Showing papers on "Goethite published in 1998"
TL;DR: In this article, the second-order kinetic expression of H2O2 over goethite surface can be described by the second order kinetic expression −d[H 2O2]/dt = k[FeOOH][H2O 2], where k = 0.031 M-1 s-1, at pH 7 in the absence of any inorganic or organic chemical species.
Abstract: This research describes the heterogeneous catalytic reactions of H2O2 with granular size goethite (α-FeOOH) particles in aqueous solution under various experimental conditions. This is an important reaction for the environment since both H2O2 and iron oxides are common constituents of natural and atmospheric waters. Furthermore, iron oxides function as catalysts in chemical oxidation processes used for treatment of contaminated waters with H2O2. The results of this study demonstrated that the decomposition rate of H2O2 over goethite surface can be described by the second-order kinetic expression −d[H2O2]/dt = k[FeOOH][H2O2], where k = 0.031 M-1 s-1, at pH 7 in the absence of any inorganic or organic chemical species. The apparent reaction rate was dominated by the intrinsic reaction rates on the oxide surfaces rather than the mass transfer rate of H2O2 to the surface. The activation energy of the reaction of H2O2 with the iron oxide surface was determined to be 32.8 kJ/M. The reaction mechanism for the de...
897 citations
TL;DR: The formation of iron biominerals is a two-step process: initially metals are electrostatically bound to the anionic surfaces of the cell wall and surrounding organic polymers, where they subsequently serve as nucleation sites for crystal growth as discussed by the authors.
412 citations
TL;DR: In this paper, the authors measured the visible to near-infrared (IR) spectra of 176 synthetic and natural samples of Fe oxides, oxyhydroxides, ferrihydrite, hematite and lepidocrocite.
Abstract: We measured the visible to near-infrared (IR) spectra of 176 synthetic and natural samples of Fe oxides, oxyhydroxides and an oxyhydroxysulfate (here collectively called "Fe oxides"), and of 56 soil samples ranging widely in goethite/hematite and goethite/lepidocrocite ratios. The positions of the second-derivative minima, corresponding to crystal-field bands, varied substantially within each group of the Fe oxide minerals. Because of overlapping band positions, goethite, maghemite and schwertmannite could not be discriminated. Using the positions of the 4Tl<----6AI, 4T2<----6AI, (4E;4AI)4---6A I and the electron pair transition (4T~ h-4Ti)<----(6Ai q-6ml) , at least 80% of the pure akaganeite, feroxyhite, ferrihydrite, hematite and lepidocrocite samples could be correctly classified by discriminant functions. In soils containing mixtures of Fe oxides, however, only hematite and magnetite could be unequivocally discriminated from other Fe oxides. The characteristic features of hematite are the lower wavelengths of the 4"171 transition (848-906 nm) and the higher wavelengths of the electron pair transition (521-565 nm) as compared to the other Fe oxides (909-1022 nm and 479-499 nm, resp.). Magnetite could be identified by a unique band at 1500 nm due to Fe(II) to Fe(III) intervalence charge transfer. As the bands of goethite and hematite are well separated, the goethite/hematite ratio of soils not containing other Fe oxides could be reasonably predicted from the amplitude of the second-derivative bands. The detection limit of these 2 minerals in soils was below 5 g kg t, which is about 1 order of magnitude lower than the detection limit for routine X-ray diffraction (XRD) analysis. This low detection limit, and the little time and effort involved in the measurements, make second-derivative diffuse reflectance spectroscopy a practical means of routinely determining goethite and hematite contents in soils. The identification of other accessory Fe oxide min- erals in soils is, however, very restricted.
364 citations
TL;DR: In this paper, the role of structural features of NOM-analogue organic acids in influencing mineral−organic acid interactions was performed, using goethite, a crystalline form of iron oxide important in geochemical systems.
Abstract: A systematic study of the role of structural features of NOM-analogue organic acids in influencing mineral−organic acid interactions was performed. Goethite, a crystalline form of iron oxide important in geochemical systems, was employed as a model mineral. Sorption experiments with a series of small molecular weight, aromatic organic acids were performed to assess the effects of particular features of compound structure on sorption to goethite and to obtain insight into the kinds of surface reactions responsible for organic acid sorption. Important functional groups of the humic acid component of NOM were examined. Experiments were also performed with Aldrich humic acid to allow comparison between sorption behavior of simple organic acids and NOM and to determine suitable analogues for modeling NOM sorption. Sorption characteristics for most organic acids were typical for anionic sorbates, with high sorption at low pH and decreasing sorption as pH increased. Sorption of simple organic acids exhibited str...
268 citations
TL;DR: In this article, naturallyoccurring iron minerals, goethite and magnetite, were used to catalyze hydrogen peroxide and initiate Fenton-like reaction of silica sand contaminated with diesel and/or kerosene in batch system.
254 citations
TL;DR: In this paper, both indirect (FTIR) and direct (X-ray Absorption Near Edge Structure (XANES) spectroscopic techniques were applied to investigate the adsorption.
Abstract: Arsenic toxicity, mobility, and bioavailability in soil-water systems are highly dependent on its oxidation states and chemical species. In this study, both indirect (FTIR) and direct (X-ray Absorption Near Edge Structure (XANES) spectroscopic techniques were applied to investigate the adsorption
217 citations
TL;DR: In this article, it was observed that limonite consists of aggregates of needle-like particles of goethite compacted together, and the grain model was finally proposed to best describe nickel dissolution kinetics.
213 citations
TL;DR: In this article, the interaction between phosphate and citrate on goethite has been studied for a better understanding of the competi tive binding of phosphate and organic matter in natural systems.
Abstract: Interaction between various species in natural systems may affect the adsorption of these species and thereby their mobility and bioavailability. Recently an ion adsorption model has been developed, based on the surface structure of metal (hydr)oxides and the structure of surface complexes identified by spectroscopy. This model has been applied to the adsorption of phosphate and weak organic acids on goethite in single anion systems. We studied the interaction between phosphate and citrate on goethite, which is important for a better understanding of the competi tive binding of phosphate and organic matter in natural systems. Citrate adsorption in the presence of phosphate decreased strongly over a wide pH range, whereas phosphate adsorption was only affected at pH values below 7, with the largest interaction around pH 5. Competition for binding sites appeared to be an important factor in determining the competitive adsorption of phosphate and citrate when taking into account that a larger number of surfa...
203 citations
TL;DR: Five moderately thermophilic iron-oxidizing bacteria, including representative strains of the three classified species, were shown to be capable of reducing ferric iron to ferrous iron when they were grown under oxygen limitation conditions.
Abstract: Five moderately thermophilic iron-oxidizing bacteria, including representative strains of the three classified species (Sulfobacillus thermosulfidooxidans, Sulfobacillus acidophilus, and Acidimicrobium ferrooxidans), were shown to be capable of reducing ferric iron to ferrous iron when they were grown under oxygen limitation conditions. Iron reduction was most readily observed when the isolates were grown as mixotrophs or heterotrophs with glycerol as an electron donor; in addition, some strains were able to couple the oxidation of tetrathionate to the reduction of ferric iron. Cycling of iron between the ferrous and ferric states was observed during batch culture growth in unshaken flasks incubated under aerobic conditions, although the patterns of oxidoreduction of iron varied in different species of iron-oxidizing moderate thermophiles and in strains of a single species (S. acidophilus). All three bacterial species were able to grow anaerobically with ferric iron as a sole electron acceptor; the growth yields correlated with the amount of ferric iron reduced when the isolates were grown in the absence of oxygen. One of the moderate thermophiles (identified as a strain of S. acidophilus) was able to bring about the reductive dissolution of three ferric iron-containing minerals (ferric hydroxide, jarosite, and goethite) when it was grown under restricted aeration conditions with glycerol as a carbon and energy source. The significance of iron reduction by moderately thermophilic iron oxidizers in both environmental and applied contexts is discussed.
175 citations
TL;DR: In this article, the authors investigated the properties of various polymorphs of iron oxide hydroxide, viz. goethite (α), akaganeite (β), lepidocrocite (γ), and feroxyhite (δ), and discussed with respect to the crystal structures proposed in the literature.
175 citations
TL;DR: In this article, the authors investigated the sorption of dissolved organic carbon (DOC) and its fractions as influenced by organic C (OC), sesquioxides amorphous Al(OH) 3, ferrihydrite, goethite, and extractable SO 4 2.
Abstract: The sorption of dissolved organic carbon (DOC) and its fractions as influenced by organic C (OC), sesquioxides amorphous Al(OH) 3 , ferrihydrite, goethite], and extractable SO 4 2 was investigated by modifying the properties of the Bs horizon of an Entic Haplorthod. Coating the Bs material with OC extracted from the Oa horizon of a mor forest floor layer decreased the sorption of total DOC. The effects on the sorption of the hydrophobic DOC fraction were little, whereas the sorption of the hydrophilic fraction was strongly reduced. At the highest OC coating levels, a net release of hydrophilic DOC occurred, indicating a displacement of indigenous hydrophilic substances from the soil by the sorbing hydrophobic dissolved organic matter (DOM). Coating of the soil material with sesquioxides clearly increased the sorption of DOC and its fractions. The strongest effects occurred when amorphous Al(OH) 3 was added, followed by ferrihydrite and goethite. Increased contents of extractable SO 4 2- did not affect the sorption of DOC on soil material. With increasing addition of DOC. however, the concentrations of SO 4 2- in the equilibrium solution increased. Adsorbing DOM seems to displace SO 4 2 bound to soil materials. Thus, DOM shows a higher affinity to sorb on soil material than SO 4 2 .
TL;DR: In this paper, Pb(II)-chloro ternary complexes were observed on goethite and γ-alumina using X-ray absorption fine structure (XAFS) spectroscopy.
TL;DR: The adsorption behavior of divalent metal ions on amphoteric surface hydroxyl groups of metal oxides is an important factor in the transport of subsurface environmental pollutants as discussed by the authors.
TL;DR: Phenanthrene, a three ring aromatic hydrocarbon, was used as a model substrate to distinguish between binding and sorption to dissolved and to mineral-associated humic acid (HA), with an emphasis on its implication to bioavailability as discussed by the authors.
TL;DR: In this article, the authors evaluate the effectiveness of common selective dissolution treatments in determining the association of minerals with potentially toxic trace elements, and find that ammonium oxalate and EDTA treatments, which are thought by some workers to dissolve only poorly crystalline phases, were found to dissolve a significant amount of crystalline goethite.
TL;DR: In this article, the effects of sulfation on the methane oxidation activity of iron oxide catalysts prepared from goethite and 2-line ferrihydrite precursors are presented.
Abstract: A study of the effects of sulfation on the methane oxidation activity of iron oxide catalysts prepared from goethite and 2-line ferrihydrite precursors is presented. Although catalytic performance is found to be dependent upon the precursor, sulfation produces general effects in both systems. Surface area is increased, oxidation activity <400°C is suppressed, and that at 500°C is enhanced leading to the production of selective products. Despite these similarities, sulfation produces different structural effects in the two systems. In the case of iron oxide prepared from goethite, extensive pitting and an increase in disorder in the cationic arrangement are observed. In contrast, a slight increase in crystallinity of the iron oxide prepared from ferrihydrite occurs. The effect of sulfation on catalytic performance is interpreted in terms of a two-site mechanism, complexation poisoning Fe3+ sites active for the lower-temperature oxidation activity with additional sites active at higher temperature being produced.
TL;DR: In this paper, the sorption behavior of uranium and selenium on a purified bentonite sample has been investigated and the results compared with those for sorption onto the unpurified material and goethite.
TL;DR: This work is concerned with the adsorption of o-phthalate (1,2-benzenedicarboxylate) at the water-metal (hydr)oxide interface, and identifies bonding mechanisms, stoichiometries, and stabilities of the formed complexes, and comparing these among the three systems.
TL;DR: In this article, it was shown that no hydroxylated hematite is formed during the early stages of the phase transformation of goethite, the presence of hydroxy ions in materials treated at high temperatures being explained by trapped water inside porous microstructure (TEM).
TL;DR: In this paper, a thermodynamic model was proposed to describe the surface charging properties of goethite in electrolyte solutions, using a linear free energy relation between the energies of the hydroxylated surface and the aqueous solvated surface to predict the acidity constants for most singly, doubly, and triply (μ3-hydroxo or μ3-oxo) coordinated surface sites.
TL;DR: In this paper, the growth rate of the (100) and (110) faces of goethite has been measured in situ with scanning force microscopy, and the growth behavior appears to be reaction controlled rather than transport controlled.
TL;DR: In this paper, a combination of rate measurements of iron(III)oxide and sulfate reduction, thermodynamic data, and pore-water and solid phase analyses was used to evaluate the relative significance of iron reduction in the sediments of an acidic strip mining lake (Lake 116, Brandenburg, Germany).
Abstract: A combination of rate measurements of iron(III)oxide and sulfate reduction, thermodynamic data, and pore-water and solid phase analyses was used to evaluate the relative significance of iron and sulfate reduction in the sediments of an acidic strip mining lake (Lake 116, Brandenburg, Germany). The rate of sulfate reduction was determined using a 35S-radiotracer method. Rates of iron turnover were quantified by mass balances based on pore-water concentration profiles. The differences in Gibbs free energy yield from reduction of iron and sulfate and from methanogenesis were calculated from individual redox couples and concentrations of reactants to account for the influence of high Fe2+ concentrations and differing mineral phases. Integrated (O-20 cm) mean rates of sulfate reduction were 1.2 (pelagial), respectively 5.2 (littoral) mmol (m2d)-1. Based on electron equivalents, the estimated iron reduction rates reached between about 50 % (pelagial) and 75 % (littoral) of the sulfate reduction rates. Compared to conditions usually assumed in the literature, in the sediments Gibbs free energy advantage of iron reduction over sulfate reduction was reduced frmm +11 KJeq-1 to a range of about +7 KJeq-1 (ferrihydrite, "reactive iron") to -6 KJeq-1 (goethite). This indicates that iron reduction was thermodynamically favored to sulfate reduction only if amorphous iron(III)oxides were available and is in accordance to the high competitiveness of sulfate reducers in the sediment. While total iron concentration in the sediments was high (up to 80% of the dryweight), reactive iron only accounted for 11-38% and was absolutely and relatively diminished in the zone of iron reduction. Pore-water concentration gradients and 137CS profiles indicated that little or no bioturbation occurred in the sediments, probably inhibiting the renewal of reactive iron. We further hypothesize that the reactivity of the iron oxide surfaces was reduced due to adsorption of DOM, suggested by IR spectra of the DOM and by a surface coverage estimate using literature data. Pelagial and littoral sediments displayed different dynamics. At the littoral relative iron reduction rate estimates were higher, iron sulfides were not accumulated and residence times of iron oxides were short compared to the pelagial. At the littoral site reoxidation of iron sulfides probably resulted in the renewal of reactive iron(III)oxides, possibly allowing for higher relative rates of iron reduction.
TL;DR: In this article, synthetic iron oxides (goethite, alpha-FeO.OH; hematite, Fe2O3; and ferrihydrite, Fe(OH)3) were used as model compounds to simulate the mineralogy of surface films on carbon steel.
Abstract: Synthetic iron oxides (goethite, alpha-FeO.OH; hematite, Fe2O3; and ferrihydrite, Fe(OH)3) were used as model compounds to simulate the mineralogy of surface films on carbon steel. Dissolution of these oxides exposed to pure cultures of the metal-reducing bacterium, Shewanella putrefaciens, was followed by direct atomic absorption spectroscopy measurement of ferrous iron coupled with microscopic analyses using confocal laser scanning and environmental scanning electron microscopies. During an 8-day exposure the organism colonized mineral surfaces and reduced solid ferric oxides to soluble ferrous ions. Elemental composition, as monitored by energy dispersive x-ray spectroscopy, indicated mineral replacement reactions with both ferrihydrite and goethite as iron reduction occurred. When carbon steel electrodes were exposed to S. putrefaciens, microbiologically influenced corrosion was demonstrated electrochemically and microscopically.
TL;DR: In this article, the degradation of a 14C-labeled poly(dimethylsiloxane) (PDMS) on 12 Ca-saturated clay minerals was investigated.
Abstract: To better understand the environmental fate of organopolysiloxanes (silicones), this study investigated the degradation of a 14C-labeled poly(dimethylsiloxane) (PDMS) on 12 Ca-saturated clay minerals. The rates and products of PDMS degradation were determined at 22 °C and 32% relative humidity, via sequential extraction and high-performance size exclusion chromatography (HPSEC). The results showed that all of the clays tested were catalysts for PDMS degradation. However, clay minerals varied substantially in their catalytic activity: kaolinite, beidellite, and nontronite were the most active; goethite and allophane were least active. In addition, PDMS degradation products bound more strongly to goethite and smectites. These results demonstrated that soil factors such as clay content and clay type are very important in determining the degradation rates of PDMS in soil.
TL;DR: In this paper, the effects of pH on the sorption of cadmium, zinc, nickel, and cobalt were studied by changing the pH of a soil and measuring sorption.
Abstract: Summary
The effects of soil pH on sorption of cadmium, zinc, nickel and cobalt were studied by changing the pH of a soil and measuring sorption. Results were compared with published results for effects of pH on sorption of cadmium, zinc and nickel by goethite. In a further experiment, the effects of pH on the uptake of zinc and cobalt by subterranean clover were measured.
Effects of pH on sorption were described in terms of the concentration of metal ions required to produce equal sorption. Where the metal ions were incubated with the soil, unit increase in pH decreased the concentration of metal ions required about 10-fold for zinc, about 7-fold for nickel, about 6-fold for cobalt, and about 4-fold for cadmium. When the soil was mixed with a large volume of solution, the effects were similar for zinc and cadmium but slightly smaller for cobalt and slightly larger for nickel. In all cases, the magnitude of the effect varied somewhat with pH. Sorption was greater with a dilute background electroiyte than with a concentrated one and the effects of pH were greater. The effects for soil were smaller than effects of pH on sorption by iron oxides for which unit increase in pH can decrease the required concentration of Zinc 35-fold and cadmium 11-fold.
These results are consistent with adsorption of divalent ions on a variable charge surface that is negatively charged. They are not consistent with the adsorption of monovalent metal ions on a variable charge surface. This mechanism requires at least a 10-fold effect of pH. They show that the change in electric potential with change in pH is smaller for reacting surfaces in soil than for goethite. The effects of changing pH on the amounts of zinc and cobalt fertilizer required for equal uptake by plants was even smaller with unit increase in pH, causing a 1.4-fold increase in the amount of fertilizer required, that is, a 1.4-fold decrease in fertilizer effectiveness.
15 Sep 1998
TL;DR: The more mobile sulfate ion is more effectively adsorbed when Pb 2+ is present, and increases Pb2+ adsorption on boehmite, and lead sulfate phases may form at the goethite surface under the authors' conditions.
Abstract: We examine the role of coadsorbed phosphate and sulfate on the adsorption of Pb2+ at the oxide-water interface of goethite and boehmite by electrophoretic mobility (EM) and X-ray absorption spectroscopy (XAS). Comparisons to Pb2+ adsorbed to pristine oxides are provided. Pb2+ binds as an inner-sphere complex to pristine goethite and groups into oxy-clusters on pristine boehmite under our conditions. The presence of either anion at the surface generally increases the ability of the solids to sorb Pb2+ but by different mechanisms. Adsorbed phosphate on both solids acts as a reactant to form lead phosphate surface phases that may be highly dispersed. The more mobile sulfate ion is more effectively adsorbed when Pb2+ is present, and increases Pb2+ adsorption on boehmite. Lead sulfate phases may form at the goethite surface under our conditions. Copyright 1998 Academic Press.
TL;DR: In this article, the pore-water chemistry of the tailings of the Copper Cliff mine has been analyzed and the presence of goethite, jarosite, gypsum, native sulfur, and a vermiculite-type clay mineral.
Abstract: The oxidation and the subsequent dissolution of sulfide minerals within the Copper Cliff tailings area have led to the release of heavy metals such as Fe, Ni, and Co to the tailings pore water. Dissolved concentrations in excess of 10 g/l Fe and 2.2 g/l Ni have been detected within the shallow pore water of the tailings, with increasing depth these concentrations decrease to or near analytical detection limits. Geochemical modelling of the pore-water chemistry suggests that pH-buffering reactions are occurring within the shallow oxidized zones, and that secondary phases are precipitating at or near the underlying hardpan and transition zones. Mineralogical study of the tailings confirmed the presence of goethite, jarosite, gypsum, native sulfur, and a vermiculite-type clay mineral. Goethite, jarosite, and native sulfur form alteration rims and pseudo-morphs of the sulfide minerals. Interstitial cements, composed of goethite, jarosite, and gypsum, locally bind the tailings particles, forming hardpan layers. Microprobe analyses of the goethite indicate that it contains up to 0.6 weight % Ni, suggesting that the goethite is a repository for Ni. Other sinks detected for heavy metals include jarosite and a vemiculite-type clay mineral which locally contains up to 1.6 weight % Ni. To estimate the mass and distribution of heavy metals associated with the secondary phases within the shallow tailings, a series of chemical extractions was completed. The experimental design permitted four fractions of the tailings to be evaluated independently. These four fractions consisted of a water-soluble, an acid-leachable, and a reducible fraction, as well as the whole-rock total. Twenty-five percent of the total mass of heavy metals was removed in the acid-leaching experiments, and 100% of the same components were removed in the reduction experiments. The data suggest that precipitation/coprecipitation reactions are providing an effective sink for most of the heavy metals released by sulfide mineral oxidation. In light of these results, potential decommissioning strategies should be evaluated with the recognition that changing the geochemical conditions may alter the stability of the secondary phases within the shallow tailings.
TL;DR: In this article, the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxide minerals, and the results suggest the following sequence of 18O-enrichment in the common hydroxides: limonite > gibbsite > goethite > brucite > diaspore.
Abstract: The modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxide minerals. The results suggest the following sequence of 18O-enrichment in the common hydroxides: limonite > gibbsite > goethite > brucite > diaspore. The hydroxides are significantly enriched in 18O relative to the corresponding oxides. The sequence of 18O-enrichment in the hydroxides and oxides of trivalent cations is as follows: M(OH)3 > MO(OH) > M2O3. There are also considerable fractionations within the polymorphos of Al(OH)3. The internally consistent fractionation factors for hydroxide–water systems are obtained for the temperature range of 0 to 1200 °C, which are comparable with the data derived from synthesis experiments and natural samples at surficial temperatures. Temperature dependence of oxygen isotope fractionations between goethite, gibbsite, boehmite and diaspore and water are significant enough for the purpose of geothermometry. Thus the hydroxide–water pairs hold great promise of serving as reliable paleothermometers in surficial geological environments.
TL;DR: In this paper, a flotation-derived, low-sulfide tailings containing approximately 0.4 wt.% S was compared with simultaneously oxidized tailings with 1.0 and 2.5 w.% s to assess their acid generating characteristics.
Abstract: Oxidation of a flotation-derived, low-sulfide tailings containing approximately 0.4 wt.% S was compared with simultaneously oxidized tailings containing 1.0 wt.% S and 2.5 wt.% S to assess their acid generating characteristics. Each tailings type was exposed to oxidation for three years in laboratory columns and in lysimeter pits in the field. In these tailings the sulfide mineral of principal concern with respect to acid generation is pyrrhotite (Fe
1-x
S). In past studies the alteration of pyrrhotite has been characterized by initial replacement with marcasite (FeS2) and ferric iron sulfates, which are followed by development of ferric oxyhydroxides such as goethite and lepidocrocite. Macroscopic characterization of the tailings shows varying and progressive degrees of oxidation correlative with the three different sulfur contents. As expected, the tailings with the lowest sulfur content are the least oxidized, and those with the highest sulfur content have reacted the most. The column tests, which represent accelerated reaction conditions relative to those for the lysimeter pits, show much higher degrees of oxidation, and a markedly more distinct boundary between the oxidized and unoxidized zones; as well, differences among the three tailings types are more pronounced.
TL;DR: In this paper, a set of experiments were conducted to evaluate: 1) sorption of plutonium-239 (Pu) on different iron oxide colloids (hematite and goethite), 2) the sorption kinetics of colloidal Pu(IV) and soluble Pu(V) onto these two colloids, and 3) desorption of colloid-loaded colloids as a function of time.
Abstract: Laboratory batch sorption experiments were conducted to evaluate: 1) sorption of plutonium-239 (Pu) on different iron oxide colloids (hematite and goethite), 2) sorption kinetics of colloidal Pu(IV) and soluble Pu(V) onto these two colloids, and 3) desorption of colloidal Pu(IV) and soluble Pu(V) from Pu-loaded colloids as a function of time. Natural groundwater and carbonate-rich synthetic groundwater were used in this study. To examine the possible influence of bicarbonate on \"Pu sorption, an additional set of experiments was conducted in sodium nitrate (NaNO,) solutions under carbon dioxide free environments. Our results show that colloidal Pu(IV) as well as soluble Pu(V) was rapidly adsorbed by hematite and goethite colloids in both natural and synthetic groundwater. The amount of Pu adsorbed by both iron oxide colloids in synthetic groundwater was higher than in natural groundwater. The presence of carbonate did not influence the sorption of Pu. While sorption of soluble Pu(V) is a slow process, sorption of colloidal Pu(IV) occurs rapidly. Desorption of Pu from iron oxide colloids is much slower than the sorption processes. Our findings suggest that different sorption and desorption behaviors of Pu by iron oxide colloids in groundwater may facilitate the transport of Pu along potential flowpaths from the areas contaminated by radionuclide and release to the accessible environment.