Tuning the surfaces of palladium nanoparticles for the catalytic conversion of Cr(VI) to Cr(III)

This work reports the feasibility of using Pd nanoparticles as innovative catalysts in the conversion of reducible contaminants from toxic to benign forms. Cr(VI) is a known carcinogen while the trivalent chromium salts are believed to be non-toxic. The ability of Pd nanoparticles to catalyze the rapid reduction of Cr(VI) to Cr(III) using reactive sulfur intermediates produced in situ was therefore studied. Using a microchamber set at 130 degrees C, the reduction mixture consists of palladium nanoparticles and sulfur (PdNPs/S), which generated highly reducing sulfur intermediates that effected the reduction of Cr(VI) to Cr(III) by 1st order reaction kinetics. UV-visible spectroscopy and cyclic voltammetry were employed to monitor the reduction process. The results showed that 99.8% of 400 microM Cr(VI) was reduced to Cr(III) by PdNPs/S in one hour compared to 2.1% by a control experiment consisting of sulfur only. The rate of Cr(VI) reduction was found to be dependent on temperature and pH and was greatly enhanced by the addition of PdNPs. Subsequent application of this approach in the reduction of Cr(VI) in soil and aqueous media was conducted. In contrast to the control experiments with and without PdNPs or sulfur, greater than 92% conversion rate was obtained in the presence of PdNPs/S within 1 hour. This represents over a 500-fold improvement in conversion rate compared to current microbial approaches. XPS analysis provided the confirmation regarding the oxidation states of Cr(VI), Cr(III) and the nature of the reactive intermediates. This work offers PdNPs/S as a new interface for the reduction of high oxidation state heavy metal pollutants.     

Geochemical assessment of metal transport in glacial till during electrokinetic remediation

This paper presents the chemical speciation and retention behavior of chromium (Cr), nickel (Ni), and cadmium (Cd) prior to and after the electrokinetic remediation in glacial till soil. The speciation of the metals was predicted using the chemical speciation program MINEQL⁺. The simulations were performed for single-contaminant with only Cr(VI) or Ni, and multi-contaminants consisting of: (1) Cr(VI), Ni, and Cd; (2) Cr(III), Ni and Cd; (3) Cr(VI), Cr(III), Ni and Cd; (4) Cr(VI), Ni, and Cd with reducing agents; and (5) Cr(III), Ni, and Cd with oxidizing agent (Mn). The results showed that the speciation and distribution of cationic metals [Ni, Cd, and Cr(III)] in glacial till soil remain unaffected or slightly affected during electrokinetics. This is attributed to the high pH buffering capacity of the glacial till, leading the metals to precipitate in the soil prior to and after electrokinetics. This study showed that during electrokinetics, Cr(VI) existed as anionic complex and migrated towards the anode and the migration is maximum in case of a single-contaminant system. The study also showed that near the anode in the absence of any reducing and oxidizing agent, Cr(VI) mostly adsorbed, and some of Cr(VI) reduced to Cr(III) and migrated towards the cathode and finally precipitated due to high pH conditions. Ni and Cd remain adsorbed or precipitated due to the high pH conditions throughout the soil. Among the reducing agents, the sulfide had significant effect on the migration of metals compared to ferrous ions. While in the presence of oxidizing agent (Mn), no noticeable Cr(VI) was found in the soil sample indicating the reduction of Cr(VI) to Cr(III) and the predominance of reducing conditions due to the presence of naturally occurring iron in the glacial till soil. Overall, this study provides a reasonable explanation of the speciation and distribution of chromium, nickel and cadmium during the electrokinetic remediation of glacial till soil.     

Sampling and analysis considerations for the determination of hexavalent chromium in workplace air

Airborne hexavalent chromium (Cr[VI]) is a known human respiratory carcinogen and allergen. Workers in a variety of industries may be exposed to airborne hexavalent chromium, with exposures frequently occurring via inhalation and/or dermal contact. Analytical methods for the measurement of Cr(VI) compounds in workplace samples, rather than for the determination of total elemental chromium in workplace air, are often desired because exposure limit values for Cr(VI) compounds are much lower than for total Cr. For years, sampling and analytical test methods for airborne Cr(VI) have been investigated so as to provide means for occupational exposure assessment to this highly toxic species. Inter-conversion of trivalent chromium (Cr[III]) and Cr(VI) can sometimes occur during sampling and sample preparation, and efforts to minimize unwanted redox reactions involving these chromium valences have been sought. Because of differences in toxicity, there is also interest in the ability to differentiate between water-soluble and insoluble forms of Cr(VI), and procedures that provide solubility information concerning Cr(VI) compounds have been developed. This paper reviews the state of the art concerning the measurement of airborne Cr(VI) compounds in workplace aerosols and related samples.     

Chromium speciation in groundwater of a tannery polluted area of Chennai City, India

Chromium speciation in groundwater of a tannery polluted area was investigated for the distribution of chromium species and the influence of redox couples such as Fe(III)/Fe(II) and Mn(IV)/Mn(II). Speciation analysis was carried out by ammonium pyrolidinedithiocarbamate (APDC)–methylisobutylketone (MIBK) procedure. The groundwater samples were analyzed for Cr(III), Cr(VI), and Cr(III)-organic complexes. The APDC could not extract the Cr(III)-organic complexes, but HNO₃ digestion of the groundwater samples released the Cr(III)-organic complexes. The groundwater of the area is relatively oxidizing with redox potential (E _h) and dissolved oxygen (DO) ranged between 65 and 299 mV and 0.25 and 4.65 mg L^???1, respectively. The Fe(II) reduction of Cr(VI) was observed in some wells, but several wells that had Fe(II)/Cr(VI) concentrations more than the stoichiometric ratio (3:1) of the reduction reaction also had appreciable concentration of Cr(VI). This could partly be due to the oxidation of Fe(II) to Fe(III) by DO. It appears that the occurrence of Mn more than the Fe(II) concentration was also responsible for the presence of Cr(VI). Other reasons could be the Fe(II) complexation by organic ligands and the loss of reducing capacity of Fe(II) due to aquifer materials, but could not be established in this study.     

Modeling nonlinearly transient zero-order bioreduction and transport of Cr(VI) in groundwater

Cr(VI) contamination of soil and groundwater is considered a major environmental concern. Bioreduction of Cr(VI) to Cr(III) can be considered a potentially effective technology in remediating Cr(VI) contaminated sites. Shewanella oneidensis MR-1 (MR-1) is one of the bacteria capable of reducing Cr(VI) to Cr(III) under anaerobic conditions. The kinetics of Cr(VI) reduction by MR-1 is defined by the dual-enzyme kinetic model which is nonlinear, transient, and zero-order. Existing transport models are not designed to simulate such reaction kinetics. The objective of this paper is to present a Petrov–Galerkin finite element model (PGFEM) to simulate transport and bioreduction of Cr(VI), by MR-1, in groundwater. The model developed is unconditionally stable and provides oscillation free accurate results for a wide range of Peclet number (Pn) and Courant number (Cn).     

Comparison of three sampling and analytical methods for the determination of airborne hexavalent chromium

A field study was conducted with the goal of comparing the performance of three recently developed or modified sampling and analytical methods for the determination of airborne hexavalent chromium (Cr(VI)). The study was carried out in a hard chrome electroplating facility and in a jet engine manufacturing facility where airborne Cr(VI) was expected to be present. The analytical methods evaluated included two laboratory-based procedures (OSHA Method ID-215 and NIOSH Method 7605) and a field-portable method (NIOSH Method 7703). These three methods employ an identical sampling methodology: collection of Cr(VI)-containing aerosol on a polyvinyl chloride (PVC) filter housed in a sampling cassette, which is connected to a personal sampling pump calibrated at an appropriate flow rate. The basis of the analytical methods for all three methods involves extraction of the PVC filter in alkaline buffer solution, chemical isolation of the Cr(VI) ion, complexation of the Cr(VI) ion with 1,5-diphenylcarbazide, and spectrometric measurement of the violet chromium diphenylcarbazone complex at 540 nm. However, there are notable specific differences within the sample preparation procedures used in three methods. To assess the comparability of the three measurement protocols, a total of 20 side-by-side air samples were collected, equally divided between a chromic acid electroplating operation and a spray paint operation where water soluble forms of Cr(VI) were used. A range of Cr(VI) concentrations from 0.6 to 960 microg m(-3), with Cr(VI) mass loadings ranging from 0.4 to 32 microg, was measured at the two operations. The equivalence of the means of the log-transformed Cr(VI) concentrations obtained from the different analytical methods was compared. Based on analysis of variance (ANOVA) results, no statistically significant differences were observed between mean values measured using each of the three methods. Small but statistically significant differences were observed between results obtained from performance evaluation samples for the NIOSH field method and the OSHA laboratory method.     

Preconcentration and speciation of Cr(III) and Cr(VI) in water and soil samples by spectrometric detection via use of nanosized alumina-coated magnetite solid phase

A novel nanomaterial has been developed for speciation of Cr(III) and Cr(VI) in water and soil samples. In this study, a new type of alumina-coated magnetite nanoparticles (Fe₃O₄/Al₂O₃ NPs) modified by the surfactant Triton X-114 has been successfully synthesized and used in magnetic mixed hemimicelles solid-phase extraction procedure. The procedure was based on the reaction of chromium(III) with 1-(2-pyridilazo)-2-naphtol as a ligand, yielding a complex, which was entrapped “in situ” in the surfactant hemimicelles. The concentration of chromium(III) was determined using flame atomic absorption spectrometry. After reduction of Cr(VI) to Cr(III) by ascorbic acid, the system was applied to the total chromium. Cr(VI) was then calculated as the difference between the total Cr and the Cr(III) content. This method can also be used for complicated matrices such as soil samples without any special pretreatment. Under the optimum conditions of parameters, the recoveries of Cr(III) by analyzing the spiked water and soil samples were between 98.6 and 100.8 % and between 96.5 and 100.7 %, respectively. Detection limits of Cr(III) were between 1.4 and 3.6 ng?mL^?1 for water samples and 5.6 ng?mg^?1 for soil samples.     

Chromium speciation in a contaminated groundwater: redox processes and temporal variability

Chromium species (Cr(III), Cr(VI), and Cr(III)-organic) in groundwater of a tannery contaminated area were monitored during pre- and post-monsoon seasons for a period of 3 years (May 2004 to January 2007). The objectives of the study were (1) to investigate the temporal variation of chromium species and other matrix constituents and (2) to study the redox processes associated with the temporal variation of chromium species. Samples were collected from 15 dug wells and analyzed for chromium species and other constituents. The results showed that the groundwater was relatively more oxidizing during post-monsoon periods than the pre-monsoon periods. Except one sample, the concentration of chromium species were found in the order of Cr(VI)>Cr(III)>Cr(III)-organic complexes during all the pre- and post-monsoon periods. In most of the wells, the concentrations of Cr(III), Cr(VI), and Cr(III)-organic decreased during post-monsoon periods compared to their pre-monsoon concentrations. However, the Cr(VI)/Cr_Total ratio still increased and the Cr(III)/Cr_Total ratio decreased during post-monsoon periods in most of the samples. The possible mechanisms for the temporal variation of chromium species were (1) Fe(II) reduction of Cr(VI) vs oxidation of Fe(II) by dissolved oxygen and (2) oxidation of Cr(III) by Mn(IV).     

Removal of hexavalent chromium from contaminated ground water using zero-valent iron nanoparticles

Batch experiments were conducted on ground water samples collected from a site contaminated with Cr(VI) to evaluate the redox potential of zero-valent iron (Fe⁰) nanoparticles for remediation of Cr(VI)-contaminated ground water. For this, various samples of contaminated ground water were allowed to react with various loadings of Fe⁰ nanoparticles for a reaction period of 60 min. Data showed 100% reduction of Cr(VI) in all the contaminated ground water samples after treatment with 0.20 gL⁻¹ of Fe⁰ nanoparticles. An increase in the reduction of Cr(VI) from 45% to 100% was noticed with the increase in the loading of Fe⁰ nanoparticles from 0.05 to 0.20 gL⁻¹. Reaction kinetics of Cr(VI) reduction showed pseudo first-order kinetics with rate constant in the range of 1.1 × 10⁻³ to 3.9 × 10⁻³ min⁻¹. This work demonstrates the potential utility of Fe⁰ nanoparticles in treatment and remediation of Cr(VI)-contaminated water source.     

Bio-concentration of chromium—an <Emphasis Type="Italic">in situ</Emphasis> phytoremediation study at South Kaliapani chromite mining area of Orissa,India

Mine waste water at South Kaliapani usually contains toxic levels of hexavalent Cr(VI). The present in situ study was conducted at South Kaliapani chromite mine area in Orissa state, India, to assess the phytoremediation ability of three plants, namely, rice (Oryza sativa L.), paragrass (Brachiaria mutica), and an aquatic weed (Eichhornia crassipes), in attenuating Cr(VI) from mine waste water and to correlate the bio-concentration factors (BCF) of Cr. Water hyacinth (E. crassipes) showed 24% to 54% reduction whereas paragrass (B. mutica) was able to reduce 18% to 33% of Cr(VI) from mine water. This reduction was studied over a period of 100 days of plant growth. The reduction was observed through a passage of a sum total of 2,000 sq. ft. cultivated plots and ponds separately. Reduction in Cr(VI) content in mine water varies with plant age as well as with the distance of passage. Cr accumulation and BCF values increased with high soil Cr levels as well as the age of plants. High BCF and transportation index (Ti) values, i.e., 10,924 and 32.09, respectively, were noted for water hyacinth. The Ti values indicated that the root-to-shoot translocation of Cr was very high after 100 days of growth. The total accumulation rate was maximum (8.29 mg Cr kg dry biomass^− 1 day ^− 1) in paragrass. The BCF values for roots were noted to be higher than those of leaves, stems, and grains of the 125-day-old plants. Hence, paragrass and water hyacinth may be used as tools of phytoremediation to combat the problem of in situ Cr contamination.