If something characterizes modern society is the continuing and increasing emissions of pollutants into the environment. These pollutants come especially from industrial activities, agriculture, mining, etc. they threaten the living organisms that are exposed to them. In order to avoid the exposure risk, different systems to repair the pollution impacts have been developed, including phytoremediation.
Phytoremediation comes from Greek: Phyto means “plant” and remedium means “to recover the balance”. Phytoremediation is a technique that exploits the ability of some plants to absorb, accumulate, metabolize, volatilize or stabilize contaminants in soil, air, water or sediment, heavy metals, radioactive elements, organic compounds and petroleum. It supposes an alternative to the physicochemical methods that have been traditionally used to solve environmental pollution problems.
Phytoremediation can be defined as “a set of technologies that reduces, in situ or ex situ, the concentration of various compounds from biochemical processes carried out by plants and microorganisms associated to them. “
Conventional methods can alter irreversibly the properties of soil, water and living organisms that live in them , they are often very expensive and have limited effectiveness. However, phytoremediation is a sustainable alternative, cheaper and with a high applicability to rehabilitate environment affected by natural and anthropogenic pollution.
There are six different methods of phytoremediation grouped into two sets; those used to contain pollution (rhizofiltration, phytostabilization and phytoimmobilisation); and the ones used to remove pollution (phytodegradation, and phytoextraction fitovolatilización). Depending on the type of pollutant and its quantity as well as the environment in which the pollutant is found, a method of phytoremediation or another is chosen. The most used for containment of pollutants is phytostabilisation and phytoextraction for disposal.
- Phytostabilization: this method is used to immobilize contaminants in soil through its absorption and accumulation in the roots of plants or precipitation in the rhizosphere. The first step in conducting a phytostabilization process is to select the right plants, usually species that accumulate few pollutants. As plants grow, the change and stabilize the soil, so that the mobility of contaminants is reduced thereby preventing their migration to other media such as water or air. Phytostabilisation is applied mainly to large tracts of land where there superficial pollution and has the advantage over other methods of soil remediation that it is easy to implement, has low cost and is also aesthetically pleasant. Also, animals can eat these plants without any problem. Some of the plants used for phytostabilization are, among others, Anthyllis vulnerary for zinc, cadmium and lead; Lupinus albus for cadmium and arsenic; and Brassica juncea for cadmium, zinc, copper, manganese, iron and lead.
- Phytoextraction: phytoremediation method, also known as phytoaccumulation, it consists in the absorption of contaminating metals by the plant roots and their accumulation in stems and leaves. To do this, the first step is to select the suitable plants according to the metals that can be found in the area as well as the site characteristics. Once the plant have growth, it is cut and incinerated and the ashes are brought to a landfill. One of the Phytoextraction advantages is that the process can be repeated unlimited times until a contaminant concentration in the medium within acceptable limits is achieved. Some of the plants used in this method are, among others, Thlaspi caerulescens for cadmium; Vertiveria zizanioides for zinc, cadmium and lead; and Pistia stratiotes for arsenic, cadmium, chromium, copper, mercury, nickel, lead and zinc.
Other methods of phytoremediation are:
- Rhizofiltration: it is used for the removal of contaminants from water environment through plant roots. When the root system of plants is well developed plants are introduced into the contaminated water count the roots absorb and accumulate metals. When the roots are saturated, the plants are harvested and available for treatment.
- Phytovolatilization: as plants and trees grow, they absorb organic and inorganic pollutants and some of them can get to the leaves and evaporate or volatilize into the atmosphere.
- Phytodegradation: plants and microorganisms that are associated with them degrade pollutants into harmless products.
- Phytostimulation: the roots of plants stimulate the microorganisms in the rhizosphere to degrade the contaminants.
The time phytoremediation takes to make its effect depends of each place, the kind of contaminant, the amount, the method of phytoremediation and the type of plant to be used. Despite the many advantages that accompany this system there are some limitations, since it can only be used efficiently in cases of more or less superficial pollution and low pollutant concentration. Therefore, further research on phytoremediation is necessary to carry out this method of recovery more efficiently and in an extensive way.