Heavy Metal Partitioning by SSE and SEM as Analytical Tools for Soil Remediation

by

Rosa Galvez-Cloutier and Jean-S. Dubé

International Workshop on Analysis, Treatment Methodologies and Remediation of Polluted Soils - Interchimie’01

Paris-Nord, VillepinteFRANCE, 2001

Laval UniversityCivil Eng. Dept.Quebec, Canada

Due to their chemical reactivity, soils can become sinks for heavy metals such asPb, Cd, Cu, Zn. These contaminants are of particular concern because they cannot be naturally degraded or eliminated. Lysimetric studies have evaluated the persistence of Pb in soil as ranging from 740 to 5900 years (Kabata-Pendias and Pendias 1991). Furthermore, soil constituents (i.e. clay minerals, secondary minerals, organic matter, and primary crystalline minerals) can concentrate heavy metals to levels higher than found in the interstitial water. They can retain heavy metals by 1) ion exchange, 2) (co)precipitation, 3) complexation, and 4)chemisorption. Heavy metal retention results in a distribution pattern among the soil constituents representing the relative proportions of heavy metals associatedto each geochemical constituent (figure 1).

There are conditions which may enhance the mobility of heavy metals accumulated in soil. In particular, acidic conditions are known to decrease heavy metal retention by soil constituents. The soluble form of a heavy metal is knownto be very mobile, readily (bio)available, and highly toxic. Acid landfill leachate,acid rain, acid spills can inhibit the capacity of the soil to act as a buffer against heavy metal transfer to groundwater and the biota. The partition of heavy metals among soil constituents should dictate the sensitivity of heavy metal particulate species to soil pH variations, the latter being controlled by the buffering capacityof the soil.

Contaminated soil’s origin:Testing fields of a military garrison in MontrealActivities: lead refining and storage of used batteries

Microanalysis by SEM was coupled to a SSE to:Validate SSE by:

identifying geochemical phases containing h.m.study sensitivity to chemical attacks during SSEstudy morphology (SEM)study chemical composition (SEM-EDS)

Heavy metal partition and SSE procedureGeochemical

heavy metal species

Dissolved heavy metals

Metalsulfides

ExchangeableMetals Metal

carbonatesMetal

Oxides/hydroxides

Adsorbedmetals

Organo-metalliccomplexes

Pote

ntia

lly

avai

labl

eH

ighl

yav

aila

ble

Soluble under pH variations

Exchangeable

Associated tocarbonates

Associated toamorphous

oxides/hydroxides

Associated toorganic matter

i AAS

ii

iii

iv

v

I) H2O, ii) KNO3, iii) NaOC+ HOAc, iv) HH, v) H2O2

Soil properties and contaminantsAmorphous Pb (A) atop CaCO3 (B) and Silicate (C)

Parameter Value

Contaminants (mg/kg)Cadmium 12 Chromium 31Copper 82Nickel 21 Lead 2090Zinc 99 Soluble heavy metals not detected

BC A

10um

CaCO3 26 % equivalent (w/w)AM 0.7 % Fe 2O3, 0.1 % MnO (w/w)OM 3.8 % (w/w)CEC 5.8 meq/100g soil

Mineralcomposition

Calcite > quartz > feldspar >kaolinite, illite, chlorite

DescriptivepH 8.0

Results: SSE, SSE-pH

020406080

100

Frac

tion

(%)

Soluble Exchangeable Carbonates Oxides/Hydroxides Organic Residual

• Soil titration curve

02468

101214

-200-1000100200300400500QA (+)/QB (-) (cmol/kg dry soil)

Soil:

solu

tion

Susp

ensi

on p

H

Titration Curve of the Longue Pointe SoilTheoretical Titration of Calcite (CaCO3)

Acid added

Initial pH = 8.0

Base added

Pb Cd Cu Zn Ni Cr Fe Ca

Figure 4.12 Micrographie de cristaux de Pb-Sb (A), de Sn (B) et de Sb (C)

Figure 4.14 Micrographie d’une particule de matière organique

Figure 4.10 Micrographie d’un gel contenant Pb et Sb (fentes de dessication (A))

Figure 4.16 Micrographie d’anthracite montrant des fractures conchoïdales (A)

Results: Pb and Zn partition vs pH

0

20

40

60

80

100

12,812,512118,06,46,15,54,94,53,63,42,61,81,3pH

PbFr

actio

n (%

)

0

20

40

60

80

100

12,812,51211,38,06,46,15,54,94,53,63,42,61,81,3

pH

Zn

Frac

tion

(%)

pH<5

pH<5Only 55% Soluble at pH 1,3

Oxide fraction sensibleto pH changes

Up to 50% retained within the residual fraction

85% Pb soluble at pH 1,3

Pb- oxide highlysensible to pH

~ 40% Pb retainedas carbonates

Set remediation levels and develop h.m. extraction

procedure.

Use h.m. partition results to:

Results: Cd and Zn partition vs pH

0

20

40

60

80

100

12,812,512,111,38,06,46,15,54,94,53,63,42,61,41,3

pH

Cd

Frac

tion

(%) 80% Cd soluble at pH 1,3

Cd - oxide highlysensible to pH

~ 25% Cd retainedas carbonates

pH<5.5

0

20

40

60

80

100

12,812,512,111,38,36,456,15,54,94,53,63,42,61,41,3

pH

Cr

Frac

tion

(%)

Cr almost unsoluble

Cr-oxide stable

80%Cr retained as mineral

Results: Pb and Cd extraction

0

10

20

30

40

50

0 5 10 15 20

Cumulative Pore Volumes ( 1 PV = 400 mL )Pb

Ext

rac.

(cum

. %)

Pb -carbonates = 36%

Cumulative Pore Volumes ( 1 PV = 400 mL )

0

10

20

30

40

0 5 10 15 20

Cd

Ext

rac.

(cum

. %

) Cadmium - carbonates = 34% 1 Vv, 10 cycles

3 Vv, 6 cycles

Continuous Leaching

Conclusions:Geochemical associations established by SSE for Pb, Zn, Cd were confirmed by SEM

SEM observations generally validated the SSE procedure if not, complemented information

The availability of heavy metals could be inferredfrom their initial partitioning or as function of pHStandard remediation criteria were far stricter than actually required/feasible compared to SSE

SSE provided valid information for the design ofa h.m. extraction procedure