Case Study:Case Study:Heavy metal bioavailability in a Heavy metal bioavailability in a

soil affected by mineral soil affected by mineral sulphides contamination sulphides contamination

following the mine spillage at following the mine spillage at AznalcAznalcóóllars (Spain)llars (Spain)

Clemente et al., Clemente et al., BiodegradationBiodegradation, , 20032003

Aryani Sumoondur Aryani Sumoondur Environmental Geosciences, Spring Environmental Geosciences, Spring 20052005

Los Frailes tailings dam failure, Los Frailes tailings dam failure, Aznalcóllar, Spain (April, 1998)Aznalcóllar, Spain (April, 1998)

OverviewOverview April 1998: 5 million April 1998: 5 million

mm33 of an acidic of an acidic highly toxic pyrite highly toxic pyrite waste spread along waste spread along the Guadiamar river the Guadiamar river and 45 kmand 45 km22 of of arable landarable land

solid phase (9 × solid phase (9 × 105 m105 m33) spread 37 ) spread 37 km downstreamkm downstream

MineralsMinerals %%

Pyrite(FeSPyrite(FeS22)) 83.183.1

Sphalerite Sphalerite (Zn,Fe)S (Zn,Fe)S 5.45.4

Galena(Galena( PbS ) PbS ) 2.12.1

Chalcopyrite(Chalcopyrite(CuFeSCuFeS22

))1.41.4

Arsenopyrite(Arsenopyrite(FeAsS FeAsS ))

0.90.9

Several trace Several trace metalsmetals

N/AN/A

Table 1: Composition of Sludge

Effect on SoilEffect on Soil In some areas, heavy metal levels (esp. Zn, In some areas, heavy metal levels (esp. Zn,

Cd, Cu) still present at phytotoxic levels even Cd, Cu) still present at phytotoxic levels even though most of the sludge and the topsoil was though most of the sludge and the topsoil was removedremoved

Source (Zn, Cd, Cu) : solution phase of spill Source (Zn, Cd, Cu) : solution phase of spill and solid phase for the other elementsand solid phase for the other elements

Under suitable aeration + moisture conditions,Under suitable aeration + moisture conditions,

sulphides are oxidised to Hsulphides are oxidised to H22SOSO44((lower pH!)lower pH!)

4FeS4FeS22 + 14H + 14H22O + 15OO + 15O2 2 →→

4Fe(OH)4Fe(OH)33 + 8SO + 8SO442-2-+ +

16H16H++

Aim of StudyAim of Study Assess effect of organic amendment and Assess effect of organic amendment and

lime (CaO) addition on the lime (CaO) addition on the bioavailabilitybioavailability of of heavy metals in soils contaminated by the heavy metals in soils contaminated by the mine spillmine spill

Factors controlling the solubility and Factors controlling the solubility and bioavailabilitybioavailability of heavy metals of heavy metals1) Soil pH1) Soil pH 2) Redox potential 2) Redox potential

3) Soil texture 3) Soil texture 4) Electrical Conductivity4) Electrical Conductivity

5) Organic matter (OM) content5) Organic matter (OM) content 14 months field experiment where the 14 months field experiment where the

evolution of soil pH and sulphate formation evolution of soil pH and sulphate formation were monitored in particular were monitored in particular

How to study bioavailabilty?How to study bioavailabilty? Metal fractions are bioavailable when they are Metal fractions are bioavailable when they are

in chemical forms which can be taken up by soil in chemical forms which can be taken up by soil organisms and plantsorganisms and plants

Common method: use a chemical extractant or Common method: use a chemical extractant or sequential leaching to predict bioavailability of sequential leaching to predict bioavailability of toxic metals in soilstoxic metals in soils

Particular chemical phases of metals in the soil Particular chemical phases of metals in the soil are extracted, which correlate well with are extracted, which correlate well with amounts of metals taken up by plants grown in amounts of metals taken up by plants grown in the soilthe soil

Methods and SamplingMethods and Sampling Soil type: non-calcareous, Soil type: non-calcareous,

19.7% clay, 34.3% silt, 46% 19.7% clay, 34.3% silt, 46% sand and ~ 1.1% OMsand and ~ 1.1% OM

Treatment :12 plots of Treatment :12 plots of 32m32m22 3 plots: cow manure 3 plots: cow manure

(soluble and easily (soluble and easily mineralisable OM)mineralisable OM)

3 plots: mature compost 3 plots: mature compost with highly humified OMwith highly humified OM

rest: controlrest: control lime: applied to highly lime: applied to highly

acidic plotsacidic plots 2 crops of 2 crops of Brassica juncea Brassica juncea

were grownwere grown 2 organic amendments 2 organic amendments

were added 1 month before were added 1 month before each sowing and fertilizedeach sowing and fertilized

After 1st crop, all plots After 1st crop, all plots were divided into 2-3 were divided into 2-3 subplots due to the great subplots due to the great variation of contamination variation of contamination and pH within plotsand pH within plots

Plots showing excessive Plots showing excessive soil acidification were soil acidification were limed pH to about 6.0limed pH to about 6.0

0–20 cm deep samples 0–20 cm deep samples were taken on March, May were taken on March, May and Dec 2000 and April and Dec 2000 and April 20012001

Samples were air dried Samples were air dried

and sieved at <2 mm and sieved at <2 mm

Analytical MethodsAnalytical Methods Total metal conc. in plant material and soil were Total metal conc. in plant material and soil were

determined following HNOdetermined following HNO33/HClO/HClO44 digestion digestion Bioavailable metals were analysed after extraction Bioavailable metals were analysed after extraction

with DTPA-CaClwith DTPA-CaCl22-triethanolamine -triethanolamine Analysis: Atomic Absorption Spectrometry (AAS)Analysis: Atomic Absorption Spectrometry (AAS) Soil pH was measured in a saturated soil pasteSoil pH was measured in a saturated soil paste EC was determined in a 1:5 aqueous soil extractEC was determined in a 1:5 aqueous soil extract SOSO44

2- 2- content was determined by turbidimetry with content was determined by turbidimetry with BaClBaCl22

Plant growth(fresh and dry weight) were also Plant growth(fresh and dry weight) were also determineddetermined

ResultsResults

Wide variation Wide variation in total metal in total metal conc. conc. between and between and within plotswithin plots

Zn, Pb and Cu Zn, Pb and Cu were principal were principal pollutantspollutants

Removal of Removal of sludge was sludge was not effective not effective

pH levels during experimentpH levels during experiment

Mar00: wide rangeMar00: wide range May00: lower pH May00: lower pH

(1st harvest) due to (1st harvest) due to sulphide oxidationsulphide oxidation

Dec00: higher pH Dec00: higher pH values, adequate for values, adequate for plant growth ( liming plant growth ( liming and dry summer and dry summer conditions )conditions )

April01: low OM and April01: low OM and CaCOCaCO3,3,, limited , limited buffering, soil pH buffering, soil pH changes drasticallychanges drastically

[SO[SO442-2-]] affected pH affected pH

values of the soilvalues of the soil pH decreased due to pH decreased due to

sulphide oxidationsulphide oxidation [SO[SO44

2-2-]] show a close relationship with EC

Plots with pH 7 have Plots with pH 7 have lowest [SOlowest [SO44

2-2-]] Liming decreased

[SO42-] by increasing

pH and precipitation of pH and precipitation of soluble SOsoluble SO44

2-2- as CaSO as CaSO44

SOSO442-2- , EC and pH , EC and pH

In April 2001, sulphate concentrations were at the lowest level

With time, the concentration of oxidisable sulphides decreased, which contributed to pH stabilisation

OM which is more readily oxidised could also have affected the redox conditions by reducing sulphide oxidation

B. junceaB. junceasurvival and biomass survival and biomass

productionproduction

pH < 3.0, plant pH < 3.0, plant survival and survival and biomass biomass production is production is zerozero

Addition of Addition of organic organic amendments amendments improved improved productionproduction

TypeType 1st 1st harvestharvest

2nd 2nd harvestharvest

Cow Cow manuremanure

2869 ± 2869 ± 507 kg 507 kg haha−1−1

518 ± 518 ± 135 kg 135 kg haha−1−1

ComposCompostt

977 ± 977 ± 195 kg 195 kg haha−1−1

2186±52186±563 kg 63 kg haha−1−1

ControlControl 1354 ± 1354 ± 470 kg 470 kg haha−1−1

1631±91631±946 kg 46 kg haha−1−1

DTPA-extracted heavy DTPA-extracted heavy metalsmetals

May 2000 April 2001

Behaviour of different heavy Behaviour of different heavy metals metals

Zn, Cu, Fe, Mn are in a wide range in all Zn, Cu, Fe, Mn are in a wide range in all samplings due to the differing total metal samplings due to the differing total metal concentrations in each plot concentrations in each plot

After 1st harvest, highest values of Zn and Cu After 1st harvest, highest values of Zn and Cu were found in zones of very low pH were found in zones of very low pH

After the 2nd harvest, soil conc. of Zn, Fe and After the 2nd harvest, soil conc. of Zn, Fe and Mn decreased, even in zones where pH was Mn decreased, even in zones where pH was low, indicating immobilisation of metals low, indicating immobilisation of metals

[Zn],[Mn] were directly correlated with [SO[Zn],[Mn] were directly correlated with [SO4 4 2−2− ] ]

No correlation for [Fe] and [SONo correlation for [Fe] and [SO4 4 2−2− ], as Fe forms ], as Fe forms

secondary mineralssecondary minerals

Behaviour of different heavy Behaviour of different heavy metalsmetals

% Pb extracted as low, (0.8%) although % Pb extracted as low, (0.8%) although total [Pb] is high total [Pb] is high

Pb shows inverse relationship with [SO4 Pb shows inverse relationship with [SO4 2− ] due to formation of insoluble Pb cpds 2− ] due to formation of insoluble Pb cpds and adsortion on surfaces of Fe-oxidesand adsortion on surfaces of Fe-oxides

OM generally promoted fixation of heavy OM generally promoted fixation of heavy metals in non-available soil fractions (Zn metals in non-available soil fractions (Zn decreased from 44.2% to 26.7%)decreased from 44.2% to 26.7%)

Cu bioavailability did not decrease after Cu bioavailability did not decrease after second harvest due to formation of stable second harvest due to formation of stable Cu complexes with soluble OMCu complexes with soluble OM

ConclusionsConclusions Soil was highly contaminated by Zn, Cu and Soil was highly contaminated by Zn, Cu and

Pb, with a wide range of pHPb, with a wide range of pH Plant survival, biomass production and heavy Plant survival, biomass production and heavy

metal contents and bioavailability were metal contents and bioavailability were conditioned by soil pHconditioned by soil pH

Effect of the organic amendments on the Effect of the organic amendments on the bioavailability of metals was difficult to bioavailability of metals was difficult to observe (great variability of total metal observe (great variability of total metal concentration and pH) but OM improved plant concentration and pH) but OM improved plant growthgrowth

Liming successfully controlled soil Liming successfully controlled soil acidificationacidification

Effect of OM and lime on soilEffect of OM and lime on soil

Lime: Raises soil pHLime: Raises soil pH Humified OM and lime immobilise Humified OM and lime immobilise

heavy metals, improving soil qualityheavy metals, improving soil quality Soluble OM in fresh manure increases Soluble OM in fresh manure increases

short-term short-term solubility of heavy metalssolubility of heavy metals However, effect of OM on heavy However, effect of OM on heavy

metal bioavailability in calcareous metal bioavailability in calcareous soils is not related to the OM soils is not related to the OM composition or degree of humificationcomposition or degree of humification