JOURNAL OF MATERIALS SCIENCE: MATERIALS 1N ELECTRONICS 5(1994) 168-172 Formation of goethite by oxidative hydrolysis of iron(ll) sulphate D. ANDREEVA, I. MITOV, T. TABAKOVA, A. ANDREEV Institute of Kinetics and Catalysis, Bulgarian Academy of Sciences, "'Acad. G. Bontchev'" street, bL 1 I, 1113 Sofia, Bulgaria The kinetics and the mechanism of goethite formation in the process of iron(ll)-sulphate oxidative hydrolysis at pH = 4.5 and temperatures of 30 ° and 60°C were studied by chemical methods, transmission electron microscopy and Mfssbauer spectroscopy. Experimental evidence was found for the role of iron(ll) and the hydrolysis intermediates in the formation of a crystalline goethite phase. 1. Introduction In a previous study on the oxidation hydrolysis of iron(II) sulphate [ 1] the mechanism of goethite forma- tion from primary aggregates ("oriented crystal growth") was discussed. According to the concept under consideration, the primary aggregates form polymer species and crystallites by the participation of iron(II) undergoing oxidative hydrolysis. This particular stage is of vital importance in the overall process of controlled precursors formation for iron(Ill) oxide with definite properties. The aim of the present study was to find experimental evidence to clarify the basic details of the mechanism of the goethite-phase formation by oxidative hydrolysis of iron(II) sulphate at a low pH and at low temperatures. For this purpose the process of hydrolysis-oxidative hydrolysis of mixed solution of iron(III) and iron(II) was applied by a complex of methods - kinetic investigations, chemicals methods, transmission elec- tron microscopy (TEM) and M6ssbauer spectroscopy. 2. Experimental procedure The kinetics of formation and transformation of the phases during the oxidative hydrolysis of iron(II) sulphate 4FeSO4 + 8NH 3 + 6H20 + 02 *-* 4FeOOH + 4(NH4)2SO 4 was studied at a definite rate of introduction of an iron-containing solution (30 g1-1) to the reactor, a stirring rate of 400 r.p.m (revolutions per minute) and an air blow rate of 51min -1. The pH of the suspension was maintained at 4.5 + 0.1, regulating the ammonium-solution flow. The hydrolysis-oxidative hydrolysis of the mixed iron(III)-iron(II) solution was carried out at the same total iron concentration. The experiment was carried out at 30 ° and 60°C. The reagents used - FeSO4.7H20 , Fe2(SO4) 3.5H20 and NH4OH - were of analytical grade. Each experiment was performed in a laboratory reactor "Contalab" (Contraves, Switzerland) allowing complete control of 168 the process parameters - temperature, pH, stirring rate, rate of reagents introduction - thus determining the high reproducibility of the results. The following experiments were performed. 1. Hydrolysis of mixed solutions of the iron(II)- 5 wt % and iron(III)-95 wt % sulphates at temper- II II1 atures 30 ° and 60°C, designed as F%Fe9530 and I1 III FesFe9560, respectively. 2. Hydrolysis of a mixed solution of iron(II)- 20 wt % and iron(III)-80 wt % sulphates at 60°C, II 11I designated as FezoFe8o60. 3. Hydrolysis of iron(Ill) sulphate with ammonia together with oxidative hydrolysis of iron(II) sulphate at temperatures of 30 ° and 60 °C in the following way. The iron(II)-sulphate solution was placed in the reactor in advance, and the solutions of iron(III) sulphate and ammonia were introduced at rates equal to those in the previous experiments. These: experiments I1 111 were designated as SFesFe9530 and SFe~Fe~J560. The kinetics of the formation and transformation of the phases formed were monitored by chemical meth- ods and M6ssbauer spectroscopy of subsamples taken in definite time intervals. The amount of the amorphous ferrihydrite in the subsamples studied was estimated by the quantity of iron dissolved in an oxalate buffer (pH = 3.0) by the method of Schwertmann [2]. In each sample the oxalate-soluble iron (Fee) and the total amount of iron (Fet) were spectrophotometrically determined. The amount of ferrihydrite phase at each definite moment was estimated by the ratio Feo/Fe t and the amount of the goethite phase was estimated by Feg = Fe t - Fee. TEM micrographs were taken on a JEM-100 M microscope. The M6ssbauer spectra were recorded on an elec- tromechanical spectrometer working in a constant- acceleration mode using a 5VCo source in a Cr-matrix. ~-Fe folio was used as a standard for comparison. The spectra were obtained at room temperature (RT) and in some cases at liquid-nitrogen temperature (LNT). 0957M522 © 1994 Chapman & Hall