A new Israeli study presented at the 49th Annual Science and Environment Conference in Tel Aviv last July revealed another surprising ecological service that some plants can perform – the ability to absorb and remove an unusually large amount of toxic metals from contaminated strain. Cadmium, a common metallic component in fertilizers, is a common example.
Strangely, the researchers found that these plants can purify soil even faster if the plants are tricked into thinking they are being attacked or eaten. Have you heard of a natural phenomenon in which some plants are able to communicate with each other through signaling? Chemical. The list of unusual abilities displayed by plants does not end here.
How polluted is the soil in the world?
Both in Israel and around the world, soil pollution is caused by man, and is the result of industrial activity and modern infrastructure. As a result of fuel refining, the use of pesticides and agricultural fertilizers, military activity and the presence of municipal sewage and landfills, heavy metals penetrate the groundwater and soil layers. Not only does it poison the environment, but it also puts us at risk of direct exposure, through drinking water or consumption of agricultural produce.
So, to what extent is the soil contaminated with heavy metals? With industry, agriculture and urbanization all on the rise, it's hard to give this exact value because reports of soil contamination are actively submitted. In fact, there are over 10 million sites with contaminated soil, reported worldwide so far. And more than half of those sites are contaminated with heavy metals and/or metalloid.
According to Grossman, about 28% of the European continent's soil is currently contaminated with one or more heavy metals in concentrations above the threshold values set by Finland's Ministry of Environmental Protection (MoEP).
How many contaminated sites are there in Israel?
This appears to be a common environmental dilemma that extends to all semi-industrialized countries, including Israel. Following a soil contamination survey conducted in 2014, the Israeli Ministry of Health identified 23,100 local areas across Israel that are or potentially contaminated by current or past human activity.
Israel's coastal aquifer is an excellent example of this. Between gas stations, industrial plants, military bases, and landfills, the coastal plain has been severely contaminated with heavy metals over the years; So much so that by 2018, 8% of the available drinking water in the coastal plain was contaminated, and nearly 200 groundwater drilling sites (about a quarter of Israel's drilling sites) were shut down.
The heavy metals and costs
Repairing the land is no small task; Nor cheap. Between excavation, transportation and treatment of contaminated soils, the cost of land rehabilitation in Israel alone amounts to about NIS 9.8 billion ($3.035 billion). An amount paid by the polluters themselves, for the most part. Variants of this financial burden appear in every industrialized country, causing a cumulative global economic impact exceeding $10 billion annually.
"The problem with soil-polluting metals is that they don't decompose," says Eyal Grossman (a research student). Without proper treatment of the soil and treatment of the source of pollution, the concentration of heavy metals in the soil will remain and is likely to increase. However, the metals do not stay in one place within the ground, and this is where the problem begins. The metals are taken through plants and inevitably reach up the food chain. In agricultural areas, for example, hazardous metals from fertilizers, pesticides, irrigated, biological, and manure can accumulate in crops and livestock before they reach our plates. In other words, we should first prevent the metals from seeping into our groundwater, so as not to contaminate the drinking water that will reach our cups.
Either way, the environment suffers, and public health can be at serious risk, especially when it comes to lead, arsenic , or cadmium – all common metallic pollutants that are considered particularly toxic.
Let nature take its course; Plants draw poison from the soil
Defined by the IARC (International Agency for Research on Cancer) as a definite carcinogen in humans, cadmium exposure, in particular, can lead to health complications such as kidney damage and bone defects. In terms of the environment, high levels of cadmium, for example, impair the photosynthesis capabilities of the tree.
For the new Israeli study, the researchers engaged in phytoremediation, a plant-based strategy designed to reduce the bioavailability of soil pollutants through certain plants suitable for storage: some plants are hundreds or thousands of times more efficient at storing metals and metalloids in their organic tissues than is considered normal for most plants. Why do these plants do this?
"The most common hypothesis today is that over-hamsters absorb the metals into their bodies and transfer them to the leaves to keep away metal-avoidant pests," Grossman says.
So far, about 721 plant species that store pollutants have been identified, including well-known plants such as sunflower, corn and rapeseed (from which canola oil is extracted). Usually, contaminated soil is transported with the help of bulldozers and sent to a treatment facility, which costs a lot of money, or it is simply buried somewhere else, in which case the contamination is simply transferred, and the problem remains unsolved.
How does the method work?
With hundreds of absorbent plants and the well-understood process of phytoremediation, you can sit back and let nature "take its course." Based on the suspected or known metallic contaminants embedded in a particular site, it will be possible to plant the hyperabsorbent plants in this area, and these plants can ideally absorb the metals into their tissues and thus improve soil fertility over time. Then, after the plants are harvested, the metals will be removed from the environment without additional expense or damage to the soil itself.
Recycle plants that have stored pollutants
Also, these metal-soaked plants can continue to become an important raw material, instead of being closed as discarded organic waste, and fully fulfill their purpose. "A lot of times you can make biofuels from the contaminated plants," Grossman says.
However, to date, the method of removing pollutants outside the confines of laboratories and greenhouses has hardly been used. Grossman notes in his research that the main reason this method is ignored is that the plant's growth rate is relatively slow.
This means that it can take several years for all pollutants to be completely removed from the soil, but the method is not often seen as economically viable. Especially in Israel, where land rehabilitation usually takes place rapidly, due to the pressure coming out of real estate development and infrastructure. Both tend to require immediate solutions.
The manipulation to speed up the process
Because of the smaller-than-desirable time it took to revive an area of land through phytoremediation, the researchers wondered what could speed up the process to perhaps make the strategy more economically viable. The solution discovered by the researchers was unusual: damage to the plant.
The researchers injected cadmium into the potting soil of 80 common sunflower plants and then made one group of plants "think" they were being attacked by insects. To do this, make small punctures in the leaves with a toothpick and spray them with jasmonic acid, a plant hormone. "It's produced by almost all plants when pests attack them, and it activates the plant's defense mechanisms," Grossman explains.
The results of the study were more than encouraging. The amount of cadmium absorbed in the sprayed and acid-punctured sunflower group was 40% higher than in the non-treated sunflower group. According to Grossman, this is equivalent to reducing the time required for effective phytoremediation by almost half. In addition, it should be noted that the sunflowers that received the spray and puncture treatment did not suffer from inhibitory damage. Instead, the same group of treated sunflowers grows identically to the untreated group.
Require cooperation from the authorities
"We are already working on the next experiment, in which we will examine whether cadmium is also stored in the body of the plant, and not just in its leaves. In other words, the plant absorbs even more cadmium than we think," Grossman explains.
"I call on municipalities and local councils that own contaminated land that will soon be abandoned to cooperate with academia," Grossman said. "It's a shame that contaminated sites will continue to harm soil and public health when sustainable green restoration methods and technologies can instead be applied to purify the soil for the benefit of us all."