Dithionite-citrate-bicarbonate Research Articles

Constraining the colouration mechanisms of Cretaceous Oceanic Red Beds using diffuse reflectance spectroscopy

We have used diffuse reflectance spectroscopy to investigate the colouration mechanisms of hematite in Cretaceous Oceanic Red Beds (CORBs). Data for samples of CORBs from the Chuangde section in Tibet, Vispi Quarry section in Italy, and Core 12X of Ocean Drilling Program Hole 1049C in the North Atlantic were compared with calibration datasets obtained for hematite in different crystalline forms (kidney and specular hematite) and calcite matrix. Spectra for hematite in either pure form or in calibration datasets show that the centre of the reflection peak shifts to a longer wavelength and depth (D) decreases as the crystallinity of the hematite increases. Compared with specular hematite, the presence of just 0.5% of kidney hematite can cause a much deeper absorption peak and greater redness value, which indicates that kidney hematite has a higher colouration capacity than specular hematite. However, both kidney and specular hematite exhibit a good correlation between the redness value for each calibration dataset and the absorption peak depth. In all three studied sections, hematite is the main iron oxide mineral responsible for colouration. Spectral features such as absorption peak depth and peak centre reveal that hematite crystallinity gradually decreases from red shale to limestone to marl. Based on a spectral comparison of red shale in the Chuangde section before and after citrate–bicarbonate–dithionite (CBD) treatment, we found that two forms of hematite are present: a fine-grained and dispersed form, and a detrital form. The former is relatively poorly crystalline hematite, which has a much stronger colouration capacity than the detrital form. In the Vispi Quarry section and Core 12X of ODP Hole 1049C, a good correlation between the absorption peak depth of hematite and redness value indicates that the red colouration is caused by hematite of similar crystallinity in each section.

Terrain attributes and spatial distribution of soil mineralogical attributes

In soil studies, there is an increasing tendency to quantify heterogeneous soil attributes by georeferencing, leading to the pedometry. In this context, terrain attributes play important role for the characterization of distinct surfaces under different pedogenetic processes. The soil mineralogical assembly expresses factors and pedogenetic processes involved in soil formation that affect their physical and chemical characteristics. This work objective was to investigate the spatial distribution of soil mineralogical attributes, using topographic terrain attributes as tools for the characterization of high or low mineral content environments and their relationships with soil chemical characteristics. This study was carried out in the Cangüiri Experiment Station of the Federal University of Paraná, at Curitiba, State of Paraná, Brazil. Soil sampling in the studied area was collected in a regular grid of 135 points. The clay fraction, after dithionite–citrate–bicarbonate (DCB) extraction, was analyzed to determine kaolinite (Ka) and gibbsite (Gb) concentrations by thermogravimetry (TG) and their respective dehydroxylation temperature peaks. Such mineral crystallographic attributes were determined by X-ray diffraction (XRD). High amorphous Al contents tended to be related to higher organic carbon (OC) values, in which areas the terrain attributes indicated smooth relief and predominance of pedimentation processes. The terrain attributes differentiated areas with higher Ka/(Ka+Gb) ratios, that is, with higher Ka values, located near drainage channels and with silica accumulation, meanwhile areas with higher proportion of Gb were found on flat summit. The mineralogical attributes studied in this work influenced some soil chemical characteristics. Satisfactory results were obtained with the use of topographic terrain attributes to characterize distinct surfaces, since they were related to some of the mineralogical attributes formation.

Characterization of primary and secondary magnetite in marine sediment by combining chemical and magnetic unmixing techniques

We present a novel technique for quantitative unmixing of primary and secondary ferrimagnetic minerals in sediments. Hysteresis and high-resolution first-order reversal curve (FORC) measurements are performed on sediment samples before and after digestion in a citrate–bicarbonate–dithionite (CBD) solution optimized for maximum selective extraction of secondary fine-grained iron oxides. The difference between magnetic measurements of untreated and CBD-treated sample materials is used to calculate the original magnetic signature of CBD-extractable minerals. A combination of selective chemical extraction and magnetic measurements suited for the detection of single-domain particles provides a cross-check between chemical and magnetic unmixing of primary and secondary iron oxides and resolves the non-uniqueness problem of numerical unmixing methods. A quantitative magnetic characterization of secondary ferrimagnetic minerals in a magnetofossil-rich pelagic carbonate is presented for the first time. It can be used for calibration of recently developed fast magnetic unmixing techniques. CBD-based Fe extraction from sediments with minimal clastic and/or aeolian inputs, such as pelagic carbonates, is particularly suited for the search for cosmogenic 60Fe signatures from supernova explosions, because 60Fe dilution by dissolved primary Fe-bearing minerals is minimized.

Late Pliocene–Quaternary evolution of redox conditions in the western Qaidam paleolake (NE Tibetan Plateau) deduced from Mn geochemistry in the drilling core SG-1

Manganese (Mn) in lake sediments reacts strongly to changes of redox conditions. This study analyzed Mn concentrations in oxides, carbonates, and bulk phases of the calcareous lacustrine sediments of a 938.5-m-long core (SG-1) taken from the western Qaidam Basin, well dated from 2.77Ma to 0.1Ma. Comparisons of extractions from diluted hydrochloric acid, acetic acid and citrate–bicarbonate–dithionite demonstrate that variations of Mn concentrations from acetic acid leaching (MnHOAc) are mostly responsible for Mn (II) fluctuations in the carbonate phase. Taking into account the relevant processes during weathering, transportation, deposition and post-deposition of Mn-bearing rocks, we conclude that Mn input from catchment weathering and paleolake redox condition provide the primary controls on variations in the Mn records of carbonate and oxide phases. We propose MnHOAc as a new sensitive indicator of paleolake redox evolution and catchment-scale climate change. The MnHOAc variations show a long-term upward decreasing trend, indicating a long-term decrease of Mn input from catchment weathering associated with increasing oxygen content in the paleolake bottom water. The great similarities of the MnHOAc record with other regional and global records suggest that paleolake redox changes and climatic drying in the Qaidam Basin may be largely related to global cooling.

Formation of iron plaque on mangrove Kandalar. Obovata (S.L.) root surfaces and its role in cadmium uptake and translocation

In this study, a pot experiment was conducted to investigate the formation of iron plaque under Cd stress and its role in Cd uptake and translocation by mangrove Kandalar. Obovata (S.L.). Results showed:1.the Fe in dithionite–citrate–bicarbonate (DCB) extracts increased with an increasing rate of Cd treatments.2.the Cd in DCB extracts and in roots and above-ground tissues significantly increased with an increasing Cd application;3.significant positive correlation between concentration of Cd and Fe in DCB extracts existed (p<0.05); and that4.the proportion of Cd in DCB extracts was significantly lower than that in roots or above-ground tissues (p<0.001).In conclusion, formation of Fe plaque can precipitate Cd on root surfaces and impede its uptake and translocation in Kandalar. Obovata (S.L.). However, in comparison with root tissues, Fe plaque is of little significance. It is the mangrove root tissue that acts as the main buffer to Cd uptake and translocation.

Chemical properties and mineralogy of soils with plinthite and petroplinthite in Iranduba (AM), Brazil

Large areas of Plinthosols with ferruginous materials such as plinthite and/or petroplinthite are fairly common in the Brazilian Amazon basin. This work was carried out to investigate the chemical behavior, mineralogical composition and weathering stage of four representative soil profiles with plinthite and petroplinthite, in Iranduba, AM (Central Amazon). Three well-drained soil profiles at high elevations were studied (P1, Plinthic Vetic Ferralsol; P2 and P3, Vetic Endopetric Plinthosol) and a contrasting poorly drained soil (P4 Haplic Plinthosol), located at low elevation. After profile descriptions, soil samples were collected from each horizon, air-dried, sieved (2 mm), and analyzed for particle-size distribution, pH, exchangeable cations (Al3+, Ca2+, Mg2+, K+, and Na+), as well as available P and total organic carbon (TOC) content. The minerals present in the clay and sand fractions, as well as in the ferruginous materials were identified by X-ray Diffraction (XRD). The weathering stage of these soils was assessed by means of Ki and Kr indexes, and the amounts of free and amorphous Fe and Al oxides by using dithionite citrate bicarbonate (DBC) and ammonium oxalate dissolution procedures, respectively. The results showed that all soils were extremely unfertile, with pH levels ranging between strong and moderate acidity, very low sum of bases and organic matter content, and of available P. The mineralogy of the soil profiles was very similar, mainly of the well-drained soils, with predominance of kaolinite and quartz in the clay and sand fractions, respectively. In the poorly-drained P4, 2:1 clay particles were also observed. These profiles can be considered highly developed according to the Ki index, however, the Ki value of P4 was higher, indicating that this soil was less developed than the others. In summary, these profiles with plinthite and petroplinthite can be characterized as highly developed and infertile soils and are, with exception of P4, well-drained.

Quantitative phase analysis of bauxites and their dissolution products

Quantitative phase analysis for four bauxite samples sourced from different geological regions was conducted with Rietveld refinement of synchrotron sourced X-ray diffraction (XRD) data. Minor and trace phases were concentrated using (i) caustic leaching to selectively remove phases such as gibbsite, boehmite and kaolinite and (ii) dithionite–citrate–bicarbonate leaching to selectively remove the iron oxide phases such as goethite and hematite, and reanalysed. Bauxites are normally recognised as very difficult samples for quantitative phase analysis (QPA). The use of synchrotron XRD in this paper allowed characterisation with QPA, crystallite size determination and elemental substitution in some phases. Elemental composition analysis by X-ray fluorescence demonstrated close agreement with the diffraction data and allowed an estimate of the composition of the non-crystalline component of the bauxite samples. Other characterisation techniques used include thermogravimetric/differential scanning calorimetry to determine the phase transitions and mass loss, and transmission electron microscopy/energy dispersive spectroscopy to capture the morphology of the crystallites in the bauxite samples.

Chromium Speciation Associated with Iron and Manganese Oxides in Serpentine Mine Tailings

Chromium is easily trapped by Fe-oxyhydroxides when the two elements are weathered from serpentinites. Additionally, manganese oxides are the only known naturally occurring oxidant for Cr(III). To elucidate the effects of Fe and Mn oxides on Cr speciation from serpentine mine tailing, this study explored Cr(VI) generation from lithiogenic Cr that increases as a result of tailing weathering, which was caused by partitioning changes of this element with pedogenic Fe and Mn oxides. Twenty-nine tailing samples (0–5 cm) were obtained from an abandoned site of serpentine mining on Wan-Ron Hill in eastern Taiwan. Samples were characterized using chemical extraction, as well as mineralogical and statistical approaches. Based on the experimental results, exchangeable Cr(VI) was observed in all samples ranging from 34.8 to 183 μg/kg. Chromite was the main Cr-bearing mineral examined in this study. Total labile Cr release and subsequent Cr(VI) generation led to the hypothesis that the labile Cr resulted initially and mainly from chromite weathering. Dithionite–citrate–bicarbonate and oxalate extractions facilitated identifying the incongruent dissolution between Cr and Fe (and/or Mn). The Cr(VI) increased with concentrations of various forms of Fe and Mn, except for total Fe, revealing that Cr(VI) could be released from the surface of the Mn oxides, where it had been oxidized, and subsequently re-adsorbed onto the surface of the surrounding Fe oxides.

Rock magnetism, iron oxide mineralogy and geochemistry of Quaternary red earth in central China and their paleopedogenic implication

This paper presents a detailed investigation on the rock magnetism, iron oxide mineralogy and geochemistry of a Quaternary red earth (QRE) in central China. The magnetic profile of QRE showed a characteristic vertical variation pattern: the uniform red clay (URC) section (0.2–2m) of the profile had a higher magnetic susceptibility (χ) while vermiculated red clay (VRC) section (2–6m) had a very low χ (<30×10−8m3kg−1). Rock magnetism and differential XRD analysis revealed that magnetite/maghemite and hematite were the main magnetic carriers in the URC section. The major elements Fe2O3 and Al2O3 of QRE increased with depth while the SiO2 content decreased with depth, indicating a leaching of the mobile elements and an enrichment of the immobile elements. In comparison with the URC, the VRC has a greater depletion in the content of Na, Mg, K and Mg, indicating a stronger weathering intensity. The dithionite–citrate–bicarbonate (DCB) extracted iron (Fed), the ratio of Fed to total iron oxide (Fe/Fet), and the weathering indices of QRE confirmed that the VRC had experienced a stronger pedogenesis than the URC. Strong pedogenesis in the VRC resulted in a transformation of the highly magnetic maghemite to the weakly magnetic hematite, which was responsible for the weak magnetism and hard magnetic behavior of the VRC. Our studies suggest that the VRC had experienced a hot climate with a dry season following one that was warm and moister. Magnetic reduction of the VRC section resulting from the transformation of highly magnetic maghemite to weakly magnetic hematite appeared to be an important paleopedogenic process in the QRE. We concluded that the magnetic parameters, hard isothermal remanent magnetization (HIRM) and SIRM/χ of QRE, can be used for the analysis of paleopedogenic processes.

Characterizing and quantifying iron oxides in Chinese loess/paleosols: Implications for pedogenesis

To accurately decode the paleoclimatic and paleoenvironmental significance of magnetic properties in Chinese loess/paleosols, neoformation of magnetic iron oxides via pedogenesis as well as the relationship between ferrimagnetic (maghemite and magnetite) and antiferromagnetic minerals (hematite and goethite) of both detrital and pedogenic origins should be well determined. To resolve this problem, magnetic iron oxides from a contiguous loess–paleosol sequence (lower part of the loess unit L1, paleosol unit S1, and the upper part of L2) at Luochuan, Center of the Chinese Loess Plateau, were well characterized and quantified by using an integrated approach including chemical dissolution, diffuse reflectance spectroscopy and rock magnetism methods. Results showed that these magnetic minerals have distinct coercivity and dissolution behavior, which is highly grain size-dependent. More specifically, the paleosol is enriched with nano-sized pedogenic hematites with a medium coercivity of ~130mT, which are also preferentially dissolved in a weak reductive environment. In contrast, the coarse-grained micro-sized detrital hematites with a medium coercivity of ~1T are dominant in loess. In addition, these coarse-grained detrital hematites are more stoichiometric than their pedogenic counterparts as evidenced by the higher band position on the diffuse reflectance spectrum. Goethite can be well distinguished from hematite by its higher coercivity (4–7T) at low temperature (10K). The consistent dissolution behavior of goethite for both loess and paleosol samples in citrate–bicarbonate–dithionite solution indicates that goethite could be less relevant to the pedogenic processes. The reductive dissolution of pedogenic maghemite lags the dissolution of pedogenic hematite and fine-grained goethite, which suggests that average grain size of pedogenic maghemite is relatively larger than those of pedogenic hematite and fine-grained goethite. Detrital magnetite is hard to dissolve and ~50% of its initial remanence survived after dissolution. On the basis of these contrasting behaviors, pedogenic hematite, fine grained goethite and maghemite content was further quantified. Our results suggest that single domain pedogenic hematite was produced after initial formation of pedogenic maghemite. This provides direct evidence for the recently proposed pathway in which ferrihydrite is transformed into a transient maghemite-like phase before its final transformation into hematite.

Colloidal properties and potential release of water-dispersible colloids in an agricultural soil depth profile

The effects of soil depth on the colloidal properties and potential release of water-dispersible colloids (WDCs) in an agricultural soil were investigated in batch experiments. Nine soil samples were collected along a trench at different depths (from −7.5cm to −127.5cm) from the TERENO test site located in Selhausen (Germany). A physical gentle soil fractionation confirmed the importance of the solution chemistry in controlling the depth distribution of WDC in the Luvisol soil. A maximal fraction of total clay (fWDC) equal to approx. 45% was released at a depth of −37.5cm to −52.5cm below the surface. This was found to be related to the lowest electrical conductivity of the soil electrolyte phase where Ca2+ concentration played a major role. A rather constant content of metal oxyhydroxide (MO(OH)) accounting for about 10% in WDCs was found along the soil depth after a dithionite-citrate-bicarbonate (DCB) extraction. However, a decrease from 40% to 25% in the contribution of MO(OH) particles to SSA of WDC was calculated along the soil depth. Using photon correlation spectroscopy (PCS) and N2 gas adsorption method, variations were measured in colloidal hydrodynamic diameters (dz) and the mineral specific surface area (SSA) for WDC in water-dispersed and dried states, respectively. A function structure f(np) was defined to relate the particle size variations between the two states along the soil depth. An inverse correlation was established between the particle size variations which assumed that the number of primary particle in the water-dispersed state increases in subsoil samples. The effects of different soil parameters such as pH, organic carbon (OC) coverage at MO(OH) surfaces and humification index (HIX) of dissolved organic carbon (DOC) on the WDC structure variations are discussed. It can be hypothesized that a maximal surface OC coverage at MO(OH) surfaces of WDCs facilitates the release of small particles through electrosteric dispersion in topsoil. Lower pHs and WDC(OC) content would favor the release of large WDC particles in subsoil where an OC content of a more aliphatic character can also facilitate hydrophobic interactions.

Isolation and Microbial Reduction of Fe(III) Phyllosilicates from Subsurface Sediments

Fe(III)-bearing phyllosilicates can be important sources of Fe(III) for dissimilatory microbial iron reduction in clay-rich anoxic soils and sediments. The goal of this research was to isolate Fe(III) phyllosilicate phases, and if possible, Fe(III) oxide phases, from a weathered shale saprolite sediment in order to permit experimentation with each phase in isolation. Physical partitioning by density gradient centrifugation did not separate phyllosilicate and Fe(III) oxide phases (primarily nanoparticulate goethite). Hence we examined the ability of chemical extraction methods to remove Fe(III) oxides without significantly altering the properties of the phyllosilicates. XRD analysis showed that extraction with acid ammonium oxalate (AAO) or AAO in the presence of added Fe(II) altered the structure of Fe-bearing phyllosilicates in the saprolite. In contrast, citrate-dithionite-bicarbonate (CDB) extraction at room temperature or 80 °C led to minimal alteration of phyllosilicate structures. Reoxidation of CDB-extracted sediment with H(2)O(2) restored phyllosilicate mineral d-spacing and Fe redox speciation to conditions similar to that in the pristine sediment. The extent of microbial (Geobacter sulfurreducens) reduction of Fe(III) phyllosilicates isolated by CDB extraction and H(2)O(2) reoxidation (16 ± 3% reduction) was comparable to what took place in pristine sediments as determined by Mossbauer spectroscopy (20 ± 11% reduction). These results suggest that materials isolated by CDB extraction and H(2)O(2) reoxidation are appropriate targets for detailed studies of natural soil/sediment Fe(III) phyllosilicate reduction.

Manganiferous Soils in Maryland: Regional Extent and Field‐Scale Electromagnetic Induction Survey

There are some unique soils in central Maryland with extremely high quantities of Mn and Fe oxides, up to 141 g kg−1 Mn and 169 g kg−1 Fe (extracted by dithionite–citrate–bicarbonate). Such high quantities of Mn oxides have pigmented the entire soil matrix black in subsoil horizons. Previously, little was known about the extent of these manganiferous soils in Maryland. Pedons were observed in transects across several fields containing manganiferous soils to better understand their spatial variability and possible relationship to landscape and geology. Manganiferous soils are heterogeneous; some pedons show modest to moderate Mn enrichment in the form of black nodules or coatings, while other pedons show extreme Mn enrichment in the form of black, loamy material up to 8.5 m deep. The soils seem to occur in relation to areas mapped as having marble bedrock in the geologic survey. The utility of electromagnetic induction (EMI) for delineating manganiferous soil units was tested for the first time. Apparent electrical conductivity (ECa) readings from the EM38DD EMI sensor had a significant empirical relationship to Mn content of the studied soils (R2 = 0.88), so maps generated from ECa data correspond reasonably well to observations made during transect work. Electromagnetic induction is therefore a useful tool for delineating manganiferous soils, which occurred in units of approximately 1 to 2 ha in each of the three fields observed.

Kinetics and efficiency of H2O2 activation by iron-containing minerals and aquifer materials

To gain insight into factors that control H2O2 persistence and OH yield in H2O2-based in situ chemical oxidation systems, the decomposition of H2O2 and transformation of phenol were investigated in the presence of iron-containing minerals and aquifer materials. Under conditions expected during remediation of soil and groundwater, the stoichiometric efficiency, defined as the amount of phenol transformed per mole of H2O2 decomposed, varied from 0.005 to 0.28%. Among the iron-containing minerals, iron oxides were 2–10 times less efficient in transforming phenol than iron-containing clays and synthetic iron-containing catalysts. In both iron-containing mineral and aquifer materials systems, the stoichiometric efficiency was inversely correlated with the rate of H2O2 decomposition. In aquifer materials systems, the stoichiometric efficiency was also inversely correlated with the Mn content, consistent with the fact that the decomposition of H2O2 on manganese oxides does not produce OH. Removal of iron and manganese oxide coatings from the surface of aquifer materials by extraction with citrate–bicarbonate–dithionite slowed the rate of H2O2 decomposition on aquifer materials and increased the stoichiometric efficiency. In addition, the presence of 2 mM of dissolved SiO2 slowed the rate of H2O2 decomposition on aquifer materials by over 80% without affecting the stoichiometric efficiency.

Vegetation Effects on Pedogenetic Forms of Iron and Aluminium and Mineralogical Properties of Basaltic Soils in the Southern Guinea Savanna of Nigeria

This study investigates the effects of vegetation particularly fast growing exotic species like Tectona grandis on the Newer Basalt of the Jos Plateau. In particular it determines the effects exotic tree species have on the pedogenetic forms of Fe and Al and mineralogical properties of the basaltic soils age. Soil samples were determined at the depth of 0 &#8211; 30cm and 30 &#8211; 60cm under plantation of four different ages and subjected to standard laboratory analysis. The citrate bicarbonate dithionite (CBD) extractable Fe and Al slightly decreased with aging plantation, which was ascribed to the inhibitory effects of organic matter on the crystallization of Fe and Al. Similarly the active Fe value was less than unity, which implied increased crystalline forms of Fe and Al oxides are as a result of weathering intensity. Mineralogical results indicated that the soils under the fallow and 11 year old plantation had the dominance of magnetites, while those under 21 year old plantation had mixed mineralogy, consisting of magnetites, biotites and montmorillonites. The older plantations (31 and 36 year old) were dominated by kaolinites and traces of haematites and goethites. The trend suggested an increased level of pedogenesis with aging plantations. Increase in crystalline forms of Fe and Al, kaolinites, traces of haematite and goethite concludes that Tectona grandis deteriorates soil properties over time.

Effects of CBD Treatment on Magnetic Minerals of Natural Samples

AbstractLoess‐paleosol sequences are good archives for research on paleoclimate over the last 2.5 million years and even earlier. Magnetic susceptibility of loess‐paleosol sequences has been used extensively as a proxy for paleoclimate interpretations. Magnetic minerals in loess/paleosols are of lithogenic (allochthonous) origin or pedogenic (authigenic) origin. It is of great significance for loess magnetism to separate magnetic signals of them and investigate its magnetite or maghemite that mainly contributes to magnetic susceptibility enhancement in paleosol. Though the combination of CBD (Citrate‐Bicarbonate‐Dithionite) procedure with loess magnetism sheds light on these questions, the CBD method is still debated and its mechanism unclear. To better understand the effects of CBD method on magnetic minerals, CBD treatment were made to loess/paleosol samples from Western Sichuan plateau, Tianshan mountains and Kurtak, Siberia and non‐aeolian samples from Subtropics in this work. Magnetic parameters, including low‐field magnetic susceptibility, frequency dependent magnetic susceptibility, anhysteretic remanent magnetization, saturation isothermal remanent magnetization and thermomagnetic curves (J‐T curves), were measured before and after the CBD treatment. Changes in magnetic mineralogy, magnetic concentration and magnetic domain were analyzed. The results suggest that the CBD method is not apparently selective in dissolving various magnetic minerals, and the dissolving quantity is controlled by grain size distribution if the temperature and duration are given. The CBD method can efficiently remove fine grains which have higher ration of surface to volume, and dissolve a rim from the outside of the coarse grains resulting in finer grain size. There are multiple possibilities of changes in magnetic susceptibility after CBD treatment. The CBD method can be used to obtain magnetic signal of pedogenic original. It is not advisable to use the method to separate lithogenic and pedogenic magnetic minerals in loess from arid or over‐wet areas.

Sorption of 2,4-D and other phenoxy herbicides to soil, organic matter, and minerals

We review 2,4-dichlorophenoxyacetic acid (2,4-D) and other phenoxy herbicide sorption experiments. A database with 469 soil–water distribution coefficients K d (in liters per kilogram) was compiled: 271 coefficients are for the phenoxy herbicide 2,4-D, 9 for 4-(2,4-dichlorophenoxy)butyric acid, 18 for 2-(2,4-dichlorophenoxy)propanoic acid, 109 for 2-methyl-4-chlorophenoxyacetic acid, 5 for 4-(4-chloro-2-methylphenoxy)butanoic acid, and 57 for 2-(4-chloro-2-methylphenoxy)propanoic acid. The following parameters characterizing the soils, solutions, or experimental procedures used in the studies were also compiled if available: solution CaCl2 concentration, pH, pre-equilibration time, temperature, soil organic carbon content (f oc), percent sand, silt and clay, oxalate extractable aluminum, oxalate extractable iron (Oxalate Fe), dithionite–citrate–bicarbonate extractable aluminum, dithionite–citrate–bicarbonate extractable iron (DCB Fe), point of zero negative charge, anion exchange capacity, cation exchange capacity, soil type, soil horizon or depth of sampling, and geographic location. K d data were also compiled characterizing phenoxy herbicide sorption to the following well-defined sorbent materials: quartz, calcite, α-alumina, kaolinite, ferrihydrite, goethite, lepidocrocite, soil humic acid, Fluka humic acid, and Pahokee peat. The data review suggests that sorption of 2,4-D can be rationalized based on the soil parameters pH, f oc, Oxalate Fe, and DCB Fe in combination with sorption coefficients measured independently for humic acids and ferrihydrite, and goethite. Soil organic matter and iron oxides appear to be the most relevant sorbents for phenoxy herbicides. Unfortunately, few authors report Oxalate Fe and DCB Fe data.

Co-contamination of arsenic and fluoride in the groundwater of unconsolidated aquifers under reducing environments

The co-contamination of arsenic (As) and fluoride (F−) in shallow aquifers is frequently observed worldwide, and the correlations between those contaminants are different according to the redox conditions. This study geochemically explores the reasons for the co-contamination and for the redox-dependent correlations by investigating the groundwater of an alluvial aquifer in Korea. Geochemical signatures of the groundwater in the study area show that the As concentrations are enriched by the reductive dissolution of Fe-(hydr)oxides, and the correlations between As and F− concentrations are poor comparatively to those observed in the oxidizing aquifers. However, F− concentrations are strongly dependent on pH. Desorption/adsorption experiments using raw soils and citrate–bicarbonate–dithionite treated soils indicated that Fe-(hydr)oxides are the important As and F− hosts causing the co-contamination phenomenon. The weaker correlation between F− and As in reducing aquifers is likely to be associated with sulfate reduction, which removes As from groundwater without changing the F− concentration.

Mineralogia e reserva de K de Cambissolos submetidos a diferentes manejos após derrubada e queima da floresta na Amazônia Meridional

As altas temperaturas ocorrentes durante a queima na floresta Amazônica podem induzir transformações mineralógicas da fração argila. Este trabalho objetivou avaliar as alterações mineralógicas nas frações areia, silte e argila e na reserva mineral de solos submetidos a diferentes manejos (11 anos de mata secundária, pastagem e café) após a derrubada e queima de floresta nativa situada na Amazônia Meridional, município de Cacoal (RO). Uma área de floresta nativa ombrófila densa, com mesma feição pedológica, foi dividida em quatro partes, sendo três áreas submetidas ao corte e à queima e posteriormente cultivadas. Em cada manejo, os perfis de solos foram descritos e amostras dos horizontes A, AB, B1, 2B2 e 2BC foram coletadas para as análises mineralógicas das frações argila, silte e areia (difratometria de raios X) e químicas da fração argila (extração de Fe com ditionito-citrato-bicarbonato - DCB e com oxalato de amônio - OA). Os teores de K não trocáveis foram determinados nas amostras de solo após extração com HNO3 1 mol L-1 fervente. Verificou-se o predomínio da caulinita na fração argila e alta reserva mineral de K nos solos, associada à ocorrência de mica nas frações argila, silte e areia. Os diferentes manejos não afetaram os teores de Fe2O3DCB e Fe2O3OA; por outro lado, o maior valor estimado para a relação entre os teores de goethita e hematita [Gt/(Gt+Hm)] na fração argila no horizonte A da floresta indica uma possível transformação parcial de Gt em Hm durante a queima da floresta para a implantação dos manejos mata secundária, pastagem e café.

Transformation of phosphorus in highly calcareous soils under field capacity and waterlogged conditions

The aim of the present study was to evaluate the transformation of applied inorganic phosphorus (P) in highly calcareous soils under two moisture regimes. The experimental design was a factorial combination of two rates of P (0 and 300 mg P kg–1 as KH2PO4) and two moisture regimes (field capacity, FC; waterlogged, WL) in a completely randomised design in duplicate with 20 surface soil samples. The fractionation sequence of inorganic P included successive extraction with NaHCO3, NH4 acetate buffer (NH4OAc), MgCl2, NH4F, NaOH–Na2CO3 (HC), Na citrate–bicarbonate–dithionite (CBD), and H2SO4 carried out 80 and 160 days after incubation. Recovery of applied P in each fraction was calculated as the difference between samples treated and untreated with P. Results indicated that NaHCO3-P decreased from 80 to 160 days, and the decrement was higher under WL than FC moisture regime. The NH4OAc-P was lower under WL than FC at 160 days, while P associated with free and crystalline Fe and Al oxides (NH4F-P, HC-P, CBD-P) was higher under WL than FC for both incubation periods. Oxalate-, citrate-, and citrate–ascorbate-extractable iron under FC and in conjunction with oxalate- and CBD-extractable aluminium and quinone- and hydroxylamine–hydrochloride-extractable manganese were the most influential factors regulating all P fractions. Results of the present study revealed that transformation of applied P into Al- and Fe-P fractions is not as low as previously reported in highly calcareous soils and that Al- and Fe-P oxides may be important in P transformation of these soils, especially in waterlogged condition.