A radiotracer study on the kinetics of gold sorption by mineral surfaces

Aqueous solutions with about 10 ppt¹⁹⁵Au and [HCl] of 10^–2.3 and 10^–1.3 m were exposed to solid minerals for several months. The gold uptake with time was observed by time-stepped sampling and radiochemical Au analysis. Sorbants were polished thick sections of quartz, pyrite, pyrrhotite and elemental gold, as well as crushed grains and sawed mineral cubes of quartz and pyrite (all randomly oriented). The kinetics of gold sorption strongly varied with the surface area of the sorbents, the type of mineral and the pH of the solution. Mineral-specific differences in reaction rates were observed only at experimental pH values around 2.3, where sorption on pyrrhotite and elemental gold was much more rapid than by quartz and pyrite. At pH around 1.3 gold sorption was rapid on all minerals. This finding is thought to reflect the gold speciation, i.e. neutral hydroxo-gold complexes above pH 1.5, for which only chemisorption is possible, versus dominantly AuCl ₄ ^– below pH 1.5, where unspecific electrostatic interaction enhances reaction rates with all protonated mineral surfaces.     

Towards finding an efficient sorbent for antimony: comparative investigations on antimony removal properties of potential antimony sorbents

Removal of antimony, classified as a pollutant of priority importance, is a challenge due to its presence in various forms in solution and the low concentration in which it is to be removed. In this study, sorption profile and the sorption mechanisms associated with the removal of antimony in its two oxidation states (+3 and +5) have been investigated in detail through batch studies involving titania sorbents, strong base anion resin, a chelating resin, and a biosorbent. Significant sorption-associated change in solution pH was observed with all the sorbents, which revealed the nature of respective sorption equilibrium involved. The results have shown that Sb(V) is removed only as anionic species, while Sb(III) is removed either as an anionic species or as cationic species according to the functional groups present in the sorbent and the solution conditions. Titania-based sorbents were found to be effective sorbents for Sb(III) and Sb(V) within a narrow pH range, while anion resin was found to be superior for removing Sb(V) as Sb(OH) ₆ ^? under a wide range of solution conditions. The suitability of the sorbents for column mode of operation has also been investigated. This report is a first attempt at a rational comparison of promising sorbents for antimony, and the results demonstrate the complexity involved in antimony sorption and give an understanding of the available options for handling the antimony removal problem in large-scale applications.     

Thermodynamic simulation of the effect of eutrophication on the behavior of heavy metals in bottom sediments by the example of the Inan’kovsky water reservoir

A thermodynamic model of the behavior of heavy metals in bottom sediments during their eutrophication is developed. The results show that liberation of heavy metals from the reduced ferrous sorbent is accompanied by sorption on clay and organic sorbents. The process is complicated by competitive correlations between heavy metals and calcium ions, while the formation of carbonate cement in the sediments yields additional sorption. As a result, the predicted concentrations of the majority of microelements, namely heavy metals, in the interstitial water of the reduced bottom sediments do not exceed the maximum permissible concentration (MPC).     

Parameters controlling the partitioning of tributyltin (TBT) in aquatic systems

In the present study the distribution of TBT between solid and water phase as a function of several parameters was determined. Two types of clay minerals (Na-montmorillonite SWy and kaolinite KGa) and quartz sand were used as sorbents in conventional batch experiments. Sorption coefficients (K_d) followed the order montmorillonite (89 l/kg) > kaolinite (51 l/kg) > quartz (25 l/kg), while for sorption coefficients normalized to the surface area (Kd′) an opposite trend was observed, with the lowest value determined for montmorillonite (2.79 × 10⁻³ l/m²) and the highest for quartz sand (8.04 × 10⁻² l/m²). The results demonstrate that numerous environmental parameters influence the adsorption process of TBT, such as solid/solution ratio, clay content and salinity. Another important factor governing TBT adsorption is pH, because it affects both the TBT species in the water phase as well as the surface properties of the mineral phase. The maximum of TBT adsorption onto clays was always around pH 6–7. According to the data, it is evident that the content of organic matter in the solid phase plays an important role on TBT adsorption, either as particulate organic matter (POM) or organic matter adsorbed to mineral particles (AOM). Experiments were carried out with well characterized organic matter and the results showed a linear increase of K_d from 51 up to 2700 l/kg upon the addition of 5% of particulate organic matter to pure phased kaolinite. TBT adsorption onto mineral surfaces, which were previously enriched with adsorbed organic matter, was investigated at different pH. The present study points to the importance of identifying and characterizing sorbents and envrionmental conditions, in order to predict and model TBT distribution in natural systems.     

Inorganic arsenic sorption by drinking-water treatment residual-amended sandy soil: effect of soil solution chemistry

Previous studies in our laboratory have demonstrated that drinking-water treatment residuals are effective sorbents of arsenic V. However, the effect of soil solution chemistry on arsenic V sorption by drinking-water treatment residuals-amended soils remains to be explored. The current study uses a batch incubation experimental set up to evaluate the effect of soil solution pH, competing ligands, and complexing metal on arsenic V sorption by a sandy soil (Immokalee series) amended with two rates (25 and 50 g kg^?1) of aluminum and iron-based drinking-water treatment residuals. Experiments were conducted at three initial arsenic loads (125, 1,875, 3,750 mg kg^?1) and a constant solid: solution ratio of 200 g L^?1. An optimum equilibration time of 8 days, obtained from kinetic studies, was utilized for sorption experiments with both aluminum and iron drinking-water treatment residual-amended soil. Presence of phosphate decreased arsenic V sorption by both aluminum and iron drinking-water treatment residual amended soils, with a strong dependence on pH, drinking-water treatment residual types, drinking-water treatment residual application rates, and phosphate concentrations. Addition of sulfate had no effect on arsenic V sorption by aluminum or iron drinking-water treatment residual-amended soil. A complementing effect of calcium on arsenic V sorption was observed at higher pH. Results elucidating the effect of soil solution chemistry on the arsenic V sorption will be helpful in calibrating drinking-water treatment residual as a sorbent for remediation of arsenic-contaminated soils.     

Adsorption of copper and zinc by oil shale

 Oil shale is able to remove appreciable amounts of copper and zinc ions from aqueous solutions. It was noted that an increase in the adsorbent concentration with constant copper or zinc concentration resulted in greater metal removal from solution. An increase in the copper or zinc concentration with a constant sorbent concentration resulted in higher metal loading per unit weight of sorbent. For both metals, copper and zinc, equilibrium was attained after 24-h contact time. Increase in the initial pH or temperature of the metal solution resulted in an increase in the metal uptake per unit weight of the sorbent. Freundlich isotherm model was found to be applicable for the experimental data of Cu²⁺ and Zn²⁺. The results showed that oil shale could be used for the adsorption of the Cu²⁺ and Zn²⁺ with higher affinity toward Zn²⁺ ions. Addition of sodium salt to the metal solution influenced copper removal positively, but inhibited zinc removal. Received: 3 January 2000 · Accepted: 27 June 2000     

Activation and regeneration of a soil sorbent for defluoridation of drinking water

Geomaterials can be cost-effective sorbents for use in water treatment. In this study, a heavily-weathered Tertiary soil from Xinzhou, China was used as a sorbent for defluoridation of high-fluoride drinking water. The soil is composed of quartz, feldspar, illite and goethite, with an Fe oxide content of 6.75%. Batch and column experiments were done to characterize the F⁻ removal properties and to develop an optimal activation and regeneration procedure. The soil can be regenerated following a simple base-acid rinsing procedure. This can be performed in situ, i.e., by passing the rinsing solutions directly through the treatment column. The same regeneration procedure can be used to activate the pristine soil. Fluoride sorption is described by a Freundlich isotherm model and the bulk of the uptake occurs within 1.5 h. Iron oxide coatings on soil particles and perhaps ≡FeOH surface groups at particle edges of illite grains are likely responsible for the soil's F-sorption property. As collected in the field, the soil has a low permeability and is thus unsuitable for direct use in a flow-through column. Heat-treatment at 400–500°C for 2 h, however, produces a granular and permeable sorbent. Although the soil's sorption capacity (150 μg/g ) is about a quarter of the low end range of values reported for commercially-available activated alumina, the sorption for F⁻ is specific. A batch sorption experiment in the presence of Cl⁻, SO₄²⁻ and HCO₃⁻ shows little or no competition from these other anions.     

Sorption of trace metals on calcite: Applicability of the surface precipitation model

Published Sorption isotherm data of Cd²⁺, Mn²⁺, Zn²⁺, and Co²⁺ on calcite are adequately described by the surface precipitation model which was originally developed by FArley et al. (1985) for the sorption of cations on metal oxides. In addition to monolayer adsorption, the model accounts for the formation of a surface phase with a composition that is described by a solid solution having as end members the sorbent calcium carbonate mineral and a pure carbonate precipitate of the sorbing trace metal. The model thus specifies a continuum between adsorption and precipitation. This feature is supported in the literature by observations on the reaction kinetics and the amount of surface coverage during trace metal sorption on calcite. The apparent adsorption constants of these trace metals, as derived from the model, can be ranked according to the degree to which their ionic radii match the ionic radius of Ca²⁺.     

Kinetics and Equilibrium Sorption Studies of 4-Nitrophenol on pyrolyzed and activated oil shale residue

Sorption of the organic pollutant 4-nitrophenol (4-NP) by pyrolyzed and activated Jordanian oil-shale was studied. Pyrolyzed oil shale was prepared using a fluidized bed reactor at 520 °C in the presence of nitrogen. Physical activation was carried out by treating the pyrolyzed oil shale with CO₂ at 830 °C, while chemical activation was achieved by using KOH and ZnCl₂ as impregnating agents. Batch kinetics and isotherm studies were conducted to evaluate the sorption process. Effects of contact time, initial sorbate concentration, sorbent concentration, temperature, pH and inorganic salts (NaCl and KCl) on the sorption process by the different sorbents were considered. Chemically activated oil shale, pretreated with ZnCl₂, gave the highest uptake of 4-NP. The isotherm experimental data fit reasonably well to Langmuir, Freundlich and Redlich-Paterson models. Three kinetic models, namely the Morris-Weber, Lagergren, and Pseudo-Second-Order model (PSOM), were applied to represent the experimental results for both pyrolyzed and ZnCl₂-oil shale sorbents.     

The effect of fulvic acid on the sorption of actinides and fission products on granite and selected minerals

Organic material is present at low concentrations, typically 1–2 mg/l in terms of dissolved organic carbon (DOC), in groundwaters deep in granitic rock. Hydrophobic and hydrophilic acids may complex inorganic contaminants and change their sorption behaviour on geological materials. This report describes a series of experiments performed under aerobic conditions to investigate the effects of fulvic acid over a concentration range of 0–5 mg/I DOC on the sorption of⁸⁵Sr,¹³⁷Cs,²³³U,²³⁸Pu and²⁴¹'Am by crushed granite, biotite, goethite, montmorillonite and quartz. In addition, similar solutions were used to study the effects of dissolved fulvic acid on the sorption of⁹⁹Tc and¹²⁵I on each of the above solids except quartz. The fulvic acid was extracted from groundwater collected at a depth of 240 m in the granitic rock of the Underground Research Laboratory near Lac du Bonnet, Manitoba.In all experiments, the sorption of the fulvic acid by the geological materials was in the order goethite montmorillonite > biotite > granite > quartz. No sorption of Tc was observed from any of the solutions on any of the solids. Low sorption of I on montmorillonite occurred in the presence of the dissolved organic material. No sorption of I was observed with the other solids used. Only montmorillonite sorbed any appreciable amount of Sr in this study with the fulvic acid having no effect on this sorption. All the solids except quartz sorbed substantial amounts of Cs, but sorption was not affected by the organic material in solution. Each of the solids sorbed U with no difference in sorption observed due to the dissolved organic. In general, Pu sorption decreased as the concentration of dissolved fulvic acid increased. However, sorption of Pu on quartz remained at approximately the same levels regardless of the concentration of organic in solution. Generally high sorption of Am was found. Lowest sorption of Am on all solids occurred from the solution with the highest concentration of fulvic acid. Sorption of Am on granite decreased as the concentration of the organic in solution increased.This study indicated that, under aerobic conditions, the effect of dissolved organic material on sorption of radioisotopes depends on the radioisotope in question and the concentration of the organic in solution.     

Modelling of Cs sorption in natural mixed-clays and the effects of ion competition

Cs migration in the environment is mainly controlled by sorption onto mineral surfaces, in particular on clay minerals. With the objective of designing a geochemical reactive barrier to treat ¹³⁷Cs accidental pollution in an industrial waste repository, different natural clayrocks were studied to analyse their capacity to retain Cs.The simple semi-empiric K_d-approach for experimental data analysis, is unsatisfactory to describe the variability of sorption upon chemical changes. Indeed, due to the high salinity of the site, the effects of competitive ions must be evaluated and quantified. Thus, the development of sorption models, capable of reproducing experimental data obtained under conditions representative of the contaminated site, and applicable to reactive transport studies, is needed.In this study, a model for Cs sorption, which takes into account the main mineralogy of the sorbent, the composition of the natural water (and ion competition) was successfully applied to interpret the non-linear Cs sorption under natural conditions.The selectivity coefficients of Cs with respect to the most important cations present in the site water (Na, K, NH₄, Ca) were derived by means of experiments in single clay minerals and synthetic mono-component solutions. Then, these parameters were tested in systems of increasing complexity.Considering the mineralogical composition of raw materials, it was shown that the principal contribution to Cs sorption is given by the mineral illite, while smectite starts to be relevant only at very high Cs loadings. Kaolinite, even in concentrations around 10 wt% of the clayey fraction, played only a minor role.With respect to the solution composition, the model was able to predict Cs sorption in electrolyte concentrations up to twice than that of seawater and up to 500 mg/L NH₄⁺. The effect of highly competing ions, especially NH₄⁺ and K⁺, on Cs retention is more important at low ionic strengths and low Cs loadings, where adsorption is dominated by illite selective frayed edge sites, FES. Divalent cations are not especially relevant as competing cations for Cs.     

Speciation of arsenic in sulfidic waters

Formation constants for thioarsenite species have been determined in dilute solutions at 25°C, ΣH₂S from 10^-7.5 to 10^-3.0 M, ΣAs from 10^-5.6 to 10^-4.8 M, and pH 7 and 10. The principal inorganic arsenic species in anoxic aquatic systems are arsenite, As(OH)₃ ⁰, and a mononuclear thioarsenite with an S/As ratio of 3:1. Thioarsenic species with S/As ratios of 1 : 1,2 : 1, and 4 : 1 are lesser components in sulfidic solutions that might be encountered in natural aquatic environments. Thioarsenites dominate arsenic speciation at sulfide concentrations > 10^-4.3 M at neutral pH. Conversion from neutral As(OH)₃ ⁰ to anionic thioarsenite species may regulate the transport and fate of arsenic in sulfate-reducing environments by governing sorption and mineral precipitation reactions.     

Lead coprecipitation with iron oxyhydroxide nano-particles

Pb²⁺ and Fe³⁺ coprecipitation was studied with sorption edge measurements, desorption experiments, sorbent aging, High Resolution Transmission and Analytical Electron Microscopy (HR TEM-AEM), and geochemical modeling. Companion adsorption experiments were also conducted for comparison. The macroscopic chemical and near atomic scale HRTEM data supplemented our molecule scale analysis with EXAFS (Kelly et al., 2008). Coprecipitation of Pb²⁺ with ferric oxyhydroxides occurred at ∼pH 4 and is more efficient than adsorption in removing Pb²⁺ from aqueous solutions at similar sorbate/sorbent ratios and pH. X-ray Diffraction (XRD) shows peaks of lepidocrocite and two additional broad peaks similar to fine particles of 2-line ferrihydrite (2LFh). HRTEM of the Pb-Fe coprecipitates shows a mixture of 2-6 nm diameter spheres and 8-20 by 200-300 nm needles, both uniformly distributed with Pb²⁺. Geochemical modeling shows that surface complexation models fit the experimental data of low Pb:Fe ratios when a high site density is used. Desorption experiments show that more Pb²⁺ was released from loaded sorbents collected from adsorption experiments than from Pb to Fe coprecipitates at dilute EDTA concentrations. Desorbed Pb²⁺ versus dissolved Fe³⁺ data show a linear relationship for coprecipitation (CPT) desorption experiments but a parabolic relationship for adsorption (ADS) experiments.Based on these results, we hypothesize that Pb²⁺ was first adsorbed onto the nanometer-sized, metastable, iron oxyhydroxide polymers of 2LFh with domain size of 2-3 nm. As these nano-particles assembled into larger particles, some Pb²⁺ was trapped in the iron oxyhydroxide structure and re-arranged to form solid solutions. Therefore, the CPT contact method produced more efficient removal of Pb²⁺ than the adsorption contact method, and Pb²⁺ bound in CPT solids represent a more stable sequestration of Pb²⁺ in the environment than Pb²⁺ adsorbed on iron oxyhydroxide surfaces.     

Sorption behavior of heavy metal pollutants onto shales and correlation with shale geochemistry

The sorption of lead (II) and cadmium (II) on seven shales belonging to the Proterozoic Vindhyan basin, central India, and a black cotton soil, Mumbai, India, was studied and compared with sorbent geochemistry. The sorption equilibrium studies were conducted under completely mixed conditions in batch reactors (pH=5.0 and ionic strength= 0.01 M) at room temperature. The Freundlich model provided better fits to the experimental data compared to Langmuir model. High cadmium and lead sorption was observed for the calcareous shales with greater than 5% CaCO₃. The Freundlich isotherm parameter relating to sorption capacity, i.e., K_F, yielded a strong correlation with the calcium carbonate and calcium oxide content across the various geosorbents studied. The observed sorption pattern may be attributed to complex formation of CaCO₃ with Pb²⁺ and Cd²⁺ leading to surface precipitation. Moreover, the Ca²⁺ present in the sorbents may also involve in ion exchange reaction with lead and cadmium.     

Sorption of monoaromatic compounds to heated and unheated coals,humic acid,and biochar: Implication for using combustion method to quantify sorption contribution of carbonaceous geosorbents in soil

Batch sorption isotherms of two nonpolar compounds (1,3-dichlorobenzene and 1,3,5-trichlorobenzene) and two polar compounds (1,3-dinitrobenzene and 1,3,5-trinitrobenzene) to heated (at 375 °C for 24 h) and unheated coals (lignite and anthracite) were compared with those to a soil humic acid and a maize stalk derived biochar. For all test compounds, unheated lignite and anthracite exhibited much stronger sorption than humic substances (the organic carbon-normalized distribution coefficient was up to 2–3 orders of magnitude larger), but lower sorption than biochar. This sorption trend is consistent with the degree of sorbent condensation (biochar > coal > humic acid). The results indicate that sorption of the test sorbates (regardless of the difference in polarity) to soils would be dominated by carbonaceous geosorbents. Notably, the organic carbon contents of the coals were pronouncedly lowered by the heat treatment, from 47.4% to 7.3% for lignite, and from 80.1% to 58.1% for anthracite. Moreover, the heat treatment markedly decreased organic carbon-normalized distribution coefficient to coals (up to one order of magnitude), attributable to the decreased hydrophobicity of sorbents due to increased O-containing groups from oxidation. An important implication is that heat treatment, which is commonly used to quantify the content of carbonaceous geosorbents in soil and sediment, may cause significant underestimation of sorption contribution of carbonaceous geosorbents due to the combined effect of reduced organic carbon content and decreased hydrophobicity of less graphitized carbonaceous geosorbents (coals). This was illustrated using a widely adopted dual-component model that combines linear partition to humic substances (represented by humic acid) and nonlinear adsorption on condensed geosorbents (represented by biochar and coal).     

The concentration of Cu(II), Pb(II) and Zn(II) from aqueous solutions by particulate algal matter

Experimental studies of the reactions of Cu(II), Pb(II), and Zn(II) in aqueous solutions with organic matter derived from fresh samples of the green filamentous algae Ulothrix spp. and the green unicellular algae Chlamydomonas spp. and Chlorella vulgaris show that, under suitable conditions, a significant proportion of the metals is removed from solution by sorption onto the particulate organic matter of the algal suspension.The metal sorption is strongly suppressed by H⁺ but is only marginally influenced by the proportion of whole cells in the suspension and by complexing of metals in solution by the soluble organic matter. The presence of relatively small amounts of the cations Na⁺ and Mg²⁺ in solution reduces the sorption of Zn(II) to near zero, but Pb(II) and Cu(II) sorption occurs to an appreciable extent even in strong brines. This may be a means for the selective precipitation of Pb(II) from brines rich in Pb(II) and Zn(II).Metal “saturation” values indicate that particulate algal matter of the type used in these experiments could sorb sufficient quantities of metal to form an ore deposit if a weight of organic matter of similar order of magnitude to that of the inorganic sediments in the deposits was available. However, the metal sorption is an equilibrium reaction, and the experimentally determined “enrichment factors” suggest that the “saturation” values could be approached only in solutions whose metal contents were initially at least two orders of magnitude above those of normal seawater.     

Experimental modeling of platinum sorption on organic matter

In connection with the discovery of a new type of Pt deposit in low-rank brown coals and black shales, the interaction of Pt-bearing aqueous solutions with fractionated organic matter (asphaltenes and asphaltenic acids) was studied at 200–400°C and 1 kbar total pressure. It was found that chemical sorption onto the organic matter lowers Pt content in the aqueous solutions by about two orders of magnitude relative to organic-free systems. Thermal maturation of the asphaltenes leads to its aromatization and concomitant sorption of Pt from n×10⁻⁴ mPt (mol per kg of dry matter) at 200°C to n×10⁻² mPt at 400°C. Thus, the Pt chemisorption on activated carbonized organic matter may be an effective mechanism of Pt accumulation in C-bearing rocks.     

Sorption and desorption of mercury(II) in saline and alkaline soils of Bahía Blanca, Argentina

The objective of this work was to study sorption–desorption and/or precipitation–dissolution processes of Hg(II) compounds considering an eventual contact of soils with Hg-bearing wastes. In addition, this study contributes new data about Hg(II) chemistry in alkaline systems. Saline and alkaline soils with low organic matter (<1 %) and high clay content (60–70 %) were obtained near a chlor-alkali plant. Batch techniques were used to perform the experiments using 0.1 M NaNO₃ solutions. Total Hg(II) concentrations ranged from 6.2 × 10^?8 to 6.3 × 10^?3 M. Sorption of Hg(II) was evaluated at two concentration ranges: (a) 6.2 × 10^?8 to 1.1 × 10^?4 M, and (b) 6.4 × 10^?4 to 6.3 × 10^?3 M. At low Hg(II) concentrations, adsorption occurred with a maximum sorption capacity ranging from 4 to 5 mmol/kg. At high Hg(II) concentrations, sorption–precipitation reactions occurred and maximum sorption capacity ranged from 17 to 31 mmol/kg. The distribution of Hg(II) hydrolysis products showed that Hg(OH)₂ was the predominant species under soil conditions. According to sorption experiments, X-ray diffraction and chemical speciation modelling, the presence of Hg(OH)₂ in the interlayer of the interstratified clay minerals can be proposed. Hg(OH)₂ was partially desorbed by repeated equilibrations in 0.1 M NaNO₃ solution. Desorption ranged from 0.1 to 0.9 mmol/kg for soils treated with 5.8 × 10^?5 M Hg(II), whereas 2.1–3.8 mmol/kg was desorbed from soils treated with 6.3 × 10^?3 M Hg(II). Formation of soluble Hg(II) complexes was limited by low organic matter content, whereas neutral Hg(OH)₂ was retained by adsorption on clay mineral surfaces.     

Abiotic interactions of natural dissolved organic matter and carbonate aquifer rock

Abiotic interactions between natural dissolved organic matter (NDOM) and carbonate aquifer rock may be controlling factors of biogeochemical processes and contaminant fate in carbonate aquifer systems. The importance and effects of these interactions were examined using batch adsorption experiments of soil NDOM and representative carbonate sorbents from the Floridan Aquifer. Adsorption of NDOM carbon to aquifer rocks was well-described using a modified linear model and was mostly reversible. Significant adsorption was observed at higher NDOM concentrations, while the release of indigenous organic matter from the rocks occurred at lower concentrations. Longer interaction periods led to more adsorption, indicating that adsorption equilibrium was not achieved. For relatively pure carbonate rock samples, sorbent surface area was found to be the most important controlling factor of adsorption, whereas the presence of indigenous organic matter and subdominant mineral phases were more important, when they occurred. Preferential adsorption of a high over low molecular weight and humic over fulvic components of NDOM onto carbonate sorbents was detected using liquid size exclusion chromatography and excitation–emission fluorometry, respectively. The presence of NDOM inhibited mineral dissolution, though this inhibition was not proportional to NDOM concentration as surface area and mineralogy of carbonate sorbents played additional roles. Though the NDOM–carbonate rock adsorption mechanism could not be completely determined due to the heterogeneity and complexity of NDOM and sorbent surfaces, it is speculated that both rapid and weak outer-sphere bonding and stronger but slower hydrophobic interaction occur. These results have important implications for groundwater quality and hydrogeologic projects such as aquifer storage and recovery.