Effects of Heavy Metals Toxicities on Vetiver Growth

Paul N.V. Truong and Jeromy Claridge. Resource Management Institute, Queensland Department of Primary Industries, Indooroopilly, Qld, 4068, Australia.

Abstract

Environmentalists are increasingly concerned with problems caused by contaminated land to the environment. Land contaminated by heavy metals, as results of mining and industrial and urban wastes, require effective erosion and sediment control measures to stop its offsite pollution. Vetiver grass which is known to have high tolerance levels to some extreme soil and climatic conditions, is tested for its tolerance to a number of heavy metals common in mining and industrial wastes in Queensland. Results indicate that Vetiver is highly tolerant to high arsenic, cadmium, chromium, copper and nickel levels in the soil. Therefore Vetiver is highly suitable for the rehabilitation/reclamation of lands contaminated by these elements.

Introduction

In recent years, environmentalists have been increasingly critical of the contamination of the environment by by-products of industrial and mining industries. The majority of these contaminations have been caused by high levels of heavy metals which can affect flora, fauna and human population living in areas in the vicinity of or downstream from the contaminated sites. Table 1 shows the levels of heavy metals required environmental and health investigations set out by the Australian and New Zealand authorities.

Table 1: Investigation Thresholds For Contaminants In Soils
Australian and New Zealand Guidelines for Assessment and Management of Contaminated Sites, January 1992 (ANZECC/NHMRC)


THRESHOLDS (mg/kg)

HEAVY METALS Environmental Investigation Health based Investigation

Antimony (Sb) 20 -
Barium (Ba) - -
Cadmium (C) 3 20
Chromium (Cr) 50 -
Cobalt (Co) - -
Copper (Cu) 60 -
Lead (Pb) 300 300
Manganese (Mn) 500 -
Mercury (Hg) 1 -
Molybdenum (Mo) - -
Nickel (Ni) 60 -
Tin (Sn) 50 -
Zinc (Zn) 200 -

These concerns have resulted in strict guidelines to prevent the spreading of these contaminations and in some cases have delayed and even stopped some industrial and mining projects, until appropriate methods of decontamination/rehabilitation are implemented. High levels of heavy metals in the by products are unavoidable but they must be contained or at least preventing from spreading downstream.

The usual methods of decontamination are to treat the contaminants chemically or to physically remove them from sites. Both methods are often expensive and at time impossible to carry out as the volume of contaminated materials can be very large, as in the case of gold and coal mine tailings.

If these wastes cannot be economically treated or removed, they must be prevented from spreading out. Wind and water erosions and leaching are the usual causes of off-site contaminations. Therefore, an effective erosion and sediment control program is needed and rehabilitation with suitable vegetation is the most practical and economical method. However, revegetation on these sites are often very difficult and slow due to their highly adverse growing conditions including toxic levels of heavy metals.

As Vetiver is capable of growing in a wide range of adverse conditions such as toxicities of Al and Mn, highly acidic, alkaline, saline and sodic conditions, this trial was set up to establish its tolerance to a number of heavy metals commonly occurred in mining and industrial wastes in Australia. These include arsenic, cadmium, chromium, copper and nickel.

Materials and Methods

A pot trial was carried out under glass house conditions over a period of 10 weeks. Table 2 shows the treatment levels in the soil of the five heavy metals used. These levels were chosen as they represent the range commonly found in contaminated sites in Queensland.

Table 2: Treatment Levels And Chemicals Used

Treatment Heavy Metals ppm
As(1) Cd(2) Cr(3) Cu(4) Ni(S)
Control 0 0 0 0 0
1 100 5 50 25 100
2 250 10 10 50 200
3 500 20 200 100 300
4 750 60 600 200 400
5 - 120 - - 500

(1) As Di sodium methyl arsenate (DSMA)
(2) As Cadmium sulphate (Cd S04)
(3) As Dipotassium chromate (K2CrO4)
(4) As Copper Sulphate (Cu S04)
(5) As Nickel Chloride 9NiC12)

A complete nutrient solution was applied at the beginning and after 5 weeks to ensure adequate supply of nutrients to Vetiver growth. A close system was used and pots were watered to field capacity daily with de-ionized water. Tops and roots were harvested for yield and chemical analyses.

Results and Discussion

Heavy metals are generally found naturally only at very low concentrations. Elevated concentrations are commonly associated with pollution from human activities. Heavy metals can affect plant growth by interfering with enzyme activities or preventing the absorption of essential nutrients. Many plants are sensitive to heavy metals, those which are tolerant are generally tolerant of most heavy metals. Tolerant plants can be divided into three groups (Robbetal. 1983):

excluder, plants with restricted transport

index plants are those reflect soil concentration

accumulation species which have higher concentration than the soil

In general, results of this series of trial indicate that Vetiver is highly tolerant to this group of heavy metals. In a hydroponic trial, Bowen (1979) found that most vascular plants are highly sensitive to heavy metals toxicity (Table 3). Baker and Eldershaw (1993) also reported very low toxic threshold levels in the soil for As, Cd and Ni (Table 4).

Table 3: Toxic Thresholds Of The Heavy Metals Used In This Trial Under Hydroponic Conditions (Bowen, 1979).


Heavy Metals Toxic Thresholds to Plant Growth (ppm)

Arsenic 0.02 - 7.5
Cadmium 0.2 - 9.0
Chromium 0.5 - 10.0
Copper 0.5 - 8.0
Nickel 0.5 - 2.0



Table 4: Toxicity Levels In The Soil Of Some Heavy Metals (Baker And Eldershaw, 1993).


Heavy Metals Toxic Threshold(ppm)
Arsenic 2.0
Cadmium 1.5
Nickel 7.0 - 10.0


Although no distinctive symptoms were recorded in this trial, leaf tips of all affected plants became chlorotic first and the chlorosis gradually spread to the whole leaf. In the cases of As and Cd the leaf tips first became red then chlorotic.

Arsenic:

Results shown in Table 5 shows that Vetiver yield was significantly reduced when soil arsenic level is at 250 ppm and higher. Although these results did not establish the exact toxic threshold level for Vetiver, which is most likely between 100 and 250 ppm, this level is extremely high as compared with the threshold of 0.02 - 7.5 ppm (Tables 3 and 4). As arsenic soil levels at 20 ppm and 100 ppm may require rehabilitation (Table 1). Vetiver is a highly suitable species for reclaiming these sites.

Literature indicated that high level of As in the plant interferes with S and P metabolism (Kabata and Pendias, 1984).


Table 5: Dry Matter Yield Of Vetiver As Affected By Various Levels Of Arsenic In The Soil


Soil Concentration (ppm) Dry Matter Yield (g/pot) Relative Yield (%)

Control 0 43.85 a* 100
100 43.51 a 99
250 18.93 b 43
500 5.56 c 13
750 0d 0
LSD (5%) 2.95

* Treatments with the same letters are not significantly different.

Cadmium

Although results from Table 6 indicates that Vetiver growth was significantly affected by soil Cd level at and higher than 20 ppm which is extremely high as compared with the threshold shown in Tables 3 and 4.

Table 6: Dry Matter Yield Of Vetiver As Affected By Various Levels Of Cadmium In The Soil.

Soil Concentration (ppm) Dry Matter Yield (g/pot) Relative Yield (%)

Control 0 43.8 a* 100
5 37.86 a 86
10 34.08 a 78
20 33.50 a 76
60 21.06b 48
120 12.24c 28
LSD (5%) 11.59

* Treatments with the same letters are not significantly different.

Although at much reduced growth (48%), Vetiver maintained slow but continual growth at 60 ppm. At this level Vetiver can be established in most contaminated industrial wastes and gold mine tailing with the highest Cd contamination in Queensland.

These results again indicated that Vetiver is highly suitable for rehabilitation of Cd contaminated land as sites with soil levels of 3 ppm and 20 ppm may require rehabilitation (Table 1).

Chromium:

Table 7 shows Vetiver growth is not significantly affected until soil Cr exceeds 200 ppm and the toxic threshold level is between 200 and 600 ppm. This level is extraordinary high as compared with the threshold of between 0.5 and 10.0 ppm shown in Table 3.

Vetiver is therefore highly suitable for reclamation of Cr contaminated lands as sites with soil levels at 50 ppm may require rehabilitation (Table 1).

Table 7: Effects Of Soil Chromium Levels On Vetiver Growth

Soil Concentration (ppm) Dry Matter Yield (g/pot) Relative Yield (%)

Control 0 33.29a* 100
50 28.71 a 81
100 34.64a 98
200 25.80a 73
600 4.68 b 13
LSD (5%) 10.75

* Treatments with the same letters are not significantly different.

Copper

Copper is a micro nutrient for plant, but it is highly toxic to plant at higher levels in the soil. Results of Table 8 indicate that the critical toxic level for Vetiver is between 50 and 100 ppm which is very high compared with the threshold shown in Table 3. However, reasonable growth (56%) continued at soil Cu level of 100 ppm. Sites with soil Cu level at 60 ppm may require rehabilitation (Table 1).

Table 8: Effects Of Soil Copper Levels On Vetiver Growth.

Soil Concentration (ppm) Dry Matter Yield (g/pot) Relative Yield (%)

Control 0 20.53 a* 100
25 17.89 a 87
50 18.87 a 92
100 11.42 b 56
150 4.58 c 22
200 3.80 c 19
LSD (5%) 5.54

* Treatments with the same letters are not significantly different.


Nickel

Although Nickel is considered a trace element for most plants and is a constituent of the important enzyme urease, but it is extremely toxic to plant at high concentration (Kabata and Pendias, 1984).

Baker and Eldershaw (1993) reported the toxic threshold level of Ni in the soil is between 7 and 10 ppm for most plants, but from our trials, 58% of growth can still be obtained at the soil level of 100 ppm. At this level Vetiver is highly suitable for the rehabilitation of Ni contaminated lands which has 60 ppm of Ni or higher (Table 1).

Table 9: Effects Of Soil Nickel Levels On Vetiver Growth.


Soil Concentration (ppm) Dry Matter Yield (g/pot) Relative Yield (%)
Control 0 35.29 a* 100
100 20.56 b 58
200 3.21 c 9
300 7.32 c 21
400 1.31 c 4
500 0.90 c 3
LSD (5%) 10.01

* Treatments with the same letters are not significantly different.


General Discussion and Conclusion

Although results of chemical analyses of plant material are not available at the time of preparing this paper, yield data have clearly demonstrated that Vetiver is highly tolerant to this group of heavy metals and therefore a highly suitable species for the rehabilitation of contaminated lands.

Results of chemical analyses will establish whether Vetiver is an excluder an index plant or an accumulator. If Vetiver is an index plant or an accumulator with high level of heavy metals in the shoots, then the use of Vetiver in the rehabilitation of the contaminated sites will have certain implications.

On the positive side, Vetiver will not only provide a very effective means of erosion and sediment control at these sites, when harvested and removed from the sites and disposed off safely elsewhere, the level of heavy metals in the soil can be gradually lowered with time.

On the negative side, these heavy metals in the plant shoots can enter the food chain and would become a health risk if animals are allowed to grazed on the rehabilitated sites.

More research is needed to investigate the tolerance of Vetiver to mixtures of a number of heavy metals as under field conditions high heavy metal contaminations often occur in combination with others. Literature indicates that interactions between themselves and other plant nutrients can affect plant tolerant level greatly (Bowen, 1979; Kabata and Pendias, 1984; Lepp, 1981).

References:

Baker, D.E. and Eldershaw, V.3. (1993). Interpreting soil analyses for agricultural land use in Queensland. Project Report Series Q093014, QDPI, Brisbane, Australia.

Bowen, H.J.M. (1979). Plants and the Chemical Elements. (Ed.) Academic Press, London

Kabata, A and Pendias, H. (1984). Trace Elements in Soils and Plants. CRC Press, Florida

Lepp, N.W. (1981). Effects of Heavy Metal Pollution on Plants - Vol.1. Effects of Trace metals on plant flinctions. Applied Science Publishers. London

Robb, D.A. and Pierpoint, W.S. (1983). Metals and Micronutrients: Uptake and Utilisation by Plants. Academic Press. London