Studies on the Synthesis of Rare Earth Elements-Based Nanomaterials for their Application to Remove Fluoride from Aqueous Medium

Abstract

Water is the lifeblood of our planet, sustaining all forms of life. However, the reckless use of freshwater by industries and individuals has resulted in a dire situation of water scarcity and contamination. Due to this overuse and the impact of climate change, groundwater is not being replenished at the same rate. The chemical composition of fresh water is now unfit for human consumption, as it is laden with a variety of harmful organic and inorganic contaminants. It is time to take action to safeguard the most precious resource - water. There are various inorganic contaminants, with fluoride being a major concern for many health institutions. Excessive fluoride beyond the permitted limit can lead to dental and skeletal fluorosis. Among the different techniques available for removing fluoride and other harmful ions, adsorption is preferred due to its ease of operation, cost-effectiveness, and efficiency. In this study, rare-earth-based nanomaterials (LCM, YCO, and CPP-2) had been synthesized to address the reported limitations. These nanomaterials based on cerium have effectively removed fluoride from aqueous medium, up to the permissible limit. Lanthanum cerate (LCM) was synthesized using the hydrothermal method, and it exhibited a ball-shaped morphology. It possesses a high specific surface area of 142 m2/g and efficiently removes fluoride, even in the presence of different coexisting ions. In another study, cerium-based yttrium cerate (YCO) was synthesized using Pechini’s method, showing a maximum Langmuir adsorption capacity of 324 mg/g. It can be reused up to five cycles, with a fluoride removal percentage of 72.9. Furthermore, a hybrid material, ceria polypyrrole (CPP-2), was synthesized. It is stable and easy to separate in aqueous medium, with a maximum Langmuir adsorption capacity of 351.8 mg/g. Chapter 1 delves into the concerning issue of fluoride-contaminated water and its profound impact on human life. The presence of excessive fluoride in groundwater is a widespread problem that affects numerous nations across the globe. This contamination stems from both natural geological factors and human activities. The detrimental effects of elevated fluoride levels in drinking water are far-reaching, leading to fluorosis, a condition that causes irreversible damage to bone and tooth tissues, as well as long-term harm to vital organs such as the kidney, liver, thyroid, and brain. Safeguarding potable water from fluoride contamination has long been a pressing priority. The process of adsorption has emerged as a cost-effective, efficient, and reusable method for defluoridation. However, it is crucial to further explore the commercial viability and reusability of adsorbents to minimize costs and waste. Additionally, the text explores various kinetic models and adsorption isotherm models to unveil the intricate mechanisms behind defluoridation. Chapter 2 discusses about the remarkable lanthanum cerate microspheres (LCM) and their extraordinary capacity to adsorb fluoride. Through meticulous batch studies following their synthesis using the hydrothermal approach, the remarkable efficacy of LCM was uncovered. The imaging techniques, including scanning electron microscopy (SEM) and transmission electron microscopy (TEM), provided captivating visuals of the ball-shaped microsphere morphology of LCM. The LCM adsorbent's optimal pH range of 3.5 - 4.5, coupled with its exceptional adsorption capacity across the pH spectrum of 3.0-7.0. The Langmuir adsorption isotherm model emerged as the superior descriptor of the adsorption isotherm, outshining the Freundlich isotherm model. At pH 4.0, the LCM adsorbent exhibited an unprecedented Langmuir adsorption capacity of 105 mg/g, surpassing all known adsorbents. The practical analytical approaches unveiled the adsorption mechanism, affirming the LCM adsorbent's prowess in eliminating fluoride. To top it off, the analysis in real groundwater solidified the adsorbent's exceptional efficacy. Through many practical analytical methods, we meticulously dissected the adsorption mechanism and conducted precise measurements, further validating the findings. Chapter 3 discusses about the formation and detailed analysis of a new adsorbing material, yttrium cerate (Y2Ce2O7), i.e., YCO, has been synthesized through the interaction of yttrium and cerium salts via Pechini’s method. Using batch experimental procedures, we assess the material's durability and selectivity as an intriguing adsorbent for defluoridation from both aqueous and genuine samples of water. SEM and TEM pictures, confirm the successful formation of porous YCO microspheres. The sample was found to be polycrystalline and the crystallite size was 13 ± 1 nm. The surface area of YCO was found out to be 54 m2/g exhibiting its porous nature. Fluoride was found to successfully bind to the YCO adsorbent which was further validated by FTIR, EDAX and XPS analysis. Defluoridation was barely impacted by the coexisting anions. An analysis of fluoride adsorption kinetics has revealed other rate- limiting steps other than the intraparticle diffusion model. At room temperature, the maximum Langmuir capacity of adsorption observed was 323.9 mg/g. For a practical outcome, the material's reusability was tested for up to five iterations in a row. In Chapter 4, the stability and effectiveness of a hybrid material in removing fluoride from drinking water is discussed. A new hybrid composite, CPP-2, was created using in situ oxidative polymerization and Pechini's method of CeO2 (CO). Various physicochemical techniques were used to analyze the properties of the adsorbent in its original state. The hybrid composite effectively removed F- ions from the effluent, with the adsorption isotherm best fitting the Langmuir model. The maximum adsorption capacity at room temperature was found to be 351.8 mg/g at a pH of 4 ± 0.2. Kinetic data indicated that the fluoride adsorption process followed a pseudo second-order kinetic model. To understand the defluoridation mechanism, FTIR and XPS spectra were examined, showing interactions between the polypyrrole moiety's nitrogen atoms and F- ions, exchange of hydroxyl groups with F-ions, and electrostatic attraction of protonated hydroxyls on the adsorbent surface. Thermodynamic data revealed an endothermic, spontaneous, and feasible adsorption process, with the capacity to endure five cycles of adsorption and desorption. Overall, the data demonstrate that CPP-2 has great potential for defluoridation from groundwater below WHO-specified pH thresholds. Chapter 5 discusses the conclusion and future scope of synthesized materials for the removal of fluoride. Excessive fluoride in drinking water can lead to various fluoride-related illnesses in the community. It is important to note that the World Health Organization recommends a maximum of 1.5 ppm of fluoride in drinking water. Globally, almost 80% of drinking water comes from groundwater sources. Researchers have developed various methods and materials to remove fluoride from water, with adsorption being the most practical, economical, and sustainable option for community use. Other methods include osmosis and ion exchange. While many researchers have synthesized adsorbents with significant defluoridation potential, practical success has been limited for various reasons. In this study, a simplified laboratory approach was used to synthesize and assess a new type of adsorbent, drawing from literature reviews and other accessible references. The focus was on synthesizing cerium-based metal oxide/hybrid materials with excellent adsorption capacity for defluoridation. The ideal characteristics of the material include high adsorption capacity, stability, affordability, and ease of use for fluoride removal. Consequently, Ce-based materials were synthesized, characterized, and tested to evaluate their effectiveness in removing fluoride.