Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Hydroxides”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 703 records · Page 39Linked to original sources
PubMed2004

Advanced phosphorus removal from membrane filtrates by adsorption on activated aluminium oxide and granulated ferric hydroxide.

The advanced phosphorus (P) removal by adsorption was studied for its suitability as a post-treatment step for membrane bioreactor (MBR) effluents low in P concentration and particle content. Two commercial adsorbents, granulated ferric hydroxide (GFH) and activated aluminium oxide (AA), were studied in batch tests and lab-scale filter tests for P adsorption in MBR filtrates. GFH showed a higher maximum capacity for phosphate and a higher affinity at low P concentrations compared to AA. Competition by inorganic ions was negligible for both adsorbents at the original pH (8.2). When equilibrium P concentrations exceeded 2 mg L(-1) in the spiked MBR filtrates, a precipitation of calcium phosphates occurred additionally to adsorption. During column studies the effluent criteria of 50 microgL(-1) P was reached after a throughput of 8000 bed volumes for GFH and 4000 for AA. Dissolved organic carbon appears to be the strongest competitor for adsorption sites. A partial regeneration and reloading of both adsorbents could be achieved by the use of sodium hydroxide.

Adsorption↗
PubMed2004

Effects of ferric hydroxide on the anaerobic biodegradation kinetics and toxicity of vegetable oil in freshwater sediments.

Biodegradation of vegetable oil in freshwater sediments exhibits self-inhibitory characteristics when it occurs under methanogenic conditions but not under iron-reducing conditions. The basis of the protective effect of iron was investigated by comparing its effects on oil biodegradation rate and the toxicity of oil-amended sediments to those of clay and calcium, which reduce the toxicity of oil-derived long-chain fatty acids by adsorption and precipitation, respectively. Kinetic parameters for an integrated mixed-second-order model were estimated by nonlinear regression using cumulative methane production as the response variable and used to compare the effects of the three treatment factors on the rate of oil biodegradation. Ferric hydroxide was the only factor that significantly (P<0.05) increased the rate of methane production from canola oil, whereas calcium significantly reduced the oil biodegradation rate. Measurement of sediment toxicity using the Microtox Solid-Phase Test showed that inhibitory products formed within 5 days of oil addition, but the sediment toxicity decreased over time as the extent of oil mineralization increased. None of the other amendments significantly reduced the toxicity of oil-containing sediments. Since ferric hydroxide stimulated the rate of oil biodegradation without affecting the toxicity of oiled sediments, it must operate through a mechanism that is different from those previously described for clay and calcium.

Bacteria, Anaerobic↗
PubMed2003

Solid-state 1H and 27Al NMR studies of amorphous aluminum hydroxides.

Two kinds of amorphous aluminum hydroxides, a sample precipitated from admixing AlCl3 and NaOH aqueous solutions and the commercial product, were measured by 27Al and 1H solid-state NMR spectroscopy. Pentahedral and tetrahedral coordinations, as well as octahedral coordination of oxygen atoms for aluminum, are observed in 27Al magic angle spinning (MAS) spectra of both amorphous samples. In contrast, octahedral coordination is only observed in gibbsite, bayerite, and boehmite. According to 1H MAS-NMR spectra under conditions of high spinning rate (35 kHz) and high field (14.09 T), free waters and OH groups coupled with aluminum for amorphous samples are observed at approximately 5 and approximately 4.5 ppm, respectively, the latter peak being broader. This is consistent with the differential spectra between spin echo and transfer of populations in double resonance. We conclude that the subunits of AlO4, AlO5, and AlO6 in amorphous aluminum hydroxides are bound through hydrogen bonds with a wide distribution of bonding strength.

Journal Article↗
PubMed2003

Exfoliation and immobilization of anionic iron porphyrin in layered double hydroxides.

Mg-Al layered double hydroxide (LDH) intercalated with glycinate anions was synthesized through co-precipitation and exfoliated in formamide and the single-layer suspension was reacted with an aqueous iron porphyrin (FeTDFSPP) solution. The new composite was characterized by powder X-ray diffraction, UV-vis, FTIR, and electron paramagnetic spectroscopies and thermal analyses (simultaneous TG/DSC). The analyses demonstrated that glycinate anions continue to be intercalated and the anionic iron porphyrin is adsorbed at the surface of the layered double hydroxide crystals.

Journal Article↗
PubMed2004

Adsorption of Cr(VI) and Se(IV) from their aqueous solutions onto Zr4+-substituted ZnAl/MgAl-layered double hydroxides: effect of Zr4+ substitution in the layer.

Hydrotalcite-like compounds (layered double hydroxides, LDHs) containing varying amounts of Al(3+), Zr(4+), and Zn(2+) or Mg(2+) in the metal hydroxide layer have been synthesized and characterized by various physicochemical methods. The adsorption behavior of uncalcined (as-synthesized) and calcined LDHs have been investigated for Cr(2)O(7)(2-) and SeO(3)(2-). The mixed oxides, obtained on calcination at 450 degrees C, exhibit high adsorption capacities for Cr(2)O(7)(2-) (1.6-2.7 meq/g) and SeO(3)(2-) (1.1-1.5 meq/g), where adsorption occurs through rehydration. Substitution of Zr(4+) in the LDHs, for either M(2+) or Al(3+) ions, increases the adsorption capacity up to 20%, thus providing an alternative way to enhance the adsorption capacity of this type of material. The high adsorption capacity of these materials could be successfully used for removal of undesirable anions from water and also for synthesis of intercalated materials with tailored acidobasicity.

Journal Article↗
PubMed1975

Modified method of C banding using barium hydroxide.

A modified centromeric heterochromatin banding technique using barium hydroxide octahydrate is described. The relationship between slide maturity and time of denaturation by barium-hydroxide is discussed.

Barium↗
PubMed2001

Two-Dimensional detection in ion chromatography: sequential conductometry after suppression and passive hydroxide introduction.

An improved method that uses sequential suppressed and nonsuppressed IC for the sensitive detection of both common anions and very weak acid anions is described. After suppressed conductometric detection of an electrolytically generated hydroxide eluent and an electrolytic suppressor, the eluent is passed into a membrane device where KOH is passively introduced into the eluent stream using Donnan forbidden leakage. A second conductivity detector then measures the conductivity of the stream. The background conductance of the second detector is typically maintained at a relatively low level of 20-30 microS/ cm. The weak acids are converted to potassium salts that are fully ionized and are detected against a low KOH background as negative peaks. The applicability of different commercially available cation exchange membranes was studied. Device configurations investigated include planar, tubular, and a filament-filled annular helical (FFAH) device. The FFAH device provided more effective mixing of the penetrated hydroxide with the eluent stream, resulting in a noise level of < or = 7 nS/cm and a band dispersion value of less than 82 microL. Optimal design and performance data are presented.

Journal Article↗
PubMed1992

Electron spin echo envelope modulation spectroscopic study of iron-nitrogen interactions in myoglobin hydroxide and Fe(III) tetraphenylporphyrin models.

The electron-nuclear coupling in low-spin iron complexes including myoglobin hydroxide (MbOH) and two related model compounds, Fe(III) tetraphenylporphyrin(pyridine)(OR-) (R = H or CH3) and Fe(III) tetraphenylporphyrin(butylamine)(OR-) was investigated using electron spin echo envelope modulation (ESEEM) spectroscopy. The assignment of frequency components in ESEEM spectra was accomplished through the use of nitrogen isotopic substitution wherever necessary. For example, the proximal imidazole coupling in MbOH was investigated without interference from the contributions of porphyrin 14N nuclei after substitution of the heme in native Mb with 15N-labeled heme. Computer simulation of spectra using angle selected techniques enabled the assignment of parameters describing the hyperfine and quadrupole interactions for axially bound nitrogen of imidazole in MbOH, of axial pyridine and butylamine in the models, and for the porphyrin nitrogens of the heme in native MbOH. The isotropic component of axial nitrogen hyperfine interactions exhibits a trend from 5 to 4 MHz, with imidazole (MbOH) greater than pyridine greater than amine. The nuclear quadrupole interaction coupling constant e2Qq was near 2 MHz for all nitrogens in these complexes. The Qzz axis of the nuclear quadrupole interaction tensor for the proximal imidazole nitrogen in MbOH was found to be aligned near gz (gmax) in MbOH, suggesting that gz is near the heme normal. A crystal field analysis, that allows a calculation of rhombic and axial splittings for the d orbitals of the t2g set in a low-spin heme complex, based on the g tensor assignment gz greater than gy greater than gx, yielded results that are consistent with the poor pi-acceptor properties expected for the closed shell oxygen atom of the hydroxide ligand in MbOH. A discussion is presented of the unusual results reported in a linear electric field effect in EPR (LEFE) study of MbOH published previously [Mims, W. B., & Peisach, J. (1976) J. Chem. Phys. 64, 1074-1091].

Animals↗
PubMed1979

Resonance Raman examination of axial ligand bonding and spin-state equilibria in metmyoglobin hydroxide and other heme derivatives.

Resonance Raman spectra and excitation profiles have been obtained within the 5700-6300-A absorption band of purified sperm whale metmyoglobin hydroxide (MbIIIOH) solutions. A large enhancement occurs for a Raman peak at 490 cm-1 which is shown by isotopic substitution of 18O for 16O to be almost purely an Fe-O stretch. The Fe-O vibration in MbIIIOH occurs 5 cm-1 to lower energy than the corresponding vibration at 495 cm-1 in human methemoglobin hydroxide (HbIIIOH) [Asher, S., Vickery, L., Schuster, T., & Sauer, K. (1977) Biochemistry 16, 5849], reflecting differences in ligand bonding between Mb(III) and Hb(III). A larger frequency difference (10 cm-1) exists between MbIIIF and HbIIIF for the Fe-F stretch. We do not observe separate Fe-O or Fe-F stretches from the alpha and beta chains of either HbIIIOH or HbIIIF. Excitation profile measurements for MbIIOH indicate that the 5700-6300-A absorption band is composed of two separate absorption bands which result from a high- and a low-spin form of MbIIIOH. The spin-state-sensitive Raman band at 1608 cm-1 reflects the high-spin species and has an excitation profile maximum at about 6000 A while the low-spin Raman band occurs at 1644 cm-1 and shows an excitation profile maximum at 5800 A. The Fe-O stretch at 490 cm-1 has an excitation profile maximum at about 6000 A. The differences in frequency and Raman cross section between the Fe-X vibrations in MbIIIX and HbIIIX (X = OH-, F-) can be related to increases in the out-of-plane iron distance for the high-spin species of MbIIIX. The shift in the 1644-cm-1 MbIIIOH low-spin state Raman band indicative of the heme core size to 1636 cm-1 in HbIIIOH indicates a larger heme core size in HbIIIOH. Raman frequency shifts are used to estimate differences in bond strain energies between MbIIIX and HbIIIX (X = OH-, F-). Previous resonance Raman excitation profile data can be interpreted in terms of separate contributions from different spin-state species.

Animals↗
PubMed2002

Modeling silica sorption to iron hydroxide.

Experiments were conducted to investigate the fundamentals of silica sorption onto preformed ferric hydroxide at pH 5.0-9.5 and silica concentrations of 0-200 mg/L as SiO2. At all pHs studied, sorption densities exceeding monolayer sorption were observed at silica levels typical of natural waters. Under some circumstances, sorption equaled or exceeded a monolayer while the particle zeta potential remained positive, a phenomenon that is completely inconsistent with available surface complexation models. To address this deficiency, an extended surface complexation model was formulated in which soluble dimeric silica (i.e., Si2O2(OH)5-) sorbs directly to iron surface sites. This new model fit sorption density data up to 0.40 mol SiO2/mol Fe and accurately predicted trends in zeta potential and observed H+ release during silica sorption to ferric hydroxide.

Adsorption↗
PubMed2003

Immobilization mechanisms of arsenate in iron hydroxide sludge stabilized with cement.

Leaching tests, Fourier transform infrared spectroscopy (FTIR), extended X-ray absorption fine structure (EXAFS) spectroscopy, and thermodynamic modeling were performed to investigate arsenate [As(V)] immobilization mechanisms in iron hydroxide sludge stabilized with cement. The sludge from a groundwater remediation site in Tacoma, WA was mixed and immobilized with premixed cement to reach cement-to-sludge ratios of 2.5, 3.3, 5, 10, and 20 (wt premixed cement/wt dry sludge). The EXAFS analysis determined that As(V) formed bidentate mononuclear complexes on the iron hydroxide surface in the sludge. The adsorbed As(V) had a characteristic FTIR band at 830 cm(-1). Cement treatment converted the adsorbed As(V) to calcium arsenate precipitate with a FTIR peak at 860 cm(-1). The chemical forms of the As(V) were incorporated in an adsorption triple layer model (TLM) to describe the leaching behavior of As(V) in a pH range between 3 and 12. Cement treatment significantly reduced arsenic mobility because of the formation of the sparingly soluble calcium arsenate.

Arsenates↗
PubMed2005

Evaluation of density functional theory methods for studying chemisorption of arsenite on ferric hydroxides.

Understanding adsorption of arsenic on ferric hydroxide surfaces is important for predicting the fate of arsenic in the environment and in designing treatment systems for removing arsenic from potable water. This research investigated the binding of arsenite to ferric hydroxide clusters using several density functional theory methods. Comparison of calculated and experimentally measured As-O and As-Fe bond distances indicated that As(III) forms both bidentate and monodentante corner-sharing complexes with Fe(III) octahedra. Edge-sharing As(III) complexes were less energetically favorable and had As-O and As-Fe distances that deviated more from experimentally measured values than corner-sharing complexes. The hydrated bidentate complex was the most energetically favorable in the vacuum phase, while the monodentate complex was most favored in the aqueous phase. Structures optimized using the Harris and Perdew-Wang local functionals were close to both experimental data and structures optimized using the nonlocal Becke-Lee-Yang-Parr (BLYP) functional. Binding energies calculated with the gradient-corrected BLYP functional were only weakly dependent on the method used for geometry optimization. The approach of using low-level structures coupled with higher level single-point energies was found to reduce computational time by 75% with no loss in accuracy of the computed binding energies.

Absorption↗
PubMed2005

Effects of soluble ferri-hydroxide complexes on microbial neutralization of acid mine drainage.

Heterotrophic respiration of ferric iron by Acidiphilium cryptum was investigated in anoxic microcosms with initial media pH values from 1.5 to 3.5. No organic carbon consumption or iron reduction was observed with an initial pH of 1.5, indicating that A. cryptum may not be capable of iron respiration at this pH. Significant iron reduction was observed at pH 2.5 and 3.5, with different effects. When the initial pH was 3.5, pH increased to 4.7-5.5 over 60 days of incubation with simultaneous production of 0.4 g L(-1) Fe2+. However, at an initial pH of 2.5, no significant change in pH was observed during iron respiration, although the accumulation of soluble ferrous iron was significantly higher, averaging 1.1 g L(-1) Fe2+. The speciation of the ferric iron electron acceptor may explain these results. At pH values of 3.5 and higher, precipitated ferric hydroxide Fe- (OH)3 would have been the primary source of ferric iron, with reduction resulting in net production of OH- ions and the significant increases in media pH observed. However at pH 2.5, soluble complexes, FeOH2+ and Fe(OH)2+, may have been the more prevalent electron acceptors, and the alkalinity generated by reduction of complexed iron was low. The existence of charged ferri-hydroxide complexes at pH 2.5 was verified by voltammetry. Results suggest that initiation of bacterial iron reduction may result in neutralization of acid mine drainage. However, this effect is extremely sensitive to iron speciation within a relatively small and critical pH range.

Acidiphilium↗
PubMed2001

Decomposition of alkyl-substituted urea molecules at a hydroxide-bridged dinickel center.

The interactions between N-methylurea, N,N'-dimethylurea, N,N-dimethylurea, tetramethylurea, and thiourea and the hydroxide-bridged dinickel complex [Ni(2)(mu-OH)(mu-H2O)(bdptz)(H2O2](OTs)(3) were investigated. Structural characterization of [Ni(2)(mu-OH)(mu-H2O)(bdptz)(Me-urea)(CH3CN)](ClO4)(3) (1) and [Ni(2)(mu-OH)(mu-H2O)(bdptz)(thiourea)(CH3CN)](ClO4)(3) (2) provided insight into the interactions of the substrates with the dinickel center. In 1, the methylurea molecule coordinates to the dinickel complex through its carbonyl oxygen atom. Complex 2 has a similar geometry, with the thiourea molecule bound to a nickel ion through its sulfur atom. When the urea substrates are heated in the presence of the hydroxide-bridged dinickel complex, N-methylurea and N,N-dimethylurea react to form methylammonium cyanate and dimethylammonium cyanate, respectively. After long reaction times, thiourea reacts similarly, producing ammonium thiocyanate. The other substrates are unreactive. These results indicate that the dinickel complex promotes the elimination of alkylamines from urea substrates to form cyanate but cannot effect the direct hydrolysis of such substrates.

Alkanes↗
PubMed2003

Hydrolysis of 4-nitrophenyl acetate by a (N2S(thiolate))zinc hydroxide complex: a model of the catalytically active intermediate for the zinc form of peptide deformylase.

A novel zinc(II) hydroxide complex with a rare alkylthiolate donor in the coordination sphere is formed in aqueous solution from the dissolution of the zinc alkyl precursor complex (PATH)ZnCH(3) (PATH = 2-methyl-1-[methyl(2-pyridin-2-ylethyl)amino]propane-2-thiolate) in H(2)O and protonolysis of the Zn-C bond to give (PATH)ZnOH (1). The (PATH)ZnOH complex has been shown to promote the hydrolysis of 4-nitrophenyl acetate (4-NA) by a detailed kinetic study and is the first functional model for the zinc form of the enzyme peptide deformylase. From a fit of the sigmoidal pH-rate profile a kinetic pK(a) of 8.05(5) and a pH-independent second-order rate constant (k" max)) of 0.089(3) M(-1) s(-1) have been obtained. The kinetic pK(a) is similar to the pK(a) of 7.7(1) determined by a potentiometric study (25 degrees C, I = 0.1 (NaNO3)). Observation of both rate enhancement and turnover shows that 1 acts as a catalyst for the hydrolysis of 4-NA, although the turnovers are modest. Activation parameters have been obtained from a temperature-dependence study of the rate constants (E(a) = 54.8 kJ mol(-1), DeltaH++ = 52.4 kJ mol(-1), and DeltaS++ = -90.0 J mol(-1) K(-1)), and support a reaction mechanism which depends on nucleophilic attack of 1 in the rate-determining step. This is the first kinetic and thermodynamic study of a 4-coordinate zinc hydroxide complex containing a thiolate donor. In addition it is only the second time that a complete set of activation parameters have been obtained for the zinc-promoted hydrolysis of a carboxylic ester. This study puts the basicity and nucleophilicity of a (N(2)S)ZnOH complex in context with those of other L(n)()ZnOH complexes and enzymes.

Amidohydrolases↗
PubMed2004

Synthesis and characterization of new Mg(2)Al-paratungstate layered double hydroxides.

Layered double hydroxides (LDHs, or hydrotalcites) with Mg(2+) and Al(3+) cations in the mixed metal hydroxide layer and paratungstate anions in the interlayer have been prepared. Different methods have been followed: anion exchange with Mg,Al LDHs originally containing nitrate or adipate, reconstruction of the LDH structure from a mildly calcined Mg(2)Al-CO(3) LDH, and coprecipitation. In all cases, the tungsten precursor salt was (NH(4))(10)H(2)W(12)O(42). The prepared solids have been characterized by elemental chemical analysis, powder X-ray diffraction (PXRD), FT-IR spectroscopy, thermogravimetric (TG) and differential thermal (DTA) analyses, scanning electron microscopy (SEM) with EDX (energy-dispersive X-ray analysis), and nitrogen adsorption at -196 degrees C for surface area and surface texture. Most of the synthesis methods used, especially anion exchange starting from a Mg(2)Al-NO(3) precursor at low temperature and short reaction times, lead to formation of a hydrotalcite with a gallery height of 9.8 A; increasing the reaction temperature to 70-100 degrees C and maintaining short contact times leads to a solid with a gallery height of 7.8 A. Both phases have been identified as a result of the intercalation of W(7)O(24)(6)(-) species in different orientations in the interlayer space. If the time of synthesis or the temperature is increased, a more stable phase, with a gallery height of 5.2 A corresponding to a solid with intercalated W(7)O(24)(6)(-), is formed, probably with grafting of the interlayer anion on the brucite-like layers. All systems are microporous. Calcination at 300 degrees C leads to amorphous species, and crystallized MgWO(4) is observed at 700 degrees C.

Journal Article↗
PubMed1996

Layered Double Hydroxides Intercalated by Polyoxometalate Anions with Keggin (alpha-H(2)W(12)O(40)(6)(-)), Dawson (alpha-P(2)W(18)O(62)(6)(-)), and Finke (Co(4)(H(2)O)(2)(PW(9)O(34))(2)(10)(-)) Structures.

The pillaring of Mg(3)Al layered double hydroxides (LDHs) by the title polyoxometalates (POMs) was accomplished by ion exchange reaction of the LDH-hydroxide and -adipate precursors with the POM anion at ambient or refluxing temperatures. The structural, thermal and textural properties of the LDH-POM intercalates were elucidated based on XRD, FTIR, TEM, EDS, and N(2) adsorption-desorption studies. A gallery height of approximately 10 Å was observed for the LDH intercalated by the symmetrical Keggin POM, whereas two different gallery heights were found for the cylindrical Dawson (14.5 and 12.8 Å) and Finke (13.3 and 12.6 Å) anions, depending on the preparation temperature. The differences in POM orientations were rationalized in terms of different electrostatic and hydrogen-bonding interactions between the POM pillars and the LDH layers. Upon thermal treatment at >/=100 degrees C, the intercalated Dawson and Finke POM ions exhibited only one gallery orientation, regardless of synthesis conditions. The crystalline microporous structures were retained upon heating each LDH-POM intercalate in N(2) to 200 degrees C. Pillaring in all cases was accompanied by the formation of a poorly ordered Mg(2+)/Al(3+) salt impurity that formed on the external surfaces of the LDH crystals.

Journal Article↗
PubMed1997

Vanadate-Inserted Layered Double Hydroxides: A (51)V NMR Investigation of the Grafting Process.

The reduction of mixed nickel-cobalt gamma-oxide-hydroxides in an NH(4)VO(3)/H(2)O(2) medium (pH = 4) leads to layered double hydroxides (LDH's) in which vanadate species are inserted in order to compensate for the excess positive charges brought by Co(III) ions. Static (51)V NMR spectra have been recorded on such LDH's after various treatments, showing that the hyperfine magnetic interaction exerted by unpaired electrons of Ni(II) on the inserted vanadium nuclei allows us to determine whether the latter are grafted to the layer or not. For materials kept for a long time in the NH(4)VO(3)/H(2)O(2) solution, a partial grafting of the inserted metavanadate chains is evidenced which leads to a full occupancy of the interlayer space by vanadate species and water molecules. Upon thermal or vacuum treatment, a further grafting and a fragmentation of the chains are evidenced, accompanied by a decrease of the interlayer spacing. For materials removed very early from the reducing solution, the initial polycondensation step of the vanadate species can be followed by NMR, which again also shows a simultaneous partial grafting.

Journal Article↗