Modeling the adsorption of Cd(II) onto Muloorina illite and related clay minerals

The adsorption of Cd(II) onto goethite, kaolinite, and illite was measured as a function of pH (adsorption edges) and concentration (adsorption isotherms) at 25 degrees C. As the pH was increased, adsorption onto goethite occurred mainly in the pH range 5.5-8, whereas adsorption onto kaolinite occurred in two stages, separated by a plateau in the pH region 5.5 to 7. Adsorption onto illite increased steadily as the pH was increased, with far less Cd(II) adsorbing onto illite than onto goethite or kaolinite per m(2) of mineral surface area. Potentiometric titrations of suspensions of each mineral, with and without Cd(II) present, were also completed. Results from all three types of experiments were modeled using an extended constant- capacitance surface complexation model. The reactions [Formula: see text] [Formula: see text] and [Formula: see text] best described Cd(II) adsorption onto goethite, while [Formula: see text] and [Formula: see text] best described Cd(II) adsorption onto kaolinite. A combination of the first, second, and fourth of these reactions best fitted the data for Cd(II) adsorption onto illite. In each case the model fitted all experimental data well. The results suggest that adsorption onto the variable charge (SOH) sites on illite more closely resembles adsorption onto goethite than onto kaolinite.     

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.     

Removal of As(V) by Cu(II)-, Ni(II)-, or Co(II)-doped goethite samples

The present study reports removal of As(V) by adsorption onto laboratory-prepared pure and Cu(II)-, Ni(II)-, and Co(II)-doped goethite samples. The X-ray diffraction patterns showed only goethite as the crystalline phase. Doping of ions in the goethite matrix resulted in shift of d-values. Various parameters chosen for adsorption were nature of adsorbent, percentage of doped cations in goethite matrix, contact time, solution pH, and percentage of adsorbate. It was observed that the pH(pzc) of the goethite surface depended on the nature and concentration of metal ions. The surface area as well as the loading capacity increased with the increase of dopant percentage in goethite matrix. A maximum loading capacity of 19.55 mg/g was observed for 2.7% Cu(II)-doped goethite. The adsorption kinetics for Ni(II), Co(II) and for undoped goethite attained a quasi-equilibrium state after 30 min with almost negligible adsorption beyond this time. In case of Cu(II)-doped goethite samples, the quasi-equilibrium state for As(V) adsorption was observed after 60 min. At each studied pH condition, it was observed that the percentage of adsorption of As(V) decreased in the order Cu(II)-doped goethite > or = Ni(II)-doped goethite > Co(II)-doped goethite > pure goethite. The adsorption followed: Langmuir isotherm, indicating monolayer formation.     

Adsorption of cadmium(II) onto goethite and kaolinite in the presence of benzene carboxylic acids

The effect of benzene carboxylic acids on the adsorption of Cd(II) (5×10⁻⁵ M) by goethite and kaolinite has been studied in 0.005 M NaNO₃ at 25°C. The concentrations of phthalic (benzene-1,2-dicarboxylic acid), hemimellitic (1,2,3), trimellitic (1,2,4), trimesic (1,3,5), pyromellitic (1,2,4,5) and mellitic (1,2,3,4,5,6) acids varied from 2.5×10⁻⁵ to 1×10⁻³ M. Mellitic acid complexes Cd(II) strongly above about pH 3, but the other acids only at higher pH, phthalic acid forming the weakest complexes. Phthalic, trimesic and mellitic acids adsorbed strongly to goethite at pH 3, but adsorption decreased at higher pH; however, mellitic acid was still about 50% adsorbed at pH 9, by which the other two were almost entirely in solution. At 10⁻³ M all the acids enhanced the adsorption of Cd(II) to goethite, the higher members of the series being the most effective. The higher members of the series suppressed Cd(II) adsorption onto kaolinite, but phthalic and trimesic acids caused slight enhancement. The effects of mellitic acid on Cd(II) adsorption depended strongly on its concentration. The maximum enhancement of Cd(II) adsorption onto goethite was at 10⁻⁴ M. The greatest suppression of Cd(II) adsorption onto kaolinite was at 10⁻³ M, and at 2.5×10⁻⁵ M mellitic acid enhanced Cd(II) adsorption onto kaolinite at intermediate pH. The results are interpreted in terms of complexation between metal and ligand (acid), metal and substrate, ligand and substrate, and the formation of ternary surface complexes in which the ligand acts as a bridge between the metal and the surface.     

2-line ferrihydrite: synthesis, characterization and its adsorption behaviour for removal of Pb(II), Cd(II), Cu(II) and Zn(II) from aqueous solutions

Nano-structured 2-line ferrihydrite was synthesized by a pH-controlled precipitation technique at 90 °C. Chemical, X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman analyses confirmed the sample to be 2-line ferrihydrite. The nano nature of the prepared sample was studied by transmission electron microscopy (TEM). The surface area obtained by the Brunauer-Emmett-Teller (BET) method was 175.8 m(2) g(-1). The nanopowder so obtained was used to study its behaviour for the removal of Pb(II), Cd(II), Cu(II) and Zn(II) from aqueous solutions. The relative importance of experimental parameters such as solution pH, contact time and concentration of adsorbate on the uptake of various cations was evaluated. By increasing the pH from 2.0 to 5.5, adsorption of the four cations increased. The kinetics parameters were compared by fitting the contact time data to both linear as well as non-linear forms of pseudo-second-order models. Linear forms of both Langmuir and Freundlich models fitted the equilibrium data of all the cations except for Pb(II) which was also fitted to the non-linear forms of both the models as it gave a low R(2) value of 0.85 for the Langmuir model. High Langmuir monolayer capacities of 366, 250, 62.5 and 500 mg g(-1) were obtained for Pb(II), Cd(II), Cu(II) and Zn(II), respectively. Presence of chloride or sulfate had an adverse effect on cation adsorption. The interactive effects on adsorption from solutions containing two, three or four cations were studied. Surprisingly no Cd(II) adsorption was observed in Pb(II)-Cd(II), Pb(II)-Cd(II)-Zn(II) and Pb(II)-Cd(II)-Cu(II)-Zn(II) systems under the studied concentration range. The overall loading capacity of the adsorbent decreased in mixed cation systems. Metal ion loaded adsorbents were characterized by XRD, FTIR and Raman techniques. The high adsorption capability of the 2-lines ferrihydrite makes it a potentially attractive adsorbent for the removal of cations from aqueous solutions.     

Modeling the adsorption of Cd(II) onto goethite in the presence of citric acid

The adsorption of cadmium onto goethite in the presence of citric acid was measured as a function of pH and cadmium concentration at 25 degrees C. Potentiometric titrations were also performed on the system. Cadmium adsorption onto goethite was enhanced above pH 4 in the presence of 50 microM, 100 microM and 1 mM citric acid. While there was little difference between the enhancements caused by 50 and 100 microM citric acid below pH 6, above pH 6 further enhancement is seen in the presence of 100 microM citric acid. When 1 mM citric acid was present, the enhancement of cadmium adsorption was greater below pH 6, with increased Cd(II) adsorption down to pH 3.5. However, above pH 6, 1 mM of citric acid caused slightly less enhancement than the lower citric acid concentrations. ATR-FTIR spectra of soluble and adsorbed citrate-cadmium species were measured as a function of pH. At pH 4.6 there was very little difference between the ternary Cd(II)-citric acid-goethite spectrum and the binary citric acid-goethite spectrum. However, spectra of the ternary system at pH 7.0 and 8.7 indicated the presence of additional surface species. Further analysis of the spectra suggested that these were metal-ligand outer-sphere complexes. Data from the adsorption experiments and potentiometric titrations of the ternary Cd(II)-citric acid-goethite system were fitted by an extended constant-capacitance surface complexation model. The spectroscopic data were used to inform the choice of surface species. Three reactions in addition to those for the binary Cd(II)-goethite and citric acid-goethite systems were required to describe all of the data. They were [formula in text], [formula in text], and [formula in text]. Neither the spectroscopy nor the modeling suggested the formation of a ternary inner-sphere complex or a surface precipitate under the conditions used in this study.     

The influence of temperature on the adsorption of mellitic acid onto goethite

The adsorption of mellitic acid (benzene-1,2,3,4,5,6-hexacarboxylic acid) onto goethite was investigated at five temperatures between 10 and 70 degrees C. Mellitic acid adsorption increased with increasing temperature below pH 7.5, but at higher pH the effect of increasing temperature was to reduce the amount adsorbed. Potentiometric titrations were conducted and adsorption isotherms were measured over the same temperature range, and the data obtained were used in conjunction with adsorption edge data to develop an Extended Constant Capacitance Surface Complexation Model of mellitic acid adsorption. A single set of reactions was used to model the adsorption for the three different experiment types at the five temperatures studied. The adsorption reactions proposed for mellitate ion (L(6-)) adsorption at the goethite surface (SOH) involved the formation of two outer-sphere complexes: SOH + L(6-) + 3H+ <==> [(SOH2)+ (LH2)(4-)]3-, 2SOH + L(6-) + 2H+ <==> [(SOH2)2(2+) (L)(6-)]4-. This mechanism is consistent with recent ATR-FTIR spectroscopic measurements of the mellitate-goethite system. Thermodynamic parameters calculated from the temperature dependence of the equilibrium constants for these reactions indicate that the adsorption of mellitic acid onto goethite is accompanied by a large entropy increase.     

Influence of temperature on the adsorption of mellitic acid onto kaolinite

The adsorption of mellitic acid (benzene-1,2,3,4,5,6-hexacarboxylic acid) onto kaolinite was investigated at five temperatures between 10 and 70 degrees C. Mellitic acid adsorption increased with increasing temperature at low pH (below pH 5.5), but at higher pH, the effect of increasing temperature was to reduce the amount adsorbed. Potentiometric titrations were conducted, adsorption isotherms were measured over the same temperature range, and the data obtained were used in conjunction with adsorption edge and ATR-FTIR spectroscopic data to develop an extended constant capacitance surface complexation model of mellitic acid adsorption. A single set of reactions was used to model all data at the five temperatures studied. The model indicates that mellitic acid sorbs via outer-sphere complexation to surface hydroxyl (SOH) groups on the kaolinite surface rather than to permanent charge sites. The reactions proposed are SOH + L6- + 2H+ <-->[(SOH2)+(LH)5-]4- and SOH + L(6-) <--> [(SOH)(L)6-]6-. Thermodynamic parameters calculated from the temperature dependence of the equilibrium constants for these reactions indicate that the adsorption of mellitic acid onto kaolinite is accompanied by a large entropy increase.     

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 (M(2+) or MY(2-)) or of possible mixtures (M(2+)+MY(2-)) 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., [M(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 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. Copyright 2000 Academic Press.     

Synthesis of Mg(II) doped ferrihydrite-humic acid coprecipitation and its Pb(II)/Cd(II) ion sorption mechanism

A magnesium doped ferrihydrite-humic acid coprecipitation (Mg-doped Fh-HA) was synthesized by coprecipitation method. The removal of heavy metals such as Pb(II) and Cd(II) was assessed. The isotherms and kinetic studies indicated that the Mg-doped Fh-HA exhibited a remarkable Pb(II) and Cd(II) sorption capacity (maximum 120.43 mg/g and 27.7 mg/g, respectively.) in aqueous solution. The sorption of Pb(II) and Cd(II) onto best fitted pseudo-second-order kinetic equation and Langmuir model. The adsorption mechanism of Mg-doped Fh-HA on Pb(II) and Cd(II) involves surface adsorption, surface complexation and surface functional groups (such as carboxyl group, hydroxyl group). In addition, ion-exchange and precipitation cannot be ignored. The Mg-doped Fh-HA is a low-cost and high-performance adsorption material and has a wide range of application prospects.     

Adsorptive accumulation of Cd(II), Co(II), Cu(II), Pb(II), and Ni(II) from water on montmorillonite: influence of acid activation

The present work investigates the influence of acid activation of montmorillonite on adsorption of Cd(II), Co(II), Cu(II), Ni(II), and Pb(II) from aqueous medium and comparison of the adsorption capacities with those on parent montmorillonite. The clay-metal interactions were studied under different conditions of pH, concentration of metal ions, amount of clay, interaction time, and temperature. The interactions were dependent on pH and the uptake was controlled by the amount of clay and the initial concentration of the metal ions. The adsorption capacity of acid-activated montmorillonite increases for all the metal ions. The interactions were adsorptive in nature and relatively fast and the rate processes more akin to the second-order kinetics. The adsorption data fitted both Langmuir and Freundlich isotherms, indicating that strong forces were responsible for the interactions at energetically nonuniform sites. The Langmuir monolayer capacity of the acid-activated montmorillonite is more than that of the parent montmorillonite (Cd(II): 32.7 and 33.2 mg/g; Co(II): 28.6 and 29.7 mg/g; Cu(II): 31.8 and 32.3 mg/g; Pb(II): 33.0 and 34.0 mg/g; and Ni(II): 28.4 and 29.5 mg/g for montmorillonite and acid-activated montmorillonite, respectively). The thermodynamics of the rate processes showed the adsorption of Co(II), Pb(II), and Ni(II) to be exothermic, accompanied by decreases in entropy and Gibbs free energy, while the adsorption of Cd(II) and Cu(II) was endothermic, with an increase in entropy and an appreciable decrease in Gibbs free energy. The results have established the potential use for montmorillonite and its acid-activated form as adsorbents for Cd(II), Co(II), Cu(II), Ni(II), and Pb(II) ions from aqueous media.     

Impact of environmental conditions on the adsorption behavior of radionuclide Ni(II) onto hematite

In this work, adsorption of Ni(II) from aqueous solution onto hematite under various solution chemistry and temperature was investigated. The results indicated that the pseudo-second-order rate equation fitted the kinetic adsorption well. The adsorption of Ni(II) onto hematite was strongly dependent on pH and ionic strength. At low pH, the adsorption was dominated by outer-sphere surface complexation or ion exchange, whereas inner-sphere surface complexation was the main adsorption mechanism at high pH. A positive effect of FA on Ni(II) adsorption was found at pH < 8.0, whereas a negative effect was observed at pH > 8.0. The Langmuir, Freundlich, and D–R models were applied to simulate the adsorption isotherms at three different temperatures of 293.15, 313.15 and 333.15 K. The thermodynamic parameters were calculated from the temperature dependent adsorption, and the results indicated that the adsorption was endothermic and spontaneous.     

Copper (II) adsorption from aqueous solution by herbaceous peat

In this research, the herbaceous peat collected from Gavurgolu peatlands, one of the biggest Turkish peatlands, was utilized as an adsorbent for the removal of copper (II) ions from aqueous solution. Adsorption experiments were conducted under various conditions, i.e., initial concentration, temperature, and pH. While the amount of Cu (II) adsorbed on the peat increased with increasing concentration of Cu (II) ions, it was not markedly affected by temperature and pH. Percentage removal was higher at lower concentration. For example, the maximum percentage removal of Cu (II) ions for initial concentration of 3 x 10(-4) M was 97.04% at 21 degrees C and pH 5.5. The adsorption capacity (Q(0)) of the peat was 4.84 mgg(-1) from Langmuir adsorption isotherm for the concentration range of 3 x 10(-4)-6 x 10(-4) M at 21 degrees C and pH 5.5. The equilibrium time of adsorption of Cu (II) ions was 150 min and independent of concentration and temperature. The amount of Cu (II) adsorbed at equilibrium time did not considerably change with temperature and pH. It was also determined that adsorption isotherm followed both Freundlich and Langmuir. Uptake mechanism of Cu (II) ions by the peat occurs via cation exchange (especially by means of Ca(2+) and Mg(2+)) as well as copper/peat complexation. Adsorption kinetic was consistent with the pseudo-second-order model.     

Sorption of radiocobalt(II) from aqueous solutions to Na-attapulgite

The sorption of Co(II) on Na-attapulgite as a function of contact time, solid content, pH, ionic strength, foreign ions, fulvic acid (FA) and temperature under ambient conditions was studied. The kinetic of Co(II) sorption on Na-attapulgite was described well by pseudo-second-order model. The sorption of Co(II) on Na-attapulgite was strongly dependent on pH and ionic strength. The sorption of Co(II) was mainly dominated by outer-sphere surface complexation and/or ion exchange at low pH, whereas inner-sphere surface complexation or surface precipitation was the main sorption mechanism at high pH values. The presence of FA did not affect Co(II) sorption obviously at pH <7, and a negative effect was observed at pH >7. The Langmuir and Freundlich models were used to simulate the sorption data at different temperatures, and the results indicated that the Langmuir model simulated the data better than the Freundlich isotherm model. The thermodynamic parameters (∆G°, ∆S°, ∆H°) calculated from the temperature-dependent sorption isotherms indicated that the sorption of Co(II) on Na-attapulgite was an endothermic and spontaneous process. The results suggest that the attapulgite sample is a suitable material in the preconcentration and solidification of radiocobalt from large volumes of aqueous solutions.     

Kinetics and equilibrium adsorption of Cu(II), Cd(II), and Ni(II) ions by chitosan functionalized with 2[-bis-(pyridylmethyl)aminomethyl]-4-methyl-6-formylphenol

Chitosan biopolymer chemically modified with the complexation agent 2[-bis-(pyridylmethyl)aminomethyl]-4-methyl-6-formylphenol (BPMAMF) was employed to study the kinetics and the equilibrium adsorption of Cu(II), Cd(II), and Ni(II) metal ions as functions of the pH solution. The maximum adsorption of Cu(II) was found at pH 6.0, while the Cd(II) and Ni(II) maximum adsorption occurred in acidic media, at pH 2.0 and 3.0, respectively. The kinetics was evaluated utilizing the pseudo-first-order and pseudo-second-order equation models and the equilibrium data were analyzed by Langmuir and Freundlich isotherms models. The adsorption kinetics follows the mechanism of the pseudo-second-order equation for all studied systems and this mechanism suggests that the adsorption rate of metal ions by CHS-BPMAMF depends on the number of ions on the adsorbent surface, as well as on their number at equilibrium. The best interpretation for the equilibrium data was given by the Langmuir isotherm and the maximum adsorption capacities were 109 mg g-1 for Cu(II), 38.5 mg g-1 for Cd(II), and 9.6 mg g-1 for Ni(II). The obtained results show that chitosan modified with BPMAMF ligand presented higher adsorption capacity for Cu(II) in all studied pH ranges.     

Cd(II) binding by particulate low-rank coals in aqueous media: sorption characteristics and NICA-Donnan models

An experimental investigation of Cd(II) sorption onto two Australian coals was carried out in 0.1 M NaNO3 at 298.2 K. The initial concentration of Cd(II) was varied from 0.133 to 2.000 mmol/g in a series of batch adsorption experiments with an initial coal concentration of 3.75 g/L of Loy Yang (brown) or of Collie (sub-bituminous) coals in the p[H+] range 2-8. Adsorption edges were typical of metal ion adsorption onto negatively charged organic substrates, starting at p[H+] approximately 3 and increasing with increasing pH. The largest measured Cd(II) uptake capacities from these experiments were of 1.2 mmol/g for Loy Yang and 0.7 mmol/g for Collie coals. This difference is ascribed to the larger concentrations of carboxyl groups in Loy Yang coal (2.78 mmol/g) compared to Collie coal (1.34 mmol/g). An adsorption isotherm for Loy Yang coal at p[H+] 6 was collected up to a surface loading of 1.7 mmol/g of adsorbed Cd(II). These experiments also revealed a release of about 1.5-1.6 protons per adsorbed Cd(II). Zeta potentials of Loy Yang coal suspensions were not affected by Cd(II) adsorption, suggesting that the coal particles efficiently neutralize the charge of Cd(II). Collie coal, on the other hand, exhibited a zeta potential increase that may indicate a modification of the surface potentials of the coal particles. Cd(II) uptake data obtained from both batch experiments and proton balance data have been combined with p[H+] stat data for the same experimentally covered Cd(II)/coal ratios to model adsorption using the NICA-Donnan model. The modeling results suggest that both coals possess identical affinities and reaction stoichiometries. Loy Yang coal, however, possessed a narrower distribution of affinities.     

Removal of Cd(II), Cu(II), and Pb(II) by adsorption onto natural clay: a kinetic and thermodynamic study

In this work, we study the elimination of three bivalent metal ions (Cd²⁺, Cu²⁺, and Pb²⁺) by adsorption onto natural illitic clay (AM) collected from Marrakech region in Morocco. The characterization of the adsorbent was carried out by X-ray fluorescence, Fourier transform infrared spectroscopy and X-ray diffraction. The influence of physicochemical parameters on the clay adsorption capacity for ions Cd²⁺, Cu²⁺, and Pb²⁺, namely the adsorbent dose, the contact time, the initial pH imposed on the aqueous solution, the initial concentration of the metal solution and the temperature, was studied. The adsorption process is evaluated by different kinetic models such as the pseudo-first-order, pseudo-second-order, and Elovich. The adsorption mechanism was determined by the use of adsorption isotherms such as Langmuir, Freundlich, and Temkin models. Experiments have shown that heavy metals adsorption kinetics onto clay follows the same order, the pseudo-second order. The isotherms of adsorption of metal cations by AM clay are satisfactorily described by the Langmuir model and the maximum adsorption capacities obtained from the natural clay, using the Langmuir isotherm model equation, are 5.25, 13.41, and 15.90 mg/g, respectively for Cd(II), Cu(II), and Pb(II) ions. Adsorption of heavy metals on clay is a spontaneous and endothermic process characterized by a disorder of the medium. The values of ΔH are greater than 40 kJ/mol, which means that the interactions between clay and heavy metals are chemical in nature.     

Removal of Chromium(III) and Cadmium(II) Heavy Metal Ions from Aqueous Solutions Using Treated Date Seeds: An Eco-Friendly Method

The aim of the research was to prepare low-cost adsorbents, including raw date pits and chemically treated date pits, and to apply these materials to investigate the adsorption behavior of Cr(III) and Cd(II) ions from wastewater. The prepared materials were characterized using SEM, FT-IR and BET surface analysis techniques for investigating the surface morphology, particle size, pore size and surface functionalities of the materials. A series of adsorption processes was conducted in a batch system and optimized by investigating various parameters such as solution pH, contact time, initial metal concentrations and adsorbent dosage. The optimum pH for achieving maximum adsorption capacity was found to be approximately 7.8. The determination of metal ions was conducted using atomic adsorption spectrometry. The experimental results were fitted using isotherm Langmuir and Freundlich equations, and maximum monolayer adsorption capacities for Cr(III) and Cd(II) at 323 K were 1428.5 and 1302.0 mg/g (treated majdool date pits adsorbent) and 1228.5 and 1182.0 mg/g (treated sagai date pits adsorbent), respectively. It was found that the adsorption capacity of H₂O₂-treated date pits was higher than that of untreated DP. Recovery studies showed maximal metal elution with 0.1 M HCl for all the adsorbents. An 83.3–88.2% and 81.8–86.8% drop in Cr(III) and Cd(II) adsorption, respectively, were found after the five regeneration cycles. The results showed that the Langmuir model gave slightly better results than the Freundlich model for the untreated and treated date pits. Hence, the results demonstrated that the prepared materials could be a low-cost and eco-friendly choice for the remediation of Cr(III) and Cd(II) contaminants from an aqueous solution.     

Modeling of copper(II) and lead(II) adsorption on kaolinite-based clay minerals individually and in the presence of humic acid

The aim of this study is to explain how clay minerals adsorb heavy metals individually and in the presence of humic acid, and to model heavy metal adsorption specifically based on surface-metal binary and surface-metal-ligand ternary complexation. The adsorption of Cu(II) and Pb(II) on kaolinite-based clay minerals has been modeled by the aid of the FITEQL3.2 computer program using single- and double-site binding models of the Langmuir approach. Potentiometric titrations and adsorption capacity experiments were carried out in solutions containing different concentrations of the inert electrolyte NaClO4; however, the modeling of binary and ternary surface complexation was deliberately done at high ionic strength (0.1 M electrolyte) for eliminating adsorption onto the permanent negatively charged sites of kaolinite. A "two-site, two pKa" model was adapted, and as for the two surface sites responsible for adsorption, it may be arbitrarily assigned that [triple bond]S1OH sites represent silanol and organic functional groups such as carboxyl having pKa values close to that of silanol, and [triple bond]S2OH sites represent aluminol and organic functional groups such as phenolics whose pKa values are close to that of aluminol, as all the studied clays contained organic carbon. Copper(II) showed a higher adsorption capacity and higher binding constants, while lead(II), being a softer cation (in respect to HSAB theory) preferred the softer basic sites with aluminol-phenol functional groups. Heavy metal cations are assumed to bind to the clay surface as the sole (unhydrolyzed) M(II) ion and form monodentate surface complexes. Cu(II) and Pb(II) adsorption in the presence of humic acid was modeled using a double-site binding model by the aid of FITEQL3.2, and then the whole system including binary surface-metal and surface-ligand and ternary surface-metal-ligand complexes was resolved with respect to species distributions and relevant stability constants. Electrostatic effects were accounted for using a diffuse layer model (DLM) requiring minimum number of adjustable parameters. Metal adsorption onto clay at low pH increased in the presence of humic acid, and the metal adsorption vs pH curves of metal-kaolinite-humic acid suspensions were much steeper (and distinctly S shaped) compared to the wider pH-gradient curves observed in binary clay-metal systems. The clay mineral in the presence of humic acid probably behaved more like a chelating ion-exchanger sorbent for heavy metals rather than being a simple inorganic ion exchanger.     

Removal of Cd(II) and Pb(II) from aqueous solution by modified attapulgite clay

Three low-cost adsorbents (purified raw attapulgite (A-ATP), high-temperature-calcined attapulgite (T-ATP), and hydrothermal loading of MgO (MgO-ATP)) were prepared as adsorbents for the removal of Cd(II) and Pb(II). By evaluating the effect of the initial solution pH, contact time, initial solution concentration, temperature and coexistence of metal ions on Cd(II) and Pb(II) adsorption, the experimental results showed that MgO-ATP was successfully prepared by hydrothermal reaction and calcination as well as appearing to be a promising excellent adsorbent. At an initial pH of 5.0, A-ATP, T-ATP and MgO-ATP reached maximum adsorption amounts of 43.5, 53.9 and 127.6 mg/g for Pb(II) and 10.9, 11.2, and 25.3 mg/g for Cd(II) at 298 K, respectively. The Cd(II) adsorption on A-ATP was fitted by the Freundlich model, while the adsorption of Pb(II) and Cd(II) on T-ATP and MgO-ATP as well as Pb(II) adsorption on A-ATP agreed with the Langmuir model. All kinetic experimental data favored pseudo second-order model. The calculated thermodynamic parameters suggested that Pb(II) adsorption onto MgO-ATP was spontaneous and exothermic. When considering foreign metal ions, the three adsorbents all presented preferential adsorption for Pb (II). Chemical adsorption had a high contribution to the removal of Cd(II) and Pb(II) by modified attapulgite. In summary, the adsorption was greatly enhanced by the hydrothermal loading of MgO. It aimed to provide insights into the MgO-ATP, which could be able to efficiently remove Cd(II) and Pb(II) and serve as an economic and promising adsorbent for heavy metal-contaminated environmental remediation.