IRON OXIDE REMOVAL FROM SOILS AND CLAYS BY A DITHIONITE-CITRATE SYSTEM BUFFERED WITH SODIUM BICARBONATE by
IRON OXIDE REMOVAL FROM SOILS AND CLAYS BY A DITHIONITE-CITRATE SYSTEM BUFFERED WITH SODIUM BICARBONATE by
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发表时间：
2006
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影响因子：
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通讯作者：
O. Medina;AD M. L. JACKSOr
中科院分区：
文献类型：
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作者：
O. Medina;AD M. L. JACKSOr
The ox ida t ion po ten t ia l of d i th ion i te (Na~S204) increases f rom 0 .37 V to 0 .73 V wi th increase in p H f rom 6 to 9, because hyd roxy l is consumed du r ing ox ida t ion of di thioni te . A t tile same t i m e t he a m o u n t of i ron oxide dissolved in 15 m i n u t e s falls off (from 100 percen t to less t h a n 1 percen t ex t rac ted) w i t h increase in p H f rom 6 to 12 owing to solubi l i ty p roduc t re la t ionships of i ron oxides. A n o p t i m u m p H for m a x i m u m reac t ion kinet ics ocem~ a t a p p r o x i m a t e l y p H 7.3. A buffer is needed to hold t he pl~ a t the o p t i m u m level because 4 moles of OH are used up in reac t ion wi th each mole of Na2S204 oxidized. Tes t s show t h a t NaHCO3 effect ively serves as a buffer in th i s appl icat ion. Crysta l l ine h e m a t i t e d issolved in a m o u n t s of several h u n d r e d mi l l ig rams in 2 min . Crysta l l ine goe th i t e d issolved more slowly, b u t d issolved du r ing t he two or three 15 rain t r e a t m e n t s no rma l ly g i v e n for i ron oxide r emova l f rom soils a n d clays. A series of m e t h o d s for t he e x t r a c t i o n of i ron oxides f rom soils a n d clays was tes ted wi th soils h igh in free i ron oxides a n d w i t h non t ron i t e a n d o ther i ron-bear ing clays. I t was found t h a t t he bicarbonate-buffere~l Na2S2Oa-citrate s y s t e m was the m o s t effective in r emova l of free i ron oxides f rom latosolic soils, a n d the least des t ruc t ive of i ron si l icate clays as ind ica ted b y least loss in ca t ion exchange capac i ty a f te r t he i ron oxide r emova l t r e a t m e n t . W i t h soils t he decrease was ve ry litt le b u t wi th the ve ry suscept ib le W o o d y d i s t r i c t non t ron i t e , t he decrease was abou t 17 percen t as con t r a s t ed to 35-80 pe rcen t wi th o ther me thods . I N T R O D U C T I O N The removal of" amorphous coatings and crystals of free iron oxides, particularly hematite and goethite, which act as cementing agents, is important in many types of analysis of softs and clay minerals. The removal of free iron oxides aids in dispersion of the silicate portion, which is essential for effective segregation into different particle size fractions. For x-ray diffraction studies the removal of free iron oxides greatly enhances the parallel orientation of layer silicate clays and brings out some x-ray diffraction peaks that are otherwise difficult or impossible to detect. Differential and integral thermal analysis, electron micrographs and cation exchange capacity are greatly improved after removal of free iron oxides. Use of citrate chelating agent (Aguilera and Jackson, 1953) with Na~S204 not only helps with iron extraction but removes some coatings of alumina and thereby assists in the dissolution of free silica cements that are stabilized by alumina coatings, as will be shown in the present paper.
