The notion of "green chemistry " has encouraged even synthetic organic chemists to include water as a solvent. Incredible selectivities and activities can be achieved through the addition of amphiphiles with a defined structure. The morphology of supramolecular assemblies or associates formed by surfactants vary according to the temperature and concentration. As a rule, reactions are typically conducted using simple spherical aggregates, that is, micelles in the nanometer range. The strong polarity gradient present between the hydrophilic surface and the hydrophobic core of the micelle means that both nonpolar and polar reagents can be solubilized. This solubilization results in reactants becoming more concentrated within the micelle than in the surrounding water phase and leads to an acceleration of the reaction and causes selective effects. The kinetic treatment of reactions in micellar systems can be accomplished by considering them as microheterogeneous two-phase systems.

Reactions in Micellar Systems

The reduction of Cr(VI) to Cr(III) mediated by environmentally relevant carboxylic acids: State-of-the-art and perspectives.

Photocatalysis is an attractive treatment method for removing hexavalent chromium (Cr(VI)) from water. Thus far, photocatalytic reduction of Cr(VI) has been investigated mostly using TiO2 photocatalysts in acidic water solutions. Here we investigate Cr(VI) removal using zinc oxide (ZnO), tungsten trioxide (WO3), and sodium tantalate (NaTaO3), metal oxides that display good activity for other photocatalytic reactions such as water splitting, as well as titanium oxide (TiO2, Evonik P90). The efficiency for Cr(VI) removal using these photocatalysts was investigated in synthetic neutral and alkaline water, as well as in cooling tower blowdown water. The effect of several additives used in water treatment processes on the Cr(VI) removal rate was also studied. For NaTaO3, citric acid was found to have a detrimental effect to Cr(VI) removal, while sodium formate, ammonium chloride, and sodium sulfite were beneficial. While sulfite alone could chemically reduce Cr(VI), sulfite in combination with a photocatalyst resulted in faster and complete removal of Cr(VI) in 10 min using a SO32−/Cr(VI) ratio >35 in pH ∼ 8 solutions. NaTaO3 was found to display the highest Cr(VI) removal rates on a photon basis at pH 3 and in the presence of sodium sulfite, while ZnO and TiO2 showed the best performance in pH 7 and cooling tower blowdown water.

Hexavalent chromium removal using metal oxide photocatalysts

Fe(III) photocatalytic reduction of Cr(VI) by low-molecular-weight organic acids with α-OH

Reaktionen in micellaren Systemen

The kinetics and mechanism of citric acid oxidation by CrVI; catalyzed by MnII, has been studied in H2O and in the presence of anionic and cationic surfactants. A linear correlation between k
 obs
 −1 and [MnII]−1 was found, satisfying the Michaelis–Menten kinetics. The rate-determining step is the decomposition of complex HCrO4–citric acid–MnII formed between citric acid–MnII and CrVI. Based on kinetic data, a one-step three-electron oxidation mechanism has been proposed. The rate decreased with increase in concentration of the cationic surfactants cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB), while anionic sodium dodecyl sulphate (SDS) had no effect on the rate. The data have been interpreted in terms of reaction in the aqueous phase. The effect of added anions, such as chloride, bromide, nitrate, and sulphate, has been studied and discussed. The activation parameters (ΔH
 ‡ and ΔS
 ‡) were significantly affected by the presence of 10.0 × 10−4 mol dm−3 of CTAB or CPB.

Unusual rate inhibition of manganese(II) assisted oxidation of citric acid by chromium(VI) in the presence of ionic micelles

Effect of micelles on the oxidation of oxalic acid by chromium(VI) in the presence and absence of manganese(II)

Kinetics of the initial stages of oxidation of tartaric acid (TA) in the absence and presence of manganese(II) ions have been studied spectrophotometrically. The rate of the induction was slow and then gradually increased with increasing [TA] and/or [HClO4]. The reaction followed third-order kinetics; first-order with respect to each of [TA], [HClO4] and [CrVI]. The kinetic and manganese(II) effect studies are consistent with a one-step three-electron mechanism (CrVI → CrIII without passing through CrIV as an intermediate) in which a termolecular complex is formed between TA, MnII and HCrO4−. In order to obtain further insight, oxidation of glyoxylic acid (GA), an oxidation product of TA, was also studied under the similar conditions. Details of the process are discussed.

One-step three-electron oxidation of tartaric and glyoxylic acids by chromium(VI) in the absence and presence of manganese(II)

Chromium(VI) oxidation of glycolic acid in the absence and presence of cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) followed the same mechanism as shown by kinetic study. The reaction followed second-order kinetics, first-order in each reactant. The oxidation is strongly catalyzed by manganese(II) and cationic micelles of CTAB or CPB. The catalytic effect of micelles can be fitted to a model in which the reaction rate depends upon the concentration of both reactants in the micellar pseudophase. Some added inorganic salts (NaCl, NaBr, NaNO3, and Na2SO4) reduce the micellar catalysis by excluding glycolic acid from the reaction site. The reactivity of glycolic acid towards chromium(VI) has been discussed and also compared with those obtained previously for the reaction between chromium(VI) and the reductants oxalic and lactic acids. On the basis of the observed results, probable mechanisms have been proposed. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 377–386, 2001

Micellar catalysis on the redox reaction of glycolic acid with chromium(VI)

The reaction of mal ic acid wit h dic hromate studi ed in prese nce and absence of mangan esc( II ) follow s a seco nd -ordcr kin et ics with rc spect to rmali c ac idl wh ich shift s to first-o rd er in pre se nce of manga ne se( II ). Th e catalyti c effec t of manga­ nese( II ) suggests that chrom ium (IV) is not formed as an intermediate . In th e first-o rd er reaction (in pre';e nce of Ill anga­ ncse(I I)). Illali c aeid is ox idized to carbon diox ide and malo ni c acid. Th e effec t of ca ti oni c and anionic surfaeta nt s was also investi ga ted and it was foun d that ca ti oni c mice ll cs catalyse th e rea cti on whil e ani onic micell es have no effect. The inllu ­ encc of differe nt parameters sllc h as rreactantl , ISll rfactanll. tcmperature and addcd sa lt s was considered. For su rfa ctanl co n­ cC lllrat ion s we ll abol'e th e cirit icalmice lle concentralion , the ratt! COIlS l,lIlt -rsllrfacta ntl profil es ca n be in terpre ted in terms of distributi on of both th e reactant s between wa ter and th e micelles, lI sin g the bin d in g consta nt s 12 1.2 and 22.5 mor' dm~ for chrolllium (VI) 10 CPB and CTAI3, re spective ly. The catalytic crfec t of thi s reaction is grea tt! r for CPB than C rAB . Thi s is th e resul t of greater assoc iation of chrom ium(VI). not of a greate r rale constant in the mice ll e. Kin eti c and mechanistic st ud ies of ch romic ac id oxi­ dation of ex- hyd roxy ac ids abo und in the literature'. The reac ti ons fo ll ow either two- lectro n or three­ electron one-step ox id ati on of substrate by chro­ mium(VI ) depending on th e molec ul ar characteri st ic of th e form er. Orga ni c sub strates genera ll y undergo at most a two-e lectro n ox id ati on in a sin gle step". One­ step three-e lec tron reduc ti on of an oxidant in vo lves more th an one molec ul e o r an organ ic subst rate in th e ac ti va ted com pl ex. Rocek el ci/. J ha ve report ed that such three -electron ox id ati on ca n take pla ce in 1:2 chromium(V I)-sub strate complex . One substrate is an orga ni c ac id ca rr yin g a second fun cti onal group and the seco nd sub strate molecule is e ither identical with the fi rst or it may be a co mpound with a sin gle func­ tion al group suc h as an alcoho l 4

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Khaled Hartani

Papers

Unusual rate inhibition of manganese(II) assisted oxidation of citric acid by chromium(VI) in the presence of ionic micelles

Effect of micelles on the oxidation of oxalic acid by chromium(VI) in the presence and absence of manganese(II)

One-step three-electron oxidation of tartaric and glyoxylic acids by chromium(VI) in the absence and presence of manganese(II)

Micellar catalysis on the redox reaction of glycolic acid with chromium(VI)

Co-oxidation of malic acid and manganese(II) by chromium(VI) in the presence and absence of ionic surfactants