Cover Crops for Phytoremediation of Heavy Metals in Soils

Bioremediation

Bioremediation is the biological processes to mitigate unwanted environmental impact by the removal of contaminants from the biosphere. It includes biological treatment of contaminated media, including subsurface material, waste water, soil and groundwater, food & agricultural waste, by altering environmental conditions to stimulate growth of microorganisms and degrade the target pollutants.Some of the examples of bioremediation includes rhizofiltration, bioaugmentation, rhizofiltration, phytoremediation,bioleaching, landfarming, mycoremediation, composting, bioaugmentation, bioventing, bioreactor, and biostimulation.

Why do we need bioremediation?

We can’t seem to stop polluting for examples:Inorganics (Uranium, sulfur, sulfuric acid), explosives (RDX, TNT), polyaromatic hydrocarbons (creosote), chlorinated hydrocarbons (Trichlorethylene, PCBs, pentachlorophenol), petroleum hydrocarbons (Gas, gas additives (MTBE), diesel).

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We need bioremediation :

• To get rid of substances that pose to human health
• Reduce toxic substances in soil and groundwater from human activities
• To provide cleaner water since water suffers due to all of the pollutants runoff into the water
• Offers an alternative method of detoxifying contaminants which are not harmful for the environment.

Examples of disasters

Many human activities, such as industrial and energy production, mining, electroplating and transportation resulted in contamination by polluting substances, and are hazardous for the environment and ecosystem as well as harmful for human health. We need bioremediation to clean up the oceans after major oil spills and other unfortunate environmental disasters.

The Exxon Valdez oil spill occurred in Alaska, March 24, 1989, when an oil tanker spilled 10.8 million US gallons of crude oil over the next few days. It is considered to be one of the worst human-caused environmental disasters.It affected 1,300 miles of coastline and destroyed the microbial populations on the shoreline and caused the deaths of many water bodies and birds.

Another accident of soil contamination in April 1998, where a dam at the mine burst releasing around five million m³ of acidic mine tailings containing dangerous levels of several heavy metals.

Types of Soil Bioremediation

Heavy metals pollution has become one of the most serious environmental problems nowadays.There are different methods which are used to decontaminate soil from heavy metals known as soil remediation.

Biological methods

Biological methods are used to remediate soil of contaminants such as for example: Petroleum hydrocarbons, Polycyclic hydrocarbons (PAH), Phenols, Nitroaromatic and halogenated compounds .

One of the methods is PHYTOREMEDIATION ( a bioremediation process) - is the direct use of living green plants for in situ risk reduction for contaminated soil, sludges, sediments, and groundwater, through contaminant removal, degradation, or containment.Phytoremediation reduces or limits excavation and disposal of large amounts of contaminated soils and water/wastewater and contaminants are removed by harvesting the plants.

How does it work

Plant roots supply organic nutrients and energy through exudates and have great impact on soil oxidation-reduction potential either directly by transporting oxygen via roots or indirectly by changing soil porosity over time, thus providing an oxidation-reduction microenvironment optimal for growth of soil microorganisms. Vegetation stabilizes soil thus prevents erosion and pollutant movement.The selection of plant types for phytoremediation becomes an important consideration to make the remediation process more effective and efficient as well as to prevent the plants from being consumed. The use of non-edible and non-commercial fast growing with high biomass plants as remediating plants can be an alternative solution and cover crops that have characteristics that meet with this purpose.Some of the best plants for phytoremediation are :

• Cover corps - Plants planted to cover soil rather than for harvestion. It manage soil fertility, quality, erosion and pests,water, weeds, biodiversity & wildlife.
• Alfalfa - Can tolerate a heavy metal soil, which is an essential characteristic of any plant species used in phytoremediation.
• Indian mustard - Accumulate certain metals while producing high quantities of biomass. It remove Cd and reduce Pb, Se, Zn, Hg and Cu.
• Poplar tree (Populus deltoides) - Can degrade petroleum hydrocarbons.

Different types of phytoremediation

There are several different types of phytoremediation mechanisms. These are:

1. Photodegradation or Rhizosphere biodegradation. In this process, the plant releases natural substances through its roots, supplying nutrients to microorganisms in the soil. The microorganisms enhance biological degradation.
2. Phyto-stabilization. In this process, chemical compounds produced by the plant immobilize contaminants, rather than degrade them.
3. Phyto-accumulation (also called phyto-extraction). In this process, plant roots sorb the contaminants along with other nutrients and water. The contaminant mass is not destroyed but ends up in the plant shoots and leaves. This method is used primarily for wastes containing metals. At one demonstration site, water-soluble metals are taken up by plant species selected for their ability to take up large quantities of lead (Pb). The metals are stored in the plantÍs aerial shoots, which are harvested and either smelted for potential metal recycling/recovery or are disposed of as a hazardous waste. As a general rule, readily bioavailable metals for plant uptake include cadmium, nickel, zinc, arsenic, selenium, and copper. Moderately bioavailable metals are cobalt, manganese, and iron. Lead, chromium, and uranium are not very bioavailable. Lead can be made much more bioavailable by the addition of chelating agents to soils. Similarly, the availability of uranium and radio-cesium 137 can be enhanced using citric acid and ammonium nitrate, respectively.
4. Phytofiltration or Rhizofiltration - is similar to phyto-accumulation, but the plants used for cleanup are raised in greenhouses with their roots in water. This system can be used for ex-situ groundwater treatment. That is, groundwater is pumped to the surface to irrigate these plants. Typically hydroponic systems utilize an artificial soil medium, such as sand mixed with perlite or vermiculite. As the roots become saturated with contaminants, they are harvested and disposed of.
5. Phyto-volatilization. In this process, plants take up water containing organic contaminants and release the contaminants into the air through their leaves.
6. Phyto-degradation. In this process, plants actually metabolize and destroy contaminants within plant tissues.

The potential use of cover crops for phytoremediation process of heavy metals contaminated soils.

Cover crops

phytoremediation process depends on many factors such as heavy metal properties, soil properties and plant species. The selection of plants to be used as remediating plants is the most important factor affecting the effectivity and efficiency of the phytoremediation process.Cover crops are grown and unharvested in order to provide ground cover to benefit the soil and other crops in a number of ways. Cover crops can be from any family such as grasses, legumes. However, to be effective, the cover crop must be quick to establish, provide an early canopy cover, be aggressive enough to suppress weeds and possess a dense and deep root system to hold the soil and improve the macroporosity of the soil. Cover crops have benefits that are summarized as follows: Reduce runoff and soil erosion, Improve soil structure and aggregate stability, Increase soil organic matter content, Prevent the leaching of available soil nutrients, Nutrient sequestration to recycle nutrients and limit surface and groundwater contamination, Increase biological diversity and disrupts pest and disease cycles, Improve water infiltration, water absorbing capacity and conserve soil moisture, Increase residue cover, Reduce soil compaction, Control weed population, Increase beneficial insect population, Some cover crops can produce certain biochemical compounds which aid in the liberation of some soil minerals, May provide off-season pastures, Soil nitrogen accumulation from legume cover crops, As a wildlife habitat and landscape aesthetics.The Cover crops can be selected to be used as remediating plants due to their characteristics.The use of cover crops as phytoremediation plants was thought to be more efficient and effective to reduce heavy metals content in the soil as well as can improve soil fertility and productivity.

Cover corps and process of phytoremediation

The effectivity and efficiency of the phytoremediation process depend on many factors such as heavy metal properties (bioavailability of the heavy metals in the soil, speciation of heavy metals), soil properties and plants species used and plants have to have high biomass production, from native species and not too palatable to prevent the risk of being consumed and grazed, and has different root system and depth to reach the different depth of contamination.Ideal characteristics of the plants to be used for phytoremediation process, they are: have height growth rate,produce more above-ground biomass, have widely distributed and highly branched root system, more accumulate the target heavy metals from soil, translocate the accumulation heavy metals from roots to shoots, tolerant to the toxic effects of the target heavy metals, adaptive to prevailing environmental and climatic conditions, resistant to pathogens and pests, easy to cultivate and harvest, and repulsive to herbivore to avoid food chain contamination. Many previous studies reported about plants that high potential to be used as phytoremediation plants for heavy metals. However, most of the plants in the report are annual, commercial and edible crops. The use of these crops for heavy metals remediation process might be effective with heavy metals hyper-accumulation potential for some crops such as Brassica juncea L. but not highly efficient economically and practically due to their characteristics as commercial and annual that need to be planted again for continuation of the remediation process after their life cycle end. These crops also mostly highly responsive to suboptimal environmental and climatic conditions. The use of these crops also has a high risk of being harvested and consumed by humans or grazed by animals due to their high palatability. Cover crops usually have a dense with the different depth root systems, grow faster and can cover the soil fast compared to other crop types. Cover crops can produce a higher amount of biomass and increase soil organic matter. The use of cover crops also can improve the physical, chemical and biological properties of the soil. Some cover crops can produce chemical compounds that help crops to uptake the heavy metals and compart them into the root and/or shoot. Cover crops also usually have a high adaptation to a wide range of environmental and climatic conditions. The use of perennial cover crops can be more efficient for a long term process of heavy metals remediation because they do not need to be replanted after harvesting. Harvesting can be done by cutting the above-ground biomass of the plants, then the new shoots will grow and the remediation process will continue. With these reasons, the use of cover crops as phytoremediation plants was thought to be more efficient and effective to reduce heavy metals content in the soil as well as can improve the physical, biological and chemical characteristics of the soil simultaneously.

CONCLUSION

Heavy metals are one of the most serious pollutants in the environment and become a major environmental and human health problems. Phytoremediation can be an alternative solution as a green technology to treat heavy metal contaminated areas. Phytoremediation of heavy metals is a cost-effective, efficient, environment and eco-friendly based on the use of metal-accumulating plants to remove toxic metals, including radionuclides as well as organic pollutants from contaminated soils and water. With their characteristics, the use of cover crops as phytoremediation plants was thought to be more efficient and effective to reduce heavy metals content in the soil as well as can improve soil fertility and productivity simultaneously. According to many studies, several non-edible plants that can be used as cover crops are potential for phytoremediation process of heavy metals contaminated soils.

References

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