The influence of copper on the transformation of ferrihydrite (5Fe2O3 · 9H2O) into crystalline products in alkaline media
Abstract
The presence of Cu retards the transformation of ferrihydrite (5Fe2O3 · 9H2O) into crystalline products. Low levels of Cu (9 mol%) suppress formation of goethite [(α-FeO(OH)] and lead to a product consisting entirely of haematite (α-Fe2O3). Cu prevents formation of goethite by hindering dissolution of ferrihydrite rather than by interfering with nucleation and growth of goethite in solution.
Chemical analysis showed that as much as 9 mol% of Cu replaces Fe in the haematite structure. The ao value of the unit cell of haematite increases from 0.5040 mn for unsubstituted haematite to 0.5048 nm for haematite with 9 mol% Cu incorporation. Transmission electron microscopy showed that Cu-haematite grows as rhombohedral crystals instead of the hexagonal plates typically obtained from ferrihydrite.
As the level of Cu in the system increases above 9 mol%, Cu-magnetite (Fe3O4) forms together with Cu-haematite. At 33 mol% Cu, tenorite (CuO) as well as Cu-magnetite is obtained.
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