Fate Of Se Research Articles

First-Time Isotopic Characterization of Seleno-Compounds in Biota: A Pilot Study of Selenium Isotopic Composition in Top Predator Seabirds.

This study pioneers the reporting of Se isotopes in marine top predators and represents the most extensive Se isotopic characterization in animals to date. A methodology based on hydride generation─multicollector inductively coupled plasma mass spectrometry─was established for such samples. The study was conducted on various internal organs of giant petrels (Macronectes spp.), encompassing bulk tissues (δ82/78Sebulk), distinct Se-specific fractions such as selenoneine (δ82/78SeSEN), and HgSe nanoparticles (δ82/78SeNPs). The δ82/78Sebulk results (2.0-5.6‰) offer preliminary insights into the fate of Se in key internal organs of seabirds, including the liver, the kidneys, the muscle, and the brain. Notably, the liver of all individuals was enriched in heavier Se isotopes compared to other examined tissues. In nanoparticle fraction, δ82/78Se varies significantly across individuals (δ82/78SeNPs from 0.6 to 5.7‰, n = 8), whereas it exhibits remarkable consistency among tissues and individuals for selenoneine (δ82/78SeSEN, 1.7 ± 0.3‰, n = 8). Significantly, there was a positive correlation between the shift from δ82/78Sebulk to δ82/78SeSEN and the proportion of Se present as selenoneine in the internal organs. This pilot study proves that Se species-specific isotopic composition is a promising tool for a better understanding of Se species fate, sources, and dynamics in animals.

Modelling selenium behavior in aquatic systems: a review of status, challenges, and opportunities.

Selenium (Se) is an essential element for aquatic organisms as well as humans. It can be toxic to organisms depending on its concentration and chemical speciation; thus, considerable efforts have been made to unravel the biogeochemical cycling of Se in aquatic systems. Mathematical models provide an important tool to better understand the fate of Se in different environment compartments. However, a comprehensive review of modeling Se in aquatic systems with current challenges and opportunities is missing. To fill this gap, we firstly summarize the processes governing Se cycling in aquatic systems, including particle adsorption and desorption, diffusion, biological uptake, redox reactions, and volatilization. Then, we critically review the available models, identifying the compartments modelled, environmental factors considered, and the Se species and geochemical processes used in each model, providing an assessment of their advantages and limitations. Data availability for modeling studies is investigated, highlighting how to better quantify the redox reactions, estimate of Se loadings, and mass balance. For the modeling of Se cycling in aquatic systems, the ability of the models to link sources to biota concentrations under a range of hydrodynamic conditions and with mechanistic representations of transport, transformation, and uptake processes is required. The majority of the current models can conduct this task; however, to better present the uptake processes of Se in the food web, two-way coupling of the Se cycling model with a food web model is recommended.

The Ozonolysis of Methylated Selenide Compounds in the Atmosphere: Isotopes, Kinetics, Products, and Mechanisms.

Selenium (Se) is an essential micronutrient with an important atmospheric component in its biogeochemical cycle. In this cycle, phytoplankton form volatile organic Se species, such as dimethyl selenide (CH3SeCH3) and dimethyl diselenide (CH3SeSeCH3), which are emitted into the atmosphere. To predict the atmospheric fate of these methylated Se compounds, we investigated their ozonolysis reaction. We used proton-transfer-reaction time-of-flight mass spectrometry to quantify atmospheric Se and its isotopes, and used this method in kinetic and product studies. The ozonolysis of CH3SeCH3 proceeded with a rate constant of (7.4 ± 2.2) × 10-17 cm3 molec-1 s-1 at 26 ± 1 °C with an activation energy of 50 ± 14 kJ mol-1 forming dimethyl selenoxide (CH3Se(O)CH3). Comparatively, CH3SeSeCH3 reacted with O3 at (2.6 ± 0.9) × 10-17 cm3 molec-1 s-1 at 27 ± 1 °C with an activation energy of 56 ± 5 kJ mol-1 forming methylselinic acid (CH3Se(O)OH). At 20 ppbv of O3, the atmospheric lifetimes of CH3SeCH3 and CH3SeSeCH3 are 7.6 and 22 h, respectively. The Se oxidation products were confirmed by synthesis and can serve as new atmospheric tracers of methylated Se compounds. Overall, we measured Se isotopes in real time and determined the rate constants, activation energies, and oxidation products. These mechanisms can now be used to determine the quantitative atmospheric fate of Se toward O3, and thus its distribution within a changing climate.

Characterization of the differentiated reduction of selenite and tellurite by a halotolerant bacterium: Process and mechanism

Se(IV)- and Te(IV)-containing wastewater usually contains salt, which reduces the activity of conventional microorganisms and may affect their microbial treatment. So far, salt-tolerant Se(IV)- and Te(IV)-reducing bacteria are very limited. Moreover, the detailed molecular response of microbial reduction of Se(IV)/Te(IV) under saline conditions has yet to be reported. In this study, the Se(IV) and Te(IV) reductive processes and mechanisms were investigated with the employment of a marine bacterium Shewanella sp. CNZ-1 (CNZ-1) using process and kinetic analyses, enzymology and RT-qPCR analyses, and differential proteomics approaches. Our results showed that CNZ-1 can effectively reduce Se(IV) and Te(IV) to Se0 and Te0 with the max k (R2) values of 0.0412 h−1 (0.86) and 0.0292 h−1 (0.91) under 2% NaCl conditions, respectively. The mechanism study showed that CNZ-1 mediated Se(IV) reduction was an enzyme-based reductive reaction, whereas CNZ-1 mediated Te(IV) reduction was more like a resistance-related detoxification process (including both bioadsorption and bioreduction). The bioreduction of Se(IV) was mainly depended on functional membrane proteins (>4-fold; e.g., nitrite reductase, polysulfide reductase and fumarate reductase), while proteins related to efflux system and stress resistance system (>2-fold; e.g., Type II secretory pathway and phage shock proteins) were essential for the Te(IV) bioreduction process. Overall, our findings expanded the understanding the fate of Se(IV) and Te(IV) in the saline environment and provided guidance for Se(IV) or Te(IV) pollution control.

Synchrotron-Based Imaging Reveals the Fate of Selenium in Striped Marsh Frog Tadpoles.

Synchrotron-based X-ray fluorescence microscopy (XFM) coupled with X-ray absorption near-edge structure (XANES) imaging was used to study selenium (Se) biodistribution and speciation in Limnodynastes peronii tadpoles. Tadpoles were exposed to dissolved Se (30 μg/L) as selenite (SeIV) or selenate (SeVI) for 7 days followed by 3 days of depuration. High-resolution elemental maps revealed that Se partitioned primarily in the eyes (specifically the eye lens, iris, and retinal pigmented epithelium), digestive and excretory organs of SeIV-exposed tadpoles. Speciation analysis confirmed that the majority of accumulated Se was converted to organo-Se. Multielement analyses provided new information on Se colocalization and its impact on trace element homeostasis. New insights into the fate of Se on a whole organism scale contribute to our understanding of the mechanisms and risks associated with Se pollution.

Simultaneous Kinetics of Selenite Oxidation and Sorption on δ-MnO2 in Stirred-Flow Reactors.

Selenium (Se) is an essential and crucial micronutrient for humans and animals, but excessive Se brings negativity and toxicity. The adsorption and oxidation of Se(IV) on Mn-oxide surfaces are important processes for understanding the geochemical fate of Se and developing engineered remediation strategies. In this study, the characterization of simultaneous adsorption, oxidation, and desorption of Se(IV) on δ-MnO2 mineral was carried out using stirred-flow reactors. About 9.5% to 25.3% of Se(IV) was oxidized to Se(VI) in the stirred-flow system in a continuous and slow process, with the kinetic rate constant k of 0.032 h−1, which was significantly higher than the apparent rate constant of 0.0014 h−1 obtained by the quasi-level kinetic fit of the batch method. The oxidation reaction was driven by proton concentration, and its rate also depended on the Se(IV) influent concentration, flow rate, and δ-MnO2 dosage. During the reaction of Se(IV) and δ-MnO2, Mn(II) was produced and adsorbed strongly on Mn oxide surfaces, which was evidenced by the total reflectance Fourier transform infrared (ATR-FTIR) results. The X-ray photoelectron spectroscopy (XPS) data indicated that the reaction of Se(VI) on δ-MnO2 produced Mn(III) as the main product. These results contribute to a deeper understanding of the interface chemical process of Se(IV) with δ-MnO2 in the environment.

Selenium mass balance and flux in water of Pariette Wetlands, Utah (USA)

Selenium (Se) has potentially deleterious impacts on flora and fauna of aquatic ecosystems. As Se moves through a wetlands system, various processes such as sorption onto sediments, plant uptake, and volatilization into the atmosphere can attenuate Se resulting in its storage in the wetlands. A comparison of inlet and outlet Se fluxes can be used to determine the mass of Se stored in a wetlands system. Inlet and outlet total Se concentrations and water discharge were measured at the Pariette Wetlands, UT, and used to calculate Se fluxes. The difference between inputs and outputs or fluxes gave great insight into how much Se was being retained or stored in the wetlands. The average influx of Se was 1530 kg year−1 and outflux was 380 kg year−1. On average, 75% (1150 kg year−1) of Se entering the wetlands was retained or stored by some biogeochemical process. Processes associated with Se retention included bioaccumulation into the biota, volatilization by plants and sediments, precipitation of insoluble phases, and sorption to sediments, which accounted for most of the attenuated Se. Water movement through the Pariette Wetlands system did not appreciably alter annual Se attenuation rates. Input, output, storage, and fate of Se for four other wetlands were compared with Pariette Wetlands with Se storage being similar among three of the wetlands: Tulare Lake (65%), Imperial (46%), Brawley (72%), and Pariette (75%).

Adsorption of Selenite onto Bacillus subtilis: The Overlooked Role of Cell Envelope Sulfhydryl Sites in the Microbial Conversion of Se(IV).

Microbial activities play a central role in the global cycling of selenium. Microorganisms can reduce, methylate, and assimilate Se, controlling the transport and fate of Se in the environment. However, the mechanisms controlling these microbial activities are still poorly understood. In particular, it is unknown how the negatively charged Se(IV) and Se(VI) oxyanions that dominate the aqueous Se speciation in oxidizing environments bind to negatively charged microbial cell surfaces in order to become bioavailable. Here, we show that the adsorption of selenite onto Bacillus subtilis bacterial cells is controlled by cell envelope sulfhydryl sites. Once adsorbed onto the bacteria, selenite is reduced and forms reduced organo-Se compounds (e.g., R1S-Se-SR2). Because sulfhydryl sites are present within cell envelopes of a wide range of bacterial species, sulfhydryl-controlled adsorption of selenite likely represents a general mechanism adopted by bacteria to make selenite bioavailable. Therefore, sulfhydryl binding of selenite likely occurs in a wide range of oxidized Se-bearing environments, and because it is followed by microbial conversion of selenite to other Se species, the process represents a crucial step in the global cycling of Se.

Selenium(IV) Sorption Onto γ-Al2O3: A Consistent Description of the Surface Speciation by Spectroscopy and Thermodynamic Modeling.

The sorption processes of Se(IV) onto γ-Al2O3 were studied by in situ Infrared spectroscopy, batch sorption studies, zeta potential measurements and surface complexation modeling (SCM) in the pH range from 5 to 10. In situ attenuated total reflection fourier-transform infrared (ATR FT-IR) spectroscopy revealed the predominant formation of a single inner-sphere surface species at the alumina surface, supporting previously reported EXAFS results, irrespective of the presence or absence of atmospherically derived carbonate. The adsorption of Se(IV) decreased with increasing pH, and no impact of the ionic strength was observed in the range from 0.01 to 0.1 mol L-1 NaCl. Inner-sphere surface complexation was also suggested from the shift of the isoelectric point of γ-Al2O3 observed during zeta potential measurements when Se(IV) concentration was 10-4 mol L-1. Based on these qualitative findings, the acid-base surface properties of γ-Al2O3 and the Se(IV) adsorption edges were successfully described using a 1-pK CD-MUSIC model, considering one bidentate surface complex based on previous EXAFS results. The results of competitive sorption experiments suggested that the surface affinity of Se(IV) toward γ-Al2O3 is higher than that of dissolved inorganic carbon (DIC). Nevertheless, from the in situ experiments, we suggest that the presence of DIC might transiently impact the migration of Se(IV) by reducing the number of available sorption sites on mineral surfaces. Consequently, this should be taken into account in predicting the environmental fate of Se(IV).

Toxicity of selenium, application of selenium in fertilizers, selenium treatment of seeds, and selenium in edible parts of plants

AbstractAgronomic biofortification is one of the approaches which have been successfully adopted for improving the nutritional content of plant-based foods and is mainly focused on optimizing the application of mineral fertilizers and/or the improvement of the solubilization and mobilization of mineral elements in the soil. In general, mineral elements with a good dynamism in the soil and in the plant are good candidates for a prosperous agronomic biofortification. Selenium deficiency occurs in areas where soil Se is low, including parts of Europe, China, North America, Australia, New Zealand, and Southern Africa. Selenium toxicity occurs in areas where soil Se is naturally high, including areas of China, India, and the United States. Toxicity from naturally occurring Se may be intensified by irrigation of seleniferous soils, mining, and use of Se-rich fossil fuels. Then, management practices benefit from a thorough understanding of the mechanisms of plant Se uptake and the fate of Se in different plant species.

Effects of selenite on Microcystis aeruginosa: Growth, microcystin production and its relationship to toxicity under hypersalinity and copper sulfate stresses

Se laden freshwater algae that enter the Salton Sea with river water pose ecorisks to wildlife in the lake by transferring selenium (Se) to higher trophic levels. The aim of this study was to investigate impacts of Se on Microcystis aeruginosa, widely distributed in freshwater bodies, and its relationship with toxicity, such as microcystins and Se residues. When supplied with selenite, the 96 h-IC50 was calculated 2.60 mg Se/L. However, these inhibitory effects did not extend to microcystin production, and the extracellular fraction significantly increased with selenite as well as sulfate. As M. aeruginosa assimilated selenite very efficiently, 97% of the removed Se was through accumulation, compared to 3% via volatilization, raising a concern about ecotoxicity caused by the remaining Se in the algae. The XAS analysis suggests the dominant Se species accumulated in the algal cells was elemental Se (81%), which is relatively nonbioavailable to aquatic organisms. We further investigated the potential fate of Se carried into the Salton Sea by M. aeruginosa with river water. Under hypersalinity stress, the biomass Se and intracellular microcystins were released and reduced by 47% and 74%, respectively, resulting in the increasing levels of Se and microcystins in the water column. CuSO4 was then applied as an algaecide to prevent M. aeruginosa from entering the lake. The results indicate a similar response to that under hypersalinity stress: the volatilization process was blocked and the Se and microcystins were released from the damaged algal cells in the presence of CuSO4, further raising toxicity levels by 8% and 60%, respectively, in the water column within 24 h. Overall, the coexistence of selenite and M. aeruginosa in river waters might negatively impact aquatic ecosystems of the Salton Sea and further research is required on how to harvest Se from M. aeruginosa to protect local wildlife.

Selenium enrichment in Carboniferous Shales, Britain and Ireland: Problem or opportunity for shale gas extraction?

The Carboniferous Bowland Shale in England, and its correlatives in Ireland, contain anomalously high concentrations of trace elements, including selenium (Se), molybdenum (Mo) and arsenic (As). High levels of these elements reflect high sulphur contents as these elements occur as trace constituents of pyrite. Anomalous Se in particular may have a volcanic provenance, from contemporary volcanic activity and/or drainage from Ordovician volcanogenic sulphide deposits. Following concern over the release of Se and As into groundwater during shale gas extraction in the US, the potential fate of Se and As during any future shale gas extraction from the Bowland Shale merits attention. It is at least an environmental issue that must be managed, but at best it could be an opportunity for extraction of Se in an environmentally sensitive manner.

Soil‐Aggregate‐Scale Heterogeneity in Microbial Selenium Reduction

Given the role of Se as both an environmental contaminant and a micronutrient, the microbial reduction and subsequent sequestration of bioavailable Se in soils are of great ecological interest. Primary particles in surface soils are typically bound into loosely packed, microporous aggregates, which may be critical spatial units in determining the fate of Se in soils. Surrounded by macropores where preferential flow rapidly advects dissolved compounds, soil aggregates are domains of slow diffusive transport where spatial variations in chemical concentrations and biogeochemical reactions can prevail. We conducted a series of controlled flow‐through experiments utilizing three‐dimensional, artificial soil aggregates (2.5‐cm i.d.) surrounded by a macropore. Aggregates were composed of either quartz sand or ferrihydrite‐coated sand inoculated with one of two Se‐reducing bacteria (Thauera selenatis or Enterobacter cloacae SLD1a‐1). Selenite export rates varied between 0.02 ± 0.01 and 3.4 ± 0.2 nmol h−1 g−1 as a function of aeration condition and input solution composition (higher SeO42− or C‐source concentrations led to higher SeO32− export). Oxic input conditions significantly decreased Se reduction; however, the detection of SeO32− in effluent samples indicates the occurrence of anoxic microzones within aggregates. Furthermore, we found that solid‐phase concentrations of reduced Se increased toward the core of aggregates and are estimated to at least double within the first millimeter into the aggregate under all conditions investigated. This indicates that concentrations of reduced Se may generally be expected to increase with distance from the advection boundary (macropore) inside aggregates, which would imply that soils with larger aggregates retain more Se.

Selenium in volcanic environments: A review

Abstract Selenium is a chemical element with major environmental implications, due to the extremely narrow range between essential and toxic concentrations. Volcanoes emit significant amounts of Se, which has an enormous impact on the Se cycle and potentially human and animal health. This paper presents a broad overview of the current state of knowledge of major aspects of volcanic derived Se, focussing on the processes in soils and aquifers. Volcanic soils, formed when volcanic products weather rapidly, display peculiar characteristics. Selenium contents can be relatively high (1–2 orders of magnitude higher) compared to the worldwide average (0.4 mg/kg), but often with low mobility. The soils can be impacted by local deposition of volcanic emissions, which are enriched in Se due to volatilization in the magma chamber. It seems that the chemical fate of Se in volcanic soils is controlled by sorption processes on poorly crystalline phases, which presence depends on the parental materials and weathering. These sorption processes also determine leaching to groundwaters. Some volcanic aquifers have Se concentrations above the WHO guideline of 10 μg/L. This might also be related to geothermal activity. In spite of the well documented interest on Se during the last decades a significant number of uncertainties exist on the behavior of this fascinating element. A preliminary model of the Se cycle around volcanoes can be constructed, but the well-known analytical problems associated with Se determinations have produced a lack of reliable information on some processes involved in the chemical interaction between soils and water. This review, therefore, also aims to identify the processes where new efforts are needed to produce good quality data to be compared and integrated in global models.

Selenium in cereals: improving the efficiency of agronomic biofortification in the UK

Wheat, despite its relatively low selenium (Se) concentration in the UK, is still an important dietary Se source and its biofortification by use of Se fertiliser may be an efficient means to increase the relatively low Se status of the population. We need to know more about the fate of Se applied to the soil and how to ensure the efficiency of Se application, and the three studies reported in this issue of Plant and Soil are timely and informative. Selenium in soil, both globally and locally, is notoriously variable; however, the soils in these studies yielded wheat grain Se concentrations in the narrow range of 16–44 ng/g. The low plant Se levels reported here are not surprising, given that selenite is the dominant Se form in these soils. A regression equation (which used total and extractable Se and extractable S as variables) explained a high proportion of the variance in grain Se concentration. Sulphur application (a common practice on UK wheat growing soils) had variable effects on grain Se concentration, depending on soil S status, pH and possibly other factors. A fertiliser methodology study investigated ways to optimise Se application for the purpose of biofortification. It was calculated that an application of a modest 10 g Se/ha as selenate would increase the grain Se concentration of UK wheat from around 30 ng/g to 300 ng/g. The national Se fertiliser program in Finland shows that this increase would have a large effect on population Se status. However, Se recovery in grain at this application rate is only 14%, and it can be argued that large-scale agronomic biofortification of cereals with Se would be somewhat wasteful of a relatively scarce trace element. Selenium’s effects and interactions in soil, plants, animals and humans are complex and often surprising and will keep researchers busy well into the future.

Selenium biofortification of high-yielding winter wheat (Triticum aestivum L.) by liquid or granular Se fertilisation

Selenium (Se) is an essential trace element for humans and livestock. In the UK, human Se intake and status has declined since the 1980s. This is primarily due to the increased use of wheat (Triticum aestivum L.) grown in UK soils which are naturally low in Se. The aim of this study was to determine the potential for increasing grain Se concentration in a high-yielding UK wheat crop using fertilisers. The crop response of winter-wheat to Se fertilisation was determined under standard field conditions in two consecutive years at up to 10 sites. Selenium fertilisers were applied as high-volume drenches of sodium selenate solution, or as granular Se-containing products. Yield and harvest index were unaffected by Se fertilisation. Under all treatments, grain Se concentration increased by 16–26 ng Se g−1 fresh weight (FW) per gram Se ha−1 applied. An application of 10 g Se ha−1 would thereby increase the Se concentration of most UK wheat grain 10-fold from current ambient levels and agronomic biofortification of UK-grown wheat is feasible. Total recovery (grain and straw) of applied Se was 20–35%. The fate of Se in the food-chain and in the soil must be determined in order to optimize the efficiency of this process.

Effect of aeration on mobility of selenium in columns of aggregated soil as influenced by straw amendment and tomato plant growth

Selenium is an essential nutrient that is potentially toxic: one of its radio-elements is also a component of long-lived radioactive waste for which long-term deep geological storage is envisaged. The chemistry of Se in soils is complex and very sensitive to redox potential and microbial activity which largely determine its oxidation state and chemical form. The dynamics of Se have been extensively studied in soils where it is deficient, and even more so when concentrations are potentially toxic. In contrast, relatively little information is available on the fate of Se in soils at intermediate concentrations (1–5 mg kg − 1 ). Some chemical reactions and biological processes that influence Se dynamics may be strongly concentration dependent. We have followed microbial activity by monitoring soil gas composition and Se volatilization and measured changes in Se fractionation using chemical extractions in a column of aggregated soil. A small proportion of soil Se was accumulated in the leaves, stems and fruits of tomato plants. Net Se volatilization losses were small (0.12% in a two-month period). There was a considerable upward movement of freshly added Se, but not of native soil Se. This vertical mobility was greater than that predicted from solute movement driven by evaporation. Selenium was strongly immobilized at the water-saturated, anoxic base of the soil columns. Straw amendment and the growth of a tomato plant did not lead to stronger association with soil organic matter. It was not possible to correlate changes in fractionation of Se between treatments and along a soil profile with the calculated fraction of anoxia, except in the completely anoxic zone.

Characterizing kinetics of transport and transformation of selenium in water–sediment microcosm free from selenium contamination using a simple mathematical model

This study developed a seven-compartment model for predicting the fate of selenium (Se) in an aquatic environment containing a water–sediment boundary. Speciation of Se in water–sediment microcosms under microaerobic conditions was measured to evaluate first-order kinetics of Se transportation and transformation. The microcosm consisted of a 10-ml solution containing 1 mM soluble Se as selenate (Se 6+) or selenite (Se 4+) and 8 g wet sediment that was free from Se contamination, sampled from the Senri, Yamato, or Yodo Rivers in Osaka, Japan. Stepwise reaction coefficients describing transportation and transformation were determined using an inverse method on this model which includes: selenate ( Se W 6 + ) and selenite ( Se W 4 + ) in ponded water; selenate ( Se S 6 + ) and selenite ( Se S 4 + ) , elemental Se (Se 0), organic Se (Se 2−) in sediment; and gaseous Se (DMSe). During this 1-month experiment, soluble Se was transported from ponded water to the sediment and Se was transformed sequentially to other Se species through biochemical reactions. Experimental and kinetic analyses indicated quantitatively that the Yamato River microcosm, with its high organic matter content, had a high adsorption rate of soluble Se. The Yodo River microcosm had a low adsorption rate for Se 6+ and a low Se reduction rate. The Senri River microcosm had an apparent high volatilization rate of DMSe. The model developed in this study is extremely useful for predicting fate of Se in aquatic environment in the field.

Effect of heat processing of protein sources on the disappearance of their selenium from mobile bags in the digestive tract of dairy cows

It was proposed that the organic selenium (Se) in ruminant feedstuffs, a large proportion of which is present in the protein molecule, should become available for digestion to the same extent as the protein in a feedstuff when passing through the digestive tract of the ruminant. Factors such as heat processing of feedstuffs that decrease protein degradability in the rumen should also reduce the proportion of its Se metabolised during microbial fermentation in the rumen. This should increase the proportion of unchanged dietary Se reaching the lower digestive tract. Six plant protein sources were roasted in a laboratory scale electric drum roaster at different temperatures and time intervals, and four fish meal and four blood meal samples were obtained from different processing plants. The mobile bag technique was used to measure the disappearance of crude protein (CP) and Se from the feedstuffs when incubated in the rumen and passing through the lower digestive tract of dairy cows. For the plant protein sources, the disappearances of Se and CP in the rumen and lower digestive tract were correlated, with coefficients of determination ( r 2) of 0.718 and 0.717, respectively. However, for the animal protein sources these correlations were low. The ruminal disappearance of Se from all unprocessed plant proteins was higher ( P<0.05) than those from their corresponding heat-processed samples. Ruminal disappearances of CP in the same samples showed the same trend as Se, viz. that heat processing reduced ( P<0.05) the ruminal disappearance of CP from these feedstuffs. Only small proportions of Se in the unprocessed plant proteins disappeared from the bags in the lower digestive tract, but after heat treatment these proportions were significantly higher. Protein disappearances followed a similar pattern. It is evident from this study that the fate of Se in the organic form in feeds is affected by the extent of exposure of the feed to ruminal fermentation. Factors affecting ruminal protein degradation such as heat processing will determine also whether Se will be subjected to microbial reduction or incorporation into microbial proteins, or would escape rumen microbial fermentation. Evidence suggested that the overprocessing of some of the test feeds reduced the availability of their Se in the digestive tract. It is concluded that heat processing of feedstuffs can contribute to the factors affecting the bio-availability of Se in feeds.

Influence of Organic Acids on Selenite Sorption by Poorly Ordered Aluminum Hydroxides

AbstractCompetition between low molecular weight (LMW) organic acids and selenite for sorption sites on hydrous metal oxides is an important factor affecting the dynamics of Se in soils and associated environments. Equilibrium and kinetic studies on the competition between selenite and 12 LMW organic acids (acetic, aspartic, citric, formic, glycine, glycolic, p‐hydroxybenzoic, malic, oxalic, salicylic, succinic, and tartaric) for the sorption sites on short‐range ordered Al hydroxides were investigated using the batch method. The equilibrium and kinetic results showed that the influence of the organic acids on selenite sorption by Al hydroxides depended on the concentration, structure, and functionality of the organic acids. The competitiveness of the organic acids with selenite for the sorption sites increased with increasing organic acid concentration. Generally, the larger the stability constant of the Al‐organic solution complexes (KAl‐L), the more effective the organic acid was in competing with selenite for the sorption sites of the Al hydroxides. However, some of the organic acids competed less successfully than expected based on their KAl‐L values. This is attributed to the stereochemical and electrostatic effects originating from both the surface of the Al hydroxides and the organic acids, which lowered the ability of some organic acids to compete with selenite for the sorption sites of the Al hydroxides. Moreover, the kinetic results revealed that the organic acids competed with selenite for the sorption sites primarily in the early reaction period (&lt;3 h), and subsequently, it was this period that was largely responsible for the equilibrium level of selenite sorbed by the Al hydroxides. The findings are of fundamental significance in understanding the role of organic acids in influencing the fate of Se in the environment.