- Email: [email protected]
A combined electrochemical-irradiation treatment of highly colored and polluted industrial wastewater
Radiation Physics and Chemistry 67 (2003) 657–663
A combined electrochemical-irradiation treatment of highly colored and polluted industrial wastewater * * b, E. Camposa, C. Barrera-D!ıaza,*, F. Urena-Nu nez M. Palomar-Pardave! c, M. Romero-Romoc a
! ! interseccion, ! Paseo Tollocan S/N, Facultad de Qu!ımica, Universidad Autonoma del Estado de M!exico, Paseo Colon C.P. 50120, Toluca, Estado de M!exico, Mexico b Instituto Nacional de Investigaciones Nucleares, Km 36.5 Carretera, Toluca, Mexico c ! Departamento de Materiales, Universidad Autonoma Metropolitana-Aztcapotzalco, Av. San Pablo 180 Col, Reynosa-Tamaulipas, C.P. 02200. Mexico, D.F Received 14 August 2002; accepted 1 December 2002
Abstract This study reports on the attainment of optimal conditions for two electrolytic methods to treat wastewater: namely, electrocoagulation and particle destabilization of a highly polluted industrial wastewater, and electrochemically induced oxidation induced by in situ generation of Fenton’s reactive. Additionally, a combined method that consisted of electrochemical treatment plus g-irradiation was carried out. A typical composition of the industrial effluent treated was COD 3400 mg/l, color 3750 Pt/Co units, and fecal coliforms 21000 MPN/ml. The best removal efficiency was obtained with electrochemical oxidation induced in situ, that resulted in the reduction of 78% for the COD, 86% color and 99.9% fecal coliforms removal. A treatment sequence was designed and carried out, such that after both electrochemical processes, a g-irradiation technique was used to complete the procedure. The samples were irradiated with various doses in an ALC g-cell unit provided with a Co-60 source. The removal efficiency obtained was 95% for the COD values, 90% color and 99.9% for fecal coliforms. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Industrial wastewater treatment; Electrocoagulation; Irradiation
1. Introduction Decoloration of wastewater is a well-known technical problem that has been the object of considerable efforts, particularly on consideration of the technological diversity which has been made available to solve the difficulties associated. Colored wastewater is not only an inherent feature of textile discharges; it is produced also in other industrial operations such as *Corresponding author. Tel.: +52-55-7212-3823; fax: +5255-72117-3890. E-mail addresses: [email protected] (C. Barrera-D!ıaz), * * [email protected] (F. Urena-Nu nez), [email protected]. uam.mx (M. Palomar-Pardav!e).
coffee absorption, yeast preparation and edible oil refinery. The concentration of dye may be significantly lower than 1 ppm; however, it can remain visible even at such small concentrations, provoking that the transparency of the discharge streams can be noticeably reduced. Strong color and turbidity of the wastewater effluents are particularly troublesome because of its negative visual impact. Because dyes absorb sunlight, plants in sewage streams may perish; thus the ecosystem of streams could be seriously affected. Moreover, chronic exposure to some colorants and their intermediates has been associated to allergy affections, and to a lesser extent, with carcinogenicity effects (Horng and Huang, 1993; Lin and Lai, 2000).
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(02)00497-8
658
C. Barrera-D!ıaz et al. / Radiation Physics and Chemistry 67 (2003) 657–663
Water decoloration can be achieved by several physical–chemical and biological methods. ‘‘Oxidizing technologies’’ such as fotocatalitic UV/TiO2, UV/H2O2, a- and b-radiation and sonic radiation have been developed to process dangerous chemicals in water sewage (Laszlo, 1997; Balanosky et al., 2000). These methods gained importance a few years ago and were considered promising for the water decontamination, mainly for bleaching (Groff, 1992). Oxidative degradation, including ozonation, UV irradiation and chlorination are effective to treat wastewater that does not contain a large amount of impurities. Recently, electrochemical technology has been reported to provide an environmentally sound method for reducing, not only color, but to remove at the same time BOD, COD, total suspended solids, and heavy metal pollution from sewage (Lang et al., 1998; Zhaing and Rusling, 1993). There are a number of electrochemical options such as electro-osmosis, in which the electricity applied drives migration of dissolved ions in water, forcing them to move through capillaries. The technique is advantageously used in conjunction with pressure filtration, which allows application of aerobic and anaerobic treatment of sewage that contains dissolved solids and to establish a relationship between dosage, voltage and quantity of filtered solids (Gingerich et al., 1999). Similarly, the principle is used to remove pollutants from specific sites, using electrodialysis that employs low-level electrical currents as cleaning agent. Good removal rates for metal ions as well as organic compounds have been achieved (i.e. oxalic acid), and of a sewage that contains solid pollutants (Lin and Chen, 1997). Electrochemical methods have been used as chemical coagulation processes to remove color and cloudiness from colored industrial wastewater. In this regard, it has been observed that the electrochemical process generated numerous flocs, which permitted to attain high efficiency in clearing wastewater (Riveiro et al., 2000). The electrochemical technologies have attracted a great deal of attention because of their versatility and environmental compatibility, which makes the treatments of liquids, gases and solids possible. In fact, the main reagent is the electron, which is a ‘‘clean reagent’’ (Rajeshwar et al., 1994). The utilization of 60Co-g-irradiation in wastewater treatment is drawing much more attention around the world, because it can be applied to treat some organic systems which are quite difficult to degrade biologically, and does not cause secondary pollution: there is no trace of polluted materials left in the wastewater after final degradation. Moreover, the irradiation technique is especially effective for bacteria sterilization and killing (Panizza et al., 2000; Lipczynska-Kochany et al., 1995). In this study, we combined electrocoagulation and electro-oxidation techniques with ionizing radiation as a polishing step, seeking to design and implement a
reliable method for wastewater treatment. Presently, it would be desirable to have reliable data concerning the use of irradiation, so that in future it may be used safely for applications in environmental protection in Mexico. The wastewater sample used in the present study was taken from the sewage stream of an industrial complex.
2. Methods 2.1. Predominance-zone diagrams in solution chemistry Predominance-zone diagrams (PZDs) are presented as important aids in the comprehension of aqueous solution chemistry by applying a general donor/acceptor/particle treatment (Rojas-Hern!andez et al., 1991). In our study, this technique was used to analyze the stability of the intermediate oxidation state of iron hydrocomplexes as a function of pH. Furthermore, equilibrium conditions among dissolved and insoluble chemical aqueous solutions and possible chemical species in the solid phase are described. 2.2. Electrochemical iron reactor The reactor consisted of a cylindrical vessel and a shaft supporting a series of plain carbon steel discs. This shaft was connected to a variable speed motor for which different velocities were possible. Seven discs were connected as cathodes and seven as anodes. The ones working as anode bore a plain surface, while the cathodes had four paddle-like projections to promote turbulence and enhance mass transfer. A bucket with four baffles served as reservoir. Each electrode had an area of 65.97 cm2; thus, the total electrode surface, Ae ; was 461.82 cm2. The volume of the liquid treated each time was 5 dm3. A dc source was used to supply the system with 9 V and 5 A. 2.3. Wastewater samples treated by the three methods 1. Electrocoagulation: The specified volume of wastewater was poured into the reactor’s reservoir, then the electrodes were introduced in the reservoir and the current applied. After elapsing various time periods, samples were taken. Finally, the pH was adjusted to 10 (using Ca(OH)2, 1 M) and the remnant liquid filtered. 2. Electro-oxidation: The reservoir was filled with a specified volume of industrial wastewater, the pH adjusted to 3 (using H2SO4, 1 M). Then 1 ml of H2O2 (30% v/v) was added to produce the electroFenton’s reagent in situ. A current of 5 A and a potential difference of 7 V were supplied. Samples were taken at specified times. At the end of the treatment, the pH
C. Barrera-D!ıaz et al. / Radiation Physics and Chemistry 67 (2003) 657–663 10 Fe(OH)2 Fe(OH)
+
6 Fe2+ General Equilibrium
4 Representative Equilibrium
2
According to AWWA standards (APHA, AWWA, 1995), the values of the process parameters such as COD, color and fecal coliforms of all samples were measured before and after each treatment.
Fe(OH)3-
8
p Fe (II)
was adjusted to 7–8 (using Ca(OH)2, 1 M) and the liquid in the vessel was filtered. 3. g-Irradiation: This treatment solely consisted in irradiating raw wastewater samples in an ALC-g-cell provided with a Co-60 source, applying different doses. Finally, a combination of both, electrochemical plus irradiation processes was performed.
659
Fe(OH)42Fe(OH)2(s)
0 -2 0
2
4
6
8
10
12
14
pH
2.4. Sludge characterization The sludge produced by the electrochemical processes (electrocoagulation and electroFento’s reactive) was analyzed by scanning electron microscopy (SEM) and X-ray microanalysis. The analysis was performed on a Phillips XL-30 microscope to observe the composition and configuration of the structure. SEM provides images of rough material with resolution at fractions of a micrometer, while energy disperse X-ray spectroscopy offers in situ chemical analysis of the bulk.
3. Results and discussion 3.1. Electrocoagulation The wastewater sample was treated with the electrochemical reactor generating an in situ coagulant which destabilized colloidal pollutant particles present in the wastewater sample. Fig. 1 shows the relationship of the COD removal and electrolysis time. The COD of
Fig. 2. PZD for Fe(II) chemical species in aqueous solution. Note that, in this case, the straight line represents the solubility equilibrium of Fe(OH)2(s) and the dotted line represents the predominance limits among soluble chemical species.
wastewater decreases as a function of elapsed time. After 25 min treatment, the COD value reaches a minimum; a 68% COD removal was achieved with this condition. The data for the iron aqueous species present in the media are given in Fig. 2. The electrocoagulation process occurs in the pH range 8.5–10. Soluble iron forms hydroxocomplexes that act as particle destabilization agents and promote removal of the pollutant through solid phase precipitated, liable to be separated with ease (sludge). The general reactions that take place are: At the anode: FeðsÞ -Fe2þ ðaqÞ þ2e :
At the cathode: 2H2 OðlÞ þ2e -H2ðgÞ þ2OH ðaqÞ :
4000
Overall: FeðsÞ þ2H2 OðaqÞ -Fe2þ ðaqÞ þH2ðgÞ þ2OHðaqÞ :
3500
With the adequate energy supply, it becomes possible to add the exact amount of coagulant in order to promote destabilization of the pollutant (particle) and finally, the aggregation of the induced flocs, while minimizing the hydrogen evolution. Fig. 2 shows the pH conditions that would favor formation of iron-based hydroxocomplexes that can absorb pollutants and coprecipitate, thus aiding to improve water quality.
COD / mgL-1
3000
2500
2000
1500
1000
500
3.2. Electro-oxidation (electroFenton’s reagent)
0 0
5
10
15
20
25
30
Time / Min
Fig. 1. Pollutant removal rate using iron electrodes as an in situ coagulant in industrial wastewater (B) and using electroFenton (’).
In this case, the wastewater sample was treated with the same electrochemical reactor; however, it was required to generate a strong oxidizing agent in situ, (Fenton’s reagent (Fe/H2O2)) by adjusting the pH to 3
C. Barrera-D!ıaz et al. / Radiation Physics and Chemistry 67 (2003) 657–663
660
8
Fe(OH)2+
7
-
Fe(OH)4 Fe(OH)3
6
pFe (III)
5 4
Fe (OH)
3
2+
3+
Fe
2 1
Fe(OH) 3(s)
0 -1 -2 0
2
4
6
8
10
12
14
pH Fig. 3. PZD for Fe(III) chemical species in aqueous solution. Note that, in this case, the straight line represents the solubility equilibrium of Fe(OH)3(s) and the dotted line represents the predominance limits among soluble chemical species.
d þ 3þ H2 O2ðaqÞ þFe2þ ðaqÞ þHðaqÞ -FeðaqÞ þH2 OðlÞ þOHðaqÞ ;
4000
3500
-1
3000
2500
COD / mgL
and adding 5 ml of H2O2. Fig. 1 shows the relationship between the COD removal and electrolysis time. The wastewater COD decreases with time, as indicated before; however, in this case a COD removal of 78% was achieved. The reactions taking place are as follows:
2000
1500
2þ d þ Fe3þ ðaqÞ þH2 O2ðaqÞ -FeðaqÞ þ O2 HðaqÞ þHðaqÞ ; 1000
Fe3þ ðaqÞ
þ
d
þ O2 HðaqÞ -Fe2þ ðaqÞ þO2ðgÞ þHðaqÞ ;
H2 O2ðaqÞ þOHdðaqÞ -H2 OðlÞ þd O2 HðaqÞ ; d 3 OHdðaqÞ þFe2þ ðaqÞ -FeðaqÞ þOHðaqÞ ; d
500
0 0
5
10
15
20
25
30
35
40
Dose / kGy
Fig. 4. Pollutant removal at different g-irradiation doses.
O2 HðaqÞ þH2 O2ðaqÞ -H2 OðlÞ þOHdðaqÞ þO2ðgÞ :
This technique has been successfully used in the oxidation of chlorinated compounds (Panizza et al., 2000). The iron chemical aqueous species in the media are presented in Fig. 3. The electroFenton process occurs at pH 3, to favor a condition where soluble iron reacts as described above. After treatment, the pH of the water sample is adjusted in the range 7–8, to induce formation of a solid phase (sludge), as observed in Fig. 3. 3.3. g-irradiation Samples of wastewater were exposed to g-radiation with different doses, as can be observed in Fig. 4. Note that there is not a linear relationship between the radiation dose and the COD values. This agrees with recent research using industrial wastewater (Bao et al., 2002). The radiation effects can be limited due to the colloidal matter that contributes to the high color present in the wastewater, that remain after the
electrocoagulation process. On the other hand, when electro-oxidation is applied prior to the g-irradiation there are less pollutants left in aqueous solution giving a low colored water and enhancing the radiation effects in the wastewater. Table 1 shows a summary of the reduction in the range of values for the parameter selected when the electrocoagulation, electro-oxidation and g-radiation are applied to the raw industrial wastewater. It can be observed from Table 1, that the treatments reduced the amount of pollutants present in the industrial wastewater. The electro-oxidation technique reduces color by 90%, the COD by 77% and 99% fecal coliforms. g-radiation also diminishes the value of the selected parameters, for instance, color by 76%, COD by 60% and 99% for bacteria. 3.4. Combined treatments In order to increase removal of the pollutant from wastewater, a set of combined experiments was carried
C. Barrera-D!ıaz et al. / Radiation Physics and Chemistry 67 (2003) 657–663
661
Table 1 Final values of the three treatments on color, turbidity and fecal coliforms after process Treatment
Color (Pt–Co)
Turbidity (NFU)
COD (mg/l)
Fecal coliforms (MPN/100 ml)
Crude wastewater Electrocoagulation Electro-oxidation g-Radiation
3750710 500710 35075 800710
14075 2074 1472 4675
3400725 1050720 770718 1378732
21,000 o10 o3 o3
1200
1000
COD / mgL
-1
800
600
400
200
0 0
5
10
15
20
25
30
35
40
trical appearance, which is most likely due to the fact that the particles are somewhat consolidated when filtration is effected, does compelling them to form amorphous larger pieces, that can easily break apart without displaying crystallographic features, such as cleavage planes or facets. Table 2 shows the composition of the structures shown in Fig. 6. The presence of carbon and oxygen suggests these belong to organic compounds present in the wastewater that were adsorbed on the surface of the colloids that were destabilized and precipitated. Note that Ca and S indicate the formation of CaSO4(s) due to pH adjustment. Also, the Fe peak confirms that iron was in the aqueous solution inducing particle destabilization and oxidation.
Dose / KGy
Fig. 5. COD removal using electrocoagulation (~) and electroFenton (’) at different radiation doses.
out that consisted in applying first the electrochemical means and second the g-radiation. Fig. 5 shows a plot depicting removal of the pollutant as different radiation doses were applied to previously treated wastewater, using electrocoagulation and electro-oxidation techniques. Fig. 5 also shows the COD behavior when radiation is the polishing step in wastewater treatment. Note that when radiation is applied to previously treated water, after electrocoagulation has taken place, there does not appear a linear trend in the COD removal. This can be due to the presence of dissolved particles that still remain after the electrocoagulation process has been applied. On the other hand, when radiation is applied to water treated by electro-oxidation, it is observed that for all dosages there is a reduction in the COD value. For the electrocoagulation case, there is a 70% reduction at 25 kGy whereas a 80% is reached for the electrooxidation treatment at 35 kGy. The overall reduction efficiency for electrocoagulation is 91% and for electrooxidation is 95%. 3.5. Sludge morphology and X-ray analysis Fig. 6 shows the morphology of the sludge generated by the electrocoagulation and electro-oxidation. In both cases, the aggregates observed showed a similar geome-
4. Conclusions The use of electrochemical methods has proved to be effective to reduce significantly the amount of pollutants present in industrial highly colored wastewater. The main mechanism responsible for particle destabilization is ascribed to the liberation of iron ions into the solution containing the pollutants, which under the pH conditions established as a result of the thermodynamic study, gave rise to hydrocomplexed chemical species capable of fixing the contaminants. Then electrocoagulation ensued, and the overall solubility of the newly formed particles suspension, being a function of pH, was manipulated to enable their separation from the aqueous phase, by means of precipitation. The oxidation conditions attained by the electrooxidation process, of the organic components contained in the wastewater permitted to obtain lower COD values, compared to those obtained using electrocoagulation. g-irradiation reduced the COD values when applied directly to raw wastewater. The radiation dose did not show a linear relationship with the COD reduction values. The best conditions for treatment result when electrooxidation and g-irradiation were applied in sequence. Electro-oxidation removed all colloidal particles present in raw wastewater, and also oxidized pollutants and removed a significant amount of the initial color present in the aqueous solution. g-irradiation interacted with the
C. Barrera-D!ıaz et al. / Radiation Physics and Chemistry 67 (2003) 657–663
662
(b)
(a)
Fig. 6. Microphotographs of the sludge generated using electrochemical means, (a) Electrocoagulation and (b) ElectroFenton.
Table 2 Results from the X-ray microanalysis in the sludge in the case of both electrochemical treatments Element
Electrocoagulation element (%)
ElectroFenton element (%)
C O Ca Si S K Fe
42.8871.25 34.5770.70 7.9370.89 0.3771.01 7.4371.00 2.0770.99 4.7570.78
37.3871.15 28.8771.70 8.9170.89 0.2771.00 12.3271.34 6.6270.99 5.6370.57
remaining dispersed pollutants in water and acted upon them allowing further COD reduction. Using electrochemical means combined with g-irradiation in industrial wastewater treatment process resulted in an improved water quality method as compared with biological methods alone.
Acknowledgements The authors wish to acknowledge the support given by CONACYT and both: the Universidad Autonoma Metropolitana-Azcapotzalco, in particular, to the Departamento de Materiales (Project 2260220), and the Universidad Autonoma del Estado de Mexico, specially the Facultad de Qu!ımica (Project 1392/2000).
References APHA, AWWA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th Edition.
American Public Health Association, Washington, DC, USA. ! Balanosky, E., Herrera, F., Lopez, A., Kiwi, J., 2000. Oxidative degradation of textile wastewater. Modelling reactor performance. Water Res. 34, 582–596. Bao, H., Liu, Y., Jia, H., 2002. A study of irradiation in the treatment of wastewater. Radiat. Phys. Chem. 63, 633–635. Gingerich, L., Nuefeld, R.D., Tood, A.T., 1999. Electroosmotically enhanced sludge pressure filtration. Water Environ. Res. 71, 267–276. Groff, K.A., 1992. Textile waste. Water Environ. Res. 64, 425–427. Horng, J., Huang, S., 1993. Removal of organic dye (direct blue) from synthetic wastewater by adsorptive bubble separation techniques. Environ. Sci. Technol. 27, 1169–1175. Lang, P.S., Ching, W.K., Willderg, D.M., Hoffman, M.R., 1998. Oxidative degradation of 2,4,6-trinitrotoluene by ozone in an electrohydraulic discharge reactor. Environ. Sci. Technol. 32, 3142–3146. Laszlo, J.A., 1997. Regeneration of dyes-saturated quaternized cellulose by bisulfate-mediated borohydrate reduction of dye azo groups: an improved process for decolorization of textile wastewaters. Environ. Sci. Technol. 31, 3647–3653. Lin, S.H., Chen, M.L., 1997. Treatment textile wastewater by chemical methods for reuse. Water Res. 31, 868–876. Lin, S.H., Lai, C., 2000. Kinetic characteristics of textile wastewater ozonation in fluidized and fixed activated carbon beds. Water Res. 34, 763–772. Lipczynska-Kochany, E., Sprah, G., Harms, S., 1995. Influence of some groundwater and surface waters constituents on the degradation of 4-chlorophenol by the fenton reaction. Chemosphere 30, 9–20. * Rajeshwar, K., Ibanez, J.G., Swaine, G.M., 1994. Electrochemistry and Environment. J. Appl. Electrochem. 24, 1077–1091. Riveiro, A.B., Mateus, E.P., Ottosen, L.M., Bech-Nielsen, G., 2000. Electrodialytic removal of Cu, Cr, and As from
C. Barrera-D!ıaz et al. / Radiation Physics and Chemistry 67 (2003) 657–663 chromated copper arsenate-treated timber waste. Environ. Sci. Technol. 34, 781–788. * Rojas-Hern!andez, A., Ram!ırez, M.T., Ib!anez, J.G., Gonz!alez, I., 1991. Construction of multicomponent pourbaix diagrams using generalized species. J. Electrochem. Soc. 138, 365–371.
663
Panizza, M., Bocca, C., Cerisola, G., 2000. Electrochemical treatment of wastewater containing polyaromatic organic pollutants. Water Res. 34, 2601–2605. Zhaing, S., Rusling, J.F., 1993. Metal ion removal by electrochemical means. Environ. Sci. Technol. 27, 1375–1382.
A combined electrochemical-irradiation treatment of highly colored and polluted industrial wastewater * * b, E. Camposa, C. Barrera-D!ıaza,*, F. Urena-Nu nez M. Palomar-Pardave! c, M. Romero-Romoc a
! ! interseccion, ! Paseo Tollocan S/N, Facultad de Qu!ımica, Universidad Autonoma del Estado de M!exico, Paseo Colon C.P. 50120, Toluca, Estado de M!exico, Mexico b Instituto Nacional de Investigaciones Nucleares, Km 36.5 Carretera, Toluca, Mexico c ! Departamento de Materiales, Universidad Autonoma Metropolitana-Aztcapotzalco, Av. San Pablo 180 Col, Reynosa-Tamaulipas, C.P. 02200. Mexico, D.F Received 14 August 2002; accepted 1 December 2002
Abstract This study reports on the attainment of optimal conditions for two electrolytic methods to treat wastewater: namely, electrocoagulation and particle destabilization of a highly polluted industrial wastewater, and electrochemically induced oxidation induced by in situ generation of Fenton’s reactive. Additionally, a combined method that consisted of electrochemical treatment plus g-irradiation was carried out. A typical composition of the industrial effluent treated was COD 3400 mg/l, color 3750 Pt/Co units, and fecal coliforms 21000 MPN/ml. The best removal efficiency was obtained with electrochemical oxidation induced in situ, that resulted in the reduction of 78% for the COD, 86% color and 99.9% fecal coliforms removal. A treatment sequence was designed and carried out, such that after both electrochemical processes, a g-irradiation technique was used to complete the procedure. The samples were irradiated with various doses in an ALC g-cell unit provided with a Co-60 source. The removal efficiency obtained was 95% for the COD values, 90% color and 99.9% for fecal coliforms. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Industrial wastewater treatment; Electrocoagulation; Irradiation
1. Introduction Decoloration of wastewater is a well-known technical problem that has been the object of considerable efforts, particularly on consideration of the technological diversity which has been made available to solve the difficulties associated. Colored wastewater is not only an inherent feature of textile discharges; it is produced also in other industrial operations such as *Corresponding author. Tel.: +52-55-7212-3823; fax: +5255-72117-3890. E-mail addresses: [email protected] (C. Barrera-D!ıaz), * * [email protected] (F. Urena-Nu nez), [email protected]. uam.mx (M. Palomar-Pardav!e).
coffee absorption, yeast preparation and edible oil refinery. The concentration of dye may be significantly lower than 1 ppm; however, it can remain visible even at such small concentrations, provoking that the transparency of the discharge streams can be noticeably reduced. Strong color and turbidity of the wastewater effluents are particularly troublesome because of its negative visual impact. Because dyes absorb sunlight, plants in sewage streams may perish; thus the ecosystem of streams could be seriously affected. Moreover, chronic exposure to some colorants and their intermediates has been associated to allergy affections, and to a lesser extent, with carcinogenicity effects (Horng and Huang, 1993; Lin and Lai, 2000).
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(02)00497-8
658
C. Barrera-D!ıaz et al. / Radiation Physics and Chemistry 67 (2003) 657–663
Water decoloration can be achieved by several physical–chemical and biological methods. ‘‘Oxidizing technologies’’ such as fotocatalitic UV/TiO2, UV/H2O2, a- and b-radiation and sonic radiation have been developed to process dangerous chemicals in water sewage (Laszlo, 1997; Balanosky et al., 2000). These methods gained importance a few years ago and were considered promising for the water decontamination, mainly for bleaching (Groff, 1992). Oxidative degradation, including ozonation, UV irradiation and chlorination are effective to treat wastewater that does not contain a large amount of impurities. Recently, electrochemical technology has been reported to provide an environmentally sound method for reducing, not only color, but to remove at the same time BOD, COD, total suspended solids, and heavy metal pollution from sewage (Lang et al., 1998; Zhaing and Rusling, 1993). There are a number of electrochemical options such as electro-osmosis, in which the electricity applied drives migration of dissolved ions in water, forcing them to move through capillaries. The technique is advantageously used in conjunction with pressure filtration, which allows application of aerobic and anaerobic treatment of sewage that contains dissolved solids and to establish a relationship between dosage, voltage and quantity of filtered solids (Gingerich et al., 1999). Similarly, the principle is used to remove pollutants from specific sites, using electrodialysis that employs low-level electrical currents as cleaning agent. Good removal rates for metal ions as well as organic compounds have been achieved (i.e. oxalic acid), and of a sewage that contains solid pollutants (Lin and Chen, 1997). Electrochemical methods have been used as chemical coagulation processes to remove color and cloudiness from colored industrial wastewater. In this regard, it has been observed that the electrochemical process generated numerous flocs, which permitted to attain high efficiency in clearing wastewater (Riveiro et al., 2000). The electrochemical technologies have attracted a great deal of attention because of their versatility and environmental compatibility, which makes the treatments of liquids, gases and solids possible. In fact, the main reagent is the electron, which is a ‘‘clean reagent’’ (Rajeshwar et al., 1994). The utilization of 60Co-g-irradiation in wastewater treatment is drawing much more attention around the world, because it can be applied to treat some organic systems which are quite difficult to degrade biologically, and does not cause secondary pollution: there is no trace of polluted materials left in the wastewater after final degradation. Moreover, the irradiation technique is especially effective for bacteria sterilization and killing (Panizza et al., 2000; Lipczynska-Kochany et al., 1995). In this study, we combined electrocoagulation and electro-oxidation techniques with ionizing radiation as a polishing step, seeking to design and implement a
reliable method for wastewater treatment. Presently, it would be desirable to have reliable data concerning the use of irradiation, so that in future it may be used safely for applications in environmental protection in Mexico. The wastewater sample used in the present study was taken from the sewage stream of an industrial complex.
2. Methods 2.1. Predominance-zone diagrams in solution chemistry Predominance-zone diagrams (PZDs) are presented as important aids in the comprehension of aqueous solution chemistry by applying a general donor/acceptor/particle treatment (Rojas-Hern!andez et al., 1991). In our study, this technique was used to analyze the stability of the intermediate oxidation state of iron hydrocomplexes as a function of pH. Furthermore, equilibrium conditions among dissolved and insoluble chemical aqueous solutions and possible chemical species in the solid phase are described. 2.2. Electrochemical iron reactor The reactor consisted of a cylindrical vessel and a shaft supporting a series of plain carbon steel discs. This shaft was connected to a variable speed motor for which different velocities were possible. Seven discs were connected as cathodes and seven as anodes. The ones working as anode bore a plain surface, while the cathodes had four paddle-like projections to promote turbulence and enhance mass transfer. A bucket with four baffles served as reservoir. Each electrode had an area of 65.97 cm2; thus, the total electrode surface, Ae ; was 461.82 cm2. The volume of the liquid treated each time was 5 dm3. A dc source was used to supply the system with 9 V and 5 A. 2.3. Wastewater samples treated by the three methods 1. Electrocoagulation: The specified volume of wastewater was poured into the reactor’s reservoir, then the electrodes were introduced in the reservoir and the current applied. After elapsing various time periods, samples were taken. Finally, the pH was adjusted to 10 (using Ca(OH)2, 1 M) and the remnant liquid filtered. 2. Electro-oxidation: The reservoir was filled with a specified volume of industrial wastewater, the pH adjusted to 3 (using H2SO4, 1 M). Then 1 ml of H2O2 (30% v/v) was added to produce the electroFenton’s reagent in situ. A current of 5 A and a potential difference of 7 V were supplied. Samples were taken at specified times. At the end of the treatment, the pH
C. Barrera-D!ıaz et al. / Radiation Physics and Chemistry 67 (2003) 657–663 10 Fe(OH)2 Fe(OH)
+
6 Fe2+ General Equilibrium
4 Representative Equilibrium
2
According to AWWA standards (APHA, AWWA, 1995), the values of the process parameters such as COD, color and fecal coliforms of all samples were measured before and after each treatment.
Fe(OH)3-
8
p Fe (II)
was adjusted to 7–8 (using Ca(OH)2, 1 M) and the liquid in the vessel was filtered. 3. g-Irradiation: This treatment solely consisted in irradiating raw wastewater samples in an ALC-g-cell provided with a Co-60 source, applying different doses. Finally, a combination of both, electrochemical plus irradiation processes was performed.
659
Fe(OH)42Fe(OH)2(s)
0 -2 0
2
4
6
8
10
12
14
pH
2.4. Sludge characterization The sludge produced by the electrochemical processes (electrocoagulation and electroFento’s reactive) was analyzed by scanning electron microscopy (SEM) and X-ray microanalysis. The analysis was performed on a Phillips XL-30 microscope to observe the composition and configuration of the structure. SEM provides images of rough material with resolution at fractions of a micrometer, while energy disperse X-ray spectroscopy offers in situ chemical analysis of the bulk.
3. Results and discussion 3.1. Electrocoagulation The wastewater sample was treated with the electrochemical reactor generating an in situ coagulant which destabilized colloidal pollutant particles present in the wastewater sample. Fig. 1 shows the relationship of the COD removal and electrolysis time. The COD of
Fig. 2. PZD for Fe(II) chemical species in aqueous solution. Note that, in this case, the straight line represents the solubility equilibrium of Fe(OH)2(s) and the dotted line represents the predominance limits among soluble chemical species.
wastewater decreases as a function of elapsed time. After 25 min treatment, the COD value reaches a minimum; a 68% COD removal was achieved with this condition. The data for the iron aqueous species present in the media are given in Fig. 2. The electrocoagulation process occurs in the pH range 8.5–10. Soluble iron forms hydroxocomplexes that act as particle destabilization agents and promote removal of the pollutant through solid phase precipitated, liable to be separated with ease (sludge). The general reactions that take place are: At the anode: FeðsÞ -Fe2þ ðaqÞ þ2e :
At the cathode: 2H2 OðlÞ þ2e -H2ðgÞ þ2OH ðaqÞ :
4000
Overall: FeðsÞ þ2H2 OðaqÞ -Fe2þ ðaqÞ þH2ðgÞ þ2OHðaqÞ :
3500
With the adequate energy supply, it becomes possible to add the exact amount of coagulant in order to promote destabilization of the pollutant (particle) and finally, the aggregation of the induced flocs, while minimizing the hydrogen evolution. Fig. 2 shows the pH conditions that would favor formation of iron-based hydroxocomplexes that can absorb pollutants and coprecipitate, thus aiding to improve water quality.
COD / mgL-1
3000
2500
2000
1500
1000
500
3.2. Electro-oxidation (electroFenton’s reagent)
0 0
5
10
15
20
25
30
Time / Min
Fig. 1. Pollutant removal rate using iron electrodes as an in situ coagulant in industrial wastewater (B) and using electroFenton (’).
In this case, the wastewater sample was treated with the same electrochemical reactor; however, it was required to generate a strong oxidizing agent in situ, (Fenton’s reagent (Fe/H2O2)) by adjusting the pH to 3
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660
8
Fe(OH)2+
7
-
Fe(OH)4 Fe(OH)3
6
pFe (III)
5 4
Fe (OH)
3
2+
3+
Fe
2 1
Fe(OH) 3(s)
0 -1 -2 0
2
4
6
8
10
12
14
pH Fig. 3. PZD for Fe(III) chemical species in aqueous solution. Note that, in this case, the straight line represents the solubility equilibrium of Fe(OH)3(s) and the dotted line represents the predominance limits among soluble chemical species.
d þ 3þ H2 O2ðaqÞ þFe2þ ðaqÞ þHðaqÞ -FeðaqÞ þH2 OðlÞ þOHðaqÞ ;
4000
3500
-1
3000
2500
COD / mgL
and adding 5 ml of H2O2. Fig. 1 shows the relationship between the COD removal and electrolysis time. The wastewater COD decreases with time, as indicated before; however, in this case a COD removal of 78% was achieved. The reactions taking place are as follows:
2000
1500
2þ d þ Fe3þ ðaqÞ þH2 O2ðaqÞ -FeðaqÞ þ O2 HðaqÞ þHðaqÞ ; 1000
Fe3þ ðaqÞ
þ
d
þ O2 HðaqÞ -Fe2þ ðaqÞ þO2ðgÞ þHðaqÞ ;
H2 O2ðaqÞ þOHdðaqÞ -H2 OðlÞ þd O2 HðaqÞ ; d 3 OHdðaqÞ þFe2þ ðaqÞ -FeðaqÞ þOHðaqÞ ; d
500
0 0
5
10
15
20
25
30
35
40
Dose / kGy
Fig. 4. Pollutant removal at different g-irradiation doses.
O2 HðaqÞ þH2 O2ðaqÞ -H2 OðlÞ þOHdðaqÞ þO2ðgÞ :
This technique has been successfully used in the oxidation of chlorinated compounds (Panizza et al., 2000). The iron chemical aqueous species in the media are presented in Fig. 3. The electroFenton process occurs at pH 3, to favor a condition where soluble iron reacts as described above. After treatment, the pH of the water sample is adjusted in the range 7–8, to induce formation of a solid phase (sludge), as observed in Fig. 3. 3.3. g-irradiation Samples of wastewater were exposed to g-radiation with different doses, as can be observed in Fig. 4. Note that there is not a linear relationship between the radiation dose and the COD values. This agrees with recent research using industrial wastewater (Bao et al., 2002). The radiation effects can be limited due to the colloidal matter that contributes to the high color present in the wastewater, that remain after the
electrocoagulation process. On the other hand, when electro-oxidation is applied prior to the g-irradiation there are less pollutants left in aqueous solution giving a low colored water and enhancing the radiation effects in the wastewater. Table 1 shows a summary of the reduction in the range of values for the parameter selected when the electrocoagulation, electro-oxidation and g-radiation are applied to the raw industrial wastewater. It can be observed from Table 1, that the treatments reduced the amount of pollutants present in the industrial wastewater. The electro-oxidation technique reduces color by 90%, the COD by 77% and 99% fecal coliforms. g-radiation also diminishes the value of the selected parameters, for instance, color by 76%, COD by 60% and 99% for bacteria. 3.4. Combined treatments In order to increase removal of the pollutant from wastewater, a set of combined experiments was carried
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Table 1 Final values of the three treatments on color, turbidity and fecal coliforms after process Treatment
Color (Pt–Co)
Turbidity (NFU)
COD (mg/l)
Fecal coliforms (MPN/100 ml)
Crude wastewater Electrocoagulation Electro-oxidation g-Radiation
3750710 500710 35075 800710
14075 2074 1472 4675
3400725 1050720 770718 1378732
21,000 o10 o3 o3
1200
1000
COD / mgL
-1
800
600
400
200
0 0
5
10
15
20
25
30
35
40
trical appearance, which is most likely due to the fact that the particles are somewhat consolidated when filtration is effected, does compelling them to form amorphous larger pieces, that can easily break apart without displaying crystallographic features, such as cleavage planes or facets. Table 2 shows the composition of the structures shown in Fig. 6. The presence of carbon and oxygen suggests these belong to organic compounds present in the wastewater that were adsorbed on the surface of the colloids that were destabilized and precipitated. Note that Ca and S indicate the formation of CaSO4(s) due to pH adjustment. Also, the Fe peak confirms that iron was in the aqueous solution inducing particle destabilization and oxidation.
Dose / KGy
Fig. 5. COD removal using electrocoagulation (~) and electroFenton (’) at different radiation doses.
out that consisted in applying first the electrochemical means and second the g-radiation. Fig. 5 shows a plot depicting removal of the pollutant as different radiation doses were applied to previously treated wastewater, using electrocoagulation and electro-oxidation techniques. Fig. 5 also shows the COD behavior when radiation is the polishing step in wastewater treatment. Note that when radiation is applied to previously treated water, after electrocoagulation has taken place, there does not appear a linear trend in the COD removal. This can be due to the presence of dissolved particles that still remain after the electrocoagulation process has been applied. On the other hand, when radiation is applied to water treated by electro-oxidation, it is observed that for all dosages there is a reduction in the COD value. For the electrocoagulation case, there is a 70% reduction at 25 kGy whereas a 80% is reached for the electrooxidation treatment at 35 kGy. The overall reduction efficiency for electrocoagulation is 91% and for electrooxidation is 95%. 3.5. Sludge morphology and X-ray analysis Fig. 6 shows the morphology of the sludge generated by the electrocoagulation and electro-oxidation. In both cases, the aggregates observed showed a similar geome-
4. Conclusions The use of electrochemical methods has proved to be effective to reduce significantly the amount of pollutants present in industrial highly colored wastewater. The main mechanism responsible for particle destabilization is ascribed to the liberation of iron ions into the solution containing the pollutants, which under the pH conditions established as a result of the thermodynamic study, gave rise to hydrocomplexed chemical species capable of fixing the contaminants. Then electrocoagulation ensued, and the overall solubility of the newly formed particles suspension, being a function of pH, was manipulated to enable their separation from the aqueous phase, by means of precipitation. The oxidation conditions attained by the electrooxidation process, of the organic components contained in the wastewater permitted to obtain lower COD values, compared to those obtained using electrocoagulation. g-irradiation reduced the COD values when applied directly to raw wastewater. The radiation dose did not show a linear relationship with the COD reduction values. The best conditions for treatment result when electrooxidation and g-irradiation were applied in sequence. Electro-oxidation removed all colloidal particles present in raw wastewater, and also oxidized pollutants and removed a significant amount of the initial color present in the aqueous solution. g-irradiation interacted with the
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(b)
(a)
Fig. 6. Microphotographs of the sludge generated using electrochemical means, (a) Electrocoagulation and (b) ElectroFenton.
Table 2 Results from the X-ray microanalysis in the sludge in the case of both electrochemical treatments Element
Electrocoagulation element (%)
ElectroFenton element (%)
C O Ca Si S K Fe
42.8871.25 34.5770.70 7.9370.89 0.3771.01 7.4371.00 2.0770.99 4.7570.78
37.3871.15 28.8771.70 8.9170.89 0.2771.00 12.3271.34 6.6270.99 5.6370.57
remaining dispersed pollutants in water and acted upon them allowing further COD reduction. Using electrochemical means combined with g-irradiation in industrial wastewater treatment process resulted in an improved water quality method as compared with biological methods alone.
Acknowledgements The authors wish to acknowledge the support given by CONACYT and both: the Universidad Autonoma Metropolitana-Azcapotzalco, in particular, to the Departamento de Materiales (Project 2260220), and the Universidad Autonoma del Estado de Mexico, specially the Facultad de Qu!ımica (Project 1392/2000).
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