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Although Fe(II) salts have been widely used as coagulants in water treatment for many years, the underlying mechanisms of their performance remain unclear. Here, we present a detailed study of the coagulation behavior of Fe(II) salts and crystallization of flocs, and investigate the effect of humic acid (HA) under different DO concentrations and pH conditions. The behavior of Fe(II) and Fe(III) coagulants was found to be markedly different with the flocs from Fe(II) consisting of planar-like crystalline γ-FeOOH in contrast to the small amorphous spherical-like flocs from Fe(III) dosing. The effect of HA on Fe(II) coagulation was different under different DO concentrations and pH, whereby the growth of γ-FeOOH was inhibited by the presence of HA, but independent of DO concentration and pH. It was found that Fe(II) was present within the internal structure of γ-FeOOH, and a plausible formation mechanism is proposed. Firstly, planar nanoparticles of Fe(OH)2 were formed via Fe(II) ion hydrolysis which then served as the nucleus for subsequent crystal growth. With oxidation, Fe(II) on the surface of nanoparticles transformed to Fe(III). Finally, the formation of γ-FeOOH in Fe(II) coagulation was accompanied by a change in solution colour to yellow.
Figure 1. Effect of coagulant type on the growth of flocs at pH 7 (a and b); SEM images of flocs formed at pH 7: Fe(II) flocs (c) and Fe(III) flocs (d). (The dosage of coagulant was 0.1 mM Fe(II)/Fe(III); scale bar: 100 nm).
Figure 2. SEM images of flocs formed at pH 7 and DO=2.0 mg/L with different HA concentrations in bulk solution: (a) HA=0 mg/L, (b) HA=1 mg/L, (c) HA=5 mg/L, (d) SEM images of initial flocs (about 300 s) without HA; (e) TEM images of flocs formed at pH 7 and DO=2.0 mg/L without HA and SAED images of lepidocrocite (Inset); (f) XRD spectra of flocs formed at pH 7 and DO=2.0 mg/L with different HA concentrations (Scale bar on SEM images are 100 nm).
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