Kinetics of 2,6-dimethyl-aniline degradation and iron crystallization in fluidized-bed fenton process

Abstract

This research determined the rate constant between 2,6-dimethy-aniline (2,6-DMA) and hydroxyl radicals(OH) by using Fenton reaction and competitive kinetics technique. Regardless of experimental conditions either batch or continuous mode, presence or absence of media, complete suspension or fluidized-bed reactor, the intrinsic 2nd-order rate constant between 2}6-DMA and OH were found to be consistent in between 1.59x10[subscript 10] and 1.80x10[subscript 10] M[subscript -1]sec[subscript -1] with the average and 95% confidence interval of 1.70±0.04x10[subscript 10] M[subscript -1]sec[subscript -1]. Aromatic intermediates from 2,6-DMA oxidation by OH were 2}6dimethy1-nitrobenzene, 2,6-dimethy-phenol, 2,6-dimethy1-nitrophenol, 2,6-dimethy1-hydroquinone, 2,6-dimethy1-benzoquinone, and 2,6-dimethy1-3-hydroxy-benzoquinone indicating the methyl groups on the benzene ring were less susceptible to OH attack than the amino and hydroxide groups. Maleic, lactic, oxalic, acetic, and formic acids were also identified as the carboxylic intermediates. Degradation mechanism of 2,6-DMA oxidation by OH was also proposed. Considering iron crystallization, it was found that ferrous (Fe[subscript 2+) was immediately transformed to ferric (Fe[subscript 3+]) after the initiation of Fenton reaction and sequentially precipitated out in the form of Fe(OH)3 due to its low solubility even in the acidic solution. However, if precipitation occurred in the labile zone, Fe(OH)3 was formed via homogeneous nucleation and crystallized very slowly onto the fluidized media. On the other hand, if it happened in the metastable zone, Fe(OH)3 crystallized onto the fluidized media more rapidly via heterogeneous nucleation and iron was removed significantly from the aqueous phase. Crystallization was controlled by the transport step rather than the surface interaction step. Rate of crystal growth followed the orthokinetic flocculation when the mixing was sufficiently provided; however, it became under the influence of molecular diffusion or perikinetic flocculation depending on the crystallite size under the stagnant conditions. Presence of organic intermediates which could form complex with Fe[subscript 3+] increased the solubility of Fe[subscript 3+] and sequentially deteriorated the crystallization process. Crystallization rate onto fluidized media decreased with time; thus, continuously replace certain portion of coated solids with fresh media will maintain satisfactory crystallization rate. Catalytic activity of the Fe(OH)3-coated sand with 3.5% iron content by weight was only equivalent to 4% of the goethite.