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Synthetic Iron Oxides as an Indicator of Reduction in Soils (IRIS)

Dep. of Natural Resource Sci., Univ. of Maryland, College Park, MD 20742-5821

View larger version (23K): [in a new window]   Fig. 1. Titration curve of pH versus quantity of base added per mol of ferric salt. Note that both chloride and nitrate salts behave identically.

View larger version (21K): [in a new window]   Fig. 2. Munsell dry color of synthetic Fe oxides stored as aqueous suspensions for various periods of time.

View larger version (16K): [in a new window]   Fig. 3. Ratio of oxalate extractable Fe (Feo) to dithionite extractable Fe (Fed) (as percent) for synthetic Fe oxides stored for various periods of time (in the dark) as aqueous suspensions. pH was not monitored carefully during the sysnthesis of the three older iron oxide samples.

View larger version (29K): [in a new window]   Fig. 4. X-ray diffractograms of synthetic Fe oxides stored as aqueous suspensions (in the dark) for various periods of time. Fh–ferrihydrite; G–goethite; H–hematite.

View larger version (32K): [in a new window]   Fig. 5. X-ray diffractograms of synthetic Fe oxides 4 d old, formed by titration of Fe(III) chloride with KOH to a pH value of 4, 7.5, 11, and 12. Note two broad peaks for ferrihydrite (Fh) at 0.26 and 0.15 nm, and goethite (G) peaks are more pronounced in iron oxides formed at higher pH.

View larger version (33K): [in a new window]   Fig. 6. X-ray diffractograms of synthetic Fe oxides 7 d old, formed by titration of Fe(III) chloride with KOH to a pH value of 4, 7.5, 11, and 12. Note two broad peaks for ferrihydrite (Fh) at 0.26 and 0.15 nm and goethite (G) peaks.

View larger version (33K): [in a new window]   Fig. 7. X-ray diffractograms of synthetic Fe oxides 14 d old, formed by titration of Fe(III) chloride with KOH to a pH value of 4, 7.5, 11, and 12. Note two broad peaks for ferrihydrite (Fh) at 0.26 and 0.15 nm and goethite (G) peaks.

View larger version (38K): [in a new window]   Fig. 8. X-ray diffractograms of synthetic Fe oxides of various ages (4 to 42 d), formed by titration of Fe(III) chloride with KOH to a pH value 7.5. Only two broad peaks for 2-line ferrihydrite (Fh) are observed.

View larger version (45K): [in a new window]   Fig. 9. X-ray diffractograms of synthetic Fe oxides of various ages (4 to 42 d), formed by titration of Fe(III) chloride with KOH to a pH value 11. Note goethite (G) peaks become more pronounced over time.

View larger version (46K): [in a new window]   Fig. 10. X-ray diffractograms of synthetic Fe oxides of various ages (4 to 42 d), formed by titration of Fe(III) chloride with KOH to a pH value 12. Note goethite (G) peaks become more pronounced over time.

View larger version (34K): [in a new window]   Fig. 11. Ratio of oxalate extractable Fe (Feo) to total extractable Fe (Fet) in synthetic Fe oxides formed by titrating to four pH values (4, 7.5, 11, and 12) and stored as aqueous suspensions in the dark over a period of 56 d. Note similarity of Feo/Fet for iron oxides synthesized at pH 4 and pH 7.5 and that the Feo/Fet ratio is very near to 1.0. Pairs of lines represent oxides synthesized from chloride and nitrate salts.

View larger version (16K): [in a new window]   Fig. 12. Change in pH of synthetic Fe oxide suspensions (formed by titration to pH 12) as affected by removal of salts by centrifuge washing and dialysis over a period of three and a half days. Values by points are time (d) following original synthesis.

View larger version (52K): [in a new window]   Fig. 13. X-ray diffractograms of synthetic Fe oxides sampled over a three and a half day period while salts were being removed by centrifuge washing and dialysis. Letters correspond to column 2 in Table 2. Goethite (G) peaks first begin to become visible in XRD scans d and e. Broad ferrihydrite (Fh) peaks are evident at 0.26 and 0.15 nm.

View larger version (21K): [in a new window]   Fig. 14. Durability of Fe oxide paint when applied to IRIS tubes as a function of goethite content. A minimum of 30 to 40% goethite is necessary for painted tubes to have a durability index of five.