Effect of preformed and non-preformed Al_(13) species on evolution of floe size, strength and fractal nature of humic acid flocs in coagulation process

摘要

Although Al_(13) polymer (AlO4Al_(12)(OH)_(24)(H2O)_(12)~(7+)) has been identified as an effective aluminum polyca-tion in coagulation of natural organic matter (NOM), the coagulation behavior of Al_(13) polymer largely depends on its preparation conditions and characteristics. The performances of pre-hydrolyzed Al_(13) and in situ formed Al_(13) in humic acid (HA) coagulation were comparatively investigated in this study. Floc properties, which were evaluated in terms of floc size, strength and compaction degree, were measured using a laser diffraction particle sizing device. Additionally, variations of these floc properties with coagulation time were also studied. The results showed that, AlCl3 showed the best HA removal efficiency of above 92% at pH 5.5-6.0, where in situ formed Al_(13) was present. The preformed Al_(13) species gave rise to better removal efficiency than AlCl3 across the pH range with the exception of pH 5.5-6.0. This indicated that both preformed and in situ formed Al_(13) polymer could enhance the HA removal efficiency. Flocs formed by the preformed Al_(13) were stronger, as reflected by the lower γ' value of 0.53; while the γ' value for the non-preformed Al_(13) was 0.72. Additionally, the pre-hydrolyzed Al_(13) species tended to produce more compact flocs with larger D_f values than the in situ formed.