Ultrasonic desorption of mercury from contaminated sediments: influence of the chelating agent DMPS

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.