Biosorption is a process in which contaminants can be adsorbed on the surface of biological materials, and has applications in environmental cleanup and metal recycling. Precious metals, such as gold, platinum and palladium, can sometimes be found in trace levels in environmental samples, and although their potential health and ecological risks are not well understood, removing these metals through biosorption can be difficult due to competition from higher concentration contaminants like iron and copper. In order to better understand how these precious metals interact with the surfaces of cells during biosorption processes, researchers at the University of Tsukuba combined two analytical techniques – X-ray absorption fine structure (XAFS) spectroscopy and single-cell inductively coupled plasma mass spectrometry (scICP-MS) – gaining new insights into the mechanisms of metal adsorption and how pH differences can alter the process.
While there are massive datasets already available tracking the sorption efficiency and capacity of different biomaterials, these findings are typically averaged over an entire cell population without assessing adsorption at the single-cell level. In this study, the researchers studied the adsorption of trace amounts of precious metals by the unicellular red algae Galdieria sulphuraria. scICP-MS allowed them to observe the number of cells that adsorbed the metals and in what concentrations per cell, depending on the type of metal and the pH of the solution in which the biosorption took place. XAFS analysis revealed more specific details about how elements on the cell surface interacted and formed complexes with specific metals.
The experiments showed that in low acidity conditions, gold interacted with the sulfur-containing groups on the cell surface, forming inner-sphere complexes with sulfur. Platinum and palladium, on the other hand, formed inner-sphere complexes with both sulfur and nitrogen. When high acidity conditions were used, platinum was no longer adsorbed, and gold and palladium only interacted with sulfur and not nitrogen. Additionally, the distribution pattern of palladium-absorbing cells changed drastically at the lower pH, both in terms of the number of cells that adsorbed palladium and the amount of palladium that was adsorbed. According to the researchers, this is the first study to report a link between these chemical interactions and the behavior of cell populations during biosorption. This research was published in the Journal of Hazardous Materials.
“The insight achieved is expected to contribute to future engineering of cell surfaces to provide enhanced metal adsorption,” said lead author Ayumi Minoda. “Optimizing the performance of biologically-derived precious metal adsorbents is expected to significantly improve the environmental sustainability of metal recycling and remediation.”