Selective removal of Cr(VI) ions from aqueous solutions including Cr(VI), Cu(II) and Cd(II) ions by 4-vinly pyridine/2-hydroxyethylmethacrylate monomer mixture grafted poly(ethylene terephthalate) fiber

In this study, a new reactively fibrous adsorbent was prepared by grafting 4-vinly pyridine (4-VP) and 2-hydroxyethylmethacrylate (HEMA) monomer mixture onto poly(ethylene terephthalate) (PET) fibers for removal of Cr(VI), Cu(II) and Cd(II) metal ions from aqueous solution by using batch adsorption method. The influence of various parameters such as graft yield (GY), pH, adsorption time, initial ion concentration and adsorption temperature was investigated. The selectivity of the reactive fiber was also examined. The results show that the adsorbed amount of metal ions followed as given in the order Cr(VI) > Cd(II) > Cu(II). At pH 3, Cr(VI) was removed by 99% while the initial concentration of ions was at 5 mg L⁻¹ and by 94% at 400 mg L⁻¹. It was found that the grafted fiber is more selective for Cr(VI) ions in the mixed solution of Cr(VI)–Cu(II), Cr(VI)–Cd(II) and Cr(VI)–Cu(II)–Cd(II) at pH 3 and it was observed that the grafted fibers are stable and regenerable by acid and base without losing their activity.     

Layer-by-layer assembly of poly (N-acryloyl-N'-propylpiperazine) and poly (acrylic acid): Effect of pH and temperature

Layer-by-layer assembly technique was applied to fabricate multilayer films of poly(N-acryloyl-N'-propylpiperazine) (PAcrNPP) and poly(acrylic acid). The film buildup was followed by Quartz Crystal Microbalance, and the effect of temperature and pH on the layer-by-layer assembly was investigated. Below the lower critical solution temperature of PAcrNPP, the adsorption amount increased with increase of temperature. As the pH of dipping solutions was systematically varied from 2.0 to 7.5, adsorption amount of polymers initially increased and then decreased. The relationship between adsorption amount and degree of ionization of polymers was discussed. When an eight-bilayer film was alternately exposed to HCl aqueous solution (pH = 3.5) and neutral water (pH = 6.5), the film showed pH-response, which could be the result of polymer rearrangement.     

Removal of Pb(II) using the modified lawny grass: Mechanism,kinetics, equilibrium and thermodynamic studies

A series of new chemical-modified lawny grass adsorbents were prepared and the feasibility of absorbents to remove Pb(II) ion from aqueous solution was examined. The absorbents were characterized by FTIR spectra and elemental analysis. Kinetics, equilibrium, thermodynamics, column adsorption and mechanism were studied. The optimum pH is in the range of 5.0–5.8 for all adsorbents. The sorption system follows pseudo-second-order kinetic model and equilibrium time is obtained after 60 min. The maximum adsorption capacities obtained from Langmuir–Freundlich model are 1.55 and 1.26 mol/kg by using 1 CG and 0.6 CG (lawny grass modified by 1 mol/L or 0.6 mol/L citric acid, respectively). Thermodynamic parameters such as ΔG, ΔH and ΔS are evaluated for the adsorption process. The results indicates that the adsorption of Pb(II) is spontaneous and endothermic. The breakthrough point is achieved at 100 BV (bed volume) by a column of 0.6 CG. Desorption of Pb(II) and regeneration of the column is achieved by 0.1 mol/L HCl elution. After 3 adsorption/desorption cycles, the breakthrough point remains around 100 BV, which shows that grass adsorbent is regenerated easily and used repeatedly. Above results indicates that lawny grass have a good potential for removal of lead from dilute aqueous solution in the future.     

Effect of competitive ions on the arsenic removal by mesoporous hydrous zirconium oxide from drinking water

Adsorption properties of 302-type commercially available hydrous zirconium oxide (302-HZO) towards arsenic and some competitive anions and cations have been studied under batch and column conditions. Due to amphoteric properties, anion exchange performance of hydrous zirconium oxide is pH dependent. Media exhibits high affinity towards arsenic in a broad pH range, with high adsorption capacity at pH < 8. It was shown that silicate and phosphate ions are arsenic's main competitors affecting media adsorption capacity. Presence of transition metal cations in <1 ppm does not affect 302-HZO capacity on arsenic, whereas alkaline-earth cations improve arsenic removal. The possibility for significant increase of 302-HZO adsorption capacity on arsenic at pH > 8 by using “solid acidifier” technique is discussed. Results of 302-HZO field trials are presented.     

Study of adsorption behaviors of meso-tetrakis (4-N-Methylpyridyl) porphine p-Toluenesulfonate at indium-tin-oxide electrode/solution interface by in-situ internal reflection spectroscopy and cyclic voltammetry

In situ electrochemical-absorption spectra of meso-tetrakis (4-N-Methylpyridyl) porphine p-Toluenesulfonate (TMPyP) adsorbed on the surface of an ITO (indium-tin-oxide) electrode was observed by simultaneous measurements using cyclic voltammetry (CV) and internal reflection spectroscopy. The CV experiment was performed using ITO coated on glass as the working electrode, platinum wire as the counter electrode and silver/silver chloride (Ag/AgCl) electrode as the reference electrode. It was found that, in 1 M HCl and pH = 2 Britton-Robinson solutions, TMPyP were adsorbed on ITO as P(0)H₄²⁺ state and P(0)H₂ state, respectively. Moreover, when the potential was applied on the ITO electrode, the redox reactions of TMPyP in 1 M HCl were also different from those in pH = 2 Britton-Robinson solutions. The adsorption rate, desorption rate and equilibrium constants as well as the adsorption free energy of TMPyP were determined by fitting adsorption process between TMPyP and ITO using Langmuirian adsorption model. The effect of pH on the internal reflection spectra was also explored, in which it was shown that the increase of pH lead to the enhancement of the intensity of the internal reflection spectra and the blue shift of the peak position. With the increase of ionic strength of TMPyP solution, the intensity of the internal reflection spectra decreased.     

Quantifying effects of pH and surface loading on arsenic adsorption on NanoActive alumina using a speciation-based model

Arsenic (As) poses a significant water quality problem and challenge for the environmental engineers and scientists throughout the world. Batch tests were carried out in this study to investigate the adsorption of As(V) on NanoActive alumina. The arsenate adsorption envelopes on NanoActive alumina exhibited broad adsorption maxima when the initial As(V) loading was less than a 50 mg g⁻¹ sorbent. As the initial As(V) loading increased to 50 mg g⁻¹ sorbent, a distinct adsorption maximum was observed at pH 3.2–4.6. FTIR spectra revealed that only monodentate complexes were formed upon the adsorption of arsenate on NanoActive alumina over the entire pH range and arsenic loading conditions examined in this study. A speciation-based adsorption model was developed to describe arsenate adsorption on NanoActive alumina and it could simulate arsenate adsorption very well in a broad pH range of 1–10, and a wide arsenic loading range of 0.5–50 mg g⁻¹ adsorbent. Only four adjustable parameters, including three adsorption constants, were included in this model. This model offers a substantial improvement over existing models in accuracy and simplification in quantifying pH and surface loading effects on arsenic adsorption.     

Use of rice straw as biosorbent for removal of Cu(II), Zn(II), Cd(II) and Hg(II) ions in industrial effluents

Adsorption experiments were carried out using waste rice straw of several kinds as a biosorbent to adsorb Cu(II), Zn(II), Cd(II) and Hg(II) ions from aqueous solutions at room temperature. To achieve the best adsorption conditions the influence of pH and contact time were investigated. The isotherms of adsorption were fitted to the Freundlich equation. Based on the experimental data and Freundlich model, the adsorption order was Cd(II) > Cu(II) > Zn(II) > Hg(II) on the rice straw. This quick adsorption process reached the equilibrium before 1.5 h, with maximum adsorptions at pH 5.0. Thermodynamic aspects of the adsorption process were investigated. The biosorbent material was used in columns for the removal of ions Cu, Zn, Cd and Hg of real samples of industrial effluent and its efficiency was studied.     

Exploring proton doping in poly-3-methylpyrrole by infrared spectroscopy

Structural changes induced by electrochemical redox processes or by pH variations on conducting poly-3-methylpyrrole, electrochemically synthesized in NaClO₄ acetonitrile solution, have been studied by infrared spectroscopy. With this aim infrared spectra of perchlorate doped poly-3-methylpyrrole films at different oxidation states as well as after immersion in acid (pH = 1) and basic (pH = 12.6) aqueous solutions have been analysed. The existence of proton-doping mechanism in this polymer has been confirmed from the comparative study of spectra of oxidized/reduced and acid/basic treated polymer.     

Optimization of the mechanical properties of calcium phosphate/multi-walled carbon nanotubes/bovine serum albumin composites using response surface methodology

Response surface methodology (RSM) coupled with the central composite design (CCD) was used to optimize the mechanical properties of calcium phosphate cement/multi-walled carbon nanotubes/bovine serum albumin (CPC/MWCNTs/BSA) composites. In this study, CPC composites were reinforced by multi-walled carbon nanotubes (MWCNTs) and bovine serum albumin (BSA) in order to induce high mechanical properties in the CPC/MWCNTs/BSA system. The effect of various process parameters on the compressive strength of CPC/MWCNTs/BSA composites was studied using design of experiments (DOE). The process parameters studied were: wt.% of MWCNTs (0.2–0.5 wt.%), wt.% of BSA (5–15 wt.%) and type of MWCNTs (e.g. as-pristine MWCNT (MWCNT-AP), hydroxyl group functionalized MWCNT (MWCNT-OH) and carboxyl group functionalized MWCNT (MWCNT-COOH)). Based on the CCD, a quadratic model was obtained to correlate the process parameters to the compressive strength of CPC/MWCNTs/BSA composites. From the analysis of variance (ANOVA), the most significant factor affected on the experimental design response was identified. The predicted compressive strength after process optimization was found to agree well with the experimental value. The results revealed that at 0.5 wt.% of MWCNT-OH and 15 wt.% of BSA, the highest compressive strength of 14 MPa was obtained.     

pH-dependant structural and morphology evolution of Ni(OH)2 nanostructures and their morphology retention upon thermal annealing to NiO

Nickel hydroxide nanosheets, nanobelts and nanorods were prepared by hydrothermal treatment of the precipitates obtained at different pH values. The morphology and crystal structure of the products could be controlled simply by adjusting the pH value at precipitation. Interconnected nanosheets of hexagonal β-Ni(OH)₂ with thickness around 10–20 nm were formed at pH ∼ 11, whereas nanobelts with typical widths around 40–80 nm, and nanorods with diameters around 50–60 nm of phase pure α-Ni(OH)₂ containing intercalated sulphate ions were obtained in the pH range ∼9.5–8.5. Thermal annealing of the hydroxides at 500 °C yielded cubic phase NiO with morphologies similar to their hydroxide precursors. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and energy dispersive X-ray (EDX) analysis were used to characterize the as-prepared products. The role of pH in controlling the phase and morphology of the products was discussed.     

pH and iodide ion effect on corrosion inhibition of histidine self-assembled monolayer on copper

Self-assembled monolayer (SAM) of histidine (His) was prepared on copper surface at various pH values. The effect of KI additives on corrosion protection efficiency of His SAM was also studied. The protection abilities of these films against copper corrosion in 0.5 M HCl aqueous solution were investigated using electrochemical impedance spectroscopy and polarization techniques. The results show that the film formed on the electrode is more stable at pH = 10 than that at other pH values. When the iodide ions were added into the His self-assembly solution (pH = 10), protection efficiency was further improved. The inhibition mechanism has been discussed by quantum chemical calculations.     

Grass waste: A novel sorbent for the removal of basic dye from aqueous solution

The aim of the present work was to investigate the feasibility of grass waste (GW) for methylene blue (MB) adsorption. The adsorption of MB on GW material was studied as a function of GW dose (0.05–1.20 g), solution pH 3–10, contact time and initial concentration (70–380 mg/L). The influence of these parameters on the adsorption capacity was studied using the batch process. The experimental data were analyzed by the Langmuir and Freundlich isotherms. The adsorption isotherm was found to follow the Langmuir model. The monolayer adsorption capacity was found to be 457.640 mg/g. The kinetic data were fitted to the pseudo-first-order and pseudo-second-order models, and were found to follow closely the pseudo-second-order kinetic model. The results revealed that GW adsorbent is potentially low-cost adsorbent for adsorption of MB.     

Adsorption of Cr(VI) using silica-based adsorbent prepared by radiation-induced grafting

Silica-based adsorbent was prepared by radiation-induced grafting of dimethylaminoethyl methacrylate (DMAEMA) onto the silanized silica followed by a protonation process. The FTIR spectra and XPS analysis proved that DMAEMA was grafted successfully onto the silica surface. The resultant adsorbent manifested a high ion exchange capacity (IEC) of ca. 1.30 mmol/g and the Cr(VI) adsorption behavior of the adsorbent was further investigated, revealing the recovery of Cr(VI) increased with the adsorbent feed and the equilibrium adsorption could be achieved within 40 min. The adsorption capacity, strongly depended on the pH of the solution, reached a maximum Cr(VI) uptake (ca. 68 mg/g) as the pH was in the range of 2.5–5.0. Furthermore, even in strong acidic (4.0 mol/L HNO₃) or alkaline media (pH 11.0), the adsorbent had a sound Cr(VI) uptake capacity (ca. 22 and 30 mg/g, respectively), and the adsorption followed Langmuir mode. The results indicated that this adsorbent, prepared via a convenient approach, is applicable for removing heavy-metal-ion pollutants (e.g. Cr(VI)) from waste waters.     

Removal of chromium from aqueous solution by using oxidized multiwalled carbon nanotubes

The batch removal of hexavalent chromium (Cr(VI)) from aqueous solution by using oxidized multiwalled carbon nanotubes (MWCNTs) was studied under ambient conditions. The effect of pH, initial concentration of Cr(VI), MWCNT content, contact time and ionic strength on the removal of Cr(VI) was also investigated. The removal was favored at low pH with maximum removal at pH <2. The adsorption kinetics was modeled by first-order reversible kinetics, pseudo-first-order kinetics, pseudo-second-order kinetics, and intraparticle diffusion models, respectively. The rate constants for all these kinetic models were calculated, and the results indicate that pseudo-second-order kinetics model was well suitable to model the kinetic adsorption of Cr(VI). The removal of chromium mainly depends on the occurrence of redox reaction of adsorbed Cr(VI) on the surface of oxidized MWCNTs to the formation of Cr(III), and subsequent the sorption of Cr(III) on MWCNTs appears as the leading mechanism for chromium uptake to MWCNTs. The presence of Cr(III) and Cr(VI) on oxidized MWCNTs was confirmed by the X-ray photoelectron spectroscopic analysis. The application of Langmuir and Freundlich isotherms are applied to fit the adsorption data of Cr(VI). Equilibrium data were well described by the typical Langmuir adsorption isotherm. Overall, the study demonstrated that MWCNTs can effectively remove Cr(VI) from aqueous solution under a wide range of experimental conditions, without significant Cr(III) release.     

Preparation of adsorbent for phosphate recovery from aqueous solutions based on condensed tannin gel

We have synthesized an iron-loaded tannin gel as an adsorbent for phosphate recovery in aqueous solutions. The use of the tannin gel prepared from condensed tannin, which is a ubiquitous and inexpensive natural polymer, is not only cost effective and environment-friendly, but interesting because the phosphate-adsorbed gel can be expected to use directly as a fertilizer. The amount of iron loaded into the tannin gel oxidized by nitric acid was much larger than that into the non-oxidized tannin gel. This increase in the amount of the loaded iron resulted in the significant increase in the adsorption amount of phosphate onto the gel. Mössbauer spectroscopy indicated that the morphology of iron in the gel is a mono-type complex, which is formed as a result of the reaction between Fe(III) and the oxidized tannin gel with carbonyl groups. The iron-loaded tannin gel showed the adsorption selectivity for phosphate over other anions and the pH independence of phosphate adsorption in the wide range of initial pH 3-12. The phosphate adsorption isotherm for the iron-loaded tannin gel followed the Freundlich equation with constants of K_F = 2.66 and 1/n = 0.31, rather than the Langmuir equation. The adsorption amount of phosphate on iron weight basis for the iron-loaded tannin gel is 31.3 mg-P/g-Fe, which indicates that iron in the gel was efficiently used for the phosphate adsorption compared with other phosphate adsorbents, such as iron hydroxides.     

Isotherms and thermodynamics for the sorption of heavy metal ions onto functionalized sporopollenin

In this study, sporopollenin of Lycopodium clavatum spores was used for the sorption experiment. Glutaraldehyde (GA) immobilized sporopollenin (Sp), is employed as a sorbent in sorption of selected heavy metal ions. The sorbent prepared by sequential treatment of sporopollenin by silanazing compound and glutaraldehyde is suggested for sorption of Cu(II), Zn(II) and Co(II) from aqueous solutions. Experimental conditions for effective sorption of heavy metal ions were optimized with respect to different experimental parameters using batch method in detail. Optimum pH range of Cu(II) has occurred at pH ≥ 5.5 and Zn(II), Co(II) at pH ≥ 5.0, for the batch method. All of the metal ions can be desorbed with 10 cm³ of 0.5 mol dm⁻³ of ethylenediaminetetraacetic acid (EDTA) solution. Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm equations were applied to the experimental data. Thermodynamic parameters such as free energy (ΔG^o), entropy (ΔS^o) and enthalpy (ΔH^o) were also calculated from the sorption results used to explain the mechanism of the sorption. The results indicated that this sorbent is successfully employed in the separation of trace Cu(II), Zn(II) and Co(II) from the aqueous solutions.     

Magnetite–hematite nanoparticles prepared by green methods for heavy metal ions removal from water

Mixed magnetite–hematite nanoparticles were synthesized via different routes such as, coprecipitation in air and N₂ atmosphere, citrate–nitrate, glycine–nitrate and microwave-assisted citrate methods. The prepared samples were characterized by X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), BET measurements and magnetic hysteresis. XRD data showed the formation of magnetite–hematite mixture with different compositions according to the synthesis method. The particle size was in the range of 4–52 nm for all the prepared samples. From HRTEM micrographs, it was found that, the synthesis method affects the moropholgy of the prepared samples in terms of crystallinity and porosity. The magnetite–hematite mixture was employed as a sorbent material for removal of some heavy metal ions from water such as lead(II), cadmium(II) and chromium(III). The effects of pH value and the contact time on the adsorption process were studied and optimized in order to obtain the highest possible adsorption efficiency of the magnetite–hematite mixture. The effect of the synthesis method of the magnetite–hematite mixture on the adsorption process was also investigated. It was found that samples prepared by the coprecipitation method had better adsorption efficiency than those prepared by other combustion methods.     

Size and morphology controlled synthesis of gold nanoparticles in green solvent: Effect of reducing agents

The effect of reducing agents on the synthesis of Au(0) metallic nanoparticles (Au NPs) prepared in green solvent medium of β-d-glucose-water dispersions has been reported first. The different equivalent amounts of NaBH₄ and pH values adjusted by NaOH were tested for the reduction of Au salt (HAuCl₄·3H₂O (hydrogen tetrachloroaurate (III) trihydrate) to obtain Au NPs. The type and the amount of reducing agent and the pH of the solution affected the size and morphology of the NPs. Addition of 4 equivalents of NaBH₄ produced homogeneously dispersed 5.3 nm (σ = 0.7) diameter particles. Excess addition of NaBH₄ caused the NPs to settle down as the precipitate forming mesh or wire structure. When salt was reduced by the addition of NaOH (pH = 8.0) the particles were larger (14.2 nm) and less homogeneous (σ = 2.8). At pH = 12.2 the NPs settled at the bottom of the vial when preparation was left overnight. The wire and mesh like structures were obtained at higher pH = 12.2.     

Nitrate removal from aqueous solutions by cross-linked chitosan beads conditioned with sodium bisulfate

The investigation of adsorption of nitrate onto chitosan beads modified by cross-linking with epichlorohydrin (ECH) and surface conditioning with sodium bisulfate was performed. The results indicated that both cross-linking and conditioning increased adsorption capacity compared to normal chitosan beads. The maximum adsorption capacity was found at a cross-linking ratio of 0.4 and conditioning concentration of 0.1 mM NaHSO₄. The maximum adsorption capacity was 104.0 mg g⁻¹ for the conditioned cross-linked chitosan beads at pH 5, while it was 90.7 mg g⁻¹ for normal chitosan beads. The Langmuir isotherm model fit the equilibrium data better than the Freundlich model. The mean adsorption energies obtained from the Dubinin-Radushkevich isotherm model for all adsorption systems were in the range of 9.55–9.71 kJ mol⁻¹, indicating that physical electrostatic force was potentially involved in the adsorption process.     

Equilibrium,kinetic and thermodynamic studies on the adsorption of 2-nitroaniline onto activated carbon prepared from cotton stalk fibre

Activated carbon prepared from cotton stalk fibre has been utilized as an adsorbent for the removal of 2-nitroaniline from aqueous solutions. The influence of adsorbent mass, contact time and temperature on the adsorption was investigated by conducting a series of batch adsorption experiments. The equilibrium data at different temperatures were fitted with the Langmuir, Freundlich, Tempkin, Redlich–Peterson and Langmuir–Freundlich models. The Langmuir–Freundlich isotherm was found to best describe the experimental data. The adsorption amount increased with increasing temperature. The maximum adsorption capacity of 2-nitroaniline was found to be 383 mg/g for initial 2-nitroaniline concentration of 200 mg/L at 45 °C. The kinetic rates were modeled by using the Lagergren-first-order, pseudo-second-order and Elovich models. The pseudo-second-order model was found to explain the adsorption kinetics most effectively. It was also found that the pore diffusion played an important role in the adsorption, and intraparticle diffusion was the rate-limiting step at the first 30 min for the temperatures of 25, 35 and 45 °C. FTIR and ¹³C NMR study revealed that the amino and isocyanate groups present on the surface of the adsorbent were involved in chemical interaction with 2-nitroaniline. The negative change in free energy (ΔG°) and positive change in enthalpy (ΔH°) indicated that the adsorption was a spontaneous and endothermic process.