Flocculation of Both Kaolin and Hematite Suspensions Using the Starch-Based Flocculants and Their Floc Properties

In this work, two kinds of starch-based flocculants with the same chemically modified functional groups but far different substitution degrees, (2-hydroxypropyl)trimethylammonium chloride etherified carboxymethyl starch (denoted as CMS-CTA-P and CMS-CTA-N, respectively) were successfully prepared. CMS-CTA-P and CMS-CTA-N bear opposite surface charge properties in water under most of the measured pH range. The flocculation performance of these two flocculants has been systematically studied using kaolin and hematite suspensions as synthetic wastewater under various pH conditions. CMS-CTA-P and CMS-CTA-N exhibited inverse flocculation behavior, but the commonness was that the starch-based flocculants with charge opposite to those of the contaminants in water always had higher flocculation efficiency. On the basis of the apparent flocculation performance and zeta-potential measurement, patching, a special charge neutralization mechanism, was significant here. Moreover, the floc properties including the floc size, fractal structure, and regrowth ability of flocs have been systematically studied by an in situ light-scattering technique in combination with fractal theory for further investigation of the flocculation mechanisms during the flocculation processes. The pH dependence of the floc properties and conformational changes of the polymeric flocculants indicated that bridging also made adequate contributions to efficient flocculation.