J. Cent. South Univ. Technol. (2011) 18: 96-100

DOI: 10.1007/s11771-011-0665-y

Degradation of polyvinyl-alcohol wastewater by Fenton’s reagent: Condition optimization and enhanced biodegradability

XIAO Yu-tang(肖羽堂), XU Shuang-shuang(许双双), LI Zhi-hua(李志花)

School of Environmental Science and Engineering, Nankai University, Tianjin 300071, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: The pretreatment of refractory polyvinyl-alcohol (PVA) wastewater with low value of COD_Cr by Fenton’s reagent was investigated to enhance the biodegradability. The effects of operating conditions such as pH of the solution, Fe²⁺ dosage, H₂O₂ dosage, reaction time and initial PVA concentration on the removal efficiency of COD_Cr were discussed. It is demonstrated that the optimum value of pH for removal of COD_Cr is 5 and the most suitable dosages of H₂O₂ (2%) and FeSO₄ (10 mg/L) are 5% and 8.0%, respectively. When the initial COD_Cr value of the PVA water is 760 mg/L, the favorable reaction time is 110 min. Under these optimum conditions, the removal ratio of COD_Cr is 58.6%-61.4%, and the value of biodegradability (COD_b/COD_Cr) increases markedly from 8.9%-9.7% to 62.6%-68.3%.

Key words: Fenton’s reagent; polyvinyl-alcohol; pretreatment; biodegradability

1 Introduction

Polyvinyl-alcohol (PVA), a well-known recalcitrant pollutant, is widely used in the textile industry as a sizing agent and in the pharmaceutical industry as an ophthalmic lubricant [1]. The large amount of discharged PVA from industrial effluents is harmful to human health and to the environment. In the view of significant pollution problem [2-3], lots of scientific studies on the removal of PVA have been carried out. The most commonly used is the biological method. An anaerobic baffled reactor (ABR) was conducted by XU et al [4] for the removal of PVA, and CHOI et al [5] investigated the degradation of PVA contained in dyeing wastewater by a mixed culture. All of these attempts have achieved certain effects, but in general, conventional biological technologies do not offer an effective treatment of PVA because the degrading capacity of most microorganisms is extremely restricted and specific for PVA [6-7]. It was reported that after 48 days of incubation, only about 40% of PVA was mineralized [8]. Membrane separation system is also adopted to treat water containing PVA. Nevertheless, it is restricted by the membrane fouling problem. Other studies on the removal of PVA focus on methods such as photochemically initiated degradation processes [9], ultrasonic techniques [10], radiation- induced degradation [11], and adsorption by various materials [12-13]. All these methods have their limitations in application. Therefore, it is imperative to pursue an effective method in the removal or pretreatment of the PVA wastewater.

Presently, wet catalytic oxidation process has been successfully adopted to treat these kinds of wastewater with high concentration of refractory organics [14-16]. However, the strict operation conditions and high primary investment hinder its popular application to the treatment of PVA wastewater with low COD_Cr value. In the last decade, increasing attention has been paid to Fenton’s reagent [17-20], which is considered one of the most promising and simplest techniques for partial oxidation of bio-refractory organic wastewater with low concentration into more biologically amenable intermediates. In order to enhance the biodegradability of the wastewater, many researchers have applied the Fenton’s reagent to the pretreatment of industry wastewater that contains refractory pollutants [21-24]. Among these applications, Fenton oxidation was used to break toxic and recalcitrant compounds into molecules that are suitable for biotreatment.

Fenton’s reagent is one of the best-known metal catalyzed oxidation reactions of water-miscible organic compounds. It consists of ferrous salt such as FeSO₄ or any other ferrous complex and H₂O₂. This mixture, at low pH, results in Fe²⁺ catalytic decomposition of H₂O₂ and proceeds via a free radical chain process which produces hydroxyl radicals (?OH). The mechanism of Fenton chemistry can be described as

Fe²⁺+H₂O₂→ Fe³⁺+OHˉ+ ?OH                   (1)

In this work, Fenton’s reagent system was employed to pretreat the bio-refractory organic wastewater of PVA with low COD_Cr value in order to remove the COD_Cr and biological toxicity and make the follow-up biological treatment possible and easy. The optimum conditions to break down PVA and the biodegradability of the effluent were examined.

2 Experimental

Both 10 mg/L FeSO₄ and 30% H₂O₂ (diluted to 2% in the experiment) were chemically pure reagents. PVA was an analytically pure reagent. Ca(OH)₂ was home- made reagent (CP). The active sludge was taken from a municipal wastewater plant in Dalian, China. After the large inert particles and debris in the sludge were removed, the sludge was cultured and tamed with a certain proportion of municipal sewage and pretreated wastewater (pH 7-8) for 7 d; then, further cultured with pretreated wastewater for 8 d before being used in the biodegradation test.

The biodegradability was evaluated by the procedure that a certain proportion of wastewater was mixed with active sludge (ratio of wastewater to sludge of 8:2) and was cultured at 30 °C in a shaker (HQL150-B, Wuhan, China) operated at 150 r/min, centrifuged at   400 r/min for 15 min. Then, the upper liquid sample was withdrawn before COD_Cr was determined according to the national standard. The COD_B is calculated by

COD_B=COD₀-COD₁                          (2)

where COD_B is the biodegradable COD_Cr of wastewater, COD₀ is the COD_Cr of the wastewater sample before culture in the shaker, and COD₁ is the COD_Cr after culture [25].

3 Results and discussion

3.1 Effect of pH on removal efficiency of COD_Cr

Fig.1 illustrates the effect of pH on the removal of COD_Cr when the pH value ranges from 3 to 8. The control experimental results indicate that the maximum removal efficiency of COD_Cr is obtained when pH is 5. Therefore, an optimal pH of 5 is established, since the removal efficiency of COD_Cr increases with pH increasing from 3 to 5 and decreases with pH increasing from 5 to 8.

pH plays an important role in catalyzed oxidation reaction and Fenton’s reagent produces the maximum amount of hydroxyl radicals under acidic conditions. However, the reaction of certain Fe²⁺ complexes with H₂O₂ at pH above 8 might not result in the formation of ?OH, since the state of Fe ion depends on pH value of wastewater and too high pH value limits the yield of ?OH. Furthermore, catalytic action of Fe ion is inhibited because it is in the state of Fe(OH)₃ at high pH value. On the other hand, when pH is below 5, the reaction (Fe³⁺+ H₂O₂→Fe²⁺+HO₂?+H⁺) is inhibited by too high concentration of H⁺, so it is difficult to reduce Fe³⁺ to Fe²⁺, leading to the limitation of the catalytic reaction, and the decreased oxidation efficiency of Fenton’s reagent.

Fig.1 Effect of pH on removal efficiency of COD_Cr (Experimental conditions: FeSO₄ (10 mg/L) dosage 5%; COD_Cr 760 mg/L; H₂O₂ (2%) dosage 3%; reaction time 120 min)

3.2 Effect of FeSO₄ dosage on removal efficiency of COD_Cr

Fig.2 shows the effect of FeSO₄ dosage on the removal efficiency of COD_Cr. The results suggest that the best dose of Fe²⁺ is 8.0% to achieve the optimal degradation efficiency. Furthermore, Fig.2 also demonstrates, within a certain range, the more the dose of Fe²⁺ is, the higher the removal efficiency of COD_Cr is, but when the dose of Fe²⁺ is over 8.0%, the effect of FeSO₄ on the removal efficiency of COD_Cr is limited.

Fig.2 Effect of FeSO₄ dosage on removal efficiency of COD_Cr (Experimental conditions: pH=5; H₂O₂ (2%) dosage 3%;   COD_Cr 760 mg/L; reaction time 120 min)

These can be explained by the fact that no ?OH generates in the absence of Fe²⁺. The concentration of ?OH depends on the initial concentration of Fe²⁺ and increases as the initial concentration of Fe²⁺ increases. That is, at low initial concentration of Fe²⁺, the yield of ?OH and the formation rate of ?OH are very small because the reaction rate for (Fe²⁺+H₂O₂→Fe³⁺+OHˉ+  ?OH) is too slow and the degradation action is inhibited. However, at higher initial Fe²⁺ concentration (>10%), Fe²⁺ is oxidized into Fe³⁺ by reducing H₂O₂ and ?OH could be consumed by the excess Fe²⁺ (Fe²⁺+?OH→ Fe³⁺+OHˉ). Moreover, the color of effluent increases due to the increased concentration of Fe²⁺. Therefore, excess dose of Fe²⁺ is unsuitable.

3.3 Effect of H₂O₂ dosage on removal efficiency of COD_Cr

Fig.3 depicts the effect of H₂O₂ dosage on the removal efficiency of COD_Cr. It can be seen from Fig.3 that the optimal dose of H₂O₂ (2%) for Fenton oxidation of PVA with a COD_Cr of 760 mg/L is 5%.

Fig.3 Effect of H₂O₂ dosage on removal efficiency of COD_Cr (Experimental conditions: pH=5; FeSO₄ (10 mg/L) dosage 5%; COD_Cr 760 mg/L; reaction time 120 min)

It may be concluded that ?OH increases with increasing the initial concentration of H₂O₂ at lower ratio of H₂O₂/Fe²⁺, during which the H₂O₂ is consumed steadily by reacting with adequate FeSO₄. Nevertheless, excess dose of H₂O₂ would be detrimental to the catalytic reaction by the oxidation of Fe²⁺ into Fe³⁺. Thereby, the yield of ?OH is restrained despite the consumption of plentiful H₂O₂. Furthermore, the unused portion of H₂O₂ could increase the COD_Cr of the effluent due to its reducibility in the determination of COD_Cr.

3.4 Effect of reaction time on removal efficiency of COD_Cr

Fig.4 illustrates the effect of reaction time on the removal efficiency of COD_Cr. As shown in Fig.4, the removal efficiency of COD_Cr increases with the increase of time and keeps approximately linear relation with reaction time at the beginning of the reaction; nevertheless it almost keeps stable after about 110 min. This can be attributed to the formation of some intermediates that could not be oxidized by ?OH in the course of Fenton reaction. Therefore, the optimal reaction time of 110 min has to be maintained to achieve the maximum degradation efficiency.

Fig.4 Effect of reaction time on removal efficiency of COD_Cr (Experimental conditions: pH=5; FeSO₄ (10 mg/L) dosage 8.0%; COD_Cr 760 mg/L; H₂O₂ (2%) dosage 5.0%)

3.5 Effect of COD_Cr of PVA concentration on removal efficiency of COD_Cr

The removal efficiency of COD_Cr of PVA wastewater is strongly inhibited with increasing COD_Cr value of the PVA wastewater, as shown in Fig.5. Fig.5 shows that the higher the COD_Cr of PVA wastewater is, the lower the removal efficiency of COD_Cr is. In other words, the lower the COD_Cr of PVA wastewater is, the higher the removal efficiency of COD_Cr is.

Fig.5 Effect of COD_Cr of PVA concentration on removal efficiency of COD_Cr (Experimental conditions: pH=5; FeSO₄ (10 mg/L) dosage 8.0%; H₂O₂ (2%) dosage 5.0%; reaction time 110 min)

When the COD_Cr of the PVA wastewater is about 760 mg/L, the removal efficiency of Fenton oxidation is 60% or so and the follow-up biological process can meet the national wastewater discharge standards. However, Fenton oxidation can only reach a removal efficiency of below 50% while the COD_Cr of PVA wastewater is over  1 g/L, and it is infeasible for the follow-up biological process to reach the national wastewater discharge standards.

To sum up, higher COD_Cr value of PVA wastewater results in the lower COD_Cr removal efficiency and higher COD_Cr of the effluent, thus it is infeasible for the follow- up biological process to meet the national wastewater discharge standards. Therefore, for COD_Cr removal, it is reasonable for Fenton oxidation to set COD_Cr of the PVA wastewater at 760 mg/L or so under the optimal conditions aforementioned.

3.6 Removal efficiency of COD_Cr under optimum conditions

In this work, Fenton’s regent is applied in order to reduce the COD_Cr of PVA wastewater and enhance the biodegradability. For Fenton oxidation, the COD_Cr value of PVA wastewater at 760 mg/L is found to be a suitable dosage. An initial pH of 5, a FeSO₄ (10 mg/L) dosage of 8.0%, an H₂O₂ (2%) dosage of 5% and a total reaction time of 110 min are selected to achieve the optimal oxidation. Under these optimal conditions, three parallel tests were carried out. The experimental results are given in Table 1.

Table 1 Removal efficiency of COD_Cr under optimum conditions of treatment

Table 1 presents the removal efficiency of COD_Cr under the optimal conditions when the COD_Cr value of the PVA wastewater is 760 mg/L. It can be seen from Table 1 that the removal efficiency of COD_Cr is 58.6%, 60.2% and 61.4%, respectively. So, the removal of COD_Cr by Fenton’s regent is effective.

3.7 Comparison tests of biodegradability

The comparison of biodegradability was made between the refractory wastewater of PVA and its effluent from pretreatment by Fenton’s reagent. The experimental results are given in Table 2.

Table 2 shows the comparison results of biodegradability under the optimum conditions for treatment for COD_Cr removal. It can be seen from Table 2 that the value of biodegradability (COD_b/COD_Cr) increases significantly from 8.9%-9.7% to 62.6%- 68.3% under the optimal conditions mentioned above, which is very favorable for the follow-up biological process.

Table 2 Comparison results of biodegradable tests

4 Conclusions

1) The COD_Cr removal efficiency of PVA wastewater is influenced greatly by the constitution of Fenton’s reagent. A FeSO₄ (10 mg/L) dosage of 8.0% and an H₂O₂ (2%) dosage of 5% are favorable for the reaction. The treatment effect is also closely related to the operation conditions. An initial pH of 5 and a total reaction time of 110 min are proved to be optimal.

2) In order to meet the national wastewater discharge standards for the follow-up biological process, it is reasonable for Fenton oxidation to set COD_Cr of the PVA wastewater at 760 mg/L or so under the optimal conditions.

3) PVA wastewater is very refractory to biodegradation, which has biodegradability (COD_b/ COD_Cr) of only 8.9%-9.7%. But after the pretreatment, the biodegradability of the PVA wastewater is enhanced to 62.6%-68.3% under the optimal conditions.

4) It is practicable for Fenton’s reagent to pretreat the refractory PVA wastewater for its biodegradability enhancement. Therefore, the combination of Fenton’s reagent and biological treatment is prospective to be applied to the treatment of other recalcitrant pollutants.

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(Edited by YANG Bing)

Foundation item: Project(08JCYBJC02600) supported by the Natural Science Foundation of Tianjin, China; Project(2008ZX07314-005-011) supported by the National Major Technological Program of China

Received date: 2009-10-10; Accepted date: 2010-03-01

Corresponding author: XIAO Yu-tang, Professor, PhD; Tel: +86-22-66229548; E-mail: xiaoyt@nankai.edu.cn