Effects of sodium hydroxide - Selective chemical treatment on samples fromsome Chilean soils

The 5 M sodium hydroxide (NaOH) chemical treatment was applied to the silt and clay fractions of three Chilean fertilized soils derived from volcanic materials tone Ultisol and two Andisols) in order to selectively concentrat e iron oxides. The treatment was sequentially applied and results were foll owed by x-ray diffraction and Mossbauer spectroscopy. In all samples, disso lution chemical treatment concentrated iron oxides in variable degrees; and both, Mossbauer spectra and x-ray diffraction patterns were significantly improved after three successive hot NaOH treatments. Best results were obta ined for the Ultisol sample which is the oldest soil with the lowest organi c matter content. The Mossbauer spectrum after three NaOH treatments of Ult isol silt fraction reveals characteristic features of partially oxidized ma gnetite, leading to two probability profiles of the hyper-fine field distri bution: one of them due to mixed valence iron (Fe3+/2+) ions in octahedral sites [delta =0.59 mm s(-1), 2 epsilon (Q)=0.02 mm s(-1), with a maximum hy per-fine field at B-hf(max)=46.2 tesla]; the other distribution profile enc ompasses contribution from Fe3+ in tetrahedral sites [delta =0.30 mm s(-1), 2 epsilon (Q)=-0.04 mm s(-1), and B-hf(max)=48.2 tesla].