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In situ and ex situ techniques

Currently, we have techniques and approaches designed to remediate both terrestrial and aquatic environments considered degraded (Lai 2015, Shishir et al. 2019). In general terms, the techniques are categorized as ex situ and in situ (Gomes et al. 2013, Lai 2015, Azubuike et al. 2016) (Figure 5).

In situ bioremediation technologies encompass the treatment and manipulation of the contaminants in the local itself (Wadgaonkar et al. 2019). Amidst the most common techniques

Illustrations of the two major groups of remediation technologies

Figure 5. Illustrations of the two major groups of remediation technologies.

categoiized as in situ, we mention the passive remediation or monitored natural attenuation, also named as “do nothing” approach as mentioned earlier. Furthermore, another major group of in situ techniques is constituted by a set of techniques that are embraced in a categoiy named “enhanced techniques” (Table 1).

Ex situ techniques are the actions and treatments that eliminate contaminants at a distinct and separate treatment facility (Wadgaonkar et al. 2019). In works involving the ex-situ bioremediation approach, we might cite the bioreactors as a usual technique (Table 2). Also, nutrients may be added in order to accelerate the chemical or physical decomposition of environmental pollutants. For instance, the ex situ remediation of heavy metals in soil is further improved by the

Table 1. Explanations of the two main categories of techniques of in situ bioremediation.

Technique

Description

Monitored, natural attenuation

Tins technique establishes a manner of reducing the mass or mobility, as well as the level of toxicity of a contaminant without human influence (EPA1999). It is based on the premise that under favorable circumstances, certain contaminants may be transformed, degraded, immobilized, as well as detoxified naturally, without any human interference (Sayler et al. 1995). The attenuation occurs due to the chemical, physical and biological processes of degradation of the contaminant (Scow and Hicks 2005). Among the mam features and assumptions of this technique are:

(i) no site manipulation is required, (u) we assume that the process of contamination is controlled and finished (no more contaminant is launched into the local environment), (m) some local environmental features (i.e., presence or absence of sunlight, level of soil moisture, soil pH, aeration and/or level of oxygen of the water (if waterbody), environmental temperature, among others) may favor transformations of the contaminant, (iv) nature and level of concentration of the contaminant.

Tins technique or approach presents as main advantages the low cost, the no necessity of interference, and the no or minimal necessity of human contact with the contaminant and/or contaminated environment (Naeem and Qazi 2019). On the other hand, as mam disadvantages, we cite the long-term to adequately transform the contaminant into a non-toxic product, and the dependence of specific site (or local) conditions to execute the work.

Enhanced

techniques

Biosluiping and Bioventing

Constitutes a mix of two processes: bioventing and vacuum-enhanced free-product recovery. Biosluiping is an excellent alternative in in situ treatment as it also remediates floating waste elements on top of the groundwater. This method applies a vacuum to extract, water, soil vapor, and additional free products from the subsurface Next, such products are separated and then treated for biodegradation (Varshney 2019) . On the other hand, the process of bioventing complements bioshupmg, once it involves the injection of an into the contaminated environment (pumps the an only into the unsaturated or vadose zone) at a rate established to maxmuze local biodegradation and minimize (or eluninate) the off-gassing of volatilized elements to the atmospheric environment. This process is also capable of degrading less volatile organic molecules. Due to the reduced volume of air required, it allows for the treatment of less permeable soils (Khan et al. 2004).

Biosparging

It involves pumping an and, if necessary, nutrients into the saturated zone (Varshney 2019). Tins technique is very similar to bioventmg in that an is injected into soil subsurface to activate microbial activities, in order to stimulate pollutant elnnination from polluted sites. However, different from bioventing, a volume of air is mjected in the saturated layer or zone. Such process may cause upward dislocation of volatile organic molecules to the unsaturated zone to facilitate the process of biodegradation (Maitra 2018).

Phytoremediation

It means the use of plants and, habitually, then associated microbes for environmental cleanup, whatever the kind of substrate: solid, liquid or gaseous (Pilon-Smiths 2005). Rluzobactena constitutes an unportant group of microorganisms that live in the plant root system and the interaction with the plants is usually mutual or symbiotic (i.e., they are not pathogenic microorganisms), since the microorganisms produce and liberate some plant growth products as hormones.

Table 2. Description of the major groups of techniques of ex situ bioi emediation.

Technique

Description

Windrow (or Biopiles) composting

It is a technique for purifying contaminated soils or dried sediments and consists of using appropnate bacteria communities to eluninate pollutants. We can cite the main advantages of the biopile technology: (a) windrow systems are relatively easy to design and make;

(b) remediation can be finished in a reasonably short tmie (3 to 6 months); (c) windrows may be cost-competitive with landfilling and are preferred over landfilling; (d) windrow technology is effective on orgamc contaminants that are difficult to desorb. On the other hand, tlie technology presents as the chief limitation the fact that it may not be effective for high contaminant concentrations of some kmd of contaminants (Gomes et al. 2013).

There are two basic categories of biopiles for bioremediation of contaminated soils, where the main difference is related to the aeration system: (i) static windrow, where the aeration process is forced by means of perforated pipes installed on the bottom of the pile. They are connected to a blower (vacuum pump or an exhaust-driven wind are normally considered too);

(ii) dynamic windrow, where the aeration is executed by means mtervallic soil tillage, analogous to the procedure applied to the composting windrows (Lopes et al. 2014).

Windrow composting

This technique has been performed using the foliowmg steps: (i) initially, contaminated material (soils) is excavated and sieved to remove large-sized debris; next (ri) the sieved material is transported to a composting pad with a provisional protected structure to provide protection from climate extremes. Additional materials, such as straw, manure, agricultural wastes, and wood chips, may be used as bulking facilitators and also as a supplemental carbon. Hence, the materials (soil and amendments) are arranged m long piles, named windrows.

The windrow is systematically mixed by filming with an adequate tractor. Indicators such as moisture, temperature, pH, and gas concentration are usually monitored. Once the process of composting is finalized (after approximately 40 days), the windrows are disassembled and the compost is taken to the final disposal (ishnoi and Dixit 2019).

Bioreactor

Commonly considered in clayed soils. The most common type of bioreactors for treatment contaminated soils are mud reactors or slimy reactors. In this technology, after excavation and screening, the contaminated soil is mixed with an aqueous phase (which may contain microorganisms and/or nutrients and or surfactants). The “mud” generated contams more or fewer solids (from 10 to 40%) depending on soil type, Stirling equipment and aeration system available. The treated sludge is usually dehydrated or alternatively may undergo solid-phase bioremediation. Another option in terms of bioreactor configuration is the reactor’s solid phase, where it works with reduced soil moisture contents (approximately 15%) (Tnpathi et al. 2017, Naeem and Qazi 2019).

Land fanning

This technique does not require an extensive preliminary assessment of polluted sites prior to remediation. Tins makes the preliminary stage short, and the work simpler and cheaper (Azubuike et al. 2016). It is a method that consists of the biological degradation of contaminants into an upper layer of soil that is periodically turned over for aeration.

The spreading of contaminating oily material on the soil and incorporation into the arable layer, also named reactive layer, may duectly and differentially affect the microorganisms responsible for biodegradation. Microbial biodegradation, which is the primary mechanism for tlie elimination of organic pollutants from the environment, forms the basis of this treatment, and the maintenance of an active heterotropluc microbial community is very unportant (Bicki and Felsot 2018).

addition of organic amendments like biosolid, compost, and municipal solid waste, which is used as both nutrients and conditioners (Varjani et al. 2018).

The induction of the process of bioremediation

In terms of induction of the process of transformation by microorganisms, three approaches can be considered (Adams et al. 2015, Shishir et al. 2019): bio-augmentation, bio-stimulation and bio-enhancement (Table 3). Such approaches might be considered for work with indigenous microorganisms or by means of inoculation of exogenous microbial species that were previously isolated in culture media.

Table 3. Mechanisms of induction of bioremediation.

Name of the mechanism of induction

Description

Bio-augmentation

It is the inoculation at the contaminated site with microorganisms selected to augment the remediation process.

Bio-stimulation

It consists of the alteration of the environmental conditions to stimulate the degradation of contaminants. Tins service usually facilitates the work of microorganisms.

Bio-enhancement

When nutrients are provided to enhance the site for the native microorganisms. Furthermore, other envuonmental factors may be modified m order to improve the working conditions of microorganisms.

Source: Information compiled from Adams et al. (2015) and Shishir et al. (2019).

Table 4. Technologies embraced by phytoremediation. Explanation and comparison.

Approach

Description

Phytostabilization

Usually applied in orgamcs and metals. Because the contaminant is treated in situ (retained), land cover (vegetation) is maintained.

Phytodegradation/Rhizodegradation

Mostly for organic products, which are attenuated in situ. Vegetation cover is preserved. Complementarity, rhizodegradation occurs in a region surrounding the plant roots. It is an integrative approach, where that the exudates from plants stimulate rhizosphere bacteria to improve biodegradation of pollutants.

Phytovolatilization

Mostly for organic products, which are removed in situ. Vegetation cover is preserved.

Phytoextraction

Used frequently for metals, which are removed in situ. Vegetation cover is harvested continually.

Source: Modified from Mahar et al. (2016).

Regarding phytoremediation, this kind of technology is accessible for various environments and categories of contaminants. However, this technology has limited application where the concentrations of contaminants are toxic to plants (Mahar et al. 2016). When we consider vascular plants, the literature points out four basic mechanisms (Gratao et al. 2005, Pilon-Smitlis 2005, Mahar et al. 2016): phytostabilization, phytodegradation (some researchers consider rhizodegradation as part of this category), phytovolatilization, and phytoextraction (Table 4).

 
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