O P Mehra

Bio: O P Mehra is an academic researcher from University of Wisconsin-Madison. The author has contributed to research in topics: Dithionite & Iron oxide. The author has an hindex of 1, co-authored 1 publications receiving 2043 citations.

Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate

Abstract: The oxidation potential of dithionite (Na 2 S 2 O 4 ) increases from 0.37 V to 0.73 V with increase in pH from 6 to 9, because hydroxyl is consumed during oxidation of dithionite. At the same time the amount of iron oxide dissolved in 15 minutes falls off (from 100 percent to less than 1 percent extracted) with increase in pH from 6 to 12 owing to solubility product relationships of iron oxides. An optimum pH for maximum reaction kinetics occurs at approximately pH 7.3. A buffer is needed to hold the pH at the optimum level because 4 moles of OH are used up in reaction with each mole of Na 2 S 2 O 4 oxidized. Tests show that NaHCO 3 effectively serves as a buffer in this application. Crystalline hematite dissolved in amounts of several hundred milligrams in 2 min. Crystalline geothite dissolved more slowly, but dissolved during the two or three 15 min treatments normally given for iron oxide removal from soils and clays. A series of methods for the extraction of iron oxides from soils and clays was tested with soils high in free iron oxides and with nontronite and other iron-bearing clays. It was found that the bicarbonate-buffered Na 2 S 2 O 4 -citrate system was the most effective in removal of free iron oxides from latosolic soils, and the least destructive of iron silicate clays as indicated by least loss in cation exchange capacity after the iron oxide removal treatment. With soils the decrease was very little but with the very susceptible Woody district nontronite, the decrease was aboout 17 percent as contrasted to 35-80 percent with other methods.

Particle-size analysis.

Abstract: Book Chapter describing methods of particle-size analysis for soils. Includes a variety of classification schemes. Standard methods for size distributions using pipet and hydrometer techniques are described. New laser-light scattering and related techniques are discussed. Complete with updated references.

Kinetic control of dissolved phosphate in natural rivers and estuaries: A primer on the phosphate buffer mechanism1

Abstract: The primary mode of interaction of dissolved phosphate with fluvial inorganic suspended particles is via a reversible two-step sorption process. The first step, adsorption/desorption on surfaces, has fast kinetics (minutes-hours). The second step, solid-state diffusion of adsorbed phosphate from the surface into the interior of particles, has slow kinetics (days-months) and is dependent on the time history of the previous surface sorption and the chemistry of the solid diffusional layer. Natural clay particles with a surficial armoring of reactive iron and aluminum hydroxyoxides resulting from chemical weathering of rocks and soils have a high capacity for absorbing phosphate in the second step and for maintaining low “equilibrium phosphate concentrations” in solution. Extrapolation of laboratory sorption and extraction experiments with natural soils and suspended sediments to the environment suggests that the phosphate concentrations of unperturbed turbid rivers (SPM > 50 mg liter I) are controlled near the dynamic equilibrium phosphate concentration of their particles (EPC, = 0.2-l .5 PM) and that fluvial suspended particles “at equilibrium” contain up to 10 pmol-P g-l that is desorbable. Release of this phosphate from particles entering the sea produces the characteristic shape and magnitude of input profiles of dissolved phosphate observed in unperturbed estuaries. On a global scale, fluvial particulates could transport from 1.4 to 14 x 1O’O mol yr-I of reactive phosphate to the sea, some 2-5 times more than that in the dissolved load alone.

First-order reversal curve diagrams: A new tool for characterizing the magnetic properties of natural samples

Abstract: Paleomagnetic and environmental magnetic studies are commonly conducted on samples containing mixtures of magnetic minerals and/or grain sizes. Major hysteresis loops are routinely used to provide information about variations in magnetic mineralogy and grain size. Standard hysteresis parameters, however, provide a measure of the bulk magnetic properties, rather than enabling discrimination between the magnetic components that contribute to the magnetization of a sample. By contrast, first-order reversal curve (FORC) diagrams, which we describe here, can be used to identify and discriminate between the different components in a mixed magnetic mineral assemblage. We use magnetization data from a class of partial hysteresis curves known as first-order reversal curves (FORCs) and transform the data into contour plots (FORC diagrams) of a two-dimensional distribution function. The FORC distribution provides information about particle switching fields and local interaction fields for the assemblage of magnetic particles within a sample. Superparamagnetic, single-domain, and multidomain grains, as well as magnetostatic interactions, all produce characteristic and distinct manifestations on a FORC diagram. Our results indicate that FORC diagrams can be used to characterize a wide range of natural samples and that they provide more detailed information about the magnetic particles in a sample than standard interpretational schemes which employ hysteresis data. It will be necessary to further develop the technique to enable a more quantitative interpretation of magnetic assemblages; however, even qualitative interpretation of FORC diagrams removes many of the ambiguities that are inherent to hysteresis data.

Preservation of organic matter in sediments promoted by iron

Abstract: About one-fifth of organic carbon in sediments is bound to reactive iron phases, which are metastable over geological timescales and may therefore serve as a sink for the long-term storage of organic carbon. It is well known that solid iron phases can preserve organic carbon in soils, but it remains uncertain whether significant amounts of organic carbon can be preserved by iron in sediments. Yves Gelinas et al. study a range of freshwater and marine sediments and find that almost one-quarter of the organic carbon in the sediments tested is directly bound to reactive iron phases. They further estimate that about 22% of the total surface marine sedimentary organic carbon is preserved by its association with iron, which suggests that reactive iron phases are a key factor in the long-term storage of organic carbon. This 'rusty sink' links the global cycles of carbon, oxygen and sulphur. The biogeochemical cycles of iron and organic carbon are strongly interlinked. In oceanic waters, organic ligands have been shown to control the concentration of dissolved iron1. In soils, solid iron phases shelter and preserve organic carbon2, but the role of iron in the preservation of organic matter in sediments has not been clearly established. Here we use an iron reduction method previously applied to soils3 to determine the amount of organic carbon associated with reactive iron phases in sediments of various mineralogies collected from a wide range of depositional environments. Our findings suggest that 21.5 ± 8.6 per cent of the organic carbon in sediments is directly bound to reactive iron phases. We further estimate that a global mass of (19–45) × 1015 grams of organic carbon is preserved in surface marine sediments as a result of its association with iron4. We propose that these associations between organic carbon and iron, which are formed primarily through co-precipitation and/or direct chelation, promote the preservation of organic carbon in sediments. Because reactive iron phases are metastable over geological timescales, we suggest that they serve as an efficient ‘rusty sink’ for organic carbon, acting as a key factor in the long-term storage of organic carbon and thus contributing to the global cycles of carbon, oxygen and sulphur5.