EVALUATION OF NOVEL GLYCOLIPIDS WITH METALS AND STRATEGIES FOR USE IN AQUEOUS MINING

Abstract

As global populations grow and urban centers expand, the demand for clean water and critical metals is rising in tandem. Access to safe water is essential for public health, agriculture, and ecological resilience, while metals are indispensable for powering the technologies that support modern infrastructure and the transition to low-carbon energy systems. Traditional mining practices have long been the backbone of global metal supply and these practices face growing challenges related to environmental impact, water and energy usage, and the difficulty of extracting rare earth elements, which often occur in low concentrations and require advanced separation technologies. Aqueous mining, i.e., recovering metals from groundwater, brine waters, industrial effluents, and mining-influenced waters, offers a promising complement to conventional approaches for producing important metals while also producing cleaner water resources to meet increasing demand. Aqueous mining is particularly interesting as metal ions are in a labile form free of their original solid matrix and often exist at concentrations high enough to be economically viable (Can Sener et al., 2021; Du et al., 2023; Panagopoulos & Giannika, 2022). However, the development of selective, efficient, and environmentally friendly technologies for metal recovery remains a significant challenge (DuChanois et al., 2023; Qasem et al., 2021).Glycolipids, particularly rhamnolipids, have emerged as promising ligands for metal complexation. Biosynthetic rhamnolipids produced by Pseudomonas aeruginosa have shown preferential binding to rare earth elements and heavy metals over common ions like potassium and magnesium (Hogan et al., 2017; Munoz‐Cupa et al., 2022; Ochoa-Loza et al., 2001). Due to these characteristics, these materials have potential for use as active groups in metal recovery applications such as hydrogels, precipitation, and ion flotation. However, their commercial use is hindered by the variability of biosynthetic mixtures, which can contain over 100 congeners depending on growth conditions (Abdel-Mawgoud et al., 2010; Dos Santos et al., 2016; Zhang et al., 2014). Advances in synthetic organic chemistry have enabled the production of pure, single congener rhamnolipids and novel glycolipids with customizable structures (Bauer et al., 2006; McCawley et al., 2023; Palos Pacheco et al., 2017). These synthetic molecules allow precise control over lipid tail length, sugar head type, and glycosidic bond configuration, which are features that may significantly influence glycolipid metal-binding capability. Preliminary studies comparing synthetic glycolipids to biosynthetic monorhamnolipids have shown promising results using conditional stability constants to quantify metal-ligand interactions (Hogan et al., 2017; Irving & Williams, 1953; Nancollas & Tomson, 1982; Ochoa-Loza et al., 2001), but this remains an area that requires additional research to fully evaluate the potential of synthetic glycolipids for metal removal applications. One area of existing data for biosynthetic and synthetic glycolipids focuses on the removal of uranium from groundwaters. This element is of particular importance in groundwater due to its prevalence across the western United States and in other uranium producing areas such as the Mansa district of Punjab, India, paired with its serious toxicological risks to both human and environmental health (Gandhi et al., 2022; Kaur Guron et al., 2025; Office of Legacy Management, 2014; Tisherman et al., 2023). Uranium in groundwater can originate from both natural and anthropogenic sources and often exceeds environmental quality standards for human consumption. Remediation methods—such as adsorption, ion exchange, membrane filtration, and biosorption—have limitations in selectivity, cost, and sustainability (Manobala et al., 2021). Prior work from the Maier Lab Group demonstrated that rhamnolipid surfactants can remove uranium via ion flotation to levels compliant with EPA maximum contaminant limits, with the added benefit of regenerable, green chemistry-based ligands (Hogan et al., 2022; Tan et al., 1994). However, unexpected inefficiencies at higher pH levels suggest that uranium speciation and carbonate interference may hinder complexation, warranting further investigation into solution chemistry and ligand-metal interactions. This research has three aims. (1) The first focus of this dissertation is to evaluate the state of hydrogel development toward the goal of aqueous mining with metal ion selectivity. It also proposes a framework for hydrogel characterization to facilitate reproducibility and comparative analysis across future studies. (2) Second, this dissertation evaluates novel glycolipid structures for their ability to selectively harvest individual metals, and to find trends in metal:glycolipid complexation to guide development of new glycolipids. This includes metal ion and glycolipid complexation trends based on sugar head(s), number and length of lipid tails, and glycosidic linkage. (3) Finally, the research aims to further understand rhamnolipid:uranium complexation with ion flotation, specifically uranium ion speciation based on solution pH and carbonate concentration, in hopes of providing a practical method for removing uranium from groundwater and other complex aqueous mixtures. Experiments were conducted for aims 2 and 3 above. Laboratory work studied novel glycolipid complexation capabilities including, a cation exchange resin competition assay, solution analysis with inductively coupled plasma – mass spectrometry (ICP-MS) by the Arizona Lab or Emerging Contaminants (ALEC), and data analysis to determine conditional stability constants for three metals and six glycolipids. Computational modeling also helped determine the interaction of the glycolipid and the metal ions. Dynamic light scattering provided information about the size(s) of complexation aggregates. This work focused on the effects of head and tail groups to determine the impact of these glycolipid moieties in metal-glycolipid interactions. A second set of laboratory experiments was aimed at further understanding the complexation pocket of the glycolipids and evaluated the effect of ‘bite-size’ of the binding pocket in the glycolipid structure. This was accomplished by testing molecules chosen specifically for the designed differences, such as placement of the glycosyl linkage, the number of sugar heads, and the number of lipid tails. Eight molecules were tested with three metal ions using Ion Specific Electrodes (ISE) to determine the amount of metal complexed by the glycolipids. Computational modeling was used to calculate the size of the oxygen pocket. Analyzing the data helped determine molecule design factors that could help with future molecules and the effort to build a library of molecules for selective ion separation from aqueous solutions. Lastly, the role of pH and carbonates in uranium ion speciation and how they affected uranium removal from groundwater samples with the use of ion flotation with rhamnolipid was evaluated. Total organic carbon (TOC), total inorganic carbon (TIC), pH, and uranium concentration were tested. This student used inductively coupled plasma- optical emissions spectroscopy (ICP-OES) to determine uranium concentration and ALEC analyzed TOC and TIC samples. Geochemical modeling used the data collected to predict uranium ion speciation which helped determine effective groundwater composition and pH ranges for this technology. This dissertation addresses material and knowledge gaps in selective metal recovery technologies and advance the design of environmentally friendly materials. Specifically, it furthered our understanding of the structure-function relationship of glycolipids with select metals to advance our development of molecular structures suitable for targeted metal remediation/capture and/or tailored glycolipids for application specific remediation technologies. It also continued prior work to advance the use of glycolipids for the remediation of uranium contaminated groundwater using ion flotation technology. The experimental work and geochemical modelling helped parameterize the solution physicochemical conditions necessary for effective treatment which will thereby improve the potential of this technology to be transferred from the laboratory to real word implementation. Taken together, this body of work contributes to the development and future implementation of glycolipid-based, sustainable solutions for water purification, resource recovery, and environmental remediation. Finally, looking beyond, ion flotation there are myriad potential applications for this technology. One of great interest is the use of hydrogels. The literature review that begins this dissertation is focused on hydrogels. Hydrogels are based on polyacrylamide (PAM) and have long been used in environmental applications due to their water absorption capacity and ability to sorb contaminants. Crosslinked PAMs are used in horticulture and wastewater treatment, while linear PAMs are employed for flocculation and sludge dewatering (Frantz et al., 2005; Hennecke et al., 2018; Xiong et al., 2018). Some PAM formulations have demonstrated the ability to remove organic dyes and heavy metals and even catalyze the breakdown of toxic compounds (Barakat, 2011; Javed et al., 2018). Yet, most studies challenge these hydrogels with single-metal solutions, leaving a critical gap in understanding their selectivity in mixed-metal environments (Ahmad et al., 2022). The lack of standardized characterization protocols further limits comparative analysis and slows technological progress. Building a systematic evaluation framework may provide structure for more rapid advancement of these technologies.