Preparation for Optimal Conformation of Lysozyme with Nanodiamond and Nanosilica as Carriers

Solutions of suspended particles of nanodiamond and nanosilica in deionized water 25 mg/10 mL were prepared for the purpose of achieving saturation adsorption. The respective saturation thresholds and adsorption reaction constants of lysozyme of extremely diluted concentrations 0–1000 nmol/L in 7 mmol/L (potassium phosphate buffer solution) PPBS on the surfaces of nanodiamond and nanosilica of 100 nm diameter have been measured. The adsorbed quantities of lysozyme on a unit surface area of both nanoparticles can be derived. The possible influences by the adsorption surface profile and adsorption surface area, adsorption ability of the nanoparticles, strength and activity of lysozyme have been discussed.     

Adsorption and desorption of lysozyme on nano-sized magnetic particles and its conformational changes

Adsorption and desorption of lysozyme on nano-sized magnetic particles and its conformational change were studied in this work. Adsorption of lysozyme on nano-sized magnetic particles (Fe₃O₄) was carried out at different pH. Maximum adsorption of lysozyme (4.65 mg/m²) occurred at its isoelectric point (pI=11.1). Differential scanning calorimetry (DSC) results show that the lysozyme adsorbed on magnetic particles did not show any thermal transition over the range 20–100 °C. High desorption of lysozyme from magnetic particles was achieved using NaH₂PO₄ (pH 4.0) (90%) and NaSCN (pH 6.0) (97%) as desorbents. The conformational change of the lysozyme desorbed by NaH₂PO₄ was small, while the lysozyme desorbed by NaSCN underwent a significant conformational change as measured by the intrinsic fluorescence. Eighty-eight and 82% activity was retained in the desorbed enzyme for desorption by NaH₂PO₄ and NaSCN, respectively.     

Concanavalin a immobilized affinity adsorbents for reversible use in yeast invertase adsorption

Concanavalin A (Con A) immobilized poly(2-hydroxyethyl methacrylate) (PHEMA) beads were investigated for specific adsorption of yeast invertase from aqueous solutions. PHEMA beads were prepared by a suspension polymerization technique with an average size of 150-200 microm, and activated by epichlorohydrin. Con A was then immobilized by covalent binding onto these beads. The maximum Con A immobilization was found to be 10 mg/g. The invertase-loading capability of the PHEMA/Con A beads was 107 mg/g. The maximum invertase adsorption capacity on the PHEMA/Con A adsorbents was observed at pH 5.0. The values of the Michaelis constant K(m) of invertase were significantly larger upon adsorption, indicating decreased affinity by the enzyme for its substrate, whereas V(max) was smaller for the adsorbed invertase. Adsorption improved the pH stability of the enzyme as well as its temperature stability. Thermal stability was found to increase with adsorption. The adsorbed enzyme activity was found to be quite stable in repeated experiments. Storage stability of adsorbed invertase.     

Characterizing protein activities on the lysozyme and nanodiamond complex prepared for bio applications

Recently, nanodiamond particles have attracted increasing attention as a promising nanomaterial for its biocompatibility, easy functionalization and conjugation with biomolecules, and its superb physical/chemical properties. Nanodiamonds are mainly used as markers for cell imaging, using its fluorescence or Raman signals for detection, and as carriers for drug delivery. For the success of these applications, the biomolecule associated with the nanodiamond has to retain its functionality. In this work, the protein activities of egg white lysozyme adsorbed on nanodiamond particles of different sizes is investigated. The lysozyme nanodiamond complex is used here as a protein model for analyzing its structural conformation changes and, correspondingly, its enzymatic activity after the adsorption. Fourier-transform infrared spectroscopy (FTIR) is used for the analysis of the sensitive protein secondary structure. To access the activities of the adsorbed lysozyme, a fluorescence-based assay is used. The process of adsorption is also analyzed using UV-visible spectroscopic measurements in combination with analysis of nanodiamond properties with FTIR, Raman spectroscopy, and ζ-potential measurements. It is found that the activity of lysozyme upon adsorption depends on the nanodiamond's size and surface properties, and that the nanodiamond particles can be selected and treated, which do not alter the lysozyme functional properties. Such nanodiamonds can be considered convenient nanoparticles for various bioapplications.     

Analysis of hen egg white lysozyme adsorption on Si(Ti)O2 ∣ aqueous solution interfaces at low ionic strength: a biphasic reaction related to solution self-association

Adsorption kinetics of hen-egg white lysozyme at concentrations of 10⁻⁷, 10⁻⁶, 10⁻⁴ M and at pH 8.0 have been measured on Si_0.8Ti_0.2O₂ surfaces by means of optical waveguide lightmode spectroscopy and were found to proceed in two distinct kinetic régimes: a fast and quasi linear increase of the surface concentration followed by slower kinetics that could be described qualitatively by the theory of random sequential adsorption. The transition between these two kinetic régimes was correlated with abrupt changes in the values of the mean refractive index and of the thickness of the adsorbed layer as well as in the desorption kinetics performed either before or after the transition from the first régime to the second. Experiments carried out at the same ionic strength but at different pH values showed that the adsorption behaviour of lysozyme is related to its self-association properties in highly concentrated solutions.     

Adsorption and protein-induced metal release from chromium metal and stainless steel

A research effort is undertaken to understand the mechanism of metal release from, e.g., inhaled metal particles or metal implants in the presence of proteins. The effect of protein adsorption on the metal release process from oxidized chromium metal surfaces and stainless steel surfaces was therefore examined by quartz crystal microbalance with energy dissipation monitoring (QCM-D) and graphite furnace atomic absorption spectroscopy (GFAAS). Differently charged and sized proteins, relevant for the inhalation and dermal exposure route were chosen including human and bovine serum albumin (HSA, BSA), mucin (BSM), and lysozyme (LYS). The results show that all proteins have high affinities for chromium and stainless steel (AISI 316) when deposited from solutions at pH 4 and at pH 7.4 where the protein adsorbed amount was very similar. Adsorption of albumin and mucin was substantially higher at pH 4 compared to pH 7.4 with approximately monolayer coverage at pH 7.4, whereas lysozyme adsorbed in multilayers at both investigated pH. The protein-surface interaction was strong since proteins were irreversibly adsorbed with respect to rinsing. Due to the passive nature of chromium and stainless steel (AISI 316) surfaces, very low metal release concentrations from the QCM metal surfaces in the presence of proteins were obtained on the time scale of the adsorption experiment. Therefore, metal release studies from massive metal sheets in contact with protein solutions were carried out in parallel. The presence of proteins increased the extent of metals released for chromium metal and stainless steel grades of different microstructure and alloy content, all with passive chromium(III)-rich surface oxides, such as QCM (AISI 316), ferritic (AISI 430), austentic (AISI 304, 316L), and duplex (LDX 2205).     

Uranium(VI) Sorption Complexes on Montmorillonite as a Function of Solution Chemistry

We have investigated the effect of changes in solution chemistry on the nature of uranyl sorption complexes on montmorillonite (SAz-1) at different surface coverages (1.43-53.6 μmol/g). Uranyl uptake onto SAz-1 between pH 3 and 7 was determined in both titration and batch-mode experiments. These pH values result in solutions that contain a range of monomeric and oligomeric aqueous uranyl species. Continuous-wave and time-resolved emission spectroscopies were used to investigate the nature of U(VI) sorbed to SAz-1. A discrete set of uranyl surface complexes has been identified over a wide range of pH values at these low to moderate coverages. For all samples, two surface complexes are detected with spectral characteristics commensurate with an inner-sphere complex and an exchange-site complex; the relative abundance of these two species is similar over these pH values at low coverage (1.43-2.00 μmol/g). In addition, surface species having spectra consistent with polymeric hydroxide-like sorption complexes form at the moderate coverages ( approximately 34-54 μmol/g), increasing in abundance as the capacity of the amphoteric surface sites is exceeded. Furthermore, a species with spectral characteristics anticipated for an outer-sphere surface complex is observed for wet paste samples at low pH (3.7-4.4) and both low ( approximately 2 μmol/g) and moderate ( approximately 40 μmol/g) coverage. There are only subtle differences in the nature of sorption complexes formed at different pH values but similar coverages, despite markedly different uranyl speciation in solution. These results indicate that the speciation in the solution has minimal influence on the nature of the sorption complex under these experimental conditions. The primary control on the nature and abundance of the different uranyl sorption complexes appears to be the relative abundance and reactivity of the different sorption sites. Copyright 2001 Academic Press.     

Adsorption of microcystin-Lr onto iron oxide nanoparticles

In this study, the adsorption of microcystin-LR onto iron oxide (maghemite) nanoparticles from water was examined. Factors influencing the sorption behavior included microcystin and maghemite concentration, pH, ionic strength, and the presence of natural organic matter. Adsorption of microcystin-LR was strongly affected by pH. The adsorption increased with decreasing pH, with a maximum adsorption around pH 3. Adsorption of microcystin-LR on maghemite was primarily attributed to electrostatic interactions, although hydrophobic interactions may also play a role. The extent of microcystin-LR adsorption onto maghemite increased with increasing ionic strength at pH 6.4, since salt ions screened the electrostatic repulsion between adsorbed microcystin molecules. Adsorption of microcystin-LR was not significantly affected by the presence of Suwannee River Fulvic acid (SRFA) below 2.5 mg/L. However, adsorption decreased at higher SRFA concentrations (2.5–25 mg/L) due to competitive adsorption between SRFA and microcystin-LR for limited sorption sites.     

Lysozyme adsorption studies at the silica/water interface using dual polarization interferometry

Lysozyme adsorption at the silica/water interface has been studied using a new analytical technique called dual polarization interferometry. This laboratory-based technique allows the build up or removal of molecular layers adsorbing or reacting on a lightly doped silicon dioxide (silica) surface to be measured in terms of thickness and refractive index changes with time. Lysozyme adsorption was studied at a range of concentrations from 0.03 to 4.0 g dm(-3) and at both pH 4 and pH 7. Adsorbed layers ranging from 14 to 43 +/- 1 A in thickness and 0.21 to 2.36 +/- 0.05 mg m(-2) in mass coverage were observed at pH 4 with increasing lysozyme concentration, indicating a strong deformation of the monolayer over the low concentration range and the formation of an almost complete sideways-on bilayer toward the high concentration of 4 g dm(-3). At pH 7, the thickness of adsorbed layers varied from 16 to 54 +/- 1 A with significantly higher surface coverage (0.74 to 3.29 +/- 0.05 mg m(-2)), again indicating structural deformation during the initial monolayer formation, followed by a gradual transition to bilayer adsorption over the high concentration end. The pH recycling performed at a fixed lysozyme concentration of 1.0 g dm(-3) indicated a broadly reversible adsorption regardless of whether the pH was cycled from pH 7 to pH 4 and back again or vice versa. These observations are in good agreement with earlier studies undertaken using neutron reflection although the fine details of molecular orientations in the layers differ subtly.     

Binary adsorption of globular proteins on ion-exchange media

Adsorption equilibrium of binary pairs of lysozyme (LYS), cytochrome c (CYC) and ribonuclease A (RNase) has been measured on different cation-exchange media at various solution conditions. Adsorption patterns largely follow the intrinsic protein–surface interactions, but can differ significantly for different pairs or even for one pair at different solution conditions. LYS/CYC adsorption shows similar behavior on all the adsorbents examined, with competitive adsorption dominated by LYS and the presence of LYS reducing the adsorption of CYC significantly. Simultaneous and sequential measurements for LYS/CYC show that the order of adsorption does not have a significant effect on the adsorption equilibrium. For LYS/RNase, LYS is consistently more strongly adsorbed. For CYC/RNase, both proteins can display significant adsorption, depending on the pH and salt concentration. A model based on colloidal energetics is developed to calculate the binary adsorption isotherms using parameter values obtained from single-component isotherms. The calculated adsorption is in good agreement with experimental results, with significantly better representation than for other commonly used binary isotherms.     

氨基功能化纳米复合材料对磷酸盐的吸附研究

合成了4种氨基(乙二胺(EDA), 二乙烯三胺(DETA), 三乙烯四胺(TETA)和四乙烯五胺(TEPA))功能化纳米复合材料(NH₂-NCMs, 分别命名为EDA-NCMs, DETA-NCMs, TETA-NCMs和TEPA-NCMs). 采用FTIR(傅立叶变换红外光谱分析), TG/DTG(热重差热分析), XPS(X-射线光电子能谱分析)等手段对其进行了表征, 并考察了其对水中磷酸盐的吸附性能. 结果表明: 溶液pH对其吸附性能影响较大, 在pH为2.5的条件下, 4种吸附剂对磷酸盐的吸附效果最佳; 在5 min内即可达到平衡吸附量的90%; 4种吸附剂对磷酸盐的吸附均符合Langmuir等温吸附模型; 吸附行为均符合准二级速率模型; 吸附反应为自发进行的放热反应和熵减的过程; 推测其吸附机理以静电作用力为主. 4种吸附剂用于磷酸盐含量为50 mg/L的废水处理的吸附率均大于97%, 均达到一级A出水的排放标准(≤1.53 mg/L, 以PO₄^3-计).     

Adsorption of zearalenone by organomodified natural zeolitic tuff

Adsorption of zearalenone (ZEN) by natural zeolitic tuff, modified with different numbers of octadecyldimethylbenzylammonium (ODMBA) ions, was investigated. The results of solid-state 1H NMR analysis of the starting material suggested that zeolitic tuff is rich in mineral clinoptilolite, confirming the results of previous thermal stability study. Three organozeolites (OZ-2, OZ-5, and OZ-10) were prepared with ODMBA surface coverages of 20, 50, and 100 mmol/100 g. The mechanism of ZEN sorption by the three organozeolites was investigated through the determination of the adsorption isotherms at pH 3, 7, and 9. Adsorption of ZEN by organozeolites was best represented by a linear type of isotherm at pH 3, while at pH 7 and 9, adsorption of ZEN by organozeolites followed a nonlinear (Langmuir) type of isotherm. The different shape of the ZEN adsorption isotherms for the three organozeolites with different levels of ODMBA at the zeolitic surface at different pH values suggests that the adsorption mechanism may be dependent on the form of ZEN in solution. Since, at pH 3, ZEN exists in solution as the neutral form, the linear isotherms at pH 3 suggested that hydrophobic interactions are probably responsible for adsorption of neutral, hydrophobic ZEN onto the hydrophobic surface of the organozeolites. At pH 7, the phenolate anion is present in water solution, while at pH 9, ZEN is almost entirely in the anionic form. The nonlinear isotherms obtained for ZEN adsorption by the three organozeolites suggest that sorption appears to be the result of the adsorption process as well as partitioning.     

Adsorption of organic matter at mineral/water interfaces. IV. Adsorption of humic substances at boehmite/water interfaces and impact on boehmite dissolution

The adsorption of Suwannee River fulvic acid (SRFA) and Pahokee peat humic acid (PPHA) at the boehmite (gamma-AlOOH)/water interface and the impact of SRFA on boehmite dissolution have been examined over a wide range of solution pH conditions (pH 2-12), SRFA surface coverages (Gamma(SRFA), total SRFA binding site concentration normalized by the boehmite surface area) of 0.0-5.33 micromol m(-2), and PPHA surface coverages (Gamma(PPHA), PPHA binding site concentration normalized by boehmite surface area) of 0.0-4.0 micromol m(-2), using macroscopic adsorption and in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. At relatively high SRFA surface coverages (Gamma(SRFA) = 5.33 micromol m(-2)), in situ ATR-FTIR spectral features of adsorbed SRFA are very similar to those measured for SRFA in solution at approximately 1-3 pH units higher. At sub-monolayer surface coverages (Gamma(SRFA) = 1.20 and 2.20 micromol m(-2)), several new peaks and enhancements of the intensities of a number of existing peaks are observed. The latter spectral changes arise from several nonorganic extrinsic species (i.e., adsorbed carbonate and water, for alkaline solution conditions), partially protonated SRFA carboxyl functional groups (near-neutral pH conditions), and small quantities of inner-spherically adsorbed SRFA carboxyl groups and/or Al(III)-SRFA complexes (for acidic conditions). The spectra of PPHA adsorbed at boehmite/water interfaces also showed changes generally consistent with our observations for SRFA sorbed on boehmite. These observations confirm that SRFA and PPHA are predominantly adsorbed at the boehmite/water interface in an outer-sphere fashion, with minor inner-sphere adsorption complexes being formed only under quite acidic conditions. They also suggest that the positively charged boehmite/water interface stabilizes SRFA and PPHA carboxyl functional groups against protonation at lower pH. Measurements of the concentration of dissolved Al(III) ions in the absence and presence of SRFA showed that the boehmite dissolution process is clearly inhibited by the adsorption of SRFA, which is consistent with previous observations that outer-spherically adsorbed organic anions inhibit Al-(oxyhydr)oxide dissolution.     

Comparison of adsorption performances of metal–chelated polyamide hollow fibre membranes in lysozyme separation

Commercially available microporous polyamide hollow fibres are modified by acid hydrolysis to activate the reactive groups and subsequently binding of the ligand, i.e. Cibacron Blue F3GA. Then the Cibacron Blue F3GA-derived hollow fibres were loaded with different metal ions (i.e. Zn(II), Cu(II), Ni(II)) to form the metal chelate. The internal polymer matrix was characterised by scanning electron microscopy. The effects of pH, initial concentration of lysozyme, metal type and temperature on the adsorption of lysozyme to the metal–chelated hollow fibres were examined in a batch reactor. The non-specific adsorption of lysozyme onto the polyamide hollow fibres was 1.8 mg/g. Cibacron Blue F3GA immobilisation increased the lysozyme adsorption up to 62.3 mg/g. Metal–chelated hollow fibres showed a significant increase of the adsorption efficiency. Lysozyme adsorption capacities of Zn(II), Cu(II) and Ni(II)-chelated hollow fibres were different. The maximum capacities of Zn(II), Cu(II) or Ni(II)-chelated hollow fibres were 144.2, 75.2 and 68.6 mg/g, respectively. Significant amount of the adsorbed lysozyme (up to 97%) was eluted in 1 h in the elution medium containing 1.0 M NaSCN at pH 8.0 and 25 mM EDTA at pH 4.9. Repeated adsorption–desorption process showed that this novel metal–chelated polyamide hollow fibres are suitable for lysozyme adsorption.     

Silica nanoparticle size influences the structure and enzymatic activity of adsorbed lysozyme

Adsorption of chicken egg lysozyme on silica nanoparticles of various diameters has been studied. Special attention has been paid to the effect of nanoparticle size on the structure and function of the adsorbed protein molecules. Both adsorption patterns and protein structure and function are strongly dependent on the size of the nanoparticles. Formation of molecular complexes is observed for adsorption onto 4-nm silica. True adsorptive behavior is evident on 20- and 100-nm particles, with the former resulting in monolayer adsorption and the latter yielding multilayer adsorption. A decrease in the solution pH results in a decrease in lysozyme adsorption. A change of protein structure upon adsorption is observed, as characterized by a loss in alpha-helix content, and this is strongly dependent on the size of the nanoparticle and the solution pH. Generally, greater loss of alpha helicity was observed for the lysozyme adsorbed onto larger nanoparticles under otherwise similar conditions. The activity of lysozyme adsorbed onto silica nanoparticles is lower than that of the free protein, and the fraction of activity lost correlates well with the decrease in alpha-helix content. These results indicate that the size of the nanoparticle, perhaps because of the contributions of surface curvature, influences adsorbed protein structure and function.     

Coverage-dependent adsorption of atomic sulfur on Fe(110): a DFT study

Adsorption of atomic sulfur at different coverages on the Fe(110) surface is examined using density functional theory (DFT) in order to investigate the effect that adsorbate-adsorbate interactions may have on the surface properties. S is adsorbed in the high-symmetry adsorption sites: 4-fold hollow, bridge, and atop sites in the following surface arrangements: c(2 x 2) and p(1 x 1) which correspond to coverages of 1/2 and 1 monolayer, respectively. The binding energy, work function change, adsorption geometry, charge density distribution, magnetic properties, and density of states are examined and compared to our previous study of S adsorbed at 1/4 monolayer coverage and p(2 x 2) arrangement [Spencer et al. Surf. Sci. 2003, 540, 420]. It was found that S forms polar covalent bonds to the surface. The bonding goes from being S-Fe dominated at the low coverages to being S-S dominated at the higher coverages where the S atoms are located closer together on the surface and interact with each other.     

FTIR spectroscopic study of low temperature NO adsorption and NO + O2 coadsorption on H-ZSM-5

Adsorption of NO and coadsorption of NO and O2 on H-ZSM-5 have been studied at low and room temperature by means of FTIR spectroscopy. For better interpretation of the spectra, experiments involving isotopic labeled molecules have been performed. Low temperature adsorption of NO on H-ZSM-5 results initially in formation of NO which is H-bonded to the zeolite acidic hydroxyls. A second NO molecule is inserted into the OH-NO species at higher coverages, thus forming OH(NO)2 complexes. Different kinds of NO dimers are also formed. Negligible amounts of oxygenated compounds have been detected. In the presence of oxygen, the (di)nitrosyl species are oxidized very fast even at 100 K to N2O3, NO+, NO2, and N2O4. Different kinds of adsorbed N2O3 species have been evidenced. With increasing temperature, NO+ migrates and occupies cationic positions. The latter species interacts with NO at low temperature to give an [ONNO]+ complex. This reaction is used to prove that the different bands in the 2206-2180 cm(-1) region are also due to NO+ species.     

New alpha-amino phenylalanine tetrazole ligand for immobilized metal affinity chromatography of proteins

A new chelating compound has been developed for use in the immobilized metal affinity chromatographic (IMAC) separation of proteins. The bidentate ligand, alpha-amino phenylalanine tetrazole, 4, was synthesized via a five-step synthesis from N-fluorenylmethoxycarbonyl phenylalanine and then immobilized onto silica through the epoxide coupling procedure. The binding behavior of the resulting IMAC sorbent, following chelation with Zn2+ to a density of 183 micromol Zn2+ ions/g silica, was characterized by the retention of proteins in the pH range of 5.0-8.0, and by the adsorption behavior of lysozyme with frontal chromatography at pH 6.0 and 8.0. The prepared column showed the separation ability to four test proteins and the retention time of these proteins increased with an increase in pH. From the derived isotherms, the adsorption capacity, qm, for the binding of lysozyme to immobilized Zn2+-alpha-amino phenylalanine tetrazole-silica was found to be 1.21 micromol/g at pH 6.0 and 1.20 micromol/g sorbent at pH 8.0, respectively, whilst the dissociation constants KD at these pH values were 5.22x10(-6) and 3.49x10(-6) M, respectively, indicating that the lysozyme was retained more stable under alkaline conditions, although the binding capacity in terms of micromole protein per gram sorbent remained essentially unchanged.     

Lysozyme Adsorption onto Different Supports: A Comparative Study

The interaction between proteins and solid surfaces has been investigated. The aim of this work is to compare three different materials (hydroxyapatite, polystyrene with core-shell structure (PE-CS) and a functionalized styrene divinylbenzene copolymer) to be used as adsorbents for lysozyme, known as a “hard” protein. Tests were performed according to an experimental design in order to compare the effects of pH, lysozyme and phosphate buffer concentration onto the adsorbed amount of protein. A 2³ factorial design and a cross design, which was performed in triplicate, were used to distinguish the most important variables and to infer about the interaction between them. Hydroxyapatite showed the best performance—higher adsorbed amount of lysozyme and smaller dispersion (72.2 ± 0.9 mg/g). However, PE-CS can be regarded as a promising support as high amounts of lysozyme are adsorbed onto this material with relatively small dispersion.     

Modeling the adsorption of Cd(II) onto kaolinite and Muloorina illite in the presence of citric acid

The adsorption of cadmium onto kaolinite and Muloorina illite in the presence of citric acid has been measured as a function of pH and cadmium concentration at 25 degrees C. When citric acid is present in the systems cadmium adsorption is slightly enhanced below pH 5, but significantly suppressed between pH 5 and 8, for both substrates. At higher citric acid concentrations very little cadmium adsorbs onto kaolinite from pH 5 to 8. Above pH 8 adsorption of Cd(II) onto illite is enhanced in the presence of citric acid, especially at lower concentrations, but this does not occur for kaolinite. Adsorption and potentiometric titration data were fitted by simple extended constant-capacitance surface complexation models for the two substrates. Enhancement of adsorption at lower pH values was ascribed to the ternary reaction [X(-)--K(+)](0)+Cd(2+)+L(3-)+2H(+) right arrow over left arrow (0)+K(+) involving outer-sphere complexation with permanently charged X(-) sites on the "silica" faces of both clay minerals. The models suggested that suppression of adsorption in the intermediate pH range was due to the formation of a strong CdL(-) solution complex which adsorbed neither on the permanently charged sites nor on the surface hydroxyl groups at the edges of the clay crystals. At higher pH values the dominant solution complex, CdLOH(2-), apparently adsorbed as an outer-sphere complex at surface hydroxyl groups on illite, SOH+2Cd(2+)+L(3-) right arrow over left arrow [SOCd(+)--CdOHL(2-)](-)+2H(+), but not on kaolinite. This difference in behavior results from the presence of =FeOH groups on the illite surface which can form surface complexes with CdLOH(2-), while the =AlOH groups on the kaolinite surface cannot.