Properties of goethites prepared under acidic and basic conditions in the presence of silicate

Goethite in natural environments usually grows in the presence of dissolved silicate, To study silicate-associated goethite with specific properties, goethite was synthesized in an Fe(III) system at RT under acidic (OH/Fe = 2; pH 1.6-1.8) and basic (OH/Fe = 4; pH 12-13) conditions at Si concentrations between 10(-5) and 1 M. The goethites were characterized by transmission (TEM) and scanning (SEM) microscopy, X-ray diffraction (XRD), Mossbauer spectroscopy (MS), and chemical analyses. Despite large differences in size and morphology, all goethite crystals were dominantly bound by 110 and 021 faces. Nanosized crystals (ca. 20 nm) were formed at low pH, where the influence of Si was weak. In the basic system, where Si retarded crystallization, much larger (tens to hundreds of nanometers) crystals were formed whose shape varied from acicular and multidomainic at low Si (10(-5) M) to monodomainic, blocky crystals at high (10(-2) M) Si concentration that had reduced growth along [001] in favor of [100] and [010]. The sizes and shapes of the crystals are discussed in terms of nuclei and grow th unit concentrations in the system. Part of the retained Si could be released by phosphate and NaOH (surface-Si), and part was only liberated into HCl congruently with Fe (up to 46 and 17 g kg(-1) for the acid and basic goethites, respectively). Neither XRD nor MS were able to prove structural Si-for-Fe substitution, probably because no tetrahedral positions are available to accommodate Si in the goethite structure. It is assumed that this nonsurface Si fraction is located between the crystal domains. (C) 1999 Academic Press.