Mechanisms of the removal of hexavalent chromium by biomaterials or biomaterial-based activated carbons

Three papers published during recent 2 years in Journal of Hazardous Materials made a mistake in analyzing chromium species in aqueous solution, resulting in incorrect elucidation of Cr(VI) biosorption; the Cr(VI) was removed from aqueous solution systems by 'anionic adsorption'. However, it has been proved that Cr(VI) is easily reduced to Cr(III) by contact with organic materials under acidic conditions because of its high redox potential value (above +1.3 V at standard condition). Therefore, it is strongly possible that the mechanism of Cr(VI) removal by biomaterials or biomaterial-based activated carbons is not "anionic adsorption" but "adsorption-coupled reduction". Thus, for researches of Cr(VI) biosorption, researchers have to analyze not only Cr(VI) but also total Cr in aqueous solution and to check the oxidation state of chromium bound on the biomaterials or activated carbons.     

Application of dolochar in the removal of cadmium and hexavalent chromium ions from aqueous solutions

Dolochar, a waste material generated in sponge iron industry, is processed and put to test as an adsorbent for removal of Cd(II) and Cr(VI) ions from aqueous solutions. The dolochar samples were characterised to determine the different phases and their distribution by reflection microscopy. The analysis indicated that the sample consists of metallic iron, fused carbon, and Ca-Mg bearing phases (Ca-Mg-silicate-oxide) along with lots of voids and pores. The fixed carbon (FC) content of the material is 13.8% with a Langmuir surface area of 81.6 m²/g and micropore area of 34.1 m²/g. Batch adsorption experiments have been conducted to study the sorption behaviour of Cd(II) and Cr(VI) ions on dolochar as a function of particle size, contact time, adsorbent dosages, pH and temperature. It is observed that higher pH and temperature enhances sorption of Cd(II) ions. In contrast, the adsorption for Cr(VI) is found to be better in acidic pH in comparison to alkaline media. The equilibrium adsorption isotherm data are tested by applying both Langmuir and Freundlich isotherm models. It is observed that Langmuir isotherm model fitted better compared to the Freundlich model indicating monolayer adsorption. The thermodynamic parameters such as ΔG°, ΔH° and ΔS° indicate the effectiveness of dolochar to remove Cd(II) and Cr(VI) ions from aqueous solution. The kinetics of adsorption is found to better fit to pseudo second order reaction.     

Biosorption of hexavalent chromium by raw and acid-treated green alga Oedogonium hatei from aqueous solutions

The hexavalent chromium, Cr(VI), biosorption by raw and acid-treated Oedogonium hatei were studied from aqueous solutions. Batch experiments were conducted to determine the biosorption properties of the biomass. The optimum conditions of biosorption were found to be: a biomass dose of 0.8 g/L, contact time of 110 min, pH and temperature 2.0 and 318 K respectively. Both Langmuir and Freundlich isotherm equations could fit the equilibrium data. Under the optimal conditions, the biosorption capacities of the raw and acid-treated algae were 31 and 35.2 mg Cr(VI) per g of dry adsorbent, respectively. Thermodynamic parameters showed that the adsorption of Cr(VI) onto algal biomass was feasible, spontaneous and endothermic under studied conditions. The pseudo-first-order kinetic model adequately describe the kinetic data in comparison to second-order model and the process involving rate-controlling step is much complex involving both boundary layer and intra-particle diffusion processes. The physical and chemical properties of the biosorbent were determined and the nature of biomass-metal ions interactions were evaluated by FTIR analysis, which showed the participation of -COOH, -OH and -NH(2) groups in the biosorption process. Biosorbents could be regenerated using 0.1 M NaOH solution, with up to 75% recovery. Thus, the biomass used in this work proved to be effective materials for the treatment of chromium bearing aqueous solutions.     

Electrothermal atomic absorption spectrometric determination of total and hexavalent chromium in atmospheric aerosols

A method was developed which allow separate determination of Cr(VI) and total Cr from the same minute sample of atmospheric aerosols. Cr(VI) was leached was with 0.1M Na(2)CO(3) and the total Cr concentrations were determined after acid digestion. The method was validated by the analysis of certified reference materials, CRM 545, Mess-3 and Pacs-2 with good agreement between certified and found values. Cr concentrations in air samples taken around the chromium smelter show concentrations that exceed the maximum allowed levels in 8h with higher values closer to the smelter. The limit of detection (LOD) of the method for Cr(VI) determination in air samples was found to be 0.2 ng m(-3), i.e. lower than offered by the commonly preferred spectrophotometric and colorimetric techniques.     

Palladium nanoparticles supported on amine-functionalized glass fiber mat for fixed-bed reactors on the effective removal of hexavalent chromium by catalytic reduction

Palladium nanoparticles were deposited on the amine-grafted glass fiber mat (GFM-NH₂) catalyst support by a conventional impregnation process followed by the borohydride reduction in aqueous solution at room temperature to create the designed Pd/GFM-NH₂ catalyst. By the use of large size glass fiber mat without nano/mesopores as the catalyst support, the internal mass transfer limitations due to the existence of nano/mesopores on the catalyst support were eliminated and the Pd/GFM-NH₂ catalyst could be easily separated from treated water due to the large size of the catalyst support. Batch experiments demonstrate its good catalytic reduction performance of Cr(VI) with formic acid as the reducing agent. It also demonstrated an efficient Cr(VI) removal and stability in a lab-prepared, packed fixed-bed tube reactor for the continuous treatment of Cr(VI)-containing water. Thus, it has a good potential for the catalytic reduction of Cr(VI) in the water treatment practice.     

Removal of chromium (VI) ions from aqueous solution by adsorption onto two marine isolates of Yarrowia lipolytica

The removal of chromium (VI) ions from aqueous solutions by the biomass of two marine strains of Yarrowia lipolytica (NCIM 3589 and 3590) was studied with respect to pH, temperature, biomass, sea salt concentration, agitation speed, contact time and initial concentration of chromium (VI) ions. Maximum biosorption was observed at pH 1.0 and at a temperature of 35 °C. Increase in biomass and sea salts resulted in a decreased metal uptake. With an agitation speed of 130 rpm, equilibrium was attained within 2 h. Under optimum conditions, biosorption was enhanced with increasing concentrations of Cr (VI) ions. NCIM 3589 and 3590 displayed a specific uptake of Cr (VI) ions of 63.73 ± 1.3 mg g⁻¹ at a concentration of 950 ppm and 46.09 ± 0.23 mg g⁻¹ at 955 ppm, respectively. Scatchard plot analysis revealed a straight line allowing the data to be fitted in the Langmuir model. The adsorption data obtained also fitted well to the Freundlich isotherm. The surface sequestration of Cr (VI) by Y. lipolytica was investigated with a scanning electron microscope equipped with an energy dispersive spectrometer (SEM-EDS) as well as with ED-X-ray fluorescence (ED-XRF). Fourier transform infrared (FTIR) spectroscopy revealed the involvement of carboxyl, hydroxyl and amide groups on the cell surfaces in chromium binding.     

Biosorption of chromium species by aquatic weeds: kinetics and mechanism studies

In this paper, we have presented the results of Cr(VI) and Cr(III) removal from aqueous phase by different aquatic weeds as biosorbents. Batch kinetic and equilibrium experiments were conducted to determine the adsorption kinetic rate constants and maximum adsorption capacities of selected biosorbents. In most of the cases, adsorption followed a second-order kinetics. For Cr(III), maximum adsorption capacity was exhibited by reed mat (7.18mg/g). In case of Cr(VI), mangrove leaves showed maximum removal/reduction capacity (8.87mg/g) followed by water lily (8.44mg/g). There was a significant difference in the concentrations of Cr(VI) and total chromium removed by the biosorbents. In case of Cr(VI) removal, first it was reduced to Cr(III) with the help of tannin, phenolic compounds and other functional groups on the biosorbent and subsequently adsorbed. Acid treatment significantly increased Cr(VI) removal capacity of the biosorbents whereas, alkali treatment reduced the Cr(VI) removal capacities of the biosorbents. FTIR spectrum showed the changes in functional groups during acid treatment and biosorption of Cr(VI) and Cr(III). Aquatic weeds seem to be a promising biosorbent for the removal of chromium ions from water environment.     

Activated carbons and low cost adsorbents for remediation of tri- and hexavalent chromium from water

Hexavalent chromium is a well-known highly toxic metal, considered a priority pollutant. Industrial sources of Cr(VI) include leather tanning, cooling tower blowdown, plating, electroplating, anodizing baths, rinse waters, etc. The most common method applied for chromate control is reduction of Cr(VI) to its trivalent form in acid (pH approximately 2.0) and subsequent hydroxide precipitation of Cr(III) by increasing the pH to approximately 9.0-10.0 using lime. Existing overviews of chromium removal only cover selected technologies that have traditionally been used in chromium removal. Far less attention has been paid to adsorption. Herein, we provide the first review article that provides readers an overview of the sorption capacities of commercial developed carbons and other low cost sorbents for chromium remediation. After an overview of chromium contamination is provided, more than 300 papers on chromium remediation using adsorption are discussed to provide recent information about the most widely used adsorbents applied for chromium remediation. Efforts to establish the adsorption mechanisms of Cr(III) and Cr(VI) on various adsorbents are reviewed. Chromium's impact environmental quality, sources of chromium pollution and toxicological/health effects is also briefly introduced. Interpretations of the surface interactions are offered. Particular attention is paid to comparing the sorption efficiency and capacities of commercially available activated carbons to other low cost alternatives, including an extensive table.     

Effective removal of zinc ions from aqueous solutions using crab carapace biosorbent

The carapace of the crab (Cancer pagurus), a waste material disposed of by the seafood industry, has recently been shown to have potential as a biosorbent for the removal of metals from aqueous media. Crab carapace in the particle size ranges 0.25-0.8mm and 0.8-1.5mm were used to investigate the effects of agitation speed, contact time, metal concentration and initial pH on the removal of Zn(2+). In sequential-batch process Zn(2+) uptakes of 105.6 and 67.6 mg/g were recorded for 0.25-0.8 mm and 0.8-1.5 mm particles, respectively, while values of 141.3 and 76.9 mg/g were recorded in fixed-bed column studies. Binary-metal studies showed that the presence of Cu(2+) or Pb(2+) significantly suppressed Zn(2+) uptake. This study confirms that crab carapace may be considered a viable and cost-effective alternative to commercial activated carbon or ion-exchange resins for the removal of metals from aqueous media.     

Evaluation of zeolite A for the sorptive removal of Cs+ and Sr2+ ions from aqueous solutions using batch and fixed bed column operations

Zeolite A was chemically synthesized and evaluated, as inorganic ion exchange material, for the removal of cesium and strontium ions from aqueous solutions in both batch and fixed bed column operations. Batch experiments were carried out as a function of pH, initial ion concentration and temperature. Simple kinetic and thermodynamic models have been applied to the rate and isotherm sorption data and the relevant kinetic and thermodynamic parameters were determined from the graphical presentation of these models. Breakthrough data were determined in a fixed bed column at room temperature (298 K) under the effect of various process parameters like bed depth, flow rate and initial ion concentration. The results showed that the total metal ion uptake and the overall bed capacity decreased with increasing flow rate and increased with increasing initial ion concentrations and bed depth. The dynamics of the ion exchange process was modeled by bed depth service time (BDST) model. The sorption rate constants (K) were found to increase with increase in flow rate indicating that the overall system kinetics was dominated by external mass transfer in the initial part of the sorption process in the column.     

Biosorption of total chromium from aqueous solution by red algae (Ceramium virgatum): equilibrium, kinetic and thermodynamic studies

This study focused on the biosorption of total chromium onto red algae (Ceramium virgatum) biomass from aqueous solution. Experimental parameters affecting biosorption process such as pH, contact time, biomass dosage and temperature were studied. Langmuir, Freundlich and Dubinin–Radushkevich (D–R) models were applied to describe the biosorption isotherms. Langmuir model fitted the equilibrium data better than the Freundlich isotherm. The biosorption capacity of C. virgatum biomass for total chromium was found to be 26.5 mg/g at pH 1.5 and 10 g/L biomass dosage, 90 min equilibrium time and 20 °C. From the D–R isotherm model, the mean free energy was calculated as 9.7 kJ/mol, indicating that the biosorption of total chromium was taken place by chemisorption. The calculated thermodynamic parameters (ΔG°, ΔH°and ΔS°) showed that the biosorption of total chromium onto C. virgatum biomass was feasible, spontaneous and exothermic at 20–50 °C. Kinetic evaluation of experimental data showed that the biosorption processes of total chromium followed well pseudo-second-order kinetics.     

Cr(VI) concentration from batch contact/tank leaching and column percolation test using fly ash with additives

Batch contact, tank leaching and column percolation tests were conducted to investigate the Cr(VI) concentration in the solution/leachate from two fly ashes (fly ash A and B) with additives. The additives used were cement, low alkalinity additive and Ariake clay. There are several factors influencing Cr(VI) concentration in solution/leachate, namely (1) properties of solid/liquid mixture (chemical composition, pH value, etc.), (2) cementation effect, (3) amount of water in contact with the solid mass (solid/liquid ratio in case of batch contact test), and (4) adsorption characteristics of the solid particles to Cr ions. The test results indicate that fly ash A has less cementation component (CaO of 1.92%) and the amount of water in contact with the fly ash played an important role. As a result, Cr(VI) concentration from the column percolation test was much higher than that of the batch contact test. Adding Ariake clay had more effect on reducing Cr(VI) concentration for fly ash A than B because the pH value of the solution from fly ash A was lower, which provided a favorable condition for Cr(VI) ions to be reduced to Cr(III) and possibly to be adsorbed by clay particles. Fly ash B has more cementation component (7.15%) and for column percolation test, curing the sample for 1 week reduced Cr(VI) concentration significantly. The test results indicate that in engineering practice, a method which closely simulates the field condition should be selected to assess possible environmental effects and corresponding countermeasure methods.     

Modeling and analysis of a packed-bed column for the effective removal of zinc from aqueous solution using dual surface-modified biomass

In this research, ultrasonic-assisted fishtail palm Caryota urens seeds (UACUS) have been prepared for the effective removal of Zn(II) ions from aqueous solution in packed-bed column studies. The effect of various operating parameters such as bed depth, initial Zn(II) ions concentration and flow rate on breakthrough curves has been investigated. The dynamic response for the adsorption of Zn(II) ions onto UACUS was described in terms of the breakthrough curves. The results demonstrated that the maximum removal of Zn(II) ions was attained at a low flow rate, influent concentration and high bed depth. The results showed that breakthrough time and exhaustion time were increased with increase of bed height and decreased with increase of flow rate and initial Zn(II) ion concentration. The experimental column data were fitted with different mathematical models, namely Adams–Bohart, Yoon–Nelson and bed depth service time (BDST) models. Amongst the models, the BDST model agreed well with the experimental data. The adsorbent was characterized by Fourier Transform Infrared Spectroscopy (FTIR), Environmental Scanning Electron Microscopy (ESEM), Energy Dispersive X-Ray Spectroscopy (EDX), and Thermogravimetric Analysis (TGA). The experimental results concluded that the prepared UACUS can be utilized as a potential and low-cost adsorbent for the removal of Zn(II) ions from the contaminated liquid.     

Ni(II) removal from aqueous solutions using cone biomass of Thuja orientalis

The biomass of terrestrial-plant materials has high removal capacities for a number of heavy metal ions. The Ni(II) biosorption capacity of the cone biomass of Thuja orientalis was studied in the batch mode. The biosorption equilibrium level was determined as a function of contact time, pH, temperature, agitation speed at several initial metal ion and adsorbent concentrations. The removal of Ni(II) from aqueous solutions increased with adsorbent concentration, temperature and agitation speed of the solution were increased. The biosorption process was very fast; 90% of biosorption occurred within 3 min and equilibrium was reached at around 7 min. It is found that the biosorption of Ni(II) on the cone biomass was correlated well (R2 > 0.99) with the Langmuir equation as compared to Freundlich, BET Temkin and D-R isotherm equation under the concentration range studied. According to Langmuir isotherm, the monolayer saturation capacity (Q(o)) is 12.42 mg g(-1). The pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models were applied to test the experimental data for initial Ni(II) and cone biomass concentrations. The pseudo-second-order kinetic model provided the best correlation of the used experimental data compared to the pseudo-first-order and intraparticle diffusion kinetic models. The activation energy of biosorption (E(a)) was determined as 36.85 kJ mol(-1) using the Arrhenius equation. This study indicated that the cone biomass of T. orientalis can be used as an effective and environmentally friendly adsorbent for the treatment of Ni(II) containing aqueous solutions.     

A comparison of low-cost biosorbents and commercial sorbents for the removal of copper from aqueous media

Three biosorbents, crab carapace, the macroalgae Fucus vesiculosus and peat were compared with two commercial materials, an activated-carbon and an ion-exchange resin for the removal of copper from aqueous media. Kinetic models of Lagergren first-order, pseudo-second order and intraparticular diffusion were used to model the data. The process for all materials is best represented by the pseudo-second order rate model. Langmuir and Freundlich isotherms were used to describe the sorption equilibrium data. Maximum uptake values were 79.4, 114.9 and 71.4 mg g(-1) for crab carapace, F. vesiculosus and ion-exchange resin, respectively. Langmuir and Freundlich isotherm models could not be fitted to the experimental data for peat and activated-carbon. Ion-exchange was calculated to contribute approximately 75%, 77% and 44% to the total biosorption by crab carapace, F. vesiculosus and peat, respectively. The removal efficiencies of crab carapace and F. vesiculosus were >95% and comparable with those achieved using ion-exchange resin. Results from this study suggest that both crab carapace and F. vesiculosus are efficient and effective biosorbent materials for the removal of copper from aqueous solutions and given that they are also low-cost, may be considered viable alternatives to activated-carbon and ion-exchange resin.     

Synthesis of interstratified graphene/montmorillonite composite material through organics-pillared,delamination and co-stacking and its application in hexavalent chromium removal from aqueous solution

Layers from two different delaminated dispersions of 3-aminopropyltriethoxysilane (APTES)-intercalated montmorillonite (Mts) and octylamine (OA)-intercalated graphene oxide (GO) could be co-stacked to obtain APTES-intercalated Mts (Mts-APTES)/OA-intercalated GO (GO-OA) interstratified composites (MAGO). The synthesized composites were characterized by XRD, FTIR, BET, TGA, TEM and XPS, which showed that MAGO had been prepared successfully. The optimal concentration of APTES was 8% in anhydrous toluene which avoided self-polymerization of APTES while facilitating the nucleophilic attack of APTES amine groups and the protic character of ethanol to compete with silane for the intimal hydroxyl groups by H-bonding. The MAGO demonstrated an extremely fast Cr(VI) removal from aqueous solution with a high removal efficiency at low pH. Data from batch studies of the adsorption process followed pseudo-second-order kinetics. The results fit a Langmuir model of adsorption, with maximum adsorption capacities of MAGO composites at pH 3.0 being 44.25 mg g^?1, 47.46 mg g^?1, 49.58 mg g^?1 under 30 °C, 40 °C, 50 °C, respectively, which were much higher than capacities of some conventional adsorbents. The reusability of the MAGO composite was also determined through adsorption-desorption studies, providing evidence for the potential use of MAGO composite in the removal of Cr(VI) from acidic wastewater.     

Modeling and evaluation of chromium remediation from water using low cost bio-char, a green adsorbent

Oak wood and oak bark chars were obtained from fast pyrolysis in an auger reactor at 400-450 °C. These chars were characterized and utilized for Cr(VI) remediation from water. Batch sorption studies were performed at different temperatures, pH values and solid to liquid ratios. Maximum chromium was removed at pH 2.0. A kinetic study yielded an optimum equilibrium time of 48 h with an adsorbent dose of 10 g/L. Sorption studies were conducted over a concentration range of 1-100mg/L. Cr(VI) removal increased with an increase in temperature (Q(Oak wood)(°): 25 °C = 3.03 mg/g; 35 °C = 4.08 mg/g; 45 °C = 4.93 mg/g and Q(Oakbark)(°): 25 °C = 4.62 mg/g; 35 °C = 7.43 mg/g; 45 °C = 7.51 mg/g). More chromium was removed with oak bark than oak wood. The char performances were evaluated using the Freundlich, Langmuir, Redlich-Peterson, Toth, Radke and Sips adsorption isotherm models. The Sips adsorption isotherm model best fits the experimental data [high regression (R(2)) coefficients]. The overall kinetic data was satisfactorily explained by a pseudo second order rate expression. Water penetrated into the char walls exposing Cr(VI) to additional adsorption sites that were not on the surfaces of dry char pores. It is remarkable that oak chars (S(BET): 1-3m(2)g(-1)) can remove similar amounts of Cr(VI) as activated carbon (S(BET): ～ 1000 m(2)g(-1)). Thus, byproduct chars from bio-oil production might be used as inexpensive adsorbents for water purification. Char samples were successfully used for chromium remediation from contaminated surface water with dissolved interfering ions.     

Electrochemical removal of chromium from aqueous solutions using electrodes of stainless steel nets coated with single wall carbon nanotubes

An electrochemical technique was adopted to investigate the removal of Cr(VI) species and total chromium (TCr) from aqueous solution at a laboratory scale. The electrodes of stainless steel nets (SSNE) coated with single wall carbon nanotubes (SWCNTs@SSNE) were used as both anode and cathode. Three parameters, including solution pH, voltage and electrolyte concentration, were studied to explore the optimal condition of chromium removal. The optimal parameters were found to be pH 4, voltage 2.5 V and electrolyte concentration 10 mg/L. Under these conditions, the Cr(VI) and TCr removal had a high correlation with the amount of SWCNTs coated on the electrodes, with coefficients of the regression equations 0.953 and 0.928, respectively. The mechanism of Cr(VI) removal was also investigated. X-ray photoelectron spectroscopy (XPS) study and scanning electron microscope (SEM) picture showed that the process of chromium removal involved the reduction of Cr(VI) to Cr(III) on the cathode, and then the adsorption of Cr(III) by SWCNTs on the cathode. The study results indicated that the proposed method provided an interesting means to remove chromium species from aqueous solution, especially Cr(VI) in acidic condition.     

Removal and recovery of heavy metals from aqueous solution using Ulmus carpinifolia and Fraxinus excelsior tree leaves

Ulmus carpinifolia and Fraxinus excelsior tree leaves, which are in great supply in Iran, were evaluated for removal of Pb(II), Cd(II) and Cu(II) from aqueous solution. Maximum biosorption capacities for U. carpinifolia and F. excelsior were measured as 201.1, 172.0 mg/g for Pb(II), 80.0, 67.2 mg/g for Cd(II) and 69.5, 33.1 mg/g for Cu(II), respectively. For both sorbents the most effective pH range was found to be 2-5 for Pb(II), 3-5 for Cd(II) and 4-5 for Cu(II). Metal ion biosorption increased as the ratio of metal solution to the biomass quantity decreased. Conversely, biosorption/g biosorbent decreased as the quantity of biomass increased. The biosorption of metal ions increased as the initial metal concentration increased. Biosorption capacities of metal ions were in the following order: Pb(II)>Cd(II)>Cu(II). The equilibrium data for Pb(II) and Cu(II) best fit the Langmuir adsorption isotherm model. Kinetic studies showed that the biosorption rates could be described by a second-order expression. Both the sorbents could be regenerated using 0.2 M HCl during repeated biosorption-desorption cycles with no loss in the efficiency of the Cu(II) removal observed. Biosorption of Pb(II), Cd(II) and Cu(II) was investigated in the presence of Na, K, Mg and Ca ions. The results from these studies show a novel way of using U. carpinifolia and F. excelsior tree leaves to remove Pb(II), Cd(II) and Cu(II) from metal-polluted waters.