Phytoremediation - unassisted and TiO2 nanoparticles assisted

• Main Author: Wang, Yamin
• Other Authors: Lam Yeng Ming
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/171841
• Institution: Nanyang Technological University

Nanoscale materials have been attracting enormous interest for their potential botanic application, for instance in environmental remediation. Phytoremediation is a sustainable technique that utilizes living plants to clean up contaminated land. However, mechanisms of interaction between nanoparticles (NPs), pollutants, and plants in the phytoremediation process are not clear. Specifically, the role of NPs takes in enhancing remediation efficiency is studied here. Furthermore, research on NPs uptake, translocation, distribution, and their effects on phytoremediation was carried out to identify potential mechanisms behind changes in plant behaviors. To identify candidate plant species, further, to study the interactions of NPs with pollutants and plants in the process of phytoremediation, a phytoremediation study was conducted. The phytoremediation study of the dissertation aimed (1) to investigate the levels of 12 heavy metals and metalloids: antimony (Sb), arsenic (As), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), manganese (Mn), magnesium (Mg), nickel (Ni), and zinc (Zn) in soils in Singapore according to the standard of Dutch Pollution Standards; (2) to assess the accumulated elemental concentrations in plant foliage and further to screen tropical plant species that can accumulate heavy metals and metalloids for phytoremediation application. In this work, soil and plant foliage samples were collected at 11 location sites and 300 sampling points in Singapore. Collected samples were digested via a microwaved-assisted acid digestion system, then analyzed by inductively coupled plasma mass spectrometry (ICP-OES). For analytes in soil, results showed that the levels of As, Cd, Cu, Mo, Sb, and Zn were greater than the target values. The levels of Cu and Sb in certain sites exceeded the intervention levels observed. Principle component analysis (PCA) and cluster analysis (CA) identified two potential sources (anthropogenic activities and natural sources); and positive matrix polarization (PMF) modeling extracted four factors (mixed effects of natural and industrial sources, natural sources, anthropogenic sources, and traffic sources) of heavy metal and metalloids in soil. For the plant part, according to bioconcentration factor (BCF) values, 12 tropical plant species were screened with accumulation abilities for phytoremediation. Results revealed heavy metal and metalloid concentrations, indicating tropical plant species which have the potential for phytoremediation. With this knowledge, designed TiO2 NPs aimed to improve the pollutant (Cd) accumulating ability in the selected plant species (Centella asiatica (L.) Urban) were studied. Several studies have reported that TiO2 NPs applied in the soil to enhance heavy metal removal by plants. However, how TiO2 NPs facilitated pollutants to be taken up by plants remains to be clarified. In this study, it aimed to employ engineered TiO2 NPs in Cd-contaminated soil, to figure out NPs effects on the phytoremediation efficiency of C. asiatica, to understand TiO2 NPs uptake, translocation, and distribution in plants, as well as NPs interaction with Cd in the plant-soil system in the phytoremediation process. Findings of the work showed that Cd uptake and accumulation were enhanced significantly in the roots and leaves of C. asiatica plants that grew in soil with assistance from TiO2 NPs. The enhancement of Cd contents in plant tissues encouraged the potential of this tropical species to be applied in the phytoremediation application. The Cd adsorption to TiO2 NPs is closely relative to their synergistic transport from soil to root. Results revealed the distribution of TiO2 NPs at the cellular level including cortex cells and the xylem region in root cells, and the air space and mesophyll cells in leaf cells. Aggregated TiO2 NPs with a size over 100 nm were found internalized inside plant cells, showing that the threshold of size limit for NPs to penetrate through the cell wall and cell membrane could be larger than 100 nm. Positively charged NPs tended to accumulate in the root, while negatively charged ones likely to be translocated from root to shoot. Findings of the work suggested that size and surface charge could be important factors that affect interactions between NPs and pollutants interaction, as well as NPs’ fate in plants.