Modeling the adsorption of Cd (II), Cu (II), Ni (II), Pb (II) and Zn (II) onto montmorillonite

The adsorption of five toxic metallic cations, Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II), onto montmorillonite was investigated as a function of pH and ionic strength and a two-site surface complexation model was used to predict the adsorption data. The results showed that in the lower pH range, 3∼6 for Cd, Cu, Ni and Zn, and 3∼4.5 for Pb, the adsorption was greatly affected by ionic strength, while in the higher pH range, the adsorption was not. In the lower pH range, the metallic cations were mainly bound through the formation of outer-sphere surface on the permanently charged basal surface sites (≡X⁻), while in the higher pH range the adsorption occurred mainly on the variably charged edge sites (≡SOH) through the formation of inner-sphere surface complexes. Acid-base surface constants and metal binding constants for the two sites were optimized using FITEQL. The adsorption affinity of the five metallic cations to the permanently charged sites of montmorillonite was Pb > Cu > Ni ≈ Zn ≈ Cd, while that to the variable charged sites was Pb ? Cu > Zn > Cd > Ni.     

Surface complexation model for multisite adsorption of copper(II) onto kaolinite

We measured the adsorption of Cu(II) onto kaolinite from pH 3-7 at constant ionic strength. EXAFS spectra show that Cu(II) adsorbs as (CuO₄H_n)ⁿ⁻⁶ and binuclear (Cu₂O₆H_n)ⁿ⁻⁸ inner-sphere complexes on variable-charge ≡AlOH sites and as Cu²⁺ on ion exchangeable ≡X--H⁺ sites. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed at least up to pH 6.5. Inner-sphere complexes are bound to the kaolinite surface by corner-sharing with two or three edge-sharing Al(O,OH)₆ polyhedra. Our interpretation of the EXAFS data are supported by ab initio (density functional theory) geometries of analog clusters simulating Cu complexes on the {110} and {010} crystal edges and at the ditrigonal cavity sites on the {001}. Having identified the bidentate (≡AlOH)₂Cu(OH)₂⁰, tridentate (≡Al₃O(OH)₂)Cu₂(OH)₃⁰ and ≡X--Cu²⁺ surface complexes, the experimental copper(II) adsorption data can be fit to the reactions

     

Surface complexation modeling of Cu(II) adsorption on mixtures of hydrous ferric oxide and kaolinite

Background  

The application of surface complexation models (SCMs) to natural sediments and soils is hindered by a lack of consistent models and data for large suites of metals and minerals of interest. Furthermore, the surface complexation approach has mostly been developed and tested for single solid systems. Few studies have extended the SCM approach to systems containing multiple solids.     

Goethite adsorption of Cu(II), Pb(II), Cd(II), and Zn(II) in the presence of sulfate: Properties of the ternary complex

Adsorption of Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ onto goethite is enhanced in the presence of sulfate. This effect, which has also been observed on ferrihydrite, is not predicted by the diffuse layer model (DLM) using adsorption constants derived from single sorbate systems. However, by including ternary surface complexes with the stoichiometry FeOHMSO₄, where FeOH is a surface adsorption site and M²⁺ is a cation, the effect of SO₄²⁻ on cation adsorption was accurately predicted for the range of cation, goethite and SO₄²⁻ concentrations studied. While the DLM does not provide direct molecular scale insights into adsorption reactions there are several properties of ternary complexes that are evident from examining trends in their formation constants. There is a linear relationship between ternary complex formation constants and cation adsorption constants, which is consistent with previous spectroscopic evidence indicating ternary complexes involve cation binding to the oxide surface. Comparing the data from this work to previous studies on ferrihydrite suggests that ternary complex formation on ferrihydrite involves complexes with the same or similar structure as those observed on goethite. In addition, it is evident that ternary complex formation constants are larger where there is a stronger metal-ligand interaction. This is also consistent with spectroscopic studies of goethite-M²⁺-SO₄²⁻ and phthalate systems showing surface species with metal-ligand bonding. Recommended values of ternary complex formation constants for use in SO₄-rich environments, such as acid mine drainage, are presented.     

Surface complexation modeling of Co(II) adsorption on mixtures of hydrous ferric oxide, quartz and kaolinite

Co sorption was measured as a function of pH, ionic strength (0.001-0.1 M NaNO₃) and sorbate/sorbent ratio on pure quartz, HFO and kaolinite and on binary and ternary mixtures of the three solids. Sorption data measured for the pure solids were used to derive internally-consistent diffuse layer surface complexation model (DLM) stability constants for Co sorption. Co sorption on HFO could be adequately modeled over a broad range of ionic strengths and sorbate/sorbent ratios with a two variable-charge site model. Fits based on a single variable-charge site model were reasonable, but were improved by using ionic-strength dependent stability constants. A single variable-charge site model with an additional permanent ion exchange site produced the best fit to Co edges measured on kaolinite over a range of ionic strength and sorbate/sorbent ratios. These DLM fits were also improved by using ionic-strength dependent stability constants. The DLM approach could not adequately describe the slope of Co sorption edges on quartz. This study demonstrates that for accurate prediction of Co sorption over wide ranges of ionic strength and sorbate/sorbent ratio, the DLM may require ionic-strength dependent stability constants. DLM stability constants for Co sorption derived for the pure solids were used to predict sorption as a function of pH and solid concentration on binary and ternary mixtures of the three solids. Discrepancies between predictions and measurements were quantitatively similar to those observed for the pure mineral systems. Thus, a simple component additivity approach provides useful predictions of metal sorption in the mixed solid systems.     

Cd adsorption onto Anoxybacillus flavithermus: Surface complexation modeling and spectroscopic investigations

Several recent studies have applied surface complexation theory to model metal adsorption behaviour onto mesophilic bacteria. However, no investigations have used this approach to characterise metal adsorption by thermophilic bacteria. In this study, we perform batch adsorption experiments to quantify cadmium adsorption onto the thermophile Anoxybacillus flavithermus. Surface complexation models (incorporating the Donnan electrostatic model) are developed to determine stability constants corresponding to specific adsorption reactions. Adsorption reactions and stoichiometries are constrained using spectroscopic techniques (XANES, EXAFS, and ATR-FTIR). The results indicate that the Cd adsorption behaviour of A. flavithermus is similar to that of other mesophilic bacteria. At high bacteria-to-Cd ratios, Cd adsorption occurs by formation of a 1:1 complex with deprotonated cell wall carboxyl functional groups. At lower bacteria-to-Cd ratios, a second adsorption mechanism occurs at pH > 7, which may correspond to the formation of a Cd-phosphoryl, CdOH-carboxyl, or CdOH-phosphoryl surface complex. X-ray absorption spectroscopic investigations confirm the formation of the 1:1 Cd-carboxyl surface complex, but due to the bacteria-to-Cd ratio used in these experiments, other complexation mechanism(s) could not be unequivocally resolved by the spectroscopic data.     

Surface complexation of Pb(II) by hexagonal birnessite nanoparticles

Natural hexagonal birnessite is a poorly crystalline layer type Mn(IV) oxide precipitated by bacteria and fungi which has a particularly high adsorption affinity for Pb(II). X-ray spectroscopic studies have shown that Pb(II) forms strong inner-sphere surface complexes mainly at two sites on hexagonal birnessite nanoparticles: triple corner-sharing (TCS) complexes on Mn(IV) vacancies in the interlayers and double edge-sharing (DES) complexes on lateral edge surfaces. Although the TCS surface complex has been well characterized by spectroscopy, some important questions remain about the structure and stability of the complexes occurring on the edge surfaces. First-principles simulation techniques such as density functional theory (DFT) offer a useful way to address these questions by providing complementary information that is difficult to obtain by spectroscopy. Following this computational approach, we used spin-polarized DFT to perform total-energy-minimization geometry optimizations of several possible Pb(II) surface complexes on model birnessite nanoparticles similar to those that have been studied experimentally. We first validated our DFT calculations by geometry optimizations of (1) the Pb-Mn oxyhydroxide mineral, quenselite (PbMnO₂OH), and (2) the TCS surface complex, finding good agreement with experimental structural data while uncovering new information about bonding and stability. Our geometry optimizations of several protonated variants of the DES surface complex led us to conclude that the observed edge-surface species is very likely to be this complex if the singly coordinated terminal O that binds to Pb(II) is protonated. Our geometry optimizations also revealed that an unhydrated double corner-sharing (DCS) species that has been proposed as an alternative to the DES complex is intrinsically unstable on nanoparticle edge surfaces, but could become stabilized if the local coordination environment is well-hydrated. A significant similarity exists in the structural parameters for the TCS complex and those for a DCS edge-surface complex that is protonated in the same manner as the optimal DES complex, which could complicate detecting the DCS complex in X-ray absorption spectra.     

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.     

Metal ion complex formation constants of some sedimentary humic acids with Zn(II), Cu(II) and Cd(II)

Metal ion complex formation constants were determined for several sedimentary humic acids (SHA) derived from a fresh water lake and several coastal marine environments, using a method based on size exclusion chromatography. Only one type of binding site was observed in all cases, and conditional log K_f values of between 5 and 7 (at pH 8, I = 0.01 M) were found. Elemental composition of the SHA was similar to soil HA, except that nitrogen content was significantly higher in the SHA. Other chemical properties of the SHA were consistent with those reported by other workers. While spectroscopic measurements indicated that the SHA may have contained significant amounts of polysaccharide compounds which were not removed by conventional separation and purification procedures, analysis indicated only very low levels of polysaccharides were present in the SHA.     

Competitive adsorption of Cd,Cu, Pb and Zn by different soils of Eastern China

Simultaneous competitive adsorption behavior of Cd, Cu, Pb and Zn onto nine soils with a wide physical–chemical characteristics from Eastern China was measured in batch experiments to assess the mobility and retention of these metals in soils. In the competitive adsorption system, adsorption isotherms for these metals on the soils exhibited significant differences in shape and in the amount adsorbed. As the applied concentration increased, Cu and Pb adsorption increased, while Cd and Zn adsorption decreased. Competition among heavy metals is very strong in acid soils with lower capacity to adsorb metal cations. Distribution coefficients (K _dmedium) for each metal and soil were calculated. The highest K _dmedium value was found for Pb and followed by Cu. However, low K _dmedium values were shown for Zn and Cd. On the basis of the K _dmedium values, the selectivity sequence of the metal adsorption is Pb > Cu > Zn > Cd and Pb > Cu > Cd > Zn. The adsorption sequence of nine soils was deduced from the joint distribution coefficients (K _dΣmedium). This indicated that acid soils with low pH value had lower adsorption capacity for heavy metals, resulting in much higher risk of heavy metal pollution. The sum of adsorbed heavy metals on the soils could well described using the Langmuir equation. The maximum adsorption capacity (Q _m) of soils ranged from 32.57 to 90.09 mmol kg⁻¹. Highly significant positive correlations were found between the K _dΣmedium and Q _m of the metals and pH value and cation exchange capacity (CEC) of soil, suggesting that soil pH and CEC were key factors controlling the solubility and mobility of the metals in soils.     

Adsorption of aqueous Pb(II), Cu(II), Zn(II) ions by amorphous tin(VI) hydrogen phosphate: an excellent inorganic adsorbent

Amorphous tin(VI) hydrogen phosphate (ATHP) was synthesized using the liquid phase precipitation method and served as an adsorbent to remove Pb(II), Cu(II), and Zn(II) from aqueous solutions. The ATHP was characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption–desorption techniques. Adsorption properties were evaluated as a function of pH, reaction time, concentration of reactants, and salinity. Their equilibrium adsorption data were modeled using Freundlich, Langmuir, and Dubinin–Kaganer–Radushkevich isotherms, respectively. The results revealed that adsorption equilibrium reached within 180 min. ATHP indicated good adsorption even below the pH_ZPC, and best adsorption at pH 5 for Pb(II) and Cu(II) and at pH 5.5 for Zn(II) was observed. Equilibrium data fitted better to the Langmuir model for Pb(II) and Cu(II) and fitted better to the Freundlich model for Zn(II). The saturated adsorption capacities deduced from the Langmuir model were 2.425, 1.801, and 0.600 mmol/g for Cu(II), Pb(II), and Zn(II), respectively, indicating an adsorption affinity order of Cu > Pb > Zn. There is a negative correlation between the concentration of NaCl and adsorption capacity of ATHP, yet ATHP still exhibited excellent adsorption having an adsorption capacity of 19.35, 15.16, 6.425 mg/g when the concentration of NaCl was 0.6 mol/L. The free energy (E) was 12.33, 10.70, and 14.74 kJ/mol for Pb(II), Cu(II), and Zn(II), respectively. An adsorption mechanism based on ion exchange between heavy metal ions and H⁺ in the ATHP is proposed. Furthermore, the used ATHP was regenerated by HCl solution and the adsorbent was used repeatedly.     

The adsorption of Cu,Pb, Zn,and Cd on goethite from major ion seawater

The adsorption of Cu, Pb, Zn, and Cd on goethite (αFeOOH) from NaNO₃ solutions and from major ion seawater was compared to assess the effect of the major ions of seawater (Na, Mg, Ca, K, Cl, and SO₄) on the adsorption behavior of the metals. Magnesium and sulphate are the principal seawater ions which enhance or inhibit adsorption relative to the inert system. Their effect, as determined from the site-binding model of Davis et al. (1978), was a combination of changing the electrostatic conditions at the interface and decreasing the available binding sites.The basic differences between the experimental system of major ion seawater and natural seawater were examined. It was concluded that: 1) although the experimental metal concentrations in major ion seawater were higher than those found in natural seawater, estimates of the binding energy of Cu, Zn, and Cd with αFeOOH for natural seawater concentrations could be made from the data, 2) Cu, Pb, Zn, and Cd showed little or no competition for surface sites on goethite, and 3) the presence of carbonate, phosphate, and silicate had little or no effect on the adsorption of Zn and Cd on goethite.     

Assessing Cd,Co, Cu,Ni, and Pb Sorption on montmorillonite using surface complexation models

Cadmium, Co, Cu, Ni and Pb adsorption is measured on montmorillonite as a function of pH (3–11), ionic strength (0.001–0.1 M NaNO₃), and sorbate concentration (0.1–10 μM metal on 0.5 g/L solid). Sorption of all metals shows strong dependence on ionic strength and sorbate concentration, as well as a break in the slope of the edge, indicative of a 2-site interaction with montmorillonite. The resulting adsorption edges are used to parameterize diffuse layer surface complexation models (DLMs) for each metal. A 2-site DLM with a bidentate variable charge surface hydroxyl site and a bidentate permanent charge exchange site produced good fits for the individual experiments, but lacked the robustness to accurately predict adsorption across the entire experimental range. Other models, such as CCM, TLM, or CD-MUSIC may be required for more accurate predictions across broad ranges of solution conditions.     

Comparison of linear and nonlinear forms of isotherm models for Zn(II) and Cu(II) sorption on a kaolinite

The most appropriate method in designing the adsorption systems and assessing the performance of the adsorption systems is to have an idea on adsorption isotherms. Comparison analysis of linear least square method and nonlinear method for estimating the isotherm parameters was made using the experimental equilibrium data of Zn(II) and Cu(II) onto kaolinite. Equilibrium data were fitted to Freundlich, Langmuir, and Redlich–Peterson isotherm equations. In order to confirm the best-fit isotherms for the adsorption system, the data set using the chi-square (χ ²), combined with the values of the determined coefficient (r ²) was analyzed. Nonlinear method was found to be a more appropriate method for estimating the isotherm parameters. The best fitting isotherm was the Langmuir and Redlich–Peterson isotherm. The Redlich–Peterson is a special case of Langmuir when the Redlich–Peterson isotherm constant g was unity. The sorption capacity of kaolinite to uptake metal ions in the increasing order was given by Cu (4.2721 mg/g)?<?Zn (4.6710 mg/g).     

Influence of operating conditions on the removal of Cu, Zn, Cd and Pb ions from wastewater by adsorption

Nile Rose Plant was used to study adsorption of several cations (Cu²+, Zn²+, Cd²+ and Pb²+) from wastewater within various experimental conditions. The dried leaves of Nile Rose Plant were used at different adsorbent/ metal ion ratios. The influence of pH, contact time, metal concentration, and adsorbent loading weight on the removal process was investigated. Batch adsorption studies were carried out at room temperature. The adsorption efficiencies were found to be pH dependent, increasing by increasing the pH in the range from 2.5 to 8.5 exept for Pb. The equilibrium time was attained within 60 to 90 min. and the maximum removal percentage was achieved at an adsorbent loading weight of 1.5 g/50 mL mixed ions solution. Isothermal studies showed that the data were best fitted to the Temkin isotherm model. The removal order was found to be Pb²⁺> Zn²⁺> Cu²⁺> Cd²⁺. The surface IR-characterization of Nile rose plant showed the presence of many functional groups capable of binding to the metal cations.     

Trace metals (Cd,Pb, Cu,Zn and Ni) in sediment of the submarine pit Dragon ear (Soline Bay,Rogoznica, Croatia)

Vertical profiles of trace metal (Cd, Pb, Zn, Cu, Ni) concentrations, organic matter content, carbonate content and granulometric composition were determined in two sediment cores from the submarine pit Dragon Ear (Middle Adriatic). Concentrations of the analyzed metals (Cd: 0.06–0.12 mg kg⁻¹, Pb: 28.5–67.3 mg kg⁻¹, Zn: 17.0-65.4 mg kg⁻¹, Cu: 21.1–51.9 mg kg⁻¹, Ni: 27.8–40.2 mg kg⁻¹) were in usual range for Adriatic carbonate marine sediments. Nevertheless, concentrations of Cu, Zn, and especially Pb in the upper layer of sediments (top 12 cm) were higher than in bottom layer, while Cd and Ni concentration profiles were uniform. Regression analysis and principal component analysis were used to interpret distribution of trace metals, organic matter and carbonate content in sediment cores. Results of both analysis showed that concentrations of all trace metals in the core below the entrance to the pit were significantly positively correlated with organic matter and negatively correlated with carbonate, while in the core more distant from the entrance only Pb showed significant positive correlation with organic matter. Obtained results indicated that, except for lead which was enriched in surface sediment, in the time of sampling (before the building of the nautical marina) investigated area belonged to unpolluted areas.     

Preparation and solubility of northupite from brine and its adsorption properties for Cu(II) and Cd(II) in seawater

The mineral northupite Na₃Mg(CO₃)₂Cl was synthesized from a solar Adriatic seawater brine pond to which Na₂CO₃ was added at 373°K. The precipitated northupite had a surface area (P) of 6.0 ± 0.4 m²g⁻¹, and the thermodynamic solubility product was estimated to be log K Na₃Mg(CO₃)₂Cl = −4.8 ± 0.3 at 25°C. This value was used to calculate the interfacial energy (σ = 50 erg cm⁻²) for the homogeneous nucleation of northupite. The solubility constant determined in this study has been used to examine the saturation state of Mahega Lake and Lake Katwe (Uganda). The waters from Lake Katwe were found to be supersaturated with respect to northupite.The adsorption of Cu and Cd onto northupite particles was studied in seawater. Both metals are strongly adsorbed. Adsorption constants and the specific area of northupite occupied by Cd and Cu using Langmuir adsorption isotherms and equilibrium constants for surface complex formation have been determined.