Photochemical oxidation of arsenic(III) to arsenic(V) using peroxydisulfate ions as an oxidizing agent

The photochemical oxidation of arsenic, As(III), to the less toxic As(V) using peroxydisulfate ions (S2O8(2-)) as the oxidizing agent under UV light irradiation was investigated. The photochemical oxidation of As(III) to As(V) assisted using peroxydisulfate ions (KPS) proved to be a simple and efficient method, and the rate of oxidation for As(III) was exceptionally high in accordance with the concentration of KPS. In this study, the UV light intensity was of primary importance for the dissociation of the KPS in generating sulfate anion radicals (SO4(-*)). Upon intense UV light irradiation, very efficient oxidation was achieved due to the complete decomposition of KPS into SO4(-*) radicals which favor a higher reaction rate. Subsequent pH variation from 3 to 9 was seen to have no influence on the photolytic cleavage of KPS, and hence, the reaction was unaltered. There was also no significant effect from the continuous purging of oxygen or dissolved oxygen before the reaction as the air-equilibrated condition was found to be sufficient for efficient oxidation. However, the continuous purging of nitrogen substantially reduced the reaction rate (20%), confirming that the dissolved oxygen plays a role in this reaction, although at high concentrations of KPS, this situation was overcome. Humic acid was also found to have no detrimental effect on the reaction rate, even at 20 ppm concentration. The resultant SO4(2-) obtained in this studywas,thus, not considered a pollutant. Moreover, there was no need for a sensitizer or other metals with highly alkaline conditions that are normally used in conjunction with KPS. Natural solar light could also effectively oxidize As(III) at room temperature. This simple technique was, thus, considered a cost-effective and safe method for the oxidation of As(III) to As(V).     

Sorption and desorption of arsenic to ferrihydrite in a sand filter

Elevated arsenic concentrations in drinking water occur in many places around the world. Arsenic is deleterious to humans, and consequently, As water treatment techniques are sought. To optimize arsenic removal, sorption and desorption processes were studied at a drinking water treatment plant with aeration and sand filtration of ferrous iron rich groundwater at Elmevej Water Works, Fensmark, Denmark. Filter sand and pore water were sampled along depth profiles in the filters. The sand was coated with a 100-300 microm thick layer of porous Si-Ca-As-contaning iron oxide (As/Fe = 0.17) with locally some manganese oxide. The iron oxide was identified as a Si-stabilized abiotically formed two-line ferrihydrite with a magnetic hyperfine field of 45.8 T at 5 K. The raw water has an As concentration of 25 microg/L, predominantly as As(II). As the water passes through the filters, As(III) is oxidized to As(V) and the total concentrations drop asymptotically to a approximately 15 microg/L equilibrium concentration. Mn is released to the pore water, indicating the existence of reactive manganese oxides within the oxide coating, which probably play a role for the rapid As(III) oxidation. The As removal in the sand filters appears controlled by sorption equilibrium onto the ferrihydrite. By addition of ferrous chloride (3.65 mg of Fe(II)/L) to the water stream between two serially connected filters, a 3 microg/L As concentration is created in the water that infiltrates into the second sand filter. However, as water flow is reestablished through the second filter, As desorbs from the ferrihydrite and increases until the 15 microg/L equilibrium concentration. Sequential chemical extractions and geometrical estimates of the fraction of surface-associated As suggest that up to 40% of the total As can be remobilized in response to changes in the water chemistry in the sand filter.     

Comparison of arsenic(V) and arsenic(III) sorption onto iron oxide minerals: implications for arsenic mobility

Arsenic derived from natural sources occurs in groundwater in many countries, affecting the health of millions of people. The combined effects of As(V) reduction and diagenesis of iron oxide minerals on arsenic mobility are investigated in this study by comparing As(V) and As(III) sorption onto amorphous iron oxide (HFO), goethite, and magnetite at varying solution compositions. Experimental data are modeled with a diffuse double layer surface complexation model, and the extracted model parameters are used to examine the consistency of our results with those previously reported. Sorption of As(V) onto HFO and goethite is more favorable than that of As(III) below pH 5-6, whereas, above pH 7-8, As(II) has a higher affinity for the solids. The pH at which As(V) and As(III) are equally sorbed depends on the solid-to-solution ratio and type and specific surface area of the minerals and is shifted to lower pH values in the presence of phosphate, which competes for sorption sites. The sorption data indicate that, under most of the chemical conditions investigated in this study, reduction of As(V) in the presence of HFO or goethite would have only minor effects on or even decrease its mobility in the environment at near-neutral pH conditions. As(V) and As(III) sorption isotherms indicate similar surface site densities on the three oxides. Intrinsic surface complexation constants for As(V) are higher for goethite than HFO, whereas As(III) binding is similar for both of these oxides and also for magnetite. However, decrease in specific surface area and hence sorption site density that accompanies transformation of amorphous iron oxides to more crystalline phases could increase arsenic mobility.     

Contrasting effects of dissimilatory iron (III) and arsenic (V) reduction on arsenic retention and transport

Reduction of arsenate As(V) and As-bearing Fe (hydr)- oxides have been proposed as dominant pathways of As release within soils and aquifers. Here we examine As elution from columns loaded with ferrihydrite-coated sand presorbed with As(V) or As(III) at circumneutral pH upon Fe and/or As reduction; biotic stimulated reduction is then compared to abiotic elution. Columns were inoculated with Shewanella putrefaciens strain CN-32 or Sulfurospirillum barnesii strain SES-3, organisms capable of As (V) and Fe (III) reduction, or Bacillus benzoevorans strain HT-1, an organism capable of As(V) but not Fe(III) reduction. On the basis of equal surface coverages, As(III) elution from abiotic columns exceeded As(V) elution by a factor of 2; thus, As(III) is more readily released from ferrihydrite under the imposed reaction conditions. Biologically mediated Asreduction induced by B. benzoevorans enhances the release of total As relative to As (V) under abiotic conditions. However, under Fe reducing conditions invoked by either S. barnesii or S. putrefaciens, approximately three times more As (V or III) was retained within column solids relative to the abiotic experiments, despite appreciable decreases in surface area due to biotransformation of solid phases. Enhanced As sequestration upon ferrihydrite reduction is consistent with adsorption or incorporation of As into biotransformed solids. Our observations indicate that As retention and release from Fe (hydr)oxide(s) is controlled by complex pathways of Fe biotransformation and that reductive dissolution of As-bearing ferrihydrite can promote As sequestration rather than desorption under conditions examined here.     

Using nitrogen and carbon dioxide molecules to probe arsenic(V) bioaccessibility in soils

Highly specialized personnel and high cost are typically required for in vivo risk assessment of arsenic (As) exposure to humans in As-contaminated soils. Arsenic bioaccessibility in soils, as determined with the aid of in vitro tests, is quite variable, and its magnitude depends upon unidentified soil properties. Use of soil chemical properties is a common practice for construction of As(V) sorption and bioaccessibility models with relative success. We propose a novel As(V) bioaccessibility model, which was tested on 17 soils. The model includes only two parameters characterizing surface properties of soils that are readily determined from N2- and CO2-based specific surface areas (SSAs), and total organic carbon (OC) content. We found that N2 and CO2 molecules act as As(V) "surrogates", probing easily accessible and relatively difficult to access soil porosity, respectively. Three interrelated linear models were constructed using two terms (CO2/N2-based SSAs and OC) that were significant (p <0.001) in explaining 51 and 95% of the variability observed in As(V) sorption and bioaccessibility, respectively. The proposed models successfully predicted bioaccessible As concentrations for 4 out of the 5 soils that were not included in the bioaccessibility models, reaching RMSE values of < or =10%.     

Arsenic(III) and arsenic(V) reactions with zerovalent iron corrosion products

Zerovalent iron (Fe0) has tremendous potential as a remediation material for removal of arsenic from groundwater and drinking water. This study investigates the speciation of arsenate (As(V)) and arsenite (As(III)) after reaction with two Fe0 materials, their iron oxide corrosion products, and several model iron oxides. A variety of analytical techniques were used to study the reaction products including HPLC-hydride generation atomic absorption spectrometry, X-ray diffraction, scanning electron microscopy-energy-dispersive X-ray analysis, and X-ray absorption spectroscopy. The products of corrosion of Fe0 include lepidocrocite (gamma-FeOOH), magnetite (Fe3O4), and/or maghemite (gamma-Fe2O3), all of which indicate Fe(II) oxidation as an intermediate step in the Fe0 corrosion process. The in-situ Fe0 corrosion reaction caused a high As(III) and As(V) uptake with both Fe0 materials studied. Under aerobic conditions, the Fe0 corrosion reaction did not cause As(V) reduction to As(III) but did cause As(III) oxidation to As(V). Oxidation of As(III) was also caused by maghemite and hematite minerals indicating that the formation of certain iron oxides during Fe0 corrosion favors the As(V) species. Water reduction and the release of OH- to solution on the surface of corroding Fe0 may also promote As(III) oxidation. Analysis of As(III) and As(V) adsorption complexes in the Fe0 corrosion products and synthetic iron oxides by extended X-ray absorption fine structure spectroscopy (EXAFS) gave predominant As-Fe interatomic distances of 3.30-3.36 A. This was attributed to inner-sphere, bidentate As(III) and As(V) complexes. The results of this study suggest that Fe0 can be used as a versatile and economical sorbent for in-situ treatment of groundwater containing As(III) and As(V).     

Arsenic(III) oxidation and arsenic(V) adsorption reactions on synthetic birnessite

The oxidation of arsenite (As(III)) by manganese oxide is an important reaction in both the natural cycling of As and the development of remediation technology for lowering the concentration of dissolved As(III) in drinking water. This study used both a conventional stirred reaction apparatus and extended X-ray absorption fine structure (EXAFS) spectroscopy to investigate the reactions of As(III) and As(V) with synthetic birnessite (MnO2). Stirred reactor experiments indicate that As(III) is oxidized by MnO2 followed by the adsorption of the As(V) reaction product on the MnO2 solid phase. The As(V)-Mn interatomic distance determined by EXAFS analysis for both As(III)- and As(V)-treated MnO2 was 3.22 A, giving evidence for the formation of As(V) adsorption complexes on MnO2 crystallite surfaces. The most likely As(V)-MnO2 complex is a bidentate binuclear corner sharing (bridged) complex occurring at MnO2 crystallite edges and interlayer domains. In the As(III)-treated MnO2 systems, reductive dissolution of the MnO2 solid during the oxidation of As(III) caused an increase in the adsorption of As(V) when compared with As(V)-treated MnO2. This suggested that As(III) oxidation caused a surface alteration, creating fresh reaction sites for As(V) on MnO2 surfaces.     

XAS and XMCD evidence for species-dependent partitioning of arsenic during microbial reduction of ferrihydrite to magnetite

Poorly crystalline Fe(III) oxyhydroxides, ubiquitously distributed as mineral coatings and discrete particles in aquifer sediments, are well-known hosts of sedimentary As. Microbial reduction of these phases is widely thought to be responsible for the genesis of As-rich reducing groundwaters found in many parts of the world, most notably in Bangladesh and West Bengal, India. As such, it is important to understand the behavior of As associated with ferric oxyhydroxides during the early stages of Fe(lll) reduction. We have used X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) to elucidate the changes in the bonding mechanism of As(III) and As(V) as their host Fe(III) oxyhydroxide undergoes bacterially induced reductive transformation to magnetite. Two-line ferrihydrite, with adsorbed As(III) or As(V), was incubated under anaerobic conditions in the presence of acetate as an electron donor, and Geobacter sulfurreducens, a subsurface bacterium capable of respiring on Fe(lll), but not As(V). In both experiments, no increase in dissolved As was observed during reduction to magnetite (complete upon 5 days incubation), consistent with our earlier observation of As sequestration by the formation of biogenic Fe(III)-bearing minerals. XAS data suggested that the As bonding environment of the As(III)-magnetite product is indistinguishable from that obtained from simple adsorption of As(lll) on the surface of biogenic magnetite. In contrast, reduction of As(V)-sorbed ferrihydrite to magnetite caused incorporation of As5+ within the magnetite structure. XMCD analysis provided further evidence of structural partitioning of As5+ as the small size of the As5+ cation caused a distortion of the spinel structure compared to standard biogenic magnetite. These results may have implications regarding the species-dependent mobility of As undergoing anoxic biogeochemical transformations, e.g., during early sedimentary diagenesis.     

Coadsorption of arsenic(III) and arsenic(V) onto hydrous ferric oxide: effects on abiotic oxidation of arsenic(III), extraction efficiency, and model accuracy

Single solute adsorption and coadsorption of As(III) and As(V) onto hydrous ferric oxide (HFO), oxidation of As(III), and extraction efficiencies were measured in 0.2 atm O2. Oxidation was negligible for single-adsorbate experiments, but significant oxidation was observed in the presence of As(V) and HFO. Single-adsorbate As(III) or As(V) were incompletely extracted (0.5 M NaOH for 20 min), but all As was recovered in coadsorbate experiments. Single-adsorbate data were well-simulated using published surface complexation models, but those models (calibrated for single-adsorbate results) provided poor fits for coadsorbate experiments. An amended model accurately simulated single- and coadsorbate results. Model predictions of significant change in As(III) surface complex speciation in coadsorbate experiments was confirmed using zeta potential measurements. Our results demonstrate that mobility of arsenic in groundwater and removal in engineered treatment systems are more complicated when both As(III) and As(V) are present than anticipated based on single-adsorbate experimental results.     

Catalyzed oxidation of arsenic(III) by hydrogen peroxide on the surface of ferrihydrite: an in situ ATR FTIR study

Knowledge of arsenic redox kinetics is crucial for understanding the impact and fate of As in the environment and for optimizing As removal from drinking water. Rapid oxidation of As(III) adsorbed to ferrihydrite (FH) in the presence of hydrogen peroxide (H2O2) might be expected for two reasons. First, the adsorbed As(III) is assumed to be oxidized more readily than the undissociated species in solution. Second, catalyzed decomposition of H2O2 on the FH surface might also lead to As(III) oxidation. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy was used to monitor the oxidation of adsorbed As(III) on the FH surface in situ. No As(III) oxidation within minutes to hours was observed prior to H2O2 addition. Initial pseudo-first-order oxidation rate coefficients for adsorbed As(III), determined at H2O2 concentrations between 8.4 microM and 8.4 mM and pH values from 4 to 8, increased with the H2O2 concentration according to the equation log k(ox) (min(-1)) = 0.17 + 0.50 log [H2O] (mol/L), n = 21, r2 = 0.87. Only a weak pH dependence of log k(ox) was observed (approximately 0.04 logarithm unit increase per pH unit). ATR-FTIR experiments with As(III) adsorbed onto amorphous aluminum hydroxide showed that Fe was necessary to induce As(III) oxidation by catalytic H2O2 decomposition. Supplementary As(III) oxidation experiments in FH suspensions qualitatively confirmed the findings from the in situ ATR-FTIR experiments. Our results indicate that the catalyzed oxidation of As(III) by H2O2 on the surface of iron (hydr)oxides might be a relevant reaction pathway in environmental systems such as surface waters, as well as in engineered systems for As removal from water.     

Resistance of casein-derived bioactive peptides to simulated gastrointestinal digestion

The resistance of six casein-derived peptides, including antihypertensive peptides RYLGY, AYFYPEL and YQKFPQY, to simulated gastrointestinal digestion and the effect on angiotensin-converting enzyme (ACE)-inhibitory activity were evaluated. After digestion, peptides RYLGY, AYFYPEL, and YQKFPQY were partly hydrolysed by the digestive enzymes. RYLGY and AYFYPEL maintained potent ACE-inhibitory activity, with IC₅₀ values as low as 9.3 and 4.7 μg mL⁻¹, respectively. Digestion fragments were sequenced and then synthesised to evaluate their activity. Several showed potent ACE-inhibitory activity, which could explain the in vitro activity of the digests. A notable antioxidant activity was also observed. Since AYFYPEL was less susceptible to digestion, we focused on the antihypertensive activity in spontaneously hypertensive rats of the main digestion fragments of RYLGY. Interestingly, these peptides showed moderate effects in vivo. This suggests that the undigested fraction could also contribute to the in vivo effects of RYLGY and AYFYPEL, and other minor fragments may also participate.     

Competing Fe (II)-induced mineralization pathways of ferrihydrite

Owing to its high surface area and intrinsic reactivity, ferrihydrite serves as a dominant sink for numerous metals and nutrients in surface environments and is a potentially important terminal electron acceptor for microbial respiration. Introduction of Fe (II), by reductive dissolution of Fe(III) minerals, for example, converts ferrihydrite to Fe phases varying in their retention and reducing capacity. While Fe(II) concentration is the master variable dictating secondary mineralization pathways of ferrihydrite, here we reveal thatthe kinetics of conversion and ultimate mineral assemblage are a function of competing mineralization pathways influenced by pH and stabilizing ligands. Reaction of Fe(II) with ferrihydrite results in the precipitation of goethite, lepidocrocite, and magnetite. The three phases vary in their precipitation extent, rate, and residence time, all of which are primarily a function of Fe(II) concentration and ligand type (Cl, SO4, CO3). While lepidocrocite and goethite precipitate over a large Fe(II) concentration range, magnetite accumulation is only observed at surface loadings greater than 1.0 mmol Fe(II)/g ferrihydrite (in the absence of bicarbonate). Precipitation of magnetite induces the dissolution of lepidocrocite (presence of Cl) or goethite (presence of SO4), allowing for Fe(III)-dependent crystal growth. The rate of magnetite precipitation is a function of the relative proportions of goethite to lepidocrocite; the lower solubility of the former Fe (hydr)oxide slows magnetite precipitation. A one unit pH deviation from 7, however, either impedes (pH 6) or enhances (pH 8) magnetite precipitation. In the absence of magnetite nucleation, lepidocrocite and goethite continue to precipitate at the expense of ferrihydrite with near complete conversion within hours, the relative proportions of the two hydroxides dependent upon the ligand present. Goethite also continues to precipitate at the expense of lepidocrocite in the absence of chloride. In fact, the rate and extent of both goethite and magnetite precipitation are influenced by conditions conducive to the production and stability of lepidocrocite. Thus, predicting the secondary mineralization of ferrihydrite, a process having sweeping influences on contaminant/nutrient dynamics, will need to take into consideration kinetic restraints and transient precursor phases (e.g., lepidocrocite) that influence ensuing reaction pathways.     

Simultaneous microbial reduction of iron(III) and arsenic(V) in suspensions of hydrous ferric oxide

Bacterial reduction of arsenic(V) and iron(III) oxides influences the redox cycling and partitioning of arsenic (As) between solid and aqueous phases in sediment-porewater systems. Two types of anaerobic bacterial incubations were designed to probe the relative order of As(V) and Fe(III) oxide reduction and to measure the effect of adsorbed As species on the rate of iron reduction, using hydrous ferric oxide (HFO) as the iron substrate. In one set of experiments, HFO was pre-equilibrated with As(V) and inoculated with fresh sediment from Haiwee Reservoir (Olancha, CA), an As-impacted field site. The second set of incubations consisted of HFO (without As) and As(III)- and As(V)- equilibrated HFO incubated with Shewanella sp. ANA-3 wild-type (WT) and ANA-3deltaarrA, a mutant unable to produce the respiratory As(V) reductase. Of the two pathways for microbial As(V) reduction (respiration and detoxification), the respiratory pathway was dominant under these experimental conditions. In addition, As(III) adsorbed onto the surface of HFO enhanced the rate of microbial Fe(III) reduction. In the sediment and ANA-3 incubations, As(V) was reduced simultaneously or prior to Fe(III), consistent with thermodynamic calculations based on the chemical conditions of the ANA-3 WT incubations.     

Arsenic(III) oxidation by iron(VI) (ferrate) and subsequent removal of arsenic(V) by iron(III) coagulation

We investigated the stoichiometry, kinetics, and mechanism of arsenite [As(III)] oxidation by ferrate [Fe(VI)] and performed arsenic removal tests using Fe(VI) as both an oxidant and a coagulant. As(III) was oxidized to As(V) (arsenate) by Fe(VI), with a stoichiometry of 3:2 [As(III):Fe(VI)]. Kinetic studies showed that the reaction of As(III) with Fe(VI) was first-order with respect to both reactants, and its observed second-order rate constant at 25 degrees C decreased nonlinearly from (3.54 +/- 0.24) x 10(5) to (1.23 +/- 0.01) x 10(3) M(-1) s(-1) with an increase of pH from 8.4 to 12.9. A reaction mechanism by oxygen transfer has been proposed for the oxidation of As(III) by Fe(VI). Arsenic removal tests with river water showed that, with minimum 2.0 mg L(-1) Fe(VI), the arsenic concentration can be lowered from an initial 517 to below 50 microg L(-1), which is the regulation level for As in Bangladesh. From this result, Fe(VI) was demonstrated to be very effective in the removal of arsenic species from water at a relatively low dose level (2.0 mg L(-1)). In addition, the combined use of a small amount of Fe(VI) (below 0.5 mg L(-1)) and Fe(III) as a major coagulant was found to be a practical and effective method for arsenic removal.     

Direct observation of tetrahedrally coordinated Fe(III) in ferrihydrite

Ferrihydrite is a common iron hydroxide nanomineral commonly found in soils, sediments, and surface waters. Reactivity with this important environmental surface often controls the fate and mobility of both essential nutrients and inorganic contaminants. Despite the critical role of ferrihydrite in environmental geochemistry, its structure is still debated. In this work, we apply bulk sensitive Fe L edge X-ray absorption spectroscopy to study the crystal field environment of the Fe in ferrihydrite and other Fe oxides of known structure. This direct probe of the local electronic structure provides verification of the presence of tetrahedrally coordinated Fe(III) in the structure of ferrihydrite and puts to rest the controversy on this issue.     

Addition of probiotic bacteria in a semi-hard goat cheese (coalho): Survival to simulated gastrointestinal conditions and inhibitory effect against pathogenic bacteria

In this study, the survival of the probiotics Lactobacillus acidophilus (LA-5), Lactobacillus casei subsp. paracasei (L. casei 01) and Bifidobacterium lactis (BB12) incorporated in a Brazilian semi-hard goat cheese (coalho) when exposed to in vitro simulated conditions of digestion was assessed. The inhibitory effects of these probiotic bacteria were also evaluated against Listeria monocytogenes and Staphylococcus aureus in the goat coalho cheese during refrigerated storage. At the end of the in vitro digestion, all of the probiotic tested strains presented decreased (p < 0.05) viable cell counts (5.5–6.0 log cfu/g) with respect to those determined before exposure to the mouth conditions (7–8 log cfu/g). L. casei subsp. paracasei presented inhibition rate of 7.87% and 23.63% against S. aureus on the 14th and 21st day of storage at 10 °C, respectively; against L. monocytogenes these values were 12.96 and 32.99%. Positive inhibition rates of B. lactis toward S. aureus were found on the 1st, 14th and 21st days of storage (16.32%, 10.12% and 3.67%, respectively); and against L. monocytogenes only on the 1st day of storage (3.28%). From these results, goat coalho cheese could be an interesting carrier of probiotic strains of L. acidophilus, L. casei subsp. paracasei and B. lactis. Moreover, L. casei subsp. paracasei, could be used as protective culture for delaying the growth of S. aureus and L. monocytogenes in goat coalho cheese.     

Changes in the antioxidant activity of loach (Misgurnus anguillicaudatus) protein hydrolysates during a simulated gastrointestinal digestion

A two-stage in vitro digestion model system (a pepsin treatment for 2 h followed by a pancreatin treatment for 2 h, both at 37 °C) was used to simulate the process of human gastrointestinal (GI) digestion to determine the changes in antioxidant activities of loach peptide previously prepared by papain digestion. Results showed that the final GI digests contained 38.1% free amino acids, with short chain peptides (<500 Da) making up the rest of the biomass. Enzymatic breakdown of the GI digests increased their hydroxyl (12% increase), 2,2′-azinobis (3-ethylbenzothiazoline-6-sulphonic acid) diammonium salt (ABTS) (5% increase) radical scavenging activity, the reducing power (77% increase) and the chelating ability of Cu²⁺ (12% increase), compared to the blank. The results showed that the digestion by gastrointestinal proteases can be used to produce antioxidant peptides, with the advantage that the peptides formed will resist physiological digestion in GI tract.     

Distinctive arsenic(V) trapping modes by magnetite nanoparticles induced by different sorption processes

Arsenic sorption onto iron oxide spinels such as magnetite may contribute to arsenic immobilization at redox fronts in soils, sediments, and aquifers, as well as in putative remediation and water treatment technologies. We have investigated As(V) speciation resulting from different sorption processes on magnetite nanoparticles, including both adsorption and precipitation, using X-ray absorption fine structure (XAFS) spectroscopy and transmission electron microscopy (TEM). XAFS results suggest that AsO(4) tetrahedra form predominantly inner-sphere bidentate corner-sharing ((2)C) complexes and outer-sphere complexes on magnetite in the adsorption experiments. In the precipitation experiments, an increasing fraction of AsO(4) tetrahedra appears to be incorporated in clusters having a magnetite-like local structure with increasing As loading, the remaining fraction of As being adsorbed at the surface of magnetite particles. In the sample with the highest As loading (15.7 μmol/m(2)) XAFS data indicate that As(V) is fully incorporated in such clusters. Such processes help to explain the significantly higher arsenic uptake in precipitation samples compared to those generated in adsorption experiments. In addition, for the precipitation samples, TEM observations indicate the formation of amorphous coatings and small (~3 nm) nanoparticles associated with larger (~20-40 nm) magnetite nanoparticles, which are absent in the adsorption samples. These results suggest that As(V) could form complexes at the surfaces of the small nanoparticles and could be progressively incorporated in their structure with increasing As loading. These results provide some of the fundamental knowledge about As(V)-magnetite interactions that is essential for developing effective water treatment technologies for arsenic.     

Bioaccessibility,changes in the antioxidant potential and colonic fermentation of date pits and apple bagasse flours obtained from co-products during simulated in vitro gastrointestinal digestion

The aims of this work were to evaluate the effect of in vitro gastrointestinal digestion (GID) on (i) the recovery and bioaccessibility indexes, (ii) the changes on antioxidant activity and (iii) the short chain fatty acid (SCFA) production of two DF rich extracts (date pits flour (DPF) and apple bagasse flour (ABF)) obtained from agro-industrial co-products. At the end of GID process, the bioaccessibility of phenolic compounds present in DPF and ABF was 78.54 and 91.58%, respectively. With regard to antioxidant activity, GID increased the chelating activity and reducing power. However, the scavenging properties were reduced. Fermentation of DPF and ABF by colonic bacteria generated SCFA such as formiate, succinate, acetate, propionate and butyrate. These findings suggest that DPF and ABF might be used as potential food ingredients due to GID produced that bioactive compounds released from the matrix which exerts antioxidant activity.