Mushtaq, Ruqyyah (2025) Investigation of Metal-Induced Marine Pollution Using THz Spectroscopy. [Tesi di dottorato]

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Abstract

This research is mainly focused on the study of the effect of metal pollution on diatomaceous algae, a family of marine organisms of fundamental importance for the correct functionality of oceanic phytoplankton, using the time domain resolved broadband Terahertz (THz) spectroscopy technique. This study aims to provide a basis for developing a rapid and sensitive detection method for biological toxicity of heavy metals, aiding in the ecological risk assessment of heavy metal pollution in the marine environment by combining THz technology, ecotoxicology, and cell biology. In chapter 1 we introduce the principles of THz generation and detection by following different mechanisms. In generation mechanisms we firstly describe the THz generation by Photoconductive antenna, secondly by Optical Rectification (OR) and thirdly by Air Plasma. In detection mechanisms we explain the THz detection by Photo conductive antenna, via Electro Optical (EO) sampling and by Air Biased Coherent Detection (ABCD). In chapter 2 we present the explanation of experimental setup used in this research work, based on THz generation via Optical Rectification (OR), through congruent lithium niobate (cLN) prism (MolTech), and THz detection by LiNbO3 crystal. We introduce the application of our THz source in THz time domain spectroscopy. There is some difficulty of using THz-TDS directly in aqueous environments due to the strong absorption of THz radiation by water molecules, which can significantly reduce the THz signal in transmission. Therefore, relatively intense THz sources with a valuable spectral bandwidth are needed for spectroscopic measurements. We emphasize that "intense" in this context still refers to values significantly below the threshold set by Raman spectroscopy, for example, and therefore, it is unable to damage diatoms through heating or ionization. An intense THz source based on optical rectification is used to investigate dilute aqueous solutions up to 500 microns thick. While less intense sources could, in principle, be effective for thinner and more concentrated samples, they would not permit the study of concentrations similar to those found in natural environments for lack of sensitivity. As good signal-to-noise ratio lies in the range of 0.2-1 THz and, thus, it will be the focus in the analysis of this research. In chapter 3 We discuss about the ecotoxicological studies, that have been increased significantly in the last decades following the exponential growth in the production and use of chemicals in agriculture, medicine, and in various industrial sectors, leading to an increasing release of toxic contaminants (heavy metal, pharmaceutical product, hydrocarbon, plastic), into water globally. Microalgae, particularly diatoms, are commonly used as reference species in ecotoxicological studies because of their small size, rapid reproduction rate, and sensitivity to environmental changes. Among pollutants, marine heavy metals pose significant risks to the marine ecological environment due to their widespread sources, difficulty in degradation, bioaccumulation, and long residual times. The International standard specifies a method for the determination of the inhibition of growth of the unicellular marine algae Skeletonema pseudocostatum and Phaeodactylum tricornutum by substances and mixtures contained in sea water or by environmental water samples (effluents, elutriates, etc), to calculate the EC50 (concentration of test substance which results in an 50 % effect in specific growth rate relative to the control) value after 72 hours. In chapter 4 we investigated the toxicity of different heavy metals on marine diatom S. pseudocostatum by combining THz spectroscopy, microscopy and ecotoxicological assays. We found that the formation of long diatom chains is significantly inhibited by the presence of lead (Pb), copper (Cu), and chromium (Cr), which disrupt their metabolism. THz absorption and refractive index spectra were not affected by diatom concentration in undoped samples, but it is observed that THz frequencies were highly sensitive to changes in diatom chain length due to heavy metals exposure. These findings suggest that this approach allows to investigate the biochemical processes involved in chain formation in S. pseudocostatum and related algae. THz spectroscopy could therefore provide deeper insights into the microscopic metabolic activity of diatoms, addressing key biochemical questions surrounding these organisms. Furthermore, we propose this novel approach for environmental pollution monitoring, since it could provide a rapid, harmless and sensitive detection method to assess heavy metal toxicity in marine diatoms, key organisms at the basis of the trophic chain. The main idea behind this research is to check if THz technology can be suitable in Ecotoxicology. THz technology has a high impact in biomedical field, for living tissues imaging because of its low photon energy, and its much safer than X-rays imaging. Despite the strong absorption of THz radiation in water, we demonstrate that changes in water absorption in the THz range due to water-diatom interaction can provide insights into diatom chain length. Marine diatoms are selected as potentially good biological indicators to detect environmental changes due to natural and anthropogenic stressors (i.e. nutrient, predators, contaminants), since their chain length can change. Among metals, we have selected copper due to its widespread presence in water and its ability to severely impact primary producers at high concentrations, chromium because it is currently a very serious and challenging issue due to industrial contamination and lead as a benchmark for highly toxic heavy-metal contaminants. In appendix a separated side-project is discussed. We use THz-TDS to investigate the alcohol/water mixtures which are indeed of great interest due to their unique structural and dynamic properties, arising from the interplay of hydrogen bonding, hydrophobic effects, and the amphiphilic nature of alcohols. Despite the increased attention, the interaction between water and solute molecules, especially in the presence of a solute with amphiphilic behaviour, is yet to be fully understood. Both hydrophilic and hydrophobic groups of an amphiphilic solute play a significant role in the intermolecular dynamics of the aqueous solution, leading to complex behaviour in their mixtures. Aqueous solutions of alcohols have provided a rich seam for scientific research for many years. Such binary mixtures are ubiquitous in many fields, including fuel cell technology and bioscience.

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