Department of Applied Chemistry and Physics Faculty of Agriculture and forestry

Remediation of lead-contaminated soils -

challenges and options

Helinä Hartikainen, Mirva Levonmäki and Salla Hartikainen

Pb as a soil pollutant in Finland

Use in gasoline ended in 1994still high concentrations on roadsides

Use in pellets and shots forbidden in hunting of aquatic birds

allowed in other type of hunting allowed on shooting ranges

Shooting ranges - a special problem 1/2

2000-2500 open-air ranges60% of them in active use

Often very heavily pollutedabout 1/3 of the ranges

can threaten groundwater

4% may cause an immediate health risk

Uneven distribution of Pb load

hot spots

Shooting ranges - a special problem 2/2

Very complex environmentsPb is continuously released from shots and pellets

of different age

large diversity of Pb species and compounds

Abandoned ranges often remain as forested fallow areas used for recreation

or colonization are a risk to biota and humans

Remediation measures are needed

Theoretical background of the study

Detrimental effect of Pb to biota depends on its species

free Pb2+ cations are more toxic than the complexed forms

Bioavailability of Pb is limited bya high tendency to be retained in soil (several

mechanisms)

low permeability of plant cells to Pb

Harmful effects of Pb may also be latent interference with ecosystem functions

Remediation options 1/2

Phytoextraction

Efficiency of depends on the translocation of Pb within plant translocation to above-ground parts is a prerequisite

Pb may have detrimental effects on plant metabolism

1. Are trees naturally present in shooting range areas able to stabilize Pb?

2. Is their growth affected by Pb?3. Can phytoextraction be enhanced by peat-derived

soluble complexing agents?

Remediation options 2/2

Chemical treatment

Can tailing material from apatite mine be used as a sorbent for free Pb cations?

• contains several components contributing to Pb retention

Soil material

Hälvälä shooting range soil

260-530 pellets in 100-g samples from the surface humic layer

total acid soluble Pb 2 000- 43 000 mg/kg (after removing the pellets)

50% in exchangeable form potentially bioavailable

complexation capacity of the humic soil layer seemed to be exceeded

Microcosm experiment

The aim was to study the uptake and allocation of Pb in pine (Pinus

sylvestris L.)

the impact of Pb on the photosynthesis and transfer of carbon to different plant parts

• to indicate the impact of Pb on plant growth

if peat is able to promote the transport of Pb to roots

MicrocosmsUncontaminated coarse-textured mineral soil in the root zone

Humic soil layer from heavily contaminated sector (acid soluble Pb ~ 21 000 mg kg-1)

pellets were not removed

Uppermost layer: peat cover of different thickness

One plant per one experimental unit

Peat

Humic soil

Mineral soil

Analyses

14CO2 fixation was measured at the end of the

experiment• 14C activity of different plant parts was determined

Pb in various plant parts was determined

Plant responses to Pb

Biomass was not affected by Pb during 11-week growing period

Mycorrhizas appeared in the rhizosphere in all unitsroots were active

No Pb tocixity symptoms were seen

Table 1. Total Pb (mg) in various plant parts of the pine seedlings in different treatments

Control 1 cm peat 2 cm peat 3 cm peat

Needles 0.14 0.014 0.015 0.010

Stems 0.16 0.023 0.014 0.063

Roots 5.41 6.44 3.61 7.06

Total 5.72 6.49 3.65 7.14

- Main part of Pb was allocated in roots- Peat addition

-tended to enhance the Pb allocation in the roots-reduced the Pb translocation to needles and stems

14C activity in various parts of pine

0

2000

4000

6000

8000

10000

0 cm 1 cm 2 cm 3 cm

Peat addition

DP

M/g

in p

ine Roots

Stems

Needles

Transfer of 14C to the roots was- reduced by high Pb in the needles

- enhanced by peat addition

Conclusions

Pb allocated in roots hardy was taken into the root cells

• maybe present as extracellular complexes

Less than 0.1% of acid soluble Pb was bioaccessible• acid soluble Pb cannot be used as a measure for the

bioavailable Pb or immediate environmental risk of Pb

Peat can be used to stabilize Pb in roots but not to enhance the phytoextraction to above-ground parts

Chemical treatment

Immobilisation of Pb by Biotite

previously a mineral name used to designate the whole mineral series (annite-phlogopite)

refers here to mixture of minerals processed from tailings produced in apatite ore enrichment

main components: phlogopite (75%) carbonate minerals (16%)other minerals (e.g. apatite) (3%)

Chemical properties of Biotite

Al- and Fe-rich silicate mineral

Al- and Fe(oxy)hydroxides are

sorbents for heavy metals

Carbonates function as Pb

sorbents and promote the retention

through

precipitation or through increase

in soil pH

Apatite is likely to form poorly

soluble Pb compounds

Siilinjärvi apatite pit

The largest phosphate mine in Western Europe produces

- apatite about 800 000 t/a

- biotite 70 000 t/a

Laboratory study

The aim was to examine

the ability of Biotite to retain Pb from aqueous solution

the impact of articificial weathering and particle size of the mineral on its retention capacity

the effect of reaction time on Pb retention by Biotite

IMMOBILIZATION OF Pb BY UNTREATED BIOTITE WITHIN 24 H

-200

0

200

400

600

800

1000

1200

0 0,5 1 1,5 2

Pb mg l-1 after 24 hours

Pb

adso

rptio

n m

g kg

-1

0 2 4 6 8 10 12

pH

Untreated Biotite > 0.2 mm Untreated Biotite < 0.2 mm

pH Untreated Biotite > 0.2 mm pH Untreated Biotite < 0.2 mm

IMMOBILIZATION OF Pb BY ACID-TREATED BIOTITE

WITHIN 24 H

-20000

0

20000

40000

60000

80000

100000

0 2 4 6 8 10

Pb mg l-1 after 24 hours

Pb

ad

so

rpti

on

mg

kg

-1

0 1 2 3 4 5 6 7

pH

Acid-treated Biotite > 0.2 mm" Acid-treated Biotite < 0.2 mm

pH Acid-treated Biotite < 0.2 mm pH Acid-treated Biotite > 0.2 mm

Conclusions

Untreated Biotite efficiently immobilises Pb from aqueous solution, the shape of the isotherm indicating precipitation

• Small particles retain Pb more than the coarse ones

Weathering increases the Pb sorption capacity retention mechanisms presumably differed from those functioning in the untreated material

Reaction time has little or no effect on the retention

Thank you!