Advanced Oxidation and Electrooxidation As Tertiary Treatment

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Advanced Oxidation and Electrooxidation As Tertiary Treatment Techniques to Improve the Purity of Tannery Wastewater P. Vijayalakshmi,† G. Bhaskar Raju,*,‡ and A. Gnanamani§ †

TamilNadu Pollution Control Board, 76, Mount Salai, Guindy, Chennai-600 032, India NML Madras Centre, CSIR Madras Complex, Taramani, Chennai-600 113, India § Central Leather Research Institute, Adyar, Chennai-600 020, India ‡

ABSTRACT: The option of electro-oxidation and advanced oxidation as tertiary treatment technique for the purification of tannery wastewater was explored. The TOC removal of 85% was achieved by UV/O3/H2O2 process, whereas it is hardly 50% by electrooxidation. However the power consumption to remove unit mass of TOC by electro-oxidation process was estimated to be 738 kW h/kg, which is ten times less than that of 7600 kW h/kg, required for advanced oxidation process. The kinetic data indicated that the degradation of organics by electro-oxidation is a current control process. To minimize the power consumption, we attempted a twostage process involving electro-oxidation in the first stage and advanced oxidation in the second stage. The results indicated that the TOC removal by advanced oxidation became sluggish, when the wastewater was processed initially by electro-oxidation. However, the effluents processed by EO were found to be completely disinfected.

1. INTRODUCTION Wastewater from the leather industry is known to contain a variety of chemicals such as sodium sulphite, basic chromium sulfate, wetting agents, bactericides, soda ash, CaO, sodium sulphide, ammonium chloride, NaCl, H2SO4, formic acid, enzymes, vegetable tannins, syntans, resins, poly urethane, dyes, fats and proteins, pigments, binders, waxes, lacquers, formaldehyde and variety of solvents and auxiliaries. Thus the tannery wastewater is a mixture of biogenic matter of hides and wide variety of organic and inorganic chemicals. Many of these materials are recalcitrant and not easily biodegradable. About 40 m3 of water and 700 kg of various chemicals are used to convert 1.0 metric tonne of hides or skins into leather. The concentration of heavy metals and toxic substances in the soil accumulated over the years had affected the fertility of the soil and altered the overall characteristics of the surface and groundwater. The groundwater regime in upper Palar basin in Tamilnadu has been severely contaminated due to discharge of effluents from a large number of tanneries. In some places, the concentration of total dissolved solids in groundwater is reported to be more than 8000 mg L1.1 In view of acute water shortage and to avoid percolation of contaminants to groundwater, strict environmental norms were imposed. Consequently, tanners in India have opted membrane technology for effective management and recycling of wastewater. Recently, electrochemical techniques are gaining importance for the treatment of wastewater containing dyes,2,3 hexavalent chromium,4 arsenic,5 and poly hydroxy phenols.6 The removal of organic pollutants from tannery effluents by electro-oxidation,79 and the influence of noble metals and metal oxides such as Ti/PtIr, Ti/PbO2, Ti/PdOCo3O4, and Ti/RbO2TiO2 as electrodes on the mineralization of organic pollutants was studied10 and suggested that electro-oxidation can be applied as a post treatment after the conventional biological process to remove the residual ammonia. The removal of proteins from tannery wastewater was observed to be significant using graphite r 2011 American Chemical Society

as anode.11 Photoelectrochemical- electrodialysis was attempted to recover water from tannery effluents.12 Recently, advanced oxidation processes (AOP) have been used to try to oxidize the organics present in textile and tannery wastewater.13 Degradation of xenobiotics originating from the textile preparation, dyeing, and finishing industry using ozonation and advanced oxidation was reported to be very effective.14 The bio degradability of textile wastewater was enhanced to 1.6 times by ozonation.15 Also, the ozonation was observed to be effective for decolorization of azo dye C.I Remazol Black 5.16 The UV/O3 process is expected to improve the treatment efficiency due to the activation of ozone molecules by UV photons and thereby facilitating the formation of hydroxyl radicals. The biodegradability of 1,4dioxane and its removal was improved in the presence of H2O2.17 The addition of H2O2 to the O3/UV process accelerates the decomposition of ozone and in turn generation of OH 3 radicals. However, H2O2 was found to affect the degradation adversely at higher concentration.18 In the present study, the option of electro-oxidation and advanced oxidation as tertiary treatment techniques was evaluated for the purification of tannery wastewater.

2. EXPERIMENTAL SECTION 2.1. Sample. Tannery wastewater was collected from a common effluent treatment plant situated at Pallavaram, Chennai. The wastewater generated from 150 tannery units, predominantly producing finished leathers from tanned leathers are connected to the common effluent treatment plant (CETP). The suspended solids are removed by chemical coagulation (primary treatment) Received: February 28, 2011 Accepted: August 2, 2011 Revised: July 13, 2011 Published: August 02, 2011 10194

dx.doi.org/10.1021/ie201039z | Ind. Eng. Chem. Res. 2011, 50, 10194–10200

Industrial & Engineering Chemistry Research

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Table 1. Physicochemical Characteristics of Tannery Wastewater, after AO and EO Treatment parameter

unit

pH

raw sample after AO

after EO

8.26

2.92

2.67

color conductivity

HU (μm hos/cm1)

500 9630

colorless 12 000

colorless 8840

TSS

(mg L1)

12

BDL

BDL

TDS

(mg L1)

7195

9020

chloride

(mg L1)

1497

1497

1161

sulfate

(mg L1)

1556

1194

853

TOC

(mg L1)

245

38

143

BOD

(mg L1)

110

15

32

TKN (mg L1) ammonical nitrogen (mg L1)

210 112

56 20

bacteria

2.6 106

BDL

(CFU)

BDL

and the supernatant water is subjected to biological treatment (secondary treatment). After biological treatment, the effluents are finally passed through an activated carbon filter (tertiary treatment) before disposal. The wastewater collected immediately after biological treatment was used in the present work. The characteristics of wastewater are given in Table 1. 2.2. Electrooxidation. The initial pH of the wastewater was adjusted to 7.0 and taken in a silica glass reactor with a working volume of 300 mL. The temperature of the reactor was maintained at 30 °C using thermo regulated water bath. Titanium mesh coated with oxides of IrO2 (25%), TaO2 (25%), and TiO2 (50%) was used as electrodes. The thickness of the coating is 8.0 μm. The electrodes were arranged in unipolar mode and the gap of 0.5 cm was maintained between the electrodes to minimize the ohmic losses. The effective surface area of anode was estimated to be 249.8 cm2. The same material with similar dimensions and surface area was used as cathode. The electrolysis experiments were conducted using a potentiostat/galvanostat system (ModelKM064, K-Pas Instronics Engineers, India). The electrodes were connected to the respective terminals of the potentiostat/galvanostat and energized for a required duration at a constant current. The voltage was found to vary between 3.0 and 8.0 V depending on the applied current. The kinetics of degradation was followed over a period of 6 h and the samples collected at different time intervals were analyzed for TOC. 2.3. UV Photoreactor. The radiation source (HML-LP 88) was a high pressure mercury vapor lamp which emits a monochromatic light at 365 nm. The sample holder is made of silica glass reactor with an inlet and outlet for ozone gas. 2.4. Ozone Generator. The corona discharge ozone generator (model L10G) supplied by Faraday Instruments, Coimbatore, India was used to carry out the experiments related to advanced oxidation. 0.3 L of wastewater with an initial pH of 7.0 was taken in the reactor and subjected to oxidation in the presence of UV/ O3 and UV/O3/H2O2. Ozone was passed through the wastewater at a constant flow rate. 2.5. Total Organic Carbon Analyzer (TOC). The degradation of organics was determined using TOC analyzer (Shimadzu VCSN/CPN Model). The homogenized diluted sample was injected in to reaction chamber packed with catalyst. The carbon is oxidized to CO2 in the reaction chamber and the CO2 gas generated is quantitatively estimated by nondispersive infrared analyzer. The TOC was deduced from the measurements of total carbon and inorganic carbon.

2.6. GC-MS Analysis. The organics present in the wastewater and the products formed during advanced oxidation process were analyzed using GC-MS-QP 2010 [Shimadzu]. The pH of the aqueous samples withdrawn from the photo reactor was adjusted to 3.0 using HNO3. The sample was transferred in to a separating funnel and 5.0 mL of diethyl ether was added to the aqueous phase. The organic and aqueous phase was thoroughly shaken for a period of 15 min and allowed to separate both organic and aqueous phase. 1.0 microliter of ether phase was injected in to GC-MS system equipped with DB-5 capillary column. The oven temperature was programmed to have initially 70 °C for 2.0 min and automatically increased to 300 °C at a rate of 10 °C per min and finally 10 min hold time at 300 °C. The reaction products were identified with the help of Wiley and NIST databases. 2.7. Chloride. The standard (Argentometric titration) method suggested by American Public Health Association was adopted for the estimation of Cl, free chlorine and microbial count.19 The pH of the solution was measured using pH meter and the concentration of chlorinated organic compounds was analyzed using AOX analyzer (EUROGLAS analytical Instruments, model ECS 1200).

3. RESULTS AND DISCUSSION 3.1. Degradation by Advanced Oxidation Process. Degradation of organics present in tannery wastewater by UV/O3 process was studied at various ozone flow rates. The ozone flow rate was varied from 1.0 to 5.0 g/h. At each ozone flow rate, samples were collected for every 30 min and analyzed for TOC. From the results shown in Figure 1, it is apparent that the rate of TOC depletion is drastic in the initial stage of oxidation and become marginal beyond one hour. Also, the rate of TOC depletion was found to increase by increasing the ozone from 1.0 g/h to 3.0 g/h. Maximum TOC depletion of 86% was obtained at ozone flow rate of 3.0 g/h. Further, most of the TOC (69%) was depleted within one hour of oxidation. The decrease in TOC depletion rate beyond 1 h may be attributed to shift in wastewater pH to 2.7 where the decomposition of ozone is inhibited. The slope changes in the kinetic profile could be explained due to the changes in the chemical nature of primary substances in solution forming persistent byproducts.20 The decomposition of ozone in the presence of UV radiation and H2O2 was well-established by earlier researchers. The generation of oxygen and hydrogen peroxide from ozone can be represented as

O3 þ hν f O2 þ O

ð1Þ

O3 þ H2 O þ hν f H2 O2 þ O2

ð2Þ

Also, the hydroxyl radicals are generated by self-decomposition of H2O2 in the presence of UV light and reaction between H2O2 and O3 and also by reaction of oxygen atom with water molecule according to the following reactions. H2 O2 þ hν f 2OH

ð3Þ

O3 þ H2 O2 f 2OH þ 3O2

ð4Þ

O þ H2 O f 2OH

ð5Þ

Experiments were conducted by varying the H2O2 concentration from 0.01 M to 0.06M. The ozone flow rate was maintained at 10195

dx.doi.org/10.1021/ie201039z |Ind. Eng. Chem. Res. 2011, 50, 10194–10200


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Figure 1. (a) Depletion of TOC at various ozone flow rates (UV/O3 process). Mineralization of organics fitted in (b) zero-order and (c) firstorder kinetic model.

Figure 2. (a) Depletion of TOC at various H2O2 concentrations (UV/ O3/H2O2 process). Mineralization of organics fitted in (b) zero-order and (c) first-order kinetic model.

1.0 g/h. The results presented in Figure 2 indicate that there is no improvement in the overall depletion of TOC. However, the rate of TOC removal was slightly improved at the initial stage by the addition of H2O2. It is known that the oxidation of carbon to CO2 is relatively easy when compared to dearomatization of the organic molecules. The decrease in TOC and pH to acidic region may be attributed to the formation of acidic compounds during mineralization. Though the mineralization and eventual removal of organic contaminants by advanced oxidation process is complex and

involves number of elementary chemical steps, the overall rate of TOC removal can often be described either by zero order or first order rate expressions.21 The degradation of organics at various ozone flow rates and H2O2 concentrations was analyzed in terms of zero, first and second order rate expressions. The kinetic data was found to fit in to zero order rate expression (Figures 1b and 2b). Because the goodness of fit (R2) for second order rate equation is poor, the graphs pertaining to second order rate equation were not presented. Generally, if the contaminant concentration 10196



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is high, the reaction rate will be zero order with respect to contaminant. The demarcation between high and low concentration varies considerably with the system but is often 100 mg/L. The power consumption for advanced oxidation (batch operation) was estimated using the following equation.21 EEM ¼

Pt1000 VMðCi Cf Þ

ð6Þ

Where EEM is electric energy in kW h required to bring about the degradation of a unit mass of contaminant in polluted water. P is rated power in kW, V is the volume of water treated in L, M is molar mass of carbon in g/mol, t is time in hours, Ci and Cf are initial and final concentration of contaminant expressed in mol. L1. Because the TOC removal is very fast in the initial stage and there is no appreciable change in the degradation beyond 1.0 h, the EEM was estimated for the first hour and it was found to be 7600 kW h/kg. The Kjeldahl nitrogen was found to be decreased to 56 mg/L from its initial value of 210 mg/L. The ammoniacal nitrogen was also estimated and found to be decreased to 20 mg/L from 112 mg/L. Above all, the wastewater was found to be free from microbes. The electrophilic reactions occur mainly in aqueous solutions where the concentration of aromatic compounds is very high. The aromatic compounds substituted with electronic donor groups such as OH and NH2 impart high electronic density on the carbon atoms situated in ortho and para positions. The nucleophilic reactions are predominant on organic compounds having electron withdrawing groups such as COOH and NO2. 3.2. Degradation by Electrooxidation. The degradation of organics by electro-oxidation process was carried out at different current densities over a period of 6 h and the results are presented in Figure 3ac. It could be seen that the TOC depletion is hardly 50% and the rate of depletion was slightly increased with current density. The kinetic data on TOC removal clearly suggest that the degradation of organics follow zero-order rate expression. This implies that electro-oxidation is current control process where the pollutant molecules arrive at the anode faster than the electrochemical production of oxidant. The concentration of Cl in the wastewater after 6 h of electrolysis was analyzed. It was observed that the Cl concentration was decreased to 1271 mg/L from its initial value of 1634 mg/L (22.22%). This clearly suggests discharge of chlorine gas from anode according to the following equation. 2Cl f Cl2 þ 2e

ð7Þ

Thus both Cl and organics are oxidized at the anode. At 25 °C and normal atmospheric pressure, the chlorine gas thus liberated from anode can dissolve in water to the extent of 6.413 g L1. If its solubility exceeds locally on the surface of the electrode, then the chlorine bubbles may form and escape from aqueous phase. Above pH 3.3, the diffused Cl2 gas will be in equilibrium between with HOCl and OCl. Because the experiments were conducted around neutral pH, entire chlorine will be in the form of HOCl and OCl species whose solubility is high in aqueous solution. Cl2 ðaqÞ þ H2 O f HOCl þ Hþ þ Cl

ð8Þ

Thus the contribution of active chlorine for the oxidation of organics is evident. Serikawa et al observed strong catalytic effect in the conversion of organic pollutants to innocuous CO2 and H2O in the presence of chloride ion.22

Figure 3. (a) Depletion of TOC at various current densities (electrooxidation process). Mineralization of organics fitted in (b) zero-order and (c) first-order kinetic model.

The degradation mechanism mainly depends on the concentration of the pollutants and applied current density. The concentration of TKN and ammoniacal nitrogen in wastewater was reduced during oxidation. Calza et al observed that the formation of NH4+ predominate over NO3 if the molecule contains extractable hydrogen.23 On the other hand, it is converted to nitrate ion when there is no extractable hydrogen. As long as the carbon holds extractable hydrogen, the degradation is expected to proceed mainly through hydroxyl attack on carbon 10197



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Figure 4. TOC removal by electro-oxidation followed by advanced oxidation process.

atom resulting in the release of NH4+ ion. The radical attack on the nitrogen atom and subsequent detachment of nitrate ions are favored if there is no extractable hydrogen. The oxidation of ammonia to molecular nitrogen is favorable in alkaline medium24 and by electrochlorination path. 2NH3 þ 6OH f N2 þ 6H2 O þ 6e

2NH3 þ 6Cl f N2 þ 6HCl þ 6e

ð9Þ ð10Þ

The estimated power consumption for electro-oxidation was found to be 738 kWh/kg. This value is approximately ten times less compared to advanced oxidation. The main drawbacks of electro-oxidation are: (1) difficult to achieve complete removal of organic pollutants; (2) kinetics of degradation is relatively slow. Better removal of TOC could be achieved by AOP though it consumes more power. 3.3. Degradation by Combination of EO and AOP. To minimize the power consumption, it was decided to adopt a twostage process involving electro-oxidation in the first stage followed by AOP in the second stage. Accordingly, the wastewater was subjected to electro-oxidation and 50% of the TOC was removed. In the second stage, oxidation was continued by UV/ O3 and the results are presented in Figure 4. The results indicate that the rate of TOC removal becomes very slow when the wastewater was treated initially by electro-oxidation. It took 6 h to accomplish 80% TOC removal from its initial value of 51% Where as it took just three hours to achieve 80% TOC removal directly by AOP. It may be due to the presence of stable refractory organic intermediates formed during electro-oxidation. Thus the AOP is not attractive to treat the effluents that were already subjected to electro-oxidation particularly in chloride medium. The samples collected after electro-oxidation were analyzed for chlorinated organic compounds and Coliform bacteria and found 164 micro grams per liter of chloro organics. This clearly suggests the formation of chloroorganics during electrooxidation process. However, the effluents processed by EO were found to be completely disinfected. The organic compounds present in the raw wastewater sample and dye metabolites formed during electro-oxidation were

Figure 5. GC-MS Spectra of pollutant molecules. (Raw effluent, Metabolites formed after 60 min of advanced oxidation and Organics formed at the end of 4 h of advanced oxidation).

Table 2. Physical and Organo-Laptic Properties of Leather by Using Treated Wastewatera tensile strength

212 kg/cm2

tongue tear strength

82 kg/cm

grain bursting strength

98 kg/cm

fullness

8 out of 10

tightness

6 out of 10

smoothness

6 out of 10

a

Bath composition: Basyntan DI (BASF), 2%; Relugan F (BASF), 2.5%; Relugan RE (BASF), 2%; Lipderm liquor FB II, 1.5%; Lipderm liquor SAF (BASF), 3%; Balmol SXE (Blamol & Lawrie, 1.5%; dye (Luganil Black ER, 1%; and formic acid, 1% (the chemicals are taken on the basis of weight %).

identified using GC-MS. Figure5 shows the gas chromatogram of the raw effluent, metabolites formed after 60 min of advanced oxidation and degradation products at the end of 4 h of advanced oxidation. The possible relative fragments that matches 90% or greater against the compounds listed in NIST and WILEY libraries were identified. The quantitative estimation of individual compounds was not attempted, as the respective standards are not easily available. However relative abundance and peak area shows the concentration of possible fragments. Major compounds observed in raw wastewater are p-benzoquinone, 2-nitro-hexane, 3,3-dimethyl-2-hexanone, 3-ethyl-3-heptanol, N-tert-butylacrylamide, 2,2-dimethyldecane, naphthalene, butyl diethylene glycol acetate, tricyclo-4,9-dodecadiene-3,6-dione, ammonium laurate, di-N- nonyl phthalate, isopropyl laurate, di-isobutyl phthalate, palmitic 10198


Industrial & Engineering Chemistry Research acid, N-butyl phthalate, docosane, tricosane, tetracosane, isooctyl phthalate. Within 1 h of advanced oxidation, many of the peaks seen in raw wastewater were disappeared. A new peak with a retention time of 2.214 min ascribed to octane occupying 56% of area was seen. However, the peaks related to different hexanes and phthalate compounds were intact. Thus, it is evident that these compounds are stable and remained intact during oxidation. After 4 h of UV/O3 oxidation most of the compounds were mineralized to CO2 and water, whereas peak corresponding to octane remained intact. 3.4. Reusability of Treated Water. The reusability of treated water was assessed by taking the same water for wet finishing of leather. Treated water was used for retanning, dyeing and fat liquoring in a single bath. The physical and organo-leptic properties of leather were assessed. The physical properties of leather shown in Table 2 clearly indicate that the reuse of the treated wastewater did not bring about any adverse change in the quality of leather. The leather was assessed by four experts for organolaptic properties viz fullness, tightness, and smoothness and found to be not altered on account of reuse of treated wastewater.

4. CONCLUSIONS The option of electro-oxidation and advanced oxidation as tertiary treatment technique for the purification of tannery wastewater was explored. The effectiveness of these techniques in terms of power consumption, reusability of treated wastewater and the quality of processed leather was evaluated. Maximum TOC removal of 86% was obtained at O3 flow rate of 3 g/h. The electric energy required to bring about the degradation of a unit mass of contaminant was found to be 7600 kW h/kg. The kinetic data of degradation of organics was analyzed with reference to zero and first order rate expressions. The degradation of organics by electro-oxidation was observed to be current control process where the pollutant molecules arrive at the anode faster than the electrochemical production of oxidant. The estimated power consumption for electro-oxidation was found to be 738 kW h/kg. The samples collected after electro-oxidation were analyzed for chlorinated organic compounds and Coliform bacteria. The results indicated the formation of chloroorganics during electrooxidation process. However, the effluents processed by EO were found to be completely disinfected. Though these techniques are effective for the degradation of refractory molecules, the energy consumption was found to be prohibitively high for actual implementation. ’ AUTHOR INFORMATION Corresponding Author

*Tel:+914422542077. Fax: +914422541027. E-mail: gbraju55@ gmail.com.

’ ACKNOWLEDGMENT The authors are thankful to the Director, National Metallurgical Laboratory, for permission to publish this work. The authors acknowledge the financial grant (NWP-O44) of Council of Scientific and Industrial Research, New Delhi. ’ REFERENCES (1) Gurunatha Rao, V. V. S.; Thangarajan, M. Ground water pollution due to discharge of tannery effluents in upper Palar Basin,

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