Heavy Metal and Metalloid Contamination of Surface and Underground Water by Gupta Abhik;

Author:Gupta, Abhik;
Language: eng
Format: epub
Publisher: Taylor & Francis Group
Published: 2020-08-21T00:00:00+00:00

8  Remediation of Heavy Metals by Biota

Phytoremediation of Heavy Metal- and Metalloid-Contaminated Water

Phytoremediation has been defined in a number of ways. A very basic and inclusive definition would be: “Phytoremediation basically refers to the use of plants and associated soil microbes to reduce the concentrations or toxic effects of contaminants in the environments” (Greipsson 2011). Tangahu et al. (2011) defined phytoremediation as “the use of plants to clean up a contamination from soils, sediments, and water.” It is a cost-effective, environment-friendly, and solar-driven technology that mainly comprises phytoextraction (or phytoaccumulation), phytofiltration, phytostabilization, phytovolatilization, phytodegradation, rhizodegradation, and phytodesalinization. Of these, phytoextraction, phytofiltration, phytostabilization, and phytovolatilization are particularly useful in the removal of heavy metals and metalloids. Phytodegradation and rhizodegradation are mostly relevant for organic toxicants, and phytodesalinization for removing salinity or sodium chloride (Ali et al. 2013).

In phytoextraction, there is uptake/absorption of heavy metals/metalloids by the roots followed by translocation into aboveground parts like shoots which can be harvested and burnt to generate energy and recover the metals (Tangahu et al. 2011). In phytofiltration, heavy metals/metalloids are removed by absorption or adsorption from water or wastewater by plants through the processes of rhizofiltration (filtration by roots), blastofiltration (by seedlings), or caulofiltration (by stem or shoot). Among the different strategies or techniques of phytoremediation, removal of heavy metals and metalloids from aqueous medium is best achieved by phytofiltration, perhaps because it involves rhizofiltration, blastofiltration, and caulofiltration to remove heavy metals and metalloids. Though aquatic plants are used in phytofiltration, terrestrial plants in hydroponic solutions are often preferred because of their much larger fibrous root systems covered with extensive root hairs to give them large surface areas for the absorption/adsorption of metallic pollutants. On the other hand, most aquatic plants have small, slow-growing roots which hinder their performance. Whether terrestrial or aquatic, plants used in phytofiltration should be able to tolerate and accumulate high concentrations of heavy metals/metalloids (Padmavathiamma and Li 2007).

In contrast to phytofiltration, phytostabilization is a technique that immobilizes the heavy metals/metalloids, thereby reducing their mobility and bioavailability, and preventing their migration into groundwater or into the food chain (Tangahu et al. 2011; Ali et al. 2013). Phytovolatilization involves uptake of pollutants by plants and their subsequent conversion into volatile forms to eventually expel them into the atmosphere. However, it is mostly applicable for organic pollutants and only mercury among the heavy metals. Further, it does not sequester the pollutant, but releases it back into the atmosphere wherefrom it can once again be deposited (Ali et al. 2013).

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