Pollutants of anthropogenic origin, i.e. from human activities, can adversely affect health and the environment. Even low levels of such pollution can present risks due to the physicochemical nature of the substances released and have an impact on the various environmental compartments, such as the hydrosphere (water), the lithosphere (soil), the atmosphere (air/gas) and the biosphere (animals and plants).
When a source of contamination is identified in the subsoil (soil and groundwater), various technologies can be useful for its decontamination. But, among all those available, bioremediation is gaining strength. This type of methodology uses biological processes to degrade contamination to innocuous compounds or to reduce their concentrations to levels below the limits established by regulatory entities 1.
Given the context of climate change, loss of biodiversity and scarcity of resources in which we find ourselves, bioremediation, due to its inherent sustainability, acquires prominence as a key technology in decontamination processes.
What is phytoremediation?
La fitorremediación es un tipo de biorremediación en la que se utilizan especies vegetales, y los microorganismos del suelo asociados a sus raíces, para disminuir las concentraciones o los efectos tóxicos de los contaminantes en el medio. Así, la tecnología de la fitorremediación se puede utilizar para varios compartimentos ambientales y para diversos tipos de contaminantes, ya que puede implicar diferentes procesos de transformación o acción sobre la contaminación (Figura 1) 2,3. Por ejemplo, en el caso de los procesos de fitodegradación o fitovolatilización, se puede conseguir una disminución de las concentraciones o la completa transformación de los contaminantes a compuestos inocuos. Mediante la fitoestabilización, lo que se consigue es reducir la movilidad/biodisponibilidad de los contaminantes y, por tanto, disminuir su dispersión y el riesgo potencial sobre la salud humana y ecosistemas. Finalmente, la técnica de la fitoextracción se basa en la acumulación de los contaminantes en el tejido vegetal de las plantas utilizadas. (Tabla 1).
| Technology | Action on contaminants | Main types of contaminants | Vegetation |
| Phytostabilization | In-situ fixation | Organics and metals | Maintenance coverage |
| Phytodegradation | In situ degradation | Organics | Maintenance coverage |
| Phytovolatilization | On-site disposal | Organics and metals | Maintenance coverage |
| Phytoextraction | In situ accumulation and ex situ disposal | Metals | Repetitive harvesting |
Table 1Comparison between various phytoremediation technologies (Source: adapted from Greipsson 2).
Comparison with other decontamination technologies
Bioremediation technologies, including phytoremediation, are considered cost-effective environmental restoration strategies. This is true both at an economic and technical-environmental level, since they represent an easy-to-implement alternative to conventional remediation systems, which are also resource-intensive and usually more destructive to the soil 2. In addition, the possibility of treating the contamination in situ and not generating a final waste reduces the life cycle of these processes and the potential local toxic impacts, which makes them very attractive remediation strategies 4. For this reason, bioremediation has become one of the preferred technologies for soil and groundwater remediation 5.
In particular, phytoremediation, in turn, presents the possibility of generating very interesting synergies, including revegetation 6,7, ecological restoration of degraded areas 8 or the production of biofuel from biomass .
However, phytoremediation techniques also present some key points that must be taken into account, especially during the design phase of the remediation system, to ensure the effectiveness of the treatment. For example, it must be taken into account that the scope of phytoremediation is limited to the root zone of the plants, so it will only act on the affection located at those depths, and that its effectiveness will be reduced when the concentrations of contaminants are toxic to them 2.
Thus, although phytoremediation technologies have been studied and applied for years and many of the remediation approaches are promising 9, it is also true that some experts in the field have shown concern about their remediation capacity and efficiency 10 and there has not yet been significant commercial application of this technology and related produced crops.
Phy2Climate Project
LITOCLEAN has experience in the field of R&D and phytoremediation, and one of the active projects is currently part of the Horizon 2020 (H2020) strategy of the European Union (EU). The Phy2Climate project is entitled “A global approach for recovery of arable land through improved phytoremediation coupled with advanced liquid biofuel production and climate friendly copper smelting process” and brings together 17 partners from 10 different countries.
The objective of the project, which is divided into several work teams, is to connect and combine the processes of phytoremediation of contaminated sites with the clean production of biofuels. These biofuels will not present land use change risks, and it will be studied whether phytoremediation will decontaminate soils affected by a wide variety of contaminants, making the restored land available for agriculture, while improving the overall sustainability, legal framework and economics of the process. This will promote the revaluation of a waste (both contaminated soil and contaminated biomass generated during phytoremediation) and the generation of a product with market value (biofuel), contributing significantly to the MissionInnovation Challenge for sustainable biofuel production, the United Nations Sustainable Development Goals and the circular economy.
Technically, the Phy2Climate project investigates phytoremediation processes of contaminated sites with different characteristics (type of contamination – metals and total petroleum hydrocarbons (TPH), soil type, climate, legislation) in 5 regions of the world (South America, Europe and Asia). Once the biomass is harvested, it will undergo an innovative cascade conversion technology to produce value-added products such as direct biofuels for road and maritime transport, as well as bio-coke, as a replacement for petroleum coke in the metallurgical industry.
In particular, LITOCLEAN is collaborating with two Spanish technology partners (EXOLUM and LEITAT) in the optimization of phytoremediation strategies at a site located in Tarragona and contaminated by TPH. During last spring, a preliminary characterization of the soil under study was performed and laboratory tests with four plant species in hydrocarbon phytoremediation were initiated. Based on the results obtained, the field pilot is being designed, which is expected to start next March.
Authors: Natàlia Blázquez, Carlos Herrarte, David Garriga and Marçal Bosch
References
1. Crawford, R. L. & Crawford, D. L. Bioremediation: principles and applications. (Cambridge University Press, 2005).
Greipsson, S. Phytoremediation. Nat. Educ. Knowl. 3, 7 (2011).
3. Lee, J. H. An overview of phytoremediation as a potentially promising technology for environmental pollution control. Biotechnol. Bioprocess Eng. 18, 431-439 (2013).
4. Lemming, G. et al. Environmental impacts of remediation of a trichloroethene-contaminated site: Life cycle assessment of remediation alternatives. Environ. Sci. Technol. 44, 9163-9169 (2010).
5. Pandey, J., Chauhan, A. & Jain, R. K. Integrative approaches for assessing the ecological sustainability of in situ bioremediation. FEMS Microbiol. Rev. 33, 324-375 (2009).
6. Yan, A. et al. Phytoremediation: A Promising Approach for Revegetation of Heavy Metal-Polluted Land. Front. Plant Sci. 11, 359 (2020).
7. Tran, H.-T. et al. Revegetation on abandoned salt ponds relieves the seasonal fluctuation of soil microbiomes. BMC Genomics 20, 478 (2019).
8. Gajić, G. et al. Ecological Potential of Plants for Phytoremediation and Ecorestoration of Fly Ash Deposits and Mine Wastes. Front. Environ. Sci. 6, 124 (2018).
9. Peuke, A. D. & Rennenberg, H. Phytoremediation. EMBO Rep. 6, 497-501 (2005).
10. Conesa, H. M., Evangelou, M. W. H., Robinson, B. H. & Schulin, R. A critical view of current state of phytotechnologies to remediate soils: Still a promising tool? Sci. World J. 2012, 1-10 (2012).
