THE EFFECTS OF DICARBOXYLIC-ACIDS ON THE CRYSTAL-GROWTH OF ALPHA-FEOOH IN AQUEOUS-MEDIA

Growth of alpha-FeOOH by the reaction of O2 with Fe2+ in an aqueous hydroxide suspension was carried out in the presence of a series of dicarboxylic acids with carbon chain lengths varying from 2 to 7 carbon atoms. A second series of reactions containing dicarboxylic acids and low concentrations of Zn2+ was also carried out. Photoacoustic IR spectroscopy suggested that dicarboxylic acids coordinated to iron in the alpha-FeOOH product. Viscosity of the final reaction suspension for experiments carried out in the presence of dicarboxylic acids was reduced to values below 35 cP, compared to 60 cP for alpha-FeOOH with no acid. Microelectrophoresis showed a correlation between an increase in the isoelectric point of the alpha-FeOOH crystals and a decrease in the viscosity of the suspension. The BET surface area of alpha-FeOOH crystals grown with 4 or 5 carbon dicarboxylic acids was larger than for crystals grown with no dicarboxylic acid. TEM photomicrographs confirm that crystals from the 3-, 4-, and 5-carbon dicarboxylic acid reactions have smaller particle size and lower aspect ratios than alpha-FeOOH grown with no acid. The 2-, 6-, and 7-carbon dicarboxylic acids produce no significant change in the suspension viscosity, BET surface area, or particle size of alpha-FeOOH crystals. Time of addition experiments with malonic acid suggest that the morphology changes caused by dicarboxylic acids occurred during a window starting after one quarter of the oxidation was complete. Zn2+ increases BET surface areas of alpha-FeOOH products independently of dicarboxylic acid effects.