Peat-assisted phytoremediation of waste foundry sands: plant growth, metal accumulation and fertility aspects.

We investigated the potential of peat additions to improve plant growth and fertility and to reduce plant metal uptake in waste foundry sands (WFS) landfills. The WFS contains 78211 mg kg(-1) and 371 mg kg(-1) concentrations of Cr and Ni, respectively, and varied metal concentrations. The experiment investigated the growth of Brassica juncea plants on fertilized WFS mixed with peat at concentrations of 0, 2.5, 5, and 10% (w/w). The highest peat treatment allowed substantial plant growth and increased Ni mass in shoots, which was positively correlated to shoot biomass increments. On a concentration basis, peat additions did not increase shoot Ni values, thus suggesting that plants grown on peat-treated WFS may not increase risks to human and ecological receptors. Chromium was below detection levels in shoots for all peat treatments. Peat-treated substrates also promoted increased CEC values and higher water holding capacity, therefore improving the WFS agronomical properties. These results indicate that peat can be used as an amendment to assist in the phytoremediation of WFS landfill areas. However, there was evidence for increased mobilization of Cr and Ni in the substrate solution which can pose a threat to local groundwater.

Induced plant uptake and transport of mercury in the presence of sulphur-containing ligands and humic acid.

The induced accumulation of mercury (Hg) by plants was investigated for the species Phaseolus vulgaris (Bush bean), Brassica juncea (Indian mustard), and Vicia villosa (Hairy vetch). All plants were grown in modified Hg-contaminated mine tailings and were treated with sulphur-containing ligands to induce Hg accumulation. The effects of varied substrate Hg concentration and humic acid (HA) level on the induced plant-Hg accumulation for B. juncea were examined. Thiosulphate salts (ammonium and sodium) mobilised Hg in the substrates and caused an increase in the Hg concentration of roots and shoots of all tested plant species. Root Hg accumulation was positively correlated to extractable Hg for (NH4)2S2O3-treated B. juncea plants grown in HA-amended substrates. However, shoot Hg translocation for this species was inhibited at 1.25 g HA kg(-1) of substrate. Mercury-thiosulphate complexes could be translocated and accumulated in the upper parts of the plants up to 25 times the Hg concentration in the substrate. We conclude that shoot Hg accumulation in the presence of thiosulphate salts is dependent upon plant species characteristics (e.g. root surface area) and humic acid content.

Mercury volatilisation and phytoextraction from base-metal mine tailings.

Experiments were carried out in plant growth chambers and in the field to investigate plant-mercury accumulation and volatilisation in the presence of thiosulphate (S2O3)-containing solutions. Brassica juncea (Indian mustard) plants grown in Hg-contaminated Tui mine tailings (New Zealand) were enclosed in gastight volatilisation chambers to investigate the effect of ammonium thiosulphate ([NH4]2 S2O3) on the plant-Hg volatilisation process. Application of (NH4)2 S2O3 to substrates increased up to 6 times the Hg concentration in shoots and roots of B. juncea relative to controls. Volatilisation rates were significantly higher in plants irrigated only with water (control) when compared to plants treated with (NH4)2 S2O3. Volatilisation from barren pots (without plants) indicated that Hg in tailings is subject to biological and photochemical reactions. Addition of sodium thiosulphate (Na2S2O3) at 5 g/kg of substrate to B. juncea plants grown at the Tui mine site confirmed the plant growth chambers studies showing the effectiveness of thio-solutions at enhancing shoot Hg concentrations. Mercury extraction from the field plots yielded a maximum value of 25 g/ha. Mass balance studies revealed that volatilisation is a dominant pathway for Hg removal from the Tui mine site. A preliminary assessment of the risks of volatilisation indicated that enhanced Hg emissions by plants would not harm the local population and the regional environment.