|
COMPOST-ENHANCED PHYTOREMEDIATION OF CONTAMINATED
SOIL
(Adapted from: US
EPA, AN ANALYSIS OF COMPOSTING AS AN ENVIRONMENTAL
REMEDIATION TECHNOLOGY, Solid
Waste and Emergency Response (5306W) EPA530-B-98-001)
Phytoremediation is a developing technology in which higher
plants and microorganisms associated with plant roots are the active
agents for uptake and/or degradation of toxic inorganic and organic
compounds in soil and water. This method successfully intercepts
nitrate and prevents its transfer from groundwater to surface water.
It also is used in a number of applications with organics-contaminated
water (Table 1). Plants also reduce the erosional transport of
contaminated soil when compared to unvegetated material. Given this,
phytoremediation provides a straightforward approach to both the
degradation and containment of contaminated soil and water.
Contaminated water is stripped of contaminants
as it flows past the plant roots, as a result of waste uptake by the
plants. Depending on the contaminant, degradation might occur in the
rhizosphere (the soil adjacent to plant roots) or within the plant
itself. If the compound is not degraded, it will likely volatilize.
Regardless of the ultimate fate of the contaminant, once contact with
the plant occurs, the water is no longer contaminated. This process
might be suitable for soil remediation and/or inexpensive confinement
of shallow contaminated water.
Phytoremediation of metal-contaminated soil relies on
the ability of plants to accumulate metals at concentrations
substantially above those found in the soil in which they grow (Kelly,
1995; Brown, 1994; Brown, 1995; Cunningham, 1995; Cornish, 1995).
Since plant uptake requires that metals be in an environmentally
mobile form (Schnoor, 1995), the use of compost is likely to be an impediment to
successful phytoremediation, as compost immobilizes toxic metals.
Table 1 Phytoremediation of Contaminated Soil or
Water
|
Contaminated Material
|
Contaminants
|
Results
|
|
Water
(hydroponic system in laboratory)
|
Nitrobenzene
|
Complete
uptake from solution
|
|
Soil
|
Trinitrotoluene
|
Essentially
complete treatment
|
|
Soil
|
Trichloroethylene
|
Enhanced
mineralization
|
|
Contaminated Soil
|
Pentachlorophenol
and phenanthrene
|
Enhanced
mineralization
|
|
Soil
|
Trinitrotoluene
|
Enhanced
degradation
|
Adapted from: Schnoor, 1995.
Numerous reports indicate that plants can take up and
degrade toxic organic compounds in soil, while other work indicates
microorganisms in the rhizosphere are very competent degraders of
soil-borne organics. Rhizosphere microorganisms are able to degrade
the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) much more rapidly
than those in root-free soil and convert a higher percentage
of carbon in 2,4-D to carbon dioxide.In contrast, enhanced
mineralization of ^14C-labeled pyrene was not found in rhizosphere
soil (Schwab, 1994 and Schwab, 1995). These apparently conflicting
results are due to the relatively high mobility of 2,4-D in soil as
compared to pyrene. As a result of rapid water uptake by plants,
desorption of contaminants from soil may be the rate-limiting step for
degradation (Schnoor, 1995). Based on the examples shown in Table 1,
plants might decrease remediation time, as well as enhance the
complete destruction of target compounds. Further work is required to
define the characteristics of plants and soil systems before an
understanding of the appropriateness of phytoremediation for
particular situations can be attained.
Phytoremediation has very large economic advantages
over mechanically intensive technologies because plants require little
maintenance in comparison to machinery. The following are the major
constraints of the method:
o
Relatively slow remediation rates. The time until site
closure can be years. This constraint means that phytoremediation
cannot be the method of
choice when rapid site closure is a necessity.
o
Lack of information about the fate of compounds in
planted soil. Losses of volatile ^14C from ^14C-labeled naphthalene
are about 50% higher in planted soil than in unplanted soil (Watkins,
1994). Poor recovery is probably the result of inefficient capture of
volatile organics and/or carbon ioxide and can be solved by the
development of better test systems. The issue of whether partial
degradation of xenobiotics, followed by conversion of metabolites into
immobile forms, is a sufficient remedy for contamination. because
immobilization of carbon from xenobiotics in conjugated forms is
promoted in planted systems. The results, presented in Figure 42,
indicate that studies of the fate of xenobiotic residues when they
enter soil would be appropriate. Because of the complexity of plants,
microorganisms, and soil
systems and the uncertainties of chemical behavior in these
systems, further research is necessary before this method can be
employed on a large scale.
o
Difficulties in establishing plants in toxic,
contaminated matrices, and in compacted and barren materials that are
not conducive to plant growth. This constraint can be overcome by the
addition of compost. A small body of research indicates that compost
can reduce toxicity of contaminated soil (probably through the
adsorption of the toxic compounds to organic matter in the compost).
In the absence of compost, little weed growth occurs, but addition of
compost detoxifies the soil and good weed growth occurs. In this case,
plant growth also accelerated decontamination when compared with soil
without compost addition, as shown in Table 2.
The amount of compost needed to achieve beneficial
effects varies with the project goals. For example, 20 percent w/w
compost is sufficient to maximize plant growth in
herbicide-contaminated soil, but 40 percent compost is needed to
accelerate herbicide degradation in the same soil. The decrease in
remediation time for relatively degradable compounds like metolachlor
strongly suggests that phytoremediation--if healthy and vigorous
plants can be established--has considerable potential for enhancing
bio-remediation activities, particularly in situations such as urban
Brownfield's, where cost and time are important components in choosing
a remediation method.
Table
2 Effects of Mix Composition and Planting on Pesticide Degradation,
Following 40 Days of
Plant Growth *
|
Mixture
|
Treatment
|
Tri-fluralin
mg kg^-1 soil |
Metola- chlor
mg kg^-1 soil |
Pendi-methanlin
mg kg^-1 soil |
|
Initial concentration
|
None
|
2.2 + or - 0.9
|
3.0 + or -0.2
|
11.8 + or - 5.1
|
|
100%
contamination
|
Planted
|
0.80 + or - 0.82
(0.27)** |
3.4 + or -5.0
(0.25) |
1.6 + or - 0.4
(0.02) |
|
100% contamination
|
Not Planted
|
0.48 + or -0.77
(0.77) |
0.99+or -1.4
(0.25) |
1.8 + or - 0.4
(0.02) |
|
50:50 soil
|
Planted
|
nd***
|
nd
|
0.5
+ or - 0.6
(0.01)
|
|
50:50 soil
|
Not Planted
|
0.52 + or - 0.53
(0.07) |
0.18+ or -0.16
(<0.001)
|
1.0 + or - 0.02 (0.02)
|
|
50:50 compost
|
Planted
|
0.36 + or - 0.33
(0.02) |
nd
|
1.5
+ or - 0.6
(0.02)
|
|
50:50 compost |
Not Planted |
0.44 = or - 0.69
(0.08) |
2.8 + or -3.4
(0.29) |
2.6 + or - 3.4
(0.12) |
* Values are means + standard deviations of duplicate
extractions of four replications per treatment.
** Values in parentheses indicate the probability that
the values are less than experiences from dilution alone (based on a
one-tailed t-test for means of unequal variance).
*** nd = not detected.
Source: Liu, 1995 |
