Humic colloid-borne natural polyvalent metal ions: dissociation experiment

The natural association nature of the humic colloid-borne trace elements is investigated. Rare earth elements (REE) Th and U are chosen as naturally occurring representatives and chemical homologues for actinides of different oxidation states present in nuclear waste. Tri- and tetravalent elements in two investigated Gorleben groundwaters (Gohy-532 and -2227) almost exclusively occur as humic or fulvic colloid-borne species. Their desorption behavior from colloids is examined in the unperturbed groundwater (pH approximately 8) under anaerobic conditions (Ar/1% CO2) by addition of a chelating cation exchanger resin. Particularly, the dissociation process of naturally occurring Eu(III) in the groundwater is compared with the Eu(III) desorption from its humate complex prepared with purified Aldrich humic acid in a buffered aqueous solution at pH approximately 8. The Eu(III) dissociation from the groundwater colloids is found to be considerably slower than found for the humate complex synthesized in the laboratory. This suggests that under natural aquatic conditions the Eu(III) binding in colloids is chemically different from the simple humate complexation as observed in the laboratory experiment. The colloid characterization bythe size exclusion chromatography (SEC) and the flow field-flow fractionation (FFFF) indicates that natural colloid-borne trace elements are found predominantly in colloids of larger size (>15 nm in size), while Eu(III) in its humate complex is found mainly in colloids of hydrodynamic diameters <5 nm. The slower desorption kinetics and the larger colloid size suggest that the polyvalent metal ion binding in natural humic colloids is associated to polynucleation with other co-present trace metal ions. Radiotracer experiments reveal that isotopic equilibria with the naturally colloid-borne trace elements are not attained within a period of more than 100 days, indicating irreversible binding of at least a part of colloid-borne polyvalent trace elements. The different kinetic properties of colloid-bound Eu(III) are discussed taking the aqueous speciation based on thermodynamic data into account.     

A kinetic study of Am(III)/humic colloid interactions

The interaction kinetics of the Am(III) ion with aquatic humic colloids is investigated under near-natural conditions by column experiments with a sandy aquifer sample rich in humic substancesforthe appraisal of the migration behavior of Am. The association and dissociation kinetics of the Am ion onto and from humic colloids control the migration of colloid-borne Am. As the contact time between Am and humic colloids prior to introduction into a column is increased, the mobility of colloid-borne Am is enhanced and hence the recovery of Am in the effluent increases. On the other hand, an increase of the migration time and residence time in column, respectively, reduces the Am recovery. Considering these experimental results a refined version of the kinetic model KICAM (Kinetically Controlled Availability Model), which suggests different Am binding modes with humic colloids, was developed. Applying KICAM it is possible to predict static and dynamic experiments affected by the kinetically controlled Am/humic colloid interactions over the range of 1 h up to several months. However, to apply these experimental results to long-term conditions, the Am binding scheme as proposed in KICAM needs to be verified. This paper provides, therefore, a basis for a better understanding of the colloid-borne Am migration in porous aquifer systems.     

Coupled stream-subsurface exchange of colloidal hematite and dissolved zinc, copper, and phosphate

Previous studies have shown that stream-subsurface exchange has important implications for both colloid and reactive solute transport in streams because it increases the opportunity for the interaction of stream-born substances with bed sediments. We executed a series of laboratory flume experiments to study the coupled stream-subsurface exchange of hematite and the sorbing solutes zinc, copper, and phosphate. A fundamental process-based transport model was applied to analyze the experimental results. We found that hematite had a significant effect on the transport of all ions tested. In addition, hematite mobility was substantially modified by the presence of these solutes. Batch and column experiments showed that the difference in hematite mobility was a direct effect of zinc, copper, and phosphate sorption to the hematite surface. Sorption substantially modified the hematite surface charge and subsequently hematite filtration behavior. These results suggest that the behavior of contaminants cannot be analyzed separately from colloids in surface and groundwater systems because surface-chemical processes can cause their behavior to be coupled. In particular, our results show that general and specific interactions between contaminants and iron oxide particles can alter colloid mobility, perturb natural fine particle dynamics, and either favor or disfavor contaminant mobility.     

Trace metal levels in uncontaminated groundwater of a coastal watershed: importance of colloidal forms

Groundwater and surface water were collected using trace metal clean techniques from the upper glacial aquifer of West Neck Bay (Shelter Island) in eastern Long Island, NY, during the late spring and summer of 1999. The collection sites on Shelter Island are located in an area that is primarily residential and believed to have uncontaminated groundwater. Ultrafiltration was used to size-fractionate the dissolved (<0.45 microm) fraction into colloidal (1 kDa - 0.45 microm) and low molecular weight (<1 kDa) size pools. These fractions were analyzed for trace metals (Al, Ag, Cd, Cu, Mn, Pb, and Zn), organic carbon, and inorganic nutrients (NH4, NO3, PO4). The levels of metals and organic carbon in the groundwater were as low as those found in the open ocean, far removed from anthropogenic inputs. These findings corroborate the need to apply trace metal clean techniques in the determination of metal levels in uncontaminated groundwater. A significant fraction of dissolved metals (22-96%) and organic carbon (approximately 40%) in the groundwater and in surface waters of the Bay was found to be associated with colloids. The significance of the metal association with the colloidal fraction decreased in the order of Al > Cu > Ag > Zn = Cd = Mn and appeared to be dependent on the affinities of these metals for humic substances. In contrast, NO3 and NH4 were found to be almost entirely (approximately 98-99%) in the low molecular weight size fraction. Metal/aluminum and metal/carbon ratios measured in the colloids were similar to those reported for humic substances and significantly different from those of soils. This suggests that colloidal particles might originate from humic materials as opposed to purely inorganic minerals. These results indicate the need to consider the colloidal fraction in the fate and mobility of metals in groundwater and that, despite the low levels of organic matter (<50 microM of DOC) measured in groundwater, some groundwater colloids appear to be organic in nature.     

Colloid mobilization in water-saturated porous media under transient chemical conditions

This research focuses on the effects of transients in porewater chemistry on colloid mobilization within water-saturated porous media. We develop a model that couples equations for solute transport with those for colloid release and transport. The model accounts for heterogeneity in the interaction energies between deposited colloids and the mineral grains by dividing the immobile-phase colloid population into a series of compartments. Each compartment releases colloids at a characteristic critical solute concentration, which is assigned on the basis of a piece-wise linear distribution function. We test this model against data from column experiments in which successive step-change reductions in porewater NaCl concentrations induced pulse-type releases of silica colloids from the surfaces of quartz sand. Comparison of experimental and computed results reveals that colloid release rates vary nonlinearly with the immobile-phase colloid concentration and depend on the chemical conditions under which the colloids were deposited on the quartz sand. Our work demonstrates that colloid mobilization kinetics can be quantified given knowledge of the spatiotemporal changes in porewater chemistry.     

Low temperature XAFS investigation on the lutetium binding changes during the 2-line ferrihydrite alteration process

The time dependent changes of Lu speciation (used as Am(III) homologue), initially sorbed onto 2-line ferrihydrite at pH 5.9, during tempering (70 degrees C) to stable crystalline transformation products, goethite and hematite, is studied. Microscopies (AFM, SEM), XRD and FTIR spectroscopy confirm transformation to both goethite and hematite, with a predominance of hematite. XRD investigation of another transformation series at pH 8.0 (75 degrees C, [Lu(III)initial] 7 times higher) shows that the cell volume of hematite increases, suggesting the incorporation of Lu in the crystal structure. Extended X-ray absorption fine structure (EXAFS) (pH 5.9 series, 70 degrees C) reveals a shortening of the Lu-O bond distance and an increase in asymmetry of the first shell with increasing tempering time in the intermediate temper time samples. The intensity of the second peak in the Fourier transform (FT) of the EXAFS increases and splits into two components. The EXAFS data of the end product can be modeled well using a hematite-like cluster, with an isotropic expansion of distances to account for incorporation of Lu into the hematite structure. These results demonstrate that the Lu is incorporated in the crystal lattice of the transformation product, as opposed to being occluded or remaining a sorbed species on the surface.     

Straining of nonspherical colloids in saturated porous media

We explore the effects of colloid shape on straining kinetics by measuring the filtration of spherical and nonspherical colloids within saturated columns packed with quartz sand. Our observations demonstrate that the transport of peanut-shaped colloids matches the transport of spherical colloids with diameters equal to the minor-axis length of the peanut-shaped colloids. The straining rates of the spherical colloids vary linearly with the ratio of colloid diameter (d(p)) to sand-grain diameter (d(g)) for 0.0083 < d(p)/d(g) < 0.06. This linear relationship also quantifies the straining rates of the peanut-shaped particles provided that the particle's minor axis length is used for d(p). Results of pore-scale simulations reveal that a peanut-shaped particle adopts a preferred orientation as it approaches a pore-space constriction such that its major axis tends to align with the local flow direction. The extent of this reorientation increases with the particle's aspect ratio. Findings from this research suggest that straining is sensitive to changes in colloid shape and thatthe kinetics of this process can be approximated on the basis of measurable properties of the nonspherical colloids and porous media.     

Modeling of simultaneous exchange of colloids and sorbing contaminants between streams and streambeds

Contaminant transport in streams can be significantly modified by both stream-subsurface exchange and the presence of colloidal particles, but the interaction of these effects is notwell understood. Exchange with the hyporheic zone exposes contaminants to surface-chemical reactions with streambed sediments, while colloidal particles have a large reactive surface area that allows them to carry pollutants that would otherwise be transported primarily as dissolved species. A new theoretical model is developed to predict the role of colloids in mediating advective contaminant exchange between streams and streambeds. Bedform-induced pumping theory is applied to model physical transport, and colloid filtration and reversible contaminant sorption are used to calculate the local distributions of colloids and contaminants within the streambed. Residence time functions of both colloids and contaminants in the bed are then used to link contaminant concentrations in the pore water and streamwater. Model simulations indicate that, under conditions of low colloid filtration and strong contaminant sorption to colloids, contaminants are mobilized by colloids and there is less retention of contaminants in the streambed. This is the case of "colloid-facilitated contaminant transport" commonly considered in groundwater transport. On the other hand, when colloid filtration is high and contaminants still sorb strongly to colloids, contaminant mobility decreases and there is greater contaminant retention in the streambed. We term this case "colloid-impeded contaminant transport". Thus, we find that a variety of contaminanttransport behavior can occur depending on the concentration and mobility of suspended particles in the system and the relative affinity of contaminants for colloids and other solid phases.     

241Am migration in a sandy aquifer studied by long-term column experiments

The migration behavior of 241Am(III) in a sandy aquifer was studied under near-natural conditions by long-term column experiments of more than 1 year duration. Columns with 50 cm length and 5 cm in diameter were packed with aeolian quartz sand and equilibrated with two different groundwaters having an original dissolved organic carbon concentration (DOC) of 1.1 and 7.2 mg x dm(-3), respectively,from the Gorleben site (Lower Saxony, Germany). In each experiment, 1 cm3 of Am-spiked groundwater ([Am] = 0.2 to 2 micromol x dm(-3)) was injected into the column. The flow rate of the groundwater was adjusted to 0.28 m x d(-1). A small colloid-borne Am fraction was found to elute together with tritiated water. After 414 and 559 days, respectively, the experiments were terminated. Whereas the nonsorbing tracer of tritiated water would have covered a distance of about 350 m in that time period, the maximum of the Am activity was detected between 32 and 40 mm column length. Applying selective dissolution analysis to the sand surface, Am was found to be preferentially bound to iron hydroxide/oxide sites. From this Am distribution, a retardation factor R of about 10(4) was determined and compared to static batch experiments. The Am breakthrough was calculated forthe conditions of the column experiment     

Influence of natural organic matter on As transport and retention

Natural organic matter (NOM) can affect the behavior of arsenic within surface and subsurface environments. We used batch and column experiments to determine the effect of peat humic acids (PHA), groundwater fulvic acids (GFA), and a soil organic matter (SOM) extract on As sorption/transport in ferrihydrite-coated sand columns. A reactive transport model was used to quantitatively interpret the transport of As in flow-through column (breakthrough) experiments. We found that As(III) breakthrough was faster than As(V) by up to 18% (with OM) and 14% (without OM). The most rapid breakthrough occurred in systems containing SOM and GFA. Dialysis and ultrafiltration of samples from breakthrough experiments showed that in OM-containing systems, As was transported mostly as free (noncomplexed) dissolved As but also as ternary As-Fe-OM colloids and dissolved complexes. In OM-free systems, As was transported in colloidal form or as a free ion. During desorption, more As(III) desorbed (23-37%) than As(V) (10-16%), and SOM resulted in the highest and OM-free systems the lowest amount of desorption. Overall, our experiments reveal that (i) NOM can enhance transport/mobilization of As, (ii) different fractions of NOM are capable of As mobilization, and (iii) freshly extracted SOM (from a forest soil) had greater impact on As transport than purified GFA/PHA.     

Sorption of Am(III) onto 6-line-ferrihydrite and its alteration products: investigations by EXAFS

For the long-term performance assessment of nuclear waste repositories, knowledge about the interactions of actinide ions with mineral surfaces such as iron oxides is imperative. The mobility of released radionuclides is strongly dependent on the sorption/desorption processes at these surfaces and on their incorporation into the mineral structure. In this study the interaction of Am(III) with 6-line-ferrihydrite (6LFh) was investigated by EXAFS spectroscopy. At low pH values (pH 5.5), as well at higher pH values (pH 8.0), Am(III) sorbs as a bidentate corner-sharing species onto the surface. Investigations of the interaction of Am(III) with Fh coated silica colloids prove the sorption onto the iron coating and not onto the silica substrate. Hence, the presence of Fh, even as sediment coating, is the dominant sorption surface. Upon heating, Fh is transformed into goethite and hematite as shown by TEM and IR measurements. The results of the fit to the EXAFS data indicate the release of sorbed Am(III) at pH 5.5 during the transformation and likely a partial incorporation of Am into the Fh transformation products at pH 8.0.     

In situ colloid mobilization in Hanford sediments under unsaturated transient flow conditions: effect of irrigation pattern

Colloid transport may facilitate off-site transport of radioactive wastes at the Hanford site, Washington State. In this study, column experiments were conducted to examine the effect of irrigation schedule on releases of in situ colloids from two Hanford sediments during saturated and unsaturated transientflow and its dependence on solution ionic strength, irrigation rate, and sediment texture. Results show that transient flow mobilized more colloids than steady-state flow. The number of short-term hydrological pulses was more important than total irrigation volume for increasing the amount of mobilized colloids. This effect increased with decreasing ionic strength. At an irrigation rate equal to 5% of the saturated hydraulic conductivity, a transient multipulse flow in 100 mM NaNO3 was equivalent to a 50-fold reduction of ionic strength (from 100 mM to 2 mM) with a single-pulse flow in terms of their positive effects on colloid mobilization. Irrigation rate was more important for the initial release of colloids. In addition to water velocity, mechanical straining of colloids was partly responsible for the smaller colloid mobilization in the fine than in the coarse sands, although the fine sand contained much larger concentrations of colloids than the coarse sand.     

Arsenic mobilization through microbially mediated deflocculation of ferrihydrite

This study examined the potential impact of microbially mediated reduction of Fe in the Fe(III)-(hydr)oxide mineral ferrihydrite on the mobility of As in natural waters. In microcosm experiments, the obligately anaerobic bacterium Geobacter metallireducens reduced on average 10% of the Fe(III) in ferrihydrite with varying sorbed As(V) surface coverages, which resulted in deflocculation of initially micron-sized As-bearing ferrihydrite aggregates to nanometersized colloids. No reduction of As(V) to As(III) was observed in microcosm samples. Measurement of Fe and As within operationally defined particulate, colloidal, and dissolved fractions of microcosm slurry samples revealed that little Fe or As was released from ferrihydrite as dissolved species. Microbially induced deflocculation of ferrihydrite in the presence of G. metallireducens was correlated with more negative zeta potential of ferrihydrite nanoparticles suggesting that G. metallireducens mediated As mobilization through alteration of ferrihydrite surface charge. TEM analysis and solution chemistry conditions suggested formation of a magnetite surface layer through topotactic recrystallization of ferrihydrite (2LFH) driven by sorbed Fe(II). The formation of nanometer-sized As-bearing colloids through microbially mediated reduction of Fe-(hydr)oxides has the potential to increase human As exposure by enhancing As mobility in natural waters and hindering As removal during subsequent drinking water treatment.     

Distribution of colloid particles onto interfaces in partially saturated sand

Colloids have long been known to facilitate the transport of contaminants in soils, but few direct observations have been made of transport and retention in unsaturated porous media. Studies have typically been limited to evaluation of column breakthrough curves, resulting in differing and sometimes conflicting proposed retention mechanisms. We carried out pore scale visualization studies of colloid transport in unsaturated quartz sand to directly observe and characterize colloid retention phenomena. Synthetic hydrophilic (0.8, 2.6, and 4.8 microm carboxylated polystyrene latex) and relatively hydrophobic (5.2 microm polystyrene latex) colloidal microspheres were added to steady-state water flow (0.15 mm min(-1)) applied to an inclined infiltration chamber. Bright field microscopy was used to determine the positions and movement of water and colloids. Confocal laser scanning microscopy was used to determine water film geometry in an unsaturated horizontal chamber. We determined mechanisms of hydrophilic colloid retention at what is generally termed the air/water/solid (AWS) interface. Based on our observations, the AWS interface is here more accuratelytermed the air/water meniscus/solid (AWmS) interface, denoting the region where between-grain water meniscii diminish to thin water films on the grain surfaces. Colloids were retained at the AWmS interface where the film thickness approximately equaled colloid diameters. The greater retention for hydrophilic colloids at this interface (compared to elsewhere in the solid/water interface) can be explained by the additional surface tension capillary potentials exerted on colloids at the AWmS interface. While some 0.8-microm colloids were observed in thin water films, film straining played no significant role in the retention of larger colloids. Mechanisms for slightly hydrophobic colloids differed slightly. In addition to primary retention at the AWmS interface, hydrophobic colloids attached to others already present atthat interface resulting in apparent retention at the air/water (AW) interface. Attachment of hydrophobic colloids was also observed at water-solid interfaces, as hydrophobicity impelled the colloids to avoid water. Factors contributing to retention of slightly hydrophobic colloids were sand grain roughness and possibly a tendency for these colloids to flow near surfaces and interfaces, consonant with the enhanced retention of hydrophobic colloids (relative to hydrophilic colloids) observed in the literature.     

Effects of adsorbed arsenate on the rate of transformation of 2-line ferrihydrite at pH 10

2-Line ferrihydrite, a form of iron in uranium mine tailings, is a dominant adsorbent for elements of concern (EOC), such as arsenic. As ferrihydrite is unstable under oxic conditions and can undergo dissolution and subsequent transformation to hematite and goethite over time, the impact of transformation on the long-term stability of EOC within tailings is of importance from an environmental standpoint. Here, studies were undertaken to assess the rate of 2-line ferrihydrite transformation at varying As/Fe ratios (0.500-0.010) to simulate tailings conditions at the Deilmann Tailings Management Facility of Cameco Corporation, Canada. Kinetics were evaluated under relevant physical (~1 °C) and chemical conditions (pH ~10). As the As/Fe ratio increased from 0.010 to 0.018, the rate of ferrihydrite transformation decreased by 2 orders of magnitude. No transformation of ferrihydrite was observed at higher As/Fe ratios (0.050, 0.100, and 0.500). Arsenic was found to retard ferrihydrite dissolution and transformation as well as goethite formation.     

Transport of silica colloids through unsaturated porous media: experimental results and model comparisons

We present results on the migration of silica colloids through laboratory columns packed with partially saturated quartz sand. The transport of the silica colloids responds to changes in the steady-state volumetric moisture content (theta) and for low theta depends on the wetting history of the sand pack prior to colloid injection. A mathematical model that incorporates a first-order rate law to simulate film straining and a second-order rate law to simulate partitioning at air-water interfaces closely describes colloid transport and mass transfer over the range of experimental conditions tested. The mass-transfer parameters of the model are sensitive to changes in both the level of water saturation and the flow rate. A semiempirical expression, based on a modification of film-straining theory, accounts for the observed variation in the first-order rate coefficient with changes in theta and average porewater velocity. Our work indicates that the presence of the air phase substantially influences porewater concentrations of mineral colloids in water-unsaturated media and that the kinetics of particle removal attributed to air-water boundaries reflects the contribution of multiple mass-transfer mechanisms.     

Effect of pH and aging time on the kinetic dissociation of 243Am(III) from humic acid-coated gamma-Al2O3: a chelating resin exchange study

The chelating resin was studied to assess its influence on metal availability and mobility in the environment. The association of organic-inorganic colloid-borne trace elements was investigated in this work. The radionuclide 243Am(III) was chosen as the representative and chemical homologue for trivalent lanthanide and actinide ions present in radioactive nuclear waste. The kinetic dissociation behavior of 243Am(III) from humic acid-coated gamma-Al2O3 was studied at pH values of 4.0 +/- 0.1, 5.0 +/- 0.2, and 6.0 +/- 0.2 with a contact time of 2 days after the addition of a chelating cation exchanger resin. The concentrations of the components were: 243Am(III) 3.0 x 10(-7) mol/L, gamma-Al2O3 0.5 g/L, HA 10 mg/L (pH 4.0 +/- 0.1, 5.0 +/- 0.2, and 6.0 +/- 0.2) and 50 mg/L (pH 6.0 +/- 0.2), respectively. The kinetics of dissociation of 243Am(III) after different equilibration time with humic acid-coated gamma-Al2O3 was also investigated at pH 5.0 +/- 0.2. The experiments were carried out in air and at ambient temperature. The results suggest that the fraction of irreversible bonding of radionuclides to HA-coated Al2O3 increases with increasing pH and is independent of aging time. The assumption of two different 243Am(III)-HA-Al2O3 species, with "fast" and "slow" dissociation kinetics, is required to explain the experimental results. 243Am(III) species present on HA-Al2O3 colloids moves from the "fast" to the "slow" dissociating sites with the increase of aging time.     

Chromium(VI) reduction kinetics by zero-valent iron in moderately hard water with humic acid: iron dissolution and humic acid adsorption

In zerovalent iron treatment systems, the presence of multiple solution components may impose combined effects that differ from corresponding individual effects. The copresence of humic acid and hardness (Ca2+/Mg2+) was found to influence Cr(VI) reduction by Feo and iron dissolution in a way different from their respective presence in batch kinetics experiments with synthetic groundwater at initial pH 6 and 9.5. Cr(VI) reduction rate constants (k(obs)) were slightly inhibited by humic acid adsorption on iron filings (decreases of 7-9% and 10-12% in the presence of humic acid alone and together with hardness, respectively). The total amount of dissolved Fe steadily increased to 25 mg L(-1) in the presence of humic acid alone because the formation of soluble Fe-humate complexes appeared to suppress iron precipitation. Substantial amounts of soluble and colloidal Fe-humate complexes in groundwater may arouse aesthetic and safety concerns in groundwater use. In contrast, the coexistence of humic acid and Ca2+/Mg2+ significantly promoted aggregation of humic acid and metal hydrolyzed species, as indicated by XPS and TEM analyses, which remained nondissolved (>0.45 microm) in solution. These metal-humate aggregates may impose long-term impacts on PRBs in subsurface settings.     

Evidence for the interaction of technetium colloids with humic substances by X-ray absorption spectroscopy

Spectroscopic extended X-ray absorption fine structure (EXAFS) evidence was obtained on the chemical environment of 99Tc(IV) atoms formed upon introduction of TcO4- into four types of laboratory-scale synthetic and natural systems which mimic in situ natural reducing conditions in humic-rich geochemical environments: (a) magnetite/pyrite in synthetic groundwater in the absence of humic substances (HSs), (b) magnetite/pyrite in natural Gorleben groundwater in the presence of HSs, (c) Boom clay sediment mixed with synthetic groundwater, and (d) Gorleben sand mixed with natural Gorleben groundwater. The investigated systems obey to pH 8-9 conditions, and all measured samples show similar EXAFS spectra for Tc, which could be fitted by a hydrated TcO2 x xH2O phase. The results are interpreted as follows: upon introduction of high concentrations (millimolar to micromolar) of TcO4-to chemically reducing environments, small Tc(IV) oxidic polymers are formed, which either may aggregate into larger units (colloids) and finally precipitate or may interact in their polymeric form with (dissolved and immobile) humic substances. This latter type of interaction--Tc(IV) colloid sorption onto HSs--differs significantly from the generally accepted metal--humate complexation and therefore offers new views on the possible reaction pathways of metals and radionuclides in humic-rich environments.