Soil demercurization technique development

. The study proposes a technique of preventing the spread of mercury contamination off industrial sites into uncontaminated areas. The study provides results of soil treatment with stormwater with added surfactants. Immobilization of mercury compounds within contaminated soil solutions without soil excavation has been demonstrated..


Introduction
Mercury is a rare element of the side subgroup of the second group of the sixth period of the periodic system of chemical elements, having the atomic number 80. It is a transition metal, which is a silvery-white volatile liquid of 13.546 g/cm3 density at normal conditions, its vapors are toxic. The average content in the earth's crust and the main types of rock is estimated at 0.03-0.09 mg/kg. The mass of mercury concentrated in the 1-km-thick surface stratum of the earth's crust is 1× 10 10 t, of which only 0.02% is in the deposits. The rest of the mercury is dispersed in the rock, in the waters of the World ocean (41.1 million tons of mercury, corresponding to a concentration of 0.03 μg/l). This dispersed mercury creates a natural geochemical background that is aggravated by anthropogenic mercury pollution, forming areas affected by technogenic contamination. About 1.1 million tons of mercury waste are stored in Russia. 58% of the total waste mass contains mercury in concentrations of 10-30 mg/kg, 12% -from 100 to 5000 mg/kg, 30% -over 5000 mg/kg.
The amount of mercury in soil and dumps of chemical and other enterprises is estimated at 3,000 tonnes. Dumps, tailings and sludge ponds of the gold mining industry have accumulated up to 6,000 tons of mercury, which represents a real threat to Russia's national security. Up to 50 tons of mercury industrial waste get into the soil, and up to 3.5 tons into the air [1].
Once in the water, mercury reacts with the dissolved organic matter, and forms strongbonded soluble complexes. In low salinity water, methylmercury HgCH3+ ion is predominant, and highly mobile. Properties of methyl derivatives of mercury vary considerably. Dimethylmercury evaporates easily from water, while monomethylmercury remains in solution and, due to its ability to permeate through cell membranes and to block enzymes, is one of the strongest toxins [2]. According to current estimates, the mercury content in sedimentary rock is significantly higher than in igneous rock, which is due to the absorption of mercury by natural adsorbents: clay sediments, iron hydroxides, and humus. Sorption of mercury in the form of Hg(ОН)2 by iron hydroxides is effective at pH 7-8. In highly acidic media, Hg2+ compounds are adsorbed in humus. Alkalinization of the medium to a pH of 4.7 completely extracts Hg2+ from the solution. It has been experimentally demonstrated that humic acid reduces Hg2+ to elemental mercury. At pH = 2 fulvic acids act as a reducing agent of mercury in the soil. In this case, the reduction goes all the way to the metal. At pH of 5.5-8, about 30% of all mercury exists in this form [2].
There are many ways mercury can get into the environment [3][4][5][6][7]. Once in soil solutions, mercury contaminants are also sorbed and reduced by natural organic matter. However, the process is not fast enough to be used in the reclamation process.
Soils contaminated with mercury waste contain not only metallic mercury, but also soluble mercury forms available for biota [8]. The content of Hg2+ in soil water reaches critical values exceeding those of the MPCcf (MPC for commercial fisheries) manifold. Stormwater, meltwater can carry soluble mercury forms and transfer Hg2+ ions outside the contaminated areas. Hence, the polluted soil areas of are to be isolated, while storm water should be diverted to a separate collector and subjected to decontamination before being returned into water bodies. Decontamination of industrial sites from mercury and its compounds can be two pronged: soil drainage, and immobilization of water-soluble mercury contaminants [8][9][10][11][12][13][14].
A promising direction is to develop ways of mercury extraction by shifting its potential towards negative by using surfactants, as well as fixating soluble mercury forms as salts within the soil pores.
Therefore, the purpose of the study has been the search for ways to immobilize insoluble mercury-containing forms and the creation of a technological process of soil decontamination from metallic mercury and its subsequent use in the manufacture cycle.

Materials and methods
To study the adsorption properties of mercury surfactants, we have measured the surface tension of a mercury-drop electrode at the potential change [15]. The potentials of the mercury-drop electrode have been measured within sodium-sulfate water solutions (0.4 M) and water-soil extract with/without the OP-7 surfactant (0.014 g/l).
The soil soluble mercury forms have been immobilized with 10% solutions of stearic acid salts. To register and evaluate the structure of mercury-containing soil, before and after the chemical treatment, we have used an MBS microscope coupled with a DCM-130 computer-controlled video unit.  The calculations given in table 1 demonstrate that the surface tension at the EZCP point is minimal for solutions containing the OP-7 surfactant. Therefore, presence of OP-7 in the soil reduces the surface tension of mercury in soil solutions. Hence, soil water contains compounds affecting the surface tension of metallic mercury. However, their effect is lower than that of a synthetic surfactant.

Results and Discussion
Mechanical extraction of mercury can be the initial stage of mercury-contaminated soil remediation. The metallic mercury reclamation by mixing it with the dispersed soil solution, followed by the decontaminated soil separation, has proven to be quite effective. In the course of the experiment, it has been quite difficult to follow the dynamics of the decontamination process, since mercury is unevenly distributed within the soil strata. Likewise, the content of soluble-mercury forms fluctuates.
When soil soil is irrigated with stormwater or sewage cleared of soluble mercury compounds and containing non-ionic surfactants (e.g., OP-7), the surface tension and negative potential of metallic-mercury contaminants are reduced, resulting in the aggregation of small mercury particles into larger droplets, their separation from the soil mass, and subsequent extraction through draining. The soil samples before and after the OP-7 solution treatment at 0.014 g/l are given in figure 2.

Conclusion
Therefore, mercury decontamination of soil in the presence of surfactants within alreadydecontaminated stormwater or wastewater can be performed without excavation. The most effective surfactants are water-soluble stearates of alkaline and alkaline-earth metals. These salts, when reacting with mercury ions within soil solutions, form insoluble mercury stearate as in 2С17Н35СООNМе + Hg2+ ↔ (С17Н35СОО)2 Hg↓ + 2Ме+. Thus, to prevent the mercury-contaminated area from spreading, as well as to block mercury seepage from industrial sites into non-polluted areas through storm water, it is possible to arrest mercury compounds within the contaminated soil solutions without soil excavation [11]. The technique has practically proven that at 180-770 mg l concentration of Hg2 + in soil solutions, the decontamination efficiency is 99.9%. Following, it is possible to extract the metallic mercury from the soil. In this case, the contaminated soil, at the manufacture site, is treated with stormwater with the surfactants added (e.g., OP-7). Mercury-containing groundwater spills into a well, where metallic mercury is separated from the water. Mercury and decontaminated groundwater are directed to separate reservoirs. The contaminated water is additionally treated, while the extracted mercury after thermal processing is used to electrochemically produce chlorine and sodium hydroxide. The residue of the thermal mercury treatment is buried. Thus, the accumulated mercury-containing wastewater upon post-treatment is returned into water bodies, while the extracted mercury goes back into the manufacture cycle.