Characterization of flocculation-conditioning ferric hydroxide gel with differently charged polyacrylamides

Three types of polyacrylamide (PAM) flocculants of cationic PAM WD4960, non-ionic PAM M351, and anionic PAM WDA110 had different performances on the Ferric hydroxide gel (FHG) dewaterability and that M351 was the best one. The optimum dosages of these PAMs did not completely depend on the FHG total suspended solid (TSS) content of 0.2–1.5 g/L. The amount of PAM absorbed on the FHG surface linearly increased with an increase in its dosage, and saturation adsorption did not occur at the tested PAM dosages. The rise in PAM dosage can increase the size of conditioned FHG flocs/aggregates, while can decrease their compactness. The conditioned FHG aggregates at the optimum M351 dosage exhibited a moderate size, the smoothest surface, and the least compact structure among the three types of PAM. Conditioning the raw FHG with the cationic PAM of WD4960 increased its positive charge and decreased its hydrophilicity and specific surface area (S). However, M351 or WDA110 conditioning only increased the negative surface charge and hydrophilicity and decreased the S value of the conditioned FHG. The osmotic pressure variations indicated that, as a hydrogel, the optimum WD4960-conditioned FHG had stronger potential to absorb water from outside than the M351 or WDA110-conditioned FHG. Compared with that in the raw FHG, the free water content (FWC) reduction in the M351- or WD4960-conditioned FHG contributed entirely to the increase in bound water content (BWC) and the water release outside the hydrogel.