Photoassisted bleaching of dyes utilizing TiO2 and visible light

Titanium dioxide was shown to be generally effective as a catalyst for photobleaching many structural classes of organic dyes in aqueous solution, using visible light. However, results from study of 15 dyes indicate that photobleaching rates differ significantly from families of dyes with different functionalities, and are dependent on the light source and crystalline form of TiO2 used. Sorption characteristic on the TiO2 surface and the aqueous solubility of the dyes also play an important role in the photobleaching rate. Kinetic analysis indicates that the dye photobleaching rates can usually be approximated as pseudo-first-order kinetics. In addition to the generally proposed photocatalytic oxidation mechanism for TiO2 reactions, we observed evidence for two kinds of electron transfer mechanisms that are "photosensitized reduction" and "photosensitized oxidation". Natural sunlight was effectively used to photobleach some of the dyes.     

Decolorization of direct dyes by salt fractionated turnip proteins enhanced in the presence of hydrogen peroxide and redox mediators

The present paper demonstrates the effect of salt fractionated turnip (Brassica rapa) proteins on the decolorization of direct dyes, used in textile industry, in the presence of various redox mediators. The rate and extent of decolorization of dyes was significantly enhanced by the presence of different types of redox mediators. Six out of 10 investigated compounds have shown their potential in enhancing the decolorization of direct dyes. The performance was evaluated at different concentrations of mediator and enzyme. The efficiency of each natural mediator depends on the type of dye treated. The decolorization of all tested direct dyes was maximum in the presence of 0.6mM redox mediator at pH 5.5 and 30 degrees C. Complex mixtures of dyes were also maximally decolorized in the presence of 0.6mM redox mediator (1-hydroxybenzotriazole/violuric acid). In order to examine the operational stability of the enzyme preparation, the enzyme was exploited for the decolorization of mixtures of dyes for different times in a stirred batch process. There was no further change in decolorization of an individual dye or their mixtures after 60 min; the enzyme caused more than 80% decolorization of all dyes in the presence of 1-hydroxybenzotriazole/violuric acid. However, there was no desirable increase in dye decolorization of the mixtures on overnight stay. Total organic carbon analysis of treated dyes or their mixtures showed that these results were quite comparable to the loss of color from solutions. However, the treatment of such polluted water in the presence of redox mediators caused the formation of insoluble precipitate, which could be removed by the process of centrifugation. The results suggested that catalyzed oxidative coupling reactions might be important for natural transformation pathways for dyes and indicate their potential use as an efficient means for removal of dyes color from waters and wastewaters.     

Nanomaterials photocatalytic activities for waste water treatment: a review

Water is necessary for the survival of life on Earth. A wide range of pollutants has contaminated water resources in the last few decades. The presence of contaminants incredibly different dyes in waste, potable, and surface water is hazardous to environmental and human health. Different types of dyes are the principal contaminants in water that need sudden attention because of their widespread domestic and industrial use. The toxic effects of these dyes and their ability to resist traditional water treatment procedures have inspired the researcher to develop an eco-friendly method that could effectively and efficiently degrade these toxic contaminants. Here, in this review, we explored the effective and economical methods of metal-based nanomaterials photocatalytic degradation for successfully removing dyes from wastewater. This study provides a tool for protecting the environment and human health. In addition, the insights into the transformation of solar energy for photocatalytic reduction of toxic metal ions and photocatalytic degradation of dyes contaminated wastewater will open a gate for water treatment research. The mechanism of photocatalytic degradation and the parameters that affect the photocatalytic activities of various photocatalysts have also been reported.

     

Synergistic effects of liquid and gas phase discharges using pulsed high voltage for dyes degradation in the presence of oxygen

The technology of combined liquid and gas phase discharges (LGD) using pulsed high voltage for dyes degradation was developed in this study. Apparent synergistic effects for Acid orange II (AO) degradation in the presence of oxygen were observed. The enhancement of AO degradation rate was around 302%. Furthermore, higher energy efficiency was obtained comparing with individual liquid phase discharge (LD) or gas phase discharge process (GD). The AO degradation in the presence of oxygen by LGD proceeded through the direct ozone oxidation and the ozone decomposition induced by LD. Important operating parameters such as electrode distance, applied voltage, pulse repetition rate, and types of dyes were further investigated.     

Electrochemical degradation of Acid Blue and Basic Brown dyes on Pb/PbO2 electrode in the presence of different conductive electrolyte and effect of various operating factors

Electrocatalytic degradation of Acid Blue and Basic Brown dyes from simulated wastewater on lead dioxide anode was investigated in different conductive electrolytes. It was shown that complete degradation of these dyes is dependent primarily on type and concentration of the conductive electrolyte. The highest electrocatalytic activity was achieved in the presence of NaCl (2g/l) and could be attributed to indirect oxidation of the investigated dyes by the electrogenerated hypochlorite ions formed from the chloride oxidation. In addition, contribution from direct oxidation could also be possible via reaction of these organic compounds with the electrogenerated hydroxyl radicals adsorbed on the lead dioxide surface. In the presence of NaOH, the electrocatalytic activity of the employed anode was not comparable to that in NaCl due primarily to the absence of chloride. This indicates that dyes degradation in NaOH occurs exclusively via direct electrochemical process. However, in H2SO4, the electrode performance was poor due partially to the absence of chloride from the conductive solution. The possibility of electrode poisoning as a result of growth of adherent film on the anode surface or production of stable intermediates not easily further oxidized by direct electrolysis in H2SO4 might also be accountable for the poor performance observed in this conductive electrolyte. Optimizing the conditions that ensure effective electrochemical degradation of Acid Blue and Basic Brown dyes on lead dioxide electrode necessitates the control of all the operating factors.     

Removal of anionic azo dye from aqueous solution via an adsorption–photosensitized regeneration process on a TiO2 surface

Textile dye effluents are typically characterized by strong color and recalcitrance, even at very low concentration. The process of enrichment of anionic azo dye on the surface of TiO₂ fibers followed by photosensitization degradation under ambient air conditions was extensively investigated. Adsorption isotherms and zeta potentials were used to describe the “dye/TiO₂ surface” interface, taking into account the effects of pH on the nature and population of the surface groups on the TiO₂ fibers. The extent of the photocatalytic degradation of dye on TiO₂ surface was determined by FTIR. N₂ adsorption isotherms and optical spectra were employed to investigate the effect of photosensitization. The adsorption of dyes on the TiO₂ surface occurs via electrostatic attraction through the formation of single- or multidentate-coordinated surface complexes. Almost complete photobleaching of the absorption band at 534 nm is achieved in ~4 h. Dye-sensitized TiO₂ fiber could absorb part of the visible light spectrum (λ?<?600 nm). Interfacial electron transfer can potentially alter the degradation efficiency. The regenerated TiO₂ fiber could be reused for subsequent decolorization without a decline in adsorption efficiency compared with freshly prepared TiO₂ samples, which may be attributed to preservation of the hierarchical pore structure and restoration of the original surface properties. In summary, we propose an efficient “adsorption–photoregeneration–reuse” process applying TiO₂ fibers for the degradation of dyes in water.     

Degradation of azo dyes by algae

The degradation of azo dyes by algae was evaluated and it was found that certain algae can degrade a number of azo dyes to some extent. The reduction rate appears to be related to the molecular structure of the dyes and the species of algae used. The azo reductase of algae is responsible for degrading azo dyes into aromatic amine by breaking the azo linkage. The aromatic amine is then subjected to further metabolism by algae. It is proposed in this paper that in stabilization ponds, algae can play a direct role in the degradation of azo dyes, rather than only providing oxygen for bacterial growth.     

Biogenic sulphide plays a major role on the riboflavin-mediated decolourisation of azo dyes under sulphate-reducing conditions

The effect of high concentrations of sulphate on the reductive decolourisation of different azo dyes by anaerobic sludge was studied in batch cultures. Sludge cultures were pre-incubated under sulphate-reducing conditions prior addition of dyes. Little or no effects of sulphate (5-10 g sulphate l(-1)) on the rate of decolourisation of Reactive Orange 14 (RO14), Direct Blue 53 (DB53) and Direct Blue 71 (DB71) were observed when no external redox mediator was provided. However, an increase in sulphate concentration, in the presence of riboflavin (20 microM), enhanced the decolourisation of all dyes. The first-rate constant of decolourisation (k) was increased up to 2-, 3.6- and 2-fold for RO14, DB53 and DB71, respectively, by supplying high sulphate concentrations, compared to the controls lacking sulphate, in the presence of the redox mediator. Sulphate reduction did not take place during the course of azo reductions, but was only evident before dye addition and after complete decolourisation, suggesting azo dyes reduction out-competed sulphate reduction for the available reducing equivalents. The experimental data suggest that reduction of azo dyes by riboflavin, which had been reduced by biogenic sulphide, was the major mechanism implicated during decolourisations, which was corroborated by abiotic incubations. Riboflavin greatly accelerated the abiotic reduction of RO14, so that the k value was increased up to 44-fold compared to the control lacking riboflavin.     

The influence of FeCl3 on the photocatalytic degradation of dissolved azo dyes in aqueous TiO2 suspensions

The study concerned decolouration of solutions of azo, anionic (Acid Orange 7, Reactive Red 45, Acid Yellow 23) and cationic (Basic Blue 41 and Basic Orange 66) dyes during illumination with UV (lambdamax 366 nm) irradiation in the presence of TiO2 and FeCl3. The process of decolouration during illumination of the solutions studied containing FeCl3 underwent significant intensification in the case of anionic dyes and unfavourable inhibition in case of cationic dyes. It was also observed that FeCl3 had a diverse influence on the adsorption of the dyes studied on TiO2. The adsorption of anionic dyes and decolouration of solutions before the illumination was observed only in the presence of FeCl3. In case of cationic dyes the addition of FeCl3 caused elimination of these phenomena. An additional cause of decolouration of anionic dyes solutions before illumination was the precipitation of their poorly soluble compounds from Fe3+. The processes of degradation and mineralization of the dye that accompanied decolouration of Acid Orange 7 solutions were also observed. It was stated that similarly to the case of Acid Orange 7, the decolouration of the studied anionic dyes' solutions can depend on the concentration of FeCl3, the amount of TiO2 and the initial concentration of the dye in its solution.     

Decolorization of Orange G and Remazol Brilliant Blue R by the white rot fungus Dichomitus squalens: toxicological evaluation and morphological study

Dichomitus squalens efficiently decolorized both Orange G and Remazol Brilliant Blue R (RBBR) at concentrations of 0.5gl(-1) and 3gl(-1) in static and shaken culture and also on solid medium within 14d. The presence of the dyes in the culture medium mostly caused a decrease in biomass production and in growth rate, which was more significant in the case of RBBR. After 14d of cultivation, electron microscopy showed substantial morphological changes in mycelia of D. squalens growing in media containing dyes. The hyphae deformations were more intensively manifested in solid media than in liquid culture. In all the cases, the morphological changes were more prominent in the presence of RBBR. Higher concentrations of both dyes brought about more intensive changes. The toxicity of synthetic dyes Orange G and RBBR was tested using a bioassay based on the growth inhibition of duckweed Lemna minor. Two endpoints such as the number of fronds and their weight were studied during the bioassay. The results showed higher toxicity of RBBR than that of Orange G. The toxicity of both dyes decreased after the decolorization process.     

Comparative use of bacterial, algal and protozoan tests to study toxicity of azo- and anthraquinone dyes

Toxicity of two azo dyes (Reactive Orange 16 (RO16); Congo Red (CR)) and two anthraquinone dyes (Remazol Brilliant Blue R (RBBR); Disperse Blue 3 (DB3)) were compared using bacterium Vibrio fischeri, microalga Selenastrum capricornutum and ciliate Tetrahymena pyriformis. The following respective endpoints were involved: acute toxicity measured as bacterial luminescence inhibition, algal growth inhibition, and the effects on the protozoa including viability, growth inhibition, grazing effect and morphometric effects. In addition, mutagenicity of the dyes was determined using Ames test with bacterium Salmonella typhimurium His(-). DB3 dye was the most toxic of all dyes in the bacterial, algal and protozoan tests. In contrast to other dyes, DB3 exhibited mutagenic effects after metabolic activation in vitro in all S. typhimurium strains used. Of the methods applied, the algal test was the most sensitive to evaluate toxicity of the dyes tested.     

Biological decolourisation of simulated azo dye in aqueous phase by algae Spirogyra species

Biological decolourisation of two azo dye effluents (direct and reactive dye) were investigated using a commonly available green algae Spirogyra sp. in viable form. Batch studies revealed the potential of algal species in removing the dye colour and dye removal was dependant on initial algal inoculum, concentration and application class of the dye. Maximum dye uptake was noticed on the third day for both the dyes. Higher dye uptake was observed in the case of direct red 28 compared to reactive red 2. Dye colour removal by the algal species may be attributed to biosorption of the dye molecules onto the surface of algal cell and subsequent diffusion and participation in metabolism (bioconversion). The remaining dye molecules could be further removed from the aqueous phase by adsorption and/or chelation reaction of the exopolymers released by the algae (biocoagulation). The results of the present study reveal the potential nature of algae in treating azo dyes which in turn can be extended to oxidation pond system of wastewater treatment.     

Sorption of basic and acid dyes from aqueous solutions onto oxihumolite

Naturally occurring kind of weathered and oxidised young brown coal called oxihumolite was used for an adsorptive removal of basic (Methylene Blue, Malachite Green) as well as acid (Egacid Orange, Midlon Black) dyes from waters. It was shown that both kinds of dyes can be sorbed onto oxihumolite. The maximum sorption capacities determined from the parameters of Langmuir isotherms ranged from 0.070 mmol g-1 (for Midlon Black) to 0.278 mmol g-1 (for Malachite Green) and did not differ significantly for basic and acid dyes. The dye sorption (except of Midlon Black) increased in the presence of inorganic salt. Non-ionic surfactants, and surfactants bearing the same charge as the dye exhibited only a minor effect on the dye sorption, whereas oppositely charged surfactants enhanced the dye sorption to a certain extent. The pH value of the aqueous phase exhibited rather pronounced effect on the sorption of acid dyes causing a suppression of the sorption with increasing pH. The sorption of basic dyes, on the other hand, remained almost unchanged in the examined pH range. Oxihumolite is recommended for the treatment of acid wastewaters because of its limited stability in alkaline aqueous solutions.     

Azo dye decolourisation by anaerobic granular sludge

The decolourisation of 20 selected azo dyes by granular sludge from an upward-flow anaerobic sludge bed (UASB) reactor was assayed. Complete reduction was found for all azo dyes tested, generally yielding colourless products. The reactions followed first-order kinetics and reaction rates varied greatly between dyes: half-life times ranged from 1 to about 100 h. The slowest reaction rates were found for reactive dyes with a triazine reactive group. There was no correlation between a dye's half-life time and its molecular weight, indicating that cell penetration was probably not an important factor. Since granular sludge contains sulphide, eight dyes were also monitored for direct chemical decolourisation by sulphide. All these dyes were reduced chemically albeit at slower rates than in the presence of sludge at comparable sulphide levels. Increasing sulphide concentrations, even when present in huge excess, stimulated the azo reduction rate. The results indicate that granular sludge can decolourise a broad spectrum of azo dye structures due to non-specific extracellular reactions. Reducing agents (e.g., sulphide) in sludge play an important role. The presence of anaerobic biomass is probably beneficial for maintaining the pools of these reduced compounds.     

The contribution of azo dyes to the mutagenic activity of the Cristais River

To verify whether dyes emitted within the discharge of a dye processing plant were contributing to the mutagenicity repeatedly found in the Cristais River, Sao Paulo, Brazil, we chemically characterized the following mutagenic samples: the treated industrial effluent, raw and treated water, and the sludge produced by a Drinking Water Treatment Plant (DWTP) located approximately 6 km from the industrial discharge. Considering that 20% of the dyes used for coloring activities might be lost to wastewaters and knowing that several dyes have mutagenic activity, we decided to analyze the samples for the presence of dyes. Thin layer chromatographic analysis indicated the presence of three prevalent dyes in all samples, except for the drinking water. This combination of dyes corresponded to a commercial product used by the industry, and it tested positive in the Salmonella assay. The structures of the dye components were determined using proton magnetic resonance and mass spectrometric (MS) methods, and the dyes were tested for mutagenicity. The blue component was identified as the C.I. Disperse Blue 373, the violet as C.I. Disperse Violet 93, and the orange as C.I. Disperse Orange 37. The dyes showed mutagenic responses of 6300, 4600, and 280 revertants/microg for YG1041 with S9 respectively. A bioassay-directed fractionation/chemical analysis showed that the C.I. Disperse Blue 373 contributed 55% of the mutagenic activity of the DWTP sludge. We showed that these dyes contributed to the mutagenic activity found in the Cristais River environmental samples analyzed and are indirectly affecting the quality of the related drinking water. Therefore, we believe that this type of discharge should be more thoroughly characterized chemically and toxicologically. Additionally, human and ecological risks associated with the release of dye processing plant effluents should be more fully investigated, especially where the resultant water is taken for human consumption.     

New catalyst comprising Silicotungstic acid and MCM-22 for degradation of some organic dyes

A heterogeneous catalyst comprising Keggin type polyoxometalate, silicotungstic acid (SiW₁₂), and MCM-22 was synthesized using wet impregnation method and characterized by acidity measurement, BET, FT-IR, XRD, and SEM. Their catalytic activity was evaluated for the degradation of cationic organic dyes like methylene blue (MB), crystal violet (CV), and an azo dye Chryosidine Y (CY) in an aqueous solution. The experimental parameters such as catalyst amount, initial dye concentration, and contact time were studied for the degradation of dyes, and it was found that the cationic dyes like methylene blue and crystal violet show better activity as compared to azo dye Chryosidine Y. This may be attributed to better electrostatic interaction of these cationic dyes with the residual negative surface charge of the catalyst, due to presence of SiW₁₂ ion as it is rich in surface oxygens and surface hydroxyl groups. The control experimental results showed that the presence of SiW₁₂ at the surface of MCM-22 promoted the degradation reactions, and presence of multiple W–O bonds in polyoxometalate also played a key role in this reaction. The catalyst exhibits recycling ability without any significant loss in activity up to four cycles.

     

白腐菌J-6对五类化学结构染料的脱色性能

从白腐菌中筛出了一株能对五类不同化学结构的染料进行高效脱色的菌株——J-6。通过采用“批式”摇床培养法,测定染料脱色率,从而确定该菌株能高效脱色的最佳培养时间和连续处理染料的能力。发酵培养96h的菌体可使活性艳蓝R(A)SP在5h内脱色率达95％以上;活性翠蓝G、活性黑GRP、混合染料在9h内脱色率达90％以上;孔雀绿、Po1yR-478在24h内脱色率分别达95％、90％以上。另外,J-6对不同染料最佳脱色时间和最终脱色效率与菌体本身发酵培养的时间长短和染料的种类紧密相关。J6处理能力强,连续处理4次染料后仍保持较高的脱色率。     

Separation of the strength and selectivity of the microbiological effect of synthetic dyes by spectral mapping technique

The growth inhibitory effect of 30 synthetic dyes on 22 bacteria (test organisms) belonging to various taxonomic groups was determined. The strength (potency) and selectivity of the biological effect were separated by the spectral mapping technique, reducing the dimensionality of the selectivity maps to two by the nonlinear mapping technique. The relationship between biological effect and physicochemical parameters of dyes was elucidated by stepwise regression analysis. It has been established that the strength of the effect of anthracene and trityl derivatives was higher than that of azobenzene dyes and significantly depended on the hydrophobicity of the compound. The selectivity of the effect also depended on hydrophobicity and on the nonpolar unsaturated surface area of the dyes. Gram negative and Gram positive bacteria differed in the strength and selectivity of their response to dyes indicating the marked impact of the taxonomical position on the response. Contrary to other multivariate mathematical statistical methods biological activity may be divided by SPM into potency and selectivity values, therefore, application of the technique in future QSAR studies is highly recommended.     

硫酸盐在偶氮染料厌氧生物脱色中的作用

以沼泽红假单胞菌W1为研究对象,考察了厌氧条件下硫酸盐还原对活性黑5(Reactive Black 5,RB5)和直接黄11(Direct Yellow 11,DY11)生物脱色的影响。结果表明,硫酸盐本身对2种染料脱色无明显影响,而硫酸盐的还原产物———硫化物能通过氧化还原介体使2种染料化学脱色,其脱色过程能在3 min内迅速完成。在无介体加入的情况下,硫化物能够通过RB5自身产生的介体加速RB5的脱色;而对于不能产生氧化还原介体的DY11,硫化物对其脱色无明显影响。硫化物经染料氧化后形成的硫单质能够被菌株W1重新转化为硫化物,继续还原染料。