Formation of iron-rich natural nanoparticles by the weathering of rock materials

The formation of colloidal nanoparticles during the weathering of Fe(II)-containing mass rock material was investigated by exposing ground phyllite to Milli-Q water (a simulation of “rain water”). Secondary mineral colloids of 101 to 102 nm were detected in significant concentrations (cf. [1]). At pH of about 8.5, the solution concentration of these colloids reached up to 10 mg/L (however, acidification to pH 4.0 prevented the formation of the colloids). The mineralogical composition of the secondary mineral colloids is assumed to be a mixture of ferrihydrite, manganese oxyhydroxides, aluminosilicates, amorphous Al(OH)3 and possibly further components. The colloids were stable in solution over longer periods of time (at least several weeks).
Direct formation of iron-containing secondary mineral colloids at the rock-water interface by the weathering of rock is an alternative to the well-known mechanism of colloid formation in the bulk of water bodies by mixing of different waters or by aeration of anoxic waters [2-4]. This direct mechanism is of relevance for colloid production during the weathering of freshly crushed rock in the unsaturated zone as for instance crushed rock in gravel heaps or mine waste rock piles.
The properties (particle size, chemical composition) of the nanoparticles produced in the weathering experiments and their influence on the behaviour of an environmental contaminant (hexavalent uranium) were investigated. It was shown that the colloids are able to keep large fractions of the U(VI) (in our case up to 90%) in a colloid-borne form.

[1] H. Zänker, G. Hüttig, T. Arnold and H. Nitsche, Aquatic Geochem. 12 (2006) 299.
[2] L. E. Schemel, B. A. Kimball and K. E. Bencala. ppl. Geochem. 15 (2000) 1003.
[3] A. B. Sullivan and J. I. Drever, Appl. Geochem. 16 (2001) 1663.
[4] A. Wolthoorn, E.J.H. Temminghoff, W.H. van Riemsdijk, Appl. Geochem. 19 (2004) 1391.