Degradation Of AZO Dyes (Methyl Orange And Congo Red) By Sodium Persulfate Activated With Zero-Valent Zinc

Azo dyes are widely used in the chemical industry, mainly for textiles, leather, printing and dyeing. The conjugated system is formed by benzene rings linked with azo bond in the molecular structure of azo dyes which are carcinogenic, teratogenic and mutagenic due to aromatic amine intermediate. Owing to the complex structure, poor biodegradability of azo dyes, the traditional physical, chemical and biological methods are difficult to make them degrade efficiently. Therefore, it is of important theoretical and practical significance to seek efficient, inexpensive and environment-friendly method to deal with azo dyes. Zn0-activated persulfate (PS) is a novel advanced oxidation technology for the degradation of organic pollutants in aqueous solution. Sulfate radical (SO4·-) produced in this system can degrade most of the organic matter theoretically. In this paper, the influencing factors and the reaction mechanism for degradation of azo dyes (methyl orange and congo red) by sodium persulfate activated with zero-valent zinc were investigated.This thesis is divided into two parts:Part I:The effects of the initial solution pH, the dosages of PS and Zn0, and the temperature were investigated through a series of batch experiments using methyl orange (MO), an azo dye, as a model organic pollutant. The results demonstrated that MO could be effectively degraded by Zn0-activated PS. The chemical oxygen demand (COD) and the total organic carbon (TOC) decreased by approximately 85 and 58%, respectively, in the solution containing 98 mg/L MO at the initial pH 5 and 25℃ within 3 h. Tertiary butyl alcohol, an ·OH-specific radical scavenger, and L-histidine, a universal radical scavenger, corroborated that both ·OH and SO4-· contributed to MO degradation, and SO4-· plays a leading role. Three stages were observed during the degradation of MO at 15 and 25℃, a rapid removal of MO in the initial stage of the reaction, followed by a very slow one and then a relatively quick degradation process. In the third stage of the reaction, the degradation of MO obeyed the first-order kinetics.Part Ⅱ:A disazo dye (congo red, CR) was adopted to study the similarity and difference of the degradation of different azo dyes by Zn0-activated PS. The effects of the initial solution pH, the dosages of PS and Zn0, and the temperature were investigated by batch experiments. The results showed that decoloration effect of congo red by single zinc is good, but the mineralization extent is not ideal. However, CR could be effectively degraded by Zn0-activated PS, which means both good decoloration and mineralization effect. The degradation of Congo red reaction can be divided into two stages, and the second reaction stage obeyed the first-order kinetics. The mineralization was monitored by CODcr, TOC, HPLC analysis technology, and the free radical species in the reaction process was identified by EPR technology to discuss and verify the possible degradation mechanisms.