Using plants to clean up contaminated water, soil

By Augustine Ignatius Doronila

IT IS not unusual to see, read or hear about environmental problems these days. Pollution has become so common that we appear resigned to the fact that this is part of modernization.

At times, it may seem that the cleanup would require a very expensive and highly sophisticated process. Have you ever thought that we could use plants to clean up contaminated soil and water?

Increased interest

Over the past 20 years there has been interest in using a series of technologies called phytoremediation (phyto = plant and remediation = providing a remedy) to provide a solution to many polluted areas.

The US Environmental Protection Agency (EPA) has encouraged research on using plants to remediate manmade contaminants through several mechanisms.

Some plants destroy organic pollutants by degrading them directly through the production of acids and enzymes which attack these compounds. Other plants aid in degradation indirectly by supporting microbial communities in the soil which will decompose the pollutants. There are other plants that take up inorganic contaminants such as heavy metals from soil or water and concentrate them in the plant tissue or root.

Extraction technique

Using different plants, phytoremediation can be applied as a containment measure for decomposition of the pollutant or as a removal or extraction technique.

Through the development and evaluation of new, soft, appropriate and efficient biological processes, it is possible to remove, contain or render harmless environmental contaminants (toxic metals and difficult-to-destroy organic pollutants) in waste waters and sites heavily affected by industrial, mining or urban activities.

Attractive technology

The technology is attractive because the cost of phytoremediation techniques is estimated to be from 20 to 50 percent less than the highly engineered physical, chemical or thermal techniques.

Moreover, there are limited funds available for environmental cleanup. This alone is relevant to less economically developed countries which have suffered a legacy of chemical pollution and are unable to provide substantial funding to immediately remove the pollutant source.

Phytoremediation is still a young technology that seeks to harness the metabolic capabilities and growth habits of higher plants. Delivering a cheap, soft and safe biological treatment applicable to specific contaminated sites and wastewaters is a relatively recent development.

Low cost, low impact

The European Union through its COST Action 837 program, which presented its major findings in October 2009 in Ascona, Switzerland, showed that there was still a significant need to pursue both fundamental and applied research to provide low-cost, low-impact, visually benign and environmentally sound remediation strategies.

It is well-suited for use at very large sites where other methods of remediation are not cost-effective or practicable; at sites with low concentrations of contaminants where only “polishing treatment” is required over long periods of time; and in conjunction with other technologies where vegetation is used as a final cap and closure of the site.

The concept of using plants to clean up contaminated environments is not new. Approximately 300 years ago, plants were proposed for use in the treatment of wastewater in Berlin, Germany. Plant species have been discovered to accumulate metals to such high concentrations, usually 1,000 times, considered toxic to a typical plant.

European plant species

They have been called metal hyperaccumulating plants. At the end of the 19th century, two European plant species, the penny cress (Thlaspi caerulescens) and a small violet (Viola calaminaria), were the first plant species documented to accumulate high levels of metals in leaves.

In 1948, Tuscan scientists Minguzzi and Vergnano identified plants able to accumulate up to 1 percent Ni (nickel) in shoots which is 10,000 times more than what a typical plant would have in leaf tissues.

Bronze Age

Some unusual plants have been discovered to grow in soils which are naturally rich in metals as well as in ancient and abandoned mining sites from the time of the Bronze Age circa 3000 BC.

Toxic

Important metals for our modern lifestyles such as nickel, copper, zinc and lead are also invariably toxic if they become dissolved in water.

The idea of using plants to extract metals from contaminated soil was subsequently revived about 30 years ago and developed by Utsunamyia (Japan) and Chaney (US). The first field trial on zinc and cadmium phytoextraction was conducted in 1991 by Baker and his colleagues.

There has been extensive research in the past two decades with major developments occurring in the technologically advanced nations. Despite significant success, the understanding of how a plant does metal extraction is still emerging. The agronomic practices to improve the extraction are still being optimized.

Growing market

According to the EPA, the US phytoremediation market has grown significantly. It expanded from $30 million in 1995 to $49 million in 1999. This may also become a technology of choice for remediation projects in developing countries because it is cost-efficient and easy to implement.

It has only been in the past 10 years that phytoremediation studies have been undertaken in tropical regions. These are invariably emerging markets which are experiencing major pollution problems due to rapid industrialization. The countries which have taken the lead in harnessing this green technology are China, Thailand, Brazil, Chile and India.

Spectacular discovery

One of the most spectacular discoveries of a hyperaccumulating plant occurred in China in 1999. Prof. Tongbin Chen and his team from the Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Science discovered a species of Chinese brake fern (Pteris vitatta) [a species of pacô] which can grow healthily in arsenic-rich soils.

Before this discovery no plant in the world had been found able to hyperaccumulate arsenic to a concentration of more than 1,000 milligrams per kilogram. So far, Chen’s team has identified a total of 16 native Chinese plants able to absorb arsenic, lead, copper and other heavy metals from soil.

By 2005, Chen’s research program in the southern Chinese province of Chenzhou had achieved success. The team conducted a field trial in Dengjiatang, a township in Hunan’s Chenzhou, where land was polluted by an arsenic smelter.

As a result of heavy arsenic pollution, two people died and most of the grains harvested in the area were contaminated.

Mei Lei, one of the researchers, reported that the arsenic level in the heavily polluted soil had dramatically decreased by half. The cost of using the fern to clean up the contaminated soil was at most one-tenth of the chemical cleaning methods.

Philippine plants

What about the Philippines? Our archipelago also has metal hyperaccumulating plants. In 1986, a British scientific expedition led by Proctor and Baker discovered four nickel hyperaccumulators.

One of these plant species, Phyllanthus balgooyi, accumulated Ni to very extreme concentration of 88,000 milligrams per kg or 8.8 percent. The metal was concentrated in a jade-green sap in a layer of wood just beneath the bark.

It has been recognized that the country may actually harbor many more of these hyperaccumulator species because of the unique geology of the nickel-rich rocks where these species can be found.

Biodiversity

Some world authorities such as Baker and Reeves feel that we are on the cusp of something significant because their intuitions from work in other parts of the world strongly suggest that our country is the habitat of a very large number of these specialized species.

The other biodiverse regions for these plants are in Cuba and New Caledonia.

Zambales

At the end of February 2009, I went on a field trip with Rene Claveria, a geologist and chair of the Department of Environmental Science, Ateneo de Manila University, and two graduate students to a nickel-rich area in Acoje, Zambales.

The mine operator, Rusina Mining, provided us generous logistic support. That initial survey resulted in a discovery of a new Ni hyperaccumulator which has sparked fresh enthusiasm to systematically discover these unique species which belongs to the genus Breynia in the plant family Phyllanthaceae.

It was a thrill to follow on from my mentor, Professor Emeritus Alan Baker, after their pioneering expedition and discovery in Palawan 24 years ago.

The Ateneo environmental scientists have subsequently followed up the initial find with a series of field trips to study the ecology. They also have initiated studies to understand its propagation and the molecules synthesized by the plants to accumulate the metal.

The analytical work has showed that this species will significantly accumulate Ni with concentrations of up to 0.9 percent in the leaf dry matter. The soils from which the plants were collected only had a third of the Ni concentration of the dry leaf matter. Bear in mind that the economic minerals underneath the vegetation contains at least 1.2 percent Ni.

Fast growing

These fast-growing high biomass plants may provide a harvestable valuable “metal crop from spent mineral resources” for post-mining communities. There is an urgent need to discover plants that can be used for the phytoextraction system.

In a recent conversation with journalist Maridel Andanar-Martinez, of the Australian Multicultural Special Broadcasting Service-Filipino program “Radyo SBS,” she aptly described these plants as “[mga] tanim na sumisipsip ng nikel sa ilalim ng lupa (plants that suck nickel from the soil).”

Understanding why and how these plants tolerate toxic conditions is important in providing a better way to restore highly disturbed areas.

Sustainable land use

There are rare examples in the tropical world and the Philippines has had very few of these species discovered in a systematic way. This initial finding and several recent discoveries by other Filipino research teams provide a window of opportunity for research and development of methods which may create novel avenues for long-term and economically sustainable land uses for local communities affected by mining.

(Augustine Doronila Ph.D. is a 2009 balikscientist awardee, research fellow, biogeochemist, restoration ecologist and post-mining reclamation expert. He is connected with the Analytical and Environmental Chemistry Research Group, School of Chemistry, University of Melbourne, Australia.)