Effect of time and temperature on the chemical composition and crystallization of mixed iron and aluminum species

We studied the influence of time ageing (up to 120 d at 50 degrees C or 30d at 95 degrees C) on the mineralogical and chemical composition of hydrolytic species of mixed aluminum and iron samples formed at pH 5.0 and initial Fe/Al molar ratio (Ri) ranging from 0.1 to 10. The partitioning distribution of Fe and Al in soluble or solid phases of different sizes (<0.01, 0.2-0.01, >0.2 mu m) depended on Ri and time. The ratio of Fe to Al of the <0.2 mu m Fe-Al species of the samples at Ri less than or equal to 4 slowly increased with time. Usually the higher Ri the higher the amount of Fe + Al present in soluble or very fine solids (<0.2 mu m). With time, high percentages of Fe were found mainly in the <0.01 mu m while the Al increase in the >0.2 mu m sizes. Gibbsite, without the presence of well-crystallized Fe-oxides was formed in the samples at Ri less than or equal to 0.5 after 7-120 d at 50 degrees C. In the samples at Ri greater than or equal to 1 low-crystalline ferrihydrite was observed after greater than or equal to 60 d. Only after 120 d did gibbsite or hematite start to form in the samples at Ri = 1-10. However, even after prolonged ageing at 95 degrees C, low-crystalline ferrihydrite was still present at Ri less than or equal to 4. The Fe-Al samples at Ri greater than or equal to 1 aged 32 d at 50 degrees C dissolved almost completely by acid ammonium-oxalate (82-93%), but the samples at Ri less than or equal to 0.5 were only partially solubilized (13-60%). After further 30 d at 95 degrees C, the percentages of Fe + Al solubilized by oxalate from the samples at R greater than or equal to 0.5 was still relatively high (22-39%).