Oxalates extraction

Figure 4. Moessbauer spectrum of Fe enriched FeZSM-5 recorded at 90 K. Solid line is the best fit obtained with the two quadrupole doublets. The sample was calcined at 820 K, excess of inactive iron was removed by oxalate extraction.

Martin et al. (2007a) investigated the accumulation and potential release of arsenic in a paddy field in Bangladesh irrigated with arsenic contaminated groundwater. The oxalate-extractable fraction related to amorphous hydrous oxide-bound arsenic represented the dominant arsenic form in the surface layer (47%). A high percentage of arsenic was removed by phosphate (22%). 

The variation of As in groundwater ranges between 1 to 180 pg/L and most of elevated concentrations were recorded for the upper shallow aquifers where mostly fine sandy sediments have frequently been noticed (Fig. 8). Oxalic extractable As in sediment has also been observed high in the fine sandy grains (8 pg/g) (Fig. 9). In contrast, the deeper aquifer was found to have contained low As in groundwater as well in sediment. 

The transformation has been followed up by XRD, Mbssbauer spectroscopy, EXAFS and colorimetry. It can be monitored more conveniently, however, by the acid oxalate extraction method in which residual ferrihydrite is dissolved and the crystalline product left intact (Schwertmann Fischer, 1966). The extent of transformation at any time is given as the ratio FOo/Fet where Fe is the oxalate soluble iron (i. e. the unconverted ferrihydrite) and Fet is the total iron in the system. A plot of log (FCo/Fet) against time of aging at 100 °C is linear over 90-95 % of the reaction... 

Whereas several specific soil attributes are advocated as being responsible for DOC sorption in the mineral soil (Table V), it appears that the greater the clay or aluminum and iron oxide content of a soil, the greater its adsorptive capacity for DOC. For example, there is a positive correlation between m (the measure of the affinity of a substance for the sorbent or the partition coefficient) and soil clay content, dithionite extractable iron (Fej), and oxalate extractable aluminum (Al0) (Moore et al., 1992 Nelson et al., 1993 Kaiser and Zech, 1998). Direct measurements of the surface area of soil particles also correlate very well with DOC adsorption capacity (Nelson et al., 1993). Furthermore, Nelson et al. (1993) report that riverine DOC concentrations are negatively correlated to the clay content of watershed... 

Oxalate-extractable Al and Fe Dalva and Moore (1991) Moore et al. (1992) Qualls (2000)... 

Dissolved metals other than calcium have a minor effect on the distribution of phosphorus between the water column and sediment in this fluvial system. The two principal metals of potential interest, iron and aluminum, are present in Genesee River water almost entirely in the particulate phase ( ). Dissolved concentrations of these metals are below the detection limit (less than 50 ug/1). Iron and aluminum minimum detectable dissolved concentrations were used to estimate the saturation levels of the corresponding phosphate minerals. These calculations suggest that both iron and aluminum phosphate minerals are substantially below saturation levels. The solid surfaces exhibited by iron and aluminum hydrous oxides (as particulate material in the water column) undoubtedly serve as sites for phosphorus adsorption and incorporation in the fluvial system. Data presented for the oxalate extraction procedure in Table III demonstrate the importance of phosphorus binding by hydrous metal oxides. 

In the lower reaches of the Genesee River, the results of the extractions suggest that substances other than hydrous oxides are phosphorus sinks. This is evident where the amount of sediment phosphorus extracted by hydrochloric acid steadily increases down river, while the oxalate extractable phosphorus remains relatively constant. Schwertmann (2 ) emphasized that the results of such procedures are best considered as a measure of the relative amount of a phase or, more generally, a measure of an element s reactivity in a sediment under carefully controlled conditions. Laboratory experiments (Figure 8) show that phosphorus uptake by calcium carbonate, under simulated natural conditions, proceeds slowly. The large hydrochloric acid extractable component observed at Rochester may arise from slow uptake and subsequent mineralization of dissolved inorganic phosphorus by carbonate minerals. 

Iron extraction values show that iron speciation varies significantly between layers in the cave (Fig. 6A). Values for amorphous, total, and ferrous iron range from 2.4 to 84 pmol/g. Extraction results indicate a significant amount of goethite in the lower layers of the sequence as determined by total minus ammonium-oxalate extractable iron (52 pmol/g in the bottom yellow layer) (Schwertmann and Taylor, 1977). The upper layers have total iron values represented almost entirely by ammonium-oxalate extractable iron (83 pmol/g in the red layer and 59 pmol/g in the top orange layer) suggestive of ferrihydrite (a necessary precursor to hematite formation). The ferrihydrite in the upper layers is indicative of formation by rapid oxidation of ferrous iron (Schwertmann, 1993). The black layer contains the only cave sediment with a significant amount (42 pmol/g) of extracted ferrous iron. 

Figure It. Relationship between average amounts of oxalate-extractable iron and arsenic from sandy sediments from a range of sites across Bangladesh. The sites are divided into those with known high and low arsenic groundwaters...

Derivation Potassium formate or carbonate mixed with a small quantity of oxalate and a slight excess of alkali is heated, the oxalate extracted with water and crystallized. 

McKeague, J.A. Day, J.H. (1966) Dithionite and oxalate extractable Fe and Al as aids in differentiating various classes of soils. Canadian Journal of Soil Science 46,13-22. 

Hot buffer extracts contained mainly types B, C and D, oxalate extracts types A, B, C and D and in acid extracts types C, D and E dominated. 

Ammonium Oxalate Extraction of Carrot AAIR. 2.0 G of carrot AAIR was added to 1 L of a 1% ammonium oxalate solution, pH 5.00. The mixture was stirred and slowly heated to 80°C. over a period of one hour. The AAIR was recovered by filtration and... 

Effect of Ammonium Oxalate Extraction of Carrot AAIR on the Co-Binding of Chenodeoxycholate and Deoxycholate. 

One of the pitfalls in the interpretation of extraction results from natural sediments is caused by the fact that the presence of Fe complexed by carboxylic acid catalyzes the reduction of crystalline iron oxides such as hematite (Sulzberger et al. 1989), magnetite (Blesa et al. 1989) and goethite (Kostka and Luther 1994). In order to avoid this catalytic dissolution of well-crystallized iron oxides by Fe + during the oxalate extraction Thamdrup and Canfield (1996) air-dried the sediment in advance, thereby oxidizing FeS and FeCOj to ferrihydrite. In addition, they applied the anoxic oxalate extraction and subtracted the released amount of Fe + from the amount of Fe determined... 

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