Academic literature on the topic 'Metal hexavalent compose'

Objectives:The main goal of this study was to compare the removal efficiency of trivalent iron and hexavalent chromium by peanut shell biochars (i.e., PB), post-treated peanut shell biochars using KMnO4 (i.e., PB-Ox), and secondary post-treated peanut shell biochars using KOH (i.e., PB-Ox-A).Methods:The adsorption mechanisms of trivalent iron and hexavalent chromium by PB, PB-Ox, and PB-Ox-A were investigated using two types of adsorption kinetic and isotherm models. Furthermore, the adsorption experiments were performed under different adsorbent dosages (0.8 - 2.4 g/L), temperatures (15 - 35℃) and ion strengths (0.05 - 0.2 M NaNO₃) to identify their effects on the adsorption of trivalent iron and hexavalent chromium by PB, PB-Ox, and PB-Ox-A.Results and Discussion:Trivalent iron and hexavalent chromium could be more effectively removed by PB-Ox-A than PB and PB-Ox because of its higher contents of oxygen containing functional groups (O/C of PB = 0.064; O/C of PB-Ox = 0.058; O/C of PB-Ox-A = 0.188), higher surface area (PB = 351.5 m²/g; PB-Ox = 344.0 m²/g; PB-Ox-A = 2121.5 m²/g), and greater pore volume (PB = 0.15 cm³/g; PB-Ox = 0.15 cm³/g; PB-Ox-A = 0.96 cm³/g). The removal efficiencies of trivalent iron and hexavalent chromium by PB, PB-Ox and PB-Ox-A were increased with increasing the adsorbent dosages (PB-Ox-A > PB-Ox > PB). The adsorption kinetic experiments demonstrated that the pseudo second order rate model was suitable for the removal of trivalent iron and hexavalent chromium by PB (R² of Fe³⁺ = 0.99; R² of Cr⁶⁺ = 0.99), PB-Ox (R² of Fe³⁺ = 0.98; R² of Cr⁶⁺ = 0.98), PB-Ox-A (R² of Fe³⁺+ = 0.99; R² of Cr⁶⁺+ = 0.99). Furthermore, the removal of trivalent iron and hexavalent chromium using PB, PB-Ox and PB-Ox-A was well fitted to the Freundlich isotherm absorption model (R² of Fe³⁺ = 0.997 - 0.999; R² of Cr⁶⁺ = 0.995 - 0.998). The changes of temperature did not show significant effects on the removal of trivalent iron and hexavalent chromium by PB, PB-Ox, and PB-Ox-A. The removal efficiency of trivalent iron by PB, PB-Ox and PB-Ox-A was not influenced by the ionic strength whereas the removal efficiency of hexavalent chromium by PB, PB-Ox and PB-Ox-A was considerably decreased with increasing the ionic strength. These observations are evident that PB-Ox-A is the most effective adsorbent for the removal of trivalent iron and hexavalent chromium.Conclusions:The proposed post-treatment procedures might improve the surface properties of peanut shell biochars intimately associated with the removal of trivalent iron and hexavalent chromium. The physicochemical properties of the heavy metals and the biochars were found to be key factors governing the adsorption mechanisms of trivalent iron and hexavalent chromium by PB, PB-Ox and PB-Ox-A.

In this study, an idea of using multiple layers of adsorbents in adsorption column is proposed for treatment of synthetic wastewater for removal of Cr(VI) ions. In the present study, an effort has been made to study and compare the performance of fixed bed column with single bed adsorbent (RH only) and multi bed adsorbent of Rice Husk (RH), Saw Dust (SD) and Coir Dust (CD) in removal of Cr(VI) from synthetic wastewater and arrive at the parameters of the adsorption column that are useful for process design. From the characterization studies, it has been observed that carbon, aluminium and silica were the major components of the three natural adsorbents studied. It was found that, for better heavy metal removal, natural adsorbents with a percentage of fineness of around 54% could be used in adsorption studies. From the breakthrough curves of single bed and multi bed adsorption columns, it was evident that the multi bed adsorption column performed better. The time taken to achieve the breakthrough point and the exhaustion point in multi bed column is 2.5 times and 1.9 times greater than the time taken by single bed adsorption studies respectively. It was observed that in multi bed adsorption column, at greater bed depths, the significant increase in metal uptake capacity was due to the increase in contact time. Kinetic models viz. Thomas model, Yoon-Nelson model and Bed Depth Service Time model (BDST) were used to predict the performance of the column and sum of squares (SS) error analysis was carried out to test the accuracy of model equations. Higher R2 value and smaller SS value obtained from Thomas model proves that it is suitable to explain the adsorption of Cr(VI) in single and multi bed adsorption column of natural adsorbents. Experimental data fitted well with BDST model where R2 = 0.997. From the cost analysis, multi bed adsorption column was proven to be economical and confirmed that the locally and abundantly available agricultural wastes viz. RH, SD and CD could be used as an alternate to commercially available activated carbon for the removal of Cr(VI) from synthetic wastewater.

AbstractWe investigate the adsorption of hexavalent uranium, U(VI), on phosphorylated cellulose nanofibers (PHO-CNF) and compare the results with those for native and TEMPO-oxidized nanocelluloses. Batch adsorption experiments in aqueous media show that PHO-CNF is highly efficient in removing U(VI) in the pH range between 3 and 6. Gelling of nanofiber hydrogels is observed at U(VI) concentration of 500 mg/L. Structural changes in the nanofiber network (scanning and transmission electron microscopies) and the surface chemical composition (X-ray photoelectron spectroscopy) gave insights on the mechanism of adsorption. The results from batch adsorption experiments are fitted to Langmuir, Freundlich, and Sips isotherm models, which indicate a maximum adsorption capacity of 1550 mg/g, the highest value reported so far for any bioadsorbent. Compared to other metals (Zn, Mn, and Cu) and typical ions present in natural aqueous matrices the phosphorylated nanofibers are shown to be remarkably selective to U(VI). The results suggest a solution for the capture of uranium, which is of interest given its health and toxic impacts when present in aqueous matrices.

Various pollutants (e.g. boron and hexavalent chromium) are introduced into the aquatic environment from a variety of industrial operations causing damages to environment and affecting human health. Boron in irrigation water is of particular interest because it can have beneficial or toxic effect on plants, depending on its concentration. Pollution of the environment with hexavalent chromium (Cr(VI)) and associated toxicity to microorganisms, plants, animals and humans is of major concern. Indeed, chromium in environmentally significant concentrations is found near to tanneries and involves large volumes of wastewater. One of the most effective remediation technologies used for the removal of B(III) and Cr(VI) from aquatic systems and wastewater is their sorption on metal oxide surfaces. However, in order to understand better the mechanisms involved and improve the efficiency of remediation technologies further fundamental studies are needed. The present study is focused on the adsorption of H3BO3 and CrO42- onto Fe(O)OH at various ionic strengths (I = 0.0, 0.1 and 1.0 M NaClO4), under normal atmospheric conditions, at 22 ± 3 oC and pH 8. Additionally, competitions studies were carried out to investigate the ion-exchange mechanism and compare the individual affinities of H3BO3 and CrO42- for Fe(O)OH. The concentration of H3BO3 and CrO42- in solution was determined spectrophotometrically by means of azomethine-H and DPC (1,5-diphenylcarbazide), respectively. The results obtained indicate that the release of Cr(VI) and therefore its concentration in solution increases as the amount of B(III) is increased in the sorption system. This phenomenon is due to the replacement of Cr(VI) ions by ions B(III) on the surface of Fe(O)OH. Evaluation of the experimental data results in a value for the competition constant which is equal logK= -3.5 ± 0.2, indicating that the adsorbent surface has greater affinity for Cr(VI) than for B(III) species. The formation constant for the Cr(VI)-Fe(O)OH surface complex is calculated to be logßCr= 7.9 ± 0.2.

Mixed plumes of chlorinated organics and oxidized metals are a common contaminant at many sites. The oxidized metals can be mediated by the establishment of moderately reducing conditions. The chlorinated organics have been demonstrated to be degradable by specific dechlorinating microrganisms in anaerobic environment such as Dehalococcoides sp. Enhanced biological dechlorination requires the presence of an effective electron donor to provide molecular hydrogen (H2) to completely degrade chlorinated ethenes. Distribution of the electron donor results in the biologically mediated establishment of highly reducing conditions in the treatment zone. This process also results in the reduction and precipitation of the oxidized metals via sulphate-reducing conditions. Peroxychem LLC has developed an innovative electron donor, ELS® Microemulsion Reagent (ELS) for in situ treatment of chlorinated organics and metals. This substrate has been successfully applied at numerous sites to address a variety of contaminants. ELS® is an organic electron donor composed of an easily fermentable organic substrate based on lecithin, and designed to enhance in situ anaerobic bioremediation aquifers contaminated by organochlorine compounds and heavy metals such as hexavalent chromium Cr[VI]. The product is easy to mix, dilute and inject into the subsurface. Once in the groundwater, indigenous microorganisms utilize ELS to rapidly generate highly reducing conditions, favoring biotic dechlorination reactions and the reduction of oxidized metals such as Cr[VI]. This innovative technology was successfully applied to a former manufacturing site in the center of Italy, where groundwater was historically contaminated with Tetrachloroethylene (PCE > 5.5 milligrams per Liter; mg/L), Trichloroethylene (TCE > 2 mg/L), 1,2-Dichloroethene (1,2-DCE > 1 mg/L) and, to a lesser extent, Vinyl Chloride (VC) and 1,2-Dichloropropane (DP). A pump-and-treat system (P&T) installed in the source was active as a source containment measure and to speed up the overall groundwater remediation. However, there was concern that the pumping could affect the ELS treatment effectiveness because of the increased groundwater flow velocity and the potential for removal of the injected bioremediation substrate. To mitigate this potential some wells were switched off the flow rates of others was adjusted to ensure compatibility with the planned product injection. In particular, an upstream low-flow-rate pump and treat system was maintained over the ELS® treatment period, primarily to delay the fast-downstream diffusion of the amendments in the aquifer, thus enhancing the source treatment. Following the calibration of the P&T system, approximately 4,900 kg of ELS® concentration was injected under high pressure at 51 locations into the source area. In about 12 months from injection of ELS® Microemulsion into the groundwater in the main source area, concentrations of PCE, TCE and the recognized catabolites, such as DCE and VC, rapidly reduced, compared to the pre-treatment concentrations, until they reached the statutory national limits (CSC D.lgs 152/06) in the main monitoring piezometers of the area, also highlighting the establishment of clear and enhanced biotic reducing conditions. No rebound effects have been observed in the next three years of monitoring.

Etude des proprietes extractives de r::(2)nc(o)(ch::(2))::(3) c(o)nr::(2). Ces composes extraient u(vi) et pu(iv), les actinides et lanthanides trivalents restant en phase aqueuse. Mise en evidence des 3 complexes 2l. Hno::(3), l. Hno::(3) et l. 2hno::(3) (l-glutaramide). A faible acidite, on a pu observer les complexes l. Uo::(2)(no::(3))::(2) et l. Pu(no::(3))::(4) et a forte acidite l. Uo::(2)(no::(3))::(3)h et l. Pu(no::(3))::(6)h::(2)

Preparation des couches electrochromes de wo::(3) par oxydation anodique sous courant pulse. Caracteristiques electrooptiques. Proprietes thermodynamiques des bronzes h::(x)wo::(3) et li::(x)wo::(3) responsables du phenomene coloration-decoloration