FACTORS INFLUENCING EFFICIENCY OF UV/H2O2 ADVANCED OXIDATION PROCESS

In recent years the textile industry is characterized as qualitative fabric industry. Raw materials and process auxiliaries (as dyes and other chemicals) are used of very good quality. Dyes are developed to be very stable and resistant to physical and biological breakdown. Therefore they are hard degradable in natural conditions. Removal of dye residual from the wastewater requires also qualitative treatment solutions. As conventional wastewater treatment methods are unable to decompose hard degradable pollutants to the final breakdown products at the textile industry wastewater treatment unit, therefore there is a demand for more advanced treatment. Considerable attention is paid to the advanced oxidation methods. In this paper the research results of textile reactive dyeing a11ificial wastewater treatment with advanced oxidation method under ultraviolet light (UV) in presence of hydrogen peroxide (H202) are described. Investigated relationships are such as pH, reaction length and wastewater composition influence to the UV/He202 advanced oxidation process efficiency. From obtained results it can be concluded that UY/H202 advanced oxidation process is inhibited by high pH level, dyeing assisting chemicals, salts and anticreasing agent. The reaction rate constant with removed disturbances is obtained up to 0.4436 min ·1, textile reactive dyeing artificial wastewater complete decolourisation is achieved within 4 to 15 minutes.

(NaCl), softening, anticreasing and other auxiliaries are used. The pollutants and residual color create high environmental and aesthetical problems [7,8].
Mostly high molecular weight "colored" compounds are resistant to biological degradation, especially the reactive dyes. Usually 90% of reactive dyes concentration that present in spent textile industrial wastewater passes the conventional municipal treatment plants and enter environment, where remain in natural conditions for 40 years and longer [ 4 l In addition, the dyes and their intennediates are defined as toxic [9]. The solution for this problem could be application of biological, chemical, physicochemical and complex treatment methods [4][5][6][7][8][9][10][11][12][13].
In the last years a lot of research has been done in order to find the most sufficient treatment method for reactive dyeing wastewater purification. Most authors refer full pollutant degradation in artificial wastewater prepared according the real wastewater model [9,14].
Reactive dyeing wastewater treatment methods have been widely investigated. All of them have benefits.
The biological systems have been developed to treat the wastewater contaminated with the dyes and other hazardous auxiliaries. Though biological methods are not capable to decompose dyes completely and treatment efficiency is up to 70% [ I OJ in lab conditions. Anaerobic process for reactive dyeing wastewater treatment was used, but toxic byproducts are produced and left in the treated water [9]. Fentons, and ordinary filtration also produce toxic sludge, and the problem occurs of the toxic sludge disposal [9, 14, 1 1 ]. Advanced oxidation processes (so -called AOP's) or membrane filtration (such as ultrafiltration, nanofiltration, and hyperfiltration) -should be included to the treatment chain, in order to decompose the hard degradable aromatic compounds to the final breakdown products, and not just to convert them to colorless even more toxic intermediates [II,14].
Lately considerable attention in the reactive wastewater treatment is given to advanced oxidation processes (AOP). The methods based on complete destruction of the pollutants by oxidation reaction [ I OJ. Under UV light in presence of hydrogen peroxide (He 2 O 2 ), active •OH radicals are formed, that have the activation energy (2.8 V) almost highest from all oxidizing agents ( except active fluorine radical -3.06 V) [ 16]. The method application on reactive textile wastewater treatment is investigated widely, although following the literature analysis there are a few questions left unclear: -Why the efficiency of the UV/H 2 O 2 -AOP is dependant on the dyeing type; -Why the higher method efficiency is reached with artificial reactive dyeing wastewater; -What additional substances or their conditional processes inhibit UV /H 2 O 2 -AOP; -What is the inhibition effect of CO/ and er ions (fom1 Na 2 C0 3 and NaCl salts added) to the UV-H2O2 decolourisation process.
For the purpose to gain more information on UV /H 2 O 2 -AOP perfonnance and to be able to answer the questions given above the investigation was done. The experiments were perfonned with textile reactive dyeing artificial wastewater, the composition of which was changed according to the investigated parameters.

2,1 Artificial wastewater preparation
The artificial wastewater samples were prepared in the same way as were proposed by most other authors [ I 0, 1 1, 12,13 ]. The dye solution was prepared from commercial dye product (obtained from DyStar, Gennany) to anticipate the experiment to real conditions. The dye powder was dissolved in demineralised water (further called -dyes) and kept for at least three hours under intensive stirring; after the stirrer was stopped the solution was left in dark for 12 hours that dye molecules would be completely dissolved. The amount of each kind artificial wastewater was prepared such as it could be possible to repeat each experiment at least three times, in order to avoid accidental results.
Commonly hues and shades are obtained by mixing basic colors. In the experiment for artificial wastewater preparations three basic colors were used.
Each color artificial wastewater was investigated separately (see Table 2 experimental matrix) in order to evaluate process kinetics for separate color and their mixture. Quantities and conditions were repeated the same as are applied in industry in general to anticipate the experiment to the industrial conditions.
The concentration of dyes in ai1ificial wastewater samples were 30% of initial dye concentration (as this amount is left after dyeing in the real wastewater [4,15]). Four different colors artificial wastewater samples were analyzed.
Additional chemicals generally applied in dyeing process were also added to artificial wastewater. The concentrations of the chemical components and the functions of the auxiliary chemicals in the dyeing process are summarized in Table I 7.  In order to investigate the impact of dyeing auxiliary chemicals to AOP kinetics and decolourisation process efficiency one or more of dyeing solution components (see Table 2) were eliminated from artificial wastewater and the method perfon11ance was tested under the same conditions. For investigation of pH influence on UV /H2O2 -AOP kinetics, Na2CO3 was replaced by NaOH, and pH level was adjusted the same.
From the first data obtained with the artificial wastewater decolourisation the sharpest process characterization was obtained with RY84 type of artificial wastewater, therefore for process kinetics investigation it was decided to use this type of dye (see Table 2 -samples No 5-12).  Temperature was maintained constant 20°C ± 2 as it influences the process dynamics [9], During the process the pH level changes (2 1], the pH was measured throughout the experiment with pH meter -MT28-280-00 I (with pH electrode WTW Sen Tix 2 1, Netherlands), Sampling was continuous; reactor was connected to the peristaltic pump (Watson -Marlow 302, Netherlands) that was piping the sample through the UY/Vis spectrophotomether quvetes, The spectrophotometric color intensity was measured with Varian Cary 3E UY-Vis Spectrophotometer (Varian B,V,, Netherlands), which was connected to the computer and the data was recorded at the certain time intervals, The reaction was stopped after 15 minutes -time interval was set from the process efficiency calculations, and was constant in order to have possibility to compare separate reactions kinetic, The artificial wastewater during decolourisation process was mixed hydraulicaly at certain time intervals with no contact with oxygen in order to get better light penetration in the depth of reaction vessel.
The light was turned on before each experiment. The irradiation power in such kind of reactor when light is going from outside to inside was calculated according [ 17].
Decolourisation (color disappearance) was measured by means of change in absorption level (m-1 ) at certain wavelength (nm) with UV-Vis spectrophotometer (Carry Varian). The wavelength for measuring oxidation absorption change was set analytically, for investigation of each color artificial wastewater samples investigated. The highest absorption value (m-1 ) was defined by scanning the artificial wastewater sample in range 200-800 nm. Highest peaks obtained in this range, were used for UV /H2O2 -AOP observation in the highest signal obtained, in order to obtain the highest sensitivity of measurement. The wavelength (nm) at which the peaks were obtained (see Table 3).

Kinetic model applied
From the data obtained and literature analyzed it is clear that under UV light in presence of hydrogen peroxide during photo catalytic process by highly active •OH radicals, dyes are oxidized to lower molecular weight substances. In order to get dye concentration value from the absorbance data the calibration curves are obtained by measuring with UV-VIS spectrophotometer.
The absorbance dependency from artificial wastewater concentration (mg/I) is derived for all four color wastewater investigated by means of calculating correlation coefficient (R 2 ). Average value from four straightened curves is R 2 > 0,99. Obtained correlation coefficient value shows strong relation between the obtained dependency and the straightened curves. Correlation was used for estimation of dye concentration, converting light absorbance value (nm) to dye concentration (mg/I) in artificial wastewater investigated. The intensity of the light was assumed as constant value. All investigated artificial wastewater samples corresponds the same kinetic model. Color removal rate (absorbance decrease) is proportional to concentration of the dye (Cd y c ) and concentration of H 2 0 2 (C 11 20 2 ). For detem,ination of oxidation process kinetics the reaction rate constant relation was defined.
If assuming that the hydrogen peroxide concentration is constant value, then the reaction could be described as pseudo first order, and it is possible to express reaction rate as follows: (2) Where: r-reaction rate; k'-reaction rate constant when C 11, 0 ,is constant value (min-1 ); Cde y ,-dye concentration (mg/I).
Dye concentration is reciprocal to the color intensity, and then it is possible to rewrite equation as: The natural logarithm plot of the color shows the plot of rate expression: The ln(color 0 /color,) over the time gives the value of the reaction rate constant -k' (min-1 ). For the data expression pseudo first order reaction rate constant -k' calculated from color removal over the time was used.

A) Artificial Wastewater Decolourisation.
Oxidation kinetics were investigated of artificial wastewater samples (RY84, RR 141, RB 171, MIX) (polluted with dyes), were exposed under UV/H 2 02 AOP. Data given in Table 3 characterizes the oxidation process effectiveness. The dye decomposition performance under oxidation conditions is given in Figure 3.     Figure 3 and data given in Table 3  Anticreasing agent was included to the artificial wastewater RY84 in particular amounts -0, 0,3; 0,6; I g/1 -of dyebath concentration. From the obtained data of a11ificial wastewater decolourisation under UV /H 2 0 2 -AOP the reaction rate constant decrease was observed (see Figure 4 and Table 4).   Table 4).
The explanation could be as follows: anticreasing agent thickens the solution (it becomes of colloidal manner), then the UV radiant is dissipated [ 17), Therefore the dye molecules are not reached by UV light properly, because of are not decomposed completely. The artificial wastewater with RY84 was prepared with the full amount of salts (NaCl; Na 2 C0 3 ) . Artificial wastewater decolourisation was investigated by UV/H202 -AOP with different salts present in solution. The amounts applied in the experiment nomially are applied in the industrial dyeing process, The artificial wastewater decolourisation perfom1ance under UV/H2O2 -AOP was investigated and inhibition effect was observed in comparison with no salts present in decolorized artificial wastewater samples.
The analysis was perfonned with one or both salts in ordinary inlet concentrations (process performance is given in Figure 5 and summarized in Table 5).
]'. The highest oxidation reaction rate constant value 0.4436 min· 1 is obtained with the artificial wastewater No. I (RY84 with no salts added). When NaCl is added (artificial wastewater No. 8) the reaction rate decreases insignificantly (see Table 5). It could be posted that NaCl presence in dyeing wastewater has certain inhibition to the UV /H 2 O 2 -AOP.
The significant inhibition of UV /H2O2 -AOP is observed with added Na2CO,, the decrease in reaction rate constant is more than IO times (see Table 5).  I 0), the reaction rate is bigger than of sample No. 9, in which only Na 2 CO 3 (full concentration) is present. As CO/, er ions in literature [ 14] is derived as main process inhibitors. Though difference in decole ourisation process perfonnance when both Na 2 CO3 and NaCl (sample No. I 0) are present in the solution, it could be explained by salts influence on the pH level of the liquid, The process inhibition appears because of the changed H 2 O 2 equilibrium by means of change in pH level, The inhibition effect obtained in the experiment is significant as it was also found in literature (4, 21 , 22, 23] ,

D) Hypothesis of pH inhibition was investigated.
Two artificial wastewater samples were prepared at the same manner, only in one sample instead ofiN a 2 C0 3 (to avoid the CO/ inhibition), N aOH was added (corresponding amount to adjiust the same pH level in artificial wastewater), It is observed from experimental results (see Figure 6) that the reaction rate constants of both a11i ficial wastewaters decolourisation under UV/H2O2 advanced oxidation process reactions are very similar. Therefore it can be stated that the UV /H2O 2 -AOP reaction is inhibited of high pH level of decolorized liquid,

CONCLUSIONS
In this exiperimental work, decolouriation of artificial wastewater polluted with three type commercial dyes (RR84, RR 1 41 , RB 1 71 ) was investigated by using UV /H2O2 advanced oxidation process, Influence of operating parameters, additional substances, pH value were investigated and evaluated by means of change in reaction rate.
The fastest decolourisation in a11ificial wastewaters RY84 and RR 1 41 under UV/H2O2 advanc ed oxidation process (reaction rate constant values -0.4436 min -I and 0. 3253 min -I respectively) was obtained. The slowest decolourisation was obtained in artificial wastewater MIX (mixture of three types of dyes), where the obtained reaction rate constant was twice lower than with RY 84. From the obtained results it can be concluded that UV/H2O2 process decolourisation depends on the dye type present in the wastewater.
Anticreasing agent presence in artificial wastewater concentration above 0.3 g/e 1 (e.g.30%) sample No. 9 and sample No. IO has the significant inhibition to the UV/H2O 2 advanced oxidation process. The higher concentration of anticreasing agent makes the liquid of colloidal manner and inhibits the decolourisation process perfom1ance, the slower reaction rate occurs and half of the dye concentration remains after 15 minutes of decolourisation.
The Cr; CO/ ions presence in the solution has significant influence to pH level in the process, though no direct inhibition on UV /H 2 O 2 advanced oxidation process is observed.
The significant inhibition on UV/H 2O2 advanced oxidation process kinetics has high pH level, as the pH in the solution has great influence to H2O2 equilibrium.
It can be assumed that solution acidification prior to the UV/H 2 O 2 advanced oxidation process application is beneficial, this way the higher effectiveness of the process can be reached. In order to ascertain if the acidification can improve the UV/H 2O2 advanced oxidation process, further investigation should be perfonned.