Optimization and Interpretation of Fenton and UV/Fenton Processes for Degradation of Syringyl Lignin

The performances of syringyl lignin degradation by using Fenton and UV/Fenton processes were investigated in a self-designed cylindrical reactor. Many factors including pH, dosage of H2O2, concentration of Fe2+ initial concentration of syringyl lignin, reaction time and UV irradiation were optimized to reach the removal efficiency of syringyl lignin up to 87.5%. Specifically the optimum reactor time was 60 min, dosage of H2O2 was two times of its theoretical quantity and molar ratio of [Fe]/[H2O2] was 1:50 at pH 3. Furthermore, introducing UV-light into the Fenton system significantly improved the decomposition of syringyl lignin with the removal efficiency of 100% in first 30 min. In addition, the degradation mechanism of syringyl lignin by Fenton process was discussed in detail by using UV-visible spectroscopy and TOC analysis. It was found that syringyl lignin was oxidized by electron transfer of complex intermediates which contained high valence iron in homogeneous Fenton system.


Introduction
In China, paper industry is not only one of the most important industries for national economy increasing, but also a major consumer of water resource, which produces environmental pollutants with various organic pollutants discharged in wastewater. The Chemical Oxygen Demand (COD) emission in China is reported to arrive at 719 thousand tons in 2010 which accounts for 20.3% of the national industrial emissions. The pollutant of paper-mill wastewater accounts for 90% of the total pollutants, causing a serious threat to people's health and ecological security [1].
Among the complex compositions of discharged wastewater from papermaking industries, lignin is the most difficult compound to be biodegraded due to its structure with phenyl propane [2,3]. The present treatment technique for papermaking wastewater is still limited owing to complex production processes, high operation cost, and high concentration of toxic substances. Nowadays, physicochemical methods are widely used for the treatment of papermaking wastewater. For example, Fenton oxidation process as an advanced oxidation technology has its superiorities in dealing with non-biodegradable organic matters, due to advantages of simple operation, accessible reagents, low treating cost, free of complicated equipment and low toxicity to the subsequent biochemical treatment. Fenton reaction mechanism and reaction rates were well concluded by Sychev et al. [4]. As a result, Fenton oxidation has been successfully used in a variety of industrial wastewater treatments [5][6][7]. Meanwhile, UV is introduced into Fenton system to improve performance of advanced oxidation process, since H 2 O 2 can be efficiently decomposed into -OH under the irradiation of UV to enhance oxidation of Fenton reagent. The UV/Fenton process was reported to be more efficient than Fenton process but caused a comparatively high cost [8].
Additionally, Fe 3+ generated from the reaction can form complex ion Fe(OH) 2+ with OH at low pH. These complex ions are transformed into Fe 2+ producing -OH under UV, which not only improves Fe 2+ concentration to enhance the catalytic performance of Fenton system, but also increases the amount of -OH.
Based on preliminary experimental study for the degradation of guaiacol [9], syringyl lignin from paper making wastewater was selected as a target compound in this study, which is a significant generated substance in the papermaking process. So far, study on degradation of syringyl lignin by Fenton process is still quite limit with few results and discussion. The homogeneous Fenton oxidation process was innovatively carried out in a self-designed cylindrical reactor, by using UV-Vis spectrophotometer to quantify syringyl lignin and discover potential complex intermediates. A number of potential factors that may impact the removal efficiency of syringyl lignin were investigated and optimized, including initial pH value, dosage of H 2 O 2 , concentration of Fe 2+ , initial concentration of syringyl lignin, reaction time and UV irradiation. Furthermore, the mechanism of syringyl lignin degradation by Fenton and UV/Fenton reaction has been initially explored by analyzing the change of UV-visible spectral of syringyl lignin during the degradation and comparing removal efficiency and mineralization efficiency of syringyl lignin.

Reactor set-up
The degradation of syringyl lignin by Fenton and UV/Fenton processes was carried out in a double glass heat-resistant reactor ( Figure 1).
The outer layer was the circulated cooling water (diameter 10 cm, temperature 20°C), and the inner was the reaction zone (height 15 cm, inner diameter 8 cm and total volume 750 cm 3 ). A high pressure mercury lamp (TianJing ZiJing Special Light Source ZSZ, China) was introduced into UV/Fenton system to provide UV light. The nominal power of ultraviolet lamp was 11 w. Solution was mixed by a thermostatic magnetic mixer (Shanghai Sile Instrument 85-2, China). The formula of syringyl lignin (SL) is OHC 6 H 3 (OCH 3 ) 2 and its main structure contains a phenyl alkoxy, a phenolic hydroxyl group and a methoxy group with the maximum absorption wavelength λ max =267 nm, whose molecular structure is shown in Figure 2.

Treatment design
Syringyl lignin (analytical purity, Nanjing Odyssey Chemical Industry Co., Ltd) was dissolved in 500 mL deionized water in the photocatalytic reactor that was place on the thermostatic magnetic mixer. Sulfuric acid (analytical purity, Tianjin Yingdaxigui Chemical Reagent) or NaOH (analytical purity, Tianjin Kaitong Chemical Reagent Co., Ltd.) was used to adjust pH to be 2.0, 3.0, 4.0, 5.0, 6.0, 9.0 and 10.0, respectively. In this test, other factors were controlled to be

Physicochemical analysis
UV/Vis spectra were recorded by using an UV-Vis spectrophotometer (LabTech UV-1000, USA) pH was measured by an acidimeter (Tianjin Shengbang Technology pHS-3C, China). Total organic carbon (TOC) was measured by a TOC analyzer (OI Analytical 1010, USA). TOC values were mainly represented by nonpurgeable organic carbon (NPOC).
Absorbance value of solution was measured at a wavelength of 267 nm. The removal efficiency of syringyl lignin could be calculated according to Eq. (2) Removal efficiency=(C0-C)/C0 x 100% (2) Where C0 was the mass concentration of syringyl lignin solution before the reaction, mg/L; C was the mass concentration of syringyl lignin solution after the reaction, mg/L.
The intermediates during degradation process were difficult to be monitored by gas chromatography or liquid chromatography, because they may instantly disappear or their concentrations are too low to be detected in the organic phase after the samples were extracted with organic solvent. So, UV-visible spectrophotometer and TOC analyzer were used to investigate the degradation process in this study. The samples were scanned by UV-visible spectrophotometer at the wavelength ranging from 200 to 600 nm.

Initial pH
The effect of different initial pH on removal efficiency of syringyl lignin is shown in Figure 3, Which indicates that the removal efficiency of syringyl lignin was significantly high at pH 3.0-4.0 with the maximum value of 87.5% at pH=3.0. However, the removal efficiency of syringyl lignin was 43.2% and almost zeros at pH 6.0 and 2.0, respectively. This phenomenon could be explained by two reasons, Fe 2+ starts to precipitate under alkaline condition with the increase of pH, and thus it can significantly reduce catalytic decomposition of H 2 O 2 . Besides, H 2 O 2 is not stable in alkaline solution and is decomposed into O 2 and H 2 O and thus lose oxidative capacity [10]. Regarding with extreme low pH, it enhances the stability of H 2 O 2 and slows decomposition of H 2 O 2 to produce -OH, resulting in the decrease of removal efficiency of syringyl lignin. In addition, the high concentration of H + inhibits the reduction of Fe 3+ to Fe 2 . Therefore, H 2 O 2 and Fe 2+ are difficult to form an effective redox system under a high or extreme low pH condition. In this study, optimized pH value for Fenton process was assumed to be 3.0. Similarly, Jung et al. [11] found that pH buffered to acidic condition maintained relatively high levels of dissolved iron in the aqueous solution, contributing to

H 2 O 2 dosage
As shown in Figure 4, the removal efficiencies of syringyl lignin increased at first and then decreased along with the increase of H 2 O 2 dosage in both systems, and finally reached a maximum value at the H 2 O 2 amount of 2.0Q th . Although the high H 2 O 2 concentration was reported to be a dominating parameter for the high removal efficiency of phenol by Fenton's reagent [12]

[Fe 2+ ]/ [H 2 O 2 ] ratio
The effect of different concentrations of Fe 2+ on removal efficiency of syringyl lignin is shown in Figure 5, which indicates that the removal efficiency of syringyl lignin increased at first and then decreased as the concentration of Fe 2+ increases, and finally reached a maximum value at [Fe 2+ ]/[H 2 O 2 ]=1 : 50. Consequently, Fe 2+ was a necessary element to catalyze the generation of radical. Otherwise, H 2 O 2 was hard to decompose to generate radical without Fe 2+ . Additionally, the changing trend of curves was similar to the curve in Figure 4. When the dosage of Fe 2+ was low, the removal efficiency of syringyl lignin continuously increased due to the accumulation of generated -OH, whereas excessive Fe 2+ was detrimental to the removal efficiency because the oversupplied Fe 3+ yields Fe 2+ that may act as a significant scavenger by quenching the -OH. Another reason is that excessive concentration of Fe 2+ promotes H 2 O 2 decomposing into large amount of -OH. Thus a high concentration of -OH leads to scavenging effect by itself. Both reasons lead to a decline in utilization efficiency of -OH. Chan et al. [13]

Reaction time
The effect of reaction time on removal efficiency of syringyl lignin is shown in Figure 6 which demonstrates that the removal efficiency of syringyl lignin was more than 70% and about 54% in UV/Fenton and Fenton system, respectively after 30 min, indicating the high rate of reaction at the beginning of Fenton reagent. After 30 min, syringyl lignin was totally removed in UV/Fenton system, while the removal efficiency of syringyl lignin in Fenton reaction increased to the maximum value about 87% at 60 min. This phenomenon suggests that Fenton reaction was very quick, and -OH produced during the reaction could basically oxidize syringyl lignin in the first 60 min of the reaction, after that the concentration of -OH decreased and it slowed down the reaction rate.

Comparison in different systems
In order to understand the role of Fe 2+ and UV in Fenton system, the removal efficiency of syringyl lignin was separately investigated in H 2 O 2 , homogeneous Fenton and homogeneous UV/Fenton system. The comparison of removal efficiency of syringyl lignin in different systems is also shown in Figure 6. H 2 O 2 system only contributed to the removal efficiency being about 21.7%. But the removal efficiency was significantly improved after addition of Fe 2+ with removal efficiency up to 87.5% at 60 min, which was 4 times higher than that in H 2 O 2 system. Moreover, the removal efficiency of syringyl lignin was even higher when UV irradiation was introduced into the Fenton system, where syringyl lignin was completely removed in first 30 min. Similarly, Hu et al. [14] compared UV/Fenton, UV/H 2 O 2 and Fenton reagent to treat landfill leachate, who suggested that the UV/Fenton process was the most effective approach oxidation process to enhance the biodegradability and eliminate the color of the leachate. This is in accordance to the theory that UV irradiation can efficiently enhance the decomposition of H 2 O 2 to produce -OH, which consequently increases the oxidation of syringyl lignin. Meanwhile, complex ion Fe(OH) 2 formed by Fe 3+ and OH at pH 3 also produces -OH and Fe 2+ under UV irradiation. Thus the catalytic capacity of Fenton was enhanced to improve the removal efficiency of syringyl lignin. By comparing removal efficiencies of syringyl lignin and TOC in Fenton and UV/Fenton system in Figure 6, It can be found that the removal efficiency of TOC was lower than that of syringyl lignin in Fenton and UV/Fenton systems. This phenomenon proposes a hypothesis that syringyl lignin was degraded into some other organic intermediates first and then completely mineralized slowly instead of one-step oxidizing syringyl lignin into CO 2 and H 2 O. Therefore, further analysis was done to reveal the mechanism by analyzing UV-Vis spectra.

Interpreting degradation of syringyl lignin
The UV-Vis spectra of UV/Fenton system are shown in Figure 7a, which demonstrates the absorption peak of syringyl lignin at around 267 nm immediately disappeared once H 2 O 2 was added into the system and a strongly absorbed spectral band was detected in the range of 200-400 nm simultaneously. It can be inferred that a large amount of complex intermediates was generated with a strong absorption in UV-Vis range. The absorbance decreased rapidly in the first 30 min of reaction and then kept stable, which implies that conjugate structure of phencyclidine is destroyed by oxidation of UV and Fenton reagent. According to Figure 6, the mineralization efficiency reached 100% after 30 min, indicating that syringyl lignin was completely mineralized; however the absorption spectrum bands in ultraviolet regions still existed and the colorless solution immediately turned to brown as H 2 O 2 was added. So it can be concluded that the Fenton reaction is not a simple radical reaction. Combining with the theory of other researchers [15][16][17], an assumption was proposed that a complex (Fe is +IV or +V) with a large absorption spectra band was generated by the reaction of Fe (II) and syringyl lignin. The pollutant was oxidized by electron transfer of complex compound, which is another important pathway of Fenton reaction. The UV-Vis spectra of syringyl lignin changing with the time of Fenton reaction are shown in Figure 7b, Which shows that the UV-Vis spectral change of syringyl lignin in homogeneous Fenton system was very similar to UV/Fenton system? The absorption peak at 267 nm disappeared when H 2 O 2 was added into the system, suggesting that the reaction mechanisms of both systems are similar. However, a larger absorbed spectral band was detected in the range of 250-600 nm in Fenton system than that in UV/Fenton system in the range of 250-400 nm, suggesting that the oxidation of UV/Fenton was stronger than Fenton reaction, and the synergistic effect between UV and Fenton reagent improved the removal efficiency of organic compounds.

Conclusion
The factors in Fenton and UV/Fenton processes such as initial pH value, dosage of H 2 O 2 and Fe 2+ , reaction time and UV irradiation were optimized in a self-designed cylindrical reactor. The optimum factors including dosage of H 2 O 2 of 2.0Q th (0.6 mL), [Fe 2+ ]/[H 2 O 2 ] ratio of 1:50, initial pH of 3.0, and reaction time of 90 min gave rise to removal efficiency of syringyl lignin being 87.5%. UV irradiation increase the removal efficiency of syringyl lignin that was completely removed in first 30 min. Reaction mechanisms of both Fenton system and UV/ Fenton system were similar, that complex intermediates (Fe is +IV or +V) were generated with a large absorption spectra band when H 2 O 2 was added. Thus syringyl lignin was degraded into organic intermediates firstly and then completely mineralized slowly instead of one-step oxidized into CO 2 and H 2 O. The present study is believed to provide a valuable foundation for the treatment of papermaking wastewater by Fenton reagent.