Physicochemical methods for the remediation of contaminants in water and soils
We are glad to see you again in the course!
In this session, we will review concepts associated with environmental decontamination techniques where their particularities, advantages, and disadvantages will be analyzed. We are sure that this session will be useful and will present in an illustrated way the fundamentals of these environmental technologies.
So let’s get started with the class!
The following treatments describe different soil decontamination techniques that also combine groundwater decontamination in some cases.
They are techniques typically applied in situ that aim to separate pollutants from the soil for subsequent purification treatment. They are simple treatments that require that the soils be permeable and that the polluting substances have sufficient mobility and are not highly adsorbed on the soil. Depending on the elements with which the extraction is carried out, we talk about:
a) Air extraction
It is used to extract the pollutants adsorbed on the particles of unsaturated soils through their volatilization or evaporation through vertical and/or horizontal extraction wells that lead the air with the pollutants to the surface.
There, they can be treated in specialized plants (generally adsorbed to carbon) or be degraded in the atmosphere naturally. The volatilization of pollutants can also be favored by practices such as plowing, and irrigation it can contribute to the solubilization and desorption of contaminants that can be dragged to the surface by evaporation. This technique is indicated for soils contaminated with volatile and semi-volatile substances such as light petroleum hydrocarbons, some non-chlorinated solvents, light polycyclic aromatic hydrocarbons, and volatile organochlorine compounds. However, it is not recommended for heavy petroleum-derived hydrocarbons, PCBs, dioxins, or metals (Blum, 2011). Sometimes the performance of this treatment can be increased by stimulating the extraction of air with temperature, usually by injecting hot air.
b) Water extraction
It is a technique used mainly for contaminated aquifers that consist of extracting contaminated water from the soil and subsoil, both from the saturated zone and from the unsaturated zone. When the saturated area is treated, the water is pumped to the surface for further treatment, known as Pump & Treat (Illangasekare and Reible, 2001). When it is sought to act on the unsaturated zone, a previous injection of water is normally made, by gravity or pressure, which drags and washes the polluting elements from the soil and stores them in the saturated zone to be later pumped to the surface. This injection of water can be reinforced with the addition of solvents or chemical compounds that can favor the desorption of soil pollutants as surfactants, to remove low solubility organic compounds; sodium hydroxide, to dissolve organic matter in the soil; water-soluble solvents such as methanol; substitution of non-toxic cations for toxic cations; complexing agents such as EDTA; acids and bases, which desorb metals and some organic compounds and salts; and reducing and oxidizing agents that increase the mobility of contaminants. Thus, the type of soil, its pH, cation exchange capacity, particle size, permeability, etc., are parameters that determine the efficiency of desorption (Megharaj et al., 2011). that desorb metals and some organic compounds and salts; and reducing and oxidizing agents that increase the mobility of contaminants. Thus, the type of soil, its pH, cation exchange capacity, particle size, permeability, etc., are parameters that determine the efficiency of desorption (Megharaj et al., 2011). that desorb metals and some organic compounds and salts; and reducing and oxidizing agents that increase the mobility of contaminants. Thus, the type of soil, its pH, cation exchange capacity, particle size, permeability, etc., are parameters that determine the efficiency of desorption (Megharaj et al., 2011).
Typically Pump and Treat systems are used for groundwater contaminated with volatile and semi-volatile organic compounds, fuels, and metals. Depending on the terrain, the type of contamination and the recovery to be carried out, the extraction of water can be carried out through wells, to which a pumping system is coupled to extract large flows; drains, which allow a greater contact surface with the contaminated area and which are used especially in low permeability soils; and drainage ditches, also used in poorly permeable soils, with shallow water tables and extraction of smaller flows. In any case, once treated, the extracted water can be partially re-filtered into the soil to counteract negative effects of the extraction such as the high drop in the water table, the possible settlement of the soil, ecosystems damaged by the loss of moisture, etc. This treatment is very common but it is not the most efficient since it is not applicable in fractured soils or clay soils, the water is not completely decontaminated for human consumption and has limitations such as its high cost and execution time (Illangasekare and Reible, 2001).
c) Free phase extraction
It is applied in soils contaminated with hydrocarbons in the free phase, located above the water table. They are normally extracted by vertical wells into which the pollutant flows, at depths of the water table greater than 80 m, and which can extract only the free phase, the free phase, and water simultaneously or a mixture of both. Equipment that only extracts free phase generally uses skimmer pumps, located at a depth that corresponds to the water-free phase interface, with a filter inside that facilitates the selective passage of organic substances based on their density. Skimmers that separate light hydrocarbons operate with very small thicknesses of the water-free phase interface, whereas skimmers used to separate heavy hydrocarbons require a minimum thickness.
3 cm. Dual pump systems extract the water and the free phase separately employing two different pumps located at different depths of the well, so that the pump that extracts the water must be located below the water-free phase interface and the pump that extracts the water. free phase above. The extraction of total fluids, which extracts a mixture of water and free phase in cases of homogenization of the pollutant with water or of very small thicknesses of the free phase in poorly permeable soils, uses a single pumping installation, either submerged or pumps surface installed vacuum. This extraction requires the subsequent separation of the two phases.
d) Dense phase extraction
Some pollutants such as chlorinated solvents, some PCBs, phenanthrene, naphthalene, and phenols are denser than water and accumulate below the water table, in the area of contact with the underlying less permeable materials. These substances, poorly soluble and difficult to degrade, can generate pollution plumes that occupy large volumes. Their extraction is carried out through wells, sometimes injecting solvents into the contaminated area that favor the circulation of contaminants towards the well, and depending on the extraction method they can be extracted along with skimmer pumps, mixed with water (with the consequent decrease in extraction performance), or by dual pumping of both phases separately. On the negative side,
e) Solvent and acid extraction
This treatment applied ex-situ, is based on the extraction of pollutants by mixing in a soil tank with an organic solvent such as acetone, hexane, methanol, dimethyl ether, and triethylamine. The organic solvent carries the pollutants and is separated from the soil by evaporation, and by adding new solvents or distillation the pollutants are removed so that the organic solvent used can be reused. In turn, the treated soil is washed to remove any remaining solvent.
Soil washing is a generally ex-situ treatment in which the excavated soil is previously physically separated by sieving, density, or gravity to remove the coarser gravel particles, with little adsorption capacity, from the fine fraction and then washed with extractants. chemicals that allow pollutants to be desorbed and solubilized (van Benschoten et al., 1997). After chemical treatment, the soil is rewashed with water to remove residual contaminants and extractants and is returned to its place of origin (Peters, 1999). The effectiveness of this technique depends on the degree of adsorption of the pollutant, controlled by a series of soil properties such as pH, texture, cation exchange capacity,
Soil washing is used primarily for soils contaminated with semi-volatile organic compounds, petroleum-derived hydrocarbons, and inorganic substances such as cyanides and heavy metals, and is less effective in treating volatile organic compounds and pesticides (Trellu et al., 2016).
It consists of applying a low-intensity electric current between electrodes introduced in situ in the contaminated soil that allows the mobilization of water, ions, and small charged particles. The anions move towards the positive electrode and the cations towards the negative electrode. The oxidation of water at the anode generates protons, H +, which move towards the cathode creating an acid front. This favors the desorption of cations from the soil and forces the dissolution of precipitated pollutants such as carbonates, hydroxides, etc. On the other hand, the OH- ions generated in the cathode by the reduction of water cause the precipitation of metals (Ramírez et al., 2015). During treatment, contaminants can be transported by electromigration (movement of ions towards the electrode of opposite charge), electroosmosis (movement of water concerning the charged surface of soil particles), electrolysis (movement of ions in response to a potential difference), and electrophoresis (ionic displacement in suspension). This technique is effective for the treatment of soils with low hydraulic permeability that are difficult to recover by other means and is especially suitable for soluble metals or complexes in the soil in the form of oxides, hydroxides, and carbonates. Its advantages include low energy consumption, control over the direction of the flow of water and dissolved pollutants, and the accumulation of pollutants on the electrodes for their subsequent elimination (Fdz-Sanromán, et al., 2021) or using exchange resins ionic or by pumping (Ma et al., 2020).
It is an ex situ treatment in which pollutants are destroyed by supplying heat. The soil is subjected to high temperatures, around 1000ºC, to oxidize and volatilize polluting organic compounds. This process generates residual gases and ashes, organic (polycyclic and sulfurized aromatic hydrocarbons, oxygenated compounds, nitrogenous aromatic compounds, etc.), and inorganic (volatile heavy metals, CO2, NOx, SOx) (Vidonish et al., 2018) that must be debugged. The most widely used combustion furnaces use high-speed air (Circulating Bed Combustors and Fluidized Bed Combustors), infrared (Infrared Combustors), and rotary systems (Rotary kilns) (Riser-Roberts, 2020).
It is indicated to recover soils contaminated with explosives and hazardous waste, particularly chlorinated hydrocarbons, PCBs, and dioxins (Zhang, 2019), although its reuse is very limited because this treatment destroys the soil structure.
b) Thermal desorption
It is another ex situ heat treatment in which the soil is subjected to lower temperatures (90-320ºC, low-temperature thermal desorption; 320-560 ° C, high-temperature thermal desorption) to achieve desorption rather than destruction of pollutants pursued by incineration. The temperatures used are chosen to volatilize organic pollutants but not to oxidize them. Specifically, during low-temperature thermal desorption, the soil retains its physical properties and its organic components, which makes it possible for it to retain its ability to withstand future biological activity. Through low-temperature thermal desorption, soils contaminated with non-halogenated volatile organic compounds, fuels, and in some cases semi-volatile organic compounds can be recovered.
When we speak of soil contamination, we must consider the complexity of the system, the number of components present (biotic and abiotic), diverse weather conditions, and the complexity of pollutants present, continuously modifying the interactions between the aforementioned elements.
This, coupled with a large number of existing technologies, could represent a challenge in making decisions regarding which technology to choose and which decontamination criteria to follow (time, treatments, sequential application of treatments, etc.).
For this reason, we include the following material in which the effect of pollutant co-occurrence and its impact on soil remediation is evaluated.
We invite you to review it and identify the factors that are significantly affected so that it allows you to foresee the possible scenarios that you would face in a similar case.
Decision-making in the face of a large number of technological alternatives represents a challenge in choice, given the complexity of criteria that could emerge as suitable.
With the knowledge acquired in this class, you must take some time to think about the importance of detailed knowledge of the contaminated system (type of contaminant, time of contamination, soil characteristics) for the best decision-making regarding the remediation treatment.
You must be clear that there is no single answer and that it is possible to meet new challenges even when it was thought that the previous ones had been solved.
Anticipatory thinking allows us to confront these situations, allowing us to visualize new scenarios in the face of different conditions of the problem to be treated.
So, draw on the knowledge acquired, not only in this class but in the ones you have previously taken. It will be of great help to you.
According to the principle of operation, decontamination technologies can be classified into physical-chemical treatments and heat treatments. These treatments show great effectiveness in removing pollutants and although they are expensive compared to biological ones, they are widely used for their quick effect once applied.
The high infrastructure required for its execution gives it a high degree of training, surveillance, and control, which represents a challenge in terms of human, physical, and technical capital.
It is important to recognize the characteristics of these technologies and their applicability to guarantee their success.
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- Garbisu, C., & Alkorta, I. (2003). Basic concepts on heavy metal soil bioremediation. (European Journal of Mineral Processing and Environmental Protection), 3 (1), 58-66.
- Trejo, JAV (2002). Remediation technologies for contaminated soils. National Institute of Ecology.
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- Trellu, C., Mousset, E., Pechaud, Y., Huguenot, D., van Hullebusch, ED, Esposito, G., & Oturan, MA (2016). Removal of hydrophobic organic pollutants from soil washing/flushing solutions: a critical review. Journal of Hazardous materials, 306, 149-174.
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- Riser-Roberts, E. (2020). Remediation of petroleum contaminated soils: biological, physical, and chemical processes. CRC press.
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