How Plants can help Clean-up Heavy Metal Contamination

The Threat of Heavy Metal Pollution

Heavy metals can contaminate the soil, from where they can enter the food chain that one way or another ends up on our dinner plate. Once heavy metals are ingested, they can permanently reside within the body and cannot be removed from the system. Heavy metals naturally exist in the environment but are almost always concentrated as a result of anthropogenic activities like mining, industrial effluents, lead batteries, vehicle emissions, fertilizers and plastics. 

Common sources of heavy metal contamination include mining, industrial effluents and vehicle emissions.

Through the soil, heavy metals can contaminate crops and persists within the muscle of livestock and fish. Foods most commonly contaminated with heavy metals include: rice, lentils, spices, green leafy vegetables, chicken and fish.

Food items commonly contaminated with Heavy metals.

Common Heavy Metals in the Environment

Heavy metals most persistent in the environment, with known side effects include: Arsenic, Lead, Mercury and Copper. The major issue with heavy metal pollution is that these metals are not biodegradable, meaning that they become a permanent part of living organisms. This is quite dangerous since heavy metals can be toxic in lower concentrations for humans and animals. Crops and other plant species are also greatly affected by these heavy metal contaminants that causes  decline in crop yield, growth and produce size.

Traditional methods of Heavy Metal Removal

Heavy metals can be removed through various chemical treatments and thermal processes from the environment but are associated with heavy costs and are difficult to execute on a larger scale. These treatments can also strip the soil of useful nutrients. Contaminated soil can either be treated on-site or collected and transported to a landfill. Transportation of the contaminated soil seems to be the least viable method since it is merely removing pollution from one site and adding to another. Alternatively, soil washing can also be done, but that too comes with its drawbacks such as production of residue concentrated with heavy metals that requires additional treatment and associated costs. Since the washing process is quite intensive, soil treated through this method cannot be used for planting crops. 

Plants as an alternative to Remove Heavy Metals from the Environment

Plants can accumulate metals from the soil through the natural process of transpirational pull. Metals are thus absorbed through the roots and predominantly accumulate in the shoot. Certain plant species known as hyperaccumulators are more efficient at accumulating heavy metals. These plant species have application for  phytoremediation, 'the technical term for using plants to remove heavy metals from the environment'. Researchers have identified four mechanisms through which the process of phytoremediation takes place: phytoextraction, phytostabilization, rhizofiltration and phytovolatilization. 

Phytoextraction

This mechanism involves the uptake of heavy metals through the roots and their deposition in the shoots and leaves. The plants can then be harvested, burnt for energy and the metal rich ash is also recycled.

Phytostabilization

This mechanism immobilizes heavy metals from the groundwater and soil, by creating a concentration zone around the plant roots. This prevents heavy metal runoff into the environment and the heavy metals eventually enter the plant roots.

Rhizofiltration

This mechanism comes in handy when wastewater requires heavy metal removal. Water dwelling plants' roots accumulate heavy metals dissolved in the wastewater.

Mechanisms by which phytoremediation takes place. Credits: Research Gate.

Plant Species used for Phytoremediation

Chinese Ladder Brake (Pteris vittata) 

Effectively removes Arsenic from soil by accumulating it in its Tissues.

Carolina Mosquito Fern (Azolla caroliniana)

Removes Arsenic through Accumulation in its Fleshy Leaves. Credit: Pinterest.

Mustard Plant Species (Brassica campestris, Brassica carinata, Brassica nigra & Brassica juncea)

Various species of Mustard plant are capable of accumulating high levels of lead and Mercury in their roots and shoots.