Comparing Rhamnolipid-Metal Interactions: Effect of Rhamnolipid Structure Modifications

Abstract

Glycolipids are bio-inspired surfactants with promising potential for metal remediation and reuse technologies. They offer an environmentally friendly, low-cost alternative to current techniques that provides a solution to growing industrial demand for metals. Metal recovery from contaminated environments, such as mining impacted waters, is critical due to their inherent economic value, and ecological and human health impacts associated with metal toxicity. This study focuses on rhamnolipids, a type of glycolipid originally produced by Pseudomonas aeruginosa. These rhamnolipids can now be chemically synthesized in large-scale batches that can be structurally modified to explore a range of metal-binding affinities. The binding affinities of biologically-produced rhamnolipid with a wide variety of metals has been investigated previously, and they are expressed as log β values. In the first part of this study, the binding affinity of synthetic Rha-C10-C10 rhamnolipid was investigated with silver (Ag+) and scandium (Sc3+), with results showing relatively low complexation for these metals (log β = 2.36 and 2.60). A second part of the study was to compare the binding affinities of Rha-C10-C10 with two newly designed rhamnolipids; 2C-Rha-C14, which has a modification to the sugar-lipid linkage that adjusts the size of the metal-binding pocket, and Rha-Phos-C13, which modifies the functional group, allowing for low-pH binding capabilities. Two metals were used in this comparison, lead (Pb2+) which has been previously studied, and gallium (Ga3+) which has not. Complexation of Pb2+ with Rha-C10-C10 was strong (log β = 10.01) as previously observed. Complexation of the 2C-Rha-C14 was similar to the Rha-C10-C10 (log β = 10.085). However, complexation of Pb2+ by the Rha-Phos-C13, measured at neutral pH was much lower (log β = 6.07). For Ga3+, binding by Rha-C10-C10 was low (log β = 0.57), while binding with 2C-Rha-C14 was moderate (log β = 4.92). However, under acidic conditions, the Rha-Phos-C13 showed increased binding of Ga3+ (log β = 5.41). These results with Rha-Phos-C13 represent the first report of successful metal binding with a rhamnolipid at low pH. The third part of this study characterized the binding of aluminum (Al3+) with Rha-Phos-C13 which revealed that there were strong interactions between the buffer used in the system and Al3+. This led to an investigation of the impact of changing the buffer concentration and the addition of methanol as a solvent. Results revealed higher binding with the standard buffer concentration (log β = 3.10) in comparison to the lower buffer concentration (log β = 1.71). Overall, this study contributed toward a deeper understanding of glycolipid-metal interactions under a variety of experimental conditions. These findings have implications for the continued development of glycolipid-based systems for selective metal recovery, with strong potential for both environmental and economic benefit.