Effect of preformed and non-preformed Al13 species on evolution of floc size, strength and fractal nature of humic acid flocs in coagulation process

Although Al13 polymer (AlO4Al12(OH)24(H2O)127+) has been identified as an effective aluminum polycation in coagulation of natural organic matter (NOM), the coagulation behavior of Al13 polymer largely depends on its preparation conditions and characteristics. The performances of pre-hydrolyzed Al13 and in situ formed Al13 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 Al13 was present. The preformed Al13 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 Al13 polymer could enhance the HA removal efficiency. Flocs formed by the preformed Al13 were stronger, as reflected by the lower γ′ value of 0.53; while the γ′ value for the non-preformed Al13 was 0.72. Additionally, the pre-hydrolyzed Al13 species tended to produce more compact flocs with larger Df values than the in situ formed.

In comparison with AlCl3 at pH 3.5, the in situ formed Al13 at pH 5.5 as well as the preformed Al13 species gave a much gentler decrease in floc size with the increasing shear rate.Figure optionsDownload as PowerPoint slideResearch highlights▶ Both the preformed and in situ formed Al13 could facilitate the particles aggregation. ▶ Floc strength and fractal structure could be evidently improved by both the preformed and in situ formed Al13. ▶ The in situ formed Al13 could easily transform into octahedral Al and then lessened the floc strength and compaction degree.