Leaching Data Research Articles

Comparison of Przm and Gleams computer model predictions with field data for alachlor, metribuzin and norflurazon leaching

The ability of the pesticide root zone model (PRZM) and groundwater loading effects of agricultural management systems (GLEAMS) model to simulate movement of three herbicides was evaluated with site-specific soil, environmental, and pesticide data. Field studies examined norflurazon movement in a Dothan loamy sand in which cotton (Gossypium hirsutum L.) was grown, and alachlor and metribuzin movement in an Appling sandy loam. Actual herbicide concentrations were always greatest near the soil surface. The PRZM and GLEAMS computer simulation models accurately predicted the total herbicide present in each profile less than 20 d after application. The models overestimated herbicide movement into the soil profile in later simulations. The model predictions generally agreed with the relative location of alachlor and metribuzin in simulations less than 7 d after herbicide applications, but simulations beyond 7 d after herbicide application deviated from actual concentrations. PRZM accurately predicted that most of the applied norflurazon would remain close to the soil surface 84 d after herbicide application, whereas GLEAMS inaccurately predicted that norflurazon would be located throughout the soil profile, although the predicted depth to the limit of detection by both models was accurate.

Durability of High‐level Waste Glass in Groundwater

This paper reviews the leaching behavior of simulated high‐level‐waste glass in Japanese groundwater both on an in situ burial site and in our laboratory. During in situ burial leach tests, the glass is immersed in groundwater for up to 1 year and 7 months, retrieved, and analyzed. A leach rate obtained from the in situ burial tests is compared with other leach data under different conditions. Two factors, i.e., flow rate and composition of the groundwater, are extensively studied in the laboratory to supplement the in situ burial data. Effects of γ irradiation on the groundwater chemistry are also examined. Based on the results, mechanisms observed with the leaching of the glass in groundwater are discussed.

An analysis of the leaching of chloride tracer applied to pipe‐drained plots using a coupled unsaturated‐saturated zone model of solute transport

Abstract. The modelling of leaching of nitrate and other solutes in artificially drained soils is complicated by the need to consider both unsaturated and saturated components of the system. This work is an attempt to couple together an unsaturated zone transport model (Addiscott & Whitmore, 1991) with a steady state travel‐ time model (Ernst, 1973) for the saturated zone. The model was tested using chloride leaching data from eight hydrologically isolated plots on a pipe‐drained clay loam soil. Approaches to parameter estimation are discussed. Results were variable, for on some plots saturated zone travel time could be virtually neglected, whereas on other plots this component was apparently important.

Combustion and leaching behavior of elements in the Argonne Premium Coal Samples

Eight Argonne Premium Coal samples and two other coal samples were used to observe the effects of combustion and leaching on 30 elements. The results were used to infer the modes of occurrence of these elements. Instrumental neutron activation analysis indicates that the effects of combustion and leaching on many elements varied markedly among the samples. As much as 90% of the selenium and bromine is volatilized from the bituminous coal samples, but substantially less is volatilized from the low-rank coals. We interpret the combustion and leaching behavior of these elements to indicate that they are associated with the organic fraction. Sodium, although nonvolatile, is ion-exchangeable in most samples, particularly in the low-rank coal samples where it is likely to be associated with the organic constituents. Potassium is primarily in an ion-exchangeable form in the Wyodak coal but is in HF-soluble phases (probably silicates) in most other samples. Cesium is in an unidentified HNO 3 -soluble phase in most samples. Virtually all the strontium and barium in the low-rank coal samples is removed by NH 4 OAc followed by HCl, indicating that these elements probably occur in both organic and inorganic phases. Most tungsten and tantalum are in insoluble phases, perhaps as oxides or in organic association. Hafnium is generally insoluble, but as much as 65% is HF soluble, perhaps due to the presence of very fine grained or metamict zircon. We interpret the leaching behavior of uranium to indicate its occurrence in chelates and its association with silicates and with zircon. Most of the rare-earth elements (REE) and thorium appear to be associated with phosphates. Differences in textural relationships may account for some of the differences in leaching behavior of the REE among samples. Zinc occurs predominantly in sphalerite. Either the remaining elements occur in several different modes of occurrence (scandium, iron), or the leaching data are equivocal (arsenic, antimon

Improved expressions for modeling diffusive, fractional cumulative leaching from finite-size waste forms

The classical expressions for modeling diffusive, fractional cumulative leach data from finite-size waste forms are in the form of open series of slow convergence and difficult error control. Alternatively, closed form expressions are obtained which are entirely analytical. Simple analytical formulae for the error associated with the proposed expressions are also provided. The new expressions for reporting fractional cumulative leaching versus time are easy to implement numerically and are shown to be adequate for the modeling of typical laboratory leach data from cementitious waste forms.

A Description of the Kinetics and Mechanisms which Control the Release of Hlw Elements from Synroc

ABSTRACTLeaching tests were carried out on well-characterised Synroc C specimens containing 10 wt% simulated HLW. The effect of temperature and leachant composition on the mechanisms and kinetics of leaching were investigated by leaching Synroc specimens in deionised water at 70°C and 150°C, and in silicate and bicarbonate solutions at 70°C. SEM and TEM were used before and after leaching to characterise the primary and secondary phases in and on the Synroc samples, and to describe the morphological and chemical changes which occurred. Leachants were analysed using ICP/optical emission spectroscopy and ICP/mass spectrometry.Titaniferous surface layers were observed on the surface after exposure to leachants. Analysis of the leaching data has established that these surface layers can reduce the release of HLW elements from Synroc and that variations in temperature and leachant composition can hinder or promote the formation of secondary phases.

Transport of Exchangeable Cations in an Aggregated Clay Soil

AbstractThe objective of this study was to assess the applicability of the classical and mobile‐immobile transport models for describing cation transport in soils. The transport of exchangeable cations for binary and ternary systems in an aggregated clay soil using miscible displacement methods was examined. Packed columns of aggregates of Sharkey soil (Vertic Haplaquept) were saturated with Ca. Pulses of Mg or Mg plus Na were applied to these columns, then eluted with Ca. Chloride was the accompanying anion. These leaching experiments were conducted at constant and variable total solution concentrations. Two pore‐water velocities were used in these experiments. Effluent was collected in fractions and analyzed for concentration of cations. Exchange isotherms were determined for the cation pairs Ca‐Mg, Ca‐Na, and Mg‐Na. Experimental leaching data were compared with predicted breakthrough curves obtained using the classical (or one‐region) and mobile/immobile water (or two‐region) transport models. The two‐region model offered no improvement over predictions obtained using the one‐region model. The locations of all breakthrough peaks were well described by the models; however, peak heights for Mg breakthroughs were consistently over‐estimated. Extension of the model to include the occurrence of CaCl+ and MgCl+ and adsorption of these complexes did not improve model predictions.

Bacterial oxidation of an arsenical gold sulphide concentrate from Olympias, Greece

The application of bacterial oxidation as a pretreatment step for the extraction of gold from arsenical gold sulphide concentrates offers potentially significant economic advantages over oxidative pretreatment alternatives. In this study the bacterial oxidation of an extremely refractory gold sulphide concentrate, from Olympias, Greece, is examined. Leaching tests were conducted in air-stirred pachuca reactors in order to determine the effect of pulp density on the degree of pyrite and arsenopyrite oxidation. Cyanidation tests were conducted in order to determine the degree of gold and silver liberation in relation to the degree of each sulphide phase oxidised and to the cyanide consumed. Mathematical analysis of the leaching data allowed the gold distribution in each sulphide mineral phase to be estimated and the gold recoveries according to the degree of each mineral oxidised predicted.

A generalized model for leaching of chlorine from Illinois coal and the effect of particle size

Experimental leaching data are presented for an Illinois No. 6 coal at four temperature levels and different particle sizes. A generalized model based on diffusion and relaxation has been fitted to Sample C8601 of Illinois No. 6 coal with a wt% of chlorine of 0.42. The experimental data on this sample covered a temperature range of 297–370 K and a particle size range of − 20 to − 325 mesh. Based on the type of coal and the conditions of leaching, it was found that 40–80% of the original chlorine could be leached from the coal matrix. The model based on diffusion-relaxation concept predicted the leaching data within 5% average absolute deviation (AAD). The diffusion rate constants at different temperatures were correlated to Arrhenius type relations. Attempts made to correlate the constants in the Arrhenius equations with the chlorine content in coal and particle size are discussed. The water leaching data were used to calculate Fickian diffusivities based on the time required for 50 per cent desorption. The calculated diffusivity values ranged from 0.6 to 3 × 10 − 11 cm 2s − 1.

Artifacts associated with the chemical leaching of sediments for rare-earth elements

Using radioactive Eu as a tracer for rare-earth elements (REE), simple experiments were carried out to determine if an re-adsorption artifact occurs during the chemical leaching of marine sediments. Fourteen commonly used chemical leaching solutions were used with and without (controls) the presence of Buzzards Bay (Massachusetts, U.S.A.) sediment. All controls show 100% recovery of soluble 152Eu, indicating no adsorption or precipitation during handling. In contrast, almost all chemical leaching solutions show poor recoveries even at low (2–4) pH. Even for HCl and HNO 3, recoveries ranged from 25% to 85% at ≤ 0.2 M. Hydroxylamine hydrochloride in acetic acid, for example, shows only 33% and 57% recoveries for 0.02 M and 0.04 M concentrations, respectively. The results show that large-scale re-adsorption of REE onto sediments occurs even at low pH associated with mineral acids, organic chelators and reductive solutions. Given their large hydrolysis constants, a case is made that one would expect that REE, like Pb and Th, to exhibit significant re-adsorption onto sediments following their dissolution from specific phases. This result reinforces concerns in the literature that chemical leaching techniques have major artifacts associated with non-selectivity and re-adsorption of trace metals. Interpretations of REE diagenesis and authigenesis, based on chemical leaching data, should be viewed with caution.

Grouted Waste Leach Tests: Pursuit of Mechanisms and Data for Long-Term Performance Assessment

ABSTRACTAt Hanford, low-level liquid nuclear waste is being mixed with cementitious materials (grout) to form leach-resistant solid waste. Prior to grouting each liquid waste, an assessment must be performed to evaluate the long-term environmental impact. These predictions rely upon a diffusioncontrolled release model and short-term laboratory leach data on small grout samples. This paper describes size scale-up and inventory scale-up experiments that evaluate whether diffusion does in fact control the release of contaminants. The results of the volume scale-up test suggest that tests on grout cylinders between the sizes 3 cm dia. by 3 cm length and 30 cm dia. by 29 cm length yield comparable results. These data and other available literature suggest that extrapolation of leach results to large blocks of solidified waste is defensible.The inventory scale-up tests for 125I and selenium show a constant effective diffusivity regardless of the original inventory. This suggests that diffusion processes do in fact control the release of iodide and selenium from grout. The 99Tc effective diffusivities may not remain constant with changes in inventory.

Temporal variations in the trace element content of stream sediments: examples from Zimbabwe

Two stream sediment geochemical anomalies in N.E. Zimbabwe have been monitored over a period of two years. The results show that although strong temporal variations in the trace element content of the sediment occurred these variations were not strictly seasonal in nature but were probably related to the cessation of a period of drought extending over a number of wet-dry season cycles. The pattern of variations was not the same at each anomaly. At one anomaly an initial fall in Zn levels after one wet season was followed by a recovery during the subsequent dry season and only a small reduction over the next rains. No anomaly was seen in the cold dilute acid leach data here. At the second site a gradual reduction of more than 100 ppm in Pb concentration took place over six collections, this reduction being evident in both XRF and cold dilute acid leach AAS data. Changes in concentration in other elements allow the variations to be related to the mineralogical composition of the sediments.

Leaching of cobalt and molybdenum from a Co-Mo/γ-Al2O3 hydrodesulphurization catalyst waste with aqueous solutions of sulphur dioxide

Cobalt and molybdenum can be easily extracted from the Co-Mo/γ-Al2O3 waste hydrodesulphurization (HDS) catalyst using aqueous solutions of SO2. Both Co and Mo readily dissolve, almost completely if the waste catalyst is calcinated at 923 K for 6 h prior to dissolution. The dissolution is very rapid at ambient temperature and it is completed in only 8 min.The rate data for molybdenum leaching fits the equation: 1 − (2α3) − (1 − α)23 = kt.Diffusion through the alumina layer controls the rate.The leaching of cobalt is slower than that of molybdenum and the experimental data fits the rate equation: 1 − (2α3) − (1 − α)23 + (βro) [1 − {1 − α}13] = ktThe data indicates that the dissolution rate of cobalt is governed by a mixed kinetic control; i.e. diffusion through the alumina layer plus reaction at the interface are rate determining factors.

Leaching of Cesium from a Cement Matrix

ABSTRACTThe leaching rate of cesium has been studied in cement. The solidification matrix was a standard Portland cement with increased sulphate resisting properties mixed with 300 kg/m3 or 350 kg/m3 powdered ion exchange resins, and with or without 5 weight-% zeolite added to the grout. The leaching rates from an ordinary cement matrix and a cement matrix mixed with zeolite were measured to be (1.3 to 2.6)×10–7 kg/m2s and (1.6 to 2.1)×10−8 kg/m2s, respectively. From the leaching data the apparent diffusivity of cesium in an ordinary cement matrix with a waste load of 300–350 kg/m3 powdered ion exchange resins was measured to be (0.6 to 1.3)×10−10 m2/s, and in a cement matrix mixed with 5 weight-% zeolite and the same waste load, the diffusivity was measured to be (0.8 to 1.0)×10−12m2/s.

Fluid inclusion studies in the polymetallic ores of Gyöngyösoroszi (north Hungary) — Spatial and temporal evolution of ore-forming fluids

The PbZnCu-bearing vein-like poly-ascendent occurrence of Gyöngyösoroszi is related to Neogene volcanic formations. The characteristic curved veins occur in the so-called “middle varying andesite” sequence accompanied by typical rock alterations (potash metasomatism, silicification, clay mineralization). Fluid inclusion studies were carried out on quartz, barite, sphalerite and calcite. The identified inclusion types are as follows: (1) vapor (low-density vapor); (2) monophase liquid (high-density liquid); (3) two-phase (l + v) hydrous inclusions; (4) three-phase (l + v + s) inclusions; (5) hydrocarbon-bearing (often oil-rich) inclusions. Based on the homogenization measurement data the occurrence was formed under epi-mesothermal conditions, concordant with the mineral paragenesis and textural-structural features of the locality. Based on the aqueous leaching data and the cryoscopic measurements two types of fluid can be identified: 1. (a) the alkali-bicarbonate type in the veinlets with druses of the wall-rock in the early phase of mineralization; 2. (b) NaCaCl type in the main (vein forming) phase of mineralization. Fluids are often accompanied by CO 2 in minor quantities being of heterogeneous distribution and occasionally by hydrocarbon derivates. The salt concentration, density and vapor pressure data of the ore-forming fluids are characteristic of the young volcanic and subvolcanic vein-like mineralizations. By means of the dynamic interpretation of the physico-chemical data the spatial and temporal evolution of fluids can be followed. The spatial evolution is reflected by isotherm maps constructed from the fluid data and by gradient determinations, the temporal evolution can be followed by the fluid control of the mineral paragenesis.

The Role Of Chemical Reaction In Waste-Form Performance

AbstractThe dissolution rate of waste solids in a geologic repository is a complex function of waste form geometry, chemical reaction rate, exterior flow field, and chemical environment. We present here an analysis to determine the steady-state mass transfer rate, over the entire range of flow conditions relevant to geologic disposal of nuclear waste. The equations for steady-state mass transfer with a chemical-reaction-rate boundary condition are solved by three different mathematical techniques which supplement each other. This theory is illustrated with laboratory leach data for borosilicate-glass and a spherical spent-fuel waste form under typical repository conditions. For borosilicate glass waste in the temperature range of 57°C to 250°C, dissolution rate in a repository is determined for a wide range of chemical reaction rates and for Peclet numbers from zero to well over 100, far beyond any Peclet values expected in a repository. Spent-fuel dissolution in a repository is also investigated, based on the limited leach data now available.

Spent UO2 Fuel Corrosion in Water; Release Mechanisms

AbstractAn update of the status of the joint SKB/Studsvik fuel leaching programme is given. High burnup BWR fuel has been leached for up to 5.5 years cumulated contact time. Data for leaching of PWR fuel are also presented. The present results confirm earlier results from the programme, i.e., in groundwater and deionized water under oxidizing conditions uranium rapidly attains saturation at about 1 mg/l and 10 μg/l, respectively.Under reducing conditions, the uranium saturation levels are below 1μg/l. The plutonium saturation level under oxidizing conditions is at about 0.3 μg/l. Under reducing conditions, the plutonium concentrations in solution appear to be controlled by the solubility of the fuel matrix.Most of the detected fission products appear to be released through selective leaching of fuel/cladding gap, fuel cracks and fissures, and grain boundaries.

Intergranular Films and Pore Surfaces in Synroc C

AbstractHigh resolution electron microscopy and scanning electron microscopy were used to determine the distribution of intergranular films and microvoids in Synroc C. Diffraction contrast derived from these films, which were 1–3 nm thick, showed then to be ill-defined crystallographically, and they may be described as glassy. Pores were usually several micrometers in extent and occurred principally in rutile-rich areas. The chemical composition of these structural features was obtained using analytical transmission electron microscopy, secondary ion mass spectrometry, x-ray photoelectron spectroscopy and scanning Auger microscopy. Within intergranular films, elemental enhancement of cesium, sodium, potassium and aluminium, and possibly silicon and molybdenum was observed. Enhancement of cesium, sodium, phosphorous, aluminium and silicon was found at triple points. Fracture faces preferentially expose grain boundaries, and dissolution of these surfaces proceeds rapidly at ambient temperatures. During the fabrication of Synroc C, microvoids trap cesium vapour, and after cooling this condenses onto pore surfaces. Recognition of the (simulated) waste species which segregate at grain boundaries and pores, permitted the reinterpretation of published leach data for monolithic and crushed Synroc C.

A stochastic approach to models for the leaching of organic chemicals in soil

The movement of chemicals in the soil media is modelled by a stochastic diffusion process with constant infinitesimal drift and variance (i.e. a Brownian motion with drift). The first hitting time distribution of this diffusion process is used to give a statistical estimation of the parameters of the Brownian motion. Statistical tests for comparing leaching data from several soil columns are given.

Sodium Diffusion and Leaching of Simulated Nuclear Waste Glass

Oxides such as CaO, ZnO, Fe20O3, and Zr0O2 were substituted for Si0O2 into a 14 Na20‐10 B20O3‐76 Si0O2 (wt%) glass in proportion to their weight percent in PNL‐7668 simulated nuclear waste glass. No correlation was found between the Na diffusion coefficient (DNa) in the bulk glass and sodium leaching in water for any of the iron‐containing borosilicate glasses. Effective diffusion coefficients calculated from the sodium leach rates in deionized water were 500 to 2000 times larger than DNa. The experimental leaching data agreed with a diffusion‐convection model where the convective term was a measured “phase‐boundary” velocity moving into the glass.