The recovery of gallium (Ga) from its low concentrated wastewater using siderophores was attempted. The project was divided into 4 work packages. The first work package deals with the anchoring/entrapment/immobilization of the siderophores on different carrier material and characterization them. This anchoring/entrapment/immobilization was needed for the efficient solid-liquid separation. The anchoring using one of the hydroxamate group of the siderophores is not possible as it affect the complexing ability of the siderophores. Thus, the immobilization was carried out using free NH3 group of desferrioxamine B (DFOB). The coupling was carried out onto carboxylated beads.
Another method for the separation of Ga-siderophores complex from the wastewater is through reversed-phase chromatography. This was attempted, even when not mentioned in the proposal, as a natural extension. The advantage of this method is it can be applied to any siderophores and not just only those siderophores with a free functional group. This was demonstrated by separating the Ga from low concentrated wastewater in the form of Ga-DFOB complex. Further, desferrioxamine E (DFOE) was used to complex Ga from GaAs wafer fabrication industry wastewater and separated using reversed-phase chromatography.
In the second work package, the complexation of DFOB and DFOE with Ga3+ was investigated in the solution phase. The complexation of Ga3+ with DFOB/E was not affected by presence of anionic/cationic contaminants, pH, arsenite, arsenate or KCl. The DFOE forms stronger complex with Ga when compared to DFOB. These findings was revealed from Density Functional Theory (DFT) calculations. Similar observations were made when Ga3+ was complexed with DFOB immobilized onto polystyrene beads.
The molecular understanding of Ga3+ complexation with DFOB or DFOE was also revealed by means of IR spectroscopy, Nuclear Magnetic Resonance (NMR) and Density Functional Theory (DFT) calculations. The complexation, as expected, was carried out by means of 3 hydroxamate groups.
In the third work package, decomplexation of Ga-DFOB/E complex was successfully carried out using ethylenediaminetetraacetic acid (EDTA). The pH plays a major role in the decomplexation. The acidic pH is required for desorption while desorption at alkaline pH was not successful. 10 cycles of complexation and desorption were carried out using DFOB immobilized polystyrene beads. No loss of the complexation ability of Ga was observed.
In the fourth work package, semi-continuous reactor was operated, however, the attempt was not successful due to the failure to choose the right membrane for the separation of DFOB immobilized polystyrene beads from wastewater. However, there is no reason for the continuous reactor to fail when the correct membrane is chosen. The cost-benefit analysis pointed to a very low operational cost of operating DFOB immobilized polystyrene beads resulting in possible high rate of returns of the technology. In the chromatographic separation method, the operating cost is higher than DFOB immobilized polystyrene beads but still the technology is cost-effective. The major cost inputs is siderophores and if their prices are reduced to 1000 € per kg and at least 50 cycles, then the technology is commercially viable.
