Treatment Efficiency And Mechanism Of Reactive Azo Dyes Containning Wastewater By Ozonated Internal Electrolytic Process

A large amount of refractory wastewater is produced when remarkable economic benefits are brought by dyeing industry. The structure of dyes and additives is becoming more and more stable, making the treatment of dyeing wastewater even more difficult. Nowadays, the mainly used technologies for dyeing wastewater treatment are based on biological method. The discharged wastewater often exceeds the standard’s requirement because the advanced treatment technologies are rarely used. Cost-efficient advanced treatment technologies are urgent needed as the discharge standard will be even more strictly since 2015. A newly designed ozonated internal electrolysis(OIE) process based on modified IE filler was applied for the most recalcitrant azo dyes containing wastewater treatment, and hoping to improve treatment efficiency and decrease the cost. OIE process can maximize the treatment efficiencies of ozone and IE. It is believed that OIE process could solve the passivation and blocking problems of IE fillers. The treatment efficiencies of different combination way of ozone and IE w ere compared. Main byproducts of RR2 dye in IE, ozone and OIE processes were analyzed, and the degradation pathways were proposed. The reaction mechanisms, operating parameters optimization and costs estimation of OIE process were also studied.Dyes and their degradation products removal efficiencies were significantly improved when ozone and IE were combined together. OIE process has much higher treatment efficiency compared to O3-IE and IE-O3 processes. The reaction time can half reduced when the former three processes have comparable color and COD removal efficiencies. The color and COD removal rates were 99% and 82% respectively when the initial COD concentration of RR2 dye was 120mg/L and treatment time was 60 min. Excellent treatment performance were obtained when OIE process was used to treat azo dyes containing wastewater. And the concentrations of nitrobenzene and aniline can be effectively controlled to desired level. Less identified intermediates and their very low concentrations confirmed that OIE process has the ability to efficiently remove organics.OIE process was constituted by ozone, ZVI and GAC. In this complicated system, more radicals would be produced and then the oxidation potential was improved significantly. The main reaction mechanism of OIE process could be summarized as: the oxidation of molecular ozone and its’ indirect reaction through hydroxyl radicals, the redox and coagulation of IE, synergy effect between dissolved iron(Fe2+ and Fe3+) and O3, and the synergy effect between O3 and IE. The mechanisms of OIE process was proposed according to former analization and experimental verification. Among the proposed mechanisms, decolorization was mainly due to ozone oxidation, while the improvement of COD removal rates could be the synergy effect of O3 and IE. The comparative study of P-chlorobenzoic acid(p CBA) degradation trend in O3, IE and OIE systems showed that the fastest degradation rate was observed in OIE process. And it thought to be more radicals was produced in OIE process, which is the most probably reason for increasing of p CBA degradation rate. But the results were very lower than the actual outcome when p CBA was used as the quantitative indicator of radicals produced by OIE process. The intermediate phenol was recognized as the reaction product between RR2 or its’ intermediates and hydroxyl radical. It’s namely that the amount of phenol produced by OIE process can indirectly present the amount of hydroxyl radicals generated in this system. But phenol can be easily degraded by OIE process. Thus, it’s very difficult for quantitative analysis of hydroxyl radicals through the production of phenol.The dissolution rate of zero valent iron(ZVI) was greatly affected by the wastewater p H. When p H>7, the dissolved total amount of Fe2+ and Fe3+ in OIE process was less than 10 mg/L after 1 hour reaction. Dissolution amount of Fe2+/Fe3+ in our OIE process was markedly reduced because of the synergy effect of ozone and IE. According to conservative estimates, the dissolution rate of ZVI is approximately 10mg/(L?h), the calculated service life of IE filler is approximately 10 years. Although the formation of Fe(OH)2 and Fe(OH)3 is the main reason for IE filler passivation and blocking problems, these problems could be greatly relieved because the concentrations of dissolved Fe2+ and Fe3+ in present process were very low.For present OIE process, the optimum operation conditions of RR2 dye degradation were as follows: initial COD concentration was less than 120mg/L, initial p H=6-9, ozone dosage rates were 1.0 to 1.5 mg O3/(L·min). Estimated cost for the treatment of real wastewater C(from initial COD and color were 120mg/L and 400 times treated to discharge standard, and the required final COD and color were less than 60mg/L and 30 times respectively) by OIE process(0.86 RMB/ton) was much lower than Fenton(1.59 RMB/ton) and O3(1.05 RMB/ton) processes.Above all, OIE process has excellent color and COD removal performance when used as advanced treatment process of azo dyes containing wastewater. The consumption rate of IE filler could be significantly reduced by the synergy effect between ozone and IE. Thus, the service life of IE filler could be extended. The passivation and blocking problems were apparently relieved. The real wastewater treatment efficiency indicates that OIE process has attractive prospect, especialy for refractory organic wastewater treatment.