**4. Mobilization and loadings the mobilized mercury**

The mobilization of the different mercury forms can due to by evaporation and dissolution. Here we do not deal with the erosion which able to mobilize all form of mercury if it has attached to the solid particle, however in case of soils and fly as particles in the flue gas this particle associated mobilization mechanism can play important role in the mercury transport.

### **4.1. Evaporation**

and to dispersion, the latter resulting in an approximately entirely even concentration. It is found either as a native metal (near to 80 % in hydrothermal and vapors) or in cinnabar, corderoite, livingstonite and other minerals. Cinnabar (HgS) is the most common ore. Mercury ores usually occur in very young orogenic belts where rock of high density on upper mantle

Given that mercury is enriched by an extremely wide variety of geological processes (Fergusson, 1990) from the formation of hydrocarbon to hydrothermal mineral occurrences, it can be regarded as an universal geochemical indicator of young geological effects; its disper‐

Historically there were two main registered Mercury mines: Almaden (Spain) and Idrija (Slovenia) in Europe. Later new occurrences were found in California and worldwide. It was used in gold separation. In Mexico we know mercury mines from the Pre-Hispanic era (Scharek et al., 2010) and a usage in cultic fests (Figure 3). In 2005, China was the top producer of mercury with almost two-thirds global share followed by Kyrgyzstan. Several other countries are believed to have unrecorded production of mercury from copper electro winning processes

**Figure 3.** Typical cinnabar occurrences in a limestone system (Formation Las Trancas, San Joaquin, Querétaro, Mexico,

sion halos are more extensive than that of any other element (Fügedi et al, 2011).

is forced to the crust of the Earth (Ozerova, 1996).

832 Environmental Risk Assessment of Soil Contamination

and by recovery from effluents.

photo by P. Scharek)

The evaporation governed by the vapor pressure depends on the volatility of the compound and the temperature. Concerning the volatility of the different mercury compounds evapora‐ tion at ambient temperature can be significant in case of elemental and the organic mercury cases, however the volatilization of the other mercury forms (the inorganic mercury com‐ pounds) can become considerable if the temperature reaches a couple hundred degree centigrade. All of the mercury compounds have relatively low boiling points (Table 1), some of them decompose before melting, others can sublimate. Based on these data it is obvious the vaporization can play important role of the mercury compound transport and mobilization.


**Table 1.** Melting and boiling points of the mercury and mercury compounds

The partial pressure of the elemental mercury (Hg0 ) reach 1 Pa at 42 ˚C and enhances expo‐ nentially till the boiling point (Tb = 356.5 ˚ C). At 20 ˚C the Hg vapor pressure is 0.18 Pa the Hg concentration in the air saturated with the mercury is 7.64 10-8mol dm-3 = 15.3 µg m-3. Due to this high volatility the elemental mercury evaporate if stored and processed an open container. Elemental mercury can escape from solution if the oxidized mercury is able to reduce. The analytical data will be inaccurate if the sample is not preserved agents the elemental mercury formation.

Concerning the global mercury contamination till the middle of the past century evapora‐ tion of elemental mercury used to extract silver and gold was the main source of the mercury emission, (Nriahu, 1994). All the once produced and recently available elemental mercury stock (in the past five century one million tons was produced from cinnabar and from other ores) if in used either evaporation or after transformation can contribute to the mercury contamination worldwide (Hylanderand Meili, 2003). This is the reason why the elemental mercury use is banned. Recently one of the most significant sources of mercu‐ ry emission by evaporation is the coal firing. During the coal burning the mercury associated with pyrite and be organically bonded to the coal minerals are released in the combus‐ tion flame as elemental mercury, which is partially oxidized to Hg(II) in homogeneous and heterogeneous catalytic reaction governed by the chlorine and the ash content of the combustion gases (Sondreal et al, 2004).

Generally accepted view is that the evaporated oxidized forms of mercury contaminate the environment locally, close to the emission source. However they can transform to elemental mercury and depend on this transformation rate it can become part of the global mercury cycle.

The transformation of the oxidized to reduce the reduced to oxidized forms can happen both in gas and aquatic environment according to the circumstances. There is similar transformation between the inorganic and organic forms.

Different species of macro algae from the dissolved mercury can produce different methylated mercury compounds in the ocean. Because of these methylated mercury compounds have high volatility and at the dimethylated form has low solubility in ocean water they are easily emitted into the atmosphere and can contribute significantly to the global atmospheric mercury (Pongratz and Heuman, 1998). Beside this different bacteria (e.g. sulfate reducing) and in case of abiotic route the tin- alkyls and the humic acids also can transform the dissolved mercury (II) to methyl mercury form (Weber, 1993). A quite detailed set of possible transformation in gas and aquatic media and the Henry constants which inform about the dissolved compound volatility are collected by Shon at al, 2005.

During heating mercury compounds can transforms directly or via oxides to elemental mercury. Beside the elemental mercury only the halogenides and the sulfides since last have a tendency to sublimate can occur in evaporated forms. The sulfides at presence of oxygen at 600 °C transforms to Hg and SO2, however in presence of Fe and CaO the HgS also will decompose to Hg and Fe- or Ca- sulfides. Using the temperature programmed evaporation technique based on the volatility difference of mercury compounds the compound forms can be distinguished and can use for mercury speciation in solids (Lopez-Anton et al, 2010, 2011).

In a high temperature process since the mercury compounds decompose the original specia‐ tion of mercury does not preserve a new speciation can formed which determined by the gas composition. In the high temperature gases high portion of mercury exists in elemental and just a small portion in oxidized form. This is the reason why these technologies such as coal fired energy production, the cement kiln, the incineration has difficulty in the mercury capture. Focusing to the soil, the heating comes from sunlight can mobilize only the weakly sorbed elemental and organic mercury but the fire on the soil surface, for example the forest fire can evaporate the less volatile mercury forms as well. This case the contamination level of the fired soil decreases but, due to the transport, at other places the contamination becomes higher (Caldwel et al, 2000).

The volatilization can be the cause of contamination but can use for decontamination as well. Based on the volatilization of mercury compounds, mercury removal process was established from coal cleaning by mild pyrolysis (Wang et al., 2000) and for the soil cleaning by thermal treatment. In case of coals the speciation of mercury determines the maximum efficiency of the mercury removal. The efficiency of the process generally remains below 100 %, (bituminous coal case at 500 ˚C it was aprox. 75 %). Since the efficiency remains below 100 % the rest of mercury still remain in the process and pass to the flue gas after the coal burning. The speciation of the mercury in the contaminated soil also has influence on the efficiency of the thermal remediation, see more details later.

Concerning that the different mercury forms exhibit different volatility the actual distribution of the mercury species in a medium the rate of the transformation process which able to modify it together govern the mercury mobilization by evaporation.

It is well known, if elemental mercury forms in the water this elemental mercury can easily escape to the gas phase. It is quite intensive if gas bubbling through the water or the water surface is disturbed (Okouchi and Saaski, 1984). Sunlight induced H2O2 formation in alkaline condition can result reduction of the oxidized mercury forms to elemental mercury. This can explains that the Hg concentration above the lake water surface can be higher day time than night. The fulvic and humic compounds are able to complex the mercury (II) ion in aquatic media but these compounds can take part in the mercury alkylations, further at a suitable pH can work as a reducing agent. The redox potential at 0 pH for Hg (II) reduction is 0.85 V(Allard and Arsenie, 1991).

This type of mercury transformation between oxidized and reduced forms together with the alkylation will generate not only a modification between the concentrations of the mercury species in the aquatic phase but will modify the mercury transport between the phases. The mercury transformation processes are important in the technological processes used for the mercury removal since can effect they efficiency (Somoano et al., 2007).

#### **4.2. Dissolution**

analytical data will be inaccurate if the sample is not preserved agents the elemental mercury

Concerning the global mercury contamination till the middle of the past century evapora‐ tion of elemental mercury used to extract silver and gold was the main source of the mercury emission, (Nriahu, 1994). All the once produced and recently available elemental mercury stock (in the past five century one million tons was produced from cinnabar and from other ores) if in used either evaporation or after transformation can contribute to the mercury contamination worldwide (Hylanderand Meili, 2003). This is the reason why the elemental mercury use is banned. Recently one of the most significant sources of mercu‐ ry emission by evaporation is the coal firing. During the coal burning the mercury associated with pyrite and be organically bonded to the coal minerals are released in the combus‐ tion flame as elemental mercury, which is partially oxidized to Hg(II) in homogeneous and heterogeneous catalytic reaction governed by the chlorine and the ash content of the

Generally accepted view is that the evaporated oxidized forms of mercury contaminate the environment locally, close to the emission source. However they can transform to elemental mercury and depend on this transformation rate it can become part of the global mercury cycle. The transformation of the oxidized to reduce the reduced to oxidized forms can happen both in gas and aquatic environment according to the circumstances. There is similar transformation

Different species of macro algae from the dissolved mercury can produce different methylated mercury compounds in the ocean. Because of these methylated mercury compounds have high volatility and at the dimethylated form has low solubility in ocean water they are easily emitted into the atmosphere and can contribute significantly to the global atmospheric mercury (Pongratz and Heuman, 1998). Beside this different bacteria (e.g. sulfate reducing) and in case of abiotic route the tin- alkyls and the humic acids also can transform the dissolved mercury (II) to methyl mercury form (Weber, 1993). A quite detailed set of possible transformation in gas and aquatic media and the Henry constants which inform about the dissolved compound

During heating mercury compounds can transforms directly or via oxides to elemental mercury. Beside the elemental mercury only the halogenides and the sulfides since last have a tendency to sublimate can occur in evaporated forms. The sulfides at presence of oxygen at 600 °C transforms to Hg and SO2, however in presence of Fe and CaO the HgS also will decompose to Hg and Fe- or Ca- sulfides. Using the temperature programmed evaporation technique based on the volatility difference of mercury compounds the compound forms can be distinguished and can use for mercury speciation in solids (Lopez-Anton et al, 2010, 2011). In a high temperature process since the mercury compounds decompose the original specia‐ tion of mercury does not preserve a new speciation can formed which determined by the gas composition. In the high temperature gases high portion of mercury exists in elemental and just a small portion in oxidized form. This is the reason why these technologies such as coal fired energy production, the cement kiln, the incineration has difficulty in the mercury capture.

formation.

combustion gases (Sondreal et al, 2004).

834 Environmental Risk Assessment of Soil Contamination

between the inorganic and organic forms.

volatility are collected by Shon at al, 2005.

The mobilization by dissolution can arrange two groups: (a) dissolutions ways exist in the nature (b) dissolution way can be applied in the laboratory and in the remediation technology to determine the loading forms or remove the mercury from the contaminated media.

#### *4.2.1. Dissolutions ways exist in the nature*

The solubility of elemental mercury and the ore of mercury can find in the nature (cinnabar etc.) are very low in water. This low solubility result low mercury concentration level in aquatic phase and restricts the transport between phases by dissolution. However the oxidation both cases enhances these mercury forms solubility. The elemental mercury can be oxidized by ozone, halogens, some components of acid rains, or by oxy-acids in laboratory (HNO3 and the hot H2SO4) resulting a soluble form. The ozone in air if does not consumed by the other more reactive air contaminants can oxidize Hg to Hg(II) (Iverfeld and Linquist, 1986; Shonet al, 2005).

In aquatic media oxidation can occur at acidic conditions if the sunlight produces oxidative radicals OH, or peroxides. This process can play role in the trap of the physically dissolved elemental mercury in water, and also can hinder the transformation of the oxidized mercury forms towards the reduced elemental mercury direction. The oxidative transformation of elemental mercury is essential in case of many mercury capture process since the oxidized forms of mercury has higher tendency to sorb and dissolve, therefore different oxidation procedures are available and applied in the demercurysation technologies (Ko et al, 2008; Lakatos et al, 2009; Sondreal et al, 2004).

However the mercury in the natural minerals is in the oxidized forms these minerals luckily due to the very low solubility can be considered not a mobile occurrence of the mercury. The environmental risk improves if the natural processes can transform the minerals a more soluble form. One of the most significant ore transformations which effect the mercury mobilization is the sulfide ore oxidation in the air. The oxidations of sulfides to sulfate a considerable enhancement ensue in mercury solubility (Holley et al, 2007). This process, the oxidation of the tailings, can accused for the mercury contamination all around the abandoned mercury ore mines.


\*Solubility from paper of Ko et al, 2008.

**Table 2.** Solubility of different mercury compounds.

Among the mercury compounds (Table 2) the mercury-chloride and nitrates are those which have the highest solubility in water. The simple cationic form of Hg(II) is not the common form in the aquatic media, it exist only in acidic solutions, at less acidic condition the dissolved mercury appears as HgOH+ , HgOHCl, Hg(OH)2 and HgCl2 molecules and complex anions HgCl4 2- at high chloride concentration. It means that the sea water contains the oxidized mercury mainly in this chlor- complex form. Beside the chlor- complex the mercury -fulvo and -humic complexes also exists in aquatic environment. The speciation in the solution, the molecular forms govern the mercury loadings and play important role at the way and efficiency of the removal.

#### *4.2.2. Dissolution for leaching mercury from different medium*

phase and restricts the transport between phases by dissolution. However the oxidation both cases enhances these mercury forms solubility. The elemental mercury can be oxidized by ozone, halogens, some components of acid rains, or by oxy-acids in laboratory (HNO3 and the hot H2SO4) resulting a soluble form. The ozone in air if does not consumed by the other more reactive air contaminants can oxidize Hg to Hg(II) (Iverfeld and Linquist, 1986; Shonet al, 2005).

In aquatic media oxidation can occur at acidic conditions if the sunlight produces oxidative radicals OH, or peroxides. This process can play role in the trap of the physically dissolved elemental mercury in water, and also can hinder the transformation of the oxidized mercury forms towards the reduced elemental mercury direction. The oxidative transformation of elemental mercury is essential in case of many mercury capture process since the oxidized forms of mercury has higher tendency to sorb and dissolve, therefore different oxidation procedures are available and applied in the demercurysation technologies (Ko et al, 2008;

However the mercury in the natural minerals is in the oxidized forms these minerals luckily due to the very low solubility can be considered not a mobile occurrence of the mercury. The environmental risk improves if the natural processes can transform the minerals a more soluble form. One of the most significant ore transformations which effect the mercury mobilization is the sulfide ore oxidation in the air. The oxidations of sulfides to sulfate a considerable enhancement ensue in mercury solubility (Holley et al, 2007). This process, the oxidation of the tailings, can accused for the mercury contamination all around the abandoned mercury

**Compound Solubility in Water**

Hg 0.049\* HgS (cinnabar, α) 0.01 HgS (metacinnabar, β) - HgSO4 - Hg(NO3)<sup>2</sup> soluble Hg2O 51 HgO 51 Hg2Cl2 10 HgCl2 66 000\* HgBr2 5 100 HgI2 51 CH3HgCl 5 780 (CH3)2Hg -

**c, ppm**

Lakatos et al, 2009; Sondreal et al, 2004).

836 Environmental Risk Assessment of Soil Contamination

\*Solubility from paper of Ko et al, 2008.

**Table 2.** Solubility of different mercury compounds.

ore mines.

Beside the thermal way and the application of the species sensitive analytical methods for mercury analysis (XPS, EXAFS etc.) the sequential extraction is often applied technique to specify the mercury chemical form and associations in solids. The thermal methods and the species sensitive elemental analysis can distinguish the elemental Hg, the HgS forms and the organically bonded mercury forms which generally exist in the soil. However these techniques do not allow doing any estimation about mobility and bioavailability of mercury. To get the loading specific information in soil for mercury, beside the classical Tessier six step extraction used generally, different modified procedure are available for Hg which able to distinguish better the mercury forms than the Tesssier method can do (Orecchio and Polizzotto, 2013; Han et al,2006). For example it can determine mercury bounded to amorphous iron oxides (by NH4 oxalate-oxalic acid extraction), mercury bonded to crystalline iron oxides (by NH2OH•HCl- 25 % acetic acid extraction), non-cinnabar mercury (elemental mercury, organic bounded, humine bounded (by 4 M HNO3 extraction), cinnabar mercury (by extracted with saturated Na2S,Han et al, 2006). The advantage of this protocol is the ability of the separation of humic and sulfide bounded mercury which important in the soil case. Two set of sequential extraction regime can compared at Table 3 and 4.

Mobilization of mercury can occur through complex formation, ligand exchange reactions with chloride and sulfur-containing ligands which leading to enhanced Hg solubility in soil solutions. The sulfur containing ligands: tiosulfates (S2O3 2 ), thyocyanites (SCN- ) can mobilize the mercury efficiently and could improve the phytoextraction efficiency (Moreno et al, 2004).

Removal mercury by phytoextraction from soils and others soil like materials eg. waste water plants biosolids often need additives which improve the solubility of the mercury. These mobilizations agents are used in accelerate phytoextraction. One type is the chelating agents: citrate, oxalate, malate, succinate, tartarate, salicilate, acetate, and amino-poly-carboxylic acids: EDTA (Lomonte et al, 2011). Since the EDTA is persistent compound, recently the biodegrad‐ able ethylendiamine-disuccinate (EDDS) or nitrilotriacetic acid (NTA) suggested as alternative chelator instead of EDTA (Evangelu et al, 2007).

Specific compounds used the mercury extraction from tissues: they can pay role in case of poisoning for detoxification : EDTA was tested for detoxification by Aposhian (Aposhian et al,1995), 2,3- dimercapto- 1- propansulfonate was used to extract mercury from tissues of rats exposed to different mercury compounds (Buchet and Lauwerys, 1989). The EDTA was not


**Table 3.** Mobilization protocol for determination of association of mercury to soil component (Han et al, 2006).


**Table 4.** Mobilization protocol for determination of association of mercury to soil component (Orecchio and Polizzotto, 2013).

found the best for mercury removal in human application since does not the best chelator for Hg and has side effects, however there are two other compounds which suggested to keep in stock in any poison control center as mercury chelator DMPS (2,3-dimercapto-1-propane sulfonic acid, (unithiol)) and DMSA (meso-2,3 dimercapto succinic acid, succimer, Guzzi et al, 2010).

#### **4.3. Loadings of mercury**

**Mobilization Protocol Determination of Association of Mercury to Soil Component**

(1 M NH4 Exchangable mercury -acetate pH 7 set with NH4OH: solid:liquid 1:25, 30 min 25 ºC)

(0.1 M NH2OH.HCl +0.01 M HCl solid:liquid1:25, 30 min 25 ºC) Easily reducible oxides bounded mercury

Hidroxylamine – HCl

H2O2 (3 mL 0.1 M HNO3+5 mL 30 % H2O2 80 ºC 2 h; 2 mL H2O2 80 ºC 1 h; 50 mL 1M NH4- acetate)

> NH4-oxalate –oxalic acid (0.2 M oxalate buffer 1:1, pH 3.25 solid:liquid1:25)

(0.04 M NH2OH.HCl in 25 % acetic acid 97-100 ºC 3 h solid:liquid 1:25)

4 M HNO3 (4 M HNO3 80 ºC16 h solid:liquid1:25)

Na2S (4 mL saturated Na2S 12 h repeated twice)

Na-acetate (1M Na-acetate, 1 h stirred, solid:liquid1:8

(1M Na-acetate –acetic acid pH 5, 4 h stirred, solid:liquid1:8)

Hydroxylamin HCl - acetic acid 0,04 M NH2OH•HCl in 25 % Acetic acid, 96 °C, 6 h, Solid:liquid 1:8

Mineralisation by HNO3-H2O2 (a)heated previously 180 ºC - organic bounded - microwave digestion in cc HNO3-H2O2 mixture; (b) no heat - elemental + organic bounded microwave digestion in cc HNO3-H2O2 mixture)

(HCl:HNO3 3:1)

Loading form of mercury Extraction procedure Water soluble mercury NH4-acetate

Crystalline iron oxide bounded mercury Hydroxilamine – HCl- acetic acid (hot)

**Table 3.** Mobilization protocol for determination of association of mercury to soil component (Han et al, 2006).

Water soluble mercury H2O100ºC, 1h stirred, solid:liquid 1:8

Loading form of mercury Extraction procedure

Carbonate bounded mercury Na-acetate –aceticacidpH 5

Sulfid bounded mercury Aqua regia

**Table 4.** Mobilization protocol for determination of association of mercury to soil component (Orecchio and

**Mobilization Protocol Determination of Association of Mercury to Soil Component**

Carbonate bounded mercury

838 Environmental Risk Assessment of Soil Contamination

( Mn-oxides)

Elemental and organic bounded mercury

Amorphous iron- oxide bounded mercury

Non cinnabar bounded mercury ( Hg, organically bounded, humin bounded) TOT (non cinnabar mercury)

Cinnabar bounded mercury

Exchangable mercury

Fe, Mn-oxide bounded mercury

Elemental and organic bounded mercury

Polizzotto, 2013).

The mercury contaminations are in the environment can exist different phases: as vapor in the air (Hg, and compounds, particle associated), dissolved in aquatic media (Hg2+, Hg(OH)2, HgCl2, HgCl4 2-, different complexes, particle associated) and solid as precipitates or minerals and in associated forms bonded to different manner to the component of different solids (soil, fly ash, waste water sludge, etc). In the previous section it was demonstrated that how the associations can be identified.

The loadings of mercury to solid can be considered as positive or negative phenomenon. It can restrict the dispersion of the contamination one side it is positive, the negative this way it can preserve the contamination. Since the loadings depends on the character of the collector and the speciation of the mercury, difficult to establish general rules for this process. However it can state that the elemental mercury has low sorption ability, the cationic sorbs better than the anionic forms on clays, and negatively charged carbon surfaces (coals, activated carbons, humic materials) the loading is more effective to that surfaces which have contain sulfides. It was interesting findings after the cinnabar oxidation a part of liberated mercury could load to the cinnabar surface this way it can be not just the source but the collector of mercury ions. Unfortunately the most toxic forms (alkyls) have the highest ability for bioaccumulation.

The nature works against the mercury contamination. Except the alkylation it transforms the mercury toward the most stable less soluble form. Near the chlor-alkali plant the total mercury sometimes reach the four order of magnitude higher level, than the background concentration, luckily it found a non-volatile and non-soluble associations since transforms to sulfides (Bernaus et al, 2006).

The history of mercury contamination is recorded by loadings. The dept profile of mercury concentration on peat can provide a clear picture how the mercury contamination changed during the mankind history (Barraclough et al, 2002).

The loadings play important role in the environmental technologies used for decrease the mercury emission or clean the contaminated medium. Sulfur and halogen containing carbons, oxidative inorganic sorbents (Lakatos et al, 2009) were developed for elemental mercury removal from flue gas. Beside a range of, classical, functionalized sorbents, sulphur containing carbon nanotubes widen the collection one can chose among for eliminate the mercury contamination in aquatic media (Pillay et al, 2013).

The coals especially the low rank and the oxidized coals are very good mercury ion collectors. Due to this feature we must face that the coal-firing are the main source of the anthropogenic mercury contamination nowadays. However this material offers us an application for cure a slice of the mercury problem: remove the mercury from aquatic media. It can use in batch mode or dynamic systems as the reactive barrier material, by the high mercury capture coal are able to retard or remove the aquatic mercury contamination (Lakatos et al,1999).
