Surface Complexation Approach Research Articles

Metal and proton binding onto the roots of Fescue rubra

This study quantifies Pb and Cd adsorption onto the root cell walls of the grass species Festuca rubra by applying a surface complexation approach to model the observed adsorption behavior. We use potentiometric titrations to determine deprotonation constants and site concentrations for the functional groups on the root material. We model the acid/base properties of the root cell wall with a non-electrostatic model involving three discrete surface functional group types, with p K a values of 4.2 ± 0.1, 6.2 ± 0.2, and 8.8 ± 0.2. Pb adsorption kinetics experiments indicate that the experimental systems reach equilibrium within 3 h, and Pb desorption experiments indicate that the adsorption reactions are fully reversible on this time scale. Adsorption experiments conducted as a function of pH yield site-specific stability constants for the important Pb- and Cd-root surface complexes. The results of the Pb and Cd adsorption experiments indicate that the first two sites contribute to metal adsorption under the experimental conditions, with calculated log stability constant values of 4.1 ± 0.3 and 5.9 ± 0.3 for the Pb system; and 3.5 ± 0.4 and 5.0 ± 0.5 for the Cd system. Our results suggest that the stabilities of the Pb- and Cd-surface complexes are comparable to those involving other biomass surfaces, and therefore these complexes may affect the transport and distribution of metals in soil systems.

Adsorption of Heterogeneously Charged Nanoparticles on a Variably Charged Surface by the Extended Surface Complexation Approach: Charge Regulation, Chemical Heterogeneity, and Surface Complexation

Adsorption of randomly branched polyelectrolytes, "hairy" particles and internally structured macromolecules, collectively denoted as heterogeneously charged nanoparticles, on charged surfaces is important in many technological and natural processes. In this paper, we will focus on (1) the charge regulation of both the nanoparticle and the surface and (2) the surface complexation between the particle functional groups and the surface sites and will theoretically study the adsorption using the extended surface complexation approach. The model explicitly considers the electrochemical potential of a nanoparticle with an average (smeared-out) structure and charge both in bulk solution and on the surface to obtain the equilibrium adsorption. The chemical heterogeneity of the particle is described by a distribution of the protonation constant. Detailed analysis of the chemical potential of the adsorbed nanoparticle reveals that the pH and salt dependence of the adsorption can be largely explained by the balance between an energy gain resulting from the particle and surface charge regulation and the surface complexation and an energy loss from the unfavorable interparticle electrostatic repulsion close to the surface. This conclusion is also supported by the strong impacts that the chemical heterogeneity of the particle functional groups, the magnitude of the surface complexation, the number of the functional groups, and the size of the particle have on the adsorption.

Reactivity of Ferric Oxides toward H2S at Low pH

Goethite (gt), 2-line (21fh), 6-line ferrihydrite (61fh), and schwertmannite (shm) were reacted with H2S under steady-state conditions at pH 2.5-5 using a novel setup that consisted of an electrochemical sulfide generator transporting H2S into a suspension of oxides in a fluidized bed-reactor. The reactants were stoichiometrically converted into Fe2+ and So. Sulfate significantly inhibited the rates. Surface area normalized rates increased with pH. They depended on the mineral type following the order gt > 21fh > 61fh, contrasting previous observations at circumneutral pH with an inverse order. The rate of shm dissolution was highest probably due to direct reduction of dissolved ferric iron by H2S. A linear relationship between the observed rate and the surface species of H2S was derived from a surface complexation approach that allowed for the estimation of intrinsic rate constants kintr for the various oxides (38 min(-1), 1.4 min(-1), 0.29 min(-1) for gt, 21fh, and 61fh, respectively). kint decreased with increasing deltaGf for the reactions and seems to depend on the reduction potential of the solution. From this observation we derived the hypothesis that kint is determined by the flat-band potential of the mineral/solute interface at low pH while being affected by surface interactions at higher pH.

Adsorption of lanthanum to goethite in the presence of gluconate

SummaryWe have examined the effect of the gluconate anion, an analogue for cellulose degradation products, on the adsorption of trivalent lanthanum (La3+)#to goethite. Lanthanum is investigated as an analogue for the trivalent actinides. Batch pH adsorption edge experiments were used to quantify the adsorption of La3+in the absence of gluconate and in solutions where gluconate was present at a 1:1 mole ratio to lanthanum. Using available thermodynamic data, it is calculated that lanthanum is primarily present in solution as the free La3+ion at pH values up to 8.5 in the absence of gluconate. Above pH 8.5, solid La(OH)3precipitates from solution. In the presence of gluconate, complexation decreases the free La3+concentration in solution. The fraction of La3+complexed increases, from 3% to 50%, as the concentrations of La3+and gluconate were increased. Very little effect on the adsorption of La3+to goethite was observed in the presence of gluconate below pH 7. At pH values above 7, however, gluconate doubled the maximum amount of La3+adsorbed when present at concentrations that saturated the goethite adsorption sites. The presence of gluconate did not appear to inhibit the formation of La(OH)3(s) at pH 8.5 and milli molar lanthanum concentrations. Adsorption to the goethite surface was represented with a surface complexation approach using the diffuse double-layer model. Intrinsic binding constants for the surface complexes were estimated from the pH adsorption edge data using the computer code FITEQL 4.0 and visual curve fitting. Two surface reactions were used to fit the adsorption data in the absence of gluconate: 1) a strong binding site with no proton release and 2) a much higher concentration of weak binding sites with release of two protons per La3+adsorbed. In the presence of gluconate, a third surface complex was needed that involved a ternary complex of two lanthanum atoms with one gluconate molecule.

Experimental study of neptunyl adsorption onto Bacillus subtilis

The subsurface mobility of Np is difficult to predict in part due to uncertainties associated with its sorption behavior in geologic systems. In this study, we measured Np adsorption onto a common gram-positive soil bacterium, Bacillus subtilis. We performed batch adsorption experiments with Np(V) solutions as a function of pH, from 2.5 to 8, as a function of total Np concentration from 1.29 × 10 −5 M to 2.57 × 10 −4 M, and as a function of ionic strength from 0.001 to 0.5 M NaClO 4. Under most pH conditions, Np adsorption is reversible and exhibits an inverse relationship with ionic strength, with adsorption increasing with increasing pH. At low pH in the 0.1 M ionic strength systems, we observed irreversible adsorption, which is consistent with reduction of Np(V) to Np(IV). We model the adsorption reaction using a nonelectrostatic surface complexation approach to yield ionic strength dependent NpO 2 +-bacterial surface stability constants. The data require two bacterial surface complexation reactions to account for the observed adsorption behavior: R-L 1 − + NpO 2 + ↔ R-L 1-NpO 2 ° and R-L 2 − + NpO 2 + ↔ R-L 2-NpO 2 °, where R represents the bacterium to which each functional group is attached, and L 1 and L 2 represent the first and second of four discrete site types on the bacterial surface. Stability constants (log K values) for the L 1 and L 2 reactions in the 0.001 M system are 2.3 ± 0.3 and 2.3 ± 0.2, and in the 0.1 M system the values are 1.7 ± 0.2 and 1.6 ± 0.2, respectively. The calculated neptunyl-bacterial surface stability constants are not consistent with values predicted using the linear free energy correlation approach from Fein et al. (2001), suggesting that possible unfavorable steric interactions and the low charge of NpO 2 + affects Np-bacterial adsorption.

Surface properties of various powdered hydroxyapatites

Electrophoretic mobilities of various synthetic and semisynthetic hydroxyapatites (Ca 10(PO 4) 6(OH) 2, HAP) suspended in aqueous solutions have been measured as a function of pH and calcium concentration. The studied powders differ in particle size, crystallinity degree and surface contamination (carbonate). When equilibrated in mineral acids or bases, a large plateau of negative mobility is observed in the pH range 5–8, with increasing negative values at higher pH. Only in the case of the sample composed of nanoparticles, positive mobility obtains at pH < 8.9. When Ca 2+ is added, positive mobility values are observed for all samples, and a bell-shaped profile results as a function of pH. Two possible models are explored to describe the results: the Nernstian approach, which assumes solubility equilibrium and surface potentials determined by the three potential-determining ions (Ca 2+, PO 3− 4, and OH −), and the surface complexation approach, based on the idea of negligible phase transfer of structural phosphate. The Nernstian model is inadequate, whereas a very simple surface complexation model based on the equations Ca 5(PO 4) + 3 = Ca 4(PO 4) − 3 + Ca 2+, Ca 4(PO 4) − 3 + H + = Ca 4(PO 4) 2(PO 4H), Ca 5(PO 4) + 3 + OH − = Ca 5(PO 4) 3(OH), coupled with a very simple electrical double layer, model suffices to reproduce the bell-shaped profile of the mobility as a function of pH in the presence of added calcium salts. The results also show that the sample composed of nanoparticles exchanges ions more easily with the solution, without reaching the solubility equilibrium in the explored timespans. In the presence of soluble phosphate salts, it is postulated that the same surface ensembles define the surface charge, with participation of phosphate as described by the equation 4 5 Ca 5(PO 4) + 3 + 3 5 PO 3− 4 = Ca 4(PO 4) − 3. HAP is just one member of a family of calcium phosphates with different (Ca)/(P) ratios. Electrophoretic mobilities of another member, tricalcium diphosphate, Ca 3(PO 4) 2, were also measured and shown to be described by the same basic model. Comparison with previous literature data shows that the negative plateau in the mobility is a general feature of many HAP samples at low Ca 2+, again in agreement with the surface complexation model. FTIR data demonstrates that surface phosphate indeed undergoes protonation, as postulated in the model.

Surface Complexation Modeling of Proton and Cd Adsorption onto an Algal Cell Wall

This study quantifies Cd adsorption onto the cell wall of the algal species Pseudokirchneriella subcapitata by applying a surface complexation approach to model the observed adsorption behavior. We use potentiometric titrations to determine deprotonation constants and site concentrations for the functional groups on the algal cell wall. Adsorption and desorption kinetics experiments illustrate that adsorption of Cd onto the cell wall is rapid and reversible, except under low pH conditions. Adsorption experiments conducted as a function of pH and total Cd concentration yield the stoichiometry and site-specific stability constants for the important Cd-algal surface complexes. We model the acid/base properties of the algal cell wall by invoking four discrete surface functional group types, with pKa values of 3.9 +/- 0.3, 5.4 +/- 0.1, 7.6 +/- 0.3, and 9.6 +/- 0.4. The results of the Cd adsorption experiments indicate that the first, third, and fourth sites contribute to Cd adsorption under the experimental conditions, with calculated log stability constant values of 4.1 +/- 0.5, 5.4 +/- 0.5, and 6.1 +/- 0.4, respectively. Our results suggest that the stabilities of the Cd-surface complexes are high enough for algal adsorption to affect the fate and transport of Cd under some conditions and that on a per gram basis, algae and bacteria exhibit broadly similar extents of Cd adsorption.

Modeling As(V) removal by a iron oxide impregnated activated carbon using the surface complexation approach

The objective of this research was to model As(V) removal onto a iron oxide impregnated activated carbon (FeAC) using the surface complexation model (SCM) approach. As(V) removal by FeAC was due to the impregnated Fe oxide, not the base carbon material and was a strong function of pH. The two-monoprotic site-triple layer model adequately described As(V) removal using 2 fitting parameters compared with the 3 parameters needed for the diprotic site model. This, along with a better representation of the recognized As(V) removal mechanism (ligand exchange with -OH) as well as the acid-base behavior makes the two-monoprotic approach the better model for As(V) removal by the impregnated iron oxide although the diprotic model was able to describe the pH dependent removal of As(V). Both models were also able to predict As(V) removal at different adsorbent/adsorbate ratios using K(As) determined from a single FeAC adsorption experiment. Thus, fewer adsorption experiments are required in order to model As(V) removal in equilibrium and column systems. The results described in this work will be used as a foundation in developing a dynamic model to predict As(V) adsorption in a fixed-bed adsorber.

Cd and Proton Adsorption onto Bacterial Consortia Grown from Industrial Wastes and Contaminated Geologic Settings

To model the effects of bacterial metal adsorption in contaminated environments, results from metal adsorption experiments involving individual pure stains of bacteria must be extrapolated to systems in which potentially dozens of bacterial species are present. This extrapolation may be made easier because bacterial consortia from natural environments appear to exhibit similar metal binding properties. However, bacteria that thrive in highly perturbed contaminated environments may exhibit significantly different adsorptive behavior. Here we measure proton and Cd adsorption onto a range of bacterial consortia grown from heavily contaminated industrial wastes, groundwater, and soils. We model the results using a discrete site surface complexation approach to determine binding constants and site densities for each consortium. The results demonstrate that bacterial consortia from different contaminated environments exhibit a range of total site densities (approximately a 3-fold difference) and Cd-binding constants (approximately a 10-fold difference). These ranges for Cd binding constants may be small enough to suggest that bacteria-metal adsorption in contaminated environments can be described using relatively few "averaged" bacteria-metal binding constants (in conjunction with the necessary binding constants for competing surfaces and ligands). However, if additional precision is necessary, modeling parameters must be developed separately for each contaminated environment of interest.

Proton and Cd adsorption onto natural bacterial consortia: Testing universal adsorption behavior

Bacterial surface adsorption can control metal distributions in some natural systems, yet it is unclear whether natural bacterial consortia differ in their adsorption behaviors. In this study, we conduct potentiometric titration and metal adsorption experiments to measure proton and Cd adsorption onto a range of bacterial consortia. We model the experimental data using a surface complexation approach to determine thermodynamic stability constants. Our results indicate that these consortia adsorb similar extents of protons and Cd and that the adsorption onto all of the consortia can be modeled using a single set of stability constants. Consortia of bacteria cultured from natural environments also adsorb metals to lesser extents than individual strains of laboratory-cultivated species. This study suggests that a wide range of bacterial species exhibit similar adsorption behaviors, potentially simplifying the task of modeling the distribution and speciation of metals in bacteria-bearing natural systems. Current models for bacteria-metal adsorption that rely on pure strains of laboratory-cultivated species likely overpredict the amount of bacteria-metal adsorption in natural systems.

Modeling the adsorption of free and heavy metal complex-bound EDTA onto red mud by a nonelectrostatic surface complexation model

The adsorption of free and divalent heavy metal (copper, cadmium, and lead) complex-bound EDTA from metal–EDTA mixture solutions on a composite adsorbent having a heterogeneous surface, i.e., bauxite waste red mud, has been investigated and modeled with the aid of a nonelectrostatic surface complexation approach in respect to adsorbate concentration and pH dependency of EDTA adsorption. EDTA was selected as the modeling ligand in view of its wide usage as an anthropogenic chelating agent and its abundance in natural waters. The adsorption experiments were conducted for pure EDTA or metal–EDTA complexes alone, or in mixtures containing (EDTA+metal–EDTA). For all studied cases, the solid adsorbent phase concentrations of the adsorbed species were found by using the derived model equations with acceptable compatibility of experimental and theoretically generated adsorption isotherms. The model basically assumed two modes of metal bonding to the surface hydroxyls: ionic (outer-sphere) binding of the EDTA anion (H 2Y 2−) or anionic metal–EDTA complex (MY 2−) to the cationic surface site (∼SOH + 2), and outer-sphere binding of H 2Y 2− or MY 2− to the neutral ∼SOH site, possibly via hydrogen-bonding. The model was useful for EDTA and metal–EDTA mixture solutions either at their natural pH of equilibration with the sorbent, or after pH elevation with NaOH titration up to pH⩽pzc of red mud. Thus adsorption of every single species (H 2Y 2− or MY 2−) or of possible mixtures (H 2Y 2−+MY 2−) at natural pH or after NaOH titration could be calculated by the use of simple quadratic equations at low metal loadings, once the initial concentrations of the corresponding species, i.e., [H 2Y 2−] 0 or [MY 2−] 0, were known. The compatibility of theoretical and experimental data pairs of adsorbed species concentrations was verified by means of nonlinear regression analysis. The findings of this study, together with the previously developed (M 2++MY 2−) mixtures adsorption model, can be further developed to serve environmental risk assessment concerning the expansion of a metal–organic (synthetic organic ligand or soil humic acids) contaminant plume with groundwater movement in soil basically consisting of hydrated oxide-type minerals.

Quantifying Metal Adsorption onto Bacteria Mixtures: A Test and Application of the Surface Complexation Model

In this study, we measure Cd, Co, Sr, and Zn adsorption onto mixtures of Gram-positive and Gram-negative bacteria, containing the bacterial species Streptococcus faecalis , Staphyloccocs aureus , Sporosarcina ureae , Bacillus subtilis , Bacillus licheniformis , Escherichia coli , Pseudomonas aeruginosa , Enterobacter aerogenes , Proteus vulgaris , and Serratia marscecens . Metal adsorption experiments were conducted with mixtures of Gram-positive bacteria, Gram-negative bacteria, and mixtures consisting of both bacterial types. The objective is to compare the metal adsorption behavior of Gram-positive and Gram-negative bacteria, and determine if the surface complexation model can be applied to describe metal adsorption onto complex assemblages of different bacterial types. The experimental results indicate that the metal adsorption behaviors of Gram-positive mixtures, Gram-negative mixtures, and mixtures of both Gram-positive and Gram-negative bacteria are nearly identical. We describe the metal adsorption data with a site-specific chemical equilibrium model, which can successfully describe the pH and bacteria:metal ratio effects on metal adsorption. The metal binding constants determined with the bacteria mixtures in this study are similar to the binding constants that have previously been measured for B. subtilis , suggesting that adsorption constants can be estimated using thermodynamic parameters determined for a limited number of bacterial species. We then apply the metal adsorption constants determined for bacteria mixtures to a hypothetical bacteria-water-rock system to illustrate the utility of the surface complexation approach.

Modelling of Pu Sorption onto the Surface of Goethite and Magnetite as Steel Corrosion Products

ABSTRACTWe have studied the surface interaction between tetravalent plutonium and two different steel corrosion products: magnetite, which is the final product of the anoxic corrosion of steel, and goethite, which exemplifies a further oxidation of the steel surface due to the presence of active oxidants in the system. The pH of the experiments has been varied from 2 to 10 and the ionic strength from 0.001 to 0.1 M NaClO4. All the sorption experiments were carried out under N2 atmosphere. No significant effect of ionic strength was observed under the conditions studied.The pHpcz of the solids (7.78 for goethite and 6.95 for magnetite) was determined by modelling potentiometric titration data.The results of the experiments show that the sorption edge of plutonium occurs between pH 3 and 4 when using goethite as a sorbing surface and between pH 4 and 5 when magnetite is used.We have modelled the sorption data by using a simple surface complexation approach with no electrostatic term. The model used involves a reduction process of Pu(IV) to Pu(III) in the presence of magnetite, which can be explained by the interaction of the actinide with the ferrous iron present in the solid. In the case of the experiments conducted with goethite, this reduction process is not possible and, therefore, in the model we have included the sorption of tetravalent Pu.

Adsorption of cadmium ions at the electrolyte/silica interface

The equations developed by us for the triple layer surface complexation approach, taking into account energetic heterogeneity of surface oxygen, are applied here to study the heterogeneity influences on the adsorption of cadmium ions at the NaClO4/silica aqueous solution interface. The model of surface heterogeneity made it possible to fit in a quantitative way the data obtained in the four independent experiments: surface charge measurements (titration), measurements of electrophoretic mobility and monitoring radiometrically cation of inert electrolyte adsorption, together with cadmium ion adsorption at different pH values. Particularly the studies of cadmium ion adsorption show that the energetic heterogeneity of the silica surface oxygen should be taken into account.

Does metal adsorption onto bacterial surfaces inhibit or enhance aqueous metal transport? Column and batch reactor experiments on Cd– Bacillus subtilis–quartz systems

In this study, we investigate the effects of bacteria on metal transport through mineral-filled columns, and how the effect varies with pH and mineralogy. We measured Bacillus subtilis and aqueous Cd transport through quartz and Fe-coated quartz columns as a function of pH, using a separation technique to determine the absolute concentrations of aqueous and bacterially bound Cd in the effluent. Experimental results indicate that under certain conditions, bacteria enhance Cd transport through the columns. In these cases, the migration of Cd through the column is facilitated by Cd adsorption onto bacterial surfaces, and by transport of the bacteria. However, under other conditions, the transport of bacteria is inhibited, causing a retardation in Cd mobility. Under these conditions, the bacteria are immobile due to bacterial adsorption onto mineral surfaces and/or straining by the sand matrix. In separate experiments, we test whether a surface complexation modeling approach can be used to account for metal adsorption in complex systems such as these that contain both bacterial and mineral surfaces. We performed batch adsorption experiments with aqueous Cd, B. subtilis, and quartz, quantifying metal and bacterial adsorption as a function of pH. We use these experiments as a rigorous test and extension of the surface complexation approach to more realistic geologic systems than have been studied previously. The experimental results show that thermodynamic stability constants, determined from binary systems, can be used to successfully quantify the distribution of Cd between the aqueous phase and the bacterial and mineral surfaces, and can be used to estimate the distribution of mass in systems not directly studied in the laboratory. The column results indicate that modeling of contaminant transport in bacteria-bearing systems requires not only accurate flow models, but also chemical speciation models that quantify the role of bacterial adsorption under a range of subsurface conditions. Surface complexation modeling offers a means to account for the adsorption chemistry in these complex systems.

Metal adsorption onto bacterial surfaces: development of a predictive approach

Aqueous metal cation adsorption onto bacterial surfaces can be successfully modeled by means of a surface complexation approach. However, relatively few stability constants for metal-bacterial surface complexes have been measured. In order to determine the bacterial adsorption behavior of cations that have not been studied in the laboratory, predictive techniques are required that enable estimation of the stability constants of bacterial surface complexes. In this study, we use a linear free-energy approach to compare previously measured stability constants for Bacillus subtilis metal-carboxyl surface complexes with aqueous metal-organic acid anion stability constants. The organic acids that we consider are acetic, oxalic, citric, and tiron. We add to this limited data set by conducting metal adsorption experiments onto Bacillus subtilis, determining bacterial surface stability constants for Co, Nd, Ni, Sr, and Zn.The adsorption behavior of each of the metals studied here was described well by considering metal-carboxyl bacterial surface complexation only, except for the Zn adsorption behavior, which required carboxyl and phosphoryl complexation to obtain a suitable fit to the data. The best correlation between bacterial carboxyl surface complexes and aqueous organic acid anion stability constants was obtained by means of metal-acetate aqueous complexes, with a linear correlation coefficient of 0.97. This correlation applies only to unhydrolyzed aqueous cations and only to carboxyl binding of those cations, and it does not predict the binding behavior under conditions where metal binding to other bacterial surface site types occurs. However, the relationship derived in this study permits estimation of the carboxyl site adsorption behavior of a wide range of aqueous metal cations for which there is an absence of experimental data. This technique, coupled with the observation of similar adsorption behaviors across bacterial species (Yee and Fein, 2001), enables estimation of the effects of bacterial adsorption on metal mobilities for a large number of environmental and geologic applications.

Measurement of bacterial surface protonation constants for two species at elevated temperatures

This study reports the first potentiometric titrations of bacterial surfaces at elevated temperatures. We used a hydrogen electrode concentration cell to examine the protonation behavior of Bacillus subtilis (an aerobic, gram-positive species present in a wide variety of low-temperature, near-surface environments) at 30, 50, and 75°C. We also investigate the protonation of TOR-39 (an anaerobic, Fe-reducing, thermophylic bacteria isolated from a deep sedimentary basin) at 50°C and compare its surface properties to those of B.subtilis. Using a surface complexation approach that has been previously applied only to room temperature experiments involving bacteria, we determine the number of functional group types present on each cell wall, their acidity constants, and their site concentrations. We also characterize the temperature dependence of the acidity constants for B.subtilis. Our results indicate that the surface acidity can be successfully described using site-specific reactions involving discrete surface functional groups.Our results indicate that the acidity constants for the surface functional groups on B. subtilis do not change significantly over the temperature interval studied, and that the surface protonation at each temperature can be estimated using a single set of acidity constants and site densities. The best fitting model for B. subtilis is a three- site model with pKa values of 4.1, 5.8, and 7.7. Average site abundances for the three dominant functional groups on B. subtilis were determined to be 9.5 · 10−5, 1.0 · 10−4, and 8.8 · 10−5 moles of sites/gm. For TOR-39, the best fitting model was also a 3-site model with pKa values of 4.5, 5.8 and 8.2, with site abundances of 6.0 · 10−5, 5.0 · 10−5 and 3.0 · 10−5. This study suggests that bacterial surface site protonation reactions are not strongly temperature dependent, at least to 75°C. The lack of significant temperature dependence could greatly simplify the task of modeling bacteria-water-rock interactions at elevated temperature.

Quantifying the effects of bacteria on adsorption reactions in water–rock systems

This paper reviews the investigations that quantify metal–bacteria, organic–bacteria, and bacteria–mineral adsorption reactions. The studies are divided into the two main approaches used to quantify adsorption: bulk partitioning approaches and surface complexation modeling. Partitioning adsorption models, such as ones which use Langmuir or Freundlich isotherms, are more simple to apply than surface complexation models because they do not require a detailed understanding of the nature of the surfaces or adsorption mechanisms involved. These measurements can be successful in describing adsorption/desorption processes for the conditions of interest if the conditions can be directly simulated in the laboratory. However, partition coefficients are applicable only to the conditions (pH, fluid and/or mineralogical compositions) at which they were determined. Conversely, surface complexation models describe adsorption reactions explicitly, accounting for surface and aqueous speciation changes as a function of pH and solution composition. The equilibrium constants which describe the extent of adsorption in surface complexation models are invarient with respect to the parameters which affect partition coefficients. This paper reviews the experimental studies which use surface complexation modeling to quantify the observed bacterial adsorption reactions. Although the number of studies is small, the results indicate that the surface complexation approach can successfully account for metal–bacteria, organic acid–bacteria, and bacteria–mineral adsorption reactions. Therefore, it offers a powerful means for estimating the effects of bacteria on solute adsorption over a wide range of subsurface conditions.

Modeling of Copper(II), Cadmium(II), and Lead(II) Adsorption on Red Mud from Metal–EDTA Mixture Solutions

The adsorption of toxic heavy metal cations, i.e., Cu(II), Cd(II), and Pb(II), from metal–EDTA mixture solutions on a composite adsorbent having a heterogeneous surface, i.e., bauxite waste red mud, has been investigated and modeled with the aid of a modified surface complexation approach in respect to pH and complexant dependency of heavy metal adsorption. EDTA was selected as the modeling ligand in view of its wide usage as an anthropogenic chelating agent and abundance in natural waters. The adsorption experiments were conducted for metal salts (nitrates), metal–EDTA complexes alone, or in mixtures containing (metal+metal–EDTA). The adsorption equilibrium constants for the metal ions and metal–EDTA complexes were calculated. For all studied cases, the solid adsorbent phase concentrations of the adsorbed metal and metal–EDTA complexes were found by using the derived model equations with excellent compatibility of experimental and theoretically generated adsorption isotherms. The model was useful for metal and metal–EDTA mixture solutions either at their natural pH of equilibration with the sorbent, or after pH elevation with NaOH titration up to a certain pH. Thus adsorption of every single species (M2+ or MY2−) or of possible mixtures (M2++MY2−) at natural pH or after NaOH titration could be calculated by the use of simple quadratic model equations, once the initial concentrations of the corresponding species, i.e., [M2+]0 or [MY2−]0, were known. The compatibility of theoretical and experimental data pairs of adsorbed species concentrations was verified by means of nonlinear regression analysis. The findings of this study can be further developed so as to serve environmental risk assessment concerning the expansion of a heavy metal contaminant plume with groundwater move ment in soil consisting of hydrated-oxide type minerals.

Experimental Study of Uranyl Adsorption onto Bacillus subtilis

Uranyl adsorption onto the Gram-positive soil bacterium Bacillus subtilis was measured using batch experiments in 0.1 M NaClO4 as a function of pH, time, and solid:solute ratio at 25 °C. The experimental data were interpreted using a surface complexation approach. The experimental measurements constrain the stoichiometry and thermo- dynamic stabilities of the important uranyl-surface complexes. The U adsorption data require two separate adsorption reactions, with the uranyl ion forming surface complexes with the neutral phosphate functional groups and the deprotonated carboxyl functional groups of the bacterial cell wall: R−POH0 + UO22+ ⇔ R−POH−UO22+ (log K = 11.8 ± 0.2) and R−COO- + UO22+ ⇔ R−COO−UO2+ (log K = 5.4 ± 0.2). These new stability constants, in conjunction with other experimental and predicted stability constants, may be incorporated in surface complexation models to determine the mobility and fate of U in bacteria-bearing water−rock systems.