- Title
- Response of phosphorus sensitive plant species to antimony, arsenic and phosphorus: toxicity, speciation, and rhizosphere induced mineral transformations
- Creator
- Abbasi, Sepide
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2022
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Arsenic (As) and antimony (Sb) are highly toxic metalloids commonly found in the soil environment. The phytotoxicity of As and Sb depends on solubility and availability constraints and the inherent characteristics of plant species exposed. There are several factors influencing the fate and bioavailability of metalloids in soil including pH, redox, adsorption, ionic strength, colloidal flocculation, presence of oxidant/reductants, precipitation, coprecipitation, ageing, and mineral dissolution. The impact of metalloids on plants can be expressed differently due to contrasting physiological and morphological characteristics of plant species. Management of metalloids in soil and their impacts on plants require significant knowledge on the controlling factors on fate of metalloids in highly dynamic soil-plant systems. Despite the extensive research conducted on As and Sb toxicity, further investigations are required to improve risk assessment of As and Sb impacts in the environment. The overall objectives of the thesis were to investigate: the impact of long-term residential time of As on As phytotoxicity; phytotoxicity of As and Sb to plant species sensitive to phosphorus (P), and; influence of root exudates and organic matter on the mineral transformation of As and P within the rhizosphere environment. Five studies were designed in order to achieve the objectives of this thesis. The major outcomes of the five studies are outlined consecutively below: Study one, for the first time, investigated the ecotoxicity of long-term (5 years) aged As from 10 different soils to cucumber (Cucumis sativus L.). The results of this study were compared against 0.25 years aged soils. The impact of soil properties on ageing of As was also assessed and literature data were used to develop a model for As ecotoxicity. The results of this study indicated: the EC10 and EC50 values from 5 years ageing of As were respectively 4.0 and 1.76 fold higher than that from 0.25 years aged As. Only Freundlich sorption parameters were correlated to the ageing factor (EC50 (5 years)/EC50 (0.25 years) (r =0.68, P=0.028), which reflects changes in As toxicity over 5 years. The original dataset was also combined with two previously reported short term studies in order to explore the relationship of soil properties, including As ageing. The combined dataset therefore contained As contaminated soils aged for between 7 d and 1820 d. The combined model (n=54) indicated a correlation between As ecotoxicity with the oxalate extractable iron content and %clay, and arsenate ecotoxicity (EC50, mg/kg). The multivariate model was related to oxalate iron content, %clay and ageing time. The second and third studies investigated the response of P-sensitive plants to As(V) and Sb(V), respectively. Given the chemical similarly between As(V) and orthophosphate, and the sensitivity of such plant species to orthophosphate, it was hypothesised that such species would express a high level of sensitivity to As(V). Accordingly, the study used one high (Hakea prostrata R.BR) and one moderate (Banksia seminuda B.Rye) P-sensitive species and one vegetable plant species (Cucumis sativus L.) in nutrient solution containing different As(V) concentrations. The results of this study indicated As(V) toxicity was more pronounced in plants with higher sensitivity to P than plants with low sensitivity to P. The increased sensitivity has important implications in ecological risk assessment and selection of plant species for rehabilitation. In assessment of Sb(V) toxicity, Hakea prostrata, Banksia seminuda, Banksia ericifolia were selected as P-sensitive plants. The response to Sb to domesticated species was examined with Cucumis sativus, Triticum aestivum (wheat), and Lactuca sativa (Lettuce) with low/no sensitivity to P. In addition to plant growth and bioaccumulation of Sb, the impact of Sb(V) on photosynthesis, Sb speciation in plant roots were also investigated to address the knowledge gap in these areas. Linear combination fitting (LCF) of the Sb K-edge- XANES (x-ray absorption near-edge spectroscopy) indicated a minor reduction of Sb(V) in roots, with ~5-33% of Sb occurring as Sb(III). Antimony-polygalacturonic acid appeared as the predominant chemical form of Sb in all plant species (up to 95%), indicating Sb was primarily bound to the cell walls of plant roots. Shell fitting of Sb K edge EXAFS (extended x-ray absorption fine structure spectroscopy) confirmed Sb-O and Sb-C were the dominant scattering paths. The fitting indicated that Sb(V) was bound to hydroxyl functional groups of cell walls as characterised by Sb-C bond distance of 2.76-8.21 Å, via development of a local coordination environment analogous to Sb-polygalacturonic acid. This is the first study to demonstrate the key role of plant cell walls in Sb metabolism and the binding mechanism to cell walls. Studies four through six assessed the influence of plants and root exudates on mineral stability and transformation in the rhizosphere environment. Scorodite and tooeleite are generally considered stable ferric- arsenic minerals in contaminated sites with relatively low water solubility. Plants may influence the stability of minerals in the rhizosphere via root exudates. Therefore, the potential role of plant roots from Fe acquisition strategies I (harsh hakea) and II (wheat) on the transformation of scorodite and tooeleite was investigated in this study. Additionally, humic acid was applied as a model electron shuttle compound to simulate the common addition of organic waste products in revegetation and phytostabilisation management approaches. The results of the investigation on scorodite rhizotransformation indicated: ascorbic acid was the principle organic acid identified from both plants exposed to scorodite and it was succinic and fumaric acids in wheat and harsh hakea, respectively, in scorodite-humic acid treatment. The XANES analysis indicated As remained as As(V) in the solid phase of scorodite. The formation of new non-crystalline phases was identified by scanning electron microscopy, and EXAFS analysis indicated the formation of ferrihydrite and amorphous ferric arsenate (as a transit mineral phase). Given the incongruent dissolution, and precipitation of ferric phases in these systems, the resulting high As: Fe ratio in the dissolved phase appeared to promote the formation amorphous ferric arsenate, at least transiently. The greatest transformation of scorodite was observed in the harsh hakea and humic acid with 20% transformation rate to nanocrystalline arsenic-iron phases. The results of tooeleite rhizotransformation indicated a significant difference (P<0.05) in root exudates from all treatments, including tooeleite exposure, humic acid and plant species. Malic acid and oxalic acid were principle organic acids identified, respectively, in root exudates of wheat and harsh hakea (Hakea prostrata) exposed to tooeleite. Hakea exposed to tooeleite released 5 times more malic acid than control. Humic acid-tooeleite exposure reduced malic acid production in Hakea by 50% compared to the control. Tooeleite and humic acid had no significant impact on malic acid extracted from wheat. Total carbohydrate release was, on average, 4.8 times greater in hakea than the wheat. Tooeleite and tooeleite-humic acid increased carbohydrate release of hakea by 2 and 3 times, respectively, and of wheat by 20 and 40 times. The EXAFS linear combination analysis indicated As remained as As(III) in tooeleite with only 10.0 and 11.0 % of As(V)-ferrihydrite in the solid-phase of wheat and Hakea treatments, respectively. Phosphorus availability from iron modified biochars in the rhizosphere of Fe strategy I and II species were investigated in the seventh study. The application of biochar-P fertilisers in the agricultural industry has been considered as a suitable substitute for traditional fertilisers made from phosphate. This thesis studied the impact of five different soils and two different plants (cucumber and wheat) on the fate of P from biochar-biosolid (BB), Fe modified biochar (BBFe) and FePO4. The results of this study indicated magnesium phosphate, iron phosphate, and hydroxyapatite were dominant P speciation in biochars identified by XANES. Plant species had a significant role in unlocking P via exudation of different organic acids (OA). Cucumber released significantly higher root exudates and had significantly higher P and Fe content than wheat. The highest (p <0.0001) plant growth was observed in the BB treatment. The addition of FePO4 resulted in significantly (p <0.0001) higher P and Fe content in the root and shoot of both plants. Soil type had no significant impact (P>0.05) on plant growth except in sandy soil with 15% organic carbon and pH of 3.15. The soil also had a significantly higher (p <0.0001) concentration of available P due to high organic carbon and low clay content and pH.
- Subject
- arsenic; antimony; rhizosphere; phytotoxicity; native plants; phosphorus sensitive plants; mineral transformation
- Identifier
- http://hdl.handle.net/1959.13/1512216
- Identifier
- uon:56599
- Rights
- Copyright 2022 Sepide Abbasi
- Language
- eng
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