Heterogeneous Photodegradation Of Methyl Orange In The Presence Of Schwertmannite And Small Molecular Organic Acids

Azo dyes have been mainly used in textile, food and leather products, leading to a large number of azo dyes wastewater discharged into the environment. Owing to complex structure and poor biodegradability of azo dyes, the traditional physical and chemical methods are difficult to degrade them. Therefore, searching for effective treatment methods for these highly toxic contaminants have an important theoretical and practical significance. Photocatalytic oxidation can effectively destroy many of the structural stability and bio-refractory organic pollutants. It is an efficient and fast for removal of organic contaminants, and thus received extensive attention. This paper mainly investigated the factors and reaction mechanism of schwertmannite photocatalytic degradation of methyl orange in the presence of small molecular organic acids(oxalic, citric or tartaric acid).Part I:At pH4.0-8.0, the factors and reaction mechanism of schwertmannite photocatalytic degradation of methyl orange were investigated by batch experiments. The results showed that under500W UV irradiation, schwertmannite photocatalytic degradation of methyl orange was weak. The reaction of single schwertmannite photocatalytic degradation of methyl orange obeys to pseudo-zero-order kinetics. Acidic conditions promote the degradation of methyl orange, and the degradation rate constant in the presence of schwertmannite at pH4.0was2times as methyl orange itself. The pH value of reaction solution and the amount of schwertmannite were the important factors which affected the photocatalytic degradation of methyl orange. By monitoring the concentration of reaction intermediate Fe2+, it was found that schwertmannite provided a source of iron. Inspired by light reaction, hydroxyl radical(·OH) rescued from Fe(OH)2+, responsible for degradation of methyl orange.Part II:At pH4.0-8.0, schwertmannite photocatalytic degradation of methyl orange in the presence of small molecular organic acids(oxalic, citric or tartaric acid) was conducted. The results showed that in the presence of schwertmannite or small molecular organic acids(oxalic, citric or tartaric acid) alone under100W UV irradiation, the degradation of methyl orange was weak. However, when schwertmannite and organic acids co-exist, the degradation rate of methyl orange was significantly improved under UV irradiation. In this case, the reaction of photocatalytic degradation of methyl orange also obeyed to pseudo-first-order kinetics. By monitoring the concentration of reaction intermediate Fe2+, it was found that the generation of Fe2+was divided into two stages, the rapidly increasing and stabilization phases. This was consistent with the process of photocatalytic degradation of methyl orange.In addition, hydroxyl radicals(-OH) produced from the system was examined, proved to be the main reason for degradation of methyl orange.