EXPERIMENTAL STUDIES ON THE POSSIBILITY OF DEPOLLUTING HYDROCARBONS-CONTAMINATED SOILS THROUGH COMBUSTION

In the current paper the authors present some experimental researches regarding the depollution through combustion of soil samples that were contaminated with hydrocarbons. Determinations were carried out on chernozem-type soils with high germinative potential, artificially polluted with an oil-derived fraction with known characteristics and a clay-type soil accidentally polluted with an oil-derived fraction of unknown composition. Several combustion tests were carried out, varying the combustion and calcination time. The tests have shown that the calcination time should be of at least 10 minutes to eliminate the pollutant, but the actual duration does not significantly influence the result.


INTRODUCTION
Soil pollution is one of the worst environmental pollution forms, both due to its multiple negative effects and to its persistence over a long period of time.
While the pollution of the atmosphere and that of the water diminishes over time due to the dispersion of the pollutants in those environments, the rate of natural elimination of pollutants in the soil is very low [1,2].
One of the more frequent types of soil pollution is the contamination with oil-derived products (hydrocarbons), even if the affected surfaces are generally not very large.The presence of hydrocarbons in the soil tend to affect the normal biological circuits, blocking the development of plants [1,3].Some of the main causes that can lead to the soil being polluted with oil-derived products are [3]:  Accidental breaking of hydrocarbons pipelines;  Exploitation of research and exploitation wells;  Leakage from hydrocarbons storages or losses during their transportation;  Road and railway accidents that involve oil tanks;  Marine pollution due to the activity of oil rigs;  Pollution of water courses and their shores by intentional or accidental spilling.Thermal treatments are usually applied ex situ, for the decontamination of soils polluted with organic materials that can be easily oxidated and converted to a high proportion of CO 2 and H 2 O.In principle, thermal methods seek the heating of the contaminated material to various temperatures, for the extraction, neutralisation, destruction or immobilisation of pollutants.Even if the principle is simple, the currently applied thermal decontamination technologies are characterised by a high degree of complexity and technicality.Organic pollutants are well suited for this type of depollution because they are easy to extract or to convert into CO 2 and H 2 O.It has been considered for a long time that soils polluted with inorganic materials such as heavy metals cannot be decontaminated using thermal methods, due to the high volatility of Hg, Cd, Pb at certain temperatures [1,2].However, in the last years, it has been proven that these metals can be partially neutralised by vitrifying them in the soil matrix and the metals volatilized through incineration or desorption can be recovered [1,6,7].
Incineration consists in the burning of the contaminated soil at high temperatures, in the presence of oxygen.High temperatures lead to the volatilisation, combustion and ultimately destruction of contaminants.Currently, for this purpose there are used devices with fluidized bed or circulation, rotating furnaces, etc. [1, 7 -10].The chlorine, SO 2 and NO X containing gases, produced during the combustion have to be subjected to an alkaline washing [11].
State-of-the-art incineration technologies offer several advantages [1]:  Fast decontamination;  Pollutants are destroyed almost completely;  Can be applied either directly on the affected surfaces or outside of these, in specialised spaces;  High decontamination efficiency.
However, there are also some disadvantages [1,8,9]:  Incineration can transform a soil polluted with organic products (hydrocarbons) into a soil contaminated with metals.For example, if the soil naturally contains lead phosphate, which is an inoffensive product, incineration can transform it into lead oxide, which is a toxic product;  Incinerated soils become agriculturally sterile, because the entire organic matter is lost;  The costs of decontamination through pure incineration are relatively high.
Desorption on the other hand implies the heating of the soil at temperatures that are high enough (usually 200 -450 °C) and for durations that are long enough for the humidity and contaminants to be desorbed, after which the resulted gases are treated for separating and concentrating the pollutants.The treatment duration depends on the characteristics of the pollutant and of the soil and on the amount of pollutant in the soil, varying between tens of minutes and several hours.Thermal desorption is recommended as depollution method for soils contaminated with volatile or semivolatile compounds [12 -15].
In the current paper, the authors have chosen to study the feasibility of a thermal decontamination method, combining incineration and thermal desorption.

EQUIPMENT AND MATERIALS
The experimental determinations pertaining to this paper were carried out on a dedicated system realised within the Laboratory for Heat Transfer Processes and Thermoenergetics from the Department of Chemical and Petrochemical Engineering from the Petroleum-Gas University of Ploiesti.The basic structure of this equipment is presented in Figure 2. 1 -cylindrical rotating calcinator; 2 -sample subjected to calcination; 3 -combustible gas burner; 4 -combustible gas counter; 5 -combustible-air mixing system; 6 -supports with ball bearings; 7 -actuation system for the calcinator; 8 -rotation speed reduction system; 9 -electrical motor for actuation; 10 -thermocouple; 11 -measurement and recording system; 12 -support.
The main components of this system are as follows:  A metal tube resting on two supports with ball bearings that allow for a continuous or discontinuous rotation by means of an actuation system consisting of an electric motor and a speed reducer;  A burner with multiple slots, generating a succession of flames evenly distributed on a length of 75 % of the length of the incineration tube;  A temperature measurement system inside the tube, consisting of a thermocouple inserted in a sheat and of an indicator-recording device;  An air blowing system consisting of a laboratory turbocharger and a distribution device that achieves a controlled removal of burning gases resulted from burning the fuel and the pollutant in the samples.
The experimental researches were carried out on three categories of soil samples:  Samples of chernozyom-type soil, with very high germinative potential, artificially polluted in laboratory conditions with a hydrocarbons fraction of known characteristics  Samples of clay-based soil, accidentally polluted with a hydrocarbons fraction of unknown characteristics.
These samples were taken from an area near the campus of the Petroleum-Gas University of Ploiesti, from an area that had been heavily polluted and where no vegetation grew.The samples were collected to a depth of 0-50 mm, since it was noticed that at higher depths, the pollution decreases significantly. Samples of nonpolluted soil for reference.These included both nonpolluted chernozyom-type soil samples and non-polluted clay-based soil samples.The latter were collected from an area close to the one with the accidentally polluted clay-based soil, but where an abundant vegetation indicated the lack of contamination.
Using the reference samples, it was possible to study the difference between the effects of the calcination on contaminated and uncontaminated soils.

EXPERIMENTAL RESEARCHES
The developed method implied the treatment of polluted soils in two stages:  Volatilisation of pollutants at temperatures of cca.400 °C for desorption;  Destruction of pollutants at temperatures above 1000 °C by combustion.
Each analysed soil sample was first weighed and then introduced in the calcination tube, where it was evenly distributed on the length corresponding to the burner range.Then the heating of the calcination furnace was initiated.
The total heat treatment time was varied among the various soil samples from 10 minutes to 20 minutes and to 30 minutes, respectively, in order to be able to assess the effect of the treatment time on the completeness of the decontamination.
First, the samples were heated up to temperatures between 280 -300 °C.
During the first 5 to 10 minutes, there could be noticed an intense elimination of water vapors and small-size dust particles.After this, there started an intense elimination of hydrocarbons vapors.Since these vapors were evacuated through the end of the tube under which the burner is located, they ignited and the flame propagated inside the tube up to the area where the soil sample was placed.This led to the combustible part of the soil sample (the hydrocarbons contaminants) burning intensely for 3-5 minutes until it was exhausted.While during this phase it was difficult to measure the temperature inside the tube due to the nonstationary character of the system comprising the flames and the flue gases, it can be estimated, based on [6], that this temperature ranged between 1200 and 1600 °C.
Once the pollutant inside the soil sample was exhausted, the flames extinguished and the system stabilised.The temperature in the system returned to the range 280 -300 °C and the decontamination could be regarded as finished.
Based on the above-presented succession of events, it can be concluded that from a phenomenological point of view, there can be distinguished two quantitatively and qualitatively distinct stages.
In the first stage, until the ignition of the hydrocarbons vapors, there could be identified a depollution through thermal desorption.
In the next stage, that begins with the ignition of the hydrocarbons vapors; there occurred a depollution through an almost complete combustion of the polluting hydrocarbons and any organic matter in the soil.
After this depollution procedure, the remaining soil had a specific aspect, presenting particles of calcinated soil and charcoal particles resulted from the burning of organic matter.This remaining soil was in turn weighed, in order to allow the determination of the weight loss during the thermal treatment.
All soil samples subjected to calcination were tested and analysed in three different ways, in order to emphasise the effects of the chosen decontamination method:  Using the method of thermal gravimetry, analysing the weight differences between the initial samples and the calcinated samples;  Using the method of extraction with benzene;  Using a biological assessment, testing the germinative capability of the calcinated soil mixed to various proportion with uncontaminated soil.
The current paper deals only with the researches carried out in connection with the first analysis, the other two analyses, by extraction with benzene and the biological assessment, respectively, being discussed in a separate study.
The gravimetric analysis allows to estimate the mass loss (expressed as percentage of the initial mass) due to the heat treatment [1].For this, the mass of each soil sample at the end of calcination is compared to the initial mass of the respective sample.The results obtained for the analysed soil samples are presented in Table 1.In this table :  m 1 = initial mass of the sample that was then subjected to heat treatment (calcination), in g;  m 2 = final mass of the sample subjected to heat treatment, in g;  Δm = m 1 -m 2 = mass losses due to the heat treatment, in g;  P d-c = Δm/m 1 x 100 = mass losses through desorbtion and combustion, expressed as mass percentage.

DISCUSSION AND CONCLUSIONS
The analysis of the values presented in Table 1 reveals following conclusions: The influence of the length of the calcination time after the extinguishing of the flames that lead to the decontamination is insignificant, both for the artificially polluted soil samples (P 121 , P 122 , P 123 ) and for the real polluted soil samples (P 221 , P 222 , P 223 ), because the percentage of mass losses remains almost constant (18.8 -19.2 % and 25.8 -26.4 %, respectively).This is also confirmed for the unpolluted soil samples (P 111 , P 112 , P 113 ).
As a conclusion, a calcination time of around 10 minutes is enough to ensure the completion of the processes leading to the elimination of the pollutant from the soil.
The difference between the mass losses in the artificially polluted samples P 121 , P 122 , P 123 (on average 19.33 %) and the mass losses in the unpolluted samples (on average 11.16 %) represents the degree of elimination of the pollutant from the samples.Thus, this degree is of 8.16 %.
Since the artificially polluted soil samples had an initial contents of 10 % pollutant mass, the unpolluted soil lost after the treatment a total mass of 90 x 11.16/ 100 = 10.044g.The remaining soil had a mass of 90 -10.44 = 79.956g.
The artificially polluted soil had after the treatment a mass of 100 -19.33 = 80.67 g.
The difference 80.67 -79.956 = 0.714 g represents waste material originating from the burning of 10 g of pollutant and remaining in the soil as charcoal.
Thus the degree of elimination of the pollutant is: 100 -7.14 = 92.86%.
Table 2 presents an analysis of the results of gravimetric analysis in terms of average mass losses and of the degree of pollutant elimination for each category of soil samples, while Table 3 offers a comparison between the results obtained through the gravimetric method, detailed in this paper, and the results of the method using extraction with a solvent.

Indicator
Value, % Depollution degree for the artificially polluted soil, calculated with the gravimetric method, % 92.86 Depollution degree for the artificially polluted soil, calculated with the method of extraction with solvent, %

93.33
Depollution degree for the real polluted soils, calculated using the gravimetric analysis data, % 8.55 Depollution degree for the real polluted soils, calculated using the results of the method of extraction with solvent, % 4.947 Grad de depoluare calculat pentru solul poluat real, din rezultatele metodei extracţiei, % 97.71 The current paper is a part of a technical-economic reasoning for the depollution of soils through combustion and thermal desorption.It is complemented by a study regarding the impact of combustion on the soil organic matter content, that showed that the depolluted soil can be used for agricultural purposes if mixed with an adequate proportion of uncontaminated soil and that will be published as a separate paper.
Depollution through combustion can be applied without prior studies on the type of the soil or of the pollutant.Also, the technology involves simple equipment of reasonable dimensions, involving acceptable investment levels.Therefore, starting from the results presented in this paper and given the above-mentioned advantages presented by this depollution method, the authors intend to develop studies that may allow the extension of the method on an industrial scale in Romania.
Furthermore, in future experiments the experimental researches will be expanded to cover the two kinds of soils (chernozem and clay) in both situations: artificially polluted with a known hydrocarbons fraction and accidentally polluted with an unknown hydrocarbons fraction.

Figure 1
Figure1presents, for example, a historical pollution with hydrocarbons near the Neagra River in Dâmbovița County, Romania.Even though in the second image there are no more visible hydrocarbons, the damage to the soil can still be seen from the absence of any vegetation in the affected area.

Table 1 .
Results of the gravimetric analysis of soil samples.

Table 2 .
Efficiency indicators for the thermal treatment, based on the results of the gravimetric analysis.

Table 3 .
Comparison between results obtained with the gravimetric method and with the solvent extraction method.