Effect of dextran on the crystallization of ferric hydroxide

The initial nucleation and phase transformatin process of the Fe(OH)3 gel in five different mineralization systems with different dextran concentrations (from 0 similar to 2.0% (w) and the same [Fe3+]=0.012 mol.L-1) have been investigated. The mineralizational phases in the initial stage were characterized by FTIR, XRD and ICP-AES spectrometry. The results showed that the concentration of Fe3+ ions in supernatant increased suddenly in the third day during aging and then its secondary reduction stage occurred. There were the same chages of pH as [Fe3+] in these process. The initial phases formed in the systems with dextran were mainly beta-FeOOH, not alpha-FeOOH which growing rapidly in the system without dextran. The dextran molecules were adsorbed on the surfaces of iron oxide particles by complexing with Fe3+. The adsorbed dextran molecules would facilitate the phase transformation from beta-FeOOH into alpha-Fe2O3 by a dissolution-recrystallization mechanism. The secondary reduction stage of [Fe3+] corresponded to the process of the phase transformation from beta-FeOOH into alpha-Fe2O3. With the increasement of the concentrations of dextran (up to 2.0%(w)) in solution, the dissolution of beta-FeOOH and the formation of alpha-Fe2O3 in these mineralization systems were accelerated. It was clear that dextran molecules could control the nuclei type and its phase transformation of the Fe(OH)(3) gel.