Phytoremediation: Overview and Opportunities

by Steven Rock

The following Foreword to Phyto: Principles and Resources for Site Remediation and Landscape Design, Routledge, June 2015, is reprinted with permission of the author.

People have deliberately grown plants to alter their environment for at least millennia. The Roman roads were lined with poplar trees to both provide shade and to keep the roads’ foundations dry by consuming water along the edges, thus making the roads last longer.

The broadest definition of phytotechnologies includes any plantings that enhance the environmental goals for the planet. The field has grown from narrow beginnings to widespread applications and has moved from hopeful but ultimately unfounded expectations to a mature set of techniques and technologies that are commonly accepted as a part of the environmental clean-up toolbox.

Phytotechnology applications have the capacity to play a significant role in transforming contaminated urban land, providing a more sustainable choice for remediation when combined with short and long term land planning.

Phyto in the 20th Century

Before the field was named, people have been using plants to enhance their work. In the 1930s bioprospecting was used as a way to predict the presence of minerals subsurface. Prospectors, particularly in newly opened lands of Siberia, discovered that they could search for plants that grew in only in areas rich in certain minerals. It was noted that some plants were reliable indicators of minerals and that leaves and twigs found in one location could contain quantities of metals much higher than those of the same plant type found in other locations.

In the 1970s several research groups began systematically studying and classifying the relationship between metals and plants and found some plants growing in metals-rich soil to have extraordinary properties. Three researchers in particular, Drs. RR Brooks, RD Reeves, and AJM Baker and their teams, crisscrossed the globe finding and cataloging plants that grew on metal-rich soils and took up unusual quantities of those metals. Some plants were found to uptake more metals than normal plants and eventually named as accumulators and hyperaccumulators.

Increasing general environmental awareness at the time spurred traditional agricultural research to study the effects of environmental contaminants on food production crops, particularly the uptake of potentially harmful heavy metals. The practice of using biosolids from sewage sludge as fertilizer brought rural crop plants in contact with all the industrial pollutants that were flushed down urban drains. It was found that some contaminants did move into some crop plants. One USDA researcher, Dr. Rufus Chaney, suggested that while planting metals excluders might help protect the food supply, it might also be possible to clean soil by raising crops that extract and accumulate metal which could be harvested not for food but for remediation.

Also in the 70s the new field of bioremediation was exploring how to use microbes to attempt environmental clean-up of degradable contaminants. Research began into whether and how much plants enhanced the microbial degradation of pesticides and petroleum products. It was soon clear that planted systems did remediate certain contaminants sooner, deeper, and, in some cases, more completely that microbial systems alone.

Cleanup Efforts Spur Commercialization

Such fundamental research continued into the 1980s, attracting attention from university research teams, government agencies, and private industry. The newfound national and international environmental awareness of the time and the creation and passage of foundational environmental legislation such as the Clean Water Act and the CERCLA (Superfund) lead to increased funding into many possible remediation strategies. Municipalities and corporations were under pressure to reduce the discharge of toxins into the air, water, and onto the land. Cleaning of historical contamination became a new and large industry. Consulting and contracting companies sprang up everywhere, industrial and commercial enterprises started in-house remediation divisions, and government agencies were started or became larger. It is no wonder that by the late 1980s some people were turning their thoughts to commercializing this new process of using plants for remediation.

The use of phytohydraulics for contaminated groundwater management and cleanup of organic pollutants with plants is a widely accepted remediation tool.

The earliest definitions of phytoremediation in the 1990s in publications and presentations refer to environmental protection via metal uptake by plants. Terms and definitions quickly proliferated as firms tried to distinguish and differentiate themselves and their processes. Phyto hyphen anything became a way to classify increasingly specific uses of planted systems. Phyto-degradation, -extraction, -enhanced bioremediation, etc. were used to describe and differentiate aspects of the field. Other terms like rhizofiltration and hydraulic control were invented and used in specific circumstances. Phytotechnologies to this day is an umbrella term that aims to encompass all uses of plants for environmental goals.

The 90s were a time of proliferation of patents as well as companies and invented words. Some patents were for inventions, some for techniques, and some for practices that had been widely used but never patented.

One successful phyto-based patent was introduced when Edd Gatliff patented a TreeWell system that in part uses a downhole sleeve and air tube to induce and enable tree roots to penetrate deeper than they would naturally. The system allows trees to be targeted at a particular depth below ground surface and to tap into contaminated groundwater while bypassing clean water bearing layers. This innovation combines several known and novel techniques and devices and allows remediation to depths and in places otherwise unobtainable.

Other patents were not so specific and had a chilling effect on the deployment of the some remediation practices. The numbers of on-site applications and field experiments fell at the end of the decade in part due to legal concerns over patents and in part because expectations caught up with reality.

Looking to Phytoextraction

It was widely hoped that phytoremediation would solve the problem of widespread low level contamination of heavy metals in soil. Many heavy metals of concern, particularly lead, persist in soils for many decades after spills, dumping, or atmospheric deposition. Large areas of land have soil that poses a risk to residents and workers, but there are few tools that are economical, non-invasive, and effective. Phytoremediation for metals (phytoextraction) was hoped to be all those things and quite literally a green technology in the bargain.

Phytoextraction- utilizing plants to uptake metals for remediation has not lived up to its early promise, and despite continued academic and public interest is not a mainstream tool in the remediation toolkit.

There are some plants that will naturally accumulate some metals under some circumstances. These natural accumulators are often small, grow slowly, and are difficult to cultivate outside of their native and often narrow range. It was hoped and claimed that some plants that grew faster, larger, and using standard agricultural equipment and practices could be induced to take up enough metal to clean soil.

Unfortunately induced phytoextraction of metals has several flaws that have to date proved insurmountable. These include the fact that the most widely used technique relies on chemically altering the contaminant to become much more soluble than in its natural state. This more soluble metal is then more likely to be taken up by the planted phytoextraction crop; however, the soluble metal is also more likely to be washed away into surface water and groundwater where it poses an even greater risk than when it was bound into the soil, which is both morally and regulatorily unacceptable.

There were a number of highly publicized demonstration projects with optimistic reports and enticing pictures. Phytoremediation entered the public lexicon via popular articles, usually featuring a picture a field of sunflowers. After a few careful experiments it was determined that indeed the plants could be induced to take up quantities of metal that could lead to a respectable clean-up in a matter of years, but that the need to prevent the escape of the mobilized metals would prevent the process from ever becoming economically feasible.

First the industrial boosters repurposed their staff and resources. Then the contractors and consultants changed their focus. Phytoextraction remains a popular academic topic of study in the search for the plant that might naturally extract and accumulate enough contaminant to be an effective tool, or to find a safe way to induce uptake. There have been some attempts at genetic modification. Currently phytoextraction of metals has not lived up to its early promise, and despite continued academic and public interest is not a mainstream tool in the remediation toolkit.

Plants Controlling Water

However, at the same time that phytoextraction of metals was enjoying a lot of press, discussion and also some failures other phytotechnologies to mitigate contaminated groundwater plumes and treat organic pollutants such as petroleum and solvents were quietly maturing and joining the toolkit. One of the processes that plants do naturally and quite well is move water. This has been used widely and effectively in landfill covers to prevent precipitation penetration, and in subsurface applications to control contaminated groundwater plumes, and in phytoforensics, where plants are used to track subsurface contaminants.

It was soon shown that planted cover systems for landfills are generally as effective as conventional covers in many parts of the U.S. Like all plant-based systems, the actual effectiveness will be a function of location. A nationwide field study from 1999 to 2011 showed how to determine equivalency for landfill cover systems. Now that there are hundreds of plant-based covers in place and enough more on engineering firm drawing boards, such covers are no longer considered experimental or innovative and regulatory approval is regularly given.

Planted cover systems for landfills are generally as effective as conventional covers in many parts of the country and hundreds of these systems are in place in the U.S.

Planting trees to not only control water but also to enhance bioremediation of organics and light solvents is also common enough to be included in many clean-up plans. Although most metals do not move easily into plants, several other organic contaminants of interest are soluble enough to move or translocate into plants where they are often degraded, without the need for harvesting the plants.

This ability of plants in general and trees in particular to take soluble contaminants from groundwater has allowed an interesting and potentially very useful technique called phytoforensics. Since 2000 Drs. Don Vroblesky, James Landmeyer, and Joel Burken have pioneered and refined the techniques needed to remove tree cores and analyze the chemical content of the sap. Side by side studies have shown that phytoforensics can reveal the origin and direction of groundwater contamination with as great accuracy and considerably less expense and disruption than conventional testing and monitoring well drilling.

Phyto in Wetlands

No discussion of phytotechnologies is complete without including wetlands. In use for cleaning wastewater since at least the 1880s, wetland technology continues to be developed and improved. Many large environmental firms have some capacity to size, specify, and install constructed wetlands to treat industrial or municipal outflow. It is one of the most robust and frequently applied uses of planted systems to achieve such diverse environmental goals as organics degradation, metals sequestration, and wildlife habitat creation- often at the same time.

Since the first meetings to discuss these topics like the “Beneficial Effects of Vegetation in Contaminated Soil” meeting hosted by Kansas State University in 1992 to the now annual conferences of the International Phytotechnology Society, researchers, consultants, regulators, and contractors meet and talk about what works and what does not. The field has undergone a tremendous shift from fringe idea, to highly touted silver bullet, to the current state of reasonable expectations for successful application on a local site-by-site basis.

Utilizing surface flow wetlands for the cleanup of water is one of the most robust and frequently applied uses of planted systems to achieve such diverse environmental goals as organics degradation, metals sequestration, and wildlife habitat creation.

Intersection of Phytotechnologies and Managed Landscape

Phytotechnologists, landscape architects and site designers share an overlapping toolbox with plants, soils, and water as the pieces to build the constructs we are called upon to create. Often a site will employ both sets of professionals – one to clean the canvas and one to provide the finishing touches once the site’s structures are complete. This book (Phyto: Principles and Resources for Site Remediation and Landscape Design) provides a means to bridge those task areas so the means to remediate a site may be part of the final landscape site design. Each profession has a specific and distinct vocabulary as is appropriate for fields that come from widely different origins and have individual project goals and deadlines. This book will help to overcome that language gap, for the landscape architecture community and for any scientists and engineers who want to understand this design discipline.

Planting any given vegetation is neither difficult nor complex, but planting for a particular outcome that sometimes won’t be realized for years or decades requires experience and patience. Practitioners in both fields recognize the need for time on a plant scale, although the site owners and regulators sometimes don’t share that view.

In conclusion, the future of phytotechnology and its application to a wide number of sites and over a range of timescales is still evolving. Designers and scientists working in collaboration can help create the correct environments to advance the range and type of plants to be used, as well as create phased projects that can begin to demonstrate the value of phytotechnologies over time.

Ultimately, phytotechnology is about using specifically selected plants, installation techniques, and creative design approaches to rethink the landscapes of the post-industrial age. It is less about simply the beauty of plants, less about gratuitous site planning and design and the creation of individual design ideas, rather it is to focus through design on plant characteristics to sequester, uptake or break down contaminants in soils and groundwater. The purpose is to understand and include the margins of scientific research and invention to employ broader boundaries, where plant-based remediation can be used for improvement and renewal, and to plan beyond the short term for a longer vision for the contemporary environments of cities, towns and communities.

About the Author

Steven Rock is an Environmental Engineer in the Remediation and Contaminant Branch at EPA’s National Risk Management Research Laboratory in Cincinnati, Ohio and has worked for the EPA since 1994. Steve manages field projects using phytoextraction, phytodegradation, plume control and vegetative. He is the author of several phytotechnology publications, including acting as team leader on the EPA’s Introduction to Phytoremediation, and a chapter in the Standard Handbook of Environmental Engineering. He co-chairs the RTDF Action Team on Phytoremediation, and has three subgroups researching the phytoremediation issues of petroleum hydrocarbons, chlorinated solvents, and vegetative covers for waste containment. He participates in EPA in-house research, and provides technical assistance to EPA regional staff on questions of phytoremediation. Steve was a member of the Interstate Technology & Regulatory Council (ITRC) Phyto team and an instructor in the ITRC training classes. He is a member of the ITRC Phyto Revision Team. Steve earned a bachelor’s degree in Energy Systems from The Evergreen State College in Olympia, Washington, and a master’s degree in Environmental Engineering from the University of Cincinnati.

***

Each author appearing herein retains original copyright. Right to reproduce or disseminate all material herein, including to Columbia University Library’s CAUSEWAY Project, is otherwise reserved by ELA. Please contact ELA for permission to reprint.

Mention of products is not intended to constitute endorsement. Opinions expressed in this newsletter article do not necessarily represent those of ELA’s directors, staff, or members.