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Influence of cobalt ions on the precipitation of goethite in highly alkaline media

S. KREHULA^* and S. MUSI

Division of Materials Chemistry, Ruer Bokovi Institute, P.O. Box 180, HR-10002 Zagreb, Croatia

^* E-mail: krehul{at}irb.hr

(Received 28 February 2007; revised 6 July 2007)

The effects of cobalt ions on the precipitation of goethite in highly alkaline media were monitored using X-ray diffraction (XRD), Mössbauer, Fourier transform infrared (FTIR) and energy dispersive X-ray (EDS) spectroscopies and field emission scanning electron microscopy (FE-SEM) techniques. Tetramethylammonium hydroxide was used as a precipitating agent. The precipitates were collected after heating of the precipitation systems at 160°C for 2 h. The XRD results showed the formation of goethite structure as a single phase up to r = 6.98 (where r = 100·[Co]/([Co]+[Fe])). Cobalt ferrite was found as an additional phase in the precipitates obtained at r = 9.09 and 13.04. Mössbauer spectroscopy showed the formation of solid solutions of -(Fe,Co)OOH. The incorporation of cobalt ions into the goethite crystal structure was monitored by a decrease in the average hyperfine magnetic field (B_hf ). The value B_hf decreased linearly up to r = 5.66, whereas a nonlinear dependence was obtained at r = 6.98 to 13.04. Infrared (IR) bands corresponding to the bending vibrations _OH and _OH were shifted from 892 to 903 cm^–1 and from 797 to 800 cm^–1, respectively, up to r = 6.98. Fourier transform infrared spectroscopy lacked the sensitivity to monitor the effects of cobalt ions, demonstrated by the Mössbauer analysis. Cobalt incorporation into the crystal structure of goethite induced a gradual elongation of -(Fe,Co)OOH particles along the crystallographic c axis. The formation of -(Fe,Co)OOH nanowires was shown at r 6.98. These nanowires were ~700 nm long, whereas their diameter was ~15–20 nm. The majority of the cobalt ferrite particles were in the nanosize range, as observed by FE-SEM. On the basis of the Co/Fe ratio measured by EDS, it appears that the cobalt ferrite particles were not fully stoichiometric.

KEYWORDS: cobalt ions, goethite, cobalt ferrite, XRD, Mössbauer, FTIR, FE-SEM, EDS