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 pesticides 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 (see figure below). 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, spills of petroleum hydrocarbons were more common in the last few decades than nowadays. 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 vapour 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 processes, wet air oxidation and direct oxidation processes by means of novel oxidizing agents 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.
Here goes a brief description about them !
What is Fenton reagent? Fenton’s reagent is a solution of hydrogen peroxide and an iron catalyst that is used to oxidize contaminants. It was developed in the 1890s by Henry John Horstman Fenton.
Ferrous Iron(II) is oxidized by hydrogen peroxide to ferric iron(III), a hydroxyl radical, and a hydroxyl anion. Iron(III) is then reduced back to iron(II), a superoxide radical, and a proton by the same hydrogen peroxide. The net effect is a disproportionation of hydrogen peroxide to create two different oxygen-radical species, with water (H+ + OH–) as a byproduct.
Fe2+ + H2O2 +H+ → Fe3+ + HO• + H2O
Fe3+ + H2O2 → Fe2+ + HOO• + H+
Even after over 100 years of study and use in water treatment, in-situ remediation methods were slow to use Fenton’s Reagent, owing to safety concerns. Remediation of soil and groundwater contamination is accomplished by injecting this strong chemical oxidant, and a chain reaction is initiated, forming more radicals, which are very reactive and destroy chemical bonds of organic compounds. In addition, pH adjustment using a strong acid such as sulfuric acid (H2SO4) or hydrochloric acid (HCl), is common since reactions of classic Fenton’s Reagent are more rapid and efficient under low pH conditions (pH 2 to 4 is optimal).
Ozone is defined as a triatomic molecule, consisting of three oxygen atoms, and it is formed from dioxygen by the action of ultraviolet light.
Among the technologies that can be applied “in situ” or “on site” soil ozone application is catalogued as one of the most promising systems. Molecular ozone (or its primary decomposition radical HO•) steadily reacts with a high number of organic and inorganic contaminants. Injected ozone gas might directly attack target compounds, or alternatively, it can decompose over metal oxides in the surface soil to generate the non-specific hydroxyl radical which in turn can oxidise/mineralize adjacent sorbed pollutants. The efficiency of ozone in soil treatment has been assessed either at laboratory level and field scale.
Its key benefits as an oxidant in soil and groundwater remediation are: destruction of targeted pollutants; rapid reaction – process allows for a quick turnaround; contaminants are destroyed rather than transferred from one phase to another; clean reaction – no hazardous by-products produced; and micro-bubbles act to extract pollutants from both groundwater and soil pores, so acting across the total soil body.
Supercritical CO2, green solvent for the 21st century
Supercritical fluids (SCFs), in particular supercritical carbon dioxide, are progressively deserving the epithet of “green solvents for the 21st century”. SCFs offer properties that are intermediate between liquids and gases.
Carbon dioxide usually behaves as a gas in air at standard temperature and pressure (STP), or as a solid called dry ice when frozen. If the temperature and pressure are both increased from STP to be at or above the critical point for carbon dioxide, it can adopt properties midway between a gas and a liquid. More specifically, it behaves as a supercritical fluid above its critical temperature (304.25 K) and critical pressure (72.9 atm or 7.39 MPa). Its properties can be summarized in lower viscosity and thermal conductivity than in liquids and better diffusion characteristics. Carbon dioxide does not require an excessive amount of energy to get supercritical conditions. As well, other advantages include the low cost of the carbon dioxide, high chemical stability and lack of toxicity.
All of these properties make supercritical CO2 an important commercial and industrial solvent due to its role in chemical extraction in addition to its low toxicity and environmental impact. The relatively low temperature of the process and the stability of CO2 also allow most compounds to be extracted with little damage or denaturing. So, the use of supercritical CO2 in soil remediation processes is recently being considered. The advantages of using CO2 include the affinity for non-polar contaminants that are tightly adsorbed into solid particulates.
Rivas FJ, García R, García-Araya JF, & Gimeno O (2008). Promoted wet air oxidation of polynuclear aromatic hydrocarbons. Journal of hazardous materials, 153 (1-2), 792-8 PMID: 17945415