Abstract : The scarcity of clean water resources has been one of the recent problems, worldwide and in the region, which threaten the sustainability of environmental resources for next generations. Hence, investigating creative, affordable, and sustainable solution for water remediation is necessary. In this research work I have studied the effect of tuning ZnFeO4 nanoparticles (ZFN) structure and properties to enhance their efficiencies in water remediation applications, through photocatalysis and adsorption. The aim of this project is to tune the ZFN structure to make them more effective in water remediation application which are specifically photocatalysis and adsorption. This was performed through two routes, the first was through oxygen vacancies control and the second was through metal doping. We also aim to investigate these ZFN in harsh environmental conditions such as high salinity, which is the case in the Kingdom of Bahrain. The modulation of the oxygen vacancies related defects in ZFN was achieved through a simple but highly effective strategy of thermal annealing. The as-prepared nanoparticles were thermally annealed at three different temperatures (500 °C, 600 °C and 700 °C) and their phase purity was confirmed by X-ray diffraction (XRD). All samples were found to exhibit pure phases of ZFN with different crystallite sizes ranging from 10 nm to 25 nm. The transmission electron microscope (TEM) images showed well dispersed nanoparticles and a strong correlation of grain size growth with annealing temperature was established. The optical absorption and emission characteristics were estimated through UV-visible and Photoluminescence (PL) spectroscopy. Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS) confirmed the variation of oxygen vacancies in the synthesised samples' lattice. The photocatalytic activities of all samples were investigated, and the highest efficiencies were recorded for the ZFN samples annealed at 500 °C. Under high salinity condition, the organic dye degradation efficiency of the same sample remained the highest among all. The excellent dye degradation abilities in ZFN samples can be attributed to the abundance of oxygen vacancies in the crystal lattice that slow down the recombination rate during the photocatalysis process. Moreover, cytotoxicity tests revealed that all prepared ZFN samples showed insignificant cell structure effects on Picochlorum sp microalgae, as verified by Fourier-transform infrared (FTIR) spectroscopy. On the other hand, no significant changes were detected on the viable cell concentration and Chlorophyll a content. This work presents a systematic way to finely tune the crystal sizes and to modulate oxygen related defects in ZFN through a simple but highly effective annealing approach to signify their potential in industrial wastewater and seawater treatment processes. Furthermore, I have also investigated the effect of Mg doped Zinc ferrite (Mg-ZFN) nanoparticles on the individual and simultaneous adsorption of Pb, Cu³² and Cd¹¹ heavy metals in aqueous solution and in seawater samples. The Mg-doped Zinc ferrite nanoparticles were synthesized successfully applying sol gel route and varying the weight percentage of Mg to 2.5%, 5%, 7.5% and 10%. Such small amounts of doping did not disturb the high crystallinity spinel phase of the nanoparticles as shown by XRD spectrum and the unique cubic lattice was preserved. The TEM images revealed semi-spherical and well distributed grains with a size ranging from 10 to 15 nm. The structure of the samples was further characterized by FTIR, Raman and XPS that confirmed the spinel structure of Zinc ferrite and the intervention of different amount of Mg. The BET surface area revealed a considerable increase within the Mg-doped samples (39.3 m²/g) in comparison with the pure zinc ferrite sample (28. 4 m²/g). Accordingly, the Mg-doped Zinc ferrite samples have shown great adsorption capacity of Pb (143.5 mg/g), Cu³² (117 mg/g) and Cd2 (77 mg/g) within 2 hrs under certain experimental conditions. Results showed good agreement with adsorption kinetics and isotherms. The prepared samples have also shown high selectivity of Pb2 in simultaneous adsorption in aqueous solutions (85 mg/g) and real seawater samples. The selectivity of Pb ions dropped dramatically to 25 mg/g within real seawater samples due to the strong ionic strength of high salinity seawater. Recycling experiments showed minor incline in adsorption capacity over five cycles. Characterization of samples after the adsorption process confirmed the stability their chemical structure and size as well as the attachment of Pb ions within the nano-adsorbent. This work provides insights into the importance of doped spinel ferrite structure in highly efficient, rapid, and simultaneous adsorption of heavy metals. It also revealed the challenge of performing adsorption process in real seawater. Overall, this work provides insights into the importance of tuning nanomaterials properties to enhance applications, as well as considering the surrounding environmental conditions that showed major effect in the efficiency of the water remediation process. From here, emerged the importance of considering local/national aspects in developing sustainable scientific solutions to water crisis.