PNNL

Abstract

The O-H stretching region of goethite particles evaporated at different levels of acidity was investigated by Attenuated Total Reflectance (ATR) Fourier Transform InfraRed (FTIR) spectroscopy. A 2D IR correlation analysis of the spectra was used to identify correlations between different sets of discrete surface OH stretches. Four groups were identified: Group I (3648 and 3578 cm-1), Group II (3668, 3660 and 3490 cm-1), Group III (3698 and 3541 cm-1) and Group IV (3628 cm-1). Groups II and III were respectively assigned to singly- (-OH) and doubly- (??-OH) coordinated hydroxyls of mostly the {110} plane of goethite. Groups I and IV assigned to a minor sub-set of singly- (-OH) and doubly- (??-OH) coordinated hydroxyls. Their structural origins, for example whether they arise from defects of are distinct hydroxyls of the {021} plane, remains to be determined. These band assignments were supported by estimates of the surface protonation level of goethite as well as the relative and absolute O-H stretching frequency calculations of all crystallographically-available surface sites of the goethite surface using the results of previous theoretical studies. A reexamination of Temperature Programmed Desorption (TPD) FTIR data of one goethite sample evaporated from alkaline conditions [Boily et al., Geochim. Cosmochim Acta, 70, 3613-3624] also provided further constraints to this band assignment and helped formulate a possible route for surface dehydroxylation reactions. This model suggests that important cooperative effects between hydrogen-bonded surface hydroxyls play a crucial role on the variations of the position and intensity of discrete O-H stretching bands as a function of protonation level and temperature.