Citrate-bicarbonate-dithionite

Ericsson T.J., Linares, Lotse E.A. Mossbauer study of the effect of dithionite-citrate-bicarbonate treatment on a vermiculite, a smectite and a soil. Clay Miner 1984 19 85-91. 

Al substitution (0.09-0.16 mol mol ) had no definite effect on the photochemical dissolution of substituted goethite in oxalate at pH 2.6 (Cornell Schindler, 1987). On the other hand, Al substitution depressed the initial (linear) stage of dissolution of synthetic goethites and hematites in mixed dithionite/citrate/bicarbonate solutions (Fig. 12.22) (Torrent et al., 1987). As the variation in initial surface area has already been accounted for, the scatter of data in this figure is presumably due to variations in other crystal properties such as disorder and micropores. Norrish and Taylor (1961) noted that as Al substitution in soil goethites increased, the rate of reductive dissolution dropped (see also Jeanroy et al., 1991). 

Fig. 12.22 Relationship between the dissolution rate per unit surface area in Na-dithionite/citrate/bicarbonate at 25 °C and the Al substitution of 28 synthetic goethites (upper) and 24 synthetic hematites (lower) (Torrent et al., 1987, with permission).

Fig. 12.25 Di ssolution features of hematites Upper Undissolved (a) and partly dissolved (b c) synthetic Al-hematite (AI/(Fe+AI) = 0.094 mol moT j in dithionite/citrate/bicarbonate at25 (Araki Sch A/ertmann unpubl.), Lo A/er Undissolved and partly dissolved hematite from a redoxomorphic subsoil of a typical Hapludalf on Permian mudstone, Ohio (Bigham et al., 1991, A/ith permission).

Fig. 12.30 Relationship between the fraction of oxidic Fe dissolved by dithionite/citrate/bicarbonate in 30 min and the proportion of Fe in hematite in hematitic/goethitic clay fractions of some Spanish soils (Barron Torrent, 1987, with permission).

Iron oxide-sorbed Dithionite/citrate/bicarbonate 39... 

See also Section 3.2.35.18.2. MX80 Wyoming bentonite was washed with 0.5 M NaCl and 0.1 M acetic acid, extracted with dithionite-citrate-bicarbonate, treated with 3% H2O2, and then washed with 0.5 M NaCl again. 

Extracted with dithionite-citrate-bicarbonate, treated with 3% H2O2. 

FIGURE 7.12 X-ray powder diffraction patterns of a soil clay fraction untreated (top) and treated (bottom) with dithionite-citrate-bicarbonate attack, which removes all secondary Fe oxides Cn = a-AljOj (corundum) Gt = goethite Hm = hematite PS = phyllosilicates and Qz = quartz. Hematite identification is uncertain because of strong overlap with phyllosili-cate and goethite peaks, hence the question marks. Samples contained a-Al203 as an internal standard. (Reprinted from Schulze, D. G., Clays Clay Miner. 34, 6, 681-685, 1986. With permission from the Clay Minerals Society.)... 

Mehra, O.E and Jackson, M.L. (1960) Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Proc. 7th Natl. Conf. Clays,... 

To assess the correlation of iron and aluminum oxides with iodine content, we used a selective extraction to target the different fractions of Fe and A1 in the soil samples. Citrate-bicarbonate-dithionite (CBD) extractable Fe contains crystalline iron oxide minerals, such as hematite, goethite, lepidocrocite, and ferrihydrite, while ammonium oxalate (NH4—Ox) extraction targets noncrystalline ( free ) Fe oxides, including ferrihydrite and ferrihydrite-hke minerals (Loeppert and Inskeep, 1996). The CBD and NH4—Ox extractions also remove some crystaUine and noncrystalline aluminum oxide phases, respectively. 

To extract different forms of Ee, the clay fractions from the fine earth of bulk, LAR and TAR were dispersed in NaOH at pH 8.5 and treated with acidic (pH 3) NH4-oxalate (Blakemore et al., 1981), citrate-bicarbonate-dithionite (CBD) (Mehra and Jackson, 1960) and 0.1 M hydroxylamine hydrochloride (HAHC) (Chao, 1972). The Ee extracted was measured by atomic absorption with a Perkin-Elmer llOOB spectrophotometer. 

Fe extracted by acid NH4-oxalate, citrate-bicarbonate-dithionite (CBD), and hydroxy-lamine hydrochloride (HAHC) from the clay from bulk and rhizosphere soil of the 2C1 and 2C2 horizons nnder Genista aetnensis at Mount Vetore (Sicily, Italy). Standard errors in parentheses (n = 3)... 

Hunt, C. P., Singer, M. J., Kletetschka, G., TenPas, J. Verosub, K. L. (1995). Effect of citrate-bicarbonate-dithionite treatment on fine-grained magnetite and maghemite. Earth and Planetary Science Letters, 130, 87-94. 

The rate of disproportionation increases with decreasing pH and rising temperature and these conditions also favour oxide reduction. To achieve a reasonable rate of dissolution, one has to compromise on the pH. A pH of 3 is used in kaolin bleaching (Jepson, 1988), whereas in soil analysis the system is usually buffered with citrate and bicarbonate at ca. pH 7 (Mehra Jackson, 1960). Citrate also complexes the dissolved Fe " and prevents its precipitation as Fe" sulphide. For the dithionite/EDTA system, Rueda et al. (1992) found a maximum efficiency at pH 5-6 and an activation energy for goethite dissolution of 70 kj mol". They stressed the importance of adsorption of S2O4 on the surface to ensure reduction of Fe ". 

Sodium dithionite has been used in combination with sodium citrate and sodium bicarbonate in a range of concentrations (Beckett, 1989, pp. 163-164), and usually at pHs between 5.8 and 7.3, for the reduction of both crystalline and amorphous (McKeague and Day, 1966 Gupta and Chen, 1975) iron oxide phases and release of sorbed trace metals. It is little used for heavy metal studies because of contamination of the dithionite with zinc and the possibility of precipitation of metal sulfides. 

Fig. 7.16 Pore water and extraction results from hemipelagic sediments off Uruguay (redrawn from Haese et al. 2000). Dissolution and precipitation of Fe is reflected by the easy reducible iron oxyhydroxide fraction whereas less reducible iron oxides soluble by subsequent citrate/dithionite/bicarbonate (CDB) extraction remain constant. A concurrent liberation of Mn and Fe indicates dissimilatory iron reduction and subsequent iron reoxidation by manganese oxides, which results in the build-up of Mn Under these conditions the actual dissimilatory iron reduction rate is higher than deduced from iron pore water gradients.

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