To investigate the influences of metal ions on ␤-FeOOH formation, the ␤-FeOOH particles were prepared by aerial oxidation of FeCl 2 solutions in the presence of chlorides and sulfates of Ti͑IV͒, Fe͑III͒, Cu͑II͒, and Ni͑II͒, and CrCl 3 at different metal/Fe atomic ratios of 0-0.1 and characterized by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, inductively coupled plasma, thermogravimetry and differential thermal analysis, and N 2 adsorption. The produced particles contained a large quantity of Ti͑IV͒ and Cr͑III͒ but little Cu͑II͒ and Ni͑II͒. All the added metal chlorides and sulfates made the formed particles less crystalline. The sulfate and chloride of Ti͑IV͒ inhibited the formation of ␤-FeOOH at a high Ti/Fe by a marked depression of solution pH by hydrolysis of Ti͑IV͒. The metal sulfate ions effectively reduced the crystallinty of ␤-FeOOH to give highly microporous particles. The strong inhibitory effect of SO 4 2Ϫ on the crystallization of ␤-FeOOH can be ascribed to the adsorption of SO 4 2Ϫ on the primary particles. This is also evident from the high content of SO 4 2Ϫ in the formed particles.

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