Hydroxide Coatings Research Articles

Electroless amorphous alloy electrodes—the formation and properties of hydrous oxide coatings in alkaline solution

Amorphous alloys (Co 94.6P 5.4 and Co 71.6Ni 18.3P 10.1) were prepared by the electroless technique. Oxide coated amorphous alloys were produced by applying a cyclic potential routine to the amorphous alloys immersed in alkaline solution at 30°C. Oxide coatings were characterized by electrochemical techniques and ir spectroscopy. The electrocatalytic activity of oxide coated amorphous alloy electrodes was tested for the oxygen evolution reaction in alkaline solution at 30°C. The high electrocatalytic activity of these electrodes is explained through the development of a porous hydrous oxide film with a spinel-type structure.

Experimental Determination of Chlorite Dissolution Rates

AbstractCurrent concepts of the geological disposal of low- and intermediate-level radioactive wastes in the UK envisage the construction of a mined facility (incorporating cementitious engineered barriers) in chlorite-bearing rocks. To model accurately the fluid-rock reactions within the ‘disturbed zone’ surrounding a repository requires functions that describe mineral dissolution kinetics under pH conditions that vary from near neutral to highly alkaline.Therefore, an experimental study to determine the dissolution rates of Fe-rich chlorite has been undertaken as part of the Nirex Safety Assessment Research Programme. Four experiments have been carried out at 25 °C and four at 70 °C, both sets using a range of NaCl/NaOH solutions of differing pH (of nominal pH 9.0,10.3, 11.6 and 13.0 [at 25 °C]).Dissolution rates have been calculated and were found to increase with increasing pH and temperature. However, increased pH resulted in non-stoichiometric dissolution possibly due to preferential dissolution of part of the chlorite structure relative to another, or reprecipitation of some elements as thin hydroxide or oxyhydroxide surface coatings on the chlorite.These results also show that chlorite dissolution is appreciably slower than that of albite and quartz at both 25 and 70 °C, but slightly faster than that of muscovite at 70 °C.

Characterization of lead on mineral surfaces in soils contaminated by mining and smelting

Mine tailings and soils impacted by mining, milling and smelting of Cu-Pb-Zn-Ag-Au ores in a small mining town in central Colorado were analyzed by X-ray photoelectron spectroscopy (XPS), and surface analysis by laser ionization (SALI) utilizing stimulated desorption (pulsed Ar +) coupled with nonresonant photoionization (using 266 nm light) and time-of-flight mass spectrometry. These surface-sensitive techniques revealed that Pb is present on mineral grain surfaces in some soild and mine tailings. The presence of surface-bound Pb correlates with soil pH; carbonate-sulfide tailing with pH≈7 have Pb-bearing iron hydroxide (and to a lesser extent iron sulfate) coatings that are several μm thick, whereas sulfide tailings with pH≈2 have little or no detectable surface-bound Pb. Grains from residential soils (pH≈7) have thin (a few nm thick) Pb-bearing surface layers. Soils impacted by smelter aerosols and fly ash (pH = 4.5) also have thin (a few nm thick) Pb-bearing surfaces layers. Measured Pb4f 7/2 binding energies (corrected for charge shifting by referencing the Si2s peak to 153.0 eV) ranged from 137.4 to 138.2 eV and suggest that the surface-bound Pb is present as Pb-O or Pb-OH species.

The distributions of 239,240Pu, 238Pu, 241Am and 137Cs among chemically-defined components of sediments, settling particulates and net plankton of Lake Michigan

The application of selective extraction techniques to samples of consolidated sediments, surficial sediment floc and suspended particulate material collected in sediment traps from Lake Michigan shows that 239,240Pu, 238Pu and 241Am are removed primarily with the hydrous oxide coatings while 137Cs is strongly associated with the mineral fraction. Low concentrations of plutonium and americium are found associated with isolated humic acid fractions. The selective extraction of plutonium from net plankton samples results in the observation of a different distribution of plutonium among phases than that observed for sedimentary material, but, in all cases, the major fraction of the plutonium is found in the same extractant as with sediment samples. Attempts to displace 137Cs from the mineral fraction by several techniques indicate that the adsorption of 137Cs is not readily reversible.

Composition of suspended particulate matter from Scottish coastal waters—geochemical implications for the transport of trace metal contaminants

The concentrations (mg kg −1) of Fe, Mn, Zn, Pb, Cu and Cd have been determined in suspended coastal particulates. Enrichment factors (EF) have been determined with iron as the reference element to identify contaminated particulates. Decreases in EFs between the Irish Sea and west of Scotland are interpreted as being due to a combination of dilution and sedimentation of enriched particulates from the Irish Sea. Particulates from the Firth of Clyde are shown to be enriched in manganese. Trace metal enrichments within the Firth are closely correlated with manganese (but not iron), suggesting that manganese hydroxide coatings are important. The origin of such coatings and the wider implications to coastal geochemistry are discussed.

Effect of Ion-Pair Formation on Boron Adsorption by Kaolinite

AbstractBoron adsorption on 2-0.2-μm size fractions of kaolinite at 25° ± 2°C at pH values between 6.0 and 10.5 was studied. The kaolinite sample was pretreated to remove any surface oxide and hydroxide coatings. The initial concentrations of boron in solution ranged between 2 and 10 mg/liter with either KClO4 or Ca(ClO4)2 as background electrolyte at constant ionic strength of 0.09 ± 0.01 mole/liter. Boron speciation in equilibrium solutions was calculated by using the chemical equilibrium computer program GEOCHEM. The adsorption of boron on kaolinite in either medium showed similar dependence on pH and initial boron concentrations. Boron adsorption at higher pHs was noticeably higher in Ca(ClO4)2 medium as compared to KClO4 medium. These differences were attributed to the formation and adsorption of CaB(OH4)+ ion-pair species.

Adsorption and Desorption of Hexavalent Chromium in an Alluvial Aquifer Near Telluride, Colorado

AbstractA laboratory investigation of reactions between hexavalent chromium [Cr(VI)] and alluvium was conducted to evaluate reactions of Cr(VI) contaminating an alluvial aquifer near Telluride, CO and to determine the mechanisms responsible for these reactions. Uncontaminated alluvium and groundwater (spiked with CrO42−) from the study site were used in batch and column experiments. Results of these experiments show that Cr(VI) was adsorbed by the alluvium. Distribution coefficients from batch experiments ranged from 52 L/kg at an equilibrium CrO42− concentration of 0.4 µmol/L to 1.7 L/kg at an equilibrium concentration of 1400 µmol/L. The zero point of charge for the alluvium was approximately 8.3, and the alluvium had a positive net charge at the groundwater pH of 6.8. Visual and chemical evidence indicated that Fe oxide and hydroxide coatings on the alluvial particles principally were responsible for the adsorption of Cr(VI). During column experiments, Cr(VI) initially was desorbed easily from the alluvium by Cr‐free groundwater; however, the rate of desorption decreased rapidly, and > 60 pore volumes of groundwater were required to decrease the effluent concentration of Cr(VI) to 3 µmol/L [drinking water standard for Cr(VI) = 1 µmol/L]. The quantity of Cr(VI) adsorbed varied with the type and concentration of other anions in solution.

Adsorption of Molybdate Anion (MoO42−) by Sodium-Saturated Kaolinite

AbstractAdsorption of Mo(VI) on 2-0.2-μm size fraction of sodium-saturated kaolinite at 25 ± 2°C and at a constant pH of 7.00 ± 0.05 was studied. The kaolinite sample was pretreated to remove any surface oxide and hydroxide coatings. The initial concentrations of Mo in solution ranged from 1 to 11 mg/liter in a NaClO4 background electrolyte at a constant ionic strength of 0.09 ± 0.01. Calculations of speciation using the GEOCHEM computer program indicated that under experimental conditions Mo(VI) was mainly in the MoO42− form. The experimental conditions were also shown to fulfill the requirements for applying the Langmuir equation in interpreting adsorption data. The Langmuir parameter for the adsorption maximum, n°, and the affinity parameter, KMoO42−−ClO4− were computed to be 3.33 × 10−4 mole/ mole of adsorbent and 5.969 × 105, respectively. The large affinity parameter indicated that the Na-saturated kaolinite surface has a very high affinity for MoO42− ions relative to ClO4− ions.

Partitioning of heavy metals into mineral and organic fractions in sediments from a reservoir.

A sediment core was collected from S Reservoir, which is located in the western part of Okayama Prefecture. The vertical distribution of heavy metals (Fe, Mn, Zn, Ni, Cu and Cd) and fine particles (<37μm) was measured. The heavy metal contents of the sediment core are relatively high in small grain layers. Partitioning of heavy metals into NH2OH HCl soluble, H2O2 soluble and HF soluble fractions has been determined with a selective chemical leaching technique on the sediment samples. The greatest parts of Mn and Cd are contained in the NHOH HCl soluble fraction (mostly iron and manganese oxide and hydroxide coatings on silicate minerals), whereas more than 50% of Fe, Zn, Ni and Cu is present in the HF soluble fraction (silicate lattice). The variations of heavy metal contents in the sediment core are chiefly due to those in NH2OH HCl soluble and HF soluble fractions. Only manganese is significantly enriched in the NH2OH HCl soluble fraction of the settling material collected by the sediment trap near the bottom, probably due to the release of manganese from bottom sediment into overlying water. Manganese oxide and hydroxide reprecipitated under oxic conditions of the bottom water after the release might have been effectively collected by the sediment trap.

Distribution and relationship of radionuclides to streambed gravels in a small watershed

In White Oak Creek watershed in eastern Tennessee, the radionuclides60Co,90Sr, and137Cs were retained by streambed gravels on different sites:60Co was associated with manganese in the hydrous oxide coatings on rocks and minerals;90Sr occurred primarily as an exchangeable cation although small amounts were held in a nonexchangeable form; and137Cs was held very selectively, presumably by illite in shale fragments composing the sediment. The distribution of radionuclides on sediments was size dependent: the 0.85–3.35 mm fine-gravel fraction was higher in radionuclide concentration than the sand fraction. An areal survey of radionuclide concentrations on streambed gravels from throughout the watershed, located numerous contamination sources. These radionuclide concentrations, when combined with both distribution coefficients of radionuclides between gravel and water and drainage area, were used to rank subsections of the watershed by their relative contribution to the overall contamination of the watershed. For90Sr, this ranking procedure agreed with the measured discharges of subsections of the watershed which are routinely monitored.

Adsorption of Cu, Zn and Pb near sulfide deposits by hydrous manganese—Iron oxide coatings on stream alluvium

Hydrous oxides of Mn and Fe scavenge several base metals in the surficial environment by adsorption and coprecipitation. Results of laboratory investigations suggest increased adsorption compared to coprecipitation where higher concentrations of metal ions exist in solution, such as generally occur near an oxidizing mineral deposit. Stream cobbles with hydrous oxide coatings, collected from the Valley and Ridge Province in northwestern Virginia, were immersed in prepared aqueous solutions of CuZnPb for 50 hr. Substantial quantities of each metal were absorbed from solution. Considerably greater proportions of the newly adsorbed metal could be extracted by ammonium citrate than background concentrations of metal initially present in the coatings. Analysis of hydrous oxide coatings collected at varying distances upstream and downstream from a small CuZnPb-sulfide deposit produced similar results. Following the same analytical procedure, considerably greater proportions of the total Cu and Zn were extracted by ammonium citrate from samples near the mine than from those upstream or at a greater distance downstream. If adsorption of metal by hydrous oxides is generally more important compared to coprecipitation where higher concentrations of metal ions are available in solution, the relation might be of practical value. The sulfide deposit is clearly delineated if Cu and Zn extracted by ammonium citrate is expressed as a percent of total Cu or Zn. Previous studies have shown that similar deposits are not clearly delineated by analysis of hydrous oxides unless ratios of metal to Mn or Fe are calculated.

Flocculation Reduces Cation Exchange Capacity of Suspended Estuarine Sediment: ABSTRACT

The variation in the cation exchange capacity (CEC) of suspended sediment entering the Delaware Bay was determined as a function of salinity. Cation exchange capacity is based on Ca, Mg, Na, and K, the four commonest exchangeable ions. A comparison of the CEC (units in meq/100 g) of suspended sediment with salinity shows a sharp initial decrease from 73 meq/100 g at 0% salinity to a minimum value of 29 meq/100 g at 2.44% salinity, then a gradual rise to 48 meq/100g at 15.03% salinity. The observed trends between CEC and salinity were correlated with sampling conditions (depth, temperature, pH), mineralogy, iron III hydroxide and organic coatings, and the degree of particle flocculation. A very good (> .90) linear correlation exists between the degree of flocculation, e pressed as the proportion of primary (individual, < 2µ) particles, and the CEC's of the overall suspended sediment. The other variables do not show any significant correlation with the CEC-salinity trend. It is evident, therefore, that the process of flocculation leads to a decrease in CEC values. It is speculated that the process of flocculation is a function of the ionic strength of the water. The attraction or repulsion of suspended particles is a surface-charge phenomenon which is controlled by the ionic strength of the water. End_of_Article - Last_Page 1013------------

Transport phases of transition metals in the Amazon and Yukon Rivers

Samples representing yearly averages of material transported by the Amazon and Yukon Rivers were analyzed to separate the transition metals (Cr, Mn, Fe, Co, Ni, and Cu) into the following transport phases: (1) crystalline particles, (2) metal hydroxide coatings, (3) solid organic material, (4) sorbed material, and (5) those in solution. The major transport phases are crystalline particles and metal hydroxide coatings, which, combined, carry 65% to 92% of the transition metals transported. Solid organic material, the next most important phase, transports between 5% and 19% of the total transported. Material carried in solution transports 0.6% to 17% of the total transported. Sorbed transition metals account for between 0.02% and 8% of the total transported. Metal hydroxide coatings represent the major transporting mechanism potentially available to organisms, since, for the Amazon and Yukon Rivers, respectively, 87% and 78% of the Fe, 69% and 73% of the Mn, and 71% and 69% of the Ni are transported in this form. Comparing the concentrations of transition metals carried to the oceans with the concentrations on the continents, a high Cu ratio (5 to 7) indicates continental depletion or river output enrichment; a moderate ratio (1.1 to 1.7) for Ni, Co, and Cr indicates intermediate depletion or enrichment, and a near-unity ratio for Fe and Mn indicates little depletion or enrichment. The sediments transport >97% of the total mass of transition metals to the world9s oceans.

Interstitial Silica in Deep-Sea Sediments from the North Pacific

Dissolved silica concentrations in pore waters from North Pacific deep-sea sediments fall into two groups. Oxidized deposits contain 22 to 35 ppm, whereas reduced sediments contain 10 to 19 ppm. All pore waters were extracted at in situ temperatures, and all sediments contain opaline silica in the form of diatom frustules and radiolarian tests. At 2°C, it appears that the pore waters are grossly undersaturated with respect to sepiolite and undersaturated relative to amorphous silica, but are slightly supersaturated relative to talc. The reduced deposits appear nearest to equilibrium with talc, suggesting that reactions in the oxidized sediments may be inhibited by “armoring” of particles by ferric hydroxide coatings.