Biostimulation

Among bioremediation strategies, biostimulation is the most frequently used, and the one that has reported the best results and efficiency in the removal of petroleum-derived hydrocarbons in soils.[9]​[10]​[11]​ In essence, biostimulation seeks to take advantage of the catabolic capacities of the community of microorganisms already inhabiting the soil, by adapting the environmental factors that are limiting for microbial development and degradation of the contaminant. In general, the most incident factors that biostimulation usually investigates involve the availability of macronutrients (nitrogen and phosphorus), which is why biostimulation is generally linked to fertilization or the addition of these macroelements to the soil.[12] Although the concentration of N and P, and therefore the optimization of the C:N:P ratio in soil, have been usually referenced as the main variables that affect the removal of hydrocarbons when strategies are implemented of biostimulation,[13]​ there are also cases in which other characteristics need to be studied. Among them we can mention the water content in the soil (humidity), pH, oxygen availability and temperature.

Bioaugmentation

Bioaugmentation, on the other hand, consists of the inoculation of the contaminated matrix with microorganisms, or group of microorganisms (consortia), that have the catabolic capacity to partially or totally degrade the contaminant present in said matrix. It is usually applied when the rate of degradation of the contaminant in the uninoculated matrix is ​​not sufficient for it to be removed at convenient times.

The usefulness and efficiency of using bioaugmentation as a bioremediation method is controversial. Many times, the microorganism or group of microorganisms that are inoculated fail to establish themselves in the contaminated matrix, and are not detectable after a certain time of treatment.[14]​[15]​ This is mainly due to the fact that the microflora present in the soil is already adapted to the particular environmental conditions and biological interactions present in said soil, and does not allow the inoculated microorganisms to establish themselves in the soil. On the other hand, according to the literature, in most cases the use of bioaugmentation as a bioremediation technique usually presents less satisfactory results than those obtained with biostimulation. In some cases, this difficulty is overcome by adapting variables (bioaugmentation + biostimulation), while in others strains that have the capacity to produce surfactants are usually used, which seeks to increase the bioavailability of hydrocarbons for the rest of the microbial community.[16][17] This is the case, for example, of some members of the genus Rhodococcus, which produce biosurfactants that remain adhered to the membrane or cell wall, favoring the degradation of organic compounds.[18]​[3]​.

Added surfactants

In many cases, even though the soil has an adequate number and quality of microorganisms to carry out the remediation, and the physicochemical variables as well as the quantities of nutrients in the soil are adequate, the contaminants are not removed significantly. This usually happens when the contaminant is closely bound to soil particles, reducing their bioavailability. The addition of surfactants is used as a solution to this problem, because its amphiphilic character reduces the surface tension of water and aqueous compounds, favors the formation of micelles and increases the mobility of hydrophobic organic compounds. In this way, they help to solubilize contaminants, increasing their availability to the bacterial community and increasing their degradation rate.[19]​[20]​.

Contenido

La fitorremediación es la descontaminación de los suelos, la depuración de las aguas residuales o la limpieza del aire interior, usando vegetales, ya sean plantas vasculares, o algas (ficorremediación). También se incluye a los hongos (micorremediación), y por extensión a los ecosistemas que contienen estas plantas. La fitorremediación es un conjunto de tecnologías que utilizan las plantas para reducir, degradar o inmovilizar compuestos orgánicos contaminantes (naturales o sintéticos), de la tierra, del agua o del aire y que provienen de las actividades humanas. Esta técnica también puede tratar la contaminación por compuestos inorgánicos (metales pesados o radioisótopos) a través de procesos como la quimiadorción de metales pesados por medio de la celulosa presente en las plantas.[21]​.

Las plantas y sus organismos rizosféricos (asociados a la zona de la raíz) se pueden usar para fitorremediación de diferentes maneras. Existen diferentes métodos de fitorremediación que se agrupan en dos conjuntos, dependiendo si se utilizan como medio de concentración (rizofiltración, fitoestabilización y fitoinmovilización) o como medio de eliminación (fitodegradación, fitoextracción y fitovolatilización).[22]​.

Advantages

Phytoremediation has gained popularity in the last 10 years. This is based in part on its relatively low cost. Because biological processes are ultimately driven by the sun, phytoremediation is on average ten times cheaper than engineering-based methods (such as soil excavation). The fact that phytoremediation is typically carried out in situ contributes to its cost-effectiveness and can reduce exposure of contaminated substrate to humans, wildlife, and the environment. Phytoremediation is also popular among the general public as a "green" alternative to chemical plants and bulldozers.[23]​.

Although phytoremediation has advantages, it also has limitations. The plants that mediate the cleanup must be where the contaminant is and must be able to act on it. Therefore, soil properties, toxicity level and climate should allow plant growth. Phytoremediation is also limited by root depth, because plants have to be able to reach the contaminant.[22]​.

One way to optimize resources is the use of already dead and pulverized biomass for the treatment of water contaminated with heavy metals.[24]​.

Methods used in phytoremediation

Phytostabilization allows pollutants to be immobilized in the soil through their absorption and accumulation in the roots or by precipitation in the rhizosphere zone. This process reduces the mobility of contaminants and prevents their migration to groundwater or air. Phytostabilization is effective in fine-textured soils with high organic matter content. It is mainly applied to large areas where there is surface contamination. This technology has the advantages over other soil remediation methods that it is lower cost, easy to apply and aesthetically pleasing. Some plants used for phytostabilization purposes are: Hyparrhenia hirta (Pb); Zygophyllum fabago (Zn); Lupinus albus (Cd, As); Anthyllis vulneraria (Zn, Pb, Cd); Deschampsia cespitosa (Pb, Cd, Zn); Cardaminopsis arenosa (Cd, Zn); Horedeum vulgare, Lupinus angustifolius and Secale cereale (As); Lolium italicum and Festuca arundinaceae (Pb, Zn); and Brassica juncea (Cd, Zn, Cu, Mn, Fe, Pb). Eichhornia crassipes is an invasive plant used in phytostabilization processes with viable results.[25]​.

Phytoextraction or phytoaccumulation consists of the absorption of contaminating metals through the roots of plants and their accumulation in stems and leaves. The first step in applying this technique is the selection of the most suitable plant species for the metals present and the characteristics of the site. Once the vegetative development of the plant is complete, the next step is to cut them and proceed to incinerate them and transfer the ashes to a safety landfill. Phytoaccumulation can be repeated unlimitedly until the remaining concentration of metals in the soil is within the limits considered acceptable (depending on the legislation in force in that location). Some plants used for this phytocorrective technique are: Thlaspi caerulescens (Cd); Sedum alfredii, Viola baoshanensis and Vetiveria zizanioides (Zn, Cd, Pb); Alyssum murale, Trifolium nigriscens, Psychotria douarrei, Geissois pruinosa, Homalium guillainii, Hybanthus floribundus, Sebertia acuminata, Stackhousia tryonii, Pimelea leptospermoides, Aeollanthus biformifolius and Haumaniastrum robertii (Ni); Brassica juncea, Helianthus annuus, Sesbania drummondii (Pb); Brassica napus (Cu, Pb, Zn); and Pistia stratiotes (Ag, Cd, Cr, Cu, Hg, Ni, Pb, Zn).

Rhizofiltration uses plants to remove contaminants from the water environment through the roots. In rhizofiltration these plants are grown hydroponically. When the root system is well developed, plants are introduced into water contaminated with metals, where the roots absorb and accumulate them. As the roots become saturated, the plants are harvested and disposed of for final use. There are a large number of studies related to the contaminant accumulation capacity of various aquatic plants, some examples of them are: Scirpus lacustris (Cd, Cu, Pb, Mg, Fe, Se, Cr), Lemna gibba (Pb, As, Cu, Cd, Ni, Cr, Al, Fe, Zn, Mn), Azolla caroliniana (Hg, Cr Sr, Cu, Cd, Zn, Ni, Pb, Au, Pt), Elatine trianda (As), Wolffia papulifera (Cd), Polygonum punctatum (Cu, Cd, Pb, Se, As, Hg, Cr, Mn) and Myriophylhum aquaticum, Ludwigina palustris and Mentha aquatica (Cu, Zn, Mn, Fe, Ni).

Phytovolatilization occurs as trees and other growing plants absorb water along with organic and inorganic contaminants. Some of these can reach the leaves and evaporate or volatilize into the atmosphere. Through this process, contaminants such as: volatile organic compounds (benzene, nitrobenzene, toluene, ethylbenzene and xylene), As, Se and Hg have been eliminated. The plants Salicornia bigelovii, Brassica juncea, Astragalus bisulcatus and Chara canescens have been used for the remediation of sites contaminated with Se and Arabidopsis thaliana for Hg.

In phytostimulation, the roots of the plants stimulate the microorganisms present in the area to be treated, and these degrade the contaminants.

In phytodegradation, plants and the microorganisms associated with them degrade organic contaminants into harmless products or mineralize them to CO and HO. In this process, contaminants are metabolized within plant tissues, and plants produce enzymes such as dehalogenase and oxygenase, which help catalyze the degradation. Phytodegradation has been used for the removal of explosives such as TNT, halogenated hydrocarbons, Bisphenol A, PAHs, and organochlorine and organophosphate pesticides.

Hydrocarbon bioremediation is used to reduce or eliminate contamination of soils contaminated with, for example, petroleum, diesel, oil and grease from various sources, including accidental or industrial spills, using microorganisms.

Hydrocarbons are compounds formed almost exclusively by carbon and hydrogen atoms, considered basic compounds of organic chemistry. Its use has become an important driver for the development of humanity, but, nevertheless, it has given rise to different catastrophic events at an environmental level due to the contamination of terrestrial and aquatic ecosystems as a result of the spill of these petroleum compounds and their derivatives. In the case of soils, the main environmental consequences that occur after a contamination event are: the reduction or inhibition of the development of vegetation cover, changes in the population dynamics of fauna, microbial biota and contamination by infiltration into underground water bodies.[26]​.

In bioremediation, biostimulation and bioaugmentation are presented as one of its main strategies, the first being defined as the addition of nutrients, mainly sources of nitrogen and phosphorus, to promote microbial growth and development, as well as stimulation through the addition of water, oxygen and other elements that improve the development of microorganisms. The use of waste sludge (biosolids) as a source of macro- and micronutrients can favor the stimulation of native soil microorganisms and cause the degradation of hydrocarbons, which are used as a source of carbon and electron donors, coupling the oxidation-reduction reaction with oxygen, which plays the role of electron acceptor.[27]​.

La industrialización global está cumpliendo las demandas "Demanda (economía)") de la población moderna a costa de la exposición ambiental a diversos contaminantes, incluidos los metales pesados. Estos metales afectan la calidad del agua y del suelo. Además, éstos entran en la cadena alimentaria y exhiben sus efectos letales en la salud humana, incluso cuando están presentes en una concentración ligeramente más alta que la requerida para el metabolismo normal; en el peor de los casos, los metales pesados pueden volverse cancerígenos. Por esto, la extracción eficiente de metales pesados presenta una demanda creciente en todo desarrollo sustentable. El enfoque de la biorremediación presenta una alternativa espléndida para los métodos caros e ineficientes convencionales para la eliminación de metales pesados.[28]​.

Biosorption

Biosorption is a subcategory of adsorption, where the sorbent is a biological matrix. Biosorption is a process of rapid and reversible binding of ions from aqueous solutions to functional groups that are present on the surface of biomass. This process is independent of cellular metabolism. It can be performed in a wide range of pH values ​​3–9 and temperature values ​​4–90 °C. This process does not require a large capital investment, so operating costs are economical. Furthermore, biological materials are often cheap and can be obtained from agriculture or industrial waste. The characteristics of biosorption, compared to those of conventional treatment methods, include low cost, high efficiency, minimization of chemical and/or biological sludge, does not require the additional addition of nutrients, the biosorbent is regenerable, and allows the possibility of recovering metals.

In recent years, the biosorption process has become an economical and environmentally friendly alternative treatment technology in the water and wastewater industry. As a result, several biosorbents have been developed that are successfully used to treat various contaminants, including metals, dyes, phenols, fluoride and pharmaceutical products in solutions (aqueous and/or oily). A wide range of naturally available biomaterials have been used as biosorbents for the desired removal of contaminants. All types of microbial, plant and animal biomass and their derived products are of great interest regarding their relationship with a wide variety of substances. Many of these biological materials, especially bacteria, cyanobacteria, algae (including microalgae, macroalgae, kelp), yeast, fungi and lichens have attracted much attention for the removal and recovery of heavy metal ions due to their good performance, low cost and availability in large quantities. Due to the presence of abundant chelating functional groups, biological materials have greater affinity for metal ions.[29]​.

Metal precipitation

Most microorganism-mediated metal transformations, including oxidation-reduction and alkylation-dealkylation, have no known biological functions. These occur through processes whose primary functions are not related to the transformation of metals, but alter their solubility, mobility and toxicity and, therefore, can be used as remediation strategies. Basically two metal precipitation mechanisms can be distinguished: reductive precipitation and biomineralization.

• - Reductive precipitation: is one of the mechanisms that allows microorganisms to reduce the mobility and toxicity of a metal or metalloid, through its reduction to a lower redox state, offering potential applications for bioremediation. Many of the microorganisms that catalyze these reactions use metals or metalloids as electron acceptors during their anaerobic respiration, reducing highly soluble oxidized forms to insoluble elemental (reduced) forms, resulting in precipitation and/or detoxification of the contaminant. Sulfoxidizing bacteria (SSO) and sulfate-reducing bacteria (SRB) are geochemically important for reducing precipitation such as Cr and U. It has been shown that the reduction of U to U mediated by Fe-reducing microorganisms can effectively remove uranium from surfaces and water bodies; When coupled with a simple extraction technique, microbial U reduction can be used to concentrate uranium in uranium-contaminated soils.

• - Biomineralization: is the formation of insoluble metal precipitates due to interactions with products of microbial metabolism. The biomineralization of metals and metalloids in the form of sulfur, hydroxide, phosphate and carbonate minerals has potential applications for bioremediation. An example of this type of system is the use of anaerobic heterotrophic bacteria that use a series of organic substrates (ethanol, acetate, butyrate, cellulose) and sulfate as electron acceptors. The biotechnological potential of BSR lies mainly in the insolubility of toxic metal sulfides (Cu, Hg, Cd, As, Se and Pb) formed during the biological reduction of sulfate, resulting in their immobilization and detoxification. Metal sulfides are insoluble at neutral pH, and some of them are also insoluble under slightly acidic conditions. However, in certain circumstances, the immobilization of metal sulfides may be inappropriate, as is the case with the immobilization of mercury "Mercury (element)") as HgS, considered an environmentally unsafe practice.[30][31]​.

Bioleaching

Bioleaching involves the application of acidophilic microorganisms with oxidative metabolism based on Fe/S to promote the solubilization of metals from solid matrices; It is a consolidated technique in the mining industry and in biohydrometallurgy, where microorganisms mediate the solubilization of metals and metalloids from minerals and concentrates. It also has the potential for ex situ and in situ remediation of aquatic sediments that are contaminated with metals, which represents a key environmental problem of global interest. Consortia of Fe/S-oxidizing bacteria and other acidophilic microorganisms are applied in large-scale plants to enhance the extraction of precious and non-precious metals from sulfides or iron-bearing ores.

Sediments appear to be very difficult matrices compared to minerals. In particular, contaminating metals tend to associate with sediment components other than those of the primary mineral crystal lattice (e.g., adsorption on organic molecules, association as exchangeable ions). As a consequence, the knowledge and principles of bioleaching are specifically established for biomining and biohydrometallurgy, so they can only partially be applied to sediment bioleaching (e.g., metabolic pathways, metal solubilization mechanisms, microbial adaptation to minerals, etc.). Bacteria with these capabilities belong to the α-β- and γ-Proteobacteria (Acidithiobacillus, Acidiphilium, Acidiferrobacter, Ferrovum), Nitrospirae (Leptospirillum), Bacillota (Alicyclobacillus, Sulfobacillus), and Actinomycetota (Ferrimicrobium, Acidimicrobium, Ferrithrix). Archaea belong mainly to the Thermoproteota (Sulfolobus, Acidianus, Metallosphaera, Sulfurisphaera), in addition to two Fe(II) oxidizers belonging to the Methanobacteriota (Ferroplasma acidiphilum and Ferroplasma acidarmanus).[32]​.

Dyes are used by many industries (textile, leather tanning, paper, food, hair dyes, etc.). With an increasing global population, textile production remains one of the prominent industries that require large amounts of water and in turn produce highly contaminated wastewater that can cause serious ecological impact if released into the environment without proper treatment.[33] It has been reported that the use of synthetic dyes has increased rapidly in textile industries due to the cost-effectiveness in their synthesis and their high stability. Furthermore, various colors can be synthesized compared to natural dyes. However, dyes dissolved in water interfere with the penetration of sunlight (which delays photosynthesis) and inhibit the growth of aquatic flora and fauna by interfering with the solubility of gases.[34]​.

Although most dyes remain stable in the environment, the transformation or reduction of some of them has been detected in sediments. These dyes can be degraded or partially transformed in the water and sediments of rivers receiving treated wastewater discharges. In addition, some dyes and their transformation products produce carcinogenic effects.[35] Long-term exposure to carcinogenic chemicals can affect the aquatic biota of the river and also human health through drinking water. Therefore, it is important to investigate the fate of residual dyes and compounds derived from the decomposition of dyes in rivers and river sediments.[36]​.

While traditional physicochemical methods for the treatment of dyes involve remediation by chemical oxidation, adsorption, separation by membranes, separation by ion exchange and coagulation, treatments that involve the use of microorganisms as a bioremediation method include the use of various bacteria, fungi and algae.[37] For the most part, the mechanism by which they do this is through biosorption, while the rest vary between the absorption, accumulation and/or degradation of the dyes. dyes.[38]​.

Uno de los pilares de las ciencias aplicadas como la biotecnología, es la transferencia del conocimiento gestado y obtenido en el laboratorio, a instancias reales de aplicación concreta. Este salto tecnológico es frecuentemente difícil e inusualmente promovido en la ciencia. Sin embargo, la biotecnología incluye en sus cimientos la demanda de ser aplicada tanto para generar un producto, como para brindar un servicio.

En ese sentido, la biorremediación está destinada desde sus orígenes a ser pasible de ser aplicada en una locación con el fin de lograr un servicio concreto: el de la recuperación de una matriz luego de un evento de contaminación. Obviamente, el traslado al campo de las investigaciones realizadas a escala laboratorio es muchas veces complicada y es generalmente difícil de lograr, ya que las variables no controlables en el campo pueden generar un menor control sobre los experimentos, y por ende, una mayor distorsión de los resultados obtenibles.

En esta sección, solamente se hará hincapié sobre las dos estrategias de biorremediación de suelo a campo más extensamente utilizadas en la actualidad, y que mejores resultados han otorgado.

Landfarming

Landfarming is a field remediation method for soils that reduces the concentration of petroleum-derived hydrocarbons, both by evaporation and biodegradation. It involves the disposal of contaminated soil in a thin layer on the surface, favoring microbial activity through oxygenation and/or the addition of nutrients, minerals or increasing its humidity.[39] Some type of insulation is generally used underneath to prevent the migration of the contaminant to the soil on which the soil is placed. landfarming.

This strategy has proven to be effective for soil bioremediation, but given the fact that light chain hydrocarbons (VOC) are eliminated by evaporation, emissions to the atmosphere should be controlled, so that the strategy does not merely consist of the transfer of the contaminant from one matrix (soil) to another (air).

The implementation and design of this strategy is extremely simple to carry out. Likewise, it results in very low costs (30-60 USD per ton). However, it is usually not very efficient for highly contaminated soils, in addition to the fact that emissions of volatile compounds must be collected and filtered before being released into the atmosphere, making the process more expensive. This methodology has been used in a case of massive contamination in Kuwait after the Gulf War, reaching removal levels of more than 80%,[40] although due to the 50 °C temperature that can reach during the day, most of the removal was by evaporation.

Biopiles

Bioremediation using biopiles consists of excavating the contaminated soil and piling it up in “piles” in a specific treatment area, which must be isolated from the surrounding soil in order to avoid leaching of the contaminant. In these piles, microbial biodegradation is also favored through aeration (which can be by mixing or forced, in addition to the addition of agents that promote soil porosity such as wood chips, perlite, etc.), the addition of nutrients or the adjustment of humidity.[41] These piles can in turn be covered, favoring an increase in soil temperature and maintaining humidity at a stable level.

Compared to landfarming, this technique is also simple to design and carry out, but it has the advantage of generally requiring shorter treatment times, requiring a much smaller treatment area. The average cost is around $30 to $90 per ton (approximately the same as landfarming), but by allowing the design of closed systems that capture volatilizations, it becomes a more feasible and viable option. However, the use of biopiles still requires the treatment of volatile elements before their final disposal. Likewise, it is difficult for the use of this strategy to obtain removal percentages greater than 95%, given that the presence of compounds in diesel oil that are recalcitrant to biodegradation is common (at least in the first stage). The effectiveness of the biopile will largely depend on a combination of: i) the climatic variables (wind, snow, temperature), ii) the characteristics of the soil (pH, microbial load, salinity, humidity, amount of nutrients) and, iii) the characteristics of the contaminant, in terms of its volatility, structure, intrinsic concentration and toxicity.

• - Phytoremediation.

• - Biodegradability.

• - Bioremediation (Toxic Cleanup) News from Genome News Network (GNN).

• - Bioremediation Discussion Group (BioGroup).

• - Technology Focus on Bioremediation of Chlorinated Solvents USEPA Technology Innovation Program website Archived February 9, 2009 at the Wayback Machine.

• - Phytoremediation Missouri Botanical Garden website.

Biostimulation

Among bioremediation strategies, biostimulation is the most frequently used, and the one that has reported the best results and efficiency in the removal of petroleum-derived hydrocarbons in soils.[9]​[10]​[11]​ In essence, biostimulation seeks to take advantage of the catabolic capacities of the community of microorganisms already inhabiting the soil, by adapting the environmental factors that are limiting for microbial development and degradation of the contaminant. In general, the most incident factors that biostimulation usually investigates involve the availability of macronutrients (nitrogen and phosphorus), which is why biostimulation is generally linked to fertilization or the addition of these macroelements to the soil.[12] Although the concentration of N and P, and therefore the optimization of the C:N:P ratio in soil, have been usually referenced as the main variables that affect the removal of hydrocarbons when strategies are implemented of biostimulation,[13]​ there are also cases in which other characteristics need to be studied. Among them we can mention the water content in the soil (humidity), pH, oxygen availability and temperature.

Bioaugmentation

Bioaugmentation, on the other hand, consists of the inoculation of the contaminated matrix with microorganisms, or group of microorganisms (consortia), that have the catabolic capacity to partially or totally degrade the contaminant present in said matrix. It is usually applied when the rate of degradation of the contaminant in the uninoculated matrix is ​​not sufficient for it to be removed at convenient times.

The usefulness and efficiency of using bioaugmentation as a bioremediation method is controversial. Many times, the microorganism or group of microorganisms that are inoculated fail to establish themselves in the contaminated matrix, and are not detectable after a certain time of treatment.[14]​[15]​ This is mainly due to the fact that the microflora present in the soil is already adapted to the particular environmental conditions and biological interactions present in said soil, and does not allow the inoculated microorganisms to establish themselves in the soil. On the other hand, according to the literature, in most cases the use of bioaugmentation as a bioremediation technique usually presents less satisfactory results than those obtained with biostimulation. In some cases, this difficulty is overcome by adapting variables (bioaugmentation + biostimulation), while in others strains that have the capacity to produce surfactants are usually used, which seeks to increase the bioavailability of hydrocarbons for the rest of the microbial community.[16][17] This is the case, for example, of some members of the genus Rhodococcus, which produce biosurfactants that remain adhered to the membrane or cell wall, favoring the degradation of organic compounds.[18]​[3]​.

Added surfactants

In many cases, even though the soil has an adequate number and quality of microorganisms to carry out the remediation, and the physicochemical variables as well as the quantities of nutrients in the soil are adequate, the contaminants are not removed significantly. This usually happens when the contaminant is closely bound to soil particles, reducing their bioavailability. The addition of surfactants is used as a solution to this problem, because its amphiphilic character reduces the surface tension of water and aqueous compounds, favors the formation of micelles and increases the mobility of hydrophobic organic compounds. In this way, they help to solubilize contaminants, increasing their availability to the bacterial community and increasing their degradation rate.[19]​[20]​.

Contenido

La fitorremediación es la descontaminación de los suelos, la depuración de las aguas residuales o la limpieza del aire interior, usando vegetales, ya sean plantas vasculares, o algas (ficorremediación). También se incluye a los hongos (micorremediación), y por extensión a los ecosistemas que contienen estas plantas. La fitorremediación es un conjunto de tecnologías que utilizan las plantas para reducir, degradar o inmovilizar compuestos orgánicos contaminantes (naturales o sintéticos), de la tierra, del agua o del aire y que provienen de las actividades humanas. Esta técnica también puede tratar la contaminación por compuestos inorgánicos (metales pesados o radioisótopos) a través de procesos como la quimiadorción de metales pesados por medio de la celulosa presente en las plantas.[21]​.

Las plantas y sus organismos rizosféricos (asociados a la zona de la raíz) se pueden usar para fitorremediación de diferentes maneras. Existen diferentes métodos de fitorremediación que se agrupan en dos conjuntos, dependiendo si se utilizan como medio de concentración (rizofiltración, fitoestabilización y fitoinmovilización) o como medio de eliminación (fitodegradación, fitoextracción y fitovolatilización).[22]​.

Advantages

Phytoremediation has gained popularity in the last 10 years. This is based in part on its relatively low cost. Because biological processes are ultimately driven by the sun, phytoremediation is on average ten times cheaper than engineering-based methods (such as soil excavation). The fact that phytoremediation is typically carried out in situ contributes to its cost-effectiveness and can reduce exposure of contaminated substrate to humans, wildlife, and the environment. Phytoremediation is also popular among the general public as a "green" alternative to chemical plants and bulldozers.[23]​.

Although phytoremediation has advantages, it also has limitations. The plants that mediate the cleanup must be where the contaminant is and must be able to act on it. Therefore, soil properties, toxicity level and climate should allow plant growth. Phytoremediation is also limited by root depth, because plants have to be able to reach the contaminant.[22]​.

One way to optimize resources is the use of already dead and pulverized biomass for the treatment of water contaminated with heavy metals.[24]​.

Methods used in phytoremediation

Phytostabilization allows pollutants to be immobilized in the soil through their absorption and accumulation in the roots or by precipitation in the rhizosphere zone. This process reduces the mobility of contaminants and prevents their migration to groundwater or air. Phytostabilization is effective in fine-textured soils with high organic matter content. It is mainly applied to large areas where there is surface contamination. This technology has the advantages over other soil remediation methods that it is lower cost, easy to apply and aesthetically pleasing. Some plants used for phytostabilization purposes are: Hyparrhenia hirta (Pb); Zygophyllum fabago (Zn); Lupinus albus (Cd, As); Anthyllis vulneraria (Zn, Pb, Cd); Deschampsia cespitosa (Pb, Cd, Zn); Cardaminopsis arenosa (Cd, Zn); Horedeum vulgare, Lupinus angustifolius and Secale cereale (As); Lolium italicum and Festuca arundinaceae (Pb, Zn); and Brassica juncea (Cd, Zn, Cu, Mn, Fe, Pb). Eichhornia crassipes is an invasive plant used in phytostabilization processes with viable results.[25]​.

Phytoextraction or phytoaccumulation consists of the absorption of contaminating metals through the roots of plants and their accumulation in stems and leaves. The first step in applying this technique is the selection of the most suitable plant species for the metals present and the characteristics of the site. Once the vegetative development of the plant is complete, the next step is to cut them and proceed to incinerate them and transfer the ashes to a safety landfill. Phytoaccumulation can be repeated unlimitedly until the remaining concentration of metals in the soil is within the limits considered acceptable (depending on the legislation in force in that location). Some plants used for this phytocorrective technique are: Thlaspi caerulescens (Cd); Sedum alfredii, Viola baoshanensis and Vetiveria zizanioides (Zn, Cd, Pb); Alyssum murale, Trifolium nigriscens, Psychotria douarrei, Geissois pruinosa, Homalium guillainii, Hybanthus floribundus, Sebertia acuminata, Stackhousia tryonii, Pimelea leptospermoides, Aeollanthus biformifolius and Haumaniastrum robertii (Ni); Brassica juncea, Helianthus annuus, Sesbania drummondii (Pb); Brassica napus (Cu, Pb, Zn); and Pistia stratiotes (Ag, Cd, Cr, Cu, Hg, Ni, Pb, Zn).

Rhizofiltration uses plants to remove contaminants from the water environment through the roots. In rhizofiltration these plants are grown hydroponically. When the root system is well developed, plants are introduced into water contaminated with metals, where the roots absorb and accumulate them. As the roots become saturated, the plants are harvested and disposed of for final use. There are a large number of studies related to the contaminant accumulation capacity of various aquatic plants, some examples of them are: Scirpus lacustris (Cd, Cu, Pb, Mg, Fe, Se, Cr), Lemna gibba (Pb, As, Cu, Cd, Ni, Cr, Al, Fe, Zn, Mn), Azolla caroliniana (Hg, Cr Sr, Cu, Cd, Zn, Ni, Pb, Au, Pt), Elatine trianda (As), Wolffia papulifera (Cd), Polygonum punctatum (Cu, Cd, Pb, Se, As, Hg, Cr, Mn) and Myriophylhum aquaticum, Ludwigina palustris and Mentha aquatica (Cu, Zn, Mn, Fe, Ni).

Phytovolatilization occurs as trees and other growing plants absorb water along with organic and inorganic contaminants. Some of these can reach the leaves and evaporate or volatilize into the atmosphere. Through this process, contaminants such as: volatile organic compounds (benzene, nitrobenzene, toluene, ethylbenzene and xylene), As, Se and Hg have been eliminated. The plants Salicornia bigelovii, Brassica juncea, Astragalus bisulcatus and Chara canescens have been used for the remediation of sites contaminated with Se and Arabidopsis thaliana for Hg.

In phytostimulation, the roots of the plants stimulate the microorganisms present in the area to be treated, and these degrade the contaminants.

In phytodegradation, plants and the microorganisms associated with them degrade organic contaminants into harmless products or mineralize them to CO and HO. In this process, contaminants are metabolized within plant tissues, and plants produce enzymes such as dehalogenase and oxygenase, which help catalyze the degradation. Phytodegradation has been used for the removal of explosives such as TNT, halogenated hydrocarbons, Bisphenol A, PAHs, and organochlorine and organophosphate pesticides.

Hydrocarbon bioremediation is used to reduce or eliminate contamination of soils contaminated with, for example, petroleum, diesel, oil and grease from various sources, including accidental or industrial spills, using microorganisms.

Hydrocarbons are compounds formed almost exclusively by carbon and hydrogen atoms, considered basic compounds of organic chemistry. Its use has become an important driver for the development of humanity, but, nevertheless, it has given rise to different catastrophic events at an environmental level due to the contamination of terrestrial and aquatic ecosystems as a result of the spill of these petroleum compounds and their derivatives. In the case of soils, the main environmental consequences that occur after a contamination event are: the reduction or inhibition of the development of vegetation cover, changes in the population dynamics of fauna, microbial biota and contamination by infiltration into underground water bodies.[26]​.

In bioremediation, biostimulation and bioaugmentation are presented as one of its main strategies, the first being defined as the addition of nutrients, mainly sources of nitrogen and phosphorus, to promote microbial growth and development, as well as stimulation through the addition of water, oxygen and other elements that improve the development of microorganisms. The use of waste sludge (biosolids) as a source of macro- and micronutrients can favor the stimulation of native soil microorganisms and cause the degradation of hydrocarbons, which are used as a source of carbon and electron donors, coupling the oxidation-reduction reaction with oxygen, which plays the role of electron acceptor.[27]​.

La industrialización global está cumpliendo las demandas "Demanda (economía)") de la población moderna a costa de la exposición ambiental a diversos contaminantes, incluidos los metales pesados. Estos metales afectan la calidad del agua y del suelo. Además, éstos entran en la cadena alimentaria y exhiben sus efectos letales en la salud humana, incluso cuando están presentes en una concentración ligeramente más alta que la requerida para el metabolismo normal; en el peor de los casos, los metales pesados pueden volverse cancerígenos. Por esto, la extracción eficiente de metales pesados presenta una demanda creciente en todo desarrollo sustentable. El enfoque de la biorremediación presenta una alternativa espléndida para los métodos caros e ineficientes convencionales para la eliminación de metales pesados.[28]​.

Biosorption

Biosorption is a subcategory of adsorption, where the sorbent is a biological matrix. Biosorption is a process of rapid and reversible binding of ions from aqueous solutions to functional groups that are present on the surface of biomass. This process is independent of cellular metabolism. It can be performed in a wide range of pH values ​​3–9 and temperature values ​​4–90 °C. This process does not require a large capital investment, so operating costs are economical. Furthermore, biological materials are often cheap and can be obtained from agriculture or industrial waste. The characteristics of biosorption, compared to those of conventional treatment methods, include low cost, high efficiency, minimization of chemical and/or biological sludge, does not require the additional addition of nutrients, the biosorbent is regenerable, and allows the possibility of recovering metals.

In recent years, the biosorption process has become an economical and environmentally friendly alternative treatment technology in the water and wastewater industry. As a result, several biosorbents have been developed that are successfully used to treat various contaminants, including metals, dyes, phenols, fluoride and pharmaceutical products in solutions (aqueous and/or oily). A wide range of naturally available biomaterials have been used as biosorbents for the desired removal of contaminants. All types of microbial, plant and animal biomass and their derived products are of great interest regarding their relationship with a wide variety of substances. Many of these biological materials, especially bacteria, cyanobacteria, algae (including microalgae, macroalgae, kelp), yeast, fungi and lichens have attracted much attention for the removal and recovery of heavy metal ions due to their good performance, low cost and availability in large quantities. Due to the presence of abundant chelating functional groups, biological materials have greater affinity for metal ions.[29]​.

Metal precipitation

Most microorganism-mediated metal transformations, including oxidation-reduction and alkylation-dealkylation, have no known biological functions. These occur through processes whose primary functions are not related to the transformation of metals, but alter their solubility, mobility and toxicity and, therefore, can be used as remediation strategies. Basically two metal precipitation mechanisms can be distinguished: reductive precipitation and biomineralization.

• - Reductive precipitation: is one of the mechanisms that allows microorganisms to reduce the mobility and toxicity of a metal or metalloid, through its reduction to a lower redox state, offering potential applications for bioremediation. Many of the microorganisms that catalyze these reactions use metals or metalloids as electron acceptors during their anaerobic respiration, reducing highly soluble oxidized forms to insoluble elemental (reduced) forms, resulting in precipitation and/or detoxification of the contaminant. Sulfoxidizing bacteria (SSO) and sulfate-reducing bacteria (SRB) are geochemically important for reducing precipitation such as Cr and U. It has been shown that the reduction of U to U mediated by Fe-reducing microorganisms can effectively remove uranium from surfaces and water bodies; When coupled with a simple extraction technique, microbial U reduction can be used to concentrate uranium in uranium-contaminated soils.

• - Biomineralization: is the formation of insoluble metal precipitates due to interactions with products of microbial metabolism. The biomineralization of metals and metalloids in the form of sulfur, hydroxide, phosphate and carbonate minerals has potential applications for bioremediation. An example of this type of system is the use of anaerobic heterotrophic bacteria that use a series of organic substrates (ethanol, acetate, butyrate, cellulose) and sulfate as electron acceptors. The biotechnological potential of BSR lies mainly in the insolubility of toxic metal sulfides (Cu, Hg, Cd, As, Se and Pb) formed during the biological reduction of sulfate, resulting in their immobilization and detoxification. Metal sulfides are insoluble at neutral pH, and some of them are also insoluble under slightly acidic conditions. However, in certain circumstances, the immobilization of metal sulfides may be inappropriate, as is the case with the immobilization of mercury "Mercury (element)") as HgS, considered an environmentally unsafe practice.[30][31]​.

Bioleaching

Bioleaching involves the application of acidophilic microorganisms with oxidative metabolism based on Fe/S to promote the solubilization of metals from solid matrices; It is a consolidated technique in the mining industry and in biohydrometallurgy, where microorganisms mediate the solubilization of metals and metalloids from minerals and concentrates. It also has the potential for ex situ and in situ remediation of aquatic sediments that are contaminated with metals, which represents a key environmental problem of global interest. Consortia of Fe/S-oxidizing bacteria and other acidophilic microorganisms are applied in large-scale plants to enhance the extraction of precious and non-precious metals from sulfides or iron-bearing ores.

Sediments appear to be very difficult matrices compared to minerals. In particular, contaminating metals tend to associate with sediment components other than those of the primary mineral crystal lattice (e.g., adsorption on organic molecules, association as exchangeable ions). As a consequence, the knowledge and principles of bioleaching are specifically established for biomining and biohydrometallurgy, so they can only partially be applied to sediment bioleaching (e.g., metabolic pathways, metal solubilization mechanisms, microbial adaptation to minerals, etc.). Bacteria with these capabilities belong to the α-β- and γ-Proteobacteria (Acidithiobacillus, Acidiphilium, Acidiferrobacter, Ferrovum), Nitrospirae (Leptospirillum), Bacillota (Alicyclobacillus, Sulfobacillus), and Actinomycetota (Ferrimicrobium, Acidimicrobium, Ferrithrix). Archaea belong mainly to the Thermoproteota (Sulfolobus, Acidianus, Metallosphaera, Sulfurisphaera), in addition to two Fe(II) oxidizers belonging to the Methanobacteriota (Ferroplasma acidiphilum and Ferroplasma acidarmanus).[32]​.

Dyes are used by many industries (textile, leather tanning, paper, food, hair dyes, etc.). With an increasing global population, textile production remains one of the prominent industries that require large amounts of water and in turn produce highly contaminated wastewater that can cause serious ecological impact if released into the environment without proper treatment.[33] It has been reported that the use of synthetic dyes has increased rapidly in textile industries due to the cost-effectiveness in their synthesis and their high stability. Furthermore, various colors can be synthesized compared to natural dyes. However, dyes dissolved in water interfere with the penetration of sunlight (which delays photosynthesis) and inhibit the growth of aquatic flora and fauna by interfering with the solubility of gases.[34]​.

Although most dyes remain stable in the environment, the transformation or reduction of some of them has been detected in sediments. These dyes can be degraded or partially transformed in the water and sediments of rivers receiving treated wastewater discharges. In addition, some dyes and their transformation products produce carcinogenic effects.[35] Long-term exposure to carcinogenic chemicals can affect the aquatic biota of the river and also human health through drinking water. Therefore, it is important to investigate the fate of residual dyes and compounds derived from the decomposition of dyes in rivers and river sediments.[36]​.

While traditional physicochemical methods for the treatment of dyes involve remediation by chemical oxidation, adsorption, separation by membranes, separation by ion exchange and coagulation, treatments that involve the use of microorganisms as a bioremediation method include the use of various bacteria, fungi and algae.[37] For the most part, the mechanism by which they do this is through biosorption, while the rest vary between the absorption, accumulation and/or degradation of the dyes. dyes.[38]​.

Uno de los pilares de las ciencias aplicadas como la biotecnología, es la transferencia del conocimiento gestado y obtenido en el laboratorio, a instancias reales de aplicación concreta. Este salto tecnológico es frecuentemente difícil e inusualmente promovido en la ciencia. Sin embargo, la biotecnología incluye en sus cimientos la demanda de ser aplicada tanto para generar un producto, como para brindar un servicio.

En ese sentido, la biorremediación está destinada desde sus orígenes a ser pasible de ser aplicada en una locación con el fin de lograr un servicio concreto: el de la recuperación de una matriz luego de un evento de contaminación. Obviamente, el traslado al campo de las investigaciones realizadas a escala laboratorio es muchas veces complicada y es generalmente difícil de lograr, ya que las variables no controlables en el campo pueden generar un menor control sobre los experimentos, y por ende, una mayor distorsión de los resultados obtenibles.

En esta sección, solamente se hará hincapié sobre las dos estrategias de biorremediación de suelo a campo más extensamente utilizadas en la actualidad, y que mejores resultados han otorgado.

Landfarming

Landfarming is a field remediation method for soils that reduces the concentration of petroleum-derived hydrocarbons, both by evaporation and biodegradation. It involves the disposal of contaminated soil in a thin layer on the surface, favoring microbial activity through oxygenation and/or the addition of nutrients, minerals or increasing its humidity.[39] Some type of insulation is generally used underneath to prevent the migration of the contaminant to the soil on which the soil is placed. landfarming.

This strategy has proven to be effective for soil bioremediation, but given the fact that light chain hydrocarbons (VOC) are eliminated by evaporation, emissions to the atmosphere should be controlled, so that the strategy does not merely consist of the transfer of the contaminant from one matrix (soil) to another (air).

The implementation and design of this strategy is extremely simple to carry out. Likewise, it results in very low costs (30-60 USD per ton). However, it is usually not very efficient for highly contaminated soils, in addition to the fact that emissions of volatile compounds must be collected and filtered before being released into the atmosphere, making the process more expensive. This methodology has been used in a case of massive contamination in Kuwait after the Gulf War, reaching removal levels of more than 80%,[40] although due to the 50 °C temperature that can reach during the day, most of the removal was by evaporation.

Biopiles

Bioremediation using biopiles consists of excavating the contaminated soil and piling it up in “piles” in a specific treatment area, which must be isolated from the surrounding soil in order to avoid leaching of the contaminant. In these piles, microbial biodegradation is also favored through aeration (which can be by mixing or forced, in addition to the addition of agents that promote soil porosity such as wood chips, perlite, etc.), the addition of nutrients or the adjustment of humidity.[41] These piles can in turn be covered, favoring an increase in soil temperature and maintaining humidity at a stable level.

Compared to landfarming, this technique is also simple to design and carry out, but it has the advantage of generally requiring shorter treatment times, requiring a much smaller treatment area. The average cost is around $30 to $90 per ton (approximately the same as landfarming), but by allowing the design of closed systems that capture volatilizations, it becomes a more feasible and viable option. However, the use of biopiles still requires the treatment of volatile elements before their final disposal. Likewise, it is difficult for the use of this strategy to obtain removal percentages greater than 95%, given that the presence of compounds in diesel oil that are recalcitrant to biodegradation is common (at least in the first stage). The effectiveness of the biopile will largely depend on a combination of: i) the climatic variables (wind, snow, temperature), ii) the characteristics of the soil (pH, microbial load, salinity, humidity, amount of nutrients) and, iii) the characteristics of the contaminant, in terms of its volatility, structure, intrinsic concentration and toxicity.

• - Phytoremediation.

• - Biodegradability.

• - Bioremediation (Toxic Cleanup) News from Genome News Network (GNN).

• - Bioremediation Discussion Group (BioGroup).

• - Technology Focus on Bioremediation of Chlorinated Solvents USEPA Technology Innovation Program website Archived February 9, 2009 at the Wayback Machine.

• - Phytoremediation Missouri Botanical Garden website.