From Contamination to Beautification: Ferns Remove Arsenic From Soil

For Teachers

Introduction

illustration of fern anatomy

Arsenic is a chemical element that has the symbol As. Arsenic is widely distributed in nature and is associated with the ores of metals such as gold, lead, and copper. Arsenic also enters the environment through human activities, as arsenic is often used in pesticides, dyes, and chemical weapons. The arsenic compound chromated copper arsenate (CCA) has also been used as a wood preservative. At high doses, arsenic is a poison to humans and other animals which can cause death, and at lower doses over a longer time, it can also cause cancer. Arsenic contamination of soil and water is thus an environmental health issue.

In 2001, University of Florida researcher Lena Ma discovered that the Chinese brake fern (Pteris vittata) grows well in arsenic-contaminated soil. This plant accumulates large amounts of arsenic in its fronds, the portion of the fern that is above ground. This discovery led to the idea that brake fern could help clean up arsenic-contaminated soil. The use of plants to clean up contaminated soil and water is called phytoremediation. Phytoremediation offers an environmentally-friendly and cost-effective method to remove arsenic from contaminated soil. In addition to brake fern, other plants have been found to be useful in phytoremediation. Sunflowers were very effective at removing radioactive materials from the water near the site the 1986 nuclear-power-plant disaster at Chernobyl, Ukraine. Poplar trees are also useful in removing a wide range of pollutants from soil and have been used widely for this purpose.

The flowchart below illustrates how brake fern could be utilized in the remediation of arsenic-contaminated soil. It is important to note that the phytoremediation process does not result in the disappearance of the arsenic. During phytoremediation, the arsenic moves from the soil to the fern fronds. It is then easy to harvest the fern fronds and further concentrate the arsenic in a safer location. Sometimes remediation processes result in complete destruction of the contaminant, as when microorganisms degrade polyaromatic hydrocarbons completely to carbon dioxide and water.

brake fern growth in soil contaminated with arsenic -> harvest contaminated biomass -> pre-treatment: compaction, combustion, compositing -> final disposal: burning or landfill

Data Sets

These data sets are the results of experiments conducted by scientists who are investigating the Chinese brake fern and its application to phytoremediation of arsenic-contaminated soil.

A.

Method: Scientists collected brake fern plants from several uncontaminated sites and then planted them in pots containing soil. The soil contained 97 p.p.m. arsenic, and the brake fern plants were grown for 20 weeks. The concentration of arsenic was measured in the fern’s roots and fronds at various timepoints. The concentration of arsenic is expressed in p.p.m. The abbreviation p.p.m. stands for parts per million, or one milligram arsenic per kilogram of soil or plant.

Fern graph showing growth time versus arsenic concentration

Questions:

B.

Method: In another experiment, brake ferns were planted in pots containing various amounts of arsenic (50 p.p.m., 500 p.p.m., and 1500 p.p.m.). The control soil contained no added arsenic, though very low levels of arsenic are naturally present in soil. After 6 weeks of growth, the scientists measured the amount of arsenic present in the fern frond.

Fern graph showing arsenic concentration as a function of growth time

Questions:

C.

Method: In another experiment, ferns were planted in pots with arsenic-contaminated soil. After 12, 16, and 20 weeks of growth, the scientists measured the amount of arsenic present in the ferns. Specifically, they measured the arsenic concentrations in old fronds, young fronds, and the roots. Mg/kg dry weight is a concentration measurement that means mg arsenic per kg dry weight of fern.

Fern graph showing fern arsenic concentrations in young fronds, old fronds, and roots

Questions:

D.

Grasshopper herbivory and arsenic-containing ferns.

Method: Grasshoppers are herbivourous predators, which means they graze on plants. Ferns are a food source for grasshoppers. Hungry grasshoppers were placed by themselves in small boxes. They were offered a small piece of fern frond as their only food source. The fronds were taken from ferns grown in arsenic-contaminated soil (experimental) or uncontaminated soil (control). The fern fronds were weighed daily for 5 days to determine how much fern the grasshopper ate. The data is presented below as a graph and in a table.

Fern graph showing how much arsenic-containing ferns grasshoppers ate
Amount of fern frond eaten (mg)
24 h control 108
24 h arsenic-treated 16
48 h control 96
48 h arsenic-treated 9

Questions:

E.

Method: In a similar experiment, hungry grasshoppers were placed by themselves in small boxes. This time, they were offered a choice of three pieces of lettuce. The lettuce had been dipped in water for 10 seconds. One piece of lettuce was dipped in water containing a high concentration of arsenic (1.0 mM), one piece of lettuce was dipped in water containing a low concentration of arsenic (0.1 mM), and one piece of lettuce was dipped in water containing no arsenic (0 mM). After 24 h or 48 h, the amount of each piece of lettuce that had been eaten was scored on a scale of 1-10, with 10 representing a completely eaten lettuce leaf. The abbreviation mM stands for millimolar. One molar means one mole of a substance per liter of water.

Fern graph showing how much ferns grasshoppers ate

Questions:

F.

Method: This experiment tested the effect of fertilizers on brake fern’s accumulation of arsenic. The three types of fertilizers tested were Compost 1, Compost 2, and phosphate rock. Brake fern plants were grown in pots in soil with 125 mg/kg arsenic. Equal amounts of fertilizer were added to experimental pots, and no fertilizer was added to the control pots. The scientists measured the arsenic concentration in the roots and fronds after 12 weeks of growth.

Fern graph showing how different fertilizers affected the growth of ferns

Questions:

G:

Brake fern distribution

View a map of brake fern distribution in Florida.

Why might the fern be present in some counties but not in others?

Extension activity:

Fern phytoremediation of arsenic-contaminated soil

Here are some values you can use to predict how ferns might be used to phytoremediate an arsenic-contaminated soil.

Assume that ferns can remove 20 mg arsenic per kg soil in 20 weeks. At the end of 20 weeks, ferns are harvested and replanted until the desired soil arsenic concentration is reached. How many harvests would be required to phytoremediate a soil contaminated with 100 mg/kg arsenic? (answer: about 5) How long would this take? (answer: 100 weeks, or just over 2 years. This assumes the fern can grow year-round.) How many harvests would be required to phytoremediate a soil contaminated with 400 mg/kg arsenic? (answer: 20 harvests) How long would this take? (answer: 400 weeks, or about 8 years) Is it realistic to assume that the ferns will always be able to remove 20 mg arsenic per kg soil in 20 weeks? (answer: No.) Why or why not? (answer: Some of the arsenic will be “harder to get to”, and the removal efficiency will not always be ideal.) What might be some limitations of using brake ferns to phytoremediate arsenic-contaminated soil? (possible answers: The ferns will only remove arsenic from soil to the depth their roots will grow. This technique is only feasible in places where the fern will grow.)

Do research to find an arsenic-contaminated site that interests you. Using the information here and other data you find in your research, make a plan to use brake ferns to phytoremediate the site. What is the history of the site? How big is it? Where is it located? Is it contaminated with compounds other than arsenic? How many fern plants will be needed? How long will the phytoremediation take? What will be done with the land after the cleanup is complete?

Other phytoremediation applications:

Research one of these topics (or another topic of your choice) further online.

Web site resources

References: