Bacteria-induced crystallization of kaolinite

Kaolinite formed at room temperature (~ 20 °C) from Si–Al solutions (Si:Al = 2:1) containing bacteria extracted from peat-moss soil and oxalate to produce aqueous octahedral Al complexes. The solutions, saturated with respect to kaolinite, were aged between 218 and 322 days. After 2–3 days of aging, a white precipitate started to form in all the solutions. At the end of the aging period, the precipitates were characterized by X-ray diffraction (XRD), Fourier transformed infrared spectrometry (FTIR), field emission-scanning electron microscopy (SEM), and high-resolution and analytical transmission electron microscopy (HRTEM-AEM). Even though we did not detect crystalline phases by XRD and FTIR, microscopic observations revealed the presence of kaolinite within the gels formed from solutions containing oxalate and bacteria. Our findings suggest that kaolinite forms through a two-step bio-induced process: I) formation and precipitation of the aluminosilicate gel, which is favoured in the presence of oxalate and organic products (extracellular polymeric substances, biofilm, metabolites); and II) crystallization of kaolinite within the gel as a consequence of microenvironment changes induced by metabolic activity, which causes local gel dissolution or its solid state re-arrangement towards a crystalline material.