High-field FT-ICR mass spectrometry and NMR spectroscopy to characterize DOM removal through a nanofiltration pilot plant.

Ultrahigh resolution Fourier transform ion cyclotron mass spectrometry and nuclear magnetic resonance spectroscopy were combined to evaluate the molecular changes of dissolved organic matter (DOM) through an ultrafiltration-nanofiltration (UF-NF) pilot plant, using two dissimilar NF membranes tested in parallel. The sampling was performed on seven key locations within the pilot plant: pretreated water, UF effluent, UF effluent after addition of reagents, permeate NF 1, permeate NF 2, brine NF 1 and brine NF 2, during two sampling campaigns. The study showed that there is no significant change in the nature of DOM at molecular level, when the water was treated with UF and/or with the addition of sodium metabisulfite and antiscaling agents. However, enormous decrease of DOM concentration was observed when the water was treated on the NF membranes. The NF process preferentially removed compounds with higher oxygen and nitrogen content (more hydrophilic compounds), whereas molecules with longer pure aliphatic chains and less content of oxygen were the ones capable of passing through the membranes. Moreover, slight molecular selectivity between the two NF membranes was also observed.

Review of the dielectric properties of nanofiltration membranes and verification of the single oriented layer approximation.

The structuring of water at soft solid surfaces remains an area of great interest to colloid science as a whole and has many applications in relation to colloid stability, foams, and wetting films as well as being central to membrane separations. Quantitatively calculating the structural components of thin layers of water and the interaction forces of hydrated molecules with the surface of pores through a layer of water having modified structure is one of the most important challenges in the physics of surface phenomenon. In this paper these effects are reviewed and discussed in relation to the confines of a capillary pore. Membrane nanofiltration is extremely complex and is dependent on the micro-hydrodynamics and interfacial events occurring at the membrane surface and within the membrane nanopores. There is significant debate as to the exact nature of these complex phenomena and rejection is typically attributed to a combination of steric and electrical effects. The electrical effects are less well understood and in particular the contribution of dielectric exclusion. A review of the two competing descriptions of dielectric exclusion is presented along with the theories currently used in modelling this phenomena. A series of rejection experiments of 0.01 M salt solutions at the membrane isoelectric point has been performed for the NF270 and NF99HF membranes. The dielectric constants inside the nanopore are calculated and these values were consistent for three of the salts studied, indicating that a simplistic model based on Born theory is accurate enough for engineering calculations and that ion solvation is most likely to be the more appropriate dielectric exclusion mechanism for true nanofiltration membranes.

Chromium recovery from exhausted baths generated in plating processes and its reuse in the tanning industry.

Chromium plating used for functional purposes provides an extremely hard, wear and corrosion resistant layer by means of electrolytic deposition. Typical layer thicknesses range between 2.5 and 500 µm. Chromium electroplating baths contain high concentrations of Cr(VI) with chromium trioxide (CrO(3)) as the chromium source. When because of technical or economic reasons a bath gets exhausted, a waste containing mainly chromium as dichromate as well as other heavy metals is generated. Chromium may then be purified for use in other industrial processes with different requirements. In this work, a sustainable system for using galvanic wastes as reagents in the leather tanning industry, thus reducing quantity of wastes to be treated, is presented. Metal cations present in the chromium exhausted bath were precipitated with NaOH. Then, the solution containing mainly soluble Cr(VI) was separated. By means of sodium sulphite in acidic conditions, Cr(VI) was reduced to Cr(III) as chromium (III) sulphate. From chromium (III) sulphate a basic Cr(III) sulphate may be obtained, which is one of most used compounds in the tanning industry. Cr(III) concentration in the final solution allows its reuse without concentration, but with a slight dilution.

Semi-dynamic leaching tests of nickel containing wastes stabilized/solidified with magnesium potassium phosphate cements.

Herein is presented a study on the long-term leaching behaviour of nickel containing wastes stabilized/solidified with magnesium potassium phosphate cements. Two different semi-dynamic leaching tests were carried out on monolithic materials: ANS 16.1 test with liquid-to-solid ratio (L/S) of 10 dm(3) kg(-1) and increasing renewal times, and ASTM C1308 test with liquid-to-solid ratio (L/S) of 100 dm(3) kg(-1) and constant renewal time of 1 day. ASTM C1308 provides a lower degree of saturation of the leachant with respect to the leached material. The effectiveness of magnesium potassium phosphate cements for the inertization of nickel was proved. XRD analyses showed the presence of bobierrite on the monolith's surface after the leaching test, which had not been detected prior to the leaching test. In addition, the calculated cumulative release of the main components of the stabilization matrix (Mg(2+), total P and K(+)) was represented versus time in logarithmic scale and it was determined if the leaching mechanism corresponds to diffusion. Potassium is released by diffusion, while total phosphorous and magnesium show dissolution. Magnesium release in ANS 16.1 is slowed down because of saturation of the leachant. Experimental results demonstrate the importance of L/S ratio and renewal times in semi-dynamic leaching tests.

Sorption of strontium on uranyl peroxide: implications for a high-level nuclear waste repository.

Strontium-90 is considered the most important radioactive isotope in the environment and one of the most frequently occurring radionuclides in groundwaters at nuclear facilities. The uranyl peroxide studtite (UO2O2 . 4H2O) has been observed to be formed in spent nuclear fuel leaching experiments and seems to have a relatively high sorption capacity for some radionuclides. In this work, the sorption of strontium onto studtite is studied as a function of time, strontium concentration in solution and pH. The main results obtained are (a) sorption is relatively fast although slower than for cesium; (b) strontium seems to be sorbed via a monolayer coverage of the studtite surface, (c) sorption has a strong dependence on ionic strength, is negligible at acidic pH, and increases at neutral to alkaline pH (almost 100% of the strontium in solution is sorbed above pH 10). These results point to uranium secondary solid phase formation on the spent nuclear fuel as an important mechanism for strontium retention in a high-level nuclear waste repository (HLNW).

Reactive transport of arsenic(III) and arsenic(V) on natural hematite: experimental and modeling.

Natural hematite was used for the removal of arsenic(III) and arsenic(V) from aqueous solution. The experimental breakthrough curves were obtained in fixed-bed columns. The transport of arsenic in a simplified fixed-bed configuration was quantified by using the CXTFIT code, which was used to estimate the transport and sorption parameters of the convective-dispersive equation (CDE) and the two-site deterministic nonequilibrium (TSM/CDE) model by fitting the models to the experimental breakthrough curves (BTC). The prediction of the breakthrough curves performed by the two-site nonequilibrium sorption model resulted in a good fit, indicating that this model can properly describe the transport and sorption processes of arsenic on natural hematite. Additionally the parameters obtained indicate that nonequilibrium sorption governs the As(III) and As(V) uptake onto hematite in a fixed-bed column. No significant differences in the transport and sorption parameters of As(III) and As(V) on natural hematite were obtained; the retardation factor values were in the same order of magnitude for both species.

Leaching behaviour of magnesium phosphate cements containing high quantities of heavy metals.

Stabilization/solidification (S/S) is one of the most employed techniques for treating liquid wastes containing heavy metals. Magnesium potassium phosphate cements have been used in recent years as stabilizing agents with positive results. However, little information is available about the retention of the metals by magnesium potassium phosphate cements matrices with the exception of the results of compliance tests. In this work, several pastes were prepared by reaction between low grade MgO and KH(2)PO(4) in the presence of different heavy metal nitrate solutions (containing Cd(II), Cr(III), Cu(II), Ni(II), Pb(II) or Zn(II)). In all cases, the initial metal content of the dissolution was 25 g dm(-3) and the oxide-phosphate ratio of the pastes was (50:50) in weight. Four different leaching tests were conducted on magnesium potassium phosphate cement pastes: simple batch test (EN 12457-2), equilibrium leaching test, availability test (NEN 7371) and acid neutralization capacity test (ANC). The metal leachate concentration was determined by means of ICP-MS. The stabilization of metals was successful in all cases, although the immobilizing system shows a better behaviour for Pb(II) and Cr(III) under acidic or neutral conditions.

Sorption of Th(IV) onto iron corrosion products: EXAFS study.

Long-term performance assessment of nuclear waste repositories is affected by the ability of the outer barrier systems to retain radionuclides after possible corrosive leakage of waste containers. The mobility of the radionuclides released from the spent fuel depends strongly on the processes that take place in the backfill material. The interaction of steel corrosion products and radionuclides is part of such a scenario. In this work, the sorption of Th(IV) onto 2-line-ferrihydrite (FeOOH x H2O) and magnetite (Fe3O4), used as models for steel corrosion products, has been studied using EXAFS spectroscopy. Sorption samples were prepared in 0.1 M NaClO4 solutions at acidic pH (initial pH values in the range 3.0-4.2) either from undersaturation and supersaturation conditions with respect to amorphous ThO2. Two oxygen subshells, one at 2.37 A and another at 2.54 A, were observed in the first hydration sphere of Th in the case of the ferrihydrite samples. Th-Fe distances for the different ferrihydrite samples are approximately 3.60 A. These results indicate a corner sharing surface complex of Th(IV) ion onto the ferrihydrite surface where the Th atom shares one O atom with each of two coordinated octahedra. The longer Th-O distance accounts for coordinated water molecules. No significant changes in the structural environment of Th in terms of coordination numbers and distances were detected as a function of Th(IV) concentration. Magnetite samples sorbing Th(IV) also showed also a strong distortion of the O shell, but in contrast to ferrihydrite, two types of nearest Fe atoms were detected at 3.50 A and 3.70 A. These results indicate that Th(IV) ion sorbs onto the magnetite surface as bidentate-corner sharing arrangements to [FeO6] octahedra and [FeO4] tetrahedra.

Effect of heavy metals and water content on the strength of magnesium phosphate cements.

In this paper the mechanical properties of magnesium potassium phosphate cements used for the Stabilization/Solidification (S/S) of galvanic wastes were investigated. Surrogate wastes (metal nitrate dissolutions) were employed containing Cd, Cr(III), Cu, Ni, Pb or Zn at a concentration of 25 g dm(-3) and different water-to-solid (W/S) ratios (0.3, 0.4, 0.5 and 0.6 dm(3)kg(-1)) have been employed. Cements were prepared by mixing hard burned magnesia of about 70% purity with potassium dihydrogen phosphate. Compressive strength and tensile strength of specimens were determined. In addition the volume of permeable voids was measured. It was found that when comparing pastes that the volume of permeable voids increases and mechanical strength decreases with the increase of water-to-solid ratio (W/S). Nevertheless pastes with the same material proportions containing different metals show different mechanical strength values. The hydration products were analyzed by XRD. With the increase of water content not previously reported hydration compound was detected: bobierrite.

Sorption of selenium(IV) and selenium(VI) onto natural iron oxides: goethite and hematite.

Selenium is a toxic element with a relatively high mobility in the natural waters. Iron oxy-hydroxides might play an important role in the migration of this element as well as on its removal from contaminated water. In this work we study the interaction of Se(IV), and Se(VI) with natural iron oxides hematite and goethite through two series of batch experiments at room temperature. In the first series, sorption as a function of initial selenium concentration is studied and the results have been fitted with Langmuir isotherms. In a second series of experiments, sorption is studied as a function of pH, being the main trend an increase of the sorption at acidic pH. The variation of the sorption with pH has been modelled with a triple layer surface complexation model and using the FITEQL program. The experimental data have been modelled considering for the Se(IV) the formation of the FeOSe(O)O(-) complex onto the hematite surface, and a mixture of FeOSe(O)O(-), and FeOSe(O)OH onto the goethite surface. For Se(VI) the surface complex considered is FeOH(2)(+)-SeO(4)(2-) on both goethite and hematite.

Interaction of uranium with in situ anoxically generated magnetite on steel.

In the high level nuclear waste repository concept, spent nuclear fuel is designed to be encapsulated in steel canisters. Thus, it is necessary to study the influence of the steel and/or its corrosion products on the behaviour of the radionuclides released from the fuel. In this sense, the main objective of this work is to contribute to the knowledge of the influence of the steel and/or its corrosion products on the uranium(VI) retention. To this aim, magnetite (Fe(3)O(4)) has been generated by anaerobic steel corrosion in an autoclave reactor at an overpressure of 8atm of H(2)(g). After characterisation by X-ray diffraction (XRD), the obtained corroded steel coupons were contacted, at two different H(2)(g) pressures (1atm and 7.6atm), with a U(VI) solution. The evolution of the uranium concentration in solution is determined and a study of the composition of the coupons at the end of the experiments is carried out. The main conclusion obtained from this work is that magnetite generated on a steel coupon is able not only to retain uranium via sorption, but also to reduce hexavalent to tetravalent uranium in a higher extent than commercial magnetite, thus, providing an effective retardation path to the migration of uranium (and, potentially, other actinides) out of the repository.

Arsenic sorption onto natural hematite, magnetite, and goethite.

In this work the sorption of As(III) and As(V) on different natural iron oxides (hematite, magnetite, and goethite) has been studied as a function of different parameters. The sorption kinetics for the three iron oxides shows that equilibrium is reached in less than 2 days and the kinetics of sorption seems to be faster for goethite and magnetite than for hematite. The variation of the arsenic sorbed on the three different sorbents as a function of the equilibrium arsenic concentration in solution has been fitted with a non-competitive Langmuir isotherm. The main trend observed in the variation of the arsenic sorbed with pH is the decrease of the sorption on the three sorbents at alkaline pH values, which agrees with results found in the literature. Highest As(III) sorption was observed on hematite surface in all the pH range compared to goethite and magnetite. Natural minerals studied in this work had similar sorption capacities for arsenic than synthetic sorbents.