13
Introduction
Mercury is considered a priority pollutant because of its toxicity, mobility and widespread
presence in the environment. Mercury bioaccumulates and biomagnifies within the food
chain, reaching its highest concentrations in large fish and apex mammalian predators.
Adverse effects of mercury on humans concern the central nervous system and pulmonary
and kidney functions. Mercury compounds also damage chromosomes, resulting in birth
defects, including a variety of neurological impairments, from severe mental retardation to
cerebral palsy [145].
Global anthropogenic emissions of mercury into the atmosphere registered a large increase
in the last decades, jumping from an estimated 1,900 tons in 1995 [146] to an estimated
2,190 tons in 2000 [147].
As a result of anthropogenic emissions, the mercury background concentration in the
atmosphere has been rising since the industrial revolution. According to the Expert Panel
on Mercury Atmospheric Processes, pre-industrial atmospheric concentrations constitute
approximately one-third of the current atmospheric concentrations and it has been
estimated that anthropogenic emissions may currently account for 50-75 percent of the
total annual input to the global atmosphere [15].
Mercury compounds are ubiquitous in the environment; they are found in the atmosphere,
water bodies, soils, sediments and living beings. In the past, sediments were often
considered as a burial sink for contaminants. In fact, it has been demonstrated that, in some
conditions, more than 99% of the heavy metals entering a river are stored in river
sediments in various forms [148]. Thus, sediments were not regarded as a potential source
of pollution.
However, due to human activities or natural processes (such as hydrodynamic flows,
bioturbation, molecular diffusion and chemical transformation), which provoke variation
in the physical–chemical characteristics of water bodies, buried mercury may be
remobilized into the overlying water.
In this way, contaminated sediments threaten human health directly, through skin contact,
or through indirect pollution of the overlying water: human beings may drink contaminated
water or, more often, eat organisms contaminated through bioaccumulation of mercury
compounds.
Because of these reasons, contaminated sediments’ contribution to mercury pollution has
recently been reassessed. It is generally accepted that heavy metal contaminated sediments
require remediation.
Unlike many organic pollutants that may be eliminated or reduced through chemical
oxidation techniques or microbial activity, heavy metals will not degrade. Thus, two
remediation strategies have been historically adopted for remedying sediments
contaminated by heavy metals. The first is an in situ strategy which aims at increasing the
stability of metals on sediment particles in situ (through immobilization, containment,
14
vitrification or capping technologies). The second aims at extracting or separating metals
from sediment. Technologies in this group include washing, flotation, thermal desorption
and phyto-extraction [149].
All existing remediation options to deal with metal contaminated sediments have their
limitations. Stabilization techniques just improve the immobility of heavy metals on
sediment without lowering their total content. Therefore, in some special conditions, part
of these immobilized metals may be re-released into the water. On the other hand,
extraction techniques are usually carried out ‘ex situ’, implying high cost for transport,
disposal and treatment. Furthermore, chemicals used to promote desorption of metals from
sediments often causes the sediment structure to deteriorate and the physio-chemical
properties to change [149].
Considering the disadvantages of the common remediation technologies just listed, the
purpose of this study is to evaluate the effectiveness of two new technologies for mercury
contaminated sediment remediation. The first one exploits ultrasound as an
environmentally-friendly means to promote desorption of mercury from sediments. During
a similar test, He and Weavers reported re-adsorption of mercury on model sediments at
prolonged sonication times [150]. Thus, the addition of a chelating agent to the slurry
solution was tested, expecting that it will be able to complex mercury, keeping this metal
in the aqueous phase.
The second tested technology is based on low temperature thermal desorption combined
with a soil washing solution containing an iodine salt, which is supposed to enhance
mercury desorption via ion exchange.
The two technologies have been tested on two real sediments through batch laboratory
experiments. Parameters taken into consideration to find the optimum working conditions
are time of reaction, presence of the chelating agent and eventual reagent concentration.
The amount of mercury desorbed has been calculated as the difference between mercury
concentrations in the sediment before and after treatment.
In conclusion, this study may be considered a step toward a deeper understanding of
contaminated sediment treatment and toward the development of new, cost-effective and
environmentally-friendly technologies for mercury sediment remediation.
This thesis is divided into five chapters. In the first, a review of mercury properties,
toxicity, sources and distribution in the environment will be provided. Chapter 2 will
furnish a characterization of sediments, a description of transport mechanisms from water
to sediments and a discussion of the interaction between mercury and different sediment
fractions. Particular attention will be dedicated to the key role played by dissolved organic
matter, and the factors affecting mercury bonding to sediments will also be emphasized. In
the final part of this chapter an overview of current sediment remediation technologies will
be provided. Chapter 3 will give an overview of ultrasound as an environmental
remediation technology. Cavitation and its effects as well as ultrasound environmental
application will be presented in this chapter. Chapters 4 and 5 are dedicated to the
discussion of the experiments and the results, regarding the first and second new
technologies proposed, respectively.
15
1.Mercury and its environmental impact
1.1 Introduction: heavy metals and the environment
Heavy metals are metals with high atomic weights and densities; however a universally
shared definition is not available. Heavy metals are found on the lower part of the periodic
table of elements. Among these compounds, some cause environmental concern owing to
their toxicity and spread in the environment. Most heavy metals are subject to emission
limits and regulations, and their concentrations in water and air are constantly monitored.
For example, arsenic, cadmium, cobalt, chromium, copper, mercury, manganese, nickel,
lead, tin, and thallium emissions generated by waste incinerators in Europe are regulated
by the European Community (Directive 2000/76/CE).
Contrary to most organic compounds, heavy metals can not decay or be transformed into
simpler forms. Thus, heavy metal remediation only consists of their removal from
environmental media. The property of heavy metals to bioaccumulate in organisms is
related to their toxicity. Many metals also exhibit exponentially increasing concentrations
as they progress to the upper levels of the food chain, a phenomenon called
biomagnification. Biomagnification potentially yields higher toxicity for organisms at the
top of the food chain, such as human beings.
In the first part of this chapter, an account of mercury characteristics, such as physio-
chemical properties, speciation and toxicity, will be given. A review of mercury-related
disasters will follow in Section 1.6. Mercury sources and emissions and transport
mechanisms among environmental compartments will be reported in Section 1.5.
1.2 Mercury as a chemical element
Mercury is the eightieth element of the periodic table of elements. Its external shell
electrons belong to the 3d shell, and due to its position in the lower part of the periodic
table, mercury is considered a heavy metal. Mercury and bromine are the only elements
which can be found in the liquid state at standard conditions of temperature and pressure.
The ancient Romans’ trade Godhead and God’s messenger, Mercurius, lends his name to
this element. Mercury is also known as quicksilver or hydrargyrum, and its symbol in the
periodic table of elements, Hg, is derived from the latter [1].
1.2.1 Mercury physical-chemical properties
Mercury is a silver-white transition metal, and its atomic weight is 200.59 g/mol. As
mentioned above, mercury is a liquid at room temperature due to its melting and boiling
points, respectively -38.83 °C and 356.73 °C [2]. Its high volatility is due to the rapid
increase of vapour pressure with temperature (about five times per 20 °C [3]).Mercury is a
poor heat conductor but a fairly good electricity conductor. These properties, together with
its liquid state, have been exploited for mobile electric contacts.
Mercury is not very reactive, in general, because of its noble-gas like electron
configuration. However, it can be dissolved by strong acids, thus producing salts.
16
1.2.2 Speciation
Mercury is quite uncommon in its elemental state, though Hg
0
can be present in rocks. The
most common oxidation state for mercury in nature is +2. In the form of Hg
2+
ion, mercury
is present as an oxide or in inorganic salts [2]. Cinnabar, a mercury sulphide salt, is the
most common mercury compound in nature, and it is quite insoluble, similar to other salts
formed with group sixteen elements. Hg halides also exist [2]. Mercury oxide can result
from exposure of the metal to air at high temperatures for long periods. Mercury nitrate,
Hg(NO
3
)
2
is another mercury salt, which has been used in the past.
Mercury(I) usually forms dimeric cations in the form
+ 2
2
Hg
. Hg
2
Cl
2
, also known as
calomel, is a colorless solid, which can react with chlorine to give mercury chloride. It has
been used in electrochemistry and past medical practices [2].
1.2.3 Methyl-mercury formation
Mercury can form covalent bonds with some anions and small organic functional groups.
For instance, mercury can combine with two methyl groups, forming di-methylmercury,
Hg(CH
3
)
2
, a very widespread compound in the environment. Formation of di-methyl-
mercury is especially prevalent in muddy lake and river bottoms, where bacteria can
convert Hg
2+
to Hg(CH
3
)
2
under anaerobic conditions [4].
The active agent in the methylation process is a normal constituent of bacteria cells, known
as methylcobalamin, a form of B
12
vitamin. Di-methyl-mercury is quite volatile and
usually evaporates from water unless it is converted to mono-methyl mercury, a less
volatile compound.
Compounds containing only one methyl group (i.e. CH
3
HgCl and CH
3
HgOH) are often
represented by CH
3
HgX and named mono-methyl-mercury or methyl-mercury [4]. These
compounds are formed more rapidly than di-methyl-mercury, especially in acidic or
neutral aqueous solutions.
This form of mercury is probably of highest environmental concern due to its toxicity, as
will be explained in subsequent paragraphs.
1.3 Uses of mercury
Besides its natural presence in rocks and minerals, the most important source of mercury
pollution is anthropogenic. Mercury pollution is related to mercury use in a number of
industrial applications and manufactured products. The incorrect disposal and treatment of
these objects and carelessness in process management potentially result in mercury
discharge into the environment and thus its pollution.
In the past, mercury has been widely used for a number of applications and technological
devices, ranging from medical to scientific, as well as technological and industrial
applications. Because of its toxicity, mercury uses and technological applications
(activities and materials using mercury) are gradually being limited in most countries.
However, mercury and its compounds are irreplaceable in some cases, and their past use in
technological devices and processes is still a source of environmental pollution. In this
section, the numerous and most common applications of metal mercury and its salts will be
analyzed. Mercury organic compounds, despite their great environmental interest, are
17
rarely industrially synthesized but formed in the environment by microorganisms.
Exceptions include organic mercury compounds formerly used as drugs, herbicides and
wood preservatives [5,6].
1.3.1 Metallic mercury
Since mercury is a liquid at room temperature and dilates with temperature, it has been
widely exploited for the manufacture of thermometers, barometers and other medical or
scientific instruments. Nowadays, mercury is primarily used for laboratory high
temperature thermometers, while its use in medical devices has greatly declined.
Metallic mercury is used in electric or electronic devices since it is a liquid with high
electrical conductivity.
Mercury easily forms alloys and amalgams with almost all other metals. This property has
been utilized for a variety of different applications, from dental amalgams production to
object plating and gold and precious metals extraction. In the latest application, a small
quantity of mercury is added to a large volume of debris from mining activity. Combining
with gold and silver, mercury makes the extraction of these metals easier.
Another application of mercury-sodium amalgam is in the chlor-alkali process, exploited
for the production of chlorine and sodium hydroxide from NaCl by electrolysis. Metal
sodium formed during electrolysis is quickly bonded to mercury, before it can react with
water, and the amalgam is then removed from the reactor, and mercury is separated and
recycled. Mercury recovery is not complete, however, and a portion evaporates to the
atmosphere or is discharged with cooling water [7].
In the past, mercury vapour has been used for fluorescent lamps, including street lighting.
Today, the use of vapour is limited to lamps with spectroscopy uses because it emits light
on a very narrow interval of wavelengths [2]. Mercury vapour is also used in special
devices for medical and disinfection purposes: excited vapour mercury can emit in the UV
field, causing damage to bacteria’s DNA, resulting in their death [8].
1.3.2 Mercury oxide and salts
The most common mercury compound in nature is cinnabar, and its chemical formula is
HgS. Since this salt is quite insoluble, wastewater containing mercury is often treated with
another sulphur salt, allowing the formation and precipitation of HgS, through the
following reaction:
) (
2 2
s
HgS S Hg → +
− +
(1.1)
Mercury nitrate has been used in the past for skins and leather processing, while mercury
oxide, HgO, has been used in miniature batteries, especially for acoustic prosthesis.
Finally, mercury chlorides have some industrial applications: Hg
2
Cl
2
is used in
electrochemistry, while HgCl
2
has been used as an insecticide and mouse poison [9].
1.4 Mercury toxicity
Small amounts of some heavy metals, like iron, cobalt, copper, manganese, selenium,
chromium, molybdenum, and zinc, are necessary for human health. Conversely, high
concentrations of these metals can be toxic and harmful. Other heavy metals are toxic or
18
carcinogenic at any concentration; mercury, cadmium, lead and arsenic belong to this
second group [10]. In this section, the concepts of bioavailability, bioaccumulation and
biomagnification will be discussed. Finally, human toxicity of mercury and its compounds
will be analyzed.
1.4.1 Bioavailability
In order to exert their toxic effects, heavy metals must come into contact with organisms
and thus reach their habitats, through diffusion or active transport; heavy metals must also
be in forms that can potentially incorporated into organisms. In this framework, we
introduce the concept of bioavailability, defined as “the portion of a chemical in the
environment that is available for biological action, such as uptake by an organism” [11]. If
we zero in on soil chemistry, the bioavailability definition can be given as "the fraction of
the total contaminant in the interstitial water and on the sediment particles that is available
for bioaccumulation" [11].
Naturally, bioavailability is also a function of the type of organism because it is related to
the pathways by which mercury is assimilated (skin contact, ingestion, inhalation or
through gills, for aquatic organisms). On the basis of their capacity of accumulate a
specific compound, organisms can be distinguished as regulators or accumulators.
Regulators can activate internal mechanisms that allow them to have a low metal uptake,
even at high external concentrations. Accumulators, on the other hand, can retain high
metal concentrations and survive, even in non-contaminated environments.
There are many factors which influence the bioavalability of metals in aquatic-sediment
systems. Such factors include the characteristics of the compound of interest, the sediments
and the organisms.
Bioavailability is the fundamental prerequisite for bioaccumulation, which then influences
toxicity. Actually, toxicity is a function of the dose, through the dose-response curve. The
effective amount of a compound in an organism can be considered as a function of its
accumulation in the organism itself (and of the background availability). If a compound is
not removed from an organism, the organisms system concentration will increase until it
reaches the toxic dose. Reduction of bioavailability is thus one of the strategies put into
effect to reduce exposure to toxic metals [11]. Actions that could be taken involve both the
biological and the chemical aspects of the process: metal uptake can be reduced by
complexing the metal on the surface of organisms or by altering the chemical speciation of
the metal towards forms less available for organisms [11].
On the other hand, bioavailability is influenced by metal mobility and solubility. As a
consequence, all factors related to these properties (pH, soil organic matter, redox potential
for soil/sediment systems, dissolved and suspended particulate matter, ionic strength and
alkalinity) must be taken into consideration.
Toxicity can thus be reduced by limiting bioavailability. The easiest method to reduce
bioavailability of heavy metals adsorbed to sediments consists simply of covering them
with a gravel layer, creating a barrier between the sediments and organisms. This
technique, however, is only temporally effective because the superficial layer can be
removed by river or sea currents, re-exposing sediments to the aquatic environment.
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