Properties and flocculation efficiency of highly cationized starch derivatives

In this study the flocculation efficiency of cationic starch derivatives of varying degrees of substitution (DS) and different amylose/amylopectin ratios was investigated and compared to structural properties. The molar masses, M, radii of gyration, PG, and the molar mass distributions of the starch samples were determined by applying the method of flow field-flow-fractionation combined with multi-angle light scattering and interferometric refractometry. In dewatering experiments potato starch derivatives of differing DS have optimum dosages that correspond to their cationicity. The lowest flocculant dosage yielding maximum dewatering results was determined for the most strongly substituted sample (DS = 1.48), while the least substituted starch derivative (DS = 0.28) showed the highest flocculant demand. Furthermore, the molar mass distribution and coil dimensions of derivatives seem to have a great influence on flocculation efficiency, because a 1.5-fold increase in molar mass can compensate a 1.5-fold decrease in IDS. Toxicological investigations on synthetic and natural flocculants employing hens fertile egg screening test (HEST) led to the conclusion that cationic starches of DS < 0.95 are less toxic than commercial synthetic flocculants. When the results of flocculation experiments and toxicological data were taken into account, a moderate DS of 0.6 was deemed to give the best compromise between dewatering efficiency, cost-effectiveness and ecological safety.