This study investigates redox transitions associated with the adsorption of Cr(VI) on commonly occurring soil components (silicates, oxides and humic acids) and their synthetic mixtures by coupling the mechanistic surface complexation modeling with spectroscopic and isotopic analyses. The mixtures of soil components were prepared to reflect the composition of the real anthroposol sample, determined by X-ray Powder Diffraction (XRD), total organic carbon (TOC) measurement and extraction methods. The effect of different initial Cr(VI) concentrations (2×10−2, 5×10−4, 10−4, 10−5, and 10−6M), background electrolyte (10−3, 10−2, and 10−1M KNO3), pH values (3–9), and sorbate/sorbent ratios (2g/L - 20g/L) were investigated. Maghemite and ferrihydrite were confirmed to be the main phases controlling Cr(VI) adsorption with increasing Cr(VI) concentration. Humic acids were primarily responsible for Cr(VI) reduction, especially at low pH values. The reduction of Cr(VI) was also proved in case of illite and kaolinite by XAS and isotopic analyses. Illite revealed higher reduction capacity in comparison with kaolinite based on XAS measurements. Chromium isotopic fractionation, resulting from Cr(VI) reduction, was the highest in the case of humic acids, followed by kaolinite and illite. However, a dissolution of intrinsic Cr originally present within kaolinite and illite might affect the final Cr isotopic composition of the supernatants due to its different Cr isotopic signature. In general, the combination of three different approaches was confirmed to offer more comprehensive information about Cr(VI) adsorption and/or reduction in soils. Detailed studies using soil mixtures can help to predict how the soil components affect Cr(VI) behavior in natural soils and possibly could improve the environmental remediation processes.

A generic hydrous Fe oxide surface is often used in surface complexation modeling for predicting metal behavior in aqueous and soil environments. However, this approach may lead to incorrect results because there are many different naturally-occurring Fe(III) (oxyhydr) oxide mineral phases in the environment, each exhibiting distinct adsorption characteristics. This work presents a consistent dataset of surface complexation model parameters valid at 25 degrees C and 1 bar that characterize the adsorption of selected divalent metals (Cd, Cu, Pb, Zn) onto environmentally relevant Fe(III) (oxyhydr) oxides, i.e., FeOOH and Fe2O3 polymorphs (goethite, hematite, lepidocrocite, maghemite), at varying metal concentrations and ionic strengths. The parameters were obtained using a simple and unified workflow and are readily implementable into widely used geochemical codes, such as Visual MINTEQ, MINEQL+, and ORCHESTRA, enabling quantification of metal adsorption onto a range of Fe (III) oxides in various aqueous environments.

A novel sorbent made from biochar modified with an amorphous Mn oxide (AMOchar) was compared with pure biochar, pure AMO, AMO+biochar mixtures and biochar+birnessite composite for the removal of various metal(loid)s from aqueous solutions using adsorption and solid-state analyses. In comparison with the pristine biochar, both Mn oxide-biochar composites were able to remove significantly greater quantities of various metal(loid)s from the aqueous solutions, especially at a ratio 2:1 (AMO:biochar). The AMOchar proved most efficient, removing almost 99, 91 and 51% of Pb, As and Cd, respectively. Additionally, AMOchar and AMO+biochar mixture exhibited reduced Mn leaching, compared to pure AMO. Therefore, it is concluded that the synthesis of AMO and biochar is able to produce a double acting sorbent ('dorbent') of enhanced efficiency, compared with the individual deployment of their component materials.

Layered double hydroxides have been proposed as effective sorbents for As(V), but studies investigating adsorption mechanisms usually lack a comprehensive mechanistic/modeling approach. In this work, we propose coupling surface complexation modeling with various spectroscopic techniques. To this end, a series of batch experiments at different pH values were performed. Kinetic data were well fitted by a pseudo-second order kinetic model, and the equilibrium data were fitted by the Freundlich model. Moreover, the pH-dependent As(V) sorption data were satisfactorily fitted by a diffuse layer model, which described the formation of >SOAsO3H- monodentate and >(SO)2AsO2- bidentate inner-sphere complexes (">S" represents a crystallographically-bound group on the surface). Additionally, XPS analyses confirmed the adsorption mechanisms. The sorption mechanisms were affected by anion exchange, which was responsible for the formation of outer sphere complexes, as identified by XRD and FTIR analyses. Furthermore, a homogenous distribution of As(V) was determined by HR-TEM with elemental mapping. Using low-temperature Mössbauer spectroscopy on isotope 57Fe, a slight shift of the hyperfine parameters towards higher values following As(V) sorption was measured, indicating a higher degree of structural disorder. In general, mechanistic adsorption modeling coupled with solid state analyses presents a powerful approach for investigating the adsorption mechanism of As(V) on Mg-Fe LDH or other sorbents.

Comprehensive mechanistic and modeling approaches are needed to effectively evaluate sorption of metal ions from aqueous solutions. However, such a complex study using layered double hydroxides has not yet been" presented. Therefore, adsorption modeling was performed coupled with solid state analyses describing the mode of zinc and lead removal by magnesium-iron layered double hydroxides, and an excellent removal efficiency for both metal ions was observed. The maximal adsorbed concentration, as established by the Langmuir model, increased with the increasing magnesium/iron molar ratio. The pH dependent sorption was fitted by the diffuse layer model, which described the formation of monodentate inner-sphere complexes, indicating strong binding between metal ions and the layered double hydroxides surface. Based on the solid state analyses of materials with high surface concentrations of zinc (1.44 mmol g(-1)) and lead (1.65 mmol g(-1)), respectively, the whole sorption mechanism was also influenced by other processes, i.e., precipitation (lead) and surface accumulation/precipitation/isomorphic substitution (zinc). Transmission electron microscopy-based elemental mapping showed a heterogeneous distribution of zinc and lead on the surface of particles. Low-temperature Mossbauer spectra were nearly identical for the studied materials before/after zinc and lead sorption indicating no structural changes in incorporated iron. Generally, we suggest that these layered double hydroxides are highly effective sorbents for metal ions from aqueous solutions. Furthermore, we propose a comprehensive mechanistic/modeling approach as a powerful tool for describing the mechanism of metal ions binding on layered double hydroxides in contaminated waters. (C) 2017 Elsevier Ltd. All rights reserved.

This paper discusses Cd(II) and Pb(II) sorption efficiency of biochars modified by impregnation with magnetic particles. All selected biochar characteristics were significantly affected after the modification. More specifically, the cation exchange capacity increased after the modification, except for grape stalk biochar. However, the changes in the pH value, PZC, and BET surface after modification process were less pronounced. The metal loading rate was also significantly improved, especially for Cd(II) sorption on/in nut shield and plum stone biochars (10- and 16-times increase, respectively). The results indicated that cation exchange (as a metal sorption mechanism) was strengthened after Fe oxide impregnation, which limited the desorbed amount of tested metals. In contrast, the magnetization of grape stalk biochar reduced Pb(II) sorption in comparison with that of pristine biochar. Magnetic modification is, therefore, more efficient for biochars with well-developed structure and for more mobile metals, such as Cd(II).

This study investigates the mechanisms of Cr(VI) adsorption on natural clay (illite and kaolinite) and synthetic (birnessite and ferrihydrite) minerals, including its speciation changes, and combining quantitative thermodynamically based mechanistic surface complexation models (SCMs) with spectroscopic measurements. Series of adsorption experiments have been performed at different pH values (3-10), ionic strengths (0.001-0.1M KNO3), sorbate concentrations (10(-4), 10(-5), and 10(-6)M Cr(VI)), and sorbate/sorbent ratios (50-500). Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy were used to determine the surface complexes, including surface reactions. Adsorption of Cr(VI) is strongly ionic strength dependent. For ferrihydrite at pH <7, a simple diffuse-layer model provides a reasonable prediction of adsorption. For birnessite, bidentate inner-sphere complexes of chromate and dichromate resulted in a better diffuse-layer model fit. For kaolinite, outer-sphere complexation prevails mainly at lower Cr(VI) loadings. Dissolution of solid phases needs to be considered for better SCMs fits. The coupled SCM and spectroscopic approach is thus useful for investigating individual minerals responsible for Cr(VI) retention in soils, and improving the handling and remediation processes.

An amorphous Mn oxide (AMO), nanomaghemite, and nanomagnetite were used as potential amendments reducing the mobility of As in three contrasting contaminated soils differing in origin of As contamination. Adsorption experiments and XPS analyses combined with incubation batch experiments and pH-static leaching tests were used. The AMO showed excellent adsorption capacity for As(V) reaching a maximum of 1.79 mmol g−1 at pH 7 and 8. Interestingly, the adsorption capacity in this case decreases with decreasing pH, probably as a result of AMO dissolution at lower pH values. Chemical sorption of As(V) onto AMO was further confirmed with XPS. Both Fe nano-oxides proved the highest adsorption capacity at pH 4 reaching 11 mg g−1 of adsorbed As(V). The AMO was also the most efficient amendment for decreasing As concentrations in soil solutions during 8 weeks of incubation. Additionally, pH-static leaching tests were performed at pH 4, 5, 6, 7, and natural pH (not adjusted) and AMO again proved the highest ability to decrease As content in leachate. On the other hand, strong dissolution of this amendment at lower pH values (especially pH 4) was observed. For that reason, AMO appears as a promising stabilizing agent for As, especially in neutral, alkaline, or slightly acidic soils, where As(V) species are expected to be more mobile.

AbstractOrganocatalyzed Michael addition of amidomalonates, e.g. (I) and (VI), towards enals (II) followed by intramolecular hemiaminal formation provides enantioenriched piperidine derivatives (III) and (VII), resp., in up to 99% optical purity.

The synthesis of piperidines and piperidines derivatives in enantiopure fashion has been a challenging goal for organic chemists. In this report we developed a nice cascade reaction for piperidine derivatives based in an amidomalonate Michael addition to enals followed by an intramolecular hemiaminal formation with good yields and enantioselectivities. Moreover we studied the 'in situ' intramolecular cyclization of this hemiaminals with alcohols forming fused piperidine–oxazolidines.

The highly enantioselective organo-co-catalytic aza-Morita-Baylis-Hillman (MBH)-type reaction between N-carbamate-protected imines and alpha,beta-unsaturated aldehydes has been developed. The organic co-catalytic system of proline and 1,4-diazabicyclo[2.2.2]octane (DABCO) enables the asymmetric synthesis of the corresponding N-Boc- and N-Cbz-protected beta-amino-alpha-alkylidene-aldehydes in good to high yields and up to 99% ee. In the case of aza-MBH-type addition of enals to phenylprop-2-ene-1-imines, the co-catalytic reaction exhibits excellent 1,2-selectivity. The organo-co-catalytic aza-MBH-type reaction can also be performed by the direct highly enantioselective addition of alpha,beta-unsaturated aldehydes to bench-stable N-carbamate-protected alpha-amidosulfones to give the corresponding beta-amino-alpha-alkylidene-aldehydes with up to 99% ee. The organo-co-catalytic aza-MBH-type reaction is also an expeditious entry to nearly enantiomerically pure beta-amino-alpha-alkylidene-amino acids and beta-amino-alpha-alkylidene-lactams (99% ee). The mechanism and stereochemistry of the chiral amine and DABCO co-catalyzed aza-MBH-type reaction are also discussed.

ChemInformVolume 42, Issue 38 Preparative Organic Chemistry ChemInform Abstract: Asymmetric Aza-Morita—Baylis—Hillman-Type Reactions: The Highly Enantioselective Reaction Between Unmodified α,β-Unsaturated Aldehydes and N-Acylimines by Organo-Cocatalysis. Sylva Cihalova, Sylva Cihalova Dep. Org. Nucl. Chem., Fac. Sci., Charles Univ., CZ-128 43 Prague, Czech RepublicSearch for more papers by this authorPawel Dziedzic, Pawel Dziedzic Dep. Org. Nucl. Chem., Fac. Sci., Charles Univ., CZ-128 43 Prague, Czech RepublicSearch for more papers by this authorArmando Cordova, Armando Cordova Dep. Org. Nucl. Chem., Fac. Sci., Charles Univ., CZ-128 43 Prague, Czech RepublicSearch for more papers by this authorJan Vesely, Jan Vesely Dep. Org. Nucl. Chem., Fac. Sci., Charles Univ., CZ-128 43 Prague, Czech RepublicSearch for more papers by this author Sylva Cihalova, Sylva Cihalova Dep. Org. Nucl. Chem., Fac. Sci., Charles Univ., CZ-128 43 Prague, Czech RepublicSearch for more papers by this authorPawel Dziedzic, Pawel Dziedzic Dep. Org. Nucl. Chem., Fac. Sci., Charles Univ., CZ-128 43 Prague, Czech RepublicSearch for more papers by this authorArmando Cordova, Armando Cordova Dep. Org. Nucl. Chem., Fac. Sci., Charles Univ., CZ-128 43 Prague, Czech RepublicSearch for more papers by this authorJan Vesely, Jan Vesely Dep. Org. Nucl. Chem., Fac. Sci., Charles Univ., CZ-128 43 Prague, Czech RepublicSearch for more papers by this author First published: 25 August 2011 https://doi.org/10.1002/chin.201138021Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume42, Issue38September 20, 2011 RelatedInformation

An organocatalytic highly enantioselective aza-Baylis–Hillman reaction of α,β-unsaturated aldehydes with in situ generated N-Boc- and N-Cbz-imines is presented. This novel process opens the pathway for the synthesis of β-amino carbonyl compounds bearing an α-alkylidene group under mild and simple conditions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Cyclotrimerization of β- or α-ethynyl-3,5-di-O-toluoyl-2-deoxy-d-ribofuranose with α,ω-diynes proceeded smoothly under Rh-catalysis to afford the corresponding β- or α-benzene C-nucleoside derivatives. Analogous co-cyclotrimerization of α- or β-propynyl- and -phenylethynyl-3,5-di-O-toluoyl-2-deoxy-d-ribofuranose with α,ω-diynes gave the corresponding arene derivatives only under microwave irradiation in the presence of a Rh-catalyst in moderate yields. Attempted homocyclotrimerization of β- or α-ethynyl-3,5-di-O-toluoyl-2-deoxy-d-ribofuranose under Rh-catalysis led only to enynes while the use of Ru-catalyst gave the desired 1,2,4- and 1,3,5-tri-(2-deoxyribofuranose-1-yl)benzene.

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