Praveen K. Tandon

Hexacyanoferrate(III) oxidation of arsenic and its subsequent removal from the spent reaction mixture

Inorganic arsenic is the most toxic form and has been classified in group 1 as carcinogenic to humans which induces lung, urinary bladder and primary skin cancer. Worldwide concern over its presence in water bodies have prompted much research and policy development focusing on the removal of this chronic human carcinogen. It has been observed that the ash of Unio (Lamellidens marginalis--the fresh water mussel) can be used successfully for the removal of arsenic(V) from the aqueous solutions at low pH (∼9.0). Initially the kinetics of oxidation of arsenic(III) by alkaline hexacyanoferrate(III), both with and without adding iridium(III) chloride was studied. Subsequently after complete removal of ferrocyanide, the removal of arsenic(V) produced in the spent reaction mixture was taken up. Out of the five ashes obtained from different sources, the ash of Unio was found to be the best which results in decreasing arsenic(V) concentration from 1000 ppb to >10 ppb, TDS from 16.9 ppt to 8.5 ppt and conductivity from 33.8 mS to 17.1 mS. Kinetic results show the possibility of graphical separation of the reaction proceeding in the absence of iridium(III) from that proceeding in the presence of iridium(III) chloride.

Redox processes in water remediation

Groundwater is the main source of drinking water and water for agricultural and industrial usage. Therefore, groundwater contamination is prevented and contaminated groundwater is remediated to protect public health and the environment. Methods to remediate groundwater contamination have been recently developed. The use of redox processes in water remediation technologies has not been properly reviewed. Numerous water remediation technologies, such as ultrasonication, bioremediation, electrokinetics and nanotechnology, are closely related to redox processes. Redox processes control the chemical speciation, bioavailability, toxicity, mobility and adsorption of water pollutants in environment. Here, we review (1) general introduction of redox processes, (2) applicability of redox processes in water remediation, and (3) catalytic enhancement of redox potentials to explore its wide applicability in environmental remediation.

Liquid-Phase and Solventless Oxidation of Cyclohexane, Benzene, and Other Hydrocarbons by Cerium(IV) Catalyzed by Iridium(III) in Acidic Medium

Abstract Oxidation of some hydrocarbons dissolved in acetic acid by cerium(IV) sulphate at 100 °C in the presence of traces of iridium(III) chloride (catalyst-substrate 1:56818 to 151515) in the solution phase resulted in good to excellent yields of corresponding carbonyl compounds. In the cases of cyclohexane and benzene, 44% and 51.8% yields of corresponding carbonyl compounds were obtained, whereas in other cases, yield ranged from 34.9 to 99.8%. Yield decreased when reactions were performed in a microwave oven by adsorbing reactants (except acetic acid) on alumina. Decrease in the yield was probably due to the high temperature generated during the course of the reaction, resulting in the loss of organics from evaporation. Conditions were optimized for the highest yields under ambient conditions.

Liquid Phase and Microwave Assisted Oxidation of Some Hydrocarbons, Aromatic Aldehydes, and Phenols by Cerium(IV) Catalyzed by Iridium(III) in Acidic Medium

Abstract Addition of traces of iridium(III) chloride with cerium(IV) sulphate (catalyst–substrate ratio 1:75757 to 1:151515) in traditional water-bath heating resulted in the oxidation of propyl benzene, naphthalene, dimethoxy benzaldehyde, trimethoxy benzaldehydes, cresol, and quinol dissolved in acetic acid to give 70, 33, 96, 74, 49, and 66% yields respectively of the products, while phenol and resorcinol polymerized. Reactants adsorbed on alumina under solventless conditions in a microwave oven resulted in the decrease in yield. Oxidation of both hydroxyl groups takes place in quinol, whereas it was selective at the α-carbon in the side chain and at the methyl group in the case of propyl benzene and cresol, respectively. Conditions were tested for the highest yields under the experimental conditions.

Oxidation of Vicinal Diols by Cerium(IV) in Aqueous Acidic Media Catalyzed by Rhodium(III)

Potential of rhodium(III) chloride in catalyzing, in aqueous sulfuric acid medium, the oxidation of vicinal diols, and the kinetics of reaction of cerium(IV) with propane-1,2-diol and butane-2,3-diol catalyzed by rhodium(III) chloride was investigated. Data show that the reactions follow first order kinetics with respect to low cerium(IV) for lower concentrations, but a further increase in the oxidant concentration retards the reaction velocity. The reaction rate shows direct proportionality with respect to low concentrations of diol, which tends to become zero order at higher concentrations of the organic substrate. The Rate is first order in catalyst. Increase in the concentrations of hydrogen and cerium(III) ions show retarding effects while increase in chloride ion concentration and in turn ionic strength of the medium has a positive effect on the rate. Stoichiometry and spectral studies confirmed the formation of one molecule each of formaldehyde and acetaldehyde and two molecules of acetaldehyde as the products of oxidation in case of propane-1,2- diol and butane-2,3-diol respectively. Thermodynamic parameters like enthalpy of activation, free energy of activation and entropy values were calculated and it was found that the formation of activated complex in the case of butane-2,3-diol was easy compared to that in the case of propane-1,2-diol.

Co-catalysis by transition metal ions in the hydrogen ion catalysed oxidation of iodide ions. A kinetic study

The reaction between KI and [Fe(CN)6]3− ion, catalysed by hydrogen ions, was found to be catalysed further by PdCl2. Separate reactions under similar conditions, studied in the absence as well as in the presence of PdCl2 catalyst, were found to follow first order kinetics w.r. to [Fe(CN)6]3− and [H+], while the order was two w.r. to [I−]. [Fe(CN)6]4− ions were found to have a negative effect while changes in ionic strength of the medium do not effect the reaction velocity. Reaction in the presence of PdCl2 showed direct proportionality w.r. to [PdCl2]. The rate and extent of the reaction, which takes place even at zero [PdCl2] in the co-catalysed reaction, was calculated and was found to be in accordance with the rate values of the separately studied reaction at similar concentrations without adding PdCl2.

Liquid-Phase Oxidation of Some Aromatic Aldehydes, Hydrocarbons, and Alcohols by Cerium(IV) Catalyzed by Iridium(III) in Acidic Medium

Abstract Addition of traces of iridium(III) chloride with cerium(IV) sulfate (catalyst–substrate ratio (1:2994 to 1:10,000) in traditional water-bath heating resulted in the oxidation of p-chlorobenzaldehyde, p-nitrobenzaldehyde, benzyl alcohol, p-methoxy benzyl alcohol, p-xylene, and p-nitrotoluene dissolved in acetic acid to give 77%, 90%, 21.7%, 88.6%, 86.2%, and 18% yields of the products, respectively, while catechol and resorcinol polymerized. Oxidation of aldehydes and alcohols resulted as usual in the corresponding acids and aldehydes, respectively, while p-xylene and p-nitrotoluene gave p-tolualdehyde and p-nitrobenzoic acid. Conditions were obtained for getting the highest yields under the experimental conditions.

Redox Processes in Water Remediation Technologies

For life sustainability water is an essential resource present on the earth. In the current era of economical growth, groundwater is getting polluted due to the various human activities, urbanization and industrialization in addition to geogenic contamination. Water contamination by toxic and hazardous heavy metals, inorganic and organic pollutants, is a widespread environmental problem and removal of these pollutants from contaminated water is a major scientific concern of research and development. Water remediation technology is the process that is used to remove various pollutants from contaminated water. Various remediation techniques such as ultrasonication, bioremediation and nano are used for water remediation. Redox processes are chemical reactions that proceed by transfer of electrons resulting in a change in oxidation state of elements that are either oxidized to a higher oxidation state or reduced to a lower oxidation state. The use of redox processes in water remediation technologies has not been properly reviewed till now even after its versatile applications in water remediation technologies. The chemical speciation, bioavailability, toxicity, mobility and suitability for biodegradation as well as adsorption for water pollutants in environment are directly affected by redox processes that occur in water.

A Novel Aspect of Hexacyanoferrate (III)- Iodide Ion Reaction in Acidic Medium

Iridium(III) chloride further catalyses the oxidation of iodide ions by K3Fe(CN)6, catalyzed by hydrogen ions obtained from perchloric acid. Rate when the reaction is catalyzed only by the hydrogen ions was separated from the rate when iridium(III) and H+ions both catalyze the reaction. Separate reactions, studied in the presence as well as in the absence of IrCl3 under similar conditions were found to follow second order kinetics in [I ̄] with direct proportionality in [Fe(CN)6] ̄ and [IrCl3]. Rate was found to follow first order kinetics with respect to [H+] at low concentrations, tending to become second order at higher concentrations of [H+]. Externally added [Fe(CN)6] ̄ ions in the beginning strongly retard the rate effect, but further addition affects the rate to a little extent. Change in ionic strength has no effect on the rate. Arrhenius parameters were calculated and probable mechanisms were proposed.