Case Study: Heavy metal bioavailability in a soil affected by mineral sulphides contamination follow

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Case StudyHeavy metal bioavailability in a soil
affected by mineral sulphides contamination
following the mine spillage at Aznalcóllars
(Spain)Clemente et al., Biodegradation, 2003

• Aryani Sumoondur Environmental
  Geosciences, Spring 2005

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

• April 1998 5 million m3 of an acidic highly
  toxic pyrite waste spread along the Guadiamar
  river and 45 km2 of arable land
• solid phase (9 105 m3) spread 37 km downstream

Table 1 Composition of Sludge
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Effect on Soil

• In some areas, heavy metal levels (esp. Zn, Cd,
  Cu) still present at phytotoxic levels even
  though most of the sludge and the topsoil was
  removed
• Source (Zn, Cd, Cu) solution phase of spill and
  solid phase for the other elements
• Under suitable aeration moisture conditions,
• sulphides are oxidised to H2SO4(lower pH!)
• 4FeS2 14H2O 15O2 ? 4Fe(OH)3 8SO42- 16H

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Aim of Study

• Assess effect of organic amendment and lime (CaO)
  addition on the bioavailability of heavy metals
  in soils contaminated by the mine spill
• Factors controlling the solubility and
  bioavailability of heavy metals
• 1) Soil pH 2) Redox potential
• 3) Soil texture 4) Electrical Conductivity
• 5) Organic matter (OM) content
• 14 months field experiment where the evolution of
  soil pH and sulphate formation were monitored in
  particular

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How to study bioavailabilty?

• Metal fractions are bioavailable when they are in
  chemical forms which can be taken up by soil
  organisms and plants
• Common method use a chemical extractant or
  sequential leaching to predict bioavailability
  of toxic metals in soils
• Particular chemical phases of metals in the soil
  are extracted, which correlate well with amounts
  of metals taken up by plants grown in the soil

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Methods and Sampling

• Soil type non-calcareous, 19.7 clay, 34.3
  silt, 46 sand and 1.1 OM
• Treatment 12 plots of 32m2
• 3 plots cow manure (soluble and easily
  mineralisable OM)
• 3 plots mature compost with highly humified OM
• rest control
• lime applied to highly acidic plots
• 2 crops of Brassica juncea were grown
• 2 organic amendments were added 1 month before
  each sowing and fertilized

• After 1st crop, all plots were divided into 2-3
  subplots due to the great variation of
  contamination and pH within plots
• Plots showing excessive soil acidification were
  limed pH to about 6.0
• 020 cm deep samples were taken on March, May and
  Dec 2000 and April 2001
• Samples were air dried and sieved at lt2 mm

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Analytical Methods

• Total metal conc. in plant material and soil were
  determined following HNO3/HClO4 digestion
• Bioavailable metals were analysed after
  extraction with DTPA-CaCl2-triethanolamine
• Analysis Atomic Absorption Spectrometry (AAS)
• Soil pH was measured in a saturated soil paste
• EC was determined in a 15 aqueous soil extract
• SO42- content was determined by turbidimetry with
  BaCl2
• Plant growth(fresh and dry weight) were also
  determined

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Results

• Wide variation in total metal conc. between and
  within plots
• Zn, Pb and Cu were principal pollutants
• Removal of sludge was not effective

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pH levels during experiment

• Mar00 wide range
• May00 lower pH (1st harvest) due to sulphide
  oxidation
• Dec00 higher pH values, adequate for plant
  growth ( liming and dry summer conditions )
• April01 low OM and CaCO3,, limited buffering,
  soil pH changes drastically

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SO42- , EC and pH

• SO42- affected pH values of the soil
• pH decreased due to sulphide oxidation
• SO42- show a close relationship with EC
• Plots with pH 7 have lowest SO42-
• Liming decreased SO42- by increasing pH and
  precipitation of soluble SO42- as CaSO4

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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
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B. junceasurvival and biomass production

• pH lt 3.0, plant survival and biomass production
  is zero
• Addition of organic amendments improved
  production

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DTPA-extracted heavy metals
April 2001
May 2000
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Behaviour of different heavy metals

• Zn, Cu, Fe, Mn are in a wide range in all
  samplings due to the differing total metal
  concentrations in each plot
• After 1st harvest, highest values of Zn and Cu
  were found in zones of very low pH
• After the 2nd harvest, soil conc. of Zn, Fe and
  Mn decreased, even in zones where pH was low,
  indicating immobilisation of metals
• Zn,Mn were directly correlated with SO4 2-
• No correlation for Fe and SO4 2- , as Fe
  forms secondary minerals

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Behaviour of different heavy metals

• Pb extracted as low, (0.8) although total Pb
  is high
• Pb shows inverse relationship with SO4 2- due
  to formation of insoluble Pb cpds and adsortion
  on surfaces of Fe-oxides
• OM generally promoted fixation of heavy metals in
  non-available soil fractions (Zn decreased from
  44.2 to 26.7)
• Cu bioavailability did not decrease after second
  harvest due to formation of stable Cu complexes
  with soluble OM

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Conclusions

• Soil was highly contaminated by Zn, Cu and Pb,
  with a wide range of pH
• Plant survival, biomass production and heavy
  metal contents and bioavailability were
  conditioned by soil pH
• Effect of the organic amendments on the
  bioavailability of metals was difficult to
  observe (great variability of total metal
  concentration and pH) but OM improved plant
  growth
• Liming successfully controlled soil acidification

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Effect of OM and lime on soil

• Lime Raises soil pH
• Humified OM and lime immobilise heavy metals,
  improving soil quality
• Soluble OM in fresh manure increases short-term
  solubility of heavy metals
• However, effect of OM on heavy metal
  bioavailability in calcareous soils is not
  related to the OM composition or degree of
  humification