Aryani Sumoondur Environmental Geosciences, Spring 2005

1. Case Study:Heavy 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

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

3. 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

4. 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+

5. 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

6. 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

7. 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 0–20 cm deep samples were taken on March, May and Dec 2000 and April 2001 Samples were air dried and sieved at <2 mm Methods and Sampling

8. 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 1:5 aqueous soil extract • SO42- content was determined by turbidimetry with BaCl2 • Plant growth(fresh and dry weight) were also determined

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

10. 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

11. 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

12. 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

13. B. junceasurvival and biomass production • pH < 3.0, plant survival and biomass production is zero • Addition of organic amendments improved production

14. DTPA-extracted heavy metals April 2001 May 2000

15. 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

16. 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

17. 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

18. 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