Transport of hexavalent chromium through anion-exchange membranes

This papers deals with the transport of hexavalent chromium through SB-6407, AFN and ACM anion-exchange membranes in contact with different salt solutions. The effect of salt solutions on the transport of chromium(VI) was investigated. The effect of salt solutions was observed at different sequences and the transport efficiencies of the membranes was found to be in the order SB-6407>AFN>ACM. The transport was found to be highest in a single state.     

Electrochemical synthesis and redox behaviour of polypyrrole coatings on copper in salicylate aqueous solution

In this work, polypyrrole (PPy) coatings have been successfully electrodeposited on copper substrates from aqueous salicylate solutions. Homogenous and uniform films can be grown under galvanostatic or potentiostatic control without noticeable substrate dissolution. The redox behaviour obtained for the films is similar to that displayed by PPy grown under the same conditions on noble metals. Strong adherence to copper is observed as far as PPy is in its oxidised state but it is lost when the system is subjected to potentials lower than −0.5 V versus saturated calomel electrode (SCE).     

污染土样中六价铬和总铬的浸出条件探究

采用单因素实验分别考察了固液比、时间、温度、pH等因素对含铬污染土样中六价铬及总铬浸出效果的影响。结果表明,随着固液比的增大,六价铬的浸出率也相应增加,但固液比不宜超过1∶20;延长时间有利于六价铬的浸出,但浸出时间不宜超过13 h;一定的温度范围内,六价铬的浸出率随温度的升高而增加,但温度不宜超过 60 ℃;在强酸性及强碱性条件下均有利于六价铬的浸出,与强碱性条件相比,强酸性条件更有利于六价铬的浸出,浸出率可以达到87%;总铬的浸出效果随这些因素的变化并不明显。     

Removal of hexavalent chromium from aqueous solutions by micellar compounds

The recovery of toxic metal compounds is a deep concern in all industries. Hexavalent chromium is particularly worrying because of its toxic influence on human health [1], [2] and [3]. Actually the wastewater norm (0.1 mg/l) is very strict and will become more severe in the near future. We present in this paper the experimental results of the metal ion which is bound on micellar compounds and then retained by ultrafiltration membrane. A well known surfactant cetyltrimethylammonium bromide (CTABr) is used as an adsorbent to remove hexavalent chromium from wastewaters. The effects of various experimental parameters on equilibrium adsorption of Cr(VI) on the surfactant have been investigated using batch adsorption experiments. It was found that the capacity of chromium adsorption on CTABr increases with initial metal concentration and in a lesser extent with pH solution. Total chromium adsorption decreased slightly with a rise in temperature suggesting an exothermic adsorption of chromium, thermodynamic parameters are evaluated. It has also been observed that the capacity of chromium adsorption decreases with the mass of adsorbent and concentration of other ions present in the solution. The metal ion adsorption on surfactant is well represented by the Freundlich isotherm.     

Reduction of hexavalent chromium by polypyrrole deposited on different carbon substrates

Pollution due to toxic hexavalent chromium has reached dangerous levels in some parts of the world, especially in Third World countries. This work focuses on the reduction of Cr(vi) ions in aqueous media by polypyrrole deposits on porous, high area three-dimensional carbon electrodes. Since many properties of deposited polymers are known to depend on the method of synthesis we performed the electropolymerization of pyrrole either at varying potential scan rates or at constant potential on different high area carbon materials. The deposits thus obtained were then put in contact with acidic hexavalent chromium solutions; high reduction yields were obtained (up to 80%). In addition, the possibility of repeating this cycle was studied as well as the wear resistance of these deposits. Finally, the morphology of the deposits was monitored at different stages of the process by scanning electron microscopy.     

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.     

Adsorption study of hexavalent chromium using tamarind hull-based adsorbents

The adsorption characteristics of hexavalent chromium was studied with an adsorbent developed from waste tamarind hull. Experiments were conducted in batch mode to observe the influence of different parameters such as initial concentration of metal ions, adsorbent dosage, adsorbent particle size, stirrer speed, temperature and pH of the solution. Acidic pH strongly favored the adsorption. With decreasing the pH of the solution from 5.0 to 1.0, the removal of chromium was enhanced from 33% to 99%. The adsorption process was found to follow a pseudo-first-order rate mechanism and the rate constant was evaluated at 30 °C. The Freundlich, Redlich–Peterson and the Fritz–Schlunder isotherm fit the equilibrium data satisfactorily. Adsorption of chromium was found to increase with increase in the process temperature. Using an adsorbent dosage of 1.0 g/L and an acidic pH (2.0), the equilibrium adsorption capacity of the prepared adsorbent was found to be about 70 mg/g at 30 °C, which increased to about 81 mg/g at 50 °C. The entropy change, free energy change and heats of adsorption were determined for the process.     

Removal of hexavalent chromium anions via polymer enhanced ultrafiltration using a fully ionized polyelectrolyte

A fully ionized polyelectrolyte was synthesized for Cr(VI) removal from aqueous solutions by continuous polymer enhanced ultrafiltration (PEUF). Effect of operating parameters such as pH, loading, polymer, and competing ion concentrations were examined. Highest Cr(VI) retention was obtained at a loading of 0.01 at pH 4. Increasing both polymer and chromate concentrations at a fixed loading of 0.01 decreased the retention which demonstrated the effect of crowding. In the presence of competing anions such as chloride and sulfate, Cr(VI) retention decreased for all of the pH values studied. Even at high competing anion concentrations, significant retentions of Cr (VI) were obtained.     

浅析三价铬彩钝膜中含六价铬的原因

从三价铬彩钝的配方、温度及镀锌工艺等方面,分析了三价铬彩色钝化膜中六价铬产生的原因。对如何避免三价铬彩色钝化膜中六价铬的产生,提出了一些建议。     

取代重污染六价铬电镀的技术及应用

介绍了六价铬电镀及取代六价铬电镀特别是三价铬电镀的研究现状和有关的技术及政策。分析了六价铬电镀的危害及三价铬电镀的优点,以及三价铬电镀取代六价铬电镀带来的效益三价铬电镀不仅可明显减少电镀对环境的污染,节省大量的污染治理费用,而且还可提高电镀产品的质量。     

Photo-reduction of hexavalent chromium in aqueous solution in the presence of zinc oxide as semiconductor catalyst

Common sources of chromium in wastewater are electroplating and leather industries. Hexavalent chromium is more toxic and carcinogenic compared to its trivalent counterpart. Conversion of Cr(VI) to Cr(III) in an aqueous medium by photocatalytic reduction using UV radiation and ZnO semiconductor catalyst has been investigated using potassium dichromate as the model compound. Effects of the process parameters such as ZnO loading (0–3 g/L) and intensity of UV radiation (0–125 W medium pressure Hg vapour lamp) on photocatalytic reduction were investigated. Initial concentration of substrate solution was varied from 40 to 120 mg/L. A comparison of the performance of the ZnO photocatalyst achieved in this study with that reported in the literature for photocatalytic and conventional processes is presented. The initial rate of the photochemical method was found to be independent of the concentration of potassium dichromate. Methanol was added as a hole scavenger for enhancement of the photo-reduction. Initial rate of photo-reduction was found to be independent of the concentration of methanol above a particular value. A probable mechanism and the corresponding kinetic model have been proposed for the photo-reduction and tested by experimental results. An alternative rate equation based on the LHHW model compares well with the mechanistic rate equation.     

Non-linear modeling of kinetic and equilibrium data for the adsorption of hexavalent chromium by carbon nanomaterials:Dimension and functionalization

The adsorption capacities for the removal of hexavalent chromium from aqueous solutions by six carbon nanomaterials have been evaluated. Single-walled and multi-walled carbon nanotubes as received and after oxidation treatment, graphene oxide and reduced graphene oxide are the materials with different dimension and functionalization compared in this research. Carbon nanotubes have been modified using hydrogen peroxide as oxidizing agent under microwave radiation. The oxidation treatment on carbon nanotubes has a positive effect increasing the adsorbent–adsorbate interaction. Rate-controlling mechanisms and equilibrium data are analyzed using non-linear models. Non-linear method is proposed as the most suitable method for determining the kinetic and equilibrium parameters. The values of adsorption energy(E) obtained from the Dubinin–Radushkevich isotherm,have been found around 0.371 and 0.870 k J·mol~(-1), indicating physical adsorption. Therefore, the pseudo-second order model represents better the kinetic experimental data. The results show that the Langmuir isotherm provides a slightly better fit to the experimental data compared with the Freundlich isotherm, indicating homogeneous distribution of active sites on carbon nanomaterials and monolayer adsorption. The separation factors RLare found in the range of 0–1, suggesting that the adsorption process is suitable for all adsorbents. The mechanisms for hexavalent chromium removal have been proposed as electrostatic interactions and hydrogen bonding.     

腐蚀保护层中六价铬含量分析

六价铬是一种致癌物质,它在汽车和电子工业中的应用受到欧盟的严格限制。目前,在腐蚀保护层中测定Cr(VI)的分析方法已经颁布实施。本文介绍了可用于上述两个行业的Cr(VI)检测标准──EN 15205和IEC 62321,并讨论了两者的差别。     

三价铬钝化件六价铬检出阳性问题及其评价

简述了三价铬钝化层中检出六价铬的原因,评述了几种RoHS认证通常采用的六价铬检测标准,包括点滴法和水煮法.讨论了RoHS认证中三价铬钝化层中六价铬检测的有关问题.提出了一些避免三价铬钝化层中检出六价铬的建议,如:采用沸水提取法而不是碱液提取法,以降低六价铬检出几率;控制检测条件;调整钝化液配方和钝化工艺条件.     

三价铬钝化膜为什么会产生六价铬

根据三价铬和六价铬的氧化还原性质,指出在碱性介质中三价铬可以被氧气氧化成六价铬,认为三价铬钝化膜中三价铬的存在形式(某种三价铬的碱式盐)是其在空气中放置时三价铬被氧化为六价铬的内在原因.提出了采用无铬钝化工艺,钝化后封闭及加强三价铬钝化工艺管理和钝化产品的存放管理等应对措施.     

代六价铬电镀现状及趋势

简述了六价铬电镀的危害。综述了目前研究和应用广泛的代铬电镀技术,如三价铬电镀技术,锡基、镍基合金电镀技术,以及复合电镀技术,阐述了它们的特点及应用。指出了目前代铬电镀技术存在的问题,提出了今后的发展趋势。     

Electrochemical synthesis of polypyrrole in ionic liquids

Electrochemical synthesis of inherently conducting polymers such as polypyrrole is traditionally performed in a molecular solvent/electrolyte system such acetonitrile/lithium perchlorate. We report the use of ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide and N,N-butylmethylpyrrolidinium bis(trifluoromethanesulfonyl) amide, both as the growth medium and as an electrolyte for the electrochemical cycling of polypyrrole films. Use of the ionic liquid as the growth medium results in significantly altered film morphologies and improved electrochemical activities.     

硫酸盐体系三价铬镀液中六价铬离子的积累及去除

绘制了分光光度法测定六价铬的标准曲线,研究了硫酸盐体系三价铬镀液中六价铬的积累规律,并测试了不同的去除方法.结果表明,添加有机醛类稳定剂或溴化铵可以降低六价铬的生成速率,使用小电流电解和添加过氧化氢能降低三价铬镀液中六价铬的质量浓度.     

Adsorption of hexavalent chromium on cationic starches with different degree of crosslinking

The influence of the degree of crosslinking in the association with the degree of cationization on the adsorption of Cr(VI) by crosslinked cationic starches (CCS) was examined. The amount of adsorbed Cr(VI) depends on the preparation of CCS before adsorption experiments (dry or preswollen CCS) and has a higher value when dry CCS are used. The amount of crosslinks in CCS needed to reach the maximum effectiveness of the cationic groups of CCS in the binding of Cr(VI) depends also on the form in which CCS are used. The adsorption capacity for Cr(VI) decreases both with dry and preswollen CCS when the adsorption temperature increases. The thermodynamic parameters are calculated. The negative values of ΔG° and ΔH° indicated that the adsorption process in both cases was spontaneous and exothermic. If preswollen CCS is used, the entropy change has a positive value and indicates that the randomness of the system increases, whereas the negative value of entropy change when Cr(VI) species are adsorbed by dry CCS shows that in this case the order of the system increases like in the classical adsorption process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Electrochemical synthesis and characterization of N-substituted polypyrrole derivatives on nickel

1-Dodecylpyrrole (PyCH₃) and 12-(pyrrol-1-yl)dodecane-1-thiol (PySH) films have been successfully electrochemically polymerised on a nickel electrode from acetonitrile solutions containing the monomer and the lithium perchlorate as supporting electrolyte. The electrochemical study of the polymer growth has been carried out by cyclic voltammetry (CV) detecting the nickel dissolution during electropolymerisation. Several surface spectroscopic and microscopic techniques have been used to characterize the surface in term of chemical composition and polymer topography. The presence of unbound and unoxidised thiol groups at the PPySH surface has been evidenced together with a very strong adhesion to the nickel substrate. Furthermore, N-substituted pyrrole derivatives exhibited some corrosion protection properties in neutral NaCl medium.