Characterization of Fe5(AsO3)3Cl2(OH)4·5H2O, a new ferric arsenite hydroxychloride precipitated from FeCl3–As2O3–HCl solutions relevant to arsenic immobilization

Abstract

Tooeleite (Fe6(AsO3)4SO4(OH)4·4H2O) is widely precipitated for direct As(III) removal from sulfate-rich industrial effluents. However, whether or not Fe(III)–As(III)–Cl(-I) precipitate is produced in chloridizing leaching media for As immobilization is almost unknown. This work founded the existence of ferric arsenite (hydroxy)chloride as a new mineral for As(III) removal. Its chemical composition and solid characterization were subsequently studied by using scanning electron microscope with an energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), infrared (FT-IR), Raman spectroscopy and thermogravimetric (TG) curve. The results showed the formation of a yellow precipitate after 3-days reaction of Fe(III)/As(III) with molar ratio ≈ 1.7 in chloride solution at pH 2.3 neutralized with NaOH. Compared with tooeleite, chemical analysis and solid characterization indicated that Cl(-I) replaces SO4(-II) producing ferric arsenite hydroxychloride with formula Fe5(AsO3)3Cl2(OH)4·5H2O. This new plate shaped solid showed better crytallinity than tooeleite, although it has similar morphology and characteristic bands to tooeleite. The FT-IR bands at 628, 964 cm−1 and the Raman bands at 448, 610, 961 cm−1 were assigned to Fe–O or As(III)–O–Fe or As(III)–O bending/stretching vibration, indicating that both arsenite and chloride substituted for the position of sulfate for ferric arsenite hydroxychloride produced due to the lack of the SO42− vibrations. Cl-(I) also contributed to increase As removal efficiency in aqueous sulfate media under acidic pH conditions via the probable formation of sulfate-chloride ferric arsenite.