Use of PDMAEMA as a non-toxic synthetic flocculant for harvesting microalgae and evaluation of the influence of polymer size on flocculation behaviour

We synthesised a library of linear 2-(dimethylaminoethyl) methacrylate polymers (PDMAEMA) as an alternative to polyacrylamide flocculants and used them for flocculation of freshwater microalgae Chlorella vulgaris. The dose required to induce flocculation (17.2–23.5 mg L−1) was found to be independent of the molecular weight (MW) of the polymer. ζ-potential measurements indicated that flocculation occurred primarily through a charge neutralisation mechanism. Surface charge reversal of the microalgal-PDMAEMA particles and resulting dispersion restabilization was observed for high MW PDMAEMA, but not for low MW PDMAEMA. We hypothesized that charge reversal was a result of partial absorbance of long polymers to the cell surface, resulting in charged loops and tails that emerge from the cell surface. This was confirmed and visualized using AFM, which revealed an increase in surface roughness of cells when high MW PDMAEMA was added in high dose, but not when low MW PDMAEMA was used. Dispersion restabilization could be partially prevented by stepwise addition of the high MW PDMAEMA.