Dissolution Of Iron Hydr Oxides Research Articles

Iron hydroxides in soils: A review of publications

Iron hydroxides are subdivided into thermodynamically unstable (ferrihydrite, feroxyhyte, and lepidocrocite) and stable (goethite) minerals. Hydroxides are formed either from Fe3+ (as ferrihydrite) or Fe2+ (as feroxyhyte and lepidocrocite). The high amount of feroxyhyte in ferromanganic concretions is proved, which points to the leading role of variable redox conditions in the synthesis of hydroxides. The structure of iron hydroxides is stabilized by inorganic elements, i.e., ferrihydrite, by silicon; feroxyhyte, by manganese; lepidocrocite, by phosphorus; and goethite, by aluminum. Ferrihydrite and feroxyhyte are formed with the participation of biota, whereas the abiotic formation of lepidocrocite and goethite is possible. The iron hydroxidogenesis is more pronounced in podzolic soils than in chernozems, and it is more pronounced in iron-manganic nodules than in the fine earth. Upon the dissolution of iron hydroxides, iron isotopes are fractioned with light-weight 54Fe atoms being dissolved more readily. Unstable hydroxides are transformed into stable (hydr)oxides, i.e., feroxyhyte is spontaneously converted to goethite, and ferrihydrite, to hematite or goethite.

Facile methods to synthesize nanaosized iron oxide colloidal dispersion and its characterization

In this study, we develop two experimental procedures to synthesize a nanosized α-Fe 2O 3 colloidal suspension within a relatively short reaction time. In the first method, we use ferric chloride as the precursor, both NaOH and urea as the precipitant, and polyvinyl pyrrolidone (PVP) as the protecting agent; we find that by adding ferrous ion (Fe 2+), nanosized α-iron oxide colloids are formed in about 40 min. The ferrous ion accelerates the conversion of Fe(OH) 3 to α-Fe 2O 3, probably by helping with the dissolution of iron hydroxide and then its re-crystallization into iron oxide. In the second method, two thirds of the stoichiometric amount of NaOH is first used to precipitate iron hydroxide. It is then heated at boiling temperature in the presence of urea or sodium bicarbonate (NaHCO 3) to complete the transformation into hematite in less than an hour. The solution pH increases with time and the transformation from iron hydroxide to α-Fe 2O 3 probably takes place by the solid state transformation mechanism. The common point between urea and sodium bicarbonate is the generation of CO 2 during the reaction, which might play some role in helping with the solid state dehydration reaction.

Dissolution of iron hydroxides by marine bacterial siderophore

A series of laboratory experiments was conducted to investigate the dissolution behavior of goethite and poorly crystalline iron hydroxide (PCIH) in the presence of siderophore produced by a marine bacterium, Alteromonas haloplanktis. The amounts of siderophore in the experimental solutions are expressed by the iron complexing capacity (ICC). The experimental data indicate that both the dissolution rates and solubilities of ferric hydroxides increase with increasing contents of siderophore and [H +]. For example, in a solution of pH=8 and ICC=12 μM, the iron content of solution increased from <0.1 to 1.3 μM by reaction with PCIH for 80 h. This Fe content is more than 50 times the solubility value of amorphous Fe(OH) 3 in pure water at pH=8. At pH=4, with increasing ICC value from 0 to 12.9 μM, the dissolution rate of goethite increased from 1.5 to 9.5 (nM/h/m 2) and that of PCIH increased from 0.4 to 3.4 (nM/h/m 2). At ICC=12.9 μM, with increasing pH from 4 to 6.8, the dissolution rate of goethite decreased from 9.5 to 1.9 nM/h/m 2, whereas that of PCIH decreased from 3.4 to 0.7 nM/h/m 2. Our study suggests that most of the iron utilized by phytoplankton in the oceans may be liberated from ferric hydroxides in aeolian particles by siderophores generated by marine bacteria.

PH dependence of iron photoreduction in a rocky mountain stream affected by acid mine drainage

AbstractThe redox speciation of dissolved iron and the transport of iron in acidic, metal‐enriched streams is controlled by precipitation and dissolution of iron hydroxides, by photoreduction of dissolved ferric iron and hydrous iron oxides, and by oxidation of the resulting dissolved ferrous iron. We examined the pH dependence of these processes in an acidic mine‐drainage stream, St Kevin Gulch, Colorado, by experimentally increasing the pH of the stream from about 4·0 to 6·5 and following the downstream changes in iron species. We used a solute transport model with variable flow to evaluate biogeochemical processes controlling downstream transport. We found that at pH 6·4 there was a rapid and large initial loss of ferrous iron concurrent with the precipitation of aluminium hydroxide. Below this reach, ferrous iron was conservative during the morning but there was a net downstream loss of ferrous iron around noon and in the afternoon. Calculation of net oxidation rates shows that the noontime loss rate was generally much faster than rates for the ferrous iron oxidation at pH 6 predicted by Singer and Stumm (1970. Science 167: 1121). The maintenance of ferrous iron concentrations in the morning is explained by the photoreduction of photoreactive ferric species, which are then depleted by noon. Copyright © 2001 John Wiley &amp; Sons, Ltd.