Adsorption of hexavalent chromium from aqueous solutions by bio-chars obtained during biomass pyrolysis

In this study, bio-chars were evaluated as a potential adsorbent for the removal of Cr (VI) ions from aqueous solutions. The effects of some important parameters including initial pH (1.5–7), adsorbent dose (0.2–5 g/L), contact time (5–900 min) and initial Cr (VI) ion concentration (5–75 mg/L) were tested on the removal of Cr (VI) ions from aqueous solution in batch experiments. Maximum adsorption capacities of the tested bio-chars under the certain experimental conditions determined as optimal were 3.53 mg/g for NCBC, 3.97 mg/g for NZCBC and 6.08 mg/g for ACBC, respectively. Results of the kinetic and isotherm modeling studies revealed that the adsorption data fitted well with a pseudo-second order and Langmuir model. In among the tested bio-chars, the bio-char (ACBC) was largely equivalent to activated carbon: AC (9.97 mg/g) in terms of adsorption capacity. All results indicated that the bio-chars had higher adsorption capacity than some chars and activated carbons reported previously, and also that these bio-chars could be used successfully as low-cost adsorbents for the removal of chromium ions from aqueous solutions under the tested experimental conditions.     

Cationic polymer chain tethered on the pore-wall of 3-D ordered macroporous resin for the removal of hexavalent chromium from aqueous solution

A novel 3-D ordered macroporous (3DOM) adsorbent with a cationic polymer chain (poly(N,N-dimethylaminoethyl methacrylate), PDMAEMA) tethered on the pore wall was prepared by surface-initiated atom transfer radical polymerization (SI-ATRP) for the removal of toxic Cr(VI) ions from aqueous solution. In comparison with recently reported adsorbents, the adsorbent remarkably stands out owing to large adsorption capacity, relatively fast kinetics, and high stability in the regeneration process. The adsorption capacity significantly depended on the solution pH and there was a wide working pH range that is much convenient in practical application. Kinetics of Cr(VI) adsorption by the 3DOM adsorbent was studied in batch experiments, in the temperature range 298–318 K. The equilibriums were arrived within 120–130 min and a pseudo-second order model can be described well. In the adsorption isotherm study, experimental data followed the Langmuir adsorption model. The maximum adsorption capacity increased with the increase of temperature, and reached the high value of 431.0 mg/g at 308 K. Thermodynamic parameters revealed spontaneous and endothermic adsorption processes. Furthermore, the 3DOM adsorbent remained high adsorption capacity (above 90% of the original Cr(VI) loading capacity) after 15 adsorption–desorption cycles by simply using sodium hydroxide solution as the desorption liquid, which ensured the reusability of 3DOM adsorbent.     

Adsorption performance of suitable nanostructured novel composite adsorbent of poly(N-methylaniline) for removal of heavy metal from aqueous solutions

Poly(N-methylaniline) (PNMA) composite composed of N-methylaniline and poly(ethanol) was prepared by in situ polymerization technique and characterized using FTIR, SEM-EDS and XRD instruments. Characterization of product revealed that this composite is crystalline in nature and the particles size is less than100 nm. The potential of this composite in removal of Cr(VI) ions from synthetic aqueous effluents was investigated by batch sorption system. The experimental results confirmed that this adsorbent has the potential application for removal of Cr(VI) ions from aqueous solution with the sorption capacity of 125 mg/g of Cr(VI)/0.1 g of adsorbent.     

Adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin

Series of resin selection experiments were carried out and the KIP210 strong base anion exchange resin was confirmed to have the maximum equilibrium adsorption capacity to remove Cr(VI) from wastewater. The adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin were investigated completely and systematically. The static experiments were performed to study the effects of various parameters, such as shaking speed, resin dosage and pH during the adsorption process. The results indicate that the effect of external diffusion is eliminated at 160 rpm, the best pH value is 3.0 and the removal percentage of Cr(VI) increases with the increase of the resin dosage. The adsorption of Cr(VI) on KIP210 agrees well with the Langmuir isotherm and the adsorption parameters of thermodynamics are ΔH = 26.5 kJ mol⁻¹, ΔS = 126.7 J mol⁻¹ K⁻¹ and ΔG < 0. It demonstrates that the adsorption of Cr(VI) on KIP210 is a spontaneously endothermic physisorption process. Moreover, the adsorption process can be described well by a pseudo-second-order kinetic model and the activation energy is 30.9 kJ mol⁻¹. The kinetic analysis showed that the adsorption rate is controlled by intraparticle diffusion. The resin is successfully regenerated using the NaOH solutions.     

Acid modified local clay beads as effective low-cost adsorbent for dynamic adsorption of methylene blue

Locally sourced clay was harnessed to study its adsorptive potential of methylene blue (MB) in wastewater streams. The clay was modified with sulfuric acid and aluminum hydroxide. The raw and modified freeze dried clay bead RHC and MHC were subjected to batch and batch/fixed-bed adsorption studies, respectively. Elemental analysis, morphological structures were determined, and surface area of 19.3 (RHC) and 101.2 (MHC) m²/g were obtained. Langmuir, Freundlich and Redlich–Peterson isotherms models were analyzed and the modification increased adsorption capacity from 58.02 to 223.19 mg/g at 30 °C. The MB adsorption on RHC/MHC was spontaneous, exothermic and obeyed pseudo-second-order model.     

Poly(N,N-dimethylaminoethyl methacrylate)/graphene oxide hybrid hydrogels: pH and temperature sensitivities and Cr(VI) adsorption

Different amounts of graphene oxide (GO) were incorporated to N,N-dimethylaminoethyl methacrylate (DMAEMA), fabricating a series of pH and temperature dual sensitive PDMAEMA/GO hybrid hydrogels by in situ polymerization. Their microscopic network structures as well as swelling properties and Cr(VI) adsorption were characterized. The equilibrium swelling ratios (ESR) of hydrogels increased significantly with 0.5 wt% GO feeding of DMAEMA amount, and then decreased with further GO loading increasing. All hydrogels showed obvious deswelling when pH value of swelling mediums increased from 5 to 10 gradually. At pH 7, hydrogels revealed slight ESR increment with temperature up to 50 °C, above which obvious deswelling occurred. In pH 8 buffer, 0.5 wt% of GO loading triggered lower critical solution temperature (LCST) to decrease by 3 °C, and 2–7 °C increment was observed when 1–6 wt% of GO was loaded, as compared with that of GO-free PDMAEMA hydrogel. Cr(VI) adsorption of hydrogels was also improved by the introduction of GO to some extent, and the maximum Cr(VI) adsorption of 180 mg/g was realized, indicating that the obtained PDMAEMA/GO hybrid hydrogels possess excellent adsorption performance.     

Synthesis,characterization of Fe3O4@glycine doped polypyrrole magnetic nanocomposites and their potential performance to remove toxic Cr(VI)

Fe₃O₄ coated glycine doped polypyrrole magnetic nanocomposite (Fe₃O₄@gly-PPy NC) was prepared via coating of suspended Fe₃O₄ nanoparticles with gly-PPy. FE-SEM and HR-TEM images indicated that Fe₃O₄ nanoparticles were encapsulated by precipitating gly-PPy moieties. Chromium(VI) adsorption followed a Langmuir isotherm with maximum capacity of 238–303 mg/g for a temperature range of 25–45 °C at pH 2. The adsorption process was governed by the ionic interaction and the reduction of Cr(VI) to Cr(III) by the PPy moiety. Results showed that NCs are effective adsorbents for the removal of Cr(VI) from wastewater and can be separated by external magnetic field from the reactor.     

Removal of As(V) and Cr(VI) in aqueous solution by sand media simultaneously coated with Fe and Mn oxides

In this research, sand media simultaneously coated with iron and manganese (iron and manganese coated sand, IMCS) were applied to treat synthetic wastewater contaminated with both Cr(VI) and As(V). Scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD) were used to characterize the surface properties of the coated layer of IMCS. Adsorption of Cr(VI) and As(V) onto IMCS followed a typical anionic type, showing a gradual decrease of adsorption as the solution pH increased. In a single system, IMCS showed a greater adsorption capacity for As(V) than that for Cr(VI) over the entire solution pH range. In a binary system, As(V) preferentially occupied the limited adsorption sites on IMCS and therefore Cr(VI) adsorption was suppressed. This result indicates that As(V) adsorption onto the surface of IMCS occurs through a strong chemical bonding such as an inner-sphere complex. As(V) adsorption onto the IMCS was well described by second-order kinetics. From the adsorption isotherm experiments at pH 4, the maximum adsorbed amount of Cr(VI) and As(V) onto IMCS in a single system was 102 mg/kg and 455 mg/kg, respectively.     

Enhanced adsorptive removal of fluoride using mesoporous alumina

Two different kinds of mesoporous alumina samples were prepared using aluminum tri-sec-butoxide in the presence of either cetyltrimethylammonium bromide (MA-1) or stearic acid (MA-2) as a structure-directing agent, and tested for adsorptive removal of fluoride in water. Both materials contain a wormhole-like mesopore structure, but exhibited different textural properties: surface area (421 or 650 m²/g) and pore volume (0.96 or 0.59 cm³/g). These mesoporous aluminas demonstrated significantly improved adsorption capacity and faster kinetics to those of the commercial activated aluminas in fluoride removal by adsorption process. MA-2 prepared using stearic acid, in particular, demonstrated an adsorption capacity (14.26 mg/g) and initial adsorption rate (14.6 mg/g min) that were respectively 2.2 and 45 times higher than those of a commercial gamma alumina. The textural features of larger surface area and relatively smaller pore size in MA-2 compared to the activated aluminas are believed to be responsible for this enhancement in adsorption process.     

Chromium (VI) removal by methylated biomass of Spirulina platensis: The effect of methylation process

Biosorption of heavy metals is an interesting approach to treat industrial wastewaters by an environmentally friendly system. Spirulina platensis biomass, an effective biosorbent for cations, cannot be used to adsorb chromate due to its negatively charged surface close to neutral conditions; therefore, methylation of biomass was performed to increase its adsorption capacity under these conditions. Batch adsorption tests carried out varying both Cr(VI) and methylated biomass concentrations showed that 2–4 g l^?1 of biosorbent were able to remove Cr(VI) with efficiency ≥80%, while higher Cr(VI) levels (43–50 mg l^?1) showed low removal efficiency. The model of Langmuir was shown to describe the adsorption phenomenon better than the Freundlich one. The values of the overall adsorption capacity of methylated biomass suggested that increased biosorbent availability does not necessarily correspond to larger amount of adsorbed metal. FT-IR spectra of dried and methylated biomass of S. platensis allowed us monitoring the efficiency of the methylation process through the analysis of CH and COO^? vibrational stretching modes, taken as diagnostic of this process.     

Perchlorate uptake by synthetic layered double hydroxides and organo-clay minerals

The perchlorate uptake of hydrotalcite- and hydrocalumite-type layered double hydroxides (LDHs), and surfactant modified clay minerals were compared. Perchlorate uptake by both hydrotalcite- and hydrocalumite-type LDHs was in the range of 0.011 to 0.197 meq/g from a 2 mM perchlorate solution. The nitrate form of Mg:Al LDH had the highest uptake of 0.197 ± 0.033 meq/g, and the carbonate form of LDH the lowest uptake of 0.011 ± 0.003 meq/g. Octadecyltrimethylammonium (ODTMA), Dodecyltrimethylammonium (DoDTMA), and hexadecyltrimethylammonium (HDTMA) clay minerals removed perchlorate in the range of 0.025 to 0.348 meq/g from a 2 mM perchlorate solution. Synthetic HDTMA (5.0 CEC) Na-1-mica exhibited the highest adsorption of perchlorate with 0.348 ± 0.011 meq/g, while DoDTMA montmorillonite had the lowest adsorption with 0.025 ± 0.009 meq/g. Tested under the same conditions, a surfactant modified carbon showed an uptake of 0.303 ± 0.005 meq/g and this carbon is currently being used for filtration of drinking water.Hydrophobic organo-clay minerals with larger interlayer distance resulted in higher perchlorate uptake by exchange/adsorption on the residual charge of the cationic surfactants. The uptake by LDH was by anion exchange on the surfaces and, for the nitrate form of hydrotalcite, also from the interlayer spaces. The mechanisms of uptake were confirmed by X-ray diffraction (XRD) and scanning electron microscopy. This study indicated that cationic surfactant containing organo-clay minerals may be useful materials for the removal of perchlorate from water and could serve as alternatives to surfactant modified-activated carbons.     

Photocatalytic reduction of Cr(VI) to Cr(III) in solution containing ZnO or ZSM-5 zeolite using oxalate as model organic compound in environment

Photocatalytic reduction of Cr(VI) to Cr(III) in aqueous solution containing ZnO or ZSM-5 zeolite under ambient condition was studied by using oxalate as model organic compound in the natural environment. ZSM-5 zeolite was characterized by X-ray diffraction (XRD), and point of zero net proton charge (PZNPC) titration. The effect of illumination time, mass content of catalyst (m/V), Cr(VI) initial concentrations, pH, ionic strength, and oxalate concentrations on the photocatalytic reduction of Cr(VI) was determined. The results indicate that the PZNPC of ZSM-5 zeolite is at pH 3.6 ± 0.1. At C[Cr(VI)_(initial)] = 2.00 × 10^?4 mol/L, pH 7.5 ± 0.1 and after illumination time of 24 h, the reduction of Cr(VI) were 1.1 × 10^?5 mol/L (no ZSM-5 zeolite, 4.0 × 10^?3 mol/L oxalate) and 1.0 × 10^?5 mol/L (0.4 g/L ZSM-5 zeolite, no oxalate), respectively; whereas the reduction of Cr(VI) achieved 1.0 × 10^?4 mol/L in the presence of 0.4 g/L ZSM-5 zeolite and 4.0 × 10^?3 mol/L oxalate. The removal of Cr(VI) from solution is dependent on pH value. The results are important for the application of zeolites in the treatment of Cr(VI) polluted solution in the natural environment.     

Cr(VI) removal from synthetic and real wastewaters: The use of the invasive biomass Sargassum muticum in batch and column experiments

The macroalgae Sargassum muticum was selected for the treatment of solutions containing Cr(VI). Very acidic pH values were established as optimal for Cr(VI) reduction. Algae chemical modification reduced equilibrium time to 4 h. First order kinetic model was used to describe the reduction kinetic of Cr(VI). A column experiment allowed to distinguish the processes occurring during Cr(VI) elimination: its reduction to Cr(III) and the subsequent adsorption of this species formed. Under the selected conditions the biomass was capable of reducing all the incoming Cr(VI) during 77 h. Industrial wastewaters from chrome plating industry were also tested for chromium removal.     

Fabrication of copper coated polymer foam and their application for hexavalent chromium removal

The polymer foam coated with zero-valent copper (Cu⁰) was designed and prepared for the removal of hexavalent chromium (Cr(VI)) in water. Firstly, porous poly(tert-butyl acrylate) was fabricated by concentrated emulsion polymerization and then acrylic acid groups were generated on the surface of foam by hydrolysis reaction. Secondly, with the help of the large amount reactive carboxylic acid groups, polyethyleneimine (PEI) were chemically grafted onto the surface by the reaction between amine group and acrylic acid group. Finally, zero-valent copper was reduced by sodium borohydride (NaBH₄) and coated on the surface of polymer foam. Thus the copper functionalized porous adsorbent (Cu⁰–PEI–PAA) was constructed, and then applied for removing Cr(VI) from aqueous solution. The removal mechanism of Cr(VI) involved redox reaction by zero-valent copper and adsorption by amine groups, simultaneously. As a result, 99.5% of Cr(VI) could be removed within 2 h, and the maximum removal capacity for Cr(VI) of Cu⁰–PEI(1800)–PAA was 9.16 mg/g. Furthermore, the effect of initial concentration of Cr(VI), pH value, and temperature on the Cr(VI) removal was investigated. Therefore, the as-prepared zero-valent copper-loaded polymer foam could be an efficient and promising remediation material to remove Cr(VI) from wastewater.     

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 298 K 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, pH_pzc, 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 pH_pzc 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. 60 mg/g of functionalized acid-activated sepiolite and ca. 37 mg/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.     

Vapor phase adsorption of trichloroethane using organically modified montmorillonite

The vapor of 1,1,1-trichloroethane (TCA) was adsorbed to organically modified montmorillonite (organoclay). Hexadecyltrimethylammonium (HDTMA) was used to modify the surface of the clay and three types of organoclays with different HDTMA loadings were prepared. In adsorption experiments, the three types of organoclays, along with the non-modified (washed) clay were used. TCA was adsorbed from gaseous phase (nitrogen) by using a fixed adsorption bed. The adsorption breakthrough curves and the adsorption isotherms were determined at three different temperatures (24, 34, and 44 °C). The adsorption data were modeled with the Langmuir and BET isotherm equations. It was found that the isotherms of non-modified clay exhibited a favorable Type I behavior, which implies that the adsorption capacity is strongly dependent on vapor concentration at low concentration ranges. In regards to the organoclays, isotherms showed a marginally favorable Type II behavior with a reduced adsorption capacity at low concentrations, and exhibited a linear increase at elevated vapor concentrations. The temperature effect on the adsorption capacities of non-modified clay and organoclays exhibited different patterns. Desorption of TCA from clays was also performed by using pure nitrogen. The desorption rate constant k was in the order of 10^?4 min^?1 for all types of clay.     

Pre-treatment of adsorbents for waste water treatment using adsorption coupled-with electrochemical regeneration

With the aim to address waste water treatment problems, a novel and economic water treatment technology was introduced at the University of Manchester. It comprised of a unique combination of adsorption and electrochemical regeneration in a single unit. This process successfully eliminated a number of organic pollutants by using an electrically conducting adsorbent material called Nyex? which was a modified form of synthetic graphite. To expand the scope of other graphite types in waste water treatment applications, natural vein and recycled vein graphite materials were selected for electrochemical surface treatment (pre-treatment) in order to evaluate their adsorptive and electrical properties. New graphite based adsorbents were developed and characterized using a laser diffraction particle size analyser, BET surface area, SEM analysis, X-ray (EDS) elemental analysis, X-ray powder diffraction, Boehm surface titration, Zeta potential electrical bed conductivity and bulk density measurements. Boehm surface titration and EDS (X-ray) elemental analysis showed a significant increase in oxygen containing surface functional groups. Although, no significant improvement in bed electrical conductivity was found to occur after electrochemical surface treatment, however, natural vein and recycled vein graphite materials presented highest bed electrical conductivity amongst competing graphite materials. Aqueous solution of acid violet 17 as a standard pollutant was used to evaluate the comparative performance of these adsorbents. The investigations revealed that electrochemical surface treatment contributed to an increase in the adsorption capacity by a factor of two only for natural vein graphite. Un-treated recycled vein graphite adsorbent delivered the same adsorptive capacity (3.0 mg g^?1) to that of electrochemically treated natural vein graphite. The electrochemical regeneration efficiency at around 100% was obtained using a treatment time of 60 and 30 min, current density of 14 mA cm^?2, charge passed of 36 and 18 C g^?1 for synthetic graphite, natural and recycled vein graphite materials respectively. Relatively a small consumption of electrical energy, 24 J g^?1 for regenerating natural vein graphite adsorbent versus 36 J g^?1 for synthetic graphite adsorbent, was found to be required for destruction/oxidation of adsorbed acid violet 17. Multiple adsorption/regeneration cycles presented no loss in adsorptive capacity over 5 adsorption/regeneration cycles. The use of natural and recycled vein graphite adsorbents offered some advantages over graphite intercalation based adsorbents with reduced electrical energy consumption during regeneration and simpler separation of particulate adsorbent.     

Adsorptive removal of sulfur dioxide by Saskatchewan lignite and its derivatives

A non-equilibrium method using fixed bed microreactor was used to measure SO₂ adsorption characteristics of chars and activated carbons derived from Saskatchewan lignite. SO₂ breakthrough times and profiles were measured using lignite at a variety of temperatures, particle sizes and SO₂ concentrations of 75–175 °C; 2–5.6 mm and 1000–5000 ppm, respectively. Adsorption was found to be a strong function of residence time and feed SO₂ concentration, a moderate function of particle size and a weak function of temperature. There was a marginal difference in the adsorption capacity between lignite (15 mg SO₂/g lignite) and the char obtained from the same starting amount of lignite (26 mg SO₂/g char, or 17 mg SO₂/g original lignite). Activation of lignite with steam resulted in an activated carbon, which had highest adsorption capacity of 93 mg SO₂/g activated carbon.     

High-pressure adsorption of methane on montmorillonite,kaolinite and illite

Methane (CH₄) adsorption of Ca^2 +-montmorillonite (Mt), kaolinite (Kaol) and illite (Il) at 60 °C and pressures up to 18.0 MPa was investigated, during which the adsorption capacity was evaluated by the Langmuir adsorption model. The influences of adsorbed water and the interlayer distance of the clay minerals on CH₄ adsorption were explored by using heated Mt products with different interlayer distances as the adsorbent. Mt, Kaol and Il showed high CH₄ adsorption capacities, and their maximum Langmuir adsorption capacities were Mt, 6.01 cm³/g; Kaol, 3.88 cm³/g; and Il, 2.22 cm³/g, respectively. CH₄ was adsorbed only on the external surface of Kaol and Il; however, adsorption also occurred in the interlayer space of Mt, which had a larger interlayer distance than the size of a CH₄ molecule (0.38 nm). CH₄ adsorption in the interlayer space of Mt was supported by the lower CH₄ adsorption capacity of heated Mt products (with the interlayer distance < 0.38 nm) than that of Mt at high pressures despite the higher external surface areas of the heated Mt samples. The entrance of CH₄ into the interlayer space of Mt occurred at low pressures, and more CH₄ molecules entered the interlayer space at high pressures. Moreover, the adsorbed water occupied the adsorption sites of the clay minerals and decreased the CH₄ adsorption capacity. These results indicate that clay minerals play a significant role in CH₄ adsorption of shale and indicate that the structure and surface properties of clay minerals are the important parameters for estimating the gas storage capacity of shale.     

Adsorption and release of gemcitabine hydrochloride and oxaliplatin by hydroxyapatite

The aim of this study was to investigate the adsorption and the release profile of two anticancer drugs, gemcitabine hydrochloride (dFdU.HCl) and oxaliplatin (DACH-Pt), from hydroxyapatite (HAP) in order to evaluate HAP as local drug delivery system (DDS). Various initial concentrations of drug aqueous solutions were used in order to determine the maximum adsorption capacity of HAP after 48 h shaking. The maximum adsorption capacity of dFdU.HCl (400 mg/g HAP) was achieved after 40 h while the maximum adsorption capacity of DACH-Pt (49.1 mg/g HAP) was accomplished after 20 h. Adsorption processes for both drugs were found to fit the Freundlich equation. The release processes were studied by soaking the samples of loaded HAP in simulation body fluids (SBF). After only 1 h 65% of dFdU.HCl was released while the release of DACH-Pt from the HAP was more gradual since 55% of DACH-Pt was released in the first 24 h. Finally, in an attempt to understand the molecular basis of the drug action, the chemical interactions involved in the complex processes of drug delivery were studied theoretically.