Tratamiento de contaminantes derivados de la industria petroquímica en suelos. Estudios previos off site

Resumen

Contaminated soils show high concentrations of chemicals or other substances deriving from man's use of the land. Soil contaminants can influence human health, surface and groundwater quality and the nature and viability of ecosystems. Therefore, government, industry, and the public now recognize the potential risks that complex chemical mixtures such as total petroleum hydrocarbons (TPH), polychloro biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), heavy metals, and pesticide pose to human health and the environment. Approximately 300 000 sites across Europe are estimated to be contaminated by past and present human activities. As a consequence, in response to a growing need to address environmental contamination, many remediation technologies have been developed to treat soil, leachate, wastewater, and groundwater. Polycyclic Aromatic Hydrocarbons or Polynuclear Aromatic Hydrocarbons (PAHs) are chemical compounds made up of two or more fused aromatic rings in a linear or clustered arrangement. They are produced through incomplete combustion and pyrolysis of organic matter. Both natural and anthropogenic sources such as forest fires, volcanic eruptions, vehicular emissions, residential wood burning, petroleum catalytic cracking and industrial combustion of fossil fuels contribute to the release of PAHs to the environment. However, spill of petroleum hydrocarbons were more common in the last few decades than nowaday. Their distinguishing feature is that they are highly hydrophobic. PAHs are easily adsorbed onto the organic matter of solid particles being catalogued as persistent micropollutants. Hydrocarbon spillage onto soils is a matter of concern. PAHs can be removed by natural remediation processes such as photo-oxidation, evaporation, dissolution or biodegradation. Alternatively, they can be sequestered within the soil's mineral and organic matter structures. Significant amounts of contaminants are retained in soils. Degradation of contaminants shows an initial fast period which decreases with time. According to contaminant sequestration hypothesis, contaminants become less extractable and less bioavailable by sequestration within the soil matrix during aging. However, in general, three and four ring-PAH compounds show more bioavailability than five and six rings-PAHs. The latter compounds are strongly adsorbed into the microporous structure of particulates. Based on these hypotheses three- and four-ring PAH contaminated soils would pose a greater risk to the environment. It is very difficult to find an efficient method of soil cleanup. Conventional remediation technologies, such as soil vapor extraction or bioventing, require years to produce concentration reductions of 50 to 90 percent, depending on soil type and volatility or biodegradability of the contaminants. Meanwhile, biodegradation is limited by low mass-transfer rates in the soil matrix. In general, the time scale involved is relatively large, and the residual contaminant level achievable may not be always appropriate. Less conventional technologies such as chemical oxidation, CO2-based process, wet air oxidation and direct oxidation processes by means of novel oxidizing agent are promising techniques to increase the degradation rate of hydrocarbons in soils. The most significant advantages are the fast treatment period and the ability to treat contaminants present at high concentrations. 1. Polycyclic aromatic hydrocarbons sorption on soil This study is intended to provide some insights into the sorption of target PAHs on soil, either individually or in a mixture of them by using saturated aqueous solution. The kinetics of the sorption is characterized by the presence of two distinct periods: an initial fast period where the process is almost complete in just 10 - 20 min followed by a second slow stage in which the PAHs uptake steadily increases until equilibrium conditions are achieved (around 2 hours). Various kinetic models (i.e. Elvoich, Lagergren, second order and double exponential models) have been used to model the experimental data, obtaining acceptable correlations. The sorption equilibrium experiments of the system PAH-soil have been assessed in the 298 - 333 K temperature range. Thus, a non-favourable shape of isotherms according to Giles classification was obtained. Moreover, unlike acenaphthene, phenanthrene at 333 K and anthracene and fluoranthene at any temperature showed anomalous isotherms. The reason seems to rely on the trapping of dissolved PAHs by soil organic matter (SOM) released to water. This abnormal trend was not experienced when the isotherms were obtained by using a four PAHs mixture. Apparently, the most soluble acenaphthene was capable of binding all the released material so no effect was thereafter observed. Additionally, applied isotherm models such Freundlich, Oswing, Peleg and Caurie have also successfully modeled the equilibrium. 2. Fenton's treatment of PAH spiked soil. The attempt in soil treatability with Fenton's reagent was conducted with spiked soil. Hydroxyl radicals (OH¿) are capable of oxidizing hazardous pollutants, however they can be simply consumed by organic matter (SOM). Thus, the influence of hydrogen peroxide and ferrous iron dosages were analyzed by following an experimental design compiling the data from 13 runs. Experimental data showed that elucidating the optimum application conditions seems to be a difficult task. The application of Fenton's reagent showed a different tendency depending on the PAH studied. The average removal for acenaphthene was close to 50%, while remaining anthracene and fluoranthene in soil was in the order of 25% of the amount extracted from non-treated soil. The most surprising results came from phenanthrene, the amount extracted from the soil was higher than the amount extracted from non-oxidized soil. Thus, Fenton's reagent in a first stage would attack the organic material responsible of phenanthrene adsorption. Oxidation of this material would likely favour the extraction of this PAH (and probably the rest of PAHs). In contrast to the other three PAHs, phenanthrene seems to remain unaltered under the presence of Fenton's reagent. An attempt to model the remaining PAH concentration as a function of reagent's dosage has been conducted. 3. Degradation of target pollutants present on spiked soil by ozonation process. The use of ozone in the removal of PAHs from spiked soil can offer two main advantages, i.e. large volumes of waste material are not usually generated and the treatment is commonly implemented over a much shorter time frame than conventional treatments. Ozone is a highly reactive and powerful oxidant that can react with a high number of organic contaminants through two pathways: molecular ozone and/or its primary decomposition hydroxyl radical (OH¿). A three level full factorial design has been conducted to assess the influence of the gas flow-rate, ozone concentration and reaction time on the remediation of soil contaminated with target PAHs. Under the operating conditions investigated, reaction time and ozone concentration seem to exert a slight positive effect while gas flow-rate does not affect the efficiency of the process. A significant PAHs removal can be achieved in the first five minutes leading to values of 50, 70, 60 and 100 % for acenaphthene, phenanthrene, anthracene and fluoranthene eliminations respectively. It seems that under the conditions investigated, the amount of ozone fed is in large excess. Moreover, an attempt to model the ozone decomposition on soil surface was conducted by assuming a series of reactions involving easily oxidable material, recalcitrant oxidized intermediates and inorganic actives sites. As inferred from the results, three different regions can be obtained: an initial period when ozone is not decomposed by soil due to the experimental set up conformation, following by a second slow stage in which ozone attacks the oxidized soil organic matter (OX-SOM), and a third region when, the OX-SOM is gradually being removed leading to the appearance of likely active inorganic sites capable of efficiently decompose ozone. Obviously, the model used is an oversimplification of the actual mechanism taking place, however it can be hypothesized that as the ozone concentration is raised, the amount of intermediate oxidized material formed is likely lowered leading, therefore, to the generation of final products that do not consume ozone. 4. Supercritical CO2 extraction of PAHs on spiked soil and solvent regeneration by ozonation. Supercritical carbon dioxide fluid progressively deserves the epithet of green solvent for the 21 st century. This solvent shows properties that are intermediate between liquids and gases. CO2 use in soil remediation processes is recently being considered. The supercritical CO2 extraction of target PAHs from an artificially contaminated soil has been investigated. The effect of temperature, pressure and extraction time has been assessed by conducting an experimental design. Under the operating conditions investigated, the results suggest the existence of perturbation variables other than the aforementioned controlled variables leading to a significant dispersion of extraction recoveries. As a consequence it is difficult to infer and to articulate accurate effects of controlled variables. However, from the completion of an appropriate experimental design some effects can be envisaged. With the exception of anthracene, an optimum in temperature (50 ºC) is envisaged when extracting the PAHs. Analogously, with the exception of anthracene (positive effect), pressure does not have a significant influence. The recovery yield increases as extraction time is increased to a value of 120 min, although the dispersion of data points does not allow the statement of consistent conclusions. Use of co-solvents in supercritical CO2 extraction is an alternative option capable of enhancing the final extraction yield. Hence, a H2O2 aqueous solution could results in a beneficial effect, especially at the optimum working temperature. However, the presence of H2O2 did exert no clear impact in the process, likely the operating conditions used and particularly soil nature were not adequate to promote its decomposition into free radicals. Nevertheless these results do not preclude the possibilities of this oxidant to enhance the efficiency of the cleaning technology. The extraction by supercritical CO2 involves the collection of contaminants into an organic solvent (methanol) after the process. In order to make the whole cleaning process environmentally attractive, regeneration of the solvent is of paramount importance. Ozone is a suitable option, since it is a selective oxidant, more soluble in methanol than in water and with acceptable reaction rates. The results suggest that methanol can be reused with no loss of either its capacity of PAH solubilization and tozone reactivity. 5. Wet oxidation. Among chemical processes, wet oxidation is considered as an innovate technology due to the use of water as extraction-oxidation agent. This process has some advantages such as low required residence time, possibility of modifying the operating conditions to reach unavailable contaminants molecules and/or possibility of combination with an extraction stage to conduct the PAH degradation. The research has been conducted by following the next stages: (A) preliminary studies with saturated aqueous solution under moderate conditions of temperature and pressure in the presence and absence of oxygen and free radical promoters; (B) wet oxidation of contaminated soil by concentrated hydrogen peroxide as promoter at mild operating conditions; (C) hot water extraction - oxidation of contaminated soil at relatively extreme operating conditions. (A) Promoted wet air oxidation. The preliminary studies are focused to find the viability of WAO to remove PAHs. Thus, runs have been carried out with a PAH-saturated aqueous solution under WAO conditions in the presence and absence of oxygen and with the addition of free radical promoters: hydroxyl radical promoter (hydrogen peroxide) and sulfate radical promoter (OXONE®). In the absence of promoters, the process achieves PAH conversion values in the range 80 - 100 % at 190 C and 50 bars of air pressure (80 min of reaction). Similar results are obtained in the presence of hydrogen peroxide, however, in this case, the time required is just 60 min with a sharp decrease in PAH concentration in the first 10 - 20 min. Additionally, temperature can be lowered to values in the range 100 - 150 C. If potassium monopersulfate is used instead of hydrogen peroxide, an analogous behavior is experienced, in the latter case, temperatures above 120 C lead to an inhibition of anthracene oxidation, likely due to ineffective decomposition of the monopersulfate molecule. (B) Wet oxidation from contaminated soil by concentrated hydrogen peroxide as promoter at mild operating conditions. Preliminary results concluded that hydrogen peroxide seems to favour the oxidation process. Consequently, some experiments have been conducted using concentrated hydrogen peroxide at relatively low temperature under pressure slightly above than atmospheric. The system H2O2-soil was studied by completing an experimental design considering reaction time, temperature and concentrated hydrogen peroxide volume as regressors. This technology achieved acceptable PAH removal percentages from soil. Under optimum conditions, acenaphthene (the most soluble PAH) is completely removed, and the rest of PAHs are also eliminated to a high exert. Temperature and hydrogen peroxide amounts seem to play a major role. (C) Hot water extraction - oxidation from contaminated soil at relatively high pressures and temperatures. This process can, a priori, combine the extracting capacity of water and oxidation capacity of dissolved oxygen at high temperature. However, to generate an aqueous effluent free of toxic contaminants is required to optimize the process. Under the range of operating conditions investigated, as an average, PAHs removal values are slightly lower than those obtained by concentrated hydrogen peroxide. Nevertheless, PAHs have been eliminated to a high extent. The best results are obtained when high temperature is used. Elimination values above 90 % are achieved for each of the target PAHs. Moreover, temperature and water flow-rate are the most influencing parameters in this process. 6. Use of sodium percarbonate as an alternative to hydrogen peroxide in soil remediation processes. Since sodium percarbonate is as dry carrier of hydrogen peroxide, this novel oxidizing agent can be considered in the soil treatment field. Thus, sodium percarbonate (2Na2CO3¿3H2O2) is a solid showing an exceptional H2O2 storage capability presenting less risk of splashing and spilling than liquid H2O2. Accordingly, a preliminary series of SPC decomposition experiments in the presence of soil has been completed and a 20-run experimental design was considered. The results suggest the development of first order kinetics in hydrogen peroxide decomposition. Soil concentration exerts a positive effect showing an approximate reaction order of 2/3. The apparent activation energy of the process was found to be around 60 kJ mol-1. Application of SPC solution to a PAHs spiked soil led to the complete oxidation of acenaphthene and variable eliminations for the rest of PAHs depending on the operating conditions applied. The main parameters affecting the process seem to be temperature and hydrogen peroxide concentration. Initially they exert a positive effect until the inefficient hydrogen peroxide decomposition into oxygen and water predominates over the beneficial effects of increasing the aforementioned parameters. 7. Real soil treatment by chemical oxidation. Using chemical oxidation with native soil is a complex system, since the effectiveness is highly dependent on a complex interplay of factors, including the soil organic matter (SOM), soil inorganic matter (SIM), water content, pH, physical properties and the age of contaminated soils as well as specific PAH pollutant characteristics. The experimental data obtained showed a high dispersion, being reproducibility a difficult task. This part of research shows the possibility of using in situ chemical oxidation to degrade PAHs deposited onto real soil by means of hydrogen peroxide, sodium percarbonate and gas ozone applications. (A) Hydrogen peroxide application. Application of in situ chemical oxidation is an option from an economic point of view. The process uses a strong oxidant, with or without catalyst addition, to oxidize organic compound. Moreover, this technique presents some advantages such as low residence time, high biodegradability of sub-products, etc. In spite of the relatively complex dataset obtained, it is possible to draw some general conclusions. Firstly, the effect of different variables has been studied by using an experimental design considering temperature, pressure and extraction time as regressors. Some collateral effects play a very important role in the process. These collateral effects are mainly related to the contact of H2O2 with pollutants. The results suggest that the hydrogen peroxide decomposition is governed by temperature. The best conditions are obtained using average operating parameters of liquid flow-rate, H2O2 concentration and temperature. (B) Sodium percarbonate application. An alternative to hydrogen peroxide application is the use of sodium percarbonate. Similarly to the previous study, a high dispersion data was obtained likely due to channeling effects. As a consequence, accurate predictions and efficiencies are difficult to obtain and only trends can be proposed. Hence, positive effects expected by temperature and reagent dosage are in some cases hindered by a higher extent in hydrogen peroxide inefficient decomposition and scavenging reactions. (C) Gaseous ozone treatment. Treatment based on liquid injection seems to be limited by channeling effects. A solution would be soil ozonation in gas phase. Consequently, the treatment has been carried out in a similar way to the previous studies. The results indicate, firstly, that the efficiency of treatment is strongly dependent on sequestration degree. Secondly, if the results are compared to the Spanish guideline values for soils after the process, the contamination of the soil is higher than legal values, however this process improves the biodegradability of the PAH-contaminated soil if compared to the untreated raw material.