHs: carcinogenic, mutagenic effects