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Bioremediation

Soil bioremediation can be defined as “the use of biologically mediated processes to detoxify, degrade or transform pollutants to an innocuous state”. This approach is based on the capacity of many microorganisms to use hydrocarbons as a source of C and energy (biodegradation), transforming or mineralizing these pollutants into less harmfiil or nonhazardous substances, which are then incorporated into natural biogeochemical cycles. Zobell (1946) was the first who demonstrated the microbial ability of using PHCs as C source and the wide distribution of these microorganisms in nature. The advantages of bioremediation with respect to other approaches are that: (i) it is cost- effective, versatile, and easy to cany out; (ii) it uses biological inputs such as microbes (making this technique ecofriendly); and (iii) there are no side effects (Kumar and Yadav 2018).

Each soil ecosystem has an intrinsic PHC-biodegradation capacity, which is known as natural attenuation (combined result of chemical oxidation, photo-oxidation, evaporation, and microbial mineralization). This means that bioremediation can occur on its own via natural attenuation in any soil, but it would take a long time in most of the cases (Polyak et al. 2018). Three groups of factors limit soil PHC-biodegradation: (i) the characteristics of the indigenous microbial communities (in terms of taxonomy, gene regulation and expression, metabolic diversity, tolerance to metals and other toxic xenobiotics, substrate uptake or adherence mechanisms, chemotaxis, and biofilm formation); (ii) enviromnental conditions (i.e., pH, temperature, pressure, moisture, salinity, and availability of nutrients and terminal electron acceptor groups); and (iii) chemical and physicochemical properties of the PHCs (i.e., solubility, concentration, hydropliobicity, volatility, and molecular mass) (Gkorezis et al. 2016). Bioremediation seeks the way to stimulate and/or improve the biodegradation potential of a soil. Examples of approaches developed for the improvement of this potential are: phytoremediation, bioaugmentation, and biostimulation. Bioremediatiou ofPHC- contaminated soils has greatly attracted the interest of environmental scientists and engineers as demonstrated by the high number of scientific articles published in the last 20 years (period 1998— 2018) on this topic (Figure 2).

Number of publications found on Scopus database using the searching terms “soil AND bioremediation AND

Figure 2. Number of publications found on Scopus database using the searching terms “soil AND bioremediation AND

hydrocarbon” in the period 1998-2018.

Phytoremediation

Pliytoremediation consists in planting and growing living plants in PHC-coutaminated soils. A useful plant for bioremediation purposes should meet as many as possible of the following requirements: (i) a fibrous root system; (ii) its biomass above the ground shouldn't be consumed by animals; (iii) tolerant to high PHC concentrations; (iv) fast growing rate; (v) versatile to growth under different environmental conditions; (vi) quick capacity to reach the required level of PHC degradation; and (vii) resistance toward diseases and pests (Kumar and Yadav 2018). Some examples of plants showing to be usefiil for phytoremediation purposes include: (i) uative/wild grasses (e.g., Italian ryegrass (Lolium multiftorum), forage glass (Brachiaria brizantha), tall fescue (Festnca anmdinacea), or smooth flatsedge (Cyperus laevigatas)); (ii) legumes (e.g., white clover (Trifolium repeus), broad bean (Vicia faba), medik (Melilotus albas), or alfalfa {Medicago sativa)): (iii) agronomic crops (e.g., rice (Oryza sativa), wheat (Triticum aestivum), maize (Zea mays), or Chinese cabbage (Brassica chinensis)): (iv) ornamental plants (e.g., blanket flower (Gaillardia aristata), devil’s beggarticks (Bidens frondosa), or purple coneflower {Echinacea purpurea)): and (v) shrubs and trees (e.g., flame tree {Delonix regia), poplar {Populus deltoides x nigra), lebbeck {Albizia lebbeck), or cassod tree {Cassia siamea)) (Correa-Garcia et al. 2018, Hussain et al. 2018, Hunt et al. 2019). Pliytoremediation is a cost-effective technology driven by solar energy and results in a natural cleaning of the environment that can be conducted either in situ or ex situ. Plants can clean up a soil using five different mechanisms (Femandez-Luqueno et al. 2017):

  • • Phytoextraction: plants transport PHCs from the soil via roots and accumulate them into the shoots.
  • • Phytodegradation: PHCs are taken up by the plants and degraded in their tissues. Plants can also synthesize and release enzymes into the soil, which degrade PHCs.
  • • Phytovolatilization: plants take PHCs up, convert them into nontoxic volatile substances, and subsequently release them into the atmosphere.
  • • Phytostabilization: bioavailability of the PHCs gets reduced because they are bound by the plants and immobilized.
  • • Phytostimulation: abundance, diversity and PHC-degrading capability of microorganisms living in the plant rhizosphere is stimulated through the exudation by roots of different organic compounds. This process is also known as rhizoremediatiou. Rhizosphere also plays other important roles during PHC-bioremediation since it has been recognized as a hotspot for horizontal gene transfer and some root exudates have been shown to detach organic contaminants from the soil organic matter, making them more available to microbes (Correa- Garcia et al. 2018).
 
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