Decomposition of an azo dye in aqueous solution by combination of ultrasound and visible light

A critical analysis of sono-hybrid advanced oxidation process of ferrioxalate system for degradation of recalcitrant pollutants

The emerging contaminants in wastewater discharged from numerous chemical process industries, pharmaceutical industries, textile, and wineries have attracted the attention of the scientific community due to their toxicity and persistence in the environment. The conventional techniques are incompetent to treat many of such recalcitrant toxic pollutants. To achieve high mineralization, advanced oxidation processes (AOPs) are found to be more efficient for the degradation of these organic pollutants without producing secondary pollutants with no/less amount of sludge. The primary oxidation agents for AOPs are in-situ generated free radicals, which are highly reactive and effective oxidants for degrading any type of organic molecules present in the wastewater. In the past decades, the combination of AOPs or simultaneous application of more than one AOP has been investigated extensively for wastewater treatment and these hybrid-AOPs have been reported to be beneficial for high-level mineralization of organic pollutants. This paper presented the characteristics, properties and influence of parameters in sono-photo-ferrioxalate system. The primary operating parameters in sono-photo-ferrioxalate system that affect the kinetics are defined as the solution pH, temperature, molar ratio of Fe³⁺/C₂O₄^2-, H₂O₂ concentration, source of light, ultrasound intensity, dissolved gases, and size of cavitation bubble. In this process, several oxidizing radicals are generated such as HO^•, HO₂^•, C₂O₄^•-, CO₂^•- and O₂^•- which are also responsible for degradation. In this review, we have mainly addressed the degradation of recalcitrant pollutants using the sono-photo-ferrioxalate system and a critical analysis of process parameters that influence mineralization efficiency.

A comprehensive review on the integration of advanced oxidation processes with biodegradation for the treatment of textile wastewater containing azo dyes

The threat of dye contamination has achieved an unsurpassed abnormal state lately due to their massive consumption in several enterprises including textile, leather, cosmetic, plastic, and paper industries. This review focuses on the integrations of various advanced oxidation processes (AOPs), such as Fenton, photocatalysis, and ozonation, with biodegradation for the treatment of textile azo dyes. Such integrations have been explored lately by researchers to bring down the processing cost and improve the degree of mineralization of the treated dyeing wastewater. The review refers to the basic mechanisms, the influence of various process parameters, outcomes of recent works, and future research directions. All the three AOPs, independently, demonstrated substantial color reduction of 54–100%. The ozonation process, stand-alone, showed the most efficient decolorization (of 88–100%) consistently in all reviewed research works. In contrast, all three AOPs independently offered varied and inadequate COD reduction in the range of 16–80%. The AOPs, after getting integrated with biodegradation, yielded an additional reduction (of 11–70%) in the COD-levels and (of 16–80%) in the TOC-levels. Further, the integration of AOPs with biodegradation has potential to significantly reduce the treatment costs. The review suggests further research efforts in the direction of sequencing chemical and biological routes such that their synergistic utilization yield complete detoxification of the textile azo dyes economically at large-scale.

The sonochemical decolourisation of textile azo dye Orange II: effects of Fenton type reagents and UV light

The removal of Orange II (O-II) from aqueous solution under irradiation at 850 kHz has been studied. The effects of both homogeneous (with FeSO4/H2O2), and heterogeneous (Fe containing ZSM-5 zeolite/H2O2) Fenton type reagents are reported together with the effect of UV irradiation in combination with ultrasound both alone and with homogeneous Fenton-type reagent. Degrees of decolourisation of 6.5% and 28.9% were observed using UV radiation and ultrasound, respectively, whereas under the simultaneous irradiation of ultrasound and UV light, the decolourisation degree reached 47.8%, indicating a synergetic effect of ultrasound and UV light. The decolourisation was increased with the addition of Fenton's reagent with an optimal Fenton molar reagent ratio, Fe(2+):H2O2 of 1:50. In the combined process of ultrasound and UV light with the homogeneous Fenton system 80.8% decolourisation could be achieved after 2h indicating that UV improves this type of Orange II degradation. The degree of decolourisation obtained using the heterogeneous sono-Fenton system (Fe containing ZSM-5 zeolite catalysts+H2O2+ultrasound) were consistently lower than the traditional homogeneous ultrasound Fenton system. This can be attributed to the greater difficulty of the reaction between Fe ions and hydrogen peroxide. In all cases the Orange II ultrasonic decolourisation was found to follow first order kinetics.

The effects of externally applied pressure on the ultrasonic degradation of Rhodamine B

The present manuscript compares the ultrasonic degradation of Rhodamine B dye under atmospheric (1 bar) and elevated pressures (1.6 and 2 bar). The degradation was studied as a function of the bulk liquid temperature and initial dye concentration at two different values of mechanical amplitudes (ultrasonic intensities). Results indicate that at the low amplitude an increase in the applied pressure increases the dye removal rate, whereas at the high amplitude, the same increase in the pressure has a minimal effect on the degradation of the dye. Furthermore, at low amplitudes an increase in the bulk liquid temperature from 5 to 35 °C increases the dye degradation by 10%. At higher intensities, the same increase in temperature has negative or no effect on the dye removal. An increase in the initial dye concentration by one order of magnitude significantly lowers the dye degradation rate regardless of the applied amplitude. Though these results are caused by numerous physical and chemical processes taking place during ultrasonic cavitation, the number of cavitation sites, bubble temperature and consequently, the amount of oxidative species inside the bubble seem to be the most important ones in determining the extent of the degradation of molecules in the bulk liquid.

Ultrasound as a basic and auxiliary process for dye remediation: a review

Ultrasonic treatment of recalcitrant contaminants has been of utmost interest recently due to the advantages of the cavitation phenomenon, which enhances the efficiency of Advanced Oxidation Processes (AOPs). The current review summarizes the use of ultrasound with biochemical, electrochemical, ozonation, photolysis, photocatalysis and Fenton processes for the degradation of mostly textile dyes and dyebath. There is a few studies about ultrasonic degradation of textile effluents or wastewater due to highly variable contents. It was found that the most common use of ultrasonic irradiation for dye degradation is the combined with the heterogeneous catalysts/adsorbents. The reaction mechanism of the ultrasonic irradiation in heterogeneous media was well investigated and understood. However, there is still lack of information about the reaction mechanism of ultrasonic irradiation in the homogeneous solutions, especially containing ferrous ions. Fenton reaction is already fast itself and gives effective degradation during the oxidation. Therefore, addition of ultrasonic irradiation to Fenton oxidation was less effective compared to other auxiliary processes. It should also be noted that ultrasonic irradiation had a negative effect on dye degradation during combined with electro-oxidation process.

Sonophotocatalysis in advanced oxidation process: a short review

Sonophotocatalysis involves the use of a combination of ultrasonic sound waves, ultraviolet radiation and a semiconductor photocatalyst to enhance a chemical reaction by the formation of free radicals in aqueous systems. Researchers have used sonophotocatalysis in a variety of investigations i.e. from water decontamination to direct pollutant degradation. This degradation process provides an excellent opportunity to reduce reaction time and the amount of reagents used without the need for extreme physical conditions. Given its advantages, the sonophotocatalysis process has a futuristic application from an engineering and fundamental aspect in commercial applications. A detailed search of published reports was done and analyzed in this paper with respect to sonication, photocatalysis and advanced oxidation processes.

Dynamical properties of iodine release in potassium iodide solution under combination of ultrasound and light irradiations

Dynamical process of iodine release in potassium iodide solution has been investigated under the irradiations of ultrasound and visible light respectively and simultaneously. Under ultrasonic irradiation, the efficiency of iodine release increases almost linearly with the irradiation time, indicating the zero-order reaction. On the other hand, the efficiency of iodine release increases exponentially with the irradiation time under visible light irradiation and combined irradiation of ultrasound and visible light, suggesting the pseudo first-order kinetics. We further find a significant synergistic effect induced by combined irradiation of ultrasound and visible light, which may be due to the well-mixed condition in the photochemical reactor with the applying simultaneous ultrasound. We further investigate influence of the initial concentration of potassium iodide on the synergistic effect.

Degradation of Acid Orange 7 by electrochemically generated (*)OH radicals in acidic aqueous medium using a boron-doped diamond or platinum anode: a mechanistic study

A comparative study of the degradation of Acid Orange 7 (AO 7) aqueous solutions in acidic medium of pH 3.0 by electro-Fenton process using Pt or boron-doped diamond (BDD) anode was reported. The oxidative degradation of AO 7 by electrochemically generated hydroxyl radicals follows a pseudo-first order kinetic with a similar rate constant with BDD or Pt anode. The absolute rate constant of the AO 7 hydroxylation reaction was determined as (1.10+/-0.04)x10(10)M(-1)s(-1) by using the competition kinetic method. The comparative study of TOC measurements during electro-Fenton treatment showed a higher mineralization rate with BDD than Pt anode at the first hours of electrolysis because of the higher oxidizing power of this anode. The electro-Fenton degradation of AO 7 was followed by monitoring the formation and evolution of aromatic intermediates which are oxidized to aliphatic carboxylic acids before mineralization (transformation to CO(2) and inorganic ions, i.e. sulphate, nitrate and ammonium). The follow-up of the solution toxicity evolution shows the formation of intermediates more toxic than AO 7 and the connection between toxicity and aromaticity. A mineralization reaction pathway of AO 7 by electro-Fenton degradation involving all the intermediates identified was proposed.