Abstract
The United Nations’ Sustainable Development Goals have sparked growing interest in biosurfactants from many surfactant-loaded industries including those utilizing froth flotation for mineral separation. However, the interaction of biosurfactants with mineral surfaces is currently poorly understood. We bridge this gap by studying adsorption of a yeast-derived bola acidic sophorolipid (ASL) biosurfactant on djurleite (Cu1.94S). The methods used include Hallimond flotation, contact angle, adsorption isotherm, zeta potential, leaching measurements, and X-ray photoelectron spectroscopy (XPS). To facilitate the interpretation of the adsorption results, we characterize the activity of ASL at the air-water interface and measure its critical micelle concentration (CMC) at different pH using static surface tension. We find ASL to be a multifunctional surfactant with an unusual, pH-sensitive interfacial behavior. At the air-water interface, ASL is most active at pH 8, while its CMC goes through minimum as low as 40 μM at pH 7. The surfactant adsorption at the djurleite-water interface makes the sulfide surface hydrophilic at acidic pH and hydrophobic at neutral and basic pH. In addition, ASL has strong affinity to copper sulfide and demonstrates metal leaching properties. Finally, ASL demonstrates detergency properties. We offer a mechanistic interpretation of these findings. Our results provide a basis for the application of acidic glycolipids in froth flotation and have implications for their application in ion separation using hydrometallurgical routes, as well as for the chemical stability of metal sulfides in environmental systems.
Highlights
Microbial biosurfactants (100% bio-based surfactants produced by microorganisms) are increasingly being introduced into many industries including home and personal care, cosmetics, agriculture, food, environmental remediation, and pharmaceutical sectors
This study has shown that acidic sophorolipid (ASL) has a similar affinity for the interface and is distributed there as its lactonic form
Before studying adsorption on djurleite, the critical micelle concentration (CMC) of ASL was measured as a function of pH and its self-assembly at the airwater interface characterized using static surface tension
Summary
Microbial biosurfactants (100% bio-based surfactants produced by microorganisms) are increasingly being introduced into many industries including home and personal care, cosmetics, agriculture, food, environmental remediation, and pharmaceutical sectors. Their advantages include environmental compatibility, biodegradability, relatively low cytotoxicity, combined with beneficial functional properties such as detergency, complexing, and anti-microbial activity. From the scaling-up view-point, some biosurfactants are already produced quite efficiently, using yeasts through highly productive and eco-friendly bioprocess routes from renewable resources (biomass) These resources can be primary (e.g., sugar and vegetable oil) and secondary (e.g., side- and waste streams including food waste) generation substrates [1,2,3]. Substitution of conventional toxic collectors by more efficient eco-friendly microbial biosurfactants could make this separation technology environmentally more sustainable [7]
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