Sludge Watch ==> Illinois Urban Manual - Soil Quality

[maureen.reilly at sympatico.ca]

Sludgewatch Admin

This document is a good backgrounder to help people understand soil quality 
and heavy metals.
The tables wil read properly if you look at this document on line.

.....................................................................


http://www.il.nrcs.usda.gov/technical/engineer/urban/tech_notes/technote3.html

Illinois Urban Manual
Technical Note No. 3
SOIL QUALITY – URBAN TECHNICAL NOTE No. 3
Heavy Metal Soil Contamination


Introduction
Soil is a crucial component of rural and urban environments, and in both 
places land management is the key to soil quality. This series of technical 
notes examines the urban activities that cause soil degradation, and the 
management practices that protect the functions urban societies demand from 
soil. This technical note focuses on heavy metal soil contamination.


Metals in Soil
Mining, manufacturing, and the use of synthetic products (e.g. pesticides, 
paints, batteries, industrial waste, and land application of industrial or 
domestic sludge) can result in heavy metal contamination of urban and 
agricultural soils. Heavy metals also occur naturally, but rarely at toxic 
levels. Potentially contaminated soils may occur at old landfill sites 
(particularly those that accepted industrial wastes), old orchards that used 
insecticides containing arsenic as an active ingredient, fields that had 
past applications of waste water or municipal sludge, areas in or around 
mining waste piles and tailings, industrial areas where chemicals may have 
been dumped on the ground, or in areas downwind from industrial sites.

Excess heavy metal accumulation in soils is toxic to humans and other 
animals. Exposure to heavy metals is normally chronic (exposure over a 
longer period of time), due to food chain transfer. Acute (immediate) 
poisoning from heavy metals is rare through ingestion or dermal contact, but 
is possible. Chronic problems associated with long-term heavy metal 
exposures are:

Lead – mental lapse.
Cadmium – affects kidney, liver, and GI tract.
Arsenic – skin poisoning, affects kidneys and central nervous system.
The most common problem causing cationic metals (metallic elements whose 
forms in soil are positively charged cations e.g., Pb2+) are mercury, 
cadmium, lead, nickel, copper, zinc, chromium, and manganese. The most 
common anionic compounds (elements whose forms in soil are combined with 
oxygen and are negatively charged e.g., MoO42-) are arsenic, molybdenum, 
selenium, and boron.


Prevention of Heavy Metal Contamination

Preventing heavy metal pollution is critical because cleaning contaminated 
soils is extremely expensive and difficult. Applicators of industrial waste 
or sludge must abide by the regulatory limits set by the U.S. Environmental 
Protection Agency (EPA) in Table 1.

Table 1. Regulatory limits on heavy metals applied to soils (Adapted from 
U.S. EPA, 1993).

Table1 Heavy metal Maximum concentration in sludge

(mg/kg or ppm) Annual pollutant loading rates Cumulative pollutant loading 
rates
(kg/ha/yr) (lb/A/yr) (kg/ha) (lb/A)
Arsenic
75
2
1.8
41
36.6

Cadmium
85
1.9
1.7
39
34.8

Chromium
3000
150
134
3000
2,679

Copper
4300
75
67
1500
1,340

Lead
420
21
14
420
375

Mercury
840
15
13.4
300
268

Molybdenum
57
0.85
0.80
17
15

Nickel
75
0.90
0.80
18
16

Selenium
100
5
4
100
89

Zinc
7500
140
125
2800
2500


Prevention is the best method to protect the environment from contamination 
by heavy metals. With the above table, a simple equation is used to show the 
maximum amount of sludge that can be applied. For example, suppose city 
officials want to apply the maximum amount of sludge (kg/ha) on some 
agricultural land. The annual pollutant-loading rate for zinc is 140 
kg/ha/yr (from Table 1). The lab analysis of the sludge shows a zinc 
concentration of 7500 mg/kg (mg/kg is the same as parts per million). How 
much can the applicator apply (tons/A) without exceeding the 140 kg/ha/yr?

Solution:
(1) Convert mg to kg (1,000,000 mg = 1kg) so all units are the same:

7500 mg X (1 kg/1,000,000 mg) = 0.0075 kg

(2) Divide the amount of zinc that can be applied by the concentration of 
zinc

in the sludge:

(140 kg Zn/ha) / (0.0075 kg Zn/kg sludge) =18,667 kg sludge/ha

(3) Convert to lb/A: 18,667 kg/ha X 0.893 = 16,669 lbs/A

Convert lbs to tons: 16,669 lb/A / 2,000 lb/T = 8.3 T sludge per acre



Traditional Remediation of Contaminated Soil
Once metals are introduced and contaminate the environment, they will 
remain. Metals do not degrade like carbon-based (organic) molecules. The 
only exceptions are mercury and selenium, which can be transformed and 
volatilized by microorganisms. However, in general it is very difficult to 
eliminate metals from the environment.

Traditional treatments for metal contamination in soils are expensive and 
cost prohibitive when large areas of soil are contaminated. Treatments can 
be done in situ (on-site), or ex situ (removed and treated off-site). Both 
are extremely expensive. Some treatments that are available include:

High temperature treatments (produce a vitrified, granular, non-leachable 
material).
Solidifying agents (produce cement-like material).
Washing process (leaches out contaminants).
Management of Contaminated Soil
Soil and crop management methods can help prevent uptake of pollutants by 
plants, leaving them in the soil. The soil becomes the sink, breaking the 
soil-plant-animal or human cycle through which the toxin exerts its toxic 
effects (Brady and Weil, 1999).

The following management practices will not remove the heavy metal 
contaminants, but will help to immobilize them in the soil and reduce the 
potential for adverse effects from the metals – Note that the kind of metal 
(cation or anion) must be considered:

Increasing the soil pH to 6.5 or higher.
Cationic metals are more soluble at lower pH levels, so increasing the pH 
makes them less available to plants and therefore less likely to be 
incorporated in their tissues and ingested by humans. Raising pH has the 
opposite effect on anionic elements.

Draining wet soils.
Drainage improves soil aeration and will allow metals to oxidize, making 
them less soluble. Therefore when aerated, these metals are less available. 
The opposite is true for chromium, which is more available in oxidized 
forms. Active organic matter is effective in reducing the availability of 
chromium.

Applying phosphate.
Heavy phosphate applications reduce the availability of cationic metals, but 
have the opposite effect on anionic compounds like arsenic. Care should be 
taken with phosphorus applications because high levels of phosphorus in the 
soil can result in water pollution.

Carefully selecting plants for use on metal-contaminated soils
Plants translocate larger quantities of metals to their leaves than to their 
fruits or seeds. The greatest risk of food chain contamination is in leafy 
vegetables like lettuce or spinach. Another hazard is forage eaten by 
livestock.


Plants for Environmental Cleanup
Research has demonstrated that plants are effective in cleaning up 
contaminated soil (Wenzel et al., 1999). Phytoremediation is a general term 
for using plants to remove, degrade, or contain soil pollutants such as 
heavy metals, pesticides, solvents, crude oil, polyaromatic hydrocarbons, 
and landfill leacheates For example, prairie grasses can stimulate breakdown 
of petroleum products. Wildflowers were recently used to degrade 
hydrocarbons from an oil spill in Kuwait. Hybrid poplars can remove 
ammunition compounds such as TNT as well as high nitrates and pesticides 
(Brady and Weil, 1999).


Plants for Treating Metal Contaminated Soils
Plants have been used to stabilize or remove metals from soil and water. The 
three mechanisms used are phytoextraction, rhizofiltration, and 
phytostabilization. This technical note will define rhizofiltration and 
phytostabilization but will focus on phytoextraction.

Rhizofiltration is the adsorption onto plant roots or absorption into plant 
roots of contaminants that are in solution surrounding the root zone 
(rhizosphere). Rhizofiltration is used to decontaminate groundwater. Plants 
are grown in greenhouses in water instead of soil. Contaminated water from 
the site is used to acclimate the plants to the environment. The plants are 
then planted on the site of contaminated ground water where the roots take 
up the water and contaminants. Once the roots are saturated with the 
contaminant, the plants are harvested including the roots. In Chernobyl, 
Ukraine, sunflowers were used in this way to remove radioactive contaminants 
from groundwater (EPA, 1998).

Phytostabilization is the use of perennial, non-harvested plants to 
stabilize or immobilize contaminants in the soil and groundwater. Metals are 
absorbed and accumulated by roots, adsorbed onto roots, or precipitated 
within the rhizosphere. Metal-tolerant plants can be used to restore 
vegetation where natural vegetation is lacking, thus reducing the risk of 
water and wind erosion and leaching. Phytostabilization reduces the mobility 
of the contaminant and prevents further movement of the contaminant into 
groundwater or the air and reduces the bioavailability for entry into the 
food chain.


Phytoextraction
Phytoextraction is the process of growing plants in metal contaminated soil 
. Plant roots then translocate the metals into aboveground portions of the 
plant. After plants have grown for some time, they are harvested and 
incinerated or composted to recycle the metals. Several crop growth cycles 
may be needed to decrease contaminant levels to allowable limits. If the 
plants are incinerated, the ash must be disposed of in a hazardous waste 
landfill, but the volume of the ash is much smaller than the volume of 
contaminated soil if dug out and removed for treatment. (See box.)

Example of Disposal
Excavating and landfilling a 10-acre contaminated site to a depth of 1 foot 
requires handling roughly 20,000 tons of soil. Phytoextraction of the same 
site would result in the need to handle about 500 tons of biomass, which is 
about 1/40 of the mass of the contaminated soil. In this example, if we 
assume the soil was contaminated with a lead concentration of 400 ppm, six 
to eight crops would be needed, growing four crops per season (Phytotech, 
2000).

Phytoextraction is done with plants called hyperaccumulators, which absorb 
unusually large amounts of metals in comparison to other plants. 
Hyperaccumulators contain more than 1,000 milligrams per kilogram of cobalt, 
copper, chromium, lead, or nickel; or 10,000 milligrams per kilogram (1 %) 
of manganese or zinc in dry matter (Baker and Brooks, 1989). One or more of 
these plant types are planted at a particular site based on the kinds of 
metals present and site conditions. Tables 2 and 3 demonstrate the 
importance of using hyperaccumulators.

Table 2. Percentage decrease in water-extractable zinc and cadmium in three 
soils after growth of Alpine pennycress (Thlaspi caerulescens) (McGrath, 
1998).

Table 2 Site Sampled Zn Cd
Farm
28
10

Garden
17
22

Mountain
64
70


Table 3. Removal of zinc in a hypothetical 4.5 T/A (dry matter) crop growing 
in soil contaminated with 1000 (ppm) zinc with a target of 50 ppm, showing 
the importance of hyperaccumulation (>10,000 ppm zinc) (McGrath, 1998).

Table 3 ppm Zn
in plant Lbs. of Zn removed % of soil total in one crop years to target
100 0.9 0.04 2470.0
1000 9 0.38 247.0
10,000 90 3.85 24.7
20,000 179 7.69 12.4
30,000 268  11.54 8.2



Phytoextraction is easiest with metals such as nickel, zinc, and copper 
because these metals are preferred by a majority of the 400 hyperaccumlator 
plants. Several plants in the genus Thlaspi (pennycress) have been known to 
take up more than 30,000 ppm (3%)of zinc in their tissues. These plants can 
be used as ore because of the high metal concentration (Brady and Weil, 
1999).

Of all the metals, lead is the most common soil contaminant (EPA, 1993). 
Unfortunately, plants do not accumulate lead under natural conditions. A 
chelator such as EDTA (ethylenediaminetetraacetic acid) has to be added to 
the soil as an amendment. The EDTA makes the lead available to the plant. 
The most common plant used for lead extraction is Indian mustard (Brassisa 
juncea). Phytotech (a private research company) has reported that they have 
cleaned up lead-contaminated sites in New Jersey to below the industrial 
standards in 1 to 2 summers using Indian mustard (Wantanabe, 1997).

Plants are available to remove zinc, cadmium, lead, selenium, and nickel 
from soils at rates that are medium to long-term, but rapid enough to be 
useful. Many of the plants that hyperaccumulate metals produce low biomass, 
and need to be bred for much higher biomass production.

Current genetic engineering efforts at USDA in Beltsville, MD, are aimed 
toward developing pennycress (Thlaspi) that is extremely zinc tolerant. 
These taller-than-normal plants would have more biomass, thereby taking up 
larger quantities of contaminating metals (Watanabe, 1997).

Traditional cleanup in situ may cost between $10.00 and $100.00 per cubic 
meter (m3), whereas removal of contaminated material (ex situ) may cost as 
high $30.00 to $300/ m3. In comparison, phytoremediation may only cost 
$0.05/ m3 (Watanabe, 1997).


Future Prospects
Phytoremediation has been studied extensively in research and small-scale 
demonstrations, but in only a few full-scale applications. Phytoremediation 
is moving into the realm of commercialization (Watanabe, 1997). It is 
predicted that the phytoremediation market will reach $214 to $370 million 
by the year 2005 (Environmental Science & Technology, 1998).

Given the current effectiveness, phytoremediation is best suited for cleanup 
over a wide area in which contaminants are present at low to medium 
concentrations. Before phytoremediation is fully commercialized, further 
research is needed to assure that tissues of plants used for 
phytoremediation do not have adverse environmental effects if eaten by 
wildlife or used by humans for things such as mulch or firewood (EPA, 1998). 
Research is also needed to find more efficient bioaccumulators, 
hyperaccumulators that produce more biomass, and to further monitor current 
field trials to ensure a thorough understanding. There is the need for a 
commercialized smelting method to extract the metals from plant biomass so 
they can be recycled.

Phytoremediation is slower than traditional methods of removing heavy metals 
from soil but much less costly. Prevention of soil contamination is far less 
expensive than any kind of remediation and much better for the environment.



References
Baker, A.J.M., and R.R. Brooks. 1989. Terrestrial plants which 
hyperaccumulate metallic elements – a review of their distribution, ecology, 
and phytochemistry. Biorecovery 1:81:126.
Brady, N.C., and R.R. Weil. 1999. The nature and properties of soils. 12th 
ed. Prentice Hall. Upper Saddle River, NJ.
Environmental Science & Technology. 1998. Phytoremediation; forecasting. 
Environmental Science & Technology. Vol. 32, issue 17, p.399A.
McGrath, S.P. 1998. Phytoextraction for soil remediation. p. 261-287. In R. 
Brooks (ed.) Plants that hyperaccumulate heavy metals their role in 
phytoremediation, microbiology, archaeology, mineral exploration and 
phytomining. CAB International, New York, NY.
Phytotech. 2000. Phytoremediation technology. 
http://clu-in.org/PRODUCTS/SITE/ongoing/demoong/phytotec.htm
U.S. EPA. 1993. Clean Water Act, sec. 503, vol. 58, no. 32. (U.S. 
Environmental Protection Agency Washington, D.C.).
U.S. EPA. 1998. A citizen’s guide to phytoremediation. http://clu- 
in.org/PRODUCTS/CITGUIDE/Phyto2.htm
Watanabe, M.E. 1997. Phytoremediation on the brink of commercialization. 
Environmental Science & Technology/News. 31:182-186.
Wenzel, W.W., Adriano, D.C., Salt, D., and Smith, R. 1999. Phytoremediation: 
A plant-microbe based remediation system. p. 457-508. In D.C. Adriano et al. 
(ed.) Bioremediation of contaminated soils. American Society of Agronomy, 
Madison, WI.
Disclaimer
Trade names are used solely to provide specific information. Mention of a 
trade name does not constitute a guarantee of the product by the U.S. 
Department of Agriculture nor does it imply endorsement by the Department or 
the Natural Resources Conservation Service over comparable products that are 
not named.