Rhamnolipid as an ion collector in bioionflotation

Objective: Industrial wastewaters are secondary sources of many critical and base metals. Recovering these metals from such waste streams helps in resource recycling and reduce the environmental burden. However, such a recovery is challenging due to the low concentration of target metals and the presence of other unwanted metals at higher concentrations. Ion flotation is a promising separation process for resource recycling of metals from secondary sources. However, secondary pollution by used chemicals and low selectivity amongst ionic species limit its practical application so far. Thus, there is a need to develop new ion collectors, which are highly selective, efficient, and ecofriendly. Microbial biomolecules, which can act as complexing agent and can bind selectively to specific metal ionic species are attractive alternative. Biosurfactants are microbial surface-active molecules having both hydrophilic and hydrophobic groups, and therefore have surface activity and metal complexing ability making them an interesting candidate for flotation reagent. Rhamnolipid, a glycolipid type of biosurfactant, is reported to complex with variety of metals. In order to improve the application of biosurfactant in these processes, a comprehensive and systematic investigation of their performance and behavior as an ion collector is required. Our objective is to provide an insight into the surface activity, foamability and foam characterization for rhamnolipid for their application in bioionflotation. In the current study, rhamnolipid was investigated for its application as an ion collector in BioIonFlotation for Ga recovery.
Results: Rhamnolipid exhibits extensive foaming over a wide range of pH. Foam produced by rhamnolipid alone has higher foam stability making it difficult to collect the
flotation concentrates. Addition of 1,2-decanediol introduces the little destability to this foam and also aids in concentrate collection.
Aggregation studies – Mixing of rhamnolipid and Ga solutions resulted in colloidal suspension. These colloidal interactions were studied in presence and absence of Ga and/or 1,2 decaneciol by dynamic light scattering. The aggregate size of rhamnolipid molecules was largely influenced by the presence of Ga and/or 1,2 decanediol. Increase in aggregate size, indicated that interaction of rhamnolipid with either or both of them boosted the aggregation process.
Dynamic surface activity – Presence and absence of Ga and/or 1,2 decaneciol had significant impact on dynamic surface activity of rhamnolipid. Presence of Ga shifted the surface activity curve of rhamnolipids. This change in trend can be attributed to the complexation of rhamnolipid with Ga that lead to longer diffusion at the sub-surface and then to interface resulting in higher surface ages. Addition of 1,2 decanediol had no such effect, whereas presence of both contributed in further shifting of the surface activity curve (Fig. 1).
Foam characterization – Foam produced by rhamnolipid was characterized by dynamic foam analyzer. Interaction of rhamnolpid with Ga and 1,2 decanediol or both, changed the foaming behavior of rhamnolipid. Images of foam revealed the differences in size and shape of bubbles as well as their development.
Conclusion: The study investigated the role of rhamnolipid biosurfactant as an ion collector for its application in flotation. The influence of Ga, as target metal and 1,2 decanediol as additional frother on properties of rhamnolipid were evaluated and found to have a significant impact. Such studies provide in-depth insights on the parameters influencing the properties of flotation collectors and provide the basis for the development of the bioionflotation process.