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Environmental Sciences
Bioremediation is a process that uses biological organisms to remove or re-treat an environmental pollutant through metabolic processes and plants to eradicate hazardous pollutants and restore the ecosystem to its original condition.
- bioremediation
- landfill
- soil
1. Bioremediation Techniques
Bioremediation techniques have been the subject of numerous studies and experiments to evaluate their effectiveness and applicability in the removal of certain pollutants. This systematic review focuses mainly on the techniques most suitable for the bioremediation of specific pollutants such as heavy metals, hydrocarbons, polymers and their derivatives, and finally organic pollutants. In the wider applicability of the studies mentioned here, it was found that the use of microbial consortia for the bioremediation of heavy metals [11,12,13,14,15,16] and hydrocarbons [17,18,19,20,21,22,23] gave satisfactory results with significant removals. For the biodegradation of polymers, it was found that certain bacterial strains were able to biodegrade plastics in soil [19,24,25,26,27,28], in contrast to other studies that used bacterial consortia [29,30,31,32], resulting in a significant reduction in these contaminants as well as a reduction in biodegradation time by 90 days [30] and 30 days [31], demonstrating that the symbiotic capacity of bacteria can be a viable option. Finally, there is the bioremediation of organic matter, the most commonly used techniques were the isolation of bacteria [33,34], and some studies specifically investigated the bioremediation capacity of Pseudomonas [35,36,37] and bacterial mixtures [38,39,40,41].
The analysis conducted in this systematic review showed that the most commonly used techniques for bioremediation of soils contaminated by leachate are mainly bioaugmentation, microbiological bioremediation, and phytoremediation, which are discussed in the results section. Each technique has its own advantages and disadvantages, and the choice of a particular technique depends on several factors, such as the nature of the contaminants, the availability of resources, and the degree of soil contamination.
Figure 1, shows several remediation methods, among which phytoremediation [3,42,43,44,45,46,47,48] and microbial bioremediation [43,44,46,49,50] stand out, followed by biostimulation and bioaugmentation [17] among other techniques that are discussed in detail in this systematic review. The importance of the selection of the bioremediation technique, the selection of the microorganisms, the target pollutant, and the experimental scale will be addressed as they directly affect the results obtained. The analysis will focus on empirical studies conducted between January 2017 and January 2023. Although there is no single technique that can remediate different types of soils, there are autochthonous (indigenous) microorganisms that, in symbiosis with inoculated microorganisms, produce results with successful pollutant removal rates. They are the key to solving most problems related to biodegradation and bioremediation of pollutants, provided that environmental conditions are suitable for their growth and metabolism [10,51]. In this study, the importance of the most commonly used biological techniques today is highlighted in order to provide an objective comparison and to promote the development of innovative, applicable, and economically viable methods.
Figure 1. Remediation techniques [45].
2. Bioventing technique
The bioventing technique requires controlled stimulation of airflow and small amounts of oxygen to release pollutants into the atmosphere through biodegradation by increasing the activities of indigenous microorganisms. This technique has gained popularity [51,52]. Bioventing is limited by the inability to oxygenate the polluted soil and the inadequate aeration of the surface contamination [10,20,45].
3. Biosparging Technique
In this technique, air is blown below the water table to raise the oxygen content of the groundwater and accelerate bacterial bioremediation of contaminants [53]. Both techniques, bioventing and biosparging, have been used simultaneously to ensure the efficient removal of soil contaminants despite unfavorable conditions. Biosparging can also combine soil and groundwater to reduce the concentration of dissolved oil compounds in groundwater mixed with soil below the water table and within the capillary fringe. It is a simple and cost-effective method with great flexibility [10].
4. Bioaugmentation Technique
In bioaugmentation, there are specific sites where microorganisms are needed to extract the pollutants. They are also able to displace indigenous microorganisms, which means they can clean the site quickly. The removal of toxic chemicals through bioaugmentation has already been reported in environments such as soil and water. However, a number of limitations have also been documented [54]. For example, it has been observed that the number of exogenous microorganisms decreases after their addition to a polluted site due to abiotic and biotic stresses. These arise from inadequate growth nutrients such as substrates, temperature fluctuations, and pH, and competition between introduced and indigenous microorganisms [55,56].
5. Microbial Bioremediation
Bacteria and fungi are the most commonly used microorganisms to remove heavy metals from contaminated soils; although, yeasts and algae are also often used. Bioremediation using microorganisms will be successful when the cultivation of a single strain is replaced by clusters of bacterial strains. The microbes/bacteria used for bioremediation are more than 25 genera that have great potential for Municipal Solid Wastes (MSW) [8,45].
For these reasons, the objective of this review is to provide a background on the involvement of microorganisms in the decontamination of leachate-contaminated soils and their subsequent bioremediation. In addition, the factors limiting the growth of bacteria depending on the environmental conditions are highlighted. Finally, the strategies to improve the decontamination of leachate-contaminated soils based on recently published reports were evaluated through a systematic analysis of the available scientific evidence on the subject over the last five years, highlighting their principles, advantages, limitations, and possible solutions. The prospects for bioremediation are also discussed.
6. Phytoremediation Technology
Phytoremediation technology is gradually being accepted and used as an effective method for regulating ecological stability and purifying water quality [57]. Phytoremediation techniques purify contaminated soil, water, and groundwater through various pathways, such as phytoextraction, phytofiltration, phytostabilization, phytovolatilization, and phytodegradation (Figure 1) [3,45].
With over 500 million years of evolution, plants have developed highly regulated mechanisms to mitigate toxicity. Some of these heavy metal wastes such as zinc, copper, manganese, nickel, and cobalt are essential trace elements for plant growth [58], or through the unique and selective absorption, transport, and bioaccumulation of plant roots to achieve pollutant degradation and removal. Therefore, phytoremediation has also been frequently combined with other methods.
The efficiency of phytoremediation is often influenced by many factors, such as plant type, pollutant concentration, composition, accumulation, degradation site, auxiliary methods, and even the genotype of the same plant [59].
According to the available research, different plant species have different abilities to remove pollutants, and different genotypes of the same plant have different cleaning efficiency. It was found that broadleaf, composite, sunflower, hemp, and other plants were among the efficient plant metal storage plants, but their cleaning efficiency was closely related to the characteristics of plant species [60].
7. Theoretical Basis
Obtaining information about platforms is useful. However, keywords are useful for selecting large amounts of information and refining the search methodology to obtain accurate results [61].
Currently, there are different methods and strategies depending on the area in different parts of the world. Common methods include biostimulation, bioaugmentation, biopiles, and bioeradication. All bioremediation techniques have their own advantages and disadvantages, as they have their own specific application [62].
This paper deals with the applications of bioremediation of leachate-contaminated soils treated with microorganisms and the efficiency of each type of strain used, and is organized as follows:
The paper includes a description of different bioremediation techniques with microorganisms and a classification of specific soil types, as well as an analysis of the information and discussions. Finally, the conclusions indicate the contributions of the article and suggestions for future work on bioremediation systems with results according to the microorganisms or strains used, and the most recommended to obtain better treatment results in soils contaminated with landfill leachate.
This entry is adapted from the peer-reviewed paper 10.3390/microorganisms11040857
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