|Title:||The role of EDDS on soil-plant-copper interactions during chelant-assisted phytoextraction|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Soils -- Heavy metal content
|Department:||Department of Civil and Environmental Engineering|
|Pages:||xv, 188 pages : color illustrations|
|Abstract:||Soil pollution with heavy metals is a serious environmental issue around the world. To remediate heavy metals in contaminated soils, chelant-assisted phytoextraction has been paid much attention and investigated widely. However, a thorough understanding is still of lack on the complex interactions among chelants, target metals, soils, and plants. Therefore, the current study aims to investigate the role of the biodegradable chelant on the extraction and transport of a representative metal - Cu in soil and plant processes. Firstly, the performance of [S,S]-ethylenediaminedisuccinic acid (EDDS) and tetrasodium of N,N-bis(carboxymethyl) glutamic acid (GLDA) on Cu phytoextraction was compared with ryegrass and tall fescue. Results showed that, compared to GLDA, EDDS induced a higher Cu concentration in plants, showed less phytotoxicity, and degraded faster in soils. Therefore, EDDS was selected as a representative biodegradable chelant for the following study. Secondly, the impact of EDDS on soil processes in phytoextraction was investigated mainly from two aspects. The first aspect concentrated on the chemical interactions of EDDS with soils in rhizosphere of ryegrass. After application into a multi-interlayer rhizobox for 7 d, EDDS transported from non-rhizosphere to rhizosphere of ryegrass. Using synchrotron-based techniques, such as X-ray micro-fluorescence (µ-XRF) and X-ray absorption near edge structure (XANES), EDDS primarily extracted Cu from the adsorbed fraction on goethite instead of clay minerals in tested soils, which was probably associated with the EDDS-promoted dissolution of iron oxides. Transportation of Cu from non-rhizosphere to rhizosphere was also facilitated in the form of CuEDDS identified by solution speciation modelling. The second aspect focused on the impacts of EDDS to soil nutrients and microbes in the rhizosphere of ryegrass. Results showed that EDDS was beneficial to rhizosphere soil microbes, with the increase of microbial biomass C, microbial biomass N, and urease activities. The benefits of EDDS can be associated with the high concentration of soil nutrients in rhizosphere soils after the application of EDDS. Finally, the influencing mechanism of EDDS on Cu uptake and transport in ryegrass was studied. EDDS increased the Cu translocation from root to shoot of ryegrass. Cu distribution in roots by µ-XRF of showed that EDDS alleviated the deposition of Cu in meristem of root tip, and in the lateral and primary root conjunction of lateral root zone. Cu speciation by XANES revealed that EDDS formed stable CuEDDS complex, reduced the root sequestration of Cu, and thus improving the transport of Cu within plants. A conceptual model was developed to describe the mechanism of Cu uptake and transport either in the presence or absence of EDDS. Collectively, this study revealed that EDDS was effective to extract Cu from soils, facilitate Cu transportation to root surface, and improve Cu internal mobility within plants. It unravels the major mechanisms involved in chelant-assisted phytoextraction, which will promote the development and application of this technology in future.|
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