Several non-toxic and novel polymeric and cellulose-based nanomaterial flocculants were synthesised. Of these, two pDMAEMA polymers – high molecular weight (~150 kDa) and a low molecular weight (~9 kDa) were used in the first phase of the project for the flotation harvesting of algae. The operating conditions and protocols of the dissolved air flotation process were optimised by using polymers synthesised in Objective – 1. Following the DAF optimisation, several types of cellulose nanocrystal (CNCs) flocculants were synthesised – CNC-MIM (CNCs modified with imidazolium groups), CNC-PYR (CNCs modified with pyridinium groups). It is of note that different variations of the CNC-PYR were attempted including, variations of the positive and negative charges by TEMPO oxidising the CNCs prior to the modification with the pyridinium groups. Novel CNCs which were modified with glycine, dimethyl glycine, trimethyl glycine were also synthesised to be evaluated as flocculants for harvesting microalgae. Ongoing work includes the modification and evaluation of cellulose nanofibrils (CNFs), which are fibres of cellulose that are in the order of microns rather than < 300 nm which are commonly observed in traditional CNCs. This ongoing work is being carried out by PhD student co-supervised by the researcher.
The appropriate dimensions of the flotation jars were evaluated to optimise efficiency while decreasing the volume of the culture suspension used. In the same project, the repeatability of the experiments was evaluated to ensure that the results obtained were statistically reproducible. In another project, DAF was contrasted against sedimentation as a part of a two-stage harvesting-dewatering treatment train. A techno-economic assessment was undertaken to evaluate the benefits of DAF over sedimentation for microalgal harvesting. CNC-based flocculants were also evaluated for harvesting freshwater and marine microalgae. Previous studies had indicated that CNCs form small flocs when harvesting marine microalgae species that are grown in seawater. These small flocs settle very slowly (several hours) or do no settle at all, and hence, DAF was evaluated as an alternative method to harvest these small flocs.
During this phase of the work, a new area of research was also investigated – the use of DAF for restoration of lakes infested by cyanobacterial blooms. Cyanobacteria are blue-green algae which are a nuisance in recreational and shallow lakes. They also contain valuable pigments and metabolites which can be extracted post-separation. These cyanobacteria also contain gas vacuoles or air pockets which give them buoyancy. So, the DAF process which uses microbubbles takes advantage of cyanobacteria’s natural buoyancy to float them.
For reversibility, the following approaches were proposed. CNCs were modified with imidazolium groups, pyridinium, glycine, dimethyl and trimethyl glycine groups. The CNCs modified by the imidazolium groups showed pH responsive nature but did not detach effectively. So, the next step to TEMPO oxidise the CNCs prior to modifying them with the imidazolium groups is being undertaken. This gives enough negative charges on the molecule to repel the cells. The results of these were not reproducible. The glycine and trimethyl glycine modifications did not show any pH responsive nor flocculating activity. The dimethyl glycine modification in contrast showed good pH responsiveness at a ~ pH of 7-8. However, these flocculants did not show any floc formation during jar testing. It is hypothesised due to poor grafting caused non-stoichiometric amounts of reagents used.
