Bioremediation: Concepts, Management, Strategies and Applications
Introduction—Environmental degradation and approaches for environmental repairs
An ecologically healthy and equilibrated environment is the foundation of human life. It provides us with the goods and services that we need to survive and prosper. However, the planet is becoming more and more degraded (Diehl 2018, IPCC 2019). Environmental degradation is any process that reduces the aptitude of a given ecosystem to sustain life. This process is related to biological and/or physical vicissitudes that affect ecological stability.
Such alterations usually modify natural fauna and flora, sometimes causing biodiversity loss, in terrestrial or aquatic systems (Figures 1 and 2). Although they may occur due to natural factors, problems concerning environmental degradation are habitually associated with anthropogenic actions, modifying the trajectory of the evolution of the environment (Tripathi et al. 2017).
Aiming to maintain the ecological health and the quality of the ecosystems services provided by the forests, oceans, rivers, and others ecosystems, currently, there are two options: (i) conserving the remaining original, pristine ecosystems (natural capital), and (ii) restoring the degraded ones (Silva and Rodgers 2018, Arponen 2019).
Several techniques and approaches have been developed in order to fulfill the second option (restore or repair degraded ecosystem): stop the degradation process and/or repair the degradation by means of interventions that might restore the original ecological conditions of the degraded ecosystem, reclaim it, or rehabilitate it. The concepmal differences between these three approaches are depicted in Figure 3.
Figure 1. Types of degradation in soil-related systems. Source: (Lai 2015) - modified.
Figure 2. Types of degradation in water recourses-related systems. Original figure inspired by the work of Lai (2015).
Figure 3. Conceptual differences between the three approaches. Source: modified from (SER 2017).
Activities that aim to repair damaged ecosystems may range from (a) local to (b) regional scale, and from (i) efforts of benevolent volunteers to (ii) logistical projects of the multi-agencies. We find interventions varying from (a) the “do nothing” attitude (i.e., just removing the degradation factor(s) and allowing the natural succession of the environment) to (b) a variety of abiotic and biotic interventions designed at speeding up or shifting the course of ecosystem recovery (Trujillo- Miranda et al. 2018, Rydgren et al. 2019). However, even in very resilient ecosystems, when degradation is severe, advanced or prolonged (or both), the ecosystem may be impotent to entirely recover on its own. This is when restoration practitioners can step in Aronson et al. (2016).
One of the most important options for repair degraded ecosystems is a set of techniques and approaches named bioremediation. Such set of techniques consist chiefly in using biological organism (several species of plants, as well as numerous species of microorganisms) as an agent of extraction, accumulation, and/or transformation (complexation or degradation) of chemical composites, in order to diminish or eradicate the toxicity of the compost. Recovery of contaminated soils, effluent and waste treatment, and cleaning of pipelines and equipment, constitute some examples of the wide application of the bioremediation.
Concept and categories of bioremediation
The central point of the bioremediation process is the mechanism of transformation of a contaminant performed by a microorganism or plant (Varjani et al. 2018). Bioremediation embraces a set of biotreatment processes that cover diverse types of biochemical mechanisms that may lead to a humification, target’s mineralization, the partial transformation of a composite or altered redox state for metallic elements, for example (Bharagava and Saxena 2020). It is viewed as the safest method to combat some kinds of degraded environments with anthropogenic composites in ecosystems (Paliwal et al. 2012). Environmentally responsive and advantageous cost-saving feature are amongst the major advantages of bioremediation related to both chemical and physical approaches of remediation (Azubuike et al. 2016).
The primal role in bioremediation is that of the interplay of metabolic features of the plant or microbial communities living within that hampered ecosystem (Paliwal et al. 2012). Nonbiological remediation technologies (e.g., excavation, pump-and-treat systems) and bio/phytoremediation might complement each other and they’re not mutually exclusive (Pilon-Smiths 2005).
The central difference between bio, phyto, and phycoremediation is the categoiy of living organisms used in each method (Adams et al. 2015, Biswas et al. 2015, Azubuike et al. 2016). Normally the literature considers as bioremediation the microbiological-related processes of
Figure 4. Graphical depiction of the concept of bioremediation and its sub-groups. Diagram elaborated with data provided
by Velazquez-Femandez and Muniz-Hemandez (2014).
remediation, and due to this, the phytoremediation and phycoremediation are placed in a different categoiy. However, the term bioremediation is here considered as the overall set of techniques that might be sub-divided into three categories: phytoremediation and phycoremediation and micro bioremediation (Figure 4).
We have four major biological agents in bioremediation: (i) vegetation, especially the root system of vascular plants, and the microbiological community, especially (ii) bacteria, (iii) algae, and (iv) fungi. Especially in opened sites and in situ techniques (concept explained ahead) the vegetation has been considered, under the variability of environmental conditions, as an agent of acceleration of the process of degradation of organic chemical residues in soils normally in association with a microorganism community (Burges et al. 2018).