Phytoextraction of multiple heavy metals from contaminated soils

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Department of Agricultural Sciences, School of Life Sciences, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

Phytoextraction that utilizes plants to take up heavy metals is a promising approach to remediate contaminated soils. Most contaminated soils are characterised by high concentrations of multiple heavy metals and high salinity, but low concentrations of nutrients. Thus, plants for phytoextraction should not only have the ability to accumulate high concentrations of heavy metals, but also exhibit multiple tolerant traits to cope with adverse soil conditions. The objective of this thesis was to explore the potential of an Australian native succulent halophyte Carpobrotus rossii (Haw.) Schwantes for phytoextraction of heavy metals, as well as associated affecting factors and possible mechanisms. The first experiment was carried out to screen the potential candidates for phytoextraction among 14 succulent plant species exposed to the combination of Cd, Cr, Cu, Mn, Ni, Pb and Zn. The results revealed that C. rossii not only had higher tolerance and biomass production, but also exhibited greater accumulation of multiple heavy metals than other species, suggesting that C. rossii is a promising candidate for phytoextraction of multiple heavy metals. The second experiment was conducted to examine the tolerance and accumulation of Cd and Zn in C. rossii, and to investigate the interactions of Cd and Zn on phytoextraction. The results showed that the critical value of C. rossii that causes 10 % reduction in shoot biomass was 115 µg g-1 for Cd and 1300 µg g-1 for Zn, respectively. The combined addition of Cd and Zn decreased biomass production more than the addition of Cd or Zn alone, and significantly increased Cd concentration, but did not affect Zn concentration in plant parts. The results suggest that C. rossii is able to hyperaccumulate Cd and could be used for phytoextraction of Cd from polluted soils. The third experiment investigated the tolerance and accumulation of Mn and its interactions of Cd and Zn in C. rossii. It showed that C. rossii was tolerant to Mn and had a critical value of Mn in shoots, up to 6737 µg g-1 , a high bioaccumulation factor (the ratio of the Mn concentration in shoots to added Mn concentration in the soil, BF = 1.1 - 4.2), and a high translocation factor (the ratio of Mn concentration in shoots to that in roots, TF = 1.6 - 4.6). Thus, C. rossii is a Mn accumulator with a potential phytoextraction of Mn-polluted soils. The salt tolerance of the halophyte C. rossii is an advantage over glycophyte hyperaccumulators. The effects of salinity on phytoextraction of C. rossii were tested with different salts at three levels of salinity. The addition of salts not only increased plant growth, but also enhanced the accumulation of Cd in shoots up to 162 µg g-1 which almost doubled the Cd concentration (87 µg g-1 ) at the same addition level of Cd (20 µg g-1 ). The increased Cd accumulation was ascribed mainly to increased ionic strength in soils due to the addition of salts and resultantly increased the mobility of Cd. In comparison, the addition of Cl- resulted in the higher Cd accumulation in shoots than the addition of SO4 2- and NO3 - . The final two experiments investigated how the phytoextraction of C. rossii was affected by six soil organic amendments that included three wood-derived biochars, two animal manures and a brown coal. Plants were grown in a sediment polluted by heavy metals. Firstly, isotherm adsorption curves showed that the overall adsorption capacity (mmol g-1 ) followed the order of Pb - Cr - Zn - Cu - Ni - Cd, and that the effect of organic amendments on the adsorption of the heavy metals generally followed the order of chicken manures - cow manure - brown coal - golden wattle biochars - blue gum biochars - radiata pine biochars. Secondly, the addition of amendments to the sediment increased the adsorption of heavy metals in soil, with the addition of brown coal resulting in the lowest concentrations of water-extractable Cd, Cu and Zn. The two manures resulted in the highest concentrations of water-extractable heavy metals, this increasing the risk of leaching of heavy metals in the soil. Furthermore, the brown coal resulted in higher shoot accumulation of heavy metals than three wood-derived biochars, whilst the manures had the lowest shoot accumulation of heavy metals, although they significantly increased shoot biomass. These findings suggest that brown coal is suitable for phytoextraction. In summary, this thesis demonstrated for the first time that C. rossii is a promising candidate for phytoextraction of Cd and Mn for soils or sites like landfills polluted by multiple heavy metals, especially for soils with high salinity. The phytoextraction potential of C. rossii was significantly affected by other heavy metals, salinity and organic amendments. Further work is needed to examine the phytoextraction potential of C. rossii in the field and to explore the relationship of metal accumulation and salt tolerance as well as associated mechanisms at cellular and molecular levels.