Removal of anionic ions from single material solution by bauxite tailings modified with FeCl3·6H2O

The adsorbabilities of the unmodified and modified bauxite tailings for Cr(Ⅵ), As(Ⅴ) and F(Ⅰ) ions were investigated. Batch experiments were carried out to determine the removal rate as a function of adsorbent dosage, solution pH value and shaking time. The results show that the maximum removal rates of Cr(Ⅵ), As(Ⅴ) and F(Ⅰ) are respectively 99%, 99% and 90% by using the modified bauxite tailings. The isoelectric point of the unmodified bauxite railings is 3.6, and that of the modified bauxite tailings is 5.0, which shifts to lower pH values in Cr(Ⅵ) solution. This indicates a specific adsorption of the anionic species on the modified bauxite tailings. A new band of Cr2O72- appears in the FTIR, showing that Cr(Ⅵ) is adsorbed on the modified bauxite tailings in the form of chemistry adsorption. The adsorption data of Cr(Ⅵ) on the modified bauxite tailings are well described by Freundlich model. The investigations of kinetic models show that pseudo-second-order kinetic model provides the best correlation for the experimental data.     

Rapid biosorption and reduction removal of Cr（Ⅵ） from aqueous solution by dried seaweeds

Four types of common seaweeds（Laminaria japonica,Undaria pinnatifida,Porphyra haitanensis,and Gracilaria lemaneiformis） were examined to remove Cr（Ⅵ） ions from aqueous solution.The experimental parameters that affected the biosorption process including pH,biomass dosage,contact time and temperature were investigated via batch experiments.The surface characteristics of seaweeds before and after Cr（Ⅵ） adsorption were studied with scanning electron microscopy and Fourier transform infrared spectroscopy.The results show that an initial solution with the pH of 1.0 is most favorable for Cr（Ⅵ） adsorption.Rapid adsorption is observed in the initial stage and adsorption equilibrium state is reached within 1 h.The adsorption efficiency by Porphyra haitanensis is the maximum among four types of seaweed powders,followed by Laminaria japonica and Undaria pinnatifida with biosorption efficiency up to 90%.The removal rate of Gracilaria lemaneiformis is less than 60%.The kinetic data obtained using the seaweeds are found to follow pseudo-second order kinetic model.Experimental sorption data adequately correlate with the Langmuir model.FTIR indicates that amino and carboxyl groups play an important role in the process of Cr（Ⅵ） adsorption and a large percentage of Cr（Ⅵ） ions are reduced by reductive groups on the surface of seaweeds.     

Adsorption behavior of Pb^2＋ and Cd^2＋ ions on bauxite flotation tailings

The adsorption behavior of Pb^2＋ and Cd^2＋ ions on bauxite flotation tailings was investigated to demonstrate the adsorptivity of the bauxite flotation tailings. The adsorption percentage of Pb^2＋ and Cd^2＋ ions as a function of adsorbent dosage, solution pH value and shaking time were determined by batch experiments. The maximum adsorption percentage of 99.93% for Pb^2＋ ions and 99.75% for Cd^2＋ ions were obtained by using bauxite flotation tailings as adsorbent. The methods, such as zeta potentials, specific surface area measurements and the analysis of adsorption kinetics, were introduced to analyze the adsorption mechanisms of the Pb^2＋ ions on bauxite flotation tailings. The isoelectric point of bauxite flotation tailings shifts from 3.6 to 5.6 in the presence of Pb^2＋ ions. The specific surface area of bauxite flotation tailings changes from 12.57 to 20.63 m^2/g after the adsorption of Pb^2＋ ions. These results indicate that a specific adsorption of the cation species happens on the surface of bauxite flotation tailings. Adsorption data of Pb^2＋ ions on the surface of bauxite flotation tailings can be well described by Langmuir model, and the pseudo-second-order kinetic model provides the best correlation for the adsorption data of Pb^2＋ and Cd^2＋ ions on bauxite flotation tailings.     

A study on Cr（VI） reduction from aqueous solutions by bauxite

The reduction of Cr（Ⅵ） in aqueous solution by using bauxite ore was investigated.Experimental results for Cr（Ⅵ）reduction in aqueous solution depending on some factors such as time,sulfuric acid amount,bauxite dosage,initial Cr（Ⅵ）concentration,formic acid concentration,daylight and temperature were presented.The obtained results show that sulfuric acid amount,bauxite dosage and initial Cr（Ⅵ） concentration of solution are most effective parameters on the reduction process.Also,it has been found that the 60 g/L of bauxite dosage is sufficient to reduce up to 100% of Cr（Ⅵ） from acidic solution under the condition of low initial Cr（Ⅵ） mass concentrations such as 10 mg/L.The reduction reaction is completed within 30 min at 25 ℃ under the experimental conditions：bauxite dosage of 60 g/L,amount of sulfuric acid 40-60 stoichiometric and initial mass concentration of Cr（Ⅵ）,10 mg/L.It was determined that reduction percentage is decreasing with increasing initial Cr（Ⅵ） concentration.The chemical oxygen demand of bauxite ore was found to be 26 mg COD/g.     

Biosorption mechanism of Cr （Ⅳ） onto cells of Synechococcus sp.

The biosorption mechanism of Cr （Ⅳ） ions on Synechococcus sp. biosorbent was studied by analyzing the biosorption kinetics as well as speciation change and bond formation during the biosorption process. The kinetics study shows that the adsorption process of Cr （Ⅳ） consists of a very fast stage in the first several minutes, in which more than half of the saturation adsorption is attained, and a slower stage that approximately follows the first order kinetic model, basically Freundlich isotherm models were observed. Comparative studies of FT-LR spectra of K2Cr2O7, free cells of Synechococcus sp., and Cr-bound cells of Synechococcus sp show that the speciation of chromium that binds to the cells ofSynechococcus sp. is Cr （Ⅲ）, instead of Cr （Ⅳ）, and the carboxylic, alcoholic, amido and amino groups may be involved in the binding of Cr （Ⅲ）. Integrative analyses of the surface electric potential, the effect of pH value on adsorption behavior of Cr （Ⅵ）, and the results of FT-IR show that the biosorption of Cr （Ⅵ） follows two subsequent steps, biosorption of Cr2O7 ^2- by electrostatical force at the protonated active sites and reduction of Cr2O7^2- to Cr^3＋ by the reductive groups on the surface of the biosorbents.     

Enhanced atrazine adsorption from aqueous solution using chitosan-modified sepiolite

A novel clay mineral biocomposite, chitosan-modified sepiolite(CMSEP), was prepared and used as adsorbent to remove atrazine from water. The adsorption behaviors including thermodynamic and kinetic parameters, effect factors and mechanisms of atrazine adsorption on CMSEP were studied. The results show that the adsorption capacity of atrazine on CMSEP increases with increasing temperature. Protonation of chitosan in biocomposite can improve adsorption ability of the composite to a certain extent. The parameters ΔGΘ, ΔHΘ and ΔSΘ are-1.48--2.69 k J/mol, 7.54 k J/mol and 30.28 J/mol, respectively. Langmuir isotherm is proved to describe the adsorption data better than other isotherms with a maximum adsorption capacity of 17.92 mg/g, suggesting that the adsorption process is homogeneous. Pseudo-second-order kinetic model can fit the adsorption kinetic processes well although intraparticle diffusion can not be discarded. Briefly, CMSEP has potential value in the removal of atrazine from water and wastewater.     

Removal of Pb(Ⅱ) ions from aqueous solutions by litchi pericarp and its leachate

The adsorption capacity of Pb(Ⅱ) on litchi pericarps was investigated as a function of temperature,pH,and adsorbent dose using batch experiments.The experimental data obtained were evaluated using adsorption equilibrium isotherms and a kinetic model.Additionally,the removal of Pb(Ⅱ) in leachate of litchi pericarps was also evaluated.The results show that litchi pericarps exhibit a high adsorption capacity to Pb(Ⅱ),with the maximum removal efficiency occurring at a temperature of 25 ℃,a pH of 6.0-7.0 and an adsorbent dosage of 10 g/L.Langmuir and Freundlich isotherms and the pseudo-second-order kinetic model can all fit the equilibrium adsorption satisfactorily,with correlation coefficients(R~2) of 0.9935,0.9918 and 1.0,respectively.An average removal efficiency of 66.65% is found for Pb(Ⅱ) in leachate of litchi pericarps.     

Adsorption of copper（ Ⅱ ） and chromium（Ⅵ） on diaspore

The adsorption of Cu（ Ⅱ ） and Cr（Ⅵ） on diaspore was studied with the help of X-ray diffraction analysis, BET measurement, zeta potential measurement and atomic adsorption spectrometry. The adsorption equilibrium almost reaches within 60 min. The adsorption isotherms of Cu（ Ⅱ ） and Cr（Ⅵ） could be well described by the Langmuir equation. The adsorption capacities of Cu（Ⅱ） and Cr（Ⅵ） are 1.944 and 1.292 mg/g, respectively. The adsorption percentage of Cr（Ⅱ ） increases with the increment of solution pH, but the adsorption percentage of Cr（Ⅵ） decreases. This could be explained by zeta potential theoretical and electrostatic attraction between metal ions and diaspore surface.     

Biosorption mechanism of Cr (Ⅵ) onto cells of Synechococcus sp.

The biosorption mechanism of Cr (Ⅵ) ions on Synechococcus sp. biosorbent was studied by analyzing the biosorption kinetics as well as speciation change and bond formation during the biosorption process. The kinetics study shows that the adsorption process of Cr (Ⅵ) consists of a very fast stage in the first several minutes, in which more than half of the saturation adsorption is attained, and a slower stage that approximately follows the first order kinetic model, basically Freundlich isotherm models were observed. Comparative studies of FT-IR spectra of K_2Cr_2O_7, free cells of Synechococcus sp., and Cr-bound cells of Synechococcus sp. show that the speciation of chromium that binds to the cells of Synechococcus sp. is Cr (Ⅲ), instead of Cr (Ⅵ), and the carboxylic, alcoholic, amido and amino groups may be involved in the binding of Cr (Ⅲ). Integrative analyses of the surface electric potential, the effect of pH value on adsorption behavior of Cr (Ⅵ), and the results of FT-IR show that the biosorption of Cr (Ⅵ) follows two subsequent steps, biosorption of Cr_2O_(7~)2-))by electrostatical force at the protonated active sites and reduction of Cr_2O_(7~(2-)) to Cr3+ by the reductive groups on the surface of the biosorbents.     

Biosorption of methylene blue by chemically modified cellulose waste

Citric acid modifi ed cellulose waste（CMCW） was prepared via esterifi cation and used as a low-cost biosorbent for the removal of methylene blue（MB） from aqueous solutions. The effects of biosorbent concentration, initial pH of MB solution, biosorption temperature, contact time, and initial MB concentration on the biosorption of MB were investigated using batch biosorption technique under static conditions. The experimental results showed that CMCW exhibited excellent biosorption characteristics for MB. The maximum biosorption capacity of MB was up to 214.5 mg/g at an adsorption temperature of 293 K. The removal rate of MB onto CMCW reached the maximum at pH〉4 and the biosorption reached an equilibrium at about 50 min. The kinetic data can be described well with the pseudo-second-order model and the isotherm data was found to fi t the Langmuir isotherm with a monolayer adsorption capacity of 211.42 mg/g. The biosorption appears to be controlled by chemisorption and may be involved in surface adsorption and pore diffusion during the whole biosorption process.     

Single-step synthesis of magnetic chitosan composites and application for chromate(Cr(Ⅵ)) removal

Magnetic chitosan composites(Fe3O4@chitosan) were synthesized in one single-step, characterized and applied in Cr(VI) removal from water. With the increase of loading proportion of chitosan, Cr(Ⅵ) adsorption capacity of Fe3O4@chitosan composites increased from 10.771 to 21.040 mg/g. The optimum adsorption capacities of Cr(VI) on Fe3O4@chitosan-3 were found in a pH range of 3.0-5.0. Kinetic study results show that the adsorption process follows pseudo-second-order model, indicating that the rate-limiting step in the adsorption of Cr(Ⅵ) involves chemisorptions. Moreover, FT-IR spectra analysis confirms that the amine and hydroxyl groups of chitosan are predominantly responsible for binding. Results from this work demonstrate that the prepared Fe3O4@chitosan composites possess great potential in Cr(Ⅵ) removal from contaminated water.     

Equilibrium and kinetics of adsorption of Ca(Ⅱ) ions onto KCTS and HKCTS

The adsorption of Ca(Ⅱ) ions from aqueous solution by chitosan α-ketoglutaric acid(KCTS) and hydroxamated chitosan α-ketoglutaric acid(HKCTS) was studied in a batch adsorption system.The Langmuir and Freundlich adsorption models were applied to describing the equilibrium isotherms,and isotherm constants were determined.The kinetics of the adsorption with respect to the initial Ca(Ⅱ) ions concentration,temperature and pH was investigated.The pseudo-first-order and second-order kinetic models were used to describe the kinetic data and the rate constants were evaluated.The results show that the experimental data fit well to the Langmuir isotherms with a high correlation coefficient(R2).The pseudo-second-order rate expression provides the best fitting kinetic model.The pseudo second-order kinetic model is indicated with the activation energy of 26.22 kJ/mol and 6.16 kJ/mol for KCTS and HKCTS,respectively.It is suggested that the overall rate of adsorption of Ca(Ⅱ) ions is likely to be controlled by the chemical process.     

Characteristics of tailings from metal mines in Hunan Province, China

The tailing soils were from 10 mining areas in Hunan Province. To predict the potential impact of tailings on nearby environments, the characteristics such as the pH value, loss on ignition, cation exchange capacity, and the concentration and speciation of heavy metals in the railings were investigated. Based on these characteristics, the pollution index and danger index were calculated so as to evaluate the priorities of remediation. The results show that the concentrations of Pb, Cd, Cu and Zn exceed the tolerable levels of the phytotoxicity in the most railings. The large proportion of heavy metals exists in the form of residual fraction in most railings, followed by sulfides/organic and Fe/Mn oxide fractions, and a little in the exchangeable and carbonate fraction. The calculated pollution indices for the tailing samples range from 1.41 to 83.42, which indicates that all the railings contain heavy metals at a level that causes toxicity to the ecosystem. The danger indices for the tailing samples range from 0.06 to 387.00. The highest value of the danger indices is that of Yongzhou sample, reaching 387. 00; the lowest one is that of Xikuangshan sample, only 0.06. Considering the results of pollution index and danger index in combination, the priority of remediation is determined to be Yongzhou, Baoshan, Xiangtan and Lengshuijiang.     

Adsorption behavior of molybdenum onto D314 ion exchange resin

The adsorption behavior of molybdenum onto D314 was studied with the static adsorption method. The adsorption process was analyzed from thermodynamic and kinetic aspects. The experimental results show that the equilibrium adsorption data conform satisfactorily to the Langmuir equation. In the adsorption process of D314 for molybdenum, the enthalpy change ΔH is positive when temperature is in the range of 298-338 K, which indicates that the adsorption is an endothermic process, and the elevated temperature benefits to the adsorption. Kinetic analysis shows that the adsorption rate is controlled by intraparticle diffusion and chemical diffusion at the same time. The adsorption mechanism of molybdenum onto D314 was discussed based on IR spectra.     

Competitive adsorption of Cu（Ⅱ） and Pb（Ⅱ） ions from aqueous solutions by Ca-alginate immobilized activated carbon and Saccharomyces cerevisiae

To establish a theoretical foundation for simultaneous removal of multi-heavy metals,the adsorption of Cu（Ⅱ） and Pb（Ⅱ） ions from their single and binary systems by Ca-alginate immobilized activated carbon and Saccharomyces cerevisiae （CAS） was investigated.The CAS beads were characterized by Scanning electron microscope （SEM） and Fourier transformed infrared spectroscopy （FTTR）.The effect of initial pH,adsorbent dosage,contact time and initial metal ions concentration on the adsorption process was systematically investigated.The experimental maximum contents of Cu（Ⅱ） and Pb（Ⅱ） uptake capacity were determined as 64.90 and 166.31 mg/g,respectively.The pseudo-second-order rate equation and Langmuir isotherm model could explain respectively the kinetic and isotherm experimental data of Cu（Ⅱ） and Pb（Ⅱ） ions in single-component systems with much satisfaction.The experimental adsorption data of Cu（Ⅱ） and Pb（Ⅱ） ions in binary system were best described by the extended Freundlich isotherm and the extended Langmuir isotherm,respectively.The removal of Cu（lⅡ） ions was more significantly influenced by the presence of the coexistent Pb（Ⅱ） species,while the Pb（Ⅱ） removal was affected slightly by varying the initial concentration of Cu（Ⅱ）.The CAS was successfully regenerated using 1 mol/L HNO3 solution.     

Corrosion resistance of Molybdenum Nitride modified Ti6Al4V alloy in HCl solution

The Mo-N surface modified layer on Ti6Al4V alloy was obtained by the plasma surface alloying technique. The structure and composition of the Mo-N modified Ti6Al4V alloy were investigated by X-ray diffraction （XRD） and glow discharge optical emission spectroscopy （GDOES）. The Mo-N modified layer contains Mo-N coating on subsurface and diffusion layers between the subsurface and substrate. The X- ray diffraction analysis of the Mo-N modified Ti6Al4V alloy reveals that the outmost surface of the Mo-N modified Ti6Al4V alloy is composed of phase Mo2N （fcc） and Mo2N （tetr）. The electrochemical corrosion performance of the Mo-N modified Ti6Al4V alloy in 0.5 mol/L HCl solution was investigated and compared with that of Ti6Al4V alloy. The chemical corrosion performance of the Mo-N modified Ti6Al4V alloy in boiling 37% HCl solution was investigated and compared with that of Ti6Al4V alloy. Results indicate that self-corroding electric potentials and corrosion-rate of the Mo-N modified Ti6Al4V alloy are higher than that of Ti6Al4V alloy in 0.5 mol/L HCl solution. The corrosion-rate of the Mo-N modified Ti6Al4V alloy is lower than that of Ti6Al4V alloy in boiling 37% HCl solution.     

Selective leaching of chromium-containing slag by HCl

A batch of column experiments was carried out to investigate the change of Cr（Ⅵ） concentration leached out from chromium-containing slag with HCI as leaching agent, and to study influences of pH, ratio of solid mass to solution volume, flow velocity and temperature on Cr（Ⅵ） leaching. The optimal parameters were obtained for Cr（Ⅵ） leaching and a fitting model was established to describe the procedure of Cr（Ⅵ） leaching. The results show that Cr（Ⅵ） concentration in leachate increases with decreasing pH and increasing flow velocity and temperature. Moreover, Cr（Ⅵ） leaching percentage increases with increasing ratio of solid mass to solution volume. The optimal parameters for Cr（Ⅵ） selective leaching are as follows： pH=3.0, 1：5 of ratio of solid mass to solution volume, 180 mL/min of flow velocity and 40 ℃ of temperature. The procedure of Cr（Ⅵ） leaching fits well with the model： v= 1.87t^-0.54, indicating that the leaching rate of Cr（Ⅵ） declines in an exponential order of-0.54.     

Surface modification of magnesium hydroxide by γ-aminopropyltriethoxysilane

Magnesium hydroxide（MH）,which is commonly used as a halogen-free flame retardant filler in composite materials,was modified by silanization reaction with γ-aminopropyltriethoxysilane （γ-APS） in aqueous solution at different pH values （pH range from 8.0 to 12.0）. The surface properties of grafted γ-APS on MH surface as a function of solution pH value were studied using elemental analysis,Fourier transform infrared spectroscopy and zeta potential measurement. The results show that hydrolysis and condensation of γ-APS are activated in alkaline solution and lead to multilayer adsorption of γ-APS molecules on the surface of MH. The type of adsorption orientation of γ-APS on MH surface is a function of coverage density that is altered by changing solution pH value. At low coverage density （e.g. 55 nm^-2）,γ-APS molecules are preferentially adsorbed to the surface with the silicon moiety towards the surface,and increasing coverage density （e.g. 90 nm^-2） leads to parallel orientation. At an even higher coverage density （e.g. 115 nm^-2）,γ-APS molecules bond to the surface with the amino moiety towards the surface.     

Synthesis,Characterization and Adsorption Properties of Low-cost Porous Calcined Dolomite Microspheres for Removal of Dyes

Low-cost porous calcined dolomite microspheres were prepared by simple spray drying and subsequent calcination. Effects of calcination temperature on phase evolution and adsorption properties of MB were investigated systematically. Results showed that microspheres treated at 400 ℃ kept mainly calcium carbonate(CaMg(CO_3)) phase with some small pores, showing better removal efficiency for MB. With the dosage of 20 g/L under the starting concentration of 100 mg/L, the removal efficiency of the microspheres reached 95.6%. The adsorption kinetics data followed the pseudo-second-order kinetic model, and the isotherm data fit the Langmuir isotherm model. The low-cost microsphere could be applied as a promising absorbent for dyes in wastewater filtration and adsorption treatment.     

High Efficient Removal of Trace Molybdenum from Water by FeCl3: Effects of pH and Affecting Factors in the Presence of Co-existing Background Constituents

The effect of FeCl_3 coagulation-filtration on the removal of trace Mo(Ⅵ) from water is investigated in the p H range of 4. 00 to 9. 00 and the effects of sulfate,silicate,phosphate and humic acid( HA) on the process were determined. Overall,the removal of Mo(Ⅵ) is determined by two factors: the content of Fe intercepted from water( Intercepted Fe) and the affinity between Mo(Ⅵ)( or co-existing background constituents) and adsorption sites. At low p H,where the agglomeration of the iron flocs is limited,the former factor is dominant,so the methods that can promote the agglomeration of the iron flocs,such as increasing the p H,adding co-existing constituents( sulfate,phosphate or HA),can increase the removal of Mo(Ⅵ). While at high p H,the second factor dominated. Increasing the p H weakens the affinity between Mo(Ⅵ) and the iron flocs,and co-existing background constituents( sulfate,phosphate,silicate or HA) compete with Mo(Ⅵ) for adsorption sites,both effects result in a decrease in Mo(Ⅵ) removal. The Mo(Ⅵ) removal efficiency of FeCl_3 in natural water decreases as the p H increases from 4. 00 to 9. 00,and it is better to operate the coagulation process at p H 5.00 in the practical water treatment engineering.