environmental chemistry: pollutants - uni-due.de · 19.12.2013 2 3 importance of soil soil is...

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Applied Analytical ChemistryOliver J. Schmitz

Chapter 4

Environmental Chemistry: Pollutants

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Soil: Composition

The lithosphere (from the Greek lithos: stone;Sphaira: ball, globe) is an about 100 km thickrock crust of the Earth.It includes the solid crust and the upper partof the Earth` mantle.The upper part of the lithosphere is the ground, the pedosphere (Greek Pedon: soil).

This is the outermost and usually loose layer of the earth's surface including its raw materials and ground water - from a few tens of centimeters to several meters thick -in which the soil life goes on. The soil is therefore the material at the surface between the air on one side and the basement rock on the other.

(http://geographyworld.edu.tr.tc/pictures/Earth-crust.png)

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Importance of soil

Soil is formed by mineral particles, organic matter, water, air and living organisms.

It is in fact an extremely complex, variable and living medium.

Soil is the interface between the earth, the air and the water, and is a non-renewable resource which performs many vital functions:

food and other biomass production,

storage,

filtration and transformation of many substances including water, carbon, nitrogen.

Soil has a role as a habitat and gene pool, serves as a platform for human activities, landscape and heritage and acts as a provider of raw materials.

These functions are worthy of protection because of their socio-economic as well as environmental importance.

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Soil components

Air and water are relatively uniform media with a largely defined composition.Unlike the ground:

The ground is not a compact material, but a complex system of mineral and organic components, in which factors such as climate, water and other substances, soil organisms and plants interact in dynamic processes.

Soils are physically and chemically different:

Sandy ground

Clay soils

Organic and mineral soils

Brown earth

Black Earth, etc.

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Soil components

Soils are independent natural bodies:

They are transformation products of mineral and organic substances, depending on the vegetation. The plants depend again on the ground on which they grow.

Microorganisms and small animals are living in the soil.

The decomposition of dead organic matter (such as plants) by soil organisms leads to inorganic compounds (mineralization) and to the formation of humus.

Where no plants exist, e.g. in deserts or high mountains, there is no real ground.

The degradation and transformation of organic matter to humic substances by chemical reactions or with the help of microorganisms (humification) is the basis for soil formation and thus for plant growth:

Plants formed (with the help of soil organisms) their own soil, therefore it is often called: grown soil.

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No real soils

(www.reisenundmeer.net/wp-content/uploads/2008/09/wueste20marokko-300x200.jpg)

(www.biologiedidaktik.at/Bilder/Medien/wueste1.jpg)

Marocco Atacama-desert

Nanga Parbat, Berg im Westhimalaya

(http://www.sai.uni-heidelberg.de/grafiken/nangaparbat.jpg)

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Importance of soil organisms

Many processes in soil formation and soil development are dependent on the existence of life and intense activity of soil organisms.

The soil organisms are responsible for, e. g.

the mixing of soil and

the degradation and transformation of dead organic matter (e. g. plant roots, leaves and other plant remains, dead animals).

By these processes minerals and nutrients are released which are needed by plants, but also some by animals for their growth and development.

(http://croptechnology.unl.edu/Image/mmamo3/TimKettler/earthwormcutaway_300.jpg)

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Importance of soil organisms

The soil is alive with organisms, ranging from visible insects and earthworms to microscopic bacteria and fungi.

In fact, the soil can be viewed as home to a great complexity of life, rather than just a medium to support plants.

An acre (4047 m²) of living soil may contain 400 kg of earthworms, 1000 kg of fungi, 680 kg of bacteria, 60 kg of protozoa, 400 kg of arthropods and algae, and even small mammals in some cases.

Soil bacteria are the most numerous, with every gram of soil containing at least a million of these tiny one-celled organisms.

There are many different species of bacteria, each with its own role in the soil environment.

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Soil components

The soil contains several components:

inorganic (mineral) components,

dead and partially decomposed organic material,

soil organisms,

soil air and

ground water in which inorganic and organic substances are partly dissolved.

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Soil components

A typical grassland soil is based on:

ca. 25% air

25% water

45% mineral components and

5% organics

There are soils that contain more than 30% of organic substances ("Organic soils", peaty soils).

The mineral components of soil are mainly silicates and inorganic nutrients. Especially Ca2+, Mg2+ and K+ as well as nitrate and phosphate are important for plants.

(http://world-of-whisky.net/wp-content/uploads/2010/07/torf.jpg)

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Soil structure

Soil structure is determined by how individual soil granules clump or bind together and aggregate, and therefore, the arrangement of soil pores between them.

Soil structure has a major influence on water and air movement, biological activity, root growth and seedling emergence.

The quantity, distribution and arrangement of pores determines water holding capacity, infiltration, permeability, root penetration, and, respiration.

Only about 50% of soil is solid material. The remainder is pore space.

It is in these spaces that the action happens. Water is stored there. Organisms live there. Organic matter and nutrients accumulate there.

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Soil structure

The picture (magnified about 20 times) demonstrates how solids and pores might arrange in soil to give a porosity of 50 %.

(http://vro.dpi.vic.gov.au/dpi/vro/vroimages.nsf/Images/soilhealth_soil_structure/$File/soilhealth_soil_structure.gif)

Small pores within the aggregates provide storage and refuge. The larger pores (and fissures) between the aggregates are the pathways for liquids, gases, roots and organisms.

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Soil development

For soil development and thus for the soil structure and vegetation are several factors important:

Depending on the climate soils are supplied with air and water from the atmosphere.

The air in the soil affects microorganisms, the respiration of plant roots and the root fungi.

With increasing depth the oxygen content of the soil is reduced. This means that microorganisms living in the upper layers of soil under aerobic conditions, in deeper layers, in contrast under anaerobic.

Moreover, the biotic and humic fractions decrease with depth, and the mineral fraction increase correspondingly.

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Soil development

Soil formation is the process by which rocks are broken down into progressively smaller particles and mixed with decaying organic material.

Bedrock begins to disintegrate as it is subjected to freezing-thawing cycles, rain, and other environmental forces (I).

The rock breaks down into parent material, which in turn breaks into smaller mineral particles(II).

(http://the-human-race.org/images/environment/stages_development.gif)

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Soil development

The organisms in an area contribute to soil formation by facilitating the disintegration process as they live and adding organic matter to the system when they die. As soil continues to develop, layers called horizons form (III).

(http://the-human-race.org/images/environment/stages_development.gif)

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Soil profile

A soil horizon is a layer parallel to the soil surface, whose physical characteristics differ from the layers above and beneath.

Each soil type has at least one, usually three or four horizons.

Horizons are defined in most cases by obvious physical features, chiefly color and texture.

These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material, i.e. ‘coarser’ or ‘sandier’ than the horizons above and below.

(http://i695.photobucket.com/albums/vv318/ChrisC85/soil3.jpg)

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Soil horizons

O horizon from decayed and decaying plant biomass

A horizon, topsoil (typically serveral cm in thickness, maximum biological activity, contains most of the soilorganic matter, metal ion and clay particles leaching)

E horizon, layer of elution of clay and aluminium andiron oxides, generally weathered, leached

B horizon, subsoil (receives material such as organicmatter, salts and clay particles leached from topsoil)

C horizon, weathered parent rock (composed ofweathered parent rocks from which the soil originated)

R horizon (bedrock)

(http://www.enchantedlearning.com/geology/soil/soillayers.GIF)

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Humic acids, fulvic acids, and humin

Humic substances in soils and sediments can be divided into three main fractions:

humic acids,

fulvic acids, and

Humin (chemical compounds in organic soil that do not dissolve when treated with diluted alkali solutions).

Humic acid is a principal component of humic substances, which are the major organic constituents of humus, peat, coal, many upland streams, dystrophic lakes, and ocean water.

It is produced by biodegradation of dead organic matter.

It is not a single acid; rather, it is a complex mixture of many different acids containing carboxyl and phenolate groups so that the mixture behaves functionally as a dibasic acid or, occasionally, as a tribasic acid.

Humic acids can form complexes with ions that are commonly found in the environment creating humic colloids.

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A typical humic substance is a mixture of many molecules, some of which are based on a motif of aromatic nuclei with phenolic and carboxylic substituents, linked together; the illustration shows a typical structure.

Hypothetical structure of humicsubstances with aromatic rings, carboxyl and hydroxyl groups as well as a peptide side chain and sugar.The composition of humicsubstances changes because of the always formation, decomposition and reconstruction in biological and abiological processes.A specific constitution represents only a temporary condition.

(http://upload.wikimedia.org/wikipedia/commons/thumb/d/d0/Humic_acid.svg/620px-Humic_acid.svg.png)

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Model of a humic acid by Schulten (1997)

(http://ars.els-cdn.com/content/image/1-s2.0-S0277379199000876-gr1.gif)

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The functional groups that contribute most to surface charge and reactivity of humicsubstances are phenolic and carboxylic groups.

Humic acids behave as mixtures of dibasic acids, with a pK1 value around 4 for protonation of carboxyl groups and around 8 for protonation of phenolate groups.

The other important characteristic is charge density.

The molecules may form a supramolecular structure held together by non-covalent forces, such as Van-der-Waals force, π-π, and CH-π bonds.

The presence of carboxylate and phenolate groups gives the humic acids the ability to form complexes with ions such as Mg2+, Ca2+, Fe2+ and Fe3+.

Many humic acids have two or more of these groups arranged so as to enable the formation of chelate complexes.

The formation of (chelate) complexes is an important aspect of the biological role of humic acids in regulating bioavailability of metal ions.

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Fulvic acids are humic acids of lower molecular weight and higher oxygen content than other humic acids.

The humic and fulvic acids are extracted as a colloidal sol from soil and other solid phase sources into a strongly basic aqueous solution of sodium hydroxide or potassium hydroxide.

Humic acids are precipitated from this solution by adjusting the pH to 1 with hydrochloric acid, leaving the fulvic acids in solution.

This is the operational distinction between humic and fulvic acids.

Humin is insoluble in dilute alkali.

Sharp boundary between the brown water of the Amazon and the black water of the Rio Negro. The dark color of water occurs due to the high amount of dissolved humic and fulvic acids.

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Atmospheric inputs to soil

The troposphere plays an important role in the distribution of materials on the Earth's surface, and thus also for the soil.

The weather, the water cycle or the long-range transport of natural and anthropogenic emissions mainly occur in the troposphere.

The troposphere is composed of gases (nitrogen, oxygen, trace gases), water droplets (liquid particles) and solid particles.

The solid particles are natural and anthropogenic origin, and are emitted by swirlingof mineral dust and sea water (salt particles), by volcanoes, by abrasion of plants or as smoke and soot into the atmosphere.

Also, chemical reactions can cause in the air to non-volatile compounds, which condense.

Water droplets in the atmosphere are mainly caused by condensation of water vapor, partially also in the turbulence (sea-spray) of water surface ("white capping").

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Atmospheric inputs to soil

For larger liquid and solid particles (dust and drops from about 1-10 µm) the upward component of air turbulence is usually not enough to keep them in suspense for a longer time. They follow the gravity, fall down and sediment.

In contrast, smaller particles (e. g. water droplets and solid particles of about 10-2 to max. 102 µm) suspended mainly in the air (airborne particles, aerosols).

From various anthropogenic sources (combustion processes, production, application, etc.) organic and inorganic (harmful) substances emit in thetroposphere.

In the troposhere this compounds are transported and desorbed at another place on earth.

Important substances and groups in this context are sulfur dioxide, nitrogen oxides, carbon dioxide, heavy metals, VOCs (volatile organic compounds) and POPs (persistent organic pollutants).

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Distribution equilibrium

There is a distribution equilibrium of the substances which are emitted into the troposphere between the different phases in the air.

The substances are therefore present in gaseous form, bound to solid particles (aerosols) and dissolved in water droplets.

To estimate the amount of a substance in the phases air and water their Henry coefficient can be used:

At a volume ratio Vwater : Vair from 10-6

substances with a Henry coefficient of > 10-5 (cg · CW-1) are located more than

90% in the gas phase

substances with a Henry coefficient < 10-7 are located more than 90% in the water phase

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Particle-bound fraction

The particle-bound fraction is dependent on the vapor pressure of the substance as well as on the temperature, the amount of particles and the particle size distribution in the air.

It increases with decreasing vapor pressure of the substance and with decreasing temperature. A rough estimate of the particle-bound fraction is possible on the basis of the vapor pressure.

Vapor pressure at 20 °C [Pa]

Particle-bound fraction[%]

10-2 ca. 1

10-3 ca. 9

> 10-4 below 50

10-4 ca. 50

< 10-4 above 50

10-5 ca. 90

10-6 ca. 99

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Particle-bound fraction

The binding mechanism is normally adsorption, but is sometimes also absorption. Due to the fact that within a group of substances (e.g., PAHs, PCBs, PCDD/F (polychlorinated dibenzodioxins and dibenzofurans)) the physico-chemical properties of individual substances are very different in some cases, the respective gaseous and particle-bound content are different.

Since there are still differences in transport, discharge and degradation mechanisms between gaseous and particle-bound fractions, the single-substance pattern of a group of substances change during their stay in the air.

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Deposition

Atmospheric deposition means the material flow from the atmosphere to the surface, i.e., the discharge and the deposition of dissolved, particle-bound or gaseous air pollutants on surfaces (acceptors) of abiotic or biotic systems.

Biotic acceptors are the above-ground parts of plants, especially the leaves and needles of the plants.

Abiotic acceptors are soils, surface water, snow or buildings (roofs, roads, etc.).

In addition to the direct and indirect photodegradation the atmospheric deposition of pollutants is the key mechanism for their elimination from the air.

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Deposition process

The effective deposition process depends:

of the properties of a substance (vapor pressure, water solubility, polarity, etc.) and its present form (gaseous, particulate bound, solved), and

on the state of the troposphere (i.e. temperature, aerosol density, rainfall and frequency) and

of the acceptor (surface roughness, and others).

Generally very volatile gaseous substances with high Henry's law constants are only removed to a small extent by the deposition from the atmosphere.

Particle-bound substances with low vapor pressure and good water-soluble substances are removed significantly more effective by deposition.

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Reemission

The elimination of substances from the atmosphere by deposition is not a totally irreversible process.

A portion of the deposited amount of substance - the proportion is dependent mainly on the deposition mechanism, of the acceptor´s surface and the temperature - can be returned to the atmosphere via evaporation (reemission).

For long-living substances cycles are observed over a long period, which suggest an indirect, apparent emission source.

This is particularly important for such long-living substances, which will not be produced and used today and therefore no direct entry into the atmosphere (e.g. PCB) exists.

Because the rate of reemission is higher in warmer climates than in cold (evaporation) and also the microbial activity decreases at low temperature, it comes to an enrichment of such substances in the colder regions of the earth after a certaintime ("grasshopper effect").

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Grasshopper effect

(http://ars.els-cdn.com/content/image/1-s2.0-S1438463911001623-gr3.jpg)

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Dry atmospheric deposition

Dry atmospheric deposition is the discharge of pollutants by small solid particles and liquid particles (fog and cloud droplets) and gases from the atmosphere, including its deposit on surfaces of acceptors.

The dry deposition removes pollutants only from the lower part of the troposhere (a few hundred meters thick near-surface air layer).

(http://3.bp.blogspot.com/-7YhteU0Whm8/UDpihBtleVI/AAAAAAAAAAM/W_7uxIdYMSE/s1600/golden-gate-bridge-fog_21245_990x742.jpg)

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The size of solid particles or droplets mainly determined the mechanism of their deposition to soil or plant surfaces.

Larger particles with an aerodynamic diameter* greater than about 1-10 µm fall because of their weight (gravity) to bottom (sedimentation).

Very small particles with an aerodynamic diameter of <0.2 µm pass mainly by diffusion to the surface of acceptors.

For the intermediate particle sizes the surfaces of acceptors are barrier to which they adhere during transport with the turbulent air flow (impaction).

*The aerodynamic diameter is an abstract term to describe the behavior of a gas-borne particle, so for example an airborne dust particle. The aerodynamic diameter is defined as the diameter of a sphere with the normalized density of 1 g/cm3, which has the same sinking velocity in still air as the particle itself.

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Gaseous components of the air are transported mainly by diffusion to the acceptor, where they are absorbed or released. This mechanism is called gas deposition and plays in particular a non-negligible role at biotic acceptors (plants) .

(http://www.stadtentwicklung.berlin.de/umwelt/monitoring/pix/we_p0101.gif)

wet desorption (acid rain) dry desorption

Gaseouspollutants

Particulatematter

Wet desorption

Particulate matter

Gaseous pollutants

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The gas deposition is strongly depended on the acceptor and in particular its surface properties and absorption capacity.

The latter decreases with the amount of deposited material and on reaching a state of equilibrium between the acceptor´s surface and the atmosphere the gas deposition will stop.

The maintenance of an imbalance by permanently removing (for example, re-emission) is therefore a requirement for a continuity of the process.

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Wet atmospheric deposition

Wet atmospheric deposition is the discharge of dissolved and undissolved (adhering to particles) substances by aqueous precipitation (rain, snow, hail) from the atmosphere.

The wet deposition can transport substances from several kilometers thick layer of air to the surface of acceptors.

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Wet atmospheric deposition

The effluent of gaseous substances in the atmosphere by dissolving in liquid particles including subsequent deposition at a acceptor surface is called "gas-scavenging”.

The "particle scavenging“, is however, the physical removal of solid particles, including annealed substances by raindrops.

If these processes happen within the cloud, it is called "in cloud scavenging" or "rain out”.

The terms “Below cloud scavenging" or "wash out” are used if the process (wet desorption) takes place during the fall of raindrops on the way from the cloud to the surface.

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Interception deposition

A special feature of wet deposition represents the so called interception deposition.

This is the atmospheric load, which reaches the ground with the dripping rain from plants.

Above-ground parts of plants, especially leaves and needles in forests, are - even in dry periods or during fog - a significant acceptor surface.

During precipitation, the previously dry deposited and in the meantime partially metabolized substances are washed away.

The wet deposition rates on the ground are thereby increased significantly in forests, particularly in conifer forests, compared to the open land.

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Take-home message

Soil components

Soil development from rocks to horizons

Soil horizons

Humic acids, fulvic acids, and humin and their differences

Grasshopper effect

Dry and wet atmospheric deposition

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Arsenic and Mercury

Arsenic: 14 mg / 70 kg Mercury: 4 mg / 70 kg(0.2 µg/g) (0.06 µg/g)

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Building Blocks of Life

Our body is a reflection of our physical environment. Most of the chemical elementsfrom the periodic table are inside our body. Some are essential to life, otherexpendable however, many unnecessary or even toxic in larger doses. The figuresshows how much of each element contains in a person weighing 70 kg (onaverage).

Human 70 kg:

basic elements of life(96 % of the body weight)

trace elements(with biological function)

trace elements(biological function controversial)

no known biologicalfunction

common elements

Occurs in the body, most commonly in the form of water. As breathing gas oxygen is the central energy supplier. However, too much of it destroys the cells.

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common elements

Is important for the growth of muscles and bones. Control how the muscles contract.

Human 70 kg:

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common elements

Is the central element in the zinc finger proteins. Which can specifically bind to the DNA, and are therefore attractive for gene therapy.

Human 70 kg:

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Mensch 70 kg:

Mercury: 4 mg / 70 kg(0.06 µg/g)

Arsenic: 14 mg / 70 kg(0.2 µg/g)





common elements

Has no known function in the body, but is toxic to kidneys, liver and lungs.

As part of an enzyme it breaks down uric acid and prevents gout attacks.

Dispensable for normal people. Used therapeutically, it can help relieve depression.

Human 70 kg:

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.





common elements

Human 70 kg:

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Mercury

Mercury: 4 mg / 70 kg(0.06 µg/g)

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Mercury: Properties

[Xe] 4f145d106s2

Density 13,55 g/cm3

Melting point -38,9 °C

Boiling point 356,6 °C

Hg is liquid: contraction of the 6s orbital (lanthanide contraction). The reason for this contraction is located in the height of the nuclear charge.

Has a high vapor pressure (volatile)

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Mercury

Mercury is a metallic element easily distinguished from all othersby ist liquidity at even the lowest temperature occuring in moderate climates.

Mercury was unknown to the ancient Hebrews and early Greeks; and the first mention of mercury was by Theophrastus, writingaround 300 BC, stating that mercury was extracted fromcinnabar by treatment with copper and vinegar.

The most important ore of mecury, cinnabar (mercuric sulfide), has been minedcontinuously sind 415 BC. Historically, the five primary mining areas for mercurywere:

the Almeden district in Spain,

the Idrija district in Slovenia,

the Monte Amiata district in Italy,

and various locales in Peru, the United States (especially California and Texas), as well as sites in Russia, Hungary, Mexico and Austria.

(http://de.sott.net/image/image/s3/61107/full/quecksilber_fullSize_1_9913021.jpg)

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Mercury consumption

Over the past 2500 years the Almeden mines were the most important, havingproduced about 280,000 tons of mercury, or about 35 % of the estimated total global production of about 800,000 tons.

(http://www.spainisculture.com/export/sites/cultura/multimedia/galerias/monumentos/parque_minero_de_almaden_c.jpg_1306973099.jpg)

(http://www.mineralienatlas.de/VIEWmax.php?param=1285396756)

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Mercury consumption

Major producers of mercury now are China and Kyrgyzstan, Chile and Peru. In Europe Spain, former Yugoslavia, and Italy are major producers.

In the United States, mercury consumption rose from 1305 tons in 1959 to2359 tons in 1969.

In the late 1970s, world use of mercury was estimated at 10,000 to 15,000 tonsannually, of which the United States used about 18 %.

(http://mcgroup.co.uk/images/reports/Mercury%20Market%20Review.jpg)

(http://ars.els-cdn.com/content/image/1-s2.0-S0959652605000752-gr2.jpg)

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Use of mercury in medicine

Cinnabar (HgS), the main mercury ore, was used as a redpigment long before refining processes for elemental mercurywere implemented.

In the 16th century, elemental mercury in combination withother compounds was considered a powerful medicinalagent.

Mercuro zinc cyanide [Zn3Hg(CN)8], also known as Lister´santiseptic, was used in the early days of antiseptics, circa 1880s, in the form of cyanide gavage or cyanide wool in general surgery.

The stronger mercurial ointments were also used to kill cutaneous parasites and to control itching.

(http://www.chemical-engineering.co/wp-content/uploads/2011/07/CINNABAR.jpg)

(http://www.kilmerhouse.com/wp-content/uploads/2008/02/lmw-ad-cuts-and-scrapes.jpg)

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Use of mercury in medicine

Until the advent of antibiotics, mercury salts were a key treatment for syphilis andother venereal deseases.

Mercury salts, especially the chloride and iodide, are powerful antiseptics and untilthe 1940s were routinely used in surgery to kill bacteria.

The volatility of mercury and many of its compounds causes their absorption by thelungs and is an unintended consequence of external application.

(http://www.sciencemuseum.org.uk/hommedia.ashx?id=8441&size=Small)

Albarello pharmacy jar for mercury ointment, Italy, 1520-1560

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Use of mercury in the industry

In the period prior to the industrial revolution, mercury was used extensively in goldextraction and the manufacture of felt hats and mirrors.

In the 1800s, it was used in the chloralkali industry, the manufacture of electricalinstruments and since 1900, it has been used in pharmaceuticals, agriculturalfungicides, the pulp and paper industry as a slimicide and the production of plastics.

In 1892, the process of producing chlorine and caustic soda from brine (sodiumchloride) was developed. Electrolysis of brine using a liquid mercury cathode toproduce chlorine at the anode and a sodium-mercury amalgam at the cathode is still used worldwide, with significant mercury contamination of the biosphere; however, the process is increasingliy under replacement using mercury-free components.

Until the late 1940s, mercury was used in the manufacture of fur felt hats from fursof rabbit, hare, muskrat, nutria, and beaver.

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Electrolysis of brine using a liquid mercurycathode

anode sideThe anodes are placed in the aqueous NaClsolution, above the liquid Mercury; the reduction of Cl- occurs to produce chlorine gas, Cl2 (g)

cathode side

A layer of Hg(l) at the bottom of the tank serves as the cathode. With a mercury cathode, the reaction of H2O to H2 has a fairly high over potential, so the reduction of Na+ to Na occurs instead.

2Na(in Hg) + 2H2O 2 Na+ + 2 OH- + H2(g) + Hg(l)

The Hg(l) that forms is recycled back into the liquid at the bottom of the tank that acts as a cathode; H2 gas is released; NaOH is left in a very pure, aqueous form.

(http://chemwiki.ucdavis.edu/@api/deki/files/14496/=mercury.cell.png)

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Use of mercury in the chemical industry

Mercury catalysts were used in the production of acetaldehyde, acetic acid, andvinyl chloride.

The major use of mercury in the decade between 1959 and 1969 was

a cathode in the electrolytic preparation of chlorine and caustic (33% of the total US Hg consumption),

27% was used for electrical apparatus,

14% for industrial and control instruments, such as switches, thermometers, barometry and general laboratory appliances

12% for antifouling and mildew-proofing paints,

9% for dental amalgams, pulp and paper manufactorers, pharmaceuticals, catalysts, metallurgy and mining and

5% for mercury formulations to control fungal diseases of seeds, bulbs andvegetables.

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Key sources of mercury

(http://www.ec.gc.ca/doc/mercure-mercury/1241/img/image002.jpg)

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Primary natural sources of mercury include volcanic activity and natural erosion of mercury-based deposits. Mercury is emitted from volcanoes into the atmosphere, along with large quantities of lead, cadmium and bismuth. About 60 tons of mercuryare discharged into the atmosphere every year form volcanoes.

Primary anthropogenic (i.e., human) sources of mercury occur when mercury in raw materials is mobilized, through such activities as the combustion of fossil fuels (e.g., coal) and mining and ore processing.

Fossil fuel combustion for power and heating is estimated to represent approximately 46% of global anthropogenic mercury emissions. Combustion of fossil fuels such as coal and crude oil is expected to increase over the coming decades in order to meet growing energy demands, particularly in developing nations. Unless control technologies are put in place, or alternative energy sources are used, mercury emissions from rapidly growing economies such as India and China are projected to continue growing rapidly.

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Secondary anthropogenic sources of mercury refer to releases that occur during the use, disposal and incineration of mercury-containing products such as batteries, paints, thermometers, and electrical and electronic devices.

Disposal of these products, particularly via incineration, accounts for 5–7% of total estimated global anthropogenic releases. Mercury is also used and released in many industrial processes, such as gold mining and the production of chemicals.

Re-mobilization and re-emission of mercury to the atmosphere occurs when previously deposited mercury (from either anthropogenic or natural sources) is re-introduced into the atmosphere. For example, mercury that has accumulated in soil or sediments can be re-mobilized in water as a result of heavy rain or floods. Similarly, mercury accumulated in vegetation can be re-emitted to the air during forest fires.

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Natural sources

The total amount of mercury in various global reservoirs is estimated at 334.17 billion tons:

almost all of this amount (98.75 %) is in oceanic sediments

and oceanic waters (1.24 %), and

most of the rest is in soils.

Living aquatic organisms are estimates to contain only 7.0 tons of mercury.

Reservoir Mercury content [t] Residence time

Atmosphere 850 6 – 90 days

Soils 21,000,000 1000 years

Freshwater 200 ---

Freshwater biota (living) 4 ---

Ocean water 4,150,000,000 2000 years

Oceanic biota (living) 3 ---

Ocean sediments 330,000,000,000 > 1 million years

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Deposition of mercury

A total of 60 to 80 tons of mercury is deposited from the atmosphere into the Arcticeach year.

The main sources of mercury to the Arctic are Eurasia and North America fromcombustion of fossil fuels to produce electricity and heat.

However, elevated mercury concentrations in fishmuscle (0.5 to 2.5 mg/kg fresh weigth) from remote Arctic lakes over extended periods (1975 to 1993) aresometimes due to natural sources of mercury.

Atmospheric deposition of mercury into the Great lakes(US) from sources up to 2500 km distant are documentedat annual deposition rates of 15 µg Hg/m2.

In South Florida, 80 to 90 % of the annual mercurydeposition occurs during summertime wet season.

(http://www.sciencenewsforkids.org/articles/20050309/a708_1335.2.sp.fob.jpg)

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How mercury enters the environment

Cinnabar(HgS)

Elemental mercuryin air/ volatile

toxic

Hg speciesin aquatic systems (Hg2+, MeHg)

MeHgfat soluble - neurotoxic

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Mercury cycle

The cycle has six major steps:

Degassing of mercury from rock, soils, and surface waters, or emissions from volcanoes and from human activities.

Movement in gaseous form through the atmosphere.

(http://www.mercury.utah.gov/atmospheric_transport.htm)

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Mercury cycle

Deposition of mercury on land and surface waters.

Conversion of the element into insoluble mercury sulfide.

Precipitation or bioconversion into more volatile or soluble forms such as methyl mercury.

Reentry into the atmosphere or bioaccumulation in food chains.

(http://www.mercury.utah.gov/atmospheric_transport.htm)

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Mercury cycle

Mercury cycles in the environment as a result of natural (e.g. geothermal activity) and anthropogenic (human) activities.

The primary anthropogenic sources are: fossil fuel combustion and smelting activities.

Both these natural and human activities release elemental mercury vapor (Hg0) into the atmosphere.

Once in the atmosphere, the mercury vapor can circulate for up to a year, and hence become widely dispersed.

The elemental mercury vapor can then undergo a photochemical oxidation to become inorganic mercury that can combine with water vapors and travel back to the Earth’s surface as rain.

This ‘mercury-water’ is deposited in soils and bodies of water.

Once in soil, the mercury accumulates until a physical event causes it to be released again (e.g. forest fire).

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Mercury cycle

In water, inorganic mercury can be converted into insoluble mercury sulfide which settles out of the water and into the sediment, or it can be converted by bacteria that process sulfate into methyl mercury.

The conversion of inorganic mercury to methyl mercury is important for two reasons:

Methyl mercury is much more toxic than inorganic mercury.

Organisms require a long time to eliminate methyl mercury, which leads to bioaccumula.

Now the methyl mercury-processing bacteria may be consumed by the next higher organism up the food chain, or the bacteria may release the methyl mercury into the water where it can adsorb (stick) to plankton, which can also be consumed by the next higher organism up the food chain.

This pattern continues as small fish/organisms get eaten by progressively bigger and bigger fish until the fish are finally eaten by humans or other animals.

Alternatively, both elemental mercury and organic (methyl) mercury can vaporize and re-enter the atmosphere and cycle through the environment.

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Mercury in fish

Methyl mercury is found in fish everywhere, some types more than others.

The usual amount of mercury found is at levels between 0.01 ppm to 0.5 ppm, although in larger fish, such as swordfish or shark, and even some larger tuna, the levels can be elevated to regions of 1.0 ppm, which is the FDA’s limit for human consumption for “commercially important species”.

To prevent fish that contains more than 1.0 ppm of methyl mercury from being sold on the market, the FDA works with state regulators to monitor all commercial fish that are bought and sold locally, as well as monitoring imported fish by not allowing fish that exceed the FDA level of 1.0 ppm into the states.

The danger of eating fish with mercury mostly affects women who are pregnant, and young children. The methyl mercury causes damage to the nervous system. Methyl mercury accumulates in fish such as Shark, Swordfish, King mackerel, marlin and Tilefish. Because these are larger fish, they tend to prey on other fish, and will contain all of the methyl mercury from every fish eaten.

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Elemental mercury health effects

Elemental (metallic) mercury primarily causes health effects when it is breathed as a vapor where it can be absorbed through the lungs.

These exposures can occur when elemental mercury is spilled or products that contain elemental mercury break and expose mercury to the air, particularly in warm or poorly-ventilated indoor spaces.

Symptoms include these: tremors; emotional changes (e.g., mood swings, irritability, nervousness, excessive shyness); insomnia; neuromuscular changes (such as weakness, muscle atrophy, twitching); headaches; disturbances in sensations; changes in nerve responses; performance deficits on tests of cognitive function.

At higher exposures there may be kidney effects, respiratory failure and death.

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Methyl mercury health effects

For fetuses, infants, and children, the primary health effect of methyl mercury is impaired neurological development.

Methyl mercury exposure in the womb, which can result from a mother's consumption of fish and shellfish that contain methyl mercury, can adversely affect a baby's growing brain and nervous system.

Impacts on cognitive thinking, memory, attention, language, and fine motor and visual spatial skills have been seen in children exposed to methyl mercury in the womb.

Recent human biological monitoring by the Centers for Disease Control and Prevention in 1999 and 2000 shows that most people have blood mercury levels below a level associated with possible health effects.

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Outbreaks of methyl mercury poisonings have made it clear that adults, children, and developing fetuses are at risk from ingestion exposure to methyl mercury.

During these poisoning outbreaks some mothers with no symptoms of nervous system damage gave birth to infants with severe disabilities, it became clear that the developing nervous system of the fetus may be more vulnerable to methyl mercury than is the adult nervous system.

In addition to the subtle impairments noted above, symptoms of methyl mercury poisoning may include; impairment of the peripheral vision; disturbances in sensations ("pins and needles" feelings, usually in the hands, feet, and around the mouth); lack of coordination of movements; impairment of speech, hearing, walking; and muscle weakness.

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Mercury and cancer

No human data indicate that exposure to any form of mercury causes cancer, but the human data currently available are very limited.

Mercuric chloride has caused increases in several types of tumors in rats and mice, and methyl mercury has caused kidney tumors in male mice.

Scientists only observed these health effects at extremely high doses, above levels that produced other effects.

When EPA revised its Cancer Guidelines in 2005, the Agency concluded that neither inorganic mercury nor methyl mercury from environmental exposures are likely to cause cancer in humans.

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Minamata disease

Minamata disease is a neurological syndrome caused by severe mercury poisoning. Symptoms include ataxia, numbness in the hands and feet, general muscleweakness, narrowing of the field of vision and damage to hearing and speech. In extreme cases, insanity, paralysis, coma, and death follow within weeks of the onsetof symptoms. Minamata disease was first discovered in Minamata city in Kumamoto prefecture, Japan, in 1956.

It was caused by the release of methyl mercuryin the industrial wastewater from the ChissoCorporation's chemical factory, which continuedfrom 1932 to 1968. This highly toxic chemicalbioaccumulated in shellfish and fish in Minamata Bay and the Shiranui Sea, whichwhen eaten by the local population resulted in mercury poisoning.

While cat, dog, pig, and human deathscontinued over more than 30 years, thegovernment and company did little to prevent the pollution.

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Minamata disease

As of March 2001, 2,265 victims had been officially recognised (1,784 of whom haddied) and over 10,000 had received financial compensation from Chisso.

By 2004, Chisso Corporation had paid $86 million in compensation, and in the same year was ordered to clean up its contamination.

On March 29, 2010, a settlement was reached to compensate as-yet uncertifiedvictims.

A second outbreak of Minamata disease occurred in Niigata Prefecture in 1965. The original Minamata disease and Niigata Minamata disease are considered two of theFour Big Pollution Diseases of Japan.

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Dimethyl mercury

Dimethyl mercury is extremely toxic and dangerous to handle. Absorption of doses as low as 0.1 mL has proven fatal. The risks are enhanced because of the high vapor pressure of the liquid.

Dimethyl mercury passes through latex, PVC, butyl, and neoprene rapidly (within seconds) and is absorbed through the skin.

Therefore, most laboratory gloves do not provide adequate protection from it, and the only safe precaution is to handle dimethyl mercury while wearing highly resistant laminated gloves underneath long-cuffed neoprene or other heavy-duty gloves.

A long face shield and work under a fume hood are also indicated.

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Dimethyl mercury

Dimethyl mercury crosses the blood–brain barrier easily, probably owing to formation of a complex with cysteine.

It is eliminated from the organism slowly, and therefore has a tendency to bioaccumulate.

The symptoms of poisoning may be delayed by months, possibly too late for effective treatment.

The toxicity of dimethyl mercury was highlighted with the death of the inorganic chemist Karen Wetterhahn of Dartmouth College in 1997, months after spilling no more than a few drops of this compound on her latex-gloved hand.

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Dimethyl mercury

1865: first synthesis of Me2Hg

2 laboratory technicians died

1943: Leaking containers in the warehouse

2 secretaries are poisoned (several meters away)

1974: Chemist died after the synthesis of Me2Hg

1996: K. Wetterhahn

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Dimethyl mercury

Karen Wetterhahn †

Scientific American 45, 222 (1997)

14th August 1996 spillage of 0.1-0.5 mL Me2Hg

over the gloved hand

November 1996 nausea, vomiting

January 1997 disturbance of speech,

auditory defect, paresthesia,

acroataxia

February 1997 coma

Death – 298 days after exposition!

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Effects of other mercury compounds (inorganic and organic)

High exposures to inorganic mercury may result in damage to the gastrointestinal tract, the nervous system, and the kidneys.

Both inorganic and organic mercury compounds are absorbed through the gastrointestinal tract and affect other systems via this route.

However, organic mercury compounds are more readily absorbed via ingestion than inorganic mercury compounds.

Symptoms of high exposures to inorganic mercury include: skin rashes and dermatitis; mood swings; memory loss; mental disturbances; and muscle weakness.

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Arsenic

Arsenic: 14 mg / 70 kg

(0.2 µg/g)

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Arsenic: Properties

Arsenic appears in three allotropic forms:

yellow, black and grey.

The stable form is a silver-gray, brittle crystallinesolid. It tarnishes rapidly in air, and at high temperatures burns forming a white cloud of arsenic trioxide.

Arsenic is a member of group Va of the periodic table, which combines readily with many elements.

The metallic form is brittle, tharnishes and when heated it rapidly oxidizes to arsenic trioxide, which has a garlic odor.

The non metallic form is less reactive but will dissolve when heated with strong oxidizing acids and alkalis.

(http://bfsu2.de/atom/Peridensystem.jpg)

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Arsenic in the environment

Arsenic can be found naturally on Earth in small concentrations.

It occurs in soil and minerals and it may enter air, water and land through wind-blown dust and water run-off.

Arsenic in the atmosphere comes from various sources:

vulcanoes release about 3000 tonnes per year and

microorganisms release volatile methyl arsines to the extent of 20.000 tons per year, but

human activity is responsible for much more: 80.000 tons of arsenic per year are released by the burning of fossil fuels.

Despite its notoriety as a deadly poison, arsenic is an essential trace element for some animals because it plays a role in protein synthesis (and maybe even for humans), although the necessary intake may be as low as 0.01 mg/day.

It is unclear whether arsenic is a dietary mineral for humans.

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Toxicity of arsenic

Arsenic toxicity is another important characteristic.

The boundary concentration of arsenic is 2-46 ppm for freshwater algae.

The LC50 value for Daphnia magna is 7.4 ppm, and for the American oyster it is 7.5 ppm. These values encompass a time period of 48 hours.

For rats an LC50 value of 20 mg/kg body mass was established. This is the value for the carcinogenic arsenic(III)oxide. This compounds also blocks enzymatic processes, increasing its toxicity.

In mice, hamsters and rats the compound was embryo toxic and teratogenic.

(http://img690.imageshack.us/img690/1082/ratpoisoncapt.jpg)

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A little uncombined arsenic occurs naturally as microcrystalline masses, found in Siberia, Germany, France, Italy, Romania and in the USA.

Most arsenic is found in conjuction with sulfur in minerals such as arsenopyrite (AsFeS), realgar, orpiment and enargite.

Non is mined as such because it is produced as a by-product of refining the ores of other metals, such as copper and lead.

World production of arsenic, in the form of its oxide, is around 50.000 tons per year, far in excess of that required by industry.

China is the chief exporting country, followed by Chile and Mexico.

World resources of arsenic in copper and lead ores exceed 10 million tons.

arsenopyrite(http://webmineral.com/specimens/photos/Arsenopyrite.jpg)

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Arsenic compounds are abundant in the Earth's crust.

Particles are released during mining, and spread throughout the environment.

Arsenic from weathered rocks and soils dissolves in groundwater and the arsenic concentrations in groundwater are particularly high in areas with geothermal activity.

In aquatic ecosystems inorganic arsenic derived from rocks such as arsenic trioxide (As2O3), orpiment (As2S3), arsenopyrite (AsFeS) and realgar (As4S4) is most prevalent.

Arsenic is applied in different shapes and forms, and can enter water bodies as such.

Large quantities of arsenic that are released from volcanic activity and from micro organisms are relatively small compared to the quantities released from for example fossil fuel combustion.

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Environmental effects of arsenic

The arsenic cycle has broadened as a consequence of human interference and due to this, large amounts of arsenic end up in the environment and in living organisms.

Arsenic is mainly emitted by the copper producing industries, but also during lead and zinc production and in agriculture.

It cannot be destroyed once it has entered the environment, so that the amounts that we add can spread and cause health effects to humans and animals on many locations on Earth.

Through fish grind in cattle feed arsenic may enter meat, and through contaminated soils it may enter plant products.

Plants absorb arsenic fairly easily, so that high-ranking concentrations may be present in food.

The concentrations of the dangerous inorganic arsenics that are currently present in surface waters enhance the chances of alteration of genetic materials of fish.

Birds eat the fish that already contain eminent amounts of arsenic and will die as a result of arsenic poisoning as the fish is decomposed in their bodies.

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Use of arsenic

Metallic arsenic is processed in lead or copper alloys, to increase hardness.

The extremely toxic arsenic gas AsH3 plays an important role in microchip production.

Copper arsenate (Cu3(AsO4)2.4H2O) is applied as a pesticide in viticulture, but its

use is currently prohibited in many countries.

Paxite (CuAs2) is an insecticide and fungicide.

Other arsenic compounds are applied as a wood preservative, in glass processing, in chemical industries, or in semiconductor technique together with gallium and indium.

Dutch painters applied arsenic as a yellow pigment.

In the First World War arsenic was applied in chemical weapons.

In the Vietnam War dimethyl arsenic acid was applied for the destruction of rice cultures.

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Use of arsenic

Although arsenic is applied less and less, it is still present in the environment in considerable quantities.

For example, near abandoned mines soil quantities of arsenic may still be up to 30 g/kg.

Arsenic was and is applied for medical purposes.

In water from safe sources it probably aids curing asthma, haematological illnesses, dermatosis and psychosis.

In the 19th century watery solutions of potassium arsenide (Fowler solution) were applied to treat chronic bronchial asthma and other diseases.

At the beginning of the 20th century other arsenic compounds were applied to treat syphilis.

Arsenic may assist in curing sleeping sickness and leukaemia.

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Health effects of arsenic

Arsenic is one of the most toxic elements that can be found.

Humans may be exposed to arsenic through food, water and air. Exposure may also occur through skin contact with soil or water that contains arsenic.

Levels of arsenic in food are fairly low, as it is not added due to its toxicity.

But levels of arsenic in fish and seafood may be high, because fish absorb arsenic from the water they live in.

Luckily this is mainly the fairly harmless organic form of arsenic, but fish that contain significant amounts of inorganic arsenic may be a danger to human health.

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Health effects of arsenic

Exposure to inorganic arsenic can cause various health effects, such as irritation of the stomach and intestines, decreased production of red and white blood cells, skin changes and lung irritation.

It is suggested that the uptake of significant amounts of inorganic arsenic can intensify the chances of cancer development, especially the chances of development of skin cancer, lung cancer, liver cancer and lymphatic cancer.

A very high exposure to inorganic arsenic can cause infertility and miscarriages, and it can cause skin disturbances, declined resistance to infections, heart disruptions and brain damage. Finally, inorganic arsenic can damage DNA.

A lethal dose of arsenic oxide is generally regarded as 100 mg.

Organic arsenic can cause neither cancer, nor DNA damage. But exposure to high doses may cause certain effects to human health, such as nerve injury and stomachaches.

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Arsenic and water

Arsenic can be found in seawater (2-4 ppb), and in rivers (0.5-2 ppb).

Half of the arsenic present is bound to particles.

The ingestion of arsenic by consumption of seafood and algae is of similarimportance. Freshwater and seas algae contain about 1-250 ppm of arsenic, freshwater mycrophytes contain 2-1450 ppm, marine molluscs contain 1-70 ppm, marine crustaceans 0.5-69 ppm, and fishes 0.2-320 ppm (all values are based on dry mass).

More than 50 arsenic species including arsenobetaine have been found in seafoodwhose toxicological effects are largely unknown.

In some marine organisms, such as algae and shrimp, arsenic can be found in organic compounds.

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Arsenic and water

According to the WHO arsenic is considered to be the most harmful toxin in drinkingwater worldwide.

The legal limit for arsenic in water applied by the World Health Organization (WHO) is 10 μg/L.

Unfortunately, because of geogenic occurences and/or as a result of miningactivities in Bangladesh, India, Thailand, Nepal and other parts of the world, millionsof people are exposed to highly arsenic-contaminated water (sometimes over 50 μg/L) and may be at risk for arsenicosis.

This problem results in long-term chronic health effects, such as skin disease, skin cancer, and tumors in lungs, bladder, kidneys and liver.

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Reaction of arsenic with water

Elementary arsenic normally does not react with water in absence of air.

It does not react with dry air, but when it comes in contact with moist air a layer is formed.

The layer has a bronze color, and later develops a black surface.

An example of an arsenic compounds that reacts strongly with water is orpiment. This is an amorphous arsenic compound. Reaction mechanism:

As2S3 + 6 H2O 2 H3AsO3 + 3 H2S

In natural water arsenic participates in oxidation and reduction reactions, coagulation and adsorption.

Adsorption of arsenic to fine particles in water and precipitation with aluminium or iron hydroxides causes arsenic to enter sediments. After some time arsenic may dissolve once again consequential to reduction reactions.

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Solubility of arsenic and arsenic compounds

Elementary arsenic is fairly insoluble, whereas arsenic compounds may readilydissolve.

Arsenic is mainly present in watery solutions as HAsO42-(aq) and H2AsO4

- (aq), andmost likely partially as H3AsO4 (aq), AsO4

3-(aq) or H2AsO3-(aq).

Examples of solubility of arsenic compounds:

arsenic(III)hydride 700 mg/L,

arsenic(III)oxide 20 g/L,

arsenic acid (H3AsO4.1/2 H2O) 170 g/L, and

arsenic(III)sulfide 0.5 mg/L.

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Map of Bangladesh Showing the Regional Distribution of Arsenic in Groundwater

Millions of people worldwide are chronically exposed to arsenic through drinking water, including 35 - 77 million people in Bangladesh.

In 2600 villages/wards, arsenic in groundwater has been found above 50 µg/L. In a preliminary study from 255 villages,86,000 people were examined and 8500 people have been registered with arsenical skin lesions.

An arsenic patient with severe keratosis.

(http://arsenic-33.narod.ru/images/arsenic.jpg)

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Arsenic contamination also in Europe

Arsenic contamination is not unique to Bangladesh. Highly elevated levels of arsenic of natural origin have been reported in groundwater in many parts of the world.

Arsenic poisoning due to excessive exposure to natural and anthropogenic arsenic in drinking water has been reported in Argentina, China, Taiwan, Thailand, India, Mexico, USA, Ghana, Hungary, United Kingdom, Chile, New Zealand, and Russia (CSIRO, 1999).

Arsenic levels in drinking water in eastern Hungary are above EU limits and significantly higher than in either Romania or Slovakia, is the outcome of a report by a group of European researchers published recently.

Whereas only 8% of drinking water topped the EU limit of 10 µg/L in the Romanian and Slovakian counties, in eastern Hungary nearly 70% of those monitored were beyond the 10 microgram EU drinking water standard.

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Arsenic in Groundwater

Probability of occurrence of excessive arsenic concentrations in groundwater.

High

Low

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Arsenic contamination in Bangladesh

This pervasive incidence of arsenic poisoning and its link to drinking water were first identified in the early 1980s.

This was not long after the population began switching from surface water sources like rivers to groundwater from tube wells — part of a national effort to decrease the incidence of bacterial illnesses caused by contaminated drinking water.

But most of the tube wells have been drilled to less than 100 feet, where they draw water directly from the arsenic-contaminated shallow aquifer.

Scientists have struggled to understand how the arsenic, which is naturally occurring in the underground sediment of the Ganges Delta, is getting into the groundwater.

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"Our research shows that water from the ponds carries degradable organic carbon into the shallow aquifer. Groundwater flow, drawn byirrigation pumping, transports that pond water to the depth where dissolved arsenic concentrations are greatest and where it is then pumped up into the irrigation and drinking wells,“ says Harvey. “

The other interesting thing we found is that the rice fields are a sink of arsenic — more arsenic goes in with the irrigation water than comes out in the groundwater.“

Whatever may be the mechanism of arsenic leaching to the aquifer, in West Bengal—India 38,865 km2 and in Bangladesh 118,849 km2 are arsenic affected areas, and population in West Bengal - India living in arsenic affected areas is 42.7 million and 104.9 million in Bangladesh.

This does not mean that the total population (147.6 million) is drinking arsenic-contaminated water in West Bengal and Bangladesh and will suffer arsenic toxicity, but it does indicate the risk levels.

(http://cee.mit.edu/sites/default/files/harvey.jpg)

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But Bangladesh is the seventh most populous country in the world, and tens of millions of its citizens have been exposed to arsenic in their water over the past several decades.

As many as 3,000 Bangladeshis die from arsenic-induced cancer each year and today approximately 2 million people in the country live with arsenic poisoning, which manifests as skin lesions and neurological disorders, and causes cardiovascular and pulmonary diseases and cancer.

Allan H. Smith a professor of epidemiology at the University of California, Berkeley, calls it “the largest mass poisoning of a population in history.”

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Take-home message


Mercury cycle

Minamata disease

Why is dimethyl mercury so dangerous?

Arsenic cycle and sources of arsenic

environmental chemistry: pollutants - uni-due.de · 19.12.2013 2 3 importance of soil soil is...

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