Sorption Properties of Freshly Precipitated Iron(III) Hydroxide toward Nickel Ions. Part 2. Structure and Composition of Iron(III) Hydroxide Precipitates

A comparative analysis of the composition and structure of freshly precipitated iron(III) hydroxide precipitates obtained from a sodium sulfate solution (400 mg/L) at pH 7 and 8 before and after the sorption of nickel ions on them was carried out. IR, Raman, and Mössbauer spectroscopy, as well as X-ray diffraction and thermal analyses, have shown that precipitates synthesized from a solution of iron(III) chloride are a two-line ferrihydrite with the gross formula Fe 2 O 3 ⋅3H 2 O. It has been established that the sorption of nickel ions on these precipitates is not accompanied with chemisorption, i.e., the formation of mixed compounds between iron and nickel. Their doping with nickel ions also was not observed. It has been found that the ζ potential of ferrihydrite particles at pH < 5.4 has a positive value, whereas it becomes negative at pH > 5.4. The zero-charge point of the precipitate particles corresponds to pH 5.4.