Unraveling the magnesium oxide nanoparticles potential for wastewater treatment applications: Atrazine degradation and Legionella disinfection

Abstract

The growing demand for efficient water treatment technologies, driven by global economic instability and stringent environmental regulations both domestically and internationally, has directed researchers' attention toward nanomaterials, particularly transition metals and their oxides. These materials are renowned for their high efficiency in applications such as the removal of emerging organic contaminants and bacterial disinfection. However, from a practical standpoint, major concerns remain regarding toxicity, cost, and scalability. In this context, magnesium oxide nanoparticles (MgO NPs) have emerged as a promising alternative. MgO is non-toxic, environmentally friendly, cost-effective, and abundant, with notable catalytic and biocidal properties. Nevertheless, its application in water treatment has been limited due to its relatively low efficiency compared to transition metal oxides. The primary objective of this dissertation is to enhance the efficiency of MgO NPs without compromising their inherent advantages. To achieve this, two innovative approaches were implemented: (1) a novel reaction system for the remediation of emerging organic contaminants and (2) a new material synthesis technique tested for antibacterial efficacy. Atrazine (ATZ) was selected as a model organic contaminant to assess the catalytic performance of MgO in activating peroxymonosulfate (PMS) under an underexplored condition, acidic aqueous media. The MgO/PMS system demonstrated remarkable performance, degrading ATZ within minutes. Its efficiency was comparable to the well-established Fenton reaction (using hydrogen peroxide), emphasizing the potential of the MgO/PMS system. This study focused on understanding the degradation mechanism and identifying the major reactive oxygen species (ROS) involved. A scavenger assay confirmed that singlet oxygen (1O2) was the predominant ROS responsible for ATZ degradation. FTIR analysis suggested a complexion-mediated degradation pathway. The degradation intermediates were identified using LC-Q-TOF-MS/MS, and the reaction pathway was proposed. Furthermore, to ensure environmental safety, the toxicity of the ATZ transformation products was evaluated using the U.S. EPA’s Toxicity Estimation Software Tool (T.E.S.T.), which showed a decline in overall toxicity over 120 minutes. This innovative approach repositions MgO as a viable alternative to transition metal oxides for organic contaminant degradation. Simultaneously, a novel synthesis method was developed, microwave- and template-assisted sol-gel synthesis, using varying concentrations of a non-ionic, non-toxic capping agent, Tween 80 (T80). MgO NPs synthesized via this method were compared to those produced by the conventional sol-gel method in terms of structural, morphological, surface atomic configuration, and electronic properties. Only notable differences were observed in the latter two properties. The MgO prepared via the conventional sol-gel method with 6% T80 (MgO-C-6%T80) exhibited the highest antibacterial activity against Escherichia coli and was subsequently tested against Legionella pneumophila, a highly resilient waterborne pathogen. More than a 4-log reduction was achieved within 60 minutes of direct contact. The disinfection mechanism was elucidated based on experimental observations, MgO characterization data, density functional theory (DFT) analysis, and literature support. The results indicated that the primary mechanism involved physical interactions through electrostatic attraction and receptor-ligand interactions with phosphate groups in the bacterial lipopolysaccharide (LPS) layer. This innovative engineering of MgO NPs presents a promising, safe, and affordable strategy for combating even the most resilient waterborne bacteria. This dissertation represents a pioneering effort to revitalize and advance the role of MgO in wastewater treatment, with key contributions including (i) the first in-depth investigation of PMS-activated MgO catalysis under acidic conditions for organic pollutant degradation; (ii) a mechanistic exploration of singlet oxygen generation and its reactivity with heterocyclic organic compounds; (iii) novel insights into the enhancement of antibacterial activity through MgO nanoparticle engineering; and (iv) a detailed study of the MgO biocidal mechanism against Legionella, a bacterium highly resistant to conventional disinfectants. By addressing both technical and ecological challenges in advanced water treatment technologies, this dissertation introduces MgO as a technically viable, economically feasible, and environmentally friendly alternative to transition metals and their oxides.