Complexation of metal ions, including alkali-earth and lanthanide(III) ions, in aqueous solution by the ligand 2,2',6',2'-terpyridyl

Some metal-ion-complexing properties of the ligand 2,2',6',2'-terpyridyl (terpy) in aqueous solution are determined by following the π-π* transitions of 2 × 10(-5) M terpy by UV-visible spectroscopy. It is found that terpy forms precipitates when present as the neutral ligand above pH ～5, in the presence of electrolytes such as NaClO(4) or NaCl added to control the ionic strength, as evidenced by large light-scattering peaks. The protonation constants of terpy are thus determined at the ionic strength (μ) = 0 to avoid precipitation and found to be 4.32(3) and 3.27(3). The log K(1) values were determined for terpy with alkali-earth metal ions Mg(II), Ca(II), Sr(II), and Ba(II) and Ln(III) (Ln = lanthanide) ions La(III), Gd(III), and Lu(III) by titration of 2 × 10(-5) M free terpy at pH >5.0 with solutions of the metal ion. Log K(1)(terpy) was determined for Zn(II), Cd(II), and Pb(II) by following the competition between the metal ions and protons as a function of the pH. Complex formation for all of these metal ions was accompanied by marked sharpening of the broad π-π* transitions of free terpy, which was attributed to complex formation affecting ligand vibrations, which in the free ligand are coupled to the π-π* transitions and thus broaden them. It is shown that log K(1)(terpy) for a wide variety of metal ions correlates well with log K(1)(NH(3)) values for the metal ions. The latter include both experimental log K(1)(NH(3)) values and log K(1)(NH(3)) values predicted previously by density functional theory calculation. The structure of [Ni(terpy)(2)][Ni(CN)(4)]·CH(3)CH(2)OH·H(2)O (1) is reported as follows: triclinic, P1, a = 8.644(3) ?, b = 9.840(3) ?, c = 20.162(6) ?, α = 97.355(5)°, β = 97.100(5)°, γ = 98.606(5)°, V = 1663.8(9) ?(3), Z = 4, and final R = 0.0319. The two Ni-N bonds to the central N donors of the terpy ligands in 1 average 1.990(2) ?, while the four peripheral Ni-N bonds average 2.107(10) ?. This difference in the M-N bond length for terpy complexes is typical of the complexes of smaller metal ions, while for larger metal ions, the difference is reversed. The significance of the metal-ion size dependence of the selectivity of polypyridyl ligands, and the greater rigidity of ligands based on aromatic groups such as pyridyl groups, is discussed.     

Role of secondary ligands in the structure and stability of metal—cytidine complexes in solution

The interaction of Cu(II), Ni(II), Zn(II), Mn(II), Co(II), Mg and Ca ions with cytidine and the biologically important ligands histidine, histamine, glycine and oxalic acid in a 1:1:1 ratio have been investigated by potentiometric equilibrium measurements at 35°C and 0.10 M (KNO₃) ionic strength. These investigations were undertaken to assess the influence of the secondary ligands on the structure and stability of the 1:1 metal—cytidine system. The stability of the binary and ternary complexes has been compared. The enhanced stability of the ternary complexes was measured in terms of Δ log K, the difference between the ternary and binary complexes. The involvement of various donor sites of histidine in metal binding was specially discussed. A general conclusion drawn from this investigation is that aromatic ligands formed more stable complexes than aliphatic ligands. This is attributed to the stacking phenomenon.     

Cobalt(II) and Nickel(II) Transfer through Charged Polysulfonated Cation Exchange Membranes

The transport of Co(II) and Ni(II) ions through charged polysulfonated ion exchange membranes under Donnan dialysis conditions has been studied as a function of pH gradient at 25 degrees C. In the Donnan dialysis process, the membrane is bounded by two electrolyte solutions, the one side (donor phase) initially containing metal salts and the other H(2)SO(4) with no external potential field applied. The transport of metal ions through membranes was correlated with the flux data as well as with estimated diffusion coefficients and was found to depend on the interaction between the fixed groups in the membrane and the metal ions. It was observed that the pH gradient influences the transport of metals and the flux of ions increases with H ion concentration in the receiver phase. Copyright 2000 Academic Press.     

Interaction of radionickel with diatomite as a function of pH, ionic strength and temperature

Sequestration of Ni(II) on diatomite as a function of reaction time, pH, ionic strength, foreign ions and temperature were investigated by batch sorption technique. The results indicated that the sorption of Ni(II) on diatomite was quickly in the first contact time of 2 h and then slowly with increasing contact time. The interaction of Ni(II) with diatomite was strongly pH- and ionic strength-dependent at low pH values (i.e., which was dominated by ion exchange or outer-sphere surface complexation), while the pH-dependent and ionic strength-independent sorption at high pH suggested that inner-sphere or multinuclear surface complexation was the main sorption mechanism. With increasing temperature, the sorption of Ni(II) on diatomite increased and the experimental data were well fitted by Langmuir model. The sorption samples at pH 6.8 and 10.0 were also characterized by XPS spectroscopy, and the results suggested that Si atoms also participated in Ni(II) sorption on diatomite. The results are important to evaluate the physicochemical behavior of Ni(II) and other similar radionuclides and heavy metal ions in the environment.     

Adsorption of Cu(II) and Ni(II) on solid humic acid from the Azraq area, Jordan

Isotherms of adsorption of Cu(II) and Ni(II) onto solid Azraq humic acid (AZHA) were studied at different pH (2.0-3.7) values and 0.1 M NaClO4 ionic strength. The Langmuir monolayer adsorption capacity was found to range from 0.1 to 1.0 mmol metal ion/g AZHA, where Cu(II) has higher adsorptivity than Ni(II). The previously reported NICA-Donnan parameters for sorption of Cu(II) on HA fit the amount of Cu(bound) determined in the present study at pH 3.7 but underestimates those at pH values of 3.0, 2.4, and 2.0. The contribution of low affinity sites to binding of metal ions increases with decreasing pH and increasing metal ion loading. The aggregation of HA, which is facilitated by decreasing pH and increasing metal loading, may increase the ability of low-affinity sites to encapsulate metal ions. The binding of Ni(II) to HA exhibits less heterogeneity and less multidentism than that of Cu(II). AZHA loaded with Cu(II) and Ni(II) was found to be insoluble in water with no measurable amount of desorbed metal ions.     

Solid phase extraction and determination of metal ions in aqueous samples using Quercetin modified Amberlite XAD-16 chelating polymer as metal extractant

A new chelating polymer has been developed using Amberlite XAD-16 anchored with Quercetin. The modified polymer was characterised by Fourier Transform Infra Red (FTIR) spectroscopy, thermogravimetric analysis, surface area analysis and elemental analysis. The Quercetin anchored polymer showed superior binding affinity for Cr(III), Mn(II), Fe(III), Co(II), Ni(II) and Cu(II) with greater than 95% adsorption under optimum conditions. The optimum pH conditions for the quantitative sorption of metal ions were studied. The developed method showed superior extraction qualities with high metal loading capacities of 387, 313, 195, 473, 210 and 320 µmol g^?1 for Cu(II), Co(II), Cr(III), Fe(III), Mn(II) and Ni(II), respectively. The rate of metal ion uptake i.e. kinetics studies performed under optimum levels, showed t _1/2 for Co(II), Cu(II), Cr(III), Fe(III), Mn(II) and Ni(II) is 20, 15, 25, 10, 30 and 15 min, respectively. Desorption of metal ions was effective with 10 mL of 2 M HCl prior to analysis using flame atomic absorption spectrophotometer. The chelating polymer was highly ion selective in nature even in the presence of interferent ions, with a high preconcentrating ability for the metal ions of interest. The developed chelating polymer was tested on its utility with synthetic and real samples like river, tap water samples and also with multivitamin tablets. It showed relative standard deviation (R.S.D.) values of/less than 3.0% reflecting on the accuracy and reproducibility of data using the newly developed chelating polymer.     

Influence and role of metal ions in enzymatic catalysis with e.c. 1.2.3.2. xanthine oxidase Application to trace analysis

The effect of metal ions on the reductive half-reaction of xanthine oxidase (XOD) in the catalytic conversion of xanthine into uric acid has been studied spectrophotometrically in Tris-HCl buffer at pH 7.4, 37 +/- 0.1 degrees and ionic strength 0.04M. Some metal ions display inhibitor properties, the sequence of inhibiting efficiency being Ag(I) > Hg(II) > Cu(II) > Cr(VI) > V(V) > Au(III) > Tl(I) and for these the I(50) values were determined. Only Tl(I), V(V) and Cu(II) showed reversible inhibition and therefore for these the mechanisms were assessed [competitive for V(V) and Tl(I); uncompetitive for Cu(II)]. The conditional inhibition constants (K(i)) were also determined. The effect of EDTA for protection of the enzyme against metal inhibition, and for its reactivation after inhibition, was also investigated. Utilization of the linear relationship between relative enzyme activity and inhibitor concentration allowed sensitive and selective (though not specific) determination of Ag(I) and Hg(II) (10(-9)-10(-8)M), and of Cu(II) and Cr(VI) (10(-7)-10(-6)M), the maximum relative error being +/- 4%. For a few metal ions, e.g., Ag(I) and Cr(VI), in the presence of EDTA, a certain specificity is observed.     

Formation and dissociation kinetics of triaza-crown-alkanoic acid complexes of transition metal(II) and lanthanide (III)

The formation and dissociation rates of some transition metal(II) and lanthanide(III) complexes of the 1,7,13-triaza-4,10,16-trioxacyclooctadecane N',N',N'-triacetic acid (1) and 1,7,13-triaza-4,10,16-trioxacyclooctadecane-N',N',N'- trimethylacetic acid (2) have been measured by the use of stopped-flow and conventional spectrophotometry. Experimental observations were made at 25.0 +/- 0.1 degrees C and at an ionic strength of 0.10 M KCl. The complexation of Zn(2+) and Cu(2+) ions with 1 and 2 proceeds through the formation of an intermediate complex (MH(3)L(+) *) in which the metal ion is incompletely coordinated. This may then lead to a final product in the rate-determining step. Between pH 4.68 and 5.55, the diprotonated (H(2)L(-)) form is revealed to be a kinetically active species despite its low concentration. The stability constants (log K (MH (3)L (+) *)) and specific base-catalyzed rate constants (k(OH)) of intermediate complexes have been determined from the kinetic data. The dissociation reactions of 1 and 2 complexes of Co(2+), Ni(2+), Zn(2+), Ce(3+), Eu(3+) and Yb(3+) were investigated with Cu(2+) ions as a scavenger in acetate buffer. All complexes exhibit acid-independent and acid-catalyzed contributions. The buffer and Cu(2+) concentration dependence on the dissociation rate has also been investigated. The metal and ligand effects on the dissociation rate of some transition metal(II) and lanthanide(III) complexes are discussed in terms of the ionic radius of the metal ions, the side-pendant arms and the rigidity of the ligands.     

Unusual metal ion selectivities of the highly preorganized tetradentrate ligand 1,10-phenanthroline-2,9-dicarboxamide: a thermodynamic and fluorescence study

Some metal ion complexing properties of the ligand PDAM (1,10-phenanthroline-2,9-dicarboxamide) in aqueous solution are reported. Using UV-visible spectroscopy to follow the intense π-π* transitions of PDAM as a function of metal ion concentration, log K(1) values in 0.1 M NaClO(4) and at 25 °C are, for Cu(II), 3.56(5); Ni(II), 3.06(5); Zn(II), 3.77(5); Co(II), 3.8(1); Mg(II), 0.1(1); Ca(II), 1.94(4); and Ba(II), 0.7(1). For more strongly bound metal ions, competition reactions between PDAM and EDTA (ethylenedinitrilo-tetraacetic acid) or tetren (1,4,7,10,13-pentaazatridecane), monitored following the UV spectrum of PDAM, gave the following log K(1) values in 0.1 M NaClO(4) and at 25 °C: Cd(II), 7.1(1); Pb(II), 5.82(5); In(III), 9.4(1); and Bi(III), 9.4(1). The very low log K(1)(PDAM) values for small metal ions such as Cu(II) or Zn(II) are unprecedented for a phen-based ligand (phen = 1,10-phenanthroline), which is rationalized in terms of the low basicity of the N donors of the ligand (pK(a) = 0.6) and the fact that PDAM has a best-fit size corresponding to large metal ions of ionic radius ~1.0 ?. Large metal ions with ionic radius ≥1.0 ? show large increases in log K(1) relative to their phen complexes, which in turn produces unparalleled selectivities, such as a 3.5 log units greater log K(1)(PDAM) for Cd(II) than for Cu(II). PDAM shows strong fluorescence in aqueous solution, suggesting that its carboxamide groups do not produce a fluorescence-quenching photon-induced electron transfer (PET) effect. Only Ca(II) produces a weak CHEF (chelation enhanced fluorescence) effect with PDAM, while all other metal ions tested produce a decrease in fluorescence, a CHEQ (chelation enhanced quenching effect). The production of the CHEQ effect is rationalized in terms of the idea that coordination of metal ions to PDAM stabilizes a canonical form of the carboxamide groups that promotes a PET effect.     

Synthesis, potentiometric and antimicrobial studies on metal complexes of isoxazole Schiff bases

The metal complexes of Cu(II), Ni(II) and Co(II) with Schiff bases of 3-(2-hydroxy-3-ethoxybenzylideneamino)-5-methyl isoxazole [HEBMI] and 3-(2-hydroxy-5-nitrobenzylidene amino)-5-methyl isoxazole [HNBMI] which were obtained by the condensation of 3-amino-5-methyl isoxazole with substituted salicylaldehydes have been synthesized. Schiff bases and their complexes have been characterized on the basis of elemental analysis, magnetic moments, molar conductivity, thermal analysis and spectral (IR, UV, NMR and Mass) studies. The spectral data show that these ligands act in a monovalent bidentate fashion, co-ordinating through phenolic oxygen and azomethine nitrogen atoms. Chelates of Co(II), Ni(II) appear to be octahedral and Cu(II) appears to be distorted octahedral. To investigate the relationship between formation constants of binary complexes and antimicrobial activity, the dissociation constants of Schiff bases and stability constants of their binary metal complexes have been determined potentiometrically in aqueous solution at 30+/-1 degrees C and at 0.1 M KNO3 ionic strength and discussed. Antimicrobial activities of the Schiff bases and their complexes were screened. The structure-activity correlation in Schiff bases and their metal(II) complexes are discussed, based on the effect of their stability constants. It is observed that the activity enhances upon complexation and the order of activity is in accordance with stability order of metal ions.     

The mutual influence of ligands in transition metal bis-chelates

Electronic structure quantum chemical calculations of the Fe(II), Ni(II) and Cu(II) bis-chelate series are reported. The nature of the ligand mutual influence effect on these compounds is elucidated. This effect manifests itself in polarization of metal—ligand bonds. The ligands which have highly covalent bonds with the metal form bonds of even greater covalent character in mixed complexes, whereas bonds for the other ligands become more ionic.     

Transition metal complexes of poly(vinylpyridines)

The structures of the precipitates of free-radical poly(4-vinylpyridine) (Vpy), free-radical poly(2-Vpy) and isotactic poly(2-Vpy) with M(II)Cl₂ (M = Co, Ni, Cu, Zn) obtained from solution have been investigated. The polymer compounds are similar to the known crystalline monomeric Vpy complexes and, with one exception, are crosslinked by the metal dichloride. Co(II) and Zn(II) are tetrahedrally coordinated by the polymer, while the Ni(II) and Cu(II) complexes are probably tetrahedral and square-planar, respectively. Because of the constraints of the polymeric ligands the stoichiometries of the complexes are not exactly the same as those of the monomeric Vpy complexes and from one to two Vpy units per metal halide are on average not coordinated. Atactic and isotactic poly(2-Vpy) showed marked differences with regard to coordination of Ni(II). The questions of stereochemistry of the coordinated metal ion, stoichiometry of the complexes, intermolecular versus intramolecular complexation of the polymer chain, and the influence of polymer tacticity on the crystallizability of polymer complexes are discussed.     

A study of microdialysis sampling of metal ions

A study of the extraction fraction (EF) of metal ions Cd, Cr, Cu, Ni and Pb sampled by microdialysis from a quiescent aqueous solution is presented. A concentric type of microdialysis probe equipped with either one of two polysulfone membranes supplied by different manufacturers or a polyether sulfone membrane, all with a 10 mm effective dialysis length was used for these investigations. EF for metal ions achieved after microdialysis sampling were evaluated for membranes exhibiting a cut-off molecular weight of 3, 5, 10 and 30 kDa. The EF for all metal ions showed a dependency on membrane cut-off as well as membrane material. For Cr EFs of 0.70 and 0.80 were achieved at 1 μl/min using a polysulfone and polyether sulfone membrane, respectively, both with a 30 kDa cut-off molecular weight. Using the polysulfone membrane, Cr showed the highest EF and Pb had the lowest at 0.1. The polyether sulfone membrane achieved an EF of 0.95 for Ni and the lowest EF value was for Cu at 0.35. In these studies it is shown that pH as well as the inclusion of an optimal concentration (0.20 M) of 8-hydroxyquinoline (8-HQ) in the perfusion liquid can enhance the EF of metal ions. Microdialysis was also used to sample for metal ions from wastewater and from whole tomatoes grown using sewage sludge manure in order to demonstrate the potential to apply it to these complicated matrices.     

Understanding the adsorption mechanism of Ni(II) on graphene oxides by batch experiments and density functional theory studies

The graphene oxides(GOs) have attracted multidisciplinary study because of their special physicochemical properties. The high surface area and large amounts of oxygen-containing functional groups make GOs suitable materials for the efficient elimination of heavy metal ions from aqueous solutions. Herein the sorption of Ni(II) on GOs was studied using batch experiments, and the results showed that the sorption of Ni(II) is strongly dependent on p H and ionic strength at pH8, and independent of ionic strength at pH8. The sorption of Ni(II) is mainly dominated by outer-sphere surface complexation and ion exchange at low p H, and by inner-sphere surface complexation at high p H. The interaction of Ni(II) with GOs was also investigated by theoretical density functional theory(DFT) calculations, and the results show that the sorption of Ni(II) on GOs is mainly attributed to the –COH and –COC groups and the DFT calculations show that Ni(II) forms stable GO_Ni_triplet structure with the binding energy of -39.44 kcal/mol, which is in good agreement with the batch sorption experimental results. The results are important for the application of GOs as adsorbents in the efficient removal of Ni(II) from wastewater in environmental pollution cleanup.     

Ion-pair reversed-phase high-performance liquid chromatography for trace metal preconcentration followed by ion-interaction chromatography

Ion-interaction chromatography of Plasmocorinth B (a disulphonated azo dye) complexes of Co(III), Cu(II), Fe(III), Ga(III), In(III), Ni(II), V(V) and Zr(IV) was studied. The behaviour of two different reversed-phase C₁₈ columns (5 and 10 μm) was compared and an on-line enrichment procedure was developed following the optimization of eluent (pH, ligand concentration, ionic strength and organic modifier). The described technique, applied to the analysis of metal ions at μg/1 levels in natural waters, gave satisfactory precision and accuracy in comparison with inductively coupled plasma atomic emission spectroscopic results.     

Synthesis of ditopic ligands for the selective extraction of copper(II) nitrate and crystal structures of their Cu(II) complexes

We have synthesized two ditopic ligands for selective extraction of copper(II) nitrate. We also synthesized one cation-only binding analog for comparison. All three ligands were characterized by conventional techniques. Competitive two-phase metal ion solvent extraction experiments were performed at 25 °C over a period of 24 h. These ligands showed significant selectivity for Cu(II) ions, having the ditopic ligands extract 81 and 73% of the Cu(II) ions in a solution of different metal ions {Ni(II), Co(II), Cu(II), Zn(II), Cd(II), Pb(II)} at pH 5.09. Competitive transport experiments (water/chloroform/water) were undertaken employing each ligand separately as the ionophore in the membrane (chloroform) phase. No metal ion transport was observed, but a large concentration of Cu(II) was present in the membrane phase. Competitive anion extraction and transport were carried out with the ditopic ligands, yielding selective extraction and transport of nitrate. Furthermore, a pH isotherm of the best ditopic ligand (H₂L²) with Cu(II) was determined from pH 1.0 to 6.0, producing a pH_½ value of approximately 2.6. Finally, crystal structures of the ditopic ligands complexed with Cu(II) were determined and refined. The coordination geometry around the metal centers are distorted square planar and the Cu(II)-donor bond lengths fall within the normal range.     

Effective concentration difference model to study the effect of various factors on the effective diffusion coefficient in the dialysis membrane

Cellulose acetate dialysis membrane (CDM) has been used in the diffusive gradients in thin films (DGT) technique, where accurate diffusion coefficients are essential for the assessment of the concentrations of labile metal in solution. Effective concentration difference model (ECDM), based on the assumption that the effective diffusion coefficient of metal ion in the dialysis membrane is determined by the effective concentration difference (ΔC(e)) across the dialysis membrane, is proposed and applied to study the effect of ionic strength, binding agent, ligands and Donnan potential on the effective diffusion coefficient. The effective diffusion coefficients of Cd(2+) through the dialysis membrane immersed in receptor solutions with binding agent were almost the same as those in receptor solutions without binding agent at higher ionic strengths (0.01-1 M) but much higher than those at lower ionic strengths (0.001-0.0001 M). The effective diffusion coefficients of Cd(2+) through the dialysis membrane immersed in deionized water receptor solutions with binding agent were not significantly different from those in synthetic receptor solutions (receptor solutions with various ionic strengths) with binding agent. The DGT-labile fractions were measured in synthetic solutions and natural waters, which indicated that the effective diffusion coefficients, through the dialysis membrane immersed in the deionized water solution with binding agent as receptor solution and in the spiked natural water as source solution, were more suitable for DGT application.     

Adsorption behavior of metal ions onto a bovine serum albumin (BSA) membrane monitored by means of an electrode-separated piezoelectric quartz crystal.

The interaction between metal ions and bovine serum albumin (BSA) was studied by using a piezoelectric quartz crystal (PQC) arranged in the electrode-separated configuration. A silanized surface of the PQC was coated with a BSA membrane via a coupling reaction with glutaraldehyde. The frequency shifts obtained from PQC coated with a BSA membrane suggested that various kinds of metal ions could be adsorbed onto the BSA membrane from aqueous solutions containing a low concentration of metal ions (2 or 10 micromol dm(-3)), only when the BSA was denatured with an alkaline solution. Anionic species of Pt(IV) and Au(III) were adsorbed onto the denatured BSA membrane from an acetic acid solution at pH 2.2, and cationic species of Cd(II), Zn(II), Co(II), Ni(II), Cu(II), and Ag(I), and cations, such as Ca2+, Ba2+, and Mg2+, were adsorbed from ammonia buffer at pH 9.5, whereas Al(III), Cr(III), Fe(III), Hg(II), and Pb(II) were hardly adsorbed. The adsorption mechanisms of these metal ions are discussed, based on the electrostatic interaction between the metal ions and the denatured BSA membrane, and complex formation between the metal ions and amino acid residues of the denatured BSA. Further, the PQC coated with a denatured BSA membrane was applied to the determination of Pt and Cd, using large frequency shifts for Pt(IV) and Cd(II).     

An optical ionic-strength sensor based on polyelectrolyte association and fluorescence energy transfer

The optical ionic-strength sensor is based on an indicator phase consisting of an aqueous solution of fluorescein-labelled dextran and polyethyleneimine labelled with Sulforhodamine 101 (Texas Red), confined behind a dialysis membrane. At low ionic strength the polymers associate and the average distance between the fluorescein and Texas Red is short enough for efficient energy transfer to occur. With increasing ionic strength the polymers dissociate and the efficiency of energy transfer decreases. The measured parameter is the ratio of the emission intensity at 520 nm, where fluorescein fluorescence is maximal, to the intensity at 620 nm, where the Texas Red emission is strong. The increase in the intensity ratio as a function of ionic strength is similar but not quite the same for different ions, suggesting that the mechanism of response involves more than a simple ionic strength effect.     

Sorption of 63Ni(II) to montmorillonite as a function of pH, ionic strength, foreign ions and humic substances

Different kinds of clay minerals have been studied extensively in the removal of radionuclides from large volumes of aqueous solutions because of their high sorption capacity. Herein, the Na-montmorillonite was characterized by using XRD and FTIR in detail. The sorption of ⁶³Ni(II) from aqueous solution to montmorillonite as a function of pH, ionic strength, foreign ions, humic substances and temperature was studied by batch technique. The sorption of ⁶³Ni(II) on montmorillonite achieved equilibration quickly. The sorption of ⁶³Ni(II) to montmorillonite was strongly dependent on pH, and dependent on ionic strength at low pH and independent of ionic strength at high pH values. The sorption of ⁶³Ni(II) on montmorillonite was enhanced at low pH in the presence of humic acid (HA), while a negative effect of HA on ⁶³Ni(II) sorption was found at high pH values. At low pH values, the sorption of ⁶³Ni(II) was attributed to outer-sphere surface complexation or ion exchange, whereas the sorption was dominated by inner-sphere surface complexation at high pH values. The montmorillonite sample is a suitable material in the preconcentration of radionuclides from large volumes and the material can be used as backfill material in nuclear waste repository.