Content uploaded by Chandrika Varadachari
Author content
All content in this area was uploaded by Chandrika Varadachari on Nov 12, 2014
Content may be subject to copyright.
A preview of the PDF is not available
Dissolution of Iron Oxides
Abstract
This paper presents an in depth analysis of the DCB (dithionite-citrate-bicarbonate) method popularly adopted for iron oxide removal from clays as outlined by Mehra and Jackson (1960). We reviewed the reaction conditions of the DCB method and found several inconsistencies in the selection of procedures such as pH, method of washing, etc. We then carried out a theoretical analysis of the chemistry of dithionite reduction, deduced the effect of pH on the reduction potential and studied the fundamental mechanisms of iron dissolution. Analysis indicates that the basic arguments on which the DCB method is based, is not in agreement with theoretical facts. These flaws are discussed. To overcome the shortcomings of the DCB method, an alternative DCO (dithionite-carbonate-oxalate) method as proposed by Goswami et al. (1995) is briefly described here. Iron oxides in soils not only occur as discrete phases, but are also closely associated with silicate clays on the surfaces of which they may form coatings (Greenland and Hayes, 1978). By virtue of their ability to adhere to the surfaces of clay minerals, iron oxides may greatly modify their properties, particularly ion exchange behaviour and surface charge. The presence of iron oxides also complicates spectroscopic studies (XRD, IR, Mössbauer, etc.) of soil clays. Therefore, for the purpose of many investigations, the selective dissolution of iron oxides from the silicate clays, is an essential feature. Iron oxide removal procedures have been routinely adopted since several decades both for characterising silicate as well as for quantifying the influence of iron oxides on various chemical properties. All the methods proposed till date, for selective dissolution of iron oxides, utilise reducing agents or complexants or a combination of the two. The notable earliest attempt was by Tamm (1922) who proposed the use of acid ammonium oxalate at pH 3.3. This method is popularly used even today, for evaluating amorphous iron oxides in soils; crystalline materials are, however, incompletely extracted by this method. The subsequent proposal of considerable importance appears to have been by Galabutskaya and Govorova (1934) who suggested the use of sodium dithionite at 40 o C. This report left largely unrecognised by the rest of the world for about 16 years.
... UV−visible spectroscopy should not be used because residual dithionite, citrate, and organic matter complexes cause discrepancies, and H 2 O 2 reacts with organic spectroscopic reagents. 25,62 The solid fraction can be prepared for further analysis using washes of organic solvent. Trace sulfur is removed using four washes of 80−100% methanol, followed by three washes of 75% acetone and one wash of 100% acetone, and these washes are discarded. ...
... 64 There is some reported uncertainty in the effectiveness of the DCB dissolution method in dissolving iron (oxyhydr)oxides. A method review published in 2006 showed incomplete extraction of iron from various soils and indicated that more iron was extracted using dithionite−citrate−oxalate, ascorbic acid−oxalate, 62 and titanium(III)−citrate−EDTA−bicarbonate. 62,65 There is, however, an important difference in these methods; the DCB method does not target magnetite, while oxalate is effective at magnetite dissolution, as discussed below. ...
... A method review published in 2006 showed incomplete extraction of iron from various soils and indicated that more iron was extracted using dithionite−citrate−oxalate, ascorbic acid−oxalate, 62 and titanium(III)−citrate−EDTA−bicarbonate. 62,65 There is, however, an important difference in these methods; the DCB method does not target magnetite, while oxalate is effective at magnetite dissolution, as discussed below. ...
Classifying iron residing in iron-bearing minerals is an important process across numerous disciplines. Iron exists in many forms and with varying degrees of accessibility for reactions. A wide range of procedures exist for identifying and quantifying iron in various forms, but they are scattered throughout numerous journals and typically have several variations. Methods for selective mineral dissolution and iron quantification are reviewed here to improve clarity and accessibility. The methods are described from least aggressive to most aggressive and include cation exchange with salt solutions and dissolutions by acetic acid to target carbonates, hydroxylamine-HCl to target ferrihydrite and lepidocrocite, sodium dithionite to target select iron (oxyhydr)oxides, oxidizing reagents to target organic matter, oxalic acid to target magnetite, concentrated HCl to target iron (oxyhydr)oxides, HF to target silicates, and acid-Cr to target iron sulfides. The dissolution methods using acetic acid, hydroxylamine-HCl, oxalic acid under darkness, and concentrated HCl were critically analyzed for quantifying iron residing in several natural and synthetic minerals. Iron quantification by UV-visible spectroscopy is reviewed, including the use of common colorimetric reagents such as potassium thiocyanate, substituted ortho-dihydroxybenzenes, and ferroin-bearing reagents. Iron detection using ferrozine in the presence of oxalate was also critically evaluated and results indicate that oxalate inhibits the formation of the Fe(II)-ferrozine complex but also efficiently photoreduces Fe(III). Natural samples, however, present several challenges and the presence of mineral salts and redox-active species must be considered. Finally, the importance of standard reporting protocols for fostering accessibility and facilitating comparisons between data sets is discussed.
... These ferric ions bind to the surfactant molecules exacerbating adsorption in ways that are not representative of the native anaerobic, reducing conditions within the reservoir [31,32]. In order to deduce precise adsorption levels within the reservoir, it is thereby necessary to alter the redox state to reducing conditions, a restoration process that has been sufficiently achieved in previous studies using reducing agents such as erythorbic acid [33] and sodium dithionite [34,35], usually compounded with complexing agents like EDTA [36,37] and citrates [38] and/or buffers including sodium bicarbonate [39]. Wang [31] found that retention of anionic sulfate surfactants to the pore surfaces of sandstone cores significantly decreased after treatment with sodium dithionite with results closely matching field tests. ...
Surfactant-assisted oil recovery is usually employed for the production of residual oil after primary and secondary recovery techniques have been exhausted. The loss of injected surfactants via adsorption on to the porous media surfaces impede the efficiency of the procedure and greatly impact process economics. To address this issue, the choice of surfactant and aqueous fluid composition are varied. In particular, the divalent ion concentration is a property that potentially influences surfactant adsorption significantly. It is this aspect which has been addressed in this study.
In this study, atomic force microscope (AFM). tips were functionalised with a self-assembled monolayer (SAM) of the alkanethiol, sodium 11-mercaptoundecanesulfonate (otherwise known as MUS) with the –SO3-headgroups oriented outwards towards the aqueous solutions, this is a model for the well-studied surfactant, sodium dodecyl sulfate, SDS. The adhesion forces between these “surfactant-functionalised” tips and Bandera Brown sandstone surfaces were measured in brine solutions of varying CaCl2 concentrations.
Observations deviate from previous studies where adsorption of surfactant headgroups in the bulk solution is enhanced via Ca²⁺ bridging with the surface. Rather, measured adhesion forces decreased with increase in CaCl2 concentration. The results are reversible and also occurs in a reducing environment when the iron in the Bandera brown is reduced for iron (III) to iron (II). The observed behaviour is interpreted in terms of Ca²⁺ preferentially bridging between neighbouring headgroups on the tip due to the dense packing of molecules non-representative of respective bulk behaviours and acting as a barrier to adhesive contact. Thus, this supposed micellization at the tip aided by the formation of Ca²⁺ salt bridges between alternate headgroups more closely mimics a surfactant micelle, rather than dispersed individual monomers.
The findings of this study provide insight into nanoscale anionic surfactant interactions in general and within sandstone reservoirs in particular where surfactant adsorption is driven largely by monomers rather than micelles. Investigating surfactant adsorption using microscopic techniques such as AFM allow for the determination of interactions at mineral-fluid interfaces and constitute an effective screening methodology for ascertaining enhanced oil recovery process feasibility.
... The organic reducing agents also increase the mobility of metals through the enhanced dissolution of soil minerals (such as scorodite, muscovite, and feldspar) and amorphous silica, and the reduction of mineral oxides (Cheng et al. 2020). Therefore, the combination of organic reductants and EDTA can significantly improve the metal removal rate (Varadachari et al. 2006). The combination of reducing agents (ascorbic acid and oxalic acid) with EDTA increase the removal efficiencies of As, Cd, Cu, and Pb from the soil 19-30%, 6-19%, 3-12%, and 6-21% respectively compared with washing with EDTA only ). ...
Soil pollution by potentially toxic elements (PTEs) is a serious threat to human health and ecosystem function. Soil washing using EDTA is one of the permanent disposal options available to remove PTEs from soil. Based on published studies, this paper summarized the current progress of remediation techniques using EDTA to mobilize and remove PTEs from contaminated soils. Firstly, the key factors to control EDTA washing were discussed, such as the concentration of EDTA, the pH of the washing solution, the washing time, multiple washing, and the liquid/solid ratio. Afterwards, the complicated changes in soil properties after washing were discussed. The change of soil properties is inevitable, so some measures need to be taken to reduce the damage to the soil by washing. Finally, the current improvements were summarized for the problems existing in the EDTA washing process. EDTA could be used in combination with other agents such as other chelating agents, reducing agents, acid compounds, and surfactants to improve efficiency of EDTA washing. The purpose of this paper is to provide a reference for further research and practical application of remediation of PTEs contaminated soil by EDTA washing.
Graphic abstract
... The prevailing (low) pH and the influence of the ammonium oxalate, which is known to affect the process in the range around between pH 2 and pH 7, may also contribute to the high Fe o values in the investigated soils. On the other hand, several studies (e.g., Rennert, 2019;Varadachari et al., 2006;) reported that magnetite-maghemite may not completely be dissolved by dithionite-citrate-bicarbonate solution, leading to an under-estimation of Fe d . ...
Icelandic soils develop in a dynamic environment affected by both natural processes and anthropogenic impacts. We present an extensive investigation of soil mineralogy and pedogenesis in a disturbed (i.e. by solifluction) pedon under such conditions. The study focuses on two distinct tephra layers; a rhyolitic tephra from the Öræfajökull eruption in 1362 CE (Ö1362) and a basaltic Veiðivötn tephra from 1477 CE (V1477). Both tephra layers form an important parent material in the study area south of Vatnajökull, Iceland. The Andosol developed from tephra and aeolian material, rich in volcanic glass. The pH (H2O) values were between 5.5 and 6.6 and clay mineralogy displayed a predominance of allophanic material in both soil and tephra. The pedon can be seen in its early stage of chemical weathering and soil development. Despite the overall predominance of non‐ and poorly‐crystalline SRO secondary materials and Fe (hydr)oxides in the clay‐size fraction, we found indication of smectite. High exchangeable Ca2+ and Mg2+ concentrations reflect enhanced aeolian input of volcanic material, while elevated exchangeable Na+ concentrations are most likely caused by the oceanic composition of precipitation. The impact of erosion and aeolian processes on pedogenesis seemed to be higher between the deposition of Ö1362 and V1477, than in the soils above. Both, soil and tephra layers appeared disturbed by erosion/deposition processes over time. Characterized by a significantly coarser particle size composition, low SOC content, and a more diverse mineralogy, a distinct fluvial sediment layer at 10–30 cm depth appeared sharply contrasting to the other soil layers in the profile. This possibly reflects the diverse composition of glaciofluvial material from a landslide originating from a Kvíárjökull moraine in the North. Changed weathering patterns and properties in the soils above the Ö1362 tephra indicated soil degradation following the deposition of the rhyolitic tephra. This article is protected by copyright. All rights reserved.
... We followed the dithionite-citrate extraction which is performed for 16 h on a rolling shaker under room temperature and anoxically in the dark (Coward et al., 2017;Wagai et al., 2013;Wagai and Mayer, 2007) and combined different approaches to account for potential difficulties when using this extraction method as previously discussed in detail by Patzner et al. (2020) and shortly discussed in the following. Due to the instability of dithionite in solution (Varadachari et al., 2006), powdered dithionite was added to the sample to reach a final concentration of 0.1 M by adding 3.125 mL of a 0.27 M trisodium citrate, 0.11 M sodium bicarbonate solution (pH 7, N 2 :CO 2 (90:10, v:v) headspace). Sodium bicarbonate was used as a buffer (pH 7) to prevent hydrolysis and re-sorption of organic matter under acidic pH. ...
In permafrost soils, substantial amounts of organic carbon (OC) are potentially protected from microbial degradation and transformation into greenhouse gases by association with reactive iron (Fe) minerals. As permafrost environments respond to climate change, increased drainage of thaw lakes in permafrost regions is predicted. Soils will subsequently develop on these drained thaw lakes, but the role of Fe-OC associations in future OC stabilization during this predicted soil development is unknown. To fill this knowledge gap, we have examined Fe-OC associations in organic, cryoturbated and mineral horizons along a 5500-year chronosequence of drained thaw lake basins in Utqiaġvik, Alaska. By applying chemical extractions, we found that ∼17% of the total OC content in cryoturbated horizons is associated with reactive Fe minerals, compared to ∼10% in organic or mineral horizons. As soil development advances, the total stocks of Fe-associated OC more than double within the first 50 years after thaw lake drainage, because of increased storage of Fe-associated OC in cryoturbated horizons (from 8 to 75% of the total Fe-associated OC stock). Spatially-resolved nanoscale secondary ion mass spectrometry showed that OC is primarily associated with Fe(III) (oxyhydr)oxides which were identified by ⁵⁷Fe Mössbauer spectroscopy as ferrihydrite. High OC:Fe mass ratios (>0.22) indicate that Fe-OC associations are formed via co-precipitation, chelation and aggregation. These results demonstrate that, given the proposed enhanced drainage of thaw lakes under climate change, OC is increasingly incorporated and stabilized by the association with reactive Fe minerals as a result of soil formation and increased cryoturbation.
... Poorly crystalline and crystalline Fe (hydr)oxides were removed from the fractionated small SMA using the Dithionite-Citrate-Bicarbonate (DCB) method developed by Mehra and Jackson (1958), mainly extracting Fe oxyhydroxides and partially Mn, Al, and Si (Bigham et al., 1978;Kiem and Kögel-Knabner, 2002). Note that the DCB treatment did not remove Fe completely (Borggaard, 1988;Varadachari et al., 2006). Especially small sized particles may be removed, as DCB is generally known to dissolve pedogenic Fe-oxide (e.g., hematite or goethite) by reduction. ...
Several beneficial soil functions are linked to aggregates, but how the formation and stability depend on the presence of colloidal- and nanosized (1000–1 nm) bulding blocks is still understood poorly. Here, we sampled subsites from an arable toposequence with 190 and 340 g kg−1 clay, and isolated small soil microaggregates (SMA;<20 μm) from larger macroaggregate units (> 250 μm) using an ultrasonic dispersion energy of 60, 250, and 440 J mL−1, respectively. We then allowed these small SMA to reaggregated after chemical removal of organic carbon (OC) as well as of Fe- and Al (hydr)oxides, respectively. The size distribution of the reaggregated small SMA and fine colloids (< 0.45 μm) was analyzed via laser diffraction and asymmetric flow field-flow fractionation coupled to inductively coupled plasma mass spectrometry and organic carbon detection, respectively. We found elevated amounts of both finer colloids and stable SMA at subsites with higher clay contents. The size distribution of small SMA was composed of two distinctive fractions including colloids and larger microaggregates with an average size of 5 μm. The removal of Fe with Dithionite-Citrate-Bicarbonate (DCB) shifted the size of the small SMA to a larger equivalent diameter, while removal of OC with NaOCl reduced it. After three wetting and drying cycles, the concentration of colloids declined, whereas the small SMA without chemical pre-treatments reaggregated to particles with larger average diameters up to 10 μm, with the size depending on the clay content. Intriguingly, this gain in size was more pronounced after Fe removal, but it was not affected by OC removal. We suggest that Fe (hydr)oxides impacts the stability of small SMA primarily by being present in small-sized pores and thus cementing the aggregates to smaller size. In contrast, the effect of OC was restricted to the size of colloids, gluing them together to small SMAs within defined size ranges when OC was present but releasing these colloids when OC was absent.
... O método, entretanto, não é totalmente seletivo, podendo também extrair o Al ligado à matéria orgânica (Al-MO), além do Al adsorvido eletrostaticamente (García-Rodeja et al., 2004;Auxtero et al., 2012;Cunha et al., 2014;. O Al extraído pelo oxalato de amônio 0,2 mol L -1 provém principalmente da dissolução de compostos de Fe e de Al inorgânicos de baixa cristalinidade e uma pequena fração do Al da gibbsita (García-Rodeja et al., 2004;Varadachari et al., 2006) e de parte dos polímeros inorgânicos de hidroxi-Al entre camadas de argilominerais silicatados 2:1 (García- Rodeja et al., 2004). ...
Resumo O alumínio (Al) "trocável" do solo é tradicionalmente extraído com solução de KCl 1 mol L-1 (Al-KCl). O emprego desta solução pode superestimar o Al trocável, implicando uma interpretação equivocada dos efeitos tóxicos desse elemento às plantas. O estudo objetivou avaliar, por meio de extrações químicas sequenciais (ES) e não sequenciais (ENS), as possíveis frações de Al que possam estar contribuindo para a superestimação do Al trocável e se as mesmas estão relacionadas com sua toxicidade nas plantas de soja cultivadas em casa de vegetação. Também, identificar a eficácia do KCl 1 mol L-1 em extrair somente as frações tóxicas do Al e o extrator que melhor estimou o potencial tóxico do mesmo às plantas.Para isso, foram realizadas ES utilizando os extratores, na ordem, CaCl2 0,01 mol L-1 , KCl 0,1 e 1 mol L-1 e oxalato de amônio 0,2 mol L-1 e ENS com uma única extração, utilizando-se somente das três últimas soluções. O Al proveniente de polímeros amorfos foram responsáveis pela superestimação do Al-KCl. O Al extraído pelos diferentes extratores não está relacionado com sua toxicidade às plantas. Para os solos AC9 e RS não houve resposta das plantas de soja à calagem e tampouco manifestação expressiva de toxicidade de Al nas mesmas. Nos solos PE, BR e CB, o KCl 1 mol L-1 foi eficaz na estimativa do Al trocável e do seu potencial tóxico. O CaCl2 0,01 mol L-1 foi o extrator que melhor identificou o potencial tóxico do Al nos solos avaliados. Palavras-chave adicionais: alumínio trocável; extratores; métodos de extração; polímeros de Al amorfos. Abstract The soil "exchangeable" aluminum (Al) is traditionally extracted with KCl 1 mol L-1 solution (Al-KCl). The use of this solution may overestimate Al exchangeable, implying a misinterpretation of the toxic effects of this element on plants. The objective of this study was to evaluate the possible fractions of Al which may be contributing to the overestimation of Al exchangeable and whether they are related to its phytotoxicity by means of sequential (ES) and non-sequential (ENS) chemical extractions. Also, to identify the efficacy of KCl 1 mol L-1 in extracting only the toxic forms of Al and the extractor that best estimates the toxic potential of this element to soybean plants cultivated in greenhouse. For this purpose, ES was performed usingthe extractors, in order, CaCl2 0.01 mol L-1 , KCl 0.1 and 1 mol L-1 and 0.2 mol L-1 ammonium oxalate and ENS with a single extraction, using only the last three solutions. Al from amorphous polymers were responsible for the overestimation of Al-KCl. Al extracted by different extractors is not related to Al toxicity to plants in some soils. For soils AC9 and RS there was no response from soybean plants to liming and no expressive manifestation of Al toxicity in them. In the PE, BR and CB soils the 1 mol L-1 KCl was effective in the estimation of exchangeable Al and its toxic potential. The CaCl2 0.01 mol L-1 was the extractor that best identified the toxic potential of Al in the evaluated soils.
... A further, yet seldom applied dithionite-based extraction method utilises the combination of dithionite, citrate and oxalate (DCO; Goswami et al. 1995;Varadachari et al. 2006). The results presented by these authors showed a greater quantitative effectiveness of DCO compared with DCB, when applied to soils, synthetic and natural goethite and hematite, and clay minerals. ...
Wet-chemical extraction of soil is a standard procedure to characterise pedogenic aluminium (Al) and iron (Fe) species, especially oxides, allophanic minerals and metal-organic associations. This article critically reviews the suitability of commonly used extractants (e.g. dithionite, oxalate and pyrophosphate) and the potentials and restrictions in their use for species identification and in soil classification. None of the commonly used extractants is completely selective and quantitative. The degree of completeness differs between the extractants and depends on soil composition. Dithionite-based methods provide a 'pseudo-total' content of pedogenic Fe oxides, as they are not always completely dissolved. Oxalate may attack further non-target species, releasing additional Al and Fe. Therefore, the extraction of Al and Fe exclusively from poorly crystalline species is not always guaranteed. As a consequence of dispersion of aggregates, pyrophosphate solubilises both mineral particles and metals from organic associations. Thus, quantification of species based on these extractions and their implementation in pedogenic thresholds may be questionable. Alternative extractants such as citrate-ascorbate and dithionite-citrate-oxalate could be used in addition, as applicable and reliable wet-chemical extractions will be still demanded for research and practical applications. The examination of the effectiveness and selectivity of wet-chemical extraction methods by spectroscopic techniques is recommended.
Laboratory core flooding experiments performed are used to predict fluid flow behaviour in the reservoir. Most reservoirs are in a reduced state. However, iron minerals in the extracted cores may become oxidized going from the reservoir to the laboratory. Oxidation of the core can affect wettability and thereby relative permeability curves used for reservoir simulation studies. The aim of this work was to study the potential effect of core oxidation on relative permeability.
Steady state relative permeability experiments with in-situ saturation measurements and use of live oil have been performed on one composite core at 10 different saturations. The core was tested under three different conditions: oxidized (Exp. 1), reduced (Exp. 2) and re-oxidised (Exp. 3). The core was cleaned, fluid saturated and aged before each test. In Exp. 2, the core plug was chemically reduced. All fluids used in this experiment were oxygen free. In Exp. 3 the core plug was treated using fluids to oxidise the core. Fluids injected and extracted, and core samples were analysed using a range of methods.
Results showed that the measured relative permeability curves from Exp. 2 and 3 were similar and significantly different from the results from Exp. 1. The attempt to restore the core material to the initial state (oxidized) after Exp. 2 by exposing the core material to fluids with oxygen was seemingly not successful. The observation was also supported by unsteady state flooding measurements which indicated that as long as the core remained fluid saturated, it behaved as a reduced core, even after extensive exposure to oxygen containing liquids. The crossing point of the oil and water relative permeabilities indicate that the core was more water wet in reduced state compared to before the oxidized state. Differences in chemical composition were also detected between extracts from Exp. 1 - 3.
The conclusion is that significant differences in steady state relative permeabilities of oil and water in oxidized and reduced states were observed for the iron containing core investigated. The results also indicate that if the core is kept fluid saturated, effects of oxidation may be significantly delayed.
Rata-rata bahan baku kaolin di Indonesia masih mempunyai derajat keputihan yang rendah padahal kaolin harus mempunyai derajat keputihan yang tinggi (>83%) untuk dapat digunakan sebagai pelapis dalam industri kertas. Untuk meningkatkan derajat keputihan dari bahan baku kaolin tersebut, proses pemutihan perlu dilakukan. Proses pemutihan kaolin telah banyak dilakukan oleh beberapa peneliti dengan menggunakan pemutih yang berbeda-beda. Warna kuning kecokelatan pada kaolin yang menurunkan derajat keputihan kaolin disebabkan oleh kandungan besi (III) oksida dalam kaolin. Pada proses pemutihan, terjadi reduksi besi (III) oksida menjadi besi (II) oksida yang lebih mudah larut dalam air sehingga lebih mudah dipisahkan dari padatan kaolin. Pada penelitian ini, pemutihan kaolin menggunakan Na2S2O4 dan EDTA sebagai pemutih. Penelitian ini bertujuan untuk melihat pengaruh variabel operasi yaitu pH dan temperatur terhadap derajat keputihan kaolin pada proses pemutihan menggunakan kedua pemutih tersebut. Hasil percobaan menunjukkan proses pemutihan menggunakan Na2S2O4 dan EDTA dapat meningkatkan derajat keputihan kaolin sehingga memenuhi spesifikasi untuk pelapis kertas pada beberapa kondisi operasi. Proses pemutihan berhasil meningkatkan derajat keputihan kaolin hingga mencapai 85,54% pada pH 12 dan temperatur 70oC.
0.2 M ammonium oxalate at pH 3 is often used as an extractant for the quantification of active Fe oxides in soil and clay. The extraction is performed in the dark during 2-4 h. The method was also used to study the spatial distribution of Fe in uncovered soil thin sections, usually during 24 h. In this study a comparison was made between extractions on ground samples and those on uncovered thin sections, in order to find the optimum extraction time for active Fe oxides from uncovered soil thin sections. This was achieved by studying dissolution vs. time curves of 6 samples with varying Fe mineralogy, which was determined with XRD, Mossbauer spectroscopy and wet chemical methods. Photomicrographs were taken after regular periods of oxalate treatment to compare the optical and chemical effect. -from Authors
This treatment was only partly effective in removing hematite from an Oxisol. Mossbauer spectroscopy is a suitable tool for investigating the effectiveness of deferration treatments on soils and clays. However, the measurements have to be done at low T, as finely dispersed Fe-oxyhydroxides are in the superparamagnetic (or even paramagnetic) state at or room T and sometimes even at 77 K. As a consequence, Mossbauer investigations carried out at room T on natural clay minerals may be influenced by Fe-oxyhydroxides in the superparamagnetic state. Mossbauer parameters deriving from ferric iron may be modified for this reason.-D.J.M.
The oxidation potential of dithionite (Na~S204) 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 tile 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 solu- bility product relationships of iron oxides. An optimum pH for maximum reaction kinetics ocem~ at approximately pH 7.3. A buffer is needed to hold the pl~ at the optimum level because 4 moles of OH are used up in reaction with each mole of Na2S204 oxi- dized. Tests show that NaHCO3 effectively serves as a buffer in this application. Crystalline hematite dissolved in amounts of several hundred milligrams in 2 min. Crystalline goethite dissolved more slowly, but dissolved during the two or three 15 rain 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-buffere~l Na2S2Oa-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 about 17 percent as contrasted to 35-80 percent with other methods.
A synergistic effect of reductant and complexant is observed in the dissolution of goethite by dithionite and citrate or EDTA. The rate data are interpreted using the surface complexation approach to describe the interface of the reacting oxide. Adsorption of both $202 (D) and complexant (L) generates =-Fe D three surface complexes that define the dissolution behavior: ~Fe-D, -~Fe-L, and dimeric -Fe L surface complexes. The initial rate increases at lower pH values because of increased surface complexation conditional formation constants. At pH values below 4, however, the fast decomposition of $20,~ gives rise to a rapid depletion of reductant, and total dissolution is not observed. It is shown that for best analytical results in soil analysis, EDTA is a better complexant than citrate; the iron extracted in one dithionite-EDTA treatment at pH 5-6, under N2 at 315 K is not increased by increasing the number of extractions, and is equivalent to the total extractable iron found by previous procedures.
The extraction of naturally occurring free iron oxides from soils and clays with oxalate solutions is distinctly photosensitive. Apparently wave- lengths around 366 m~, the near ultra-violet region, are the most efficient; this radiation is predominant in the emission of a medium-pressure mercury arc lamp. A qualitative consideration of the dissolution process indicates that in all probability electron transfer from solute to solid via intermediary C.zO~- is involved. Such a mechanism may explain the attack of near-neutral oxalate solutions upon natural ferric oxides and the decomposition of the quite resistant hematite in slightly acid media, when irradiated. The reaction appears to be autocatalytic. Electron transfer utilizing thermal electrons may also explain the attack of dithio- nite ions upon ferric oxides, which was shown to proceed in media above neutrality. Finally, a detailed description of the method is given, in- cluding the application for estimating free iron oxides in particle-size fractions obtained during the course of mechanical analysis.