ENVIRONMENTAL RESTORATION of METALSCONTAMINATED SOILS - CHAPTER 9 pptx

Various chemical extractants such as the EDTA method Haq et al., 1980, the DTPA soil test of Lindsay and Norvell 1978, and the AB-DTPA procedure Soltanpour, 1985 have been proposed to ev

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Section II Biological Methods and Processes

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9

The Relationships between the Phytoavailability and the Extractability of Heavy Metals in Contaminated Soils

Lenom J Cajuste and Reggie J Laird

CONTENTS

9.1 Introduction 189

9.2 Background 190

9.2.1 Sources of Soil Contamination 190

9.2.2 Chelating Agents as Soil Tests for Heavy Metals 191

9.3 Materials and Methods 193

9.3.1 Field Observations 193

9.3.2 Sample Preparation and Analysis 193

9.4 Results and Discussion 194

9.4.1 Effect of Chelates on Metal Removal 194

9.4.2 Extractable Metals and Phytoavailability Relationships 194

9.5 Conclusions 197

9.6 Summary 197

References 197

Since the beginning of this century, with the era of intensive Mexican industrialization there has been considerable interest in applying the wastewater collected from the metro-politan area of Mexico City to croplands of the Mezquital Valley This municipal waste-water has been recognized as a valuable source of plant nutrients such as N, P, and minor elements that favor adequate crop growth (Cajuste et al., 1991; Juste and Mench, 1992) In spite of these beneficial effects, there is concern about the potential hazard associated with the consumption of the edible portion of crops grown in this area because of metal phyto-toxicity or transfer of the metal into the food chain (Chang, 1984; Adriano, 1986; Rappaport

et al., 1988; Miller et al., 1995) Various chemical extractants such as the EDTA method (Haq et al., 1980), the DTPA soil test of Lindsay and Norvell (1978), and the AB-DTPA procedure (Soltanpour, 1985) have been proposed to evaluate the phytoavailability of heavy metals from contaminated soils

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190 Environmental Restoration of Metals–Contaminated Soils

9.2 Background

The contamination of soil can derive from distinct activities such as (1) industrial opera-tions, (2) agricultural activities, and (3) agricultural domestic and urban activities Some environmental contaminants and their associated sources are presented in Table 9.1

As a consequence of the adverse effects of the technical civilization of the world, soil as a component of the biosphere has been viewed as a natural buffer system that controls the fate of chemical elements in the environment

Heavy metals are common sources of soil contaminants; they are also present in natural waters because of natural processes or man’s activities In this aqueous phase they do not stay under the soluble form for a long time; they are rather present as suspended colloids

or they are fixed by organic material and mineral substances (Kabata-Pendias and Pendias 1992) The anthropogenic sources of trace elements in waters are associated with mining

of coal and mineral ores, and manufacturing and municipal wastewater operations (Förstner, 1995)

Most of the regional contamination of the soil originates from industrial regions and urban areas where factories, motor vehicles, and municipal waste are the common sources

of heavy metals Furthermore, fertilizers, pesticides, and sewage sludge constitute other

applica-tion on soil composiapplica-tion are of great environmental concern because of their heavy cumu-lative load, which tends to increase the soils’ levels of Ni, Cr, Pb, Zn, Cu, and Hg because

of long-term use (Chaney, 1978; Henry and Harrison, 1992)

TABLE 9.1

Some Heavy Metals Regarded as Environmental Pollutants

Concentration Range (mg kg –1 )

Ni Oil (residual/diesel)

Coal ash Gasoline additives

— 3–1,300

—

Pb Coal ash

Motor vehicle, leaded gasoline Battery reclamation

Battery recycling

10–7,000

— 2.16–43,700 (sediment) 2–135,000 (soil) 0–140 mg L –1 (waters) 210–75,850 (soil)

Cr Wood preserving, tanneries, mining Chromium production

— 500–70,000 Adapted from Peters, R.W and L Shem, Treatment of soils contaminated with heavy metals, in Metal Speciation and Contamination of Soils, H.A Allen, C.P Huang, G.W Bailey, and A.R Bowers, Eds., Lewis Publishers, Boca Raton, FL, 1995.

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The Relationships between the Phytoavailability and the Extractability of Heavy Metals 191

In addition to sewage sources of trace pollutants, heavy metals also can derive from other soil contaminant sources For example, lead originates from motor vehicle exhaust, espe-cially in the vicinity of highways, or from smelters and mining operations Copper and chromium as soil contaminants are found in discharges from tanneries and wood preserv-ing plants Another common soil contaminant is arsenic which has found wide use in the past as a preservative of hides in tanning (Connell, 1997) All of these elements, including

Se, B, Mo, are of primary concern to plant, animal, and human health Of these, in terms of uptake and accumulation by food chain crops, cadmium poses the greatest long-term threat to human health A report from CAST (Council for Agricultural Science and Technol-ogy, 1976) summarizes the impact of these metals on plant and animal health; for example,

Cd, Cu, Ni, Zn, and Mo are the elements that plants can accumulate and then become prob-lems throughout the food chain Copper, nickel, and zinc can cause phytotoxicity at ele-vated levels in acid soils Boron, molybdenum, and arsenic are of concern for both plant and animal health, and Se is of concern for animal health Because they form sparingly sol-uble compounds in the soil, mercury and lead are not taken by plants in amounts harmful

to consumers and have not posed a direct food chain problem for land application of sew-age sludge However, through other pathways of the food chain, such as direct ingestion from soil or the edible portion of crops, they can be harmful to the consumers

A wide range of organic substances also occurs as contaminants in soils; of particular interest are petroleum products, paints, oils, waxes, solvents, plasticizers, pigments, and so forth These organic substances can be divided into several groups including pesticides, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), halogenated aliphatics, ethers, phtalate esters, phenols, monocyclic aromatics, and nitrosamines (Som-mers and Barbarick, 1988; Jorgensen and Johnsen, 1989)

All of these substances would be expected to be present in urban areas and agricultural land The concentrations of most tend to decrease with time because of environmental transformation and degradation processes; however, some, will tend to persist in the envi-ronment for many years, or even many decades

Many heavy metals (essential and nonessential) are taken up by plants at a level often without any cause for concern; however, when crops are cultivated in contaminated soils,

TABLE 9.2

Sources of Some Trace Elements in Agricultural Soils

Element

(%)

mg kg –1 dry weight

Adapted from Kabata-Pendias, A and H Pendias, Trace Elements in Soil and Plants, CRC

Press, Boca Raton, FL, 1992.

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metal plant concentrations may reach phytotoxic levels There is, therefore, a need to monitor heavy metal content of soils Considerable research has been done on the extrac-tion of heavy metals from contaminated soils by chelating agents, primarily the EDTA (ethylenediamine-tetraacetic acid) and the DTPA (diethylenetriaminepentaacetic acid) These chelates decrease the activity of metal ions in soil solution by giving rise to the formation of soluble metal chelate complexes

Initial efforts in selecting a chelate for studying the fate of metals in the environment have centered around EDTA In an experiment carried out on treatment of a soil contam-inated with Cd, Cr, Cu, Pb, and Ni, Ellis et al (1986) reported that the EDTA chelated and solubilized all of the metals to some degree On the other hand, EDTA was seen to offer good potential as soil washing additive for the removal of Pb from soils (Peters and Shem, 1995)

Elliot et al (1989) in a series of batch experiments evaluated extractive decontamination

of Pb-polluted soil using EDTA Their results showed that increasing EDTA concentration resulted in a greater Pb release and that recovery of Pb was higher under acidic than alka-line conditions

Many studies have indicated that soil-test-extractable heavy metals can be correlated with their associated levels in plants Bowman et al (1981) found that Ni was complexed and therefore was maintained in soluble form from different agricultural soils and poten-tial waste disposal sites, in the presence of a high level of Ca ions and a small amount of chelating agent EDTA

The Lindsay and Norvell (1978) procedure using DTPA is one of the most popular soil chelating extractants Haq et al (1980) in a greenhouse study compared nine extractants for plant available Zn, Cd, Ni, and Cu in 46 Canadian soils contaminated with heavy metals; they concluded that DTPA-extractable metals were best correlated with heavy metals uptake by plant

Because the DTPA soil test is to some extent tedious and laborious to perform in laboratories, Soltanpour and Schwab (1977) proposed a simplification of the method by combining the DTPA test with ammonium bicarbonate and by making some adjustments relative to the solution pH and the extracting time Barbarick and Workman (1987), by comparing DTPA and AB-DTPA-extractable Cd, Cu, Ni, Pb, and Zn with metal plant concentration from a greenhouse experiment, found a highly significant linear relation-ship between these parameters They also indicated that either extractant could predict some accumulation of these metals in the field, and that the relationships between AB-DTPA-extractable metal and their associated levels in plant were markedly influ-enced by soil pH Rappaport et al (1988) evaluated the ability of DTPA to predict heavy metal (Cd, Cu, Ni, and Zn) availability to corn grown in three soils amended with high doses of sewage sludge Corn, grain, and stover yields, and DTPA-extractable Cd, Cu, and

Ni were closely related to sludge rate, although the critical phytotoxic level of these metals could not be established

Intensive efforts have been directed (Wang 1997) at the development of sequential frac-tionation schemes that quantitatively partition the total amount of a heavy metal into soil pools that may be interpreted for predicting metal phytoavailability from contaminated soils In this regard, greenhouse and field studies have been conducted to relate these chemical fractions to plant uptake and soil test extractable levels

The objectives of this study were (1) to compare the EDTA extractant to DTPA and AB-DTPA for the removal of metals from contaminated soils; and (2) to determine the relationships between EDTA, DTPA, and AB-DTPA extractable metals of the wastewater

same fields

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9.3 Materials and Methods

Composite soil samples (Typic Calciorthids, 0 to 15 cm depth) were collected from 17 alfalfa plots located in the irrigation District 03 in the Mexican Valley of Mezquital These plots were irrigated with wastewater from domestic and industrial effluents of the metropolitan area of Mexico City (Cajuste 1991), over different periods of time (Table 9.3)

Soil samples were dried and ground for laboratory analyses Soil characterization included the determination of pH, percent clay, percent organic matter, and total Cd, Ni, and Pb Plant samples collected from the 17 plots were dried at 70°C for 48 h and ground in a Wiley

mix-ture (1:4) and analyzed for Cd, Ni, and Pb using an AA spectrophotometer (Table 9.4)

A batch experiment was conducted to determine extractable Cd, Ni, and Pb from the soils

and 8.4 Additionally, duplicates of 1-g samples of soil were analyzed using the following

TABLE 9.3

Sampling Sites as Related to Date of Wastewater Application

First Year under Irrigation

No of Years Irrigated

2-1, S-2, S-3 Tlaxcoapan Lagunilla 1912 87 2-4, S-5 Tlahualilpan Tepatepec 1912 87 2-6, S-7, S-8 Mixquiahuala Progreso 1920 79 S-9, S-10 Tepatepec Tepatepec 1954 35

S-12, S-13 Atitalaquia Lagunilla 1976 23 S-14, S-15 Clavijero Tepatepec 1976 23

TABLE 9.4

Range, Mean, and Standard Deviation of Soil Characteristics and Metal Concentrations in Alfalfa Tissue

Clay (g kg –1 ) 240–700 412.4 13.7 Soil Cd ( µ g g –1 ) 17.4–39.1 30.8 1.17 Soil Ni ( µ g g –1 ) 11.5–35.1 21.6 6.05 Soil Pb ( µ g g –1 ) 7.7–63.0 41.7 1.52 Plant Cd ( µ g g –1 ) 0.48–1.97 1.28 0.47 Plant Ni ( µ g g –1 ) 3.18–9.09 6.35 1.49 Plant Pb ( µ g g –1 ) 1.26–7.70 3.30 1.68

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respec-tively Cd, Ni, and Pb were determined by the same procedure mentioned above

Statistical analyses (correlation and regression procedures) were carried out to compare soil tests and to relate the amounts of Cd, Ni, and Pb extracted from the soils to their associated concentrations in alfalfa tissue

Amounts of metal extracted from the soils, on the average, varied widely, according to solu-tion pH and chelate concentrasolu-tions The convensolu-tional EDTA method was, in general, more effective in removing metals than the DTPA and AB-DTPA soil tests, presumably because

of its stronger solution concentration than the other solutions, thus favoring greater metal solubility (Table 9.5) An increase in chelate concentration generally resulted in greater metal release by EDTA and DTPA, but this effect was not consistent in the extraction of

com-plexation has been reported by many researchers (Ellis et al., 1986; Peters and Shem, 1995; Elliott et al., 1989; Bowman et al., 1981); they suggest that complexing agents offer the potential to be effective extractants of heavy metals from contaminated soils because of the increase in metal solubility

There was no consistent effect of solution pH on the extraction of metals; for instance, an increase in soil test pH showed a negative effect on the level of EDTA extractable Cd and the amounts of Pb removed by chelates (Table 9.5) The quantity of Ni extracted by the chelates, however, showed an opposite trend and DTPA-extractable Cd was insensitive to pH variation

presented) Correlations between the amounts of metals extracted by the conventional

TABLE 9.5

Removal of Heavy Metals from the Soils (Average Values) by Chelates at Different Solution pH Values

(µg g –1 )

a pH of the solution of the conventional method.

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and plant tissue were carried out As only the top portion of the alfalfa plant was sampled and inasmuch as dry matter was not correlated with metal concentration in alfalfa tissue, metal concentration in the plant was used instead of total metal uptake In general, total Cd and Pb correlated very significantly with soil tests (Table 9.7) Although soil organic matter had a positive and significant effect on total Cd, percent clay was not similarly correlated (Table 9.7) This seems to indicate that Cd bound to organic matter was more important as a source of soil Cd than the Cd retained on the mineral exchange complex

A significant correlation was found between plant Cd concentration and Cd removed by the three EDTA concentrations and the conventional DTPA procedure Phytoavailable Cd, however, correlated poorly with AB-DTPA-Cd and the other soil properties (Table 9.7) Likewise, total Pb and Pb concentration in the plant were not correlated with soil pH, organic matter, or percent clay (Table 9.7)

Relationships between total levels of Ni in soil and plant and their amounts complexed

by the chelates showed, to some extent, a different pattern Total Ni was directly related to

TABLE 9.6

Averaged Amounts of Cd, Ni, and Pb Removed from the Soils as Influenced by Solution of Chelates

Chelates

( µg g –1)

a Solution concentration of the conventional method.

TABLE 9.7

Correlation Coefficients (r) between Metals Extracted by the Chelates and Total Metal Content in Soil and Metal Concentration in Alfalfa Tissue

Cd

Cdtot 0.78*** 0.77*** 0.80*** 0.80*** 0.83*** 0.50* 0.75*** 0.67*** 0.68*** –0.5* 0.52* –0.10 CdPp 0.48* 0.55** 0.50* 0.52* 0.45 0.37 0.46 0.36 0.37 –0.19 0.22 0.37 Ni

Nitot 0.54** 0.52* 0.67** 0.81*** 0.78*** 0.58** 0.80*** 0.81*** 0.74*** 0.63** 0.20 –0.01 NiPp 0.72*** 0.62** 0.64** 0.46 0.18 0.51* 0.35 0.30 0.39 0.62** 0.04 0.09 Pb

Pbtot 0.78*** 0.84*** 0.89*** 0.47* 0.88*** 0.69*** 0.71*** 0.92*** 0.86*** –0.43 0.30 0.06 PbPP 0.68** 0.72** 0.67** 0.54** 0.47* 0.61** 0.34 0.56** 0.58** –0.28 0.27 –0.03

Note: ED1 to ED2, DT1 to DT3, and AB1 to AB3 mean 0.01, 0.05, and 0.1 M EDTA, 0.005, 0.01 and 0.1 M DTPA, and 0.005, and 0.01, and 0.1 M AB-DTPA, respectively.

Cdtot, Nitot, and Pbtot = Total Cd, total Ni, and total Pb in soils, respectively.

CdPp, NiPp, and PbPp = Cd, Ni, and Pb concentration in plant, respectively.

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Ni extracted by soil tests and plant Ni concentration was found to be strongly influenced

In the case of soil Ni, similar observations were made by Wang et al (1997) who found that DTPA-extractable Ni correlated significantly with soil Ni fractions as a result of a sequential fractionation analysis from a pot experiment They suggested that the DTPA-extractable Ni

in terms of phytoavailability was related to the concentrations in soil fractions

The amounts of metals removed from the soils by the conventional chelating methods over the whole period of irrigation management correlated poorly with plant metal con-centrations; extractable metals, in general, accounted for less than 50% of the variability of plant metal concentrations This indicates that data for metals extracted during the first period of irrigation management may not belong to the same population as the data from the sites irrigated during the last 20 years The separation of the data on the accumulation

of metals for sampling sites with relatively short- and long-term periods of irrigation man-agement, corresponding to time intervals of 20 to 23 and 45 to 87 years, respectively, improved the prediction of metal phytoavailability dependence, solely on extractable met-als or on these in combination with other soil variables

Regression equations between plant-available metals and their associated extractable metals for Group I (20 to 23 years of irrigation management) and Group II (45 to 87 years

of irrigation management) are shown below (Data for one site in each group were deleted for each of the three metals, as metal concentrations in alfalfa plants grown in the two sites were much higher than the other values and would have biased the regression equations.)

GROUP I

GROUP II

respectively

In Group I, the highest regression coefficient was observed when the plant concentration

This regression accounted for about 87% of the variability of plant Ni concentrations In the case of Group II, the combined effect of EDTA-extracted Pb and percent organic matter was responsible for 85% of the variability in plant Pb concentrations On the other hand, the removal of Pb by EDTA alone could explain 85% of the variation in plant Pb concentrations (Equation 1)

A somewhat different situation was observed with Cd in Group II, where about 72% of the variability in plant concentration of Cd could be explained by differences in soil pH and percent organic matter (Equation 8) EDTA extractable Cd, on the other hand, had only a

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These results are in agreement with earlier findings of some researchers (Haq et al., 1980;

Ellis et al., 1986; Peters and Shem, 1995; Elliot, 1989; Bowman et al., 1981; Soltanpur and

Schwab, 1977; Barbarick and Workman, 1987; Peters and Ku, 1987; Wang et al., 1997; Xiu

et al., 1991; Jorgensen and Johnsen, 1989) who showed that these three extractants have

proved to be reliable chemical methods for assessing the levels of toxic metals in soils

The three chelating agents appeared to be effective extractants in removing metals from

contaminated soils The EDTA procedure was the best single extracting agent for most

metals Extractable metals were, to some extent, influenced by solution pH and chelate

concentrations Correlation coefficients between soil tests were highly significant, but

extractable metals accounted for less than 50% of the variability of plant metal

concentra-tions The separation of the soils into two groups, based on the number of years of irrigation

management, improved the prediction of metal phytoavailability, solely in terms of the

amounts of extractable metals or on these in combination with other soil variables

Chelating agents are known to be effective extractants of heavy metals from contaminated

soils This study was aimed to compare the DTPA procedure with the EDTA and AB-DTPA

soil tests for extraction of Cd, Ni, and Pb, and to determine the relationships between soil

Soil samples (Typic Calciorthids) were collected from 17 unfertilized plots irrigated with

wastewater They were analyzed for metal contents using varying chelate concentrations

and at three pH values Plant samples were obtained from the sites and were analyzed for

metal content Results indicated that EDTA, in general, extracted greater amounts of metals

than DTPA, regardless of solution pH The amounts of metals removed were influenced by

extractant concentrations Correlation coefficients between soil tests were highly significant

accounted for less than 50% of the variation; however, the separation of the soil sites by

amounts of time under irrigation with wastewater improved the relationship between

these two parameters

References

Adriano, D.C., Nickel, in Trace Elements in the Terrestrial Environment, Springer-Verlag, New York,

1986, 362.

Barbarick, K.A and S.M Workman, Ammonium bicarbonate-DTPA and DTPA extractions of

sludge-amended soils, J Environ Quality, 16, 125, 1987.

Bowman, R.S., M.S Essington, and G.A O’Connor, Soil sorption of nickel: influence and composition,

Soil Sci Soc Am J., 45, 860, 1981

Tiêu đề	The Relationships Between The Phytoavailability And The Extractability Of Heavy Metals In Contaminated Soils
Tác giả	Lenom J. Cajuste, Reggie J. Laird
Trường học	CRC Press LLC
Chuyên ngành	Environmental Science
Thể loại	Chương
Năm xuất bản	2001
Thành phố	Boca Raton

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