EFFECTS OF CRYSTALLINITY OF GOETHITE .1. PREPARATION AND PROPERTIES OF GOETHITES OF DIFFERING CRYSTALLINITY

We. prepared five samples of pure goethite and three samples of alumin ous goethite with mole fractions of aluminium ranging from 0.05 to 0.2 0. One of the samples of pure goethite was repeatedly heated in an aut oclave at 150 degrees C. We studied the samples using transmission ele ctron microscopy and XRD and we measured the surface area (BET), rate of dissolution of the samples in acid, the surface charge in four conc entrations of sodium nitrate from pH 4-10, and the sorption of calcium and chloride from a dilute solution of calcium chloride. Surface area s ranged from 18 to 132 m(2) g(-1). The BET adsorption-desorption isot herms had hysteresis loops which increased as the surface area increas ed. These showed that there were pores present with diameters smaller than 2 nm and that the number of pores increased as the surface area i ncreased. All samples of goethite showed typical X-ray diffraction pat terns but with peak heights decreasing, and width increasing, as surfa ce area increased. Mean coherence lengths, as determined by XRD line b roadening, were smaller than those determined by transmission electron microscopy. This was because each crystal was composed of several dom ains, and the XRD data reflect the size of the domains. Heating one of the goethite samples in an autoclave caused most of the domains to co alesce and slightly decreased the surface area. As the surface area of the eight goethite samples increased, the mean coherence length in th e direction of the a, b and c axes decreased. The better crystallized samples dissolved more slowly in acid and the dissolution curves were sigmoid because the acid was able to penetrate between the domains all owing access to a larger surface area and consequently accelerated dis solution. The points of zero charge :increased with increasing surface area of the goethite. The charge carried by the goethite at pH values distant from the point of zero charge increased as the surface area i ncreased. This may indicate movement of protons or hydroxyl ions into pores or other defects in the crystal. The effects of pH and of salt c oncentration on surface charge were closely described by the variable- charge/variable-potential model. This model also closely described ads orption of Ca2+ and Cl- from calcium chloride solutions requiring only the allocation of constants for these ions. The substitution of alumi nium for iron did not affect the charge properties of goethite.