Treatment of Acrylonitrile Production Effluent by an Advanced Oxidation Process

By Zhang Jie, Ni Ming, Ran Xianqiang, Xue Binjie, Liu Xianghu, Fan Jianwei*
March 2011

The Authors are Professors at the State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, in Shanghai, China.
* Corresponding Author

Abstract
Fenton process could produce OH radical and other active oxygen with existence of H2O2 and Fe2+. In this paper pretreatment of acrylonitrile manufacturing wastewater by Fenton process was studied. Effects of dose of H2O2, pH value and ratio of Fe2+/H2O2 on removal efficiency of TOC and CODCr were researched. Experimental results showed that highest TOC and CODCr removal efficiency were acquired at pH 3.0, with 5.54 g/L H2O2 and Fe2+/H2O2 ratio about 0.1. In addition, BOD5/CODCr value increased from 0.18 to 0.61, which meant that Fenton process also increased susceptibility of wastewater to microorganism degrade.

Keywords: Acrylonitrile wastewater, Fenton process, TOC, COD, removal efficiency

Introduction

Acrylonitrile is a monomer widely used in industry for manufacture of plastics, rubber, nylon, and acrylic fibers1. It has also been used as a fumigant and insecticide. acrylonitrile production will exceed 6 millions tons for application in butadiene rubber and ABS polymer in 2010 all over the world. As a consequence, wastewater from acrylonitrile manufacture plants, which contains several major organic components, presents a challenge in environmental protection2. Based upon results of extensive acute toxicity data, these organic presents were generally recognized as highly toxic following oral, skin, and vapor inhalation exposure3; consequently wastewater from acrylonitrile manufacturing plants should be carefully treated before its safe discharge into environment. It is therefore of great research and practical interest in investigating technologies/processes that can effectively treat this wastewater4-7.

The acrylonitrile wastewater has mainly been treated with microbial, despite the fact that it is difficult to biodegrade. In order to increase susceptibility to microbial attack, some advanced oxidation processes (AOPs) such as ozone, electrochemical oxidation, wet oxidation, photocatalytic oxidation were used to pretreat this kind of wastewater8-10. Fenton oxidation process, being the most simple and economic one of AOPs11, has been applied to treat different pollutants including phenol12, dyes13, cresols14, gallic acids15. However, researches using AOPs has been carried out mostly with synthetic water. The synthetic water, which only consisted of acrylonitrile, was different from the actual acrylonitrile manufacturing wastewater, in which main organic component was not acrylonitrile.2 Here, our research has focused on confirming the practical possibility of Fenton oxidation process of actual acrylonitrile manufacturing wastewater.

This study attempted to investigate the effects of the amount of H2O2 and Fe2+, Fe2+/H2O2 ratio, pH value, and other experimental parameters on removal of CODCr, BOD5, and TOC of wastewater. The improvement of biodegradability of wastewater was also been studied by comparison of the ratio of BOD5 ⁄ CODCr. In addition, change of main organic components in wastewater before and after Fenton oxidation process was investigated through GC-MS method.

Methods and Materials

The analysis of acrylonitrile manufacturing wastewater

Acrylonitrile manufacturing wastewater was withdrawn from petrochemical enterprises in Jinshan, Shanghai. Main water quality parameters such as CODCr, BOD5, TOC, TN, TKN, AN (acrylonitrile) and pH were measured with the Chinese national standard method (GB-3838-2002). Main organic components in wastewater were analyzed by following procedures: 500 ml wastewater sample was pretreated with 0.45 m Millpore membrane, then 20 ml CH2Cl2 was added into the sample; after hand-shaking for 1 min, organic phase was then separated from aqueous phase; the same process was repeated for four times; at lasted the 80 ml organic phase was dried over anhydrous sodium sulfate; after separation from the drying agent, remaining organic solutions was evaporated in a Kudern-Danish (K-D) evaporator at 45°, the evaporation process was stopped when 1.0 ml solution was left. Then the left organic solution was injected into TRACE GC2000 (Thermofisher, USA) to analyze the main organic components in the wastewater. The general GC run parameters used were as follows: injector, 280°; detector, 250°; column, 50° initial, 3min hold, 7°min−1, 270° final, 10min hold; carrier gas, GC-grade helium, gas pressure, 0.5MPa; 1 µl sample was injected in a split mode, ratio of split was 1 ⁄ 50. The mass spectrometer was operated in EI mode at 70 eV.

Treatment of acrylonitrile manufacturing wastewater with Fenton process

Acrylonitrile manufacturing wastewater was treated with Fenton process by following procedures: 500 ml wastewater sample was pretreated with 0.45 m Millpore membrane; H2SO4 was added into the wastewater to adjust the pH; then FeSO4·7H2O was added into wastewater, too; H2O2 was dropwisely added into the sample with continuous stirring for 30min; then NaOH was added into above-mentioned solution to adjust value of pH to 9.0; after sedimentation for 1 hour, supernatant was collected to measure the CODCr, BOD5 and TOC. Here the CODCr and BOD5 were measured by the Chinese national standard method (GB-3838-2002). TOC was measured with Elementar LiquiTOC instrument (German).

Results

Table 1 · Properties of acrylonitrile manufacturing wastewater
Description Acrylonitrile manufacturing wastewater
CODCr (mg/L) 3590
TOC (mg/L) 2040
BOD5 (mg/L) 710
TN (mg/L) 585
TKN (mg/L) 391.2
AN (mg/L) 2.8
B/C 0.18
pH 7.18

Analysis of acrylonitrile manufacturing wastewater

Main water quality parameters of acrylonitrile manufacturing wastewater are shown in table 1. Because of the wastewater was distillate of four-effect evaporation system, CODCr and TOC were not very high. In addition, during four-effect evaporation system, acrylonitrile was recovered; concentration of acrylonitrile (AN) in wastewater was just about 2.8 mg/L; so synthetic water, which only consisted of acrylonitrile, was different from actual acrylonitrile manufacturing wastewater.

Table 2 · Main organic components in acrylonitrile manufacturing wastewater
Description Molecular formula Molecular structure
Pyrimidine C4H4N2
Fumaronitrile C4H2N2
Succinonitrile C4H4N2
3-Cyanopyridine C6H4N2

According to GC/MS analysis results, actual acrylonitrile manufacturing wastewater mainly contained 3-Cyanopyridine, Pyrimidine, Fumaronitrile, Succinonitrile and some other organic compounds. These kinds of organic compounds were persist to microorganisms degrade. Otherwise, the BOD5/CODCr (B/C) ratio was just about 0.18, which meant that this kind of wastewater was difficult to biodegrade. So some advanced oxidation processes, which could improve its susceptibility to biodegrade, such as Fenton oxidation process and ozone oxidation process were very useful for the treatment of acrylonitrile manufacturing wastewater.

Figure 1 · The effect of pH on removal of CODCr (A) and TOC (B)
Fig 1

Comparison of CODCr and TOC removal at different pH value

As shown in figure 1(A), CODCr of the supernatant in the range of pH2-7 first decreased with pH extension and then increased with further increase of pH. Correspondingly, CODCr removal efficiency in the range of pH2-7 first increased with pH extension and then decreased with further increase of pH. At pH 3.0, CODCr removal efficiency was about 31.9%, which attain to maximal in figure 1 (A), which meant that more CODCr could be removed under this condition, and CODCr of the supernatant was the lowest (2446 mg/L). In addition, it should be noted that change of TOC of the supernatant and TOC removal efficiency was almost the same as change of CODCr and CODCr removal efficiency, as shown in figure 1 (B). The TOC removal efficiency reached to the maximal (26.7 %) when pH value increased to 3; correspondingly, TOC of the supernatant reached to the lowest (1503 mg/L). Higher concentration of H2O2 was favorable for removal of CODCr and TOC.

Figure 2 · The effect of H2O2 dose on removal of CODCr (A) and TOC (B)
Fig 2

Comparison of CODCr and TOC removal at different H2O2 addition

As shown in figure 2 (A), CODCr of the supernatant increased with extension of H2O2 addition in the range of 0-6.5 g/L; CODCr removal efficiency decreased with more H2O2 addition. When H2O2 increased from 1.36 g/L to 5.44 g/L, CODCr of the supernatant decreased from 3198 mg/L to 2048 mg/L; correspondingly, CODCr removal efficiency increased from 11 % to 43.0 %. However, with further increase of H2O2 addition (6.46 g/L), CODCr of supernatant just reduced to 1994 mg/L; and CODCr removal efficiency just increased to 45%. Transformation of TOC of supernatant and TOC removal efficiency at different H2O2 concentration was almost the same as change of CODCr and CODCr removal efficiency, as shown in figure 2 (B). When concentration of H2O2 increased from 1.36 g/L to 5.44 g/L, TOC of supernatant decreased from 2029 mg/L to 1279 mg/L; correspondingly, TOC removal efficiency increased from 9.3 % to 37.7 %. TOC of supernatant just reduced to 1270 mg/L when H2O2 addition was increased to 6.46 g/L; and meanwhile TOC removal efficiency increased to 38.1 % merely. In conclusion, higher dose of H2O2 was favorable for CODCr and TOC removal, but excessively dose of H2O2 influenced efficiency of Fenton process inversely.

Figure 3 · The effect of Fe2+/H2O2 ratio on removal of CODCr (A) and TOC (B)
Fig 3

Comparison of CODCr and TOC removal at different Fe2+ ⁄ H2O2 ratio.

As shown in figure 3 (A), CODCr of supernatant increased with Fe2+ ⁄ H2O2 ratio extension; CODCr removal efficiency decreased with Fe2+addition. When Fe2+ ⁄ H2O2 ratio increased from 0.1 to 0.6, CODCr of supernatant increased from 2123 mg/L to 3046 mg/L; correspondingly, CODCr removal efficiency contemporary decreased from 40.9 % to 15.2 %. When Fe2+ ⁄ H2O2 ratio further increased to 1.0, CODCr of supernatant increased to 3184 mg/L; and CODCr removal efficiency decreased to 11.8%. Higher Fe2+ ⁄ H2O2 ratio was unfavorable for TOC removal. TOC removal efficiency decreased from 39.9 % to 15.4 % When Fe2+ ⁄ H2O2 ratio increased from 0.1 to 0.6;, TOC concentration of supernatant increased from 1229 mg/L to 1727 mg/L correspondingly. When the Fe2+/H2O2 ratio further increased to 1.0, TOC of supernatant increased to 1800 mg/L; and the TOC removal efficiency decreased to 11.9%. Higher Fe2+ ⁄ H2O2 ratio inhibited removal and degradation of organic components in actual acrylonitrile manufacturing wastewater.

Effects of H2O2, pH and Fe2+ ⁄ H2O2 ratio on BOD5 ⁄ CODCr ratio

Figure 4 · The effects of pH (B), H2O2 dose (A) and Fe2+/H2O2 ratio (C)
on BOD5/CODCr of wastewater
Fig 4

In order to increase susceptibility to microbial attack, some advanced oxidation processes were used to pretreated wastewater. Here the effect of Fenton process on BOD5 ⁄ CODCr ratio (B/C) of acrylonitrile manufacturing wastewater was studied. The effect of H2O2 addition on the B/C was shown in figure 4 (A). Addition of H2O2 could enhance B/C. B/C increased in the range of H2O2 concentration 1.36~6.46 g/L. B/C increased from 0.31 to 0.61 when the H2O2 concentration increased from 1.36 g/L to 5.54 g/L, however, B/C went to 0.62 with more H2O2 addition (6.46g/L). In addition, pH also influenced B/C of wastewater in Fenton process. As shown in figure 4 (B), B/C first increased with increasing pH value, and then it reduced with further increase of pH, the maximal B/C was about 0.54 while pH value got to 3.0. It can be seen that behavior of Fe2+ ⁄ H2O2 ratio was opposite with that of H2O2, which meant that higher Fe2+ ⁄ H2O2 ratio inhibited the enhancement of B/C, as the figure 4 (C) shown.

Discussion

Acrylonitrile manufacturing wastewater was withdrawn from petrochemical enterprises in Jinshan, Shanghai. The CODCr and TN were high in the wastewater. In addition, BOD5 ⁄ CODCr was low in this actual wastewater. Although wastewater was let out during production of acrylonitrile, main organic components in wastewater was not acrylonitrile. It was mainly composed of 3-Cyanopyridine, Pyrimidine, Fumaronitrile, Succinonitrile and some others organic components, which contained “-C=N” and “-C=N-” in their molecular structures2. It was very difficult for microorganisms to degrade these organic components. So this may be the reason for the necessary pretreatment of actual wastewater by advance oxidation processes.

For Fenton process, it produced “·OH”, “·HO2” and other active oxygen through following equations16:

Fe2+ + H2HO → Fe3+ + ·OH + OH   (1)

Fe3+ + H2HO → Fe2+ + ·OH2 + OH+   (2)

These active oxygen, especially “·OH”, had high oxidation ability, and could be used as oxidant to destroy and remove organic components for treatment of wastewater. So Fenton process was always used as pretreatment method for treatment of some kinds of organic wastewater17. Behavior of Fenton process essentially depended on pH, temperature, does of H2O2 and Fe2+ and chemical structure of organic compounds18. Higher does of H2O2 was favorable for active oxygen production, and correspondingly enhanced destruction of organic compounds in wastewater. So here increase of H2O2 dose was favorable for CODCr and TOC removal from acrylonitrile manufacturing wastewater, as experimental results in figure 1 shown. It should be noted that the above conclusion did mean that it was not necessary to control the dose of H2O2. Excessive high dose of H2O2 not only enhanced cost, but also influenced efficiency of Fenton process, because·OH radical also reacted with excess H2O2 and cause more consumption of H2O219. So here was an optimal H2O2 dose for actual Fenton process. In this experiment, the optimal H2O2 dose was about 5.54 g/L.

For the equation (1) and (2), reactions mainly took place at acid condition, because Fe2+ easily turn to Fe3+at pH values higher than 4.0, which had a tendency to produce ferric hydroxo complexes. Moreover, H2O2 was unstable and decomposing in basic solutions.19 But if H+ concentration was too high, it inhibited reaction in equation (2), reduced production of active oxygen and influenced cycle of Fe2+. It was necessary to control pH value during Fenton process. In this experiment, pH 3.0 had highest CODCr and TOC removal efficiency and highest B/C. It could be concluded that pH 3.0 significantly enhanced production of active oxygen, accelerated oxidation of organic compound and improved susceptibility to microbial degrade.

Dose of Fe2+ and Fe2+ ⁄ H2O2 ratio also influenced behavior of Fenton process. But unlike result of H2O2 dose, opposite experimental result was observed for ratio of Fe2+ ⁄ H2O2. Higher Fe2+ ⁄ H2O2 ratio was not favorable for active oxygen production and organic compounds oxidation. Fe2+ was necessary to catalyze production of active oxygen through equation (1) and (2), but excessive Fe2+ also reacted with·OH radical through following equation (3) 20:

Fe2+ + ·OH + H+ → Fe3+ + H2O   (3)

·OH radical was consumed by excessive Fe2+ through equation (3).  In this process, , the system had highest CODCr and TOC removal efficiency and highest B/C when ratio of Fe2+ ⁄ H2O2 was about 0.1. Under this condition, active oxygen was produced through equation (1) and (2) with existence of catalysis Fe2+. Active oxygen oxidized organic compounds, added to aromatic or heterocyclic rings (as well as to the unsaturated bonds of alkenes or alkynes), and made them change into CO2. With further increasing of Fe2+, excessive Fe2+ not only catalyzed production of active oxygen, but also consumed OH radical. Here the optimal Fe2+ ⁄ H2O2 ratio was about 0.1.

Conclusions

Acrylonitrile manufacturing wastewater was treated by Fenton process. During this process, some organic components in wastewater were removed by some kinds of active oxygen, which were produced at pH 3.0 with the addition of H2O2 and Fe2+. The process had highest CODCr and TOC removal efficiency at pH 3.0. Excessive Fe2+ and H2O2 both influenced removal efficiency of this process. Optimal dose of H2O2 was about 5.54 g/L; and optimal Fe2+ ⁄ H2O2 was about 0.1. The Fenton process not only remove organic components from wastewater, but also increase susceptibility of wastewater to microbial degrade. B/C changed from 0.18 to 0.61 during this process. Fenton process was an necessary pretreated method for treating acrylonitrile manufacturing wastewater.

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