Acid-activated Sepiolite Research Articles

Synthesis of smart PVA/acid-activated/Cu2O@sepiolite nanocomposite: Evaluation of thermal, mechanical, lead removal and antibacterial properties

Lead [Pb(II)] poses a serious threat to public health and the environment due to its high prevalence in industrial effluents. Herein, we developed smart poly(vinyl) alcohol (PVA) nanocomposites containing acid-activated sepiolite (ASep) and cuprous oxide (Cu2O) nanoparticles containing sepiolite using the alkaline method. Structural, thermal, physical, and mechanical properties of PVA/ASep@Cu2O nanocomposites were evaluated. The modulus of elasticity of PVA/ASep@Cu2O nanocomposites was 1.26-fold enhanced compared to the PVA membrane. The influence of adsorbent dose (0.01, 0.05, and 0.1 g), pH (3, 4, 5), time (10, 20, 30, 40, 50, and 60 min), and temperature (300.15, 310.15, and 320.15 K) on the removal of Pb(II) from aqueous media by PVA/ASep@Cu2O nanocomposites was investigated. The maximum Pb(II) adsorption capacity of PVA/ASep@Cu2O nanocomposite was 284.09 mg/g. The practical application of the smart nanocomposite for eliminating heavy metals from industrial effluent and contaminated drinking water was determined. The PVA/ASep@Cu2O nanocomposite also exhibited robust antibacterial properties against Staphylococcus aureus and Escherichia coli. Therefore, PVA/ASep@Cu2O nanocomposite could be employed as a smart approach by producing clean water to protect public health and the environment.

Sepiolite as an efficient adsorbent for ethylene gas

AbstractThe ability of Na+-, Li+-, K+-, Ca2+- and Mg2+-exchanged sepiolites and acid-activated sepiolites to remove ethylene from storage environments was examined. The sepiolite from Sivrihisar deposit, Turkey, was treated with 1.0 M NaNO3, LiNO3, KNO3, Ca(NO3)2, Mg(NO3)2, HNO3 and H2SO4 solutions at 90°C for 4 h. The mineralogical, chemical and textural properties of the materials were examined using X-ray diffraction, X-ray fluorescence, cation-exchange capacity and nitrogen gas adsorption analyses. The main mineral phase present in the materials was sepiolite, with minor dolomite, and traces of quartz and feldspar minerals and amorphous matter. Adsorption isotherms of ethylene at 273 K were measured on sepiolite samples over a pressure range of 0–100 kPa. As a result of partial blockage of the sepiolite channels, the ethylene adsorption capacity on cation-exchanged sepiolite forms (0.376–0.907 mmol g–1) was less than that of acid-activated sepiolite forms (1.279 and 1.308 mmol g–1). The ethylene adsorption capacities of the sepiolite samples were compared with those of other clay-based materials (0.167–1.817 mmol g–1) reported in previous studies of ethylene removal.

Modelling and Optimization of Sepiolite Activation with Citric Acid Using Factorial Experimental Design and Response Surface Methodology

In this study, the effects of several parameters such as the acid concentration, mixing time, temperature, solid rate, and mixing rate on the specific surface area (BET) and cation exchange capacity (CEC) of sepiolite activated by citric acid were investigated using factorial experimental design and response surface methodology. While the specific surface area and cation exchange capacity of nitric acid activated sepiolite were chosen as the responses, the acid concentration, time, temperature, solid rate, and mixing rate were the process parameters. The relationship between the process parameters and the responses was obtained and used in the determination of the optimal conditions. The result showed that the specific surface area and cation exchange capacity of the acid-activated sepiolite were strongly affected by the process parameters. The optimal conditions for achieving the maximum specific surface area and minimum cation exchange capacity were 95.73 °C, 8.64%, 4.99 N, 1 h, and 450 rpm for temperature, solid rate, acid concentration, activation time, and the mixing rate, respectively. The experimental and predicted values for the SBET and the CEC were obtained as 450.97 m2/g and 449.97 m2/g, and 6.79 meq/100 g and 6.86 meq/100 g, respectively. The results obtained from this study clearly indicated that the actual values were in good conformity with the predicted values as evidenced by small errors of 1.00% and 0.07% for the SBET and CEC, respectively. As a result, the SBET and CEC of the Sep activated by citric acid were strongly affected by the process parameters, especially the solid rate for the SBET and the temperature for CEC. It was found that citric-acid activated Sep had a SBET approximately 2.5 times higher than that of the untreated Sep while the CEC of Sep may be strongly eliminated by citric acid treatment. Thus, the mineral structure was progressively transformed into amorphous silica coming from the tetrahedral sheet of the generated fibrous silicates, whose SBET was larger than that of the raw Sep.

Influence of Dry Milling on Phase Transformation of Sepiolite upon Alkali Activation: Implications for Textural, Catalytic and Sorptive Properties.

Activation of natural sepiolite by means of grinding in a planetary mill followed by wet NaOH activation was studied for the purpose of endowing the product with enhanced basicity for potential catalytic/sorptive applications. Synthesized solids were characterized with X-ray powder diffraction (XRD), N2 adsorption/desorption, scanning electron microscopy (SEM), energy dispersive (EDX), atomic absorption (AAS), Fourier-transform infrared (FTIR) and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies. Surface basicity was determined by titration with benzoic acid. Grinding changed the pathway of sepiolite phase transformation upon NaOH treatment. The as-received sepiolite evolved to Na-sepiolite (loughlinite) with a micropore system blocked by nanocrystalline Mg(OH)2, while ground samples yielded magnesium silicate hydrate phase (MSH), with well-developed microporous texture. In unmilled sepiolite desilication involved preferential leaching of Si from the center of the structural ribbons, while in ground samples additional loss of Si from ribbon-ribbon corner linkages was observed. In all cases treatment with NaOH led to enhancement of surface basicity. Synthesized materials were tested as catalysts in a base-catalyzed aldol self-condensation of acetone and oxidation of cyclohexanone to ε-caprolactone, as well as CO2 sorbents. Catalytic trends depended not only on samples’ basicity, but also on texture and phase composition of the catalysts. Grinding combined with alkali activation proved a simple and effective method for boosting CO2-sorption capacity of sepiolite to the level comparable to amine-functionalized, acid-activated sepiolite sorbents.

A heterogeneous Fenton reaction system of N-doped TiO2 anchored on sepiolite activates peroxymonosulfate under visible light irradiation

Completely decomposing low concentrations of organic pollutants in neutral wastewater under sunlight irradiation poses an important challenge for water treatment systems. A combination of photocatalysis and Fenton techniques is expected to solve this problem. Herein, a novel heterogeneous Fenton reaction system with good photocatalytic performance was developed by activating peroxymonosulfate (PMS) via nitrogen-doped TiO2 (N-TiO2) and acid-activated sepiolite (ASep) nanocomposites (N-TiO2/ASep). The N-TiO2/ASep nanocomposites were prepared by anchoring the N-TiO2 onto ASep. The electronic structure of the Ti3+ self-doped N-doped TiO2 was regulated by calcining N-TiO2/ASep nanocomposites under different atmospheres. Methyl orange degradation, influencing factors, and reaction mechanisms under visible light irradiation were investigated in detail. The results indicated that, under visible light irradiation, N-TiO2/ASep alone had almost no photocatalytic activity, and PMS alone exhibited only 20% photocatalytic activity. However, the N-TiO2/ASep(N2)/PMS system degraded about 95% of the methyl orange within 60 min. By contrast, the reaction rate constant of N-TiO2/ASep(N2)/PMS was nearly 5.44 times higher than that of TiO2/PMS. This excellent performance was attributed to the synergy among N-TiO2, ASep, and PMS. Moreover, the calculated carrier density and photocurrent density of N-TiO2/ASep (N2) was nearly 3.75 and 2.80 times that of N-TiO2/ASep (Air), suggesting that N-TiO2/ASep(N2) had a greater carrier separation efficiency because more Ti3+ species were provided. Furthermore, radical scavenger experiments and electron spin resonance testing demonstrated that h+, O2−, SO4− and OH were the important radicals. More importantly, photocatalytic performance was optimal at pH 7 in the presence of various ions. The results of this study expanded the applications of PMS and advanced oxidation processes. This work provides the necessary scientific basis for further studies of organic pollutant remediation.

Silver nanoparticles assembled on modified sepiolite nanofibers for enhanced catalytic reduction of 4-nitrophenol

Acid-activated sepiolite nanofibers (ASep) were organically modified with (3-aminopropyl) triethoxysilane (APTES) to produce the amino-functionalized nanofibers (NH2-ASep). Due to the amino groups, highly dispersed silver nanoparticles (AgNPs) were assembled on the surface of NH2-ASep by a simple chemical reduction approach, and the AgNPs were smaller than those directly deposited onto ASep surface. The as-synthesized Ag/NH2-ASep nanofiber catalysts exhibited an excellent catalytic activity in the reduction process of 4-nitrophenol to 4-aminophenol with full conversion within 150 s in the presence of NaBH4 as the reductant. The results indicated that Ag/NH2-ASep nanofiber catalysts exhibited excellent catalytic activities because the highly dispersed of small-sized AgNPs were exposed on the surface of highly dispersed nanofibers with amino functional groups, allowing effective contact with the reactants and catalysis of the reaction. Moreover, the Ag/NH2-ASep nanofiber catalysts are stable and easy to use for the catalytic reaction due to their one-dimensional nanofiber structure with functional amino groups.

Low-cost DETA impregnation of acid-activated sepiolite for CO2 capture

In this paper, a low-cost sepiolite-based adsorbent for CO2 capture was obtained by impregnating DETA into the acid-modified sepiolite. Thermogravimetric analysis, nitrogen adsorption-desorption isotherm, Fourier transform infrared spectroscopy and X-ray diffraction were used for the characterization of the sepiolite supports before and after DETA impregnation. Thermogravimetric analysis was also employed to evaluate the CO2 adsorption performances of the adsorbents. The characterization methods revealed that DETA impregnated into the acid-modified sepiolite effectively. The CO2 adsorption experimental results indicated that acid-activation and amine-loading simultaneously improved the CO2 adsorption capacities of sorbents. For amine-loaded acid-modified sepiolite, low dosage of impregnated amine decreased the CO2 adsorption capacities of sorbents at 35 °C due to the restrictions imposed in the transport and reaction of CO2 due to pore blockage by amines. The highest CO2 adsorption performance attained 1.65 mmol/g with the acid-modified sepiolite with 0.8 g-DETA loading. 95.2% of the adsorption capacity of 0.8-DETA/SH-18 maintains after 4 adsorption-desorption cycles, and the sorbent can be almost fully recovered at 75 °C, pure N2. The kinetic studies showed that for low amine loadings the CO2 adsorption is controlled by micropore blocking. For higher loadings a more complex diffusion and reaction process leads to a two-step adsorption kinetics. By contrast, it can be found that Avrami model is in good accordance with the adsorption kinetics of CO2 on amine-functionalized sepiolite. A cost evaluation indicated that DETA-impregnated sepiolites are cheap adsorbents for CO2 capture.

Highly dispersed sepiolite-based organic modified nanofibers for enhanced adsorption of Congo red

Abstract Acid-activated sepiolite fibers (ASEP) were grafted with amino groups using (3-aminopropyl) triethoxysilane to prepare an emerging organic-inorganic hybrid nanofibers (NH2-ASEP). Owing to the high surface area and pore volume, and the highly dispersed nanofibers with functional amino-groups, the prepared NH2-ASEP exhibited good adsorption performance towards Congo red (CR) anionic dye. It displayed a large sorption capacity (522 mg/g at pH = 8.0, 25 °C), low sorption equilibrium time (within 10 min), and renewability. Equilibrium adsorption data of NH2-ASEP towards CR dye were evaluated using Freundlich, Langmuir and Sips isotherm models, and the Langmuir isotherm exhibited a better fit with a maximum adsorption capacity of 539.71 mg/g, which is very close to the experimental data and higher than those of the reported fiber adsorbents. The adsorption process of CR dye onto NH2-ASEP surface followed a pseudo-second order rate expression. Highly dispersed sepiolite-based organic modified nanofibers have great potential applications in the fields of water treatment, membrane separation, catalysis, pH responsive delivery and other environmental remediation.

Clarification of apple juice using activated sepiolite as a new fining clay

AbstractThe function of acid-activated sepiolite clay was evaluated for use in the clarification of apple juice. Optimizing the clarification process was done using response surface methodology (RSM) considering juice turbidity as a response. The efficiency of sepiolite in terms of clarification was compared to that of bentonite and a combination of these fining agents with gelatin and silica gel at optimal conditions. The conditions of 0.05% clay concentration at a temperature of 50°C for 7 h were deemed optimal in terms of juice clarification. The present results showed that treatment of sepiolitegelatin-silica gel was the most active fining agent with 99.7% reduction in apple-juice turbidity. The rate of turbidity, the viscosity, the total phenolic contents and the colour changes were fitted to first-, zero-, first- and zero-order kinetic models, respectively.

Roles of texture and acidity of acid-activated sepiolite catalysts in gas-phase catalytic dehydration of glycerol to acrolein

In the search for novel solid acid catalysts for the gas-phase catalytic dehydration of glycerol to acrolein, sepiolite clay minerals were activated with hydrochloric acid to produce acid-activated sepiolite catalysts. The effects of the activation process on the texture and acidity of acid-activated sepiolite, and their roles in gas-phase catalytic dehydration of glycerol to acrolein, were then investigated. A series of acid-activated sepiolite catalysts were made from purified natural sepiolite treated with hydrochloric acid of different concentrations (0–8mol/L) at 80°C, followed by drying at 100°C. The acid-activated sepiolite catalyst samples were characterized using X-ray diffraction, X-ray fluorescence analysis, scanning electronic microscopy, thermogravimetric analysis, N2 adsorption-desorption isotherms, NH3-temperature-programmed desorption. Fourier transform-infrared spectroscopy and pyridine adsorption followed by in situ infrared spectroscopy. The catalytic performances in gas-phase catalytic dehydration of glycerol to acrolein were studied using a vertical fixed-bed reactor. Typically, a glycerol conversion of 92.9%, an acrolein selectivity of 59.4%, and a yield of acrolein of 55.2% were achieved when the gas-phase catalytic dehydration of glycerol was conducted over HCl (2mol/L) acid-activated sepiolite catalyst at 320°C with an aqueous glycerol solution (20wt.%) at a rate of 0.10ml/min as the feedstock and air as the carrier gas at a flow rate of 20ml/min. The activation of the sepiolite with hydrochloric acid followed by drying can eliminate a portion of the zeolitic water in the tunnels, remove part of the magnesium and aluminum cations in the octahedral sheets of sepiolite, unbundle aggregated sepiolite fibers, and partly break the Si-O-Si in the tetrahedral sheets of sepiolite. These factors increased the porosity, the specific surface area and the acidity of the sepiolite. The partial leaching of magnesium and aluminum ions in the octahedral sheet generated octahedral vacancies and created MgOAl+ sites in the octahedral sheets, thereby increasing the amount of Lewis acid sites. Along with penetration of H+ cations into the interlayered space of the sepiolite by an ion-exchange reaction and physicochemical surface adsorption, some breakage of the SiOSi bonds in the tetrahedral sheets generated more SiOH+, thus increasing the quantity of Brønsted acid sites. Furthermore, the strength of acidity of the hydrochloric acid-activated sepiolite was significantly changed after the acid activation. The medium-strong Brønsted acid sites (SiOH+ and H+ adsorbed on the surface) appeared to be beneficial for the high conversion of glycerol and the selectivity to acrolein. Lewis acid sites could facilitate the formation of acetol. Under the reaction conditions mentioned above, the coking on the hydrochloric acid-activated sepiolite catalysts readily occurred, rapidly leading to the decrease of the yield of acrolein.

Improved catalytic performance of acid-activated sepiolite supported nickel and potassium bimetallic catalysts for liquid phase hydrogenation of 1,6-hexanedinitrile

Different inorganic acids were used to activate sepiolite, and the acid-activated sepiolites supported nickel and potassium bimetallic catalysts were prepared. Nitrogen adsorption-desorption, hydrogen chemisorption, ammonia temperature programmed desorption (NH3-TPD), temperature programmed reduction (TPR), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR) and energy dispersive X-ray (EDX) were used to characterize the catalysts. The catalytic performance of the acid-activated sepiolite supported K-Ni bimetallic catalysts were investigated in 1,6-hexanedinitrile (HDN) hydrogenation in liquid phase. It was revealed that the potassium could increase the alkalinity of the catalyst with the aim of inhibiting the formation of the 1-azacycloheptane (ACH). And the addition of potassium reduces the particle size of nickel and improves its dispersion. Compared with hydrochloric acid and sulfuric acid, nitric acid treatment increases more silanol groups (SiOH) on the sepiolite surface, which is helpful to nickel particles adsorption and dispersion. Nitric acid activated sepiolite supported nickel and potassium bimetallic catalysts (K-Ni/NASEP) present the best catalytic performance, the conversion of HDN comes up to 92.0% under moderate conditions of lower temperature and pressure, the selectivity to 6-aminocapronitrile (ACN) and 1,6-hexanediamine (HDA) is up to 95.2%.

Nanoscale zerovalent iron (nZVI) supported by natural and acid-activated sepiolites: the effect of the nZVI/support ratio on the composite properties and Cd2+ adsorption.

Natural (SEP) and partially acid-activated (AAS) sepiolites were used to prepare composites with nanoscale zerovalent iron (nZVI) at different (SEP or AAS)/nZVI ratios in order to achieve the best nZVI dispersibility and the highest adsorption capacity for Cd2+. Despite the higher surface area and pore volume of AAS, better nZVI dispersibility was achieved by using SEP as the support. On the other hand, a lower oxidation degree was achieved during the synthesis using AAS. X-ray photoelectron spectroscopy (XPS) analysis of the composite with the best nZVI dispersibility, before and after Cd2+ adsorption, confirmed that the surface of the nZVI was composed of oxidized iron species. Metallic iron was not present on the surface, but it was detected in the subsurface region after sputtering. The content of zerovalent iron decreased after Cd2+ adsorption as a result of iron oxidation during Cd2+ adsorption. The XPS depth profile showed that cadmium was present not only at the surface of the composite but also in the subsurface region. The adsorption isotherms for Cd2+ confirmed that the presence of SEP and AAS decreased the agglomeration of the nZVI particles in comparison to the pure nZVI, which provided a higher adsorption capacity. The results showed that the prevention of both aggregation and oxidation during the synthesis was necessary for obtaining an SEP/AAS-nZVI composite with a high adsorption capacity, but oxidation during adsorption was beneficial for Cd2+ removal. The formation of strong bonds between Cd2+ and the adsorbents sites of different energy until monolayer formation was proposed according to modeling of the adsorption isotherms.

Influence of the synthesis parameters on the properties of the sepiolite-based magnetic adsorbents

The magnetic nanocomposites (MNCs), used as adsorbents for Cd2+ ions, were prepared by a co-precipitation method using both the untreated natural (SEP) and the partially acid-activated (ASEP) sepiolites as supports for magnetite. The synthesis conditions differed as follows: NaOH or NH3 was added in the SEP or the ASEP suspension before or after the addition of Fe3+ and Fe2+. The SEP-based MNCs prepared using NaOH had a lower magnetization, but higher adsorption capacities than the MNCs synthesized using NH3. The order of the mixing of the reagents had a small influence on the properties of SEP-based MNCs, especially when NaOH was used for the synthesis. Due to acidity of the surface functional groups of the ASEP, the magnetic composite could be synthesized only when NH3 was added in the suspension of ASEP having Fe3+ and Fe2+ ions. Despite of the worse dispersibility of the magnetic particles, the adsorption capacity of the ASEP-based composite was higher than of the MNC with SEP prepared by the same procedure. All the MNCs showed a superparamagnetic behavior at room temperature and the adsorption capacities were higher than of pure compounds. The adsorption of Cd2+ on the composites was best fitted with the Sips model, which suggested the chemisorption on the heterogeneous surface at energetically non-uniform sites until a monolayer was formed.

Liquid phase hydrogenation of adiponitrile over acid-activated sepiolite supported K–La–Ni trimetallic catalysts

Acid-activated sepiolite (ASEP) supported K–La–Ni trimetallic catalysts were prepared and tested in the liquid phase hydrogenation of adiponitrile (ADN). The sepiolite was treated with dilute hydrochloric acid solution to obtain ASEP, ASEP supported K–La–Ni trimetallic catalysts were prepared by the incipient impregnation method and characterized by nitrogen adsorption–desorption, temperature programmed reduction, hydrogen chemisorption, powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and energy dispersive X-ray. It was revealed that potassium could inhibit the formation of the 1-azacycloheptane by-product by neutralizing some acid sites on the catalyst, and the potassium and lanthanum could efficiently reduce nickel particles size and improve its dispersion. Sepiolite treated with hydrochloric acid increases the number of silanol groups (Si–OH) on the sepiolite surface, which is helpful to nickel particles adsorption and dispersion. ASEP supported K–La–Ni trimetallic catalysts (K–La–Ni/ASEP) present better catalytic performance under mild reaction conditions of 373 K and 2.0 MPa, the ADN conversion reaches to 90.03 %, the selectivity to 6-aminocapronitrile and hexamethylenediamine is up to 90.22 %.

Optimization of oil bleaching parameters, using response surface methodology, for acid-activated sepiolite from Iran

AbstractAn Iranian sepiolite was activated with 1.5 M HCl. During activation, the specific surface area increased from 105 to 168 m2/g and the SiO2 content increased from 45.4 to 51.6%. The roles of temperature and time and the amount of acid-activated sepiolite used, were investigated and the optimum conditions, where chlorophyll, β-carotene, free fatty acid and peroxide were at their minimum levels, were determined for bleaching of canola oil by applying response surface methodology (RSM). Optimum bleaching conditions were obtained by adding 3% of acid-activated sepiolite at a temperature of 110°C with a bleaching time of 42.46 min during which, the amount of chlorophyll-α, β-carotene, free fatty acid and peroxide were reduced from 12.30 to 0.35 mg/kg, 49.15 to 14.98 mg/kg, 0.62 to 0.15%, and 3.87 to 2.14 meq O2/kg of oil, respectively. At optimum conditions, sepiolite displayed a greater capacity for removal of chlorophyll and β-carotene than a commercial bentonite bleaching agent.

Phospholipid adsorption from vegetable oils on acid-activated sepiolite

In this study, adsorption of natural and model phospholipids (PL) from camelina and rapeseed oils on acid-activated sepiolite has been studied. Adsorption isotherms were determined for total phosphorus and for individual PL at different conditions. Influence of acid-degumming with citric acid as well as of type and ionic form of the phospholipid was investigated. The experimental data were correlated using the Moreau isotherm. The results show that neutral PLs, such as phosphatidylcholines, adsorb as ordered monolayers with molecular area of 1.3 nm2/molecule. Phosphatidic acids have similar adsorption capacity when in free acid form but the interaction with the clay surface is much weaker. Sodium salt gives substantially lower saturation capacity because of electrostatic repulsion between the adsorbed PL molecules and between PL and the clay surface. Calcium and magnesium salts have very low capacity and seem to be incapable to form any ordered structures on the clay surface.

Synthesis of bicrystalline TiO2 supported sepiolite fibers and their photocatalytic activity for degradation of gaseous formaldehyde

Bicrystalline TiO2 supported acid activated sepiolite (TiO2/AAS) fibers have been successfully synthesized by a simple method at low temperature. The as-prepared TiO2/AAS composites were characterized by X-ray diffraction (XRD), nitrogen adsorption/desorption method, UV–vis diffuse reflectance spectrum (UV–vis DRS), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared (FT-IR) spectroscopy. The X-ray diffraction analysis indicates that the binary mixtures of anatase and rutile exist in the TiO2/AAS composites. The specific surface area of the TiO2/AAS composites is much larger than that of raw sepiolite and acid activated sepiolite (AAS) fibers. The photocatalytic activities of the TiO2/AAS composites were evaluated by the degradation of gaseous formaldehyde (a main indoor air pollutant) under UV light irradiation. The result indicates that the photocatalytic activity of the TiO2/AAS composites is excellent and superior to that of the TiO2/sepiolite (raw sepiolite) and pure TiO2. The high photocatalytic activity of the TiO2/AAS composites can be attributed to the anatase–rutile mixed phase and bimodal pore structure in the TiO2/AAS composites.

Adsorption of ferrocyanide onto raw and acid-activated clinoptilolite and sepiolite: equilibrium modelling by error minimization

AbstractThe experimental data on adsorption of Fe and CN of a ferrocyanide complex onto raw and acid-activated clinoptilolite/sepiolite on the basis of detention time and particle size was modelled by a linear and a non-linear approach. The linearized best-fit isotherm selection method and non-linear error minimization was applied through Freundlich, Langmuir and Temkin isotherms. ERRSQ, MPSD, HYBRID and ARE error functions were minimized by a developed MATLAB script to determine the isotherm parameters in non-linear optimization. The complex was not adsorbed as whole anions but the Fe and CN were adsorbed separately. 0.65 mg Fe/L. min and 4.84 mg CN/L. min initial adsorption rates were achieved with acid activated clinoptilolite. The Fe adsorption was not as successful as CN. The adsorption of Fe and CN was described by Freundlich and Langmuir isotherms respectively. The differences between the predicted isotherm parameter sets of linear models and minimized error function models indicated that both the best-fit isotherm selection and the isotherm constant determinations can be performed properly by error minimization as well as by conventional linear best fit modelling approach.

Adsorption of chromium(VI) from aqueous solutions onto amine-functionalized natural and acid-activated sepiolites

Chromium(VI) adsorption onto amine-functionalized sepiolites from aqueous solution at 298K was investigated. Natural and acid-activated sepiolites were functionalized by covalent grafting [3-(2-aminoethylamino)propyl]trimethoxy-silane to the silanol groups onto the sepiolite surface. Functionalization was proved by differential thermal analysis and the point of zero charge, pHpzc, determinations. The adsorption isotherms suggested that the adsorption capacity of the functionalized acid-activated sepiolite was higher than that of the functionalized natural sepiolite, for all investigated initial solution pH values. Adsorption efficiency of the functionalized sepiolites was improved by decreasing solution pH value. Due to the high value of pHpzc and large buffer capacity of adsorbents, a very low initial solution pH value was required to achieve high protonation of the surface amine groups and provide electrostatic attraction of Cr(VI) anionic species. The maximum chromium(VI) removal was achieved at an initial pH of 2.0; ca. 60mg/g of functionalized acid-activated sepiolite and ca. 37mg/g of functionalized natural sepiolite. Equilibrium data for both functionalized sepiolites at an initial solution pH value of 2.0 fitted well to the Freundlich model. The adsorption process followed pseudo-second-order kinetics. The values of the thermodynamic parameters indicate a spontaneous adsorption process of a prevalently physical nature.

Sorption of radiocobalt on acid-activated sepiolite: effects of pH, ionic strength, foreign ions, humic acid and temperature

The acid-activated sepiolite (ASEP) was prepared by physical purification and acid activation of natural sepiolite, and was characterized by XRD, FT-IR, SEM and N2 adsorption–desorption. The prepared ASEP was applied for the sorption of 60Co(II) from aqueous solutions. The sorption of 60Co(II) from aqueous solutions by ASEP was investigated as a function of contact time, solid content, pH, ionic strength, foreign ions, humic acid (HA) and temperature. The results indicated that the sorption of 60Co(II) on ASEP was strongly dependent on pH values. At low pH, the sorption of 60Co(II) was dominated by outer-sphere surface complexation or ion exchange, whereas inner-sphere surface complexation or surface precipitation was the main sorption mechanism at high pH. The presence of HA increased the sorption of 60Co(II) on ASEP at low pH values, and reduced the sorption at high pH values. The Langmuir and Freundlich models were applied to simulate the sorption of 60Co(II) at three temperatures of 298, 318 and 338 K. The thermodynamic parameters ( $$ \Updelta G^\circ ,\,\;\Updelta S^\circ $$ and $$ \Updelta H^\circ $$ ) calculated from the temperature dependent sorption isotherms indicated that the sorption of 60Co(II) on ASEP was an endothermic and spontaneous process. ASEP has a great application potential for cost-effective disposal of 60Co(II) from large volumes of aqueous solutions.