Soil and Sediment Extraction

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Talanta 46 (1998) 449455

Extraction procedures for the determination of heavy metals in

contaminated soil and sediment

Gemma Rauret

Dept. Qumica Analtica, Uni6ersitat de Barcelona, Barcelona, Spain

Received 25 May 1997; accepted 14 October 1997

Abstract

Extraction tests are commonly used to study the mobility of metals in soils and sediments by mimicking different

environmental conditions or dramatic changes on them. The results obtained by determining the extractable elements

are dependent on the extraction procedure applied. The paper summarises state of the art extraction procedures used

for heavy metal determination in contaminated soil and sediments. Two types of extraction are considered: single and

sequential. Special attention is paid to the Standard, Measurement and Testing projects from the European

Commission which focused on the harmonisation of the extraction procedures and on preparing soil and sediment

certified reference materials for extractable heavy metal contents. 1998 Elsevier Science B.V. All rights reserved.

Keywords: Extraction procedures; Heavy metals; Contaminated soil; Sediment; Certified reference materials

1. Introduction

Trace metals in soils and sediments may exist in

different chemical forms or ways of binding. In

unpolluted soils or sediments trace metals are

mainly bound to silicates and primary minerals

forming relatively immobile species, whereas in

polluted ones trace metals are generally more

mobile and bound to other soil or sediments

phases. In environmental studies the determina-

tion of the different ways of binding gives more

information on trace metal mobility, as well as on

their availability or toxicity, in comparison with

the total element content. However, the determi-nation of the different ways of binding is difficult

and often impossible. Different approaches are

used for soil and sediment analysis, many of them

focused on pollutant desorption from the solid

phase; others are focused on the pollutant adsorp-

tion from a solution by the solid phase. Among

those approaches based on desorption, leaching

procedures are the most widely accepted and

used.

Extraction procedures by means of a single

extractant are widely used in soil science. These

procedures are designed to dissolve a phase whose

element content is correlated with the availability

of the element to the plants. This approach is well

established for major elements and nutrients and

it is commonly applied in studies of fertility and

quality of crops, for predicting the uptake ofessential elements, for diagnosis of deficiency or

excess of one element in a soil, in studies of the

physical-chemical behaviour of elements in soils

0039-9140/98/$19.00 1998 Elsevier Science B.V. All rights reserved.

PII S 0 0 3 9 - 9 1 4 0 ( 9 7 ) 0 0 4 0 6 - 2


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G. Rauret /Talanta 46 (1998) 449 455450

and for survey purposes. To a lesser extent they

are applied to elements considered as pollutants

such as heavy metals. The application of extrac-

tion procedures to polluted or naturally contami-

nated soils is mainly focused to ascertain the

potential availability and mobility of pollutants

which is related to soil-plant transfer of pollutants

and to study its migration in a soil profile which isusually connected with groundwater problems [1].

For sediment analysis, extraction is used to

asses long term emission potential of pollutants

and to study the distribution of pollutants among

the geochemical phases. As far as heavy metals

are concerned sediments are usually a sink but

may also become a source under certain condi-

tions, especially in heavily contaminated areas or

in drastically changing environments. Chemical

extraction of sediments has proven to be adequate

for determining the metal associated with source

constituents in sedimentary deposits [2], but the

general aim of many studies involving chemical

extraction is the determination of element distri-

bution among different phases of a sediment.

Single extractants are usually chosen to evaluate a

particular release controlling mechanism such as

desorption by increasing salinity or complexing by

competing organic agents. Generally, fractions

can be isolated more specifically by using sequen-

tial extraction schemes. For sediments these pro-

cedures are frequently used and are designed in

relation to the problems arising from disposal ofdredged materials.

Extraction tests, either in soils and sediments,

are always restricted to a reduced group of ele-

ments and as far as soil is concerned they are

applied to a particular type of soil; silicious, car-

bonated or organic. In a regulatory context, two

applications for leaching tests can be recognised:

the assessment or prediction of the environmental

effects of a pollutant concentration in the environ-

ment and the promulgation of guidelines or objec-

tives for soil quality as for example for land

application of sewage sludge or dredge sediments.The data obtained when applying these tests are

used for decision makers in topics such as land

use of soil or in countermeasures application.

2. Commonly used extraction procedures in soils

During the last decades several extraction pro-cedures for extractable heavy metals in soils havebeen developed and modified. In this respect, twogroups of tests must be considered: the singlereagent extraction test, one extraction solution

and one soil sample, and in the sequential extrac-tion procedures, several extraction solutions areused sequentially to the same sample althoughthis last type of extraction is still in developmentfor soils. Both types of extraction are appliedusing not only different extracting schemes butalso different laboratory conditions. This leads tothe use of a great deal of extraction procedures.In Table 1 a summary of the most commonleaching test are given.

Table 1

Most common single extraction tests

Type and solution strength ReferenceGroup

[3]HNO3 0.432 mol l1Acid extraction

Aqua regia [4]

HCl 0.11 mol l1 [3]

CH3COOH 0.1 mol l1 [5]

Melich 1: [6]

HCl 0.05 mol l1+H2SO40.0125 mol l1

EDTA 0.010.05 mols l1 [3]Chelating

agents at different pH

[7]DTPA 0.005 mol l1+TEA

0.1 mol l1

CaCl2 0.01 mol l

1

Melich 3: [8]

CH3COOH 0.02 mol l1

NH4F 0.015 mol l1

HNO3

0.013 mol l1

EDTA 0.001 mol l1

NH4 acetate, acetic acidBuffered salt [9]

buffer pH=7; 1 mol l1solution

[3]NH4 acetate, acetic acid

buffer pH=4.8; 1 mol l1

Unbuffered salt CaCl2 0.1 mol l1 [3]

solution

CaCl2

0.05 mol l1 [3]

[3]CaCl2 0.01 mol l1

NaNO3 0.1 mol l1

[10]NH4NO3 1 mol l

1 [3]

AlCl3 0.3 mol l1 [11]

BaCl2

0.1 mol l1 [12]


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G. Rauret /Talanta 46 (1998) 449 455 451

Table 2

Extraction methods proposed for standardisation or standardised in some European countries

Method MethodCountry Reference

[15]Mobile trace element determination1 mol l1 NH4NO3Germany

[16]Available Cu, Zn and Mn evaluation for fer-France 0.01 mol l1 Na2 EDTA+1 mol l1

tilisation purposesCH3COONH4 at pH=7

DTPA 0.005 mol l1

+TEA 0.1 mol l1

+CaCl2 0.01 mol l

1 at pH=7.3

Available Cu, Zn, Fe and Mn evaluation inItaly 0.02 mol l1 EDTA+0.5 mol l1 [17]

acidic soilsCH3COONH

4at pH=4.6

DTPA 0.005 mol l1+TEA 0.1 mol l1+

CaCl2 0.01 mol l1 at pH=7.3

[18]Availability and mobility of heavy metals inCaCl2 0.1 mol l1Netherlands

polluted soils evaluation

Soluble heavy metal (Cu, Zn, Cd, Pb and [19]Switzerland NaNO3 0.1 mol l1

Ni) determination and ecotoxicity risk evalu-

ation

United Kingdom EDTA 0.05 mol l1 at pH=4 [20]Cu availability evaluation

From Table 1 it can be observed that a single

extraction including a large spectra of extractants

are used. It ranges from very strong acids, such as

aqua regia, nitric acid or hydrochloric acid, to

neutral unbuffered salt solutions, mainly CaCl2 or

NaNO3. Other extractants such as buffered salt

solutions or complexing agents are frequently ap-

plied, because of their ability to form very stable

water soluble complexes with a wide range of

cations. Hot water is also used for the extraction

of boron. Basic extraction by using sodium hy-

droxide is used to assess the influence of thedissolved organic carbon in the release of heavy

metals from soils. A large number of extractants

are reviewed by Pickering [13] and Lebourg [14].

The increasing performance of the analytical

techniques used for element determination in an

extract, together with the increasing evidence that

exchangeable metals better correlate with plant

uptake, has lead extraction methods to evolve

towards the use of less and less aggressive solu-

tions [10]. These solutions are sometimes called

soft extractants and are based on non buffered

salt solutions although diluted acids and complex-ant agents are also included in the group. Neutral

salts dissolve mainly the cation exchangeable frac-

tion although in some cases the complexing ability

of the anion can play a certain role. Diluted acids

dissolve partially trace elements associated to dif-

ferent fractions such as exchangeable, carbonates,

iron and manganese oxides and organic matter.

Complexing agents dissolve not only exchange-

able element fraction but also the element fraction

forming organic matter complexes and the ele-

ment fraction fixed on the soil hydroxides. Nowa-

days it is generally accepted that extractants are

not selective and that minor variations in analyti-

cal procedures have significant effects on the re-

sults.Some leaching procedures for soils have been

adopted officially or its adoption is under study in

different countries with different objectives [14].

An account of these methods are given on Table

2.

3. Commonly used extraction procedures in

sediments

As for soils, exchangeable metal in sediments

are selectively displaced by soft extractants. Otherextractants used are less selective and they co-ex-

tract the exchangeable fraction together with

metals bound to different sediment phases more


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G. Rauret /Talanta 46 (1998) 449 455 453

Table 4

Sequential extraction schemes

2 3 4 5Method 1

HF/HClO4NaOAc 1mol l1 H2O2 8.8 mol l

1Tessier et al. NH2OH.HCl 0.04 molMgCl2 mol l1

l1

HNO3/NH4OAc residualpH 7 pH 5 25% HOAc

silicate phaseorganic matter+sul-exchangeable carbonate Fe/Mn oxides

phide

HNO3

NH4Ox/HOx 0.1 mol H

2O2

8.8 mol l1NH2OH.HCl 0.1 molForstner NaOAc 1 mol

l1 l1 l1

residualpH 7 NH4OAcpH 5 easily reducible pH 3 in dark

silicate phasemoderately reducible organic matter+sul-exchan+carb

phide

NaOAc 1 mol l1 NH2OH.HCl 0.1 molMeguellati BaCl2 1 mol l1 H2O2 8.8 mol l

1+ ashing

l1HNO3 +HF/HCl

residualpH 5 25%HOAcorganic matter+sul-pH 7

phide

silicate phaseFe/Mn oxidesexchangeable carbonate

reau Community of Reference), has sponsored

from 1987 several projects focused on single ex-

traction for soils and sequential extraction for

soils and sediments. The project started with the

intercomparison of existing procedures tested in

an interlaboratory exercise [27]. The next step was

to adopt common procedures for single extraction

of trace metals from mineral soils. The second

step was to adopt a common procedure for se-

quential extraction of sediment. As a conclusion

of the first step, single extraction procedures using

acetic acid, 0.43 mol l1, and EDTA, 0.005 mol

l1 for mineral soils and a mixture of DTPA,

0.005 mol l1 diethylenetriamine pentaacetic acid,

0.01 mol l1 CaCl2 and 0.1 mol l1 tri-

ethanolamine for calcareous soils were adopted

for extractable Cd, Cr, Cu, Ni, Pb and Zn. In

order to improve the quality of the determination

of extractable metal content in different types of

soil using the procedures previously adopted, the

extraction procedures were validated by means of

intercomparison exercises [28,29]. Moreover the

lack of suitable certified reference materials for

this type of studies did not enable the quality of

the measurements to be controlled. With the pur-pose to overcome this problem three certified

reference materials: a terra rossa soil, a sewage

amended soil and a calcareous soil have been

prepared and their extractable trace metal con-

tents were certified (CRM 483, CRM 484 and

CRM 600) [30,31].

The second step of the EC, Standards, Mea-

surement and Testing was focused on a feasibility

study on the adoption and validation of a sequen-

tial extraction scheme for sediment samples. In a

workshop held in 1992 in Sitges (Spain) a sequen-

tial extraction scheme was proposed which in-

cludes three steps: acetic acid, hydroxylamine

hydrochloride or a reducing reagent and hydrogen

peroxide or an oxidising reagent. This procedure

is schematised in Table 5. Moreover in this work-

shop the main analytical limitations in sequential

extraction of trace metals in sediments were thor-

oughly discussed and practical recommendations

were given [32,33]. These recommendations deal

with sampling and sample pre-treatment, practical

experiences with reagents and matrices and ana-

lytical problems after extraction.

Once the scheme was designed, it was tested

through two round robin exercises using two dif-

ferent type of sediment, silicious and calcareous

[34]. In these exercises some critical parameters in

the protocol were identified such as the type andthe speed of the shaking and the need of an

optimal separation of the liquidsolid phases af-

ter the extraction. It was stated that the sediment


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G. Rauret /Talanta 46 (1998) 449 455454

should be continually in suspension during the

extraction. In these intercomparison exercises an

important decrease was noted on the acceptable

set of values for concentration in the extract lower

than 10 mg l1, which illustrates the difficulties

experienced by a number of laboratories in the

determination of such concentration levels in

these matrices. It was concluded that when elec-trothermal atomic absorption spectrometry is

used for the final determination, the method of

standard additions is strongly recommended for

calibration. The results obtained in the round

robin exercises encouraged to proceed with the

organisation of a certification campaign in order

to produce a sediment reference material follow-

ing the sequential extraction scheme adopted. So

the next step of the project was the preparation of

a sediment certified reference material for the

extractable contents of Cd, Cr, Cu, Ni. Pb and

Zn, following the three-step sequential extractionprocedure. A silicious type sediment with rather

high trace metal content was chosen for this pur-

pose. This material has been recently certified for

five metals, Cd, Cr, Ni, Pb and Zn in the first

step, Cd, Ni and Zn in the second step and Cd, Ni

and Pb in the third step [35]. Not all the elements

were certified because the lack of reproducibility

atributable to non adherence to the protocol, in

the acceptance of too large tolerances in the con-

ditions specified in it or in the existence of critical

aspects in the procedure referred mainly to the

second step. These aspects were mainly pH, redox

conditions and possible losses of sediment in the

transfer. The results obtained in the certificationexercise recommended to continue the develop-

ment of the extraction protocol in order to in-

crease reproducibility. Consequently the causes of

non reproducibility are now under study in a new

SMT project.

5. Conclusions

The advantages of a differential analysis over

investigations of total metal contents and about

the usefulness of single and sequential chemical

extraction for predicting long-term adverse effectsof heavy metals from polluted solid material, soils

and sediments, is beyond any doubt. The ad-

vances in this field, especially to make available

soil and sediment certified reference materials for

extractable element contents by using harmonised

procedures, is going to increase the quality of the

results due to the possibility of verifying the ana-

lytical quality control.

Nevertheless some problems need to be solved

with these procedures for example: (1) reactions

are not selective and are influenced by the experi-

mental conditions so it is necessary to identify the

main variables which involves a lack of reproduci-

bility when applying a procedure, to write very

well defined protocols and to validate them; (2)

labile fractions could be transformed during sam-

ple preparation and during sequential extraction

schemes application so problems encountered

when preparing certified reference materials are

not representing all the problems to be found

when working with environmental samples such

as wet sediments, some work in this area is

needed; (3) analytical problems due to the low

level of metals to be measured in the different

fractions especially when using soft extractants;and (4) the procedures need to be optimised and

validated for different type of soils, including

organic soils and sediments.

Table 5

EC Standard, Measurements and Testing procedure

ConditionsStep

1 0.11 mol l

1

HOAc,V m1 40 ml g1

temp. 20oC,

shaking overnight

2 0.1 mol l1 NH2OH.HCl (pH=2 with HNO3)

V m1 40 ml.g1

temp. 20oC

shaking overnight

8.8 mol l1 H2O2 (pH=23 with HNO3)3

V m1=10 ml g1

room temperature 1h.

New addition 10 ml g1

85oC for 1h.

reduce volume to few ml.

1 mol l1

NH4Oac (pH=2 with HNO3)V m1=50 ml g1

20oC

shaking overnight


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G. Rauret /Talanta 46 (1998) 449 455 455

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