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

Abstract

AbstractThe 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 (FESEM) techniques. Tetramethylammonium hydroxide was used as a precipitating agent. The precipitates were collected after heating of the precipitation systems at 160ºC for 2 h. The XRD results showed the formation of goethite structure as a single phase up tor= 6.98 (wherer= 100·[Co]/([Co]+[Fe])). Cobalt ferrite was found as an additional phase in the precipitates obtained atr= 9.09 and 13.04. Mössbauer spectroscopy showed the formation of solid solutions of α-(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⟩ decreased linearly up tor= 5.66, whereas a nonlinear dependence was obtained atr= 6.98 to 13.04. Infrared (IR) bands corresponding to the bending vibrations δOHand γOHwere shifted from 892 to 903 cm–1and from 797 to 800 cm–1, respectively, up tor= 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 α-(Fe,Co)OOH particles along the crystallographiccaxis. The formation of α-(Fe,Co)OOH nanowires was shown atr⩾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.