FR2751900A1 - Treatment of soils polluted with poly:aromatic hydrocarbon compounds - Google Patents

Abstract

Treating soil contaminated by polyaromatic hydrocarbons comprises: (a) extracting the contaminated soil; (b) mixing with an aerating texturing agent; and (c) adding a nutrient. Previous to the mixing of the soil and aerating agent, the agent is seeded with a micro-organism capable of bio-degrading polyaromatic hydrocarbons.

Description

 The present invention relates to a process for treating soils heavily contaminated with hydrocarbons for the rehabilitation of sites with such soils. It relates more particularly to the accelerated treatment of industrial soils contaminated in particular by polyaromatic hydrocarbons recognized as being more difficult to biodegrade than other hydrocarbon organic compounds, their chemical formulas being more condensed.

 Different types of removal of hydrocarbons from the soil are possible. The natural route is the elimination of the hydrocarbons contained in the soil by microorganisms naturally present in the soil which feed on hydrocarbons. However, the speed of such degradation depends on a large number of factors, including the concentration of hydrocarbons, their more or less condensed chemical nature, but also the presence of the nutrients necessary for these microorganisms to perform their vital functions.

 It is known that microorganisms and more particularly bacteria can consume, during active aerobic fermentation phases, 15 to 30 times more carbon than nitrogen, depending on whether the hydrocarbon present in the soil is more or less biodegradable. However, this consumption is limited by the quantity of nutrients present near the said bacteria, nitrogen for the most part, then phosphorus among the medium elements, and finally for a small part sulfur and potassium.

 To limit or even compensate for the deficit of these elements, it is known to provide bacteria with an additional nutrient in order to place them in optimal operating conditions.

By optimal conditions for optimal bacterial functioning is meant the conditions for which the weight ratios of carbon C, nitrogen N, phosphorus P and sulfur S are equal to:
C / N = 20 to 70
N / P = 2 to 10
C / P = 75 to 150
C / S = 100 to 300
With this in mind, in international patent application WO 91/19039 (Grace-Sierra Hort.Prod. Co), it is proposed to spread complete fertilizers such as those present on the market or similar formulations on these contaminated soils. identical to the method used to cover the cultures. As the process of biodegradation of hydrocarbons is significantly longer than the processes of assimilation by plants, the preferred formulations are those of delayed-release fertilizers with prolonged diffusion. However, these methods have limited effectiveness because the hydrocarbons are not uniformly distributed in the soil.

 Other soil rehabilitation methods offer a mixed treatment in that it combines the supply of secondary nutrients for microorganisms in a double oleophilic and hydrophilic form, and the use of a soil aeration agent. This is the case of the process of French patent application 93.10416 of September 1, 1993, for which the supply of nutrients is obtained by the combination of two types of nutrients, one in oleophilic form, the other in hydrophilic form , and the aeration agent is constituted by a texturing agent for aeration, such as for example straw.

 It is also known in patent WO 95/26944 to treat these soils, polluted by hydrocarbons, after washing the excavated soil with ammonia, by using the residues of treatment for removing fluff from the cotton grain in the presence of sulfuric acid and water. Here, the function of cellulose fiber is twofold, that of absorbing hydrocarbons and that of support for the development of bacteria. The major drawback of this method is the complexity of the method of preparation of the absorbent.

 The present invention aims to remedy the drawbacks mentioned above, in particular by increasing the number of microorganisms effective in destroying highly condensed polyaromatic hydrocarbons while reducing the masses of products mixed with the earth for the purpose of its treatment.

 The subject of the present invention is a method for treating soil contaminated with highly condensed polyaromatic hydrocarbons comprising a step of extracting the contaminated soil, a step of mixing at least one aerating texturing agent with the soil and a step d addition of at least one nutrient characterized in that before mixing the contaminated soil with at least one aerating texturing agent, at least one part called texturing agent is seeded with at least one microorganism chosen from the group consisting of all strains of microorganisms capable of biodegrading polyaromatic hydrocarbons.

 These microorganisms can come from two different sources. The first source is provided by natural strains of microorganisms, extracted from soil contaminated by hydrocarbons and cultivated in order to increase their number before sowing them on the texturizer. The methods of soil extraction and preculture are those usually implemented by those skilled in the art. This extraction of natural strains has the advantage of using the microorganisms which are most suitable for combating pollution by polyaromatic hydrocarbons since they are present on the polluted site, that they therefore resist pollution and that they were able to develop systems for degrading pollutants. The second source is a cultivated strain of the most suitable microorganisms from collections whose properties are well known.

In a preferred embodiment of the invention, the preferred microoganisms are the fungi belonging to the group constituted by the Phanerochaete, Cunninghamella,
Penicillium, Candida, Sporomyces, Cladosporium,
Synephalastrum and Mucor.

 As the preferred texturing agent on which these microorganisms are sown, a lignocellulosic material from the group consisting of wood chips, pieces of wood, lumber, crushed pallets, straw, various lignocellulosic debris or their mixture is used.

 The seeding of the texturing agent must be sufficient for the biodegradation process of the hydrocarbons to be sufficiently rapid. Preferably, the seeded texturing agent contains more than 107 microorganisms / g of texturing agent.

 However, this original seeding of the texturing agent is not the only parameter having an influence on the biodegradation process. Indeed, the proportions of contaminated soil, texturing agent and food or nutrients added to the mixture to allow microorganisms to proliferate, are also very important. In addition, it is obvious that we will prefer to mix the least possible texturing agent with the maximum of contaminated soil.

 In a preferred embodiment of the invention, 90 to 50% by weight of soil contaminated with polyaromatic hydrocarbons will be mixed with 10 to 50 g by weight of texturing agent sown so as to form a swath.

 Throughout the duration of the treatment of this swath, its humidity and aeration, that is to say oxygenation, levels are controlled. In a preferred embodiment of the invention, the temperature is maintained at room temperature, the humidity is maintained by adding water from 30 to 70% by weight of the swath, and preferably from 30 to 50% by weight of the swath, and finally, the oxygenation rate of the humidified swath is greater than or equal to 10 W by volume.

 To oxygenate the swath, any means can be used to introduce an oxygen source either by forced aeration or by mechanical aeration, for example, by inversion, at determined periods, in particular in the event of a sharp rise in the swath temperature.

However, to maintain the biodegradation activity of microorganisms, it is necessary to regularly add sufficient nutrients to each swath to allow them to survive and reproduce in situ. In a preferred embodiment of the invention, these nutrients are nutrients of the type
N / P / K, such that the N / P weight ratio varies from 4 to 1. In a preferred embodiment of the invention, 12.5 g of nutrient with N / P weight ratio equal to 15/7 per 100 ppm are introduced. 3 polyaromatic hydrocarbons per m of soil.

 The examples given below aim to explain the invention, without however limiting its scope.

EXAMPLE 1
The present example describes the preparation of the pre-seeded texturing agents used in mixture with the contaminated soil in the windrows to biodegrade the polyaromatic hydrocarbons.

 In the present invention, the texturing agent is inoculated with different fungi either, in test A, extracted from the contaminated soil (natural source or SN), or, in test B, from collections.

TEST A:
Soil samples were taken at different points on an industrial site containing on average 8.5 g of polyaromatic hydrocarbons per kilogram of soil.

 1 g of soil from each of the three samples is suspended in 10 ml of an L1 medium consisting of water, nutrients and a dispersant under standard conditions. The nutrients used are specific mixtures for fungi such as malt extracts, yeast extracts, bactopeptone and glucose.

 These suspensions are agitated for one hour, then 500F1 of these suspensions are spread on a support S consisting, in a manner known per se, of a gelling agent (solid agar type) moistened and containing the same specific nutrients for fungus as above and anti-bacterial agents in order to proliferate only the site fungi.

 These solid culture media containing, dispersed, the spores and fragments of mycellium to fungi, are introduced into an incubator and maintained at 28 ° C. for one week, the development of the fungi being monitored visually.

 The various microorganisms thus cultivated are taken up and suspended by dilution in the same medium Ll as that already used for the first suspension of the soil, then homogenized and finally, diluted to 10% by weight in an L2 medium containing in solution in water, specific nutrients for fungi such as malt extract, yeast, peptone and glucose. The suspension of microorganisms thus obtained is incubated for about a week with gentle shaking between 28 and 30 OC to obtain an abundant preculture.

 After homogenization of this preculture, straw is seeded by washing with this preculture.

ESSAT: B:
The mushrooms from a collection are placed in liquid culture prior to sowing the lignocellulosic texturing agent in a medium known to a person skilled in the art according to known techniques. To allow significant growth of the biomass before inoculation, this liquid culture is maintained in static or agitated condition. When the development of the culture is optimal, preferably after a week, the entire liquid culture is homogenized to form a suspension of fragments of mycellium with which the texturing agent is inoculated.

EXAMPLE II
The present example presents the implementation of the method according to the invention and the first results obtained.

 The technique chosen for windrow composting consists in excavating the soil, then treating it by kneading it with pre-crops and a texturing agent such as straw and / or wood shavings and placing this mixture in a pile or swaths on a sealed area for the biodegradation reaction to occur.

 The swaths can be prepared with varying levels of soil, texturing agent and microorganisms of different nature for a humidity level maintained at 40% by weight of the swath. Two types of oxygen aeration can be used, namely, forced aeration by means of a continuous suction of air in each swath at the rate of 15 l of air per hour, and mechanical aeration by mechanical reversal of the swaths when the measured oxygen level is less than 10% by volume.

 Significant results are not yet available at the swath level. On a laboratory scale, aromatic hydrocarbons with more than three cycles are degraded to more than 80 W by weight in 45 days.

Claims (10)

1 - Process for treating soils contaminated with  hydrocarbons, more particularly hydrocarbons  polyaromatics including a soil extraction step  contaminated, a step of mixing a texturing agent  aeration with the soil and a step of adding at least  a nutrient characterized in that prior to  mixing the contaminated soil with at least one agent  texturing aeration, at least part of it is seeded  of said texturing agent by at least one microorganism  chosen from the group consisting of all strains  microorganisms capable of biodegrading  polyaromatic hydrocarbons. 2 - Method according to claim 1 characterized in that  microorganisms are extracted from soil samples  taken from the polluted soil to be treated and then put in pre  culture in an appropriate medium. 3 - Method according to claim 1 characterized in that  microorganisms come from collections. 4 - Method according to one of claims 1 to 3 characterized  in that the microorganisms are fungi from  group formed by the Phanerochaete, Cunninghamella,  Penicillium, Candida, Sporomyces, Cladosporium,  Syncephalastrum and Mucor. 5 - Method according to one of claims 1 to 4 characterized  in that the texturing agent is a material  lignocellulosic chosen from the group consisting of  wood shavings, pieces of wood,  buchettes, crushed pallets, debris  various cellulosics, straw or a mixture thereof. 6 - Method according to claims 1 to 5 characterized in that  the seeded texturing agent contains more than 107  microorganisms spread per gram. 7 - Method according to one of claims from 1 to 6  characterized in that it consists in mixing 90 to 50% in  weight of soil contaminated by hydrocarbons  polyaromatic with 10 to 50% by weight of agent  seeded aeration texturing. 8 - Method according to one of claims 1 to 7 characterized  in that the mixture or swath is maintained for  the entire duration of treatment at room temperature at a  humidity level varying from 30 to 70%, preferably 30 to  50% by weight, with an oxygen level at least equal to 10  %. 9 - Method according to one of claims from 1 to 8  characterized in that one adds in the swath during  throughout the course of treatment a sufficient amount  at least one N / P / K type nutrient with a poderal ratio  N / P varying from 4 to 1. 10- A method according to claim 9 characterized in that at  minus 12.5 grams of N / P weight ratio nutrient  is equal to 15/7 per m3 of soil treated at 100 ppm  polyaromatic hydrocarbons.