engleski

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

The effects of cobalt ions on the precipitation of goethite in highly alkaline media were monitored using X-ray diffraction (XRD), Mossbauer, 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 º ; ; 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. Mossbauer spectroscopy showed the formation of solid solutions of a-(Fe, Co)OOH. The incorporation of cobalt ions into the goethite crystal structure was monitored by a decrease in the average hyperfine magnetic field (Bhf). The value Bhf i 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 dOH and gOH 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 Mo¨ssbauer analysis. Cobalt incorporation into the crystal structure of goethite induced a gradual elongation of a-(Fe, Co)OOH particles along the crystallographic c axis. The formation of a-(Fe, Co)OOH nanowires was shown at r 56.98. 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.

cobalt ions ; goethite ; cobalt ferrite ; XRD ; Mossbauer ; FTIR ; FE-SEM ; EDS

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