Reliable remediation techniques at low cost for contaminated soil and groundwater are required.
Bioremediation- a remediation method using microbes, and phytodegradation-a remediation method using plants are closely watched.
Phytodegradation is the method which reduces contamination in substances such as soil, sediments and groundwater, or prevents discharge of the contaminants from soil using wild flowers, woody plants and rhizosphere microbes. This method was first verified at contaminated sites in the early 1990’s, in the U.S, and since this time, tests and remediation have been performed at more than two hundreds contaminated sites 1).
Soil remediation technology using plants in order to remediate contaminated sites caused by heavy metals has been examined since the 1970’s in Japan.
In this paper, the outline of phytoremediation-a remediation technique for contaminated soil and groundwater using plants and study cases of remediation using plants for remediation of contamination such as petroleum, fluorine and arsenic are described.
1. Outline of phytoremediation
Phytoremediation is divided by the following functions 3): 1) Control of groundwater flow, 2) Phytodegradation-degradation using plants, 3) Phytoextraction-extraction using plants, 4) Phytostabilization-stabilization using plants, 5) Phytovolatilization-volatilization using plants, 6) Rhizodegradation-degradation in the rhizosphere, and 7) Rhizofiltration-filtration in the rhizosphere. Phytoremediation has a neutralizing effect on chlorinated organic compounds, heavy metals, radioactive materials, petroleum, explosives such as TNT and RDX and excess fertilizer (Fig. 1). Plants such as Indian mustard, poplars and grasses have been mainly considered. This method is also helpful as a nonproliferation technology for widespread contamination of low concentration, which meets remediation objectives.
There are the pros and cons of phytoremediation. The good points are that phytoremediation is low in cost, gentle to the environment and doesn’t harm the visual landscape. On the other hand, the bad points are that the remediation technique is limited to the areas where the roots of plants can reach, needs a long time period for application, requires proper waste disposal of the plants used for the uptake of contaminated substances, and a wide amount of space for the remediation plants to grow. 4).
Fig.１Function of phytoremediation
2. Study of remediation of petroleum contaminated soil using plants
Lolium multiflorum L. or Italian ryegrass was grown for five months in simulated diesel oil-contaminated soil with 1.5% (w/w). The relationship between the degradation effect and the growth of roots was examined 5). Measuring total petroleum hydrocabon (TPH), the number of aerobic bacteria, microbial activity and the root length every month, the relationship was analyzed. The microbial activity was evaluated by the dehydrohenase activity (DHA) of soil, which is an index of CO2 elution of soil microbes and degradation activity performed by organics containing oil.
Diesel oil content in the harvest area decreased by 55% during five months compared to the non harvest area. After the 91st day when the root length was over 60% of the maximum value after sowing, diesel oil content in the harvest area showed a significantly lower value than the non harvest area. The DHA of soil in the harvest area also showed a significantly higher value than the non harvest area (Fig. 2). The results indicate that the improved activity of Rhizosphere microbes brought about a nuetralization of the diesel oil. The analysis of the relationship between the growth of roots and the DHA of soil demonstrated a high positive-correlation. The growth of roots was determined to be a very effective remediation in the rhizosphere in the contaminated soil caused by petroleum oil.
Fig.2 Soil dehydrogenase activity during the 152-day experiment
Data shown here represent the mean of 5 replicates.
Error bars indicate the standard deviation. *P<0.05. **P<0.01
3. Study of remediation of fluoride and boron using plants
By changing fluoride content from 200mg/kg to 4000mg/kg using Brassica rapa var. peruviridis, or Komatsuna, the growth inhibition was tested. In addition, searching for plants feasible for remediation was performed at two points (2m X 6m, 2.5m X 6m) of high concentration areas of boron 6).
Plant growth test using fluoride contaminated soil is shown in Fig. 3. On the twentieth day after sowing, Komatsuna grew in the 1000mg/kg fluoride contaminated area, similar to its growth in the non-fluoride contaminated soil. However, a decrease of leaf length was observed in the higher concentration soil. No growth was seen in the contaminated soil of 4000mg/kg fluoride. Elution of fluoride was 21.4mg/L in the content of 1000mg/kg and 48.3mg/L in the content of 1500mg/kg. Although it is larger than the environmental standard deviation at 0.8mg/L, no influence of the large elution of fluoride on germination was confirmed. An influence of less than 30mg/L elution of low concentration of fluoride on plants was also confirmed.
This study found fifty species of plants in thirty different families to be feasible for remediation (Table 1). The whole vegetation rate is more than 90 % in herbaceous layer. Boron content in plants showed a high amount in the leaves. In woody plants, Mulberry, Japanese Hackberry, Zelkova, Elderberry and Oleander showed high boron concentration, while wild flowers, such as American Pokeweed, Ragweed and Commelina demonstrated high concentrations of boron.
Fig.３ Plant growth test using fluoride contaminated soil
Table 1 Plants and boron content at Boron contaminated area
In this paper, the outline of phytoremediation-a remediation technique of contaminated soil and groundwater using plants and study cases of remediation using plants concerning petroleum, fluorine and arsenic have been described.
It was found that roots are an important factor to remediate soil contaminated by oil. As regards the cleanup of fluoride and boron, it was shown that boron can be highly collected in the leaves of some kinds of plants.