The hydrous oxides of iron and manganese

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. 

Acid (pH 3) ammonium oxalate has been widely used to dissolve iron and aluminium oxides and release bound trace metals since its introduction in 1922 (Tamm, 1922) (Tamm s reagent). Typically McLaren et al. (1986) used 0.17moll-1 ammonium oxalate +0.1 moll- 1 oxalic acid. The extraction is sensitive to light (Mitchell and Mackenzie, 1954) and particularly to ultraviolet light (Endredy, 1963). Schwertmann (1964) showed that in the dark the amorphous iron oxides were mainly attacked and under ultraviolet illumination the crystalline phases were dissolved as effectively as by the dithionite reagent. Heavy metals are released, with the exception of lead and cadmium whose oxalates are poorly soluble and which coprecipitate with calcium oxalate. The use of oxalic 

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Industrial waste to the Saale River (Germany) resulted in zinc concentrations at Rubolstadt of several thousand milligrams per liter which decreased by a factor of 10 within 6 to 8 miles downstream. Surficial sediment in front of a dam 7.4 miles downstream had a zinc content of 0.86%, whereas similar sediment 21.8 miles further downstream had only 0.36% zinc (91), Since the hydrous oxides of iron and manganese are ubiquitous in surficial stream sediments, it is suggested that the zinc decontamination of the Saale River in this case may have been caused by the scavenging ability of the hydrous oxides. 

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