Treatment of semi-aerobic aged-refuse biofilter effluent from treating landfill leachate with the Fenton method

Process Safety and Environmental Protection 133 (2020) 32–40

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Treatment of semi-aerobic aged-refuse biofilter effluent from treating landfill leachate with the Fenton method Zhepei Gu a , Weiming Chen b , Qibin Li b , Aiping Zhang a,∗ a Key Laboratory of Special Wastewater Treatment of Sichuan Province Higher Education System, College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, 610066, China b Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu, 611756, China

a r t i c l e

i n f o

Article history: Received 28 March 2019 Received in revised form 16 June 2019 Accepted 17 October 2019 Available online 30 October 2019 Keywords: Semi-aerobic aged-refuse biofilter Fenton reaction Ultraviolet-visible spectra Fulvic- and humic-like substances

a b s t r a c t Semi-aerobic aged-refuse biofilter (SAARB) can achieve good treatment efficiency for organics and nitrogen ammonia, but the effluent of SAARB from treating landfill leachate (SAARB leachate) has high chemical oxygen demand (COD), compared to the discharge standard (100 mg/L), and high humification degree. In this study, the conventional Fenton method was used in the advanced treatment of SAARB leachate. Fluorescent components in SAARB leachate were identified and tracked with parallel factor (PARAFAC) analysis, and the influencing factors, i.e., Fe2+ dosage, n(H2 O2 /FeSO4 ) ratio, initial pH, and reaction time, on the removal of fluorescent components were systematically investigated. UV–Vis spectra show the structural transformation of dissolved organic matter (DOM) in SAARB leachate. Results show that SAARB leachate mainly contains two fluorescent components: fulvic-like substances [C1; excitation (Ex)/emission (Em) (nm), (245)280/405] and humic-like substances [C2; Ex/Em (nm), (260)360/435]. A higher Fe2+ dosage, longer reaction time, and acidic conditions increased the degradation efficiency of organics, but excess H2 O2 inhibited the treatment process. Under optimum Fenton parameters (Fe2+ dosage of 5 mmol/L, n(H2 O2 /FeSO4 ) ratio of 8, initial pH of 4, and reaction time of 60 min), removal rates of C1 and C2 were 52.97% and 77.81%, respectively. UV–Vis spectra indicate that the Fenton reaction can destroy unsaturated conjugated bonds, decreasing the humification degree, hydrophobicity, molecular weight, and condensation degree. Finally, the biodegradability (represented by BOD5 (biochemical oxygen demand for 5-days)/COD) greatly increased (from 0.05 to 0.36), thus benefiting further biological treatment. The results provide suggestions and guidance for practical applications in landfill leachate treatment. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction With the rapid expansion of urbanization and the improvement of living standards, the amount of municipal solid waste is growing quickly. During the landfilling of municipal solid waste, the production of a large amount of leachate is inevitable. Landfill leachate includes four main groups of pollutants, namely, dissolved organic matter (DOM), inorganic macrocomponents, heavy metals, and xenobiotic organic compounds (Kjeldsen et al., 2002). Traditional treatment of landfill leachate, such as biological, physical, and combined processes, is limited by complex processes, high cost, and poor stability (Ahmed and Lan, 2012; Campagna et al., 2013; Chaudhari and Murthy, 2010; Han et al., 2013; Renou et al.,

∗ Corresponding author. E-mail address: [email protected] (A. Zhang).

2008). There has been great interest in research into economical and effective treatment methods for landfill leachate. Semi-aerobic aged-refuse biofilter (SAARB) is a treatment technology using waste (Han et al., 2013; Zhang et al., 2018a). The combination of anaerobic, anoxic, and aerobic regions can accelerate the transformation and degradation of organics. Previous studies (Han et al., 2011) have reported that SAARB achieves a nitrogen ammonia removal rate of 98%, but its effluent leachate has high chemical oxygen demand (COD), high humic substance content, and low biodegradability and thus, need advanced treatment to meet discharge standards. Membrane separation technology can have use in the advanced treatment of leachate, but it has the shortcomings of concentrated leachate production and membrane pollution (Calabro et al., 2010; He et al., 2015). Advanced oxidation processes (AOPs), which are widely applied to the treatment of recalcitrant organic wastewater, have good treatment efficiency (Anglada et al., 2011; Chen et al.,

https://doi.org/10.1016/j.psep.2019.10.030 0957-5820/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Z. Gu, W. Chen, Q. Li et al. / Process Safety and Environmental Protection 133 (2020) 32–40

2019, 2018; de Morais and Zamora, 2005; Gu et al., 2018; Hu et al., 2016; Oulego et al., 2015, 2016; Wang et al., 2017, 2014; Wu et al., 2008; Zhang et al., 2018b). However, the treatment of SAARB effluent from treating landfill leachate (SAARB leachate) with AOPs has been rarely discussed. Traditional AOPs include ozonation, electrochemical oxidation, photocatalytic oxidation, and the Fenton method. Ozone is a green oxidant but its utilization rate is low in leachate treatment (Wang et al., 2017). Electrochemical oxidation is suitable for wastewater with low-concentration organics, but it is limited by its long reaction time (Atmaca, 2009; Ding et al., 2018; Fernandes et al., 2017; Zolfaghari et al., 2016). Photocatalytic oxidation is effective in the degradation of simple organic matter, but its degradation of complex and macro-organic matter is unsatisfactory (Gupta et al., 2011; Rojviroon et al., 2015). However, the Fenton method, as a classic and traditional AOP, is widely used in leachate treatment at ambient reaction conditions and is non-toxic (Deng and Englehardt, 2006; Gupta et al., 2014). Many recent papers report the treatment of pollutants using Fenton-like methods, such as those using Fe-impregnated biochar catalyst (Park et al., 2018), zero-valent iron/copper system (Yamaguchi et al., 2018), and CeO2 –H2 O2 system (Zang et al., 2017). Satisfactory treatment results were obtained by all of them. However, the novel Fenton-like method is limited in practical application by its high cost of catalytic materials and its process stability. Thus, it is rarely used in practical projects. Contrary to the novel Fenton-like method, the traditional Fenton method is simple in operation and uses a mild reaction; as a result, it is widely used in the treatment of recalcitrant organics (Deng and Englehardt, 2006; Hermosilla et al., 2009). SAARB leachate is markedly different from other leachates; it has relative low organic concentration (indicated as COD 1173.80 mg/L) compared with raw landfill leachate (indicated as COD 5000–6000 mg/L), but has high humification degree. In particular, SAARB leachate originating from old landfill leachate is almost entirely composed of macromolecular organic matter, which is essentially recalcitrant organic matter (Han et al., 2011). However, the degradation mechanism of recalcitrant organic matter in SAARB leachate during a traditional Fenton method has been rarely studied. On this basis, the objectives of this study were (1) to identify the recalcitrant organic matter in SAARB leachate using parallel factor (PARAFAC) analysis based on three-dimensional excitation and emission matrix (3D-EEM); (2) to optimize operational parameters of the Fenton method according to the COD, the absorbance at 254 nm (UV254 ), and color number (CN) removal efficiencies; (3) to investigate the degradation characteristics of the identified fluorescent components under different reaction conditions; (4) to reveal the transformation of DOM in SAARB leachate under optimal operational parameters by using UV–Vis spectra; and (5) to study the improvement of biodegradability of SAARB leachate in terms of biological oxygen demand (BOD5 ).

2. Materials and methods 2.1. Materials 2.1.1. Landfill leachate and SAARB leachate Mature landfill leachate (10 years) was collected from a traditional anaerobic landfill site in southwestern China. The total area of the landfill was nearly 93,334 m2 , and its treatment capacity was ∼ 450 tons per day. The collected leachate showed a dark brown color; its basic properties are shown in Table 1. SAARB leachate used this study is the effluent wastewater after SAARB pretreatment (Han et al., 2011) from mature landfill leachate; its basic properties are shown in Table 1. It is noteworthy that ammonia nitrogen was not

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detected after SAARB pretreatment; this result is consistent with our previous studies (Han et al., 2013, 2011). 2.1.2. Materials Hydrogen peroxide (H2 O2 , 30% v/v), ferrous sulfate (FeSO4 ·7H2 O, analytical grade), sulfuric acid (H2 SO4 , 98%, analytical grade), sodium hydroxide (NaOH, analytical grade), potassium titanium oxalate (K2 TiO(C2 O4 )4 , analytical grade), sodium oxalate (Na2 C2 O4 , analytical grade), and sodium thiosulfite (Na2 S2 O3 , analytical grade) were purchased from Chengdu Kelong Chemical Co. (Chengdu, China). Deionized water was used throughout the experiment. 2.2. Experimental procedure The operational parameters (FeSO4 dosage, n(H2 O2 /FeSO4 ) ratio, initial pH, and reaction time) were optimized using batch experiments. In each batch experiment, SAARB leachate was transferred into a 50 mL beaker, and then its initial pH was adjusted to 1–7 using H2 SO4 . First, the desired dose of FeSO4 (2–8 mmol/L) and H2 O2 (10–70 mmol/L) were added into the reactor to initiate the Fenton reaction. The effects of reaction time (from 5 to 60 min) were investigated. After the reaction, a certain volume of each sample was immediately taken for H2 O2 concentration determination using spectrophotometric analysis with titanium oxalate (de Laat and Gallard, 2000; Gallard and de Laat, 2000). It was then transferred into a reactor with residual sample and held in a water bath at 50 ◦ C for 5 min to stop the reaction. The pH of each sample was adjusted to 9 in order to stop the Fenton reaction, and then filtered by quantitative filter paper, and then each sample was diluted with deionized water for sequential tests. 2.3. Analytical method The pH was determined through the glass electrode method using a pH meter (Phs-25; Fangzhou, Chengdu, China). COD was determined through the microwave digestion–titration method (HJ828-2017). BOD5 was determined using the inoculation–dilution–titration method (HJ05-2009). The concentrations of iron ions (Fe2+ and Fe3+ ) were determined with the o-phenanthroline spectrophotometric method (Herrera et al., 1989; Tamura et al., 1974). The H2 O2 concentration was determined using spectrophotometric determination with titanium oxalate (de Laat and Gallard, 2000; Gallard and de Laat, 2000). CN (Eq. 1) was also determined. A436 , A525 , and A620 are the absorbances of the samples at wavelengths of 436, 525, and 620 nm, respectively. CN =

A2436 + A2525 + A2620 · A436 + A525 + A620

(1)

The relative amount of aromatic DOM in samples was characterized with UV254 . UV–Vis spectroscopy was done to record the absorbance of the samples at wavelengths of 250–500 nm by using a Perkin-Elmer Lambda 950 (Perkin-Elmer, Inc., Waltham, MA, USA); here, wavenumber range was 250–500 nm and the scan interval was 1 nm. 3D-EEM of all samples, including raw SAARB leachate (diluted 10 times in order to eliminate the influence of iron ions on fluorescence intensity quenching) were obtained with a Horiba Scientific Auqlog-UV-800-C (HORIBA Scientific, Edison, NJ, USA). Here, the excitation wavelength slit was 5 nm, the scan speed was 500 nm/min, the excitation wavelength was 200–550 nm, and the emission wavelength was 200–600 nm. Three-dimensional properties were determined using a charge coupled device detector. Deionized water served as a blank, and Rayleigh scattering and Raman scattering were eliminated by using Horiba Scientific software kit. To identify the fluorescent DOM components, PARAFAC

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Table 1 The water quality of the leachate before and after SAARB pretreatment. Samples

COD (mg/L)

B/C

UV254 (cm−1 )

NH3 -N (mg/L)

pH

Landfill leachate SAARB leachate

5000–6000 1173.80

0.1–0.3 0.04

1–2 5.96

1500–2500 ND

7.86 7.14

B/C is the BOD-to-COD ratio, which represents the biodegradability of the sample; UV254 represents the absorbance of the sample at a wavelength of 254 nm; ND represents not detected.

Fig. 1. Fluorescent components in SAARB leachate identified by PARAFAC analysis based on 3D-EEM spectra: (a and b) component 1 (C1) and (c and d) component 2 (C2). Ex and Em represent Excitation and Emission respectively.

analysis based on 3D-EEM was used. The data of the 3D-EEM samples was analyzed by data packets using the DOM Fluor toolbox in MATLAB 2007 (MathWorks, Inc., Natick, MA, USA). The first step used the 3D-EEM of DOM to remove Rayleigh scattering and secondary Rayleigh scattering. Samples could be isolated using two different fluorescent components, the component drawing was produced using Origin 2019b (OriginLab Corp.) software.

3. Results and discussion 3.1. Identification of recalcitrant organic matter by PARAFAC analysis based on 3D-EEM spectra Two fluorescent components were identified with PARAFAC analysis (Fig. 1). The maximum Ex/Em values of the two fluorescent components are shown in Table 2.

Because of the peak overlap in the 3D-EEM spectra, PARAFAC analysis was used to differentiate peaks and to study the effects of the Fenton method on the degradation of SAARB leachate. Two fluorescent components were identified, namely, component 1 (C1) and component 2 (C2). C1 showed two excitation maxima at 245 and 280 nm, corresponding to the same emission maxima at 405 nm. According to previous reports (Baker and Curry, 2004; He et al., 2011; Yu et al., 2010), this could be ascribed to fulvic-like substances that extensively exist in landfill leachate, domestic sewage, lake, and soil. C2 showed excitation peaks at 260 and 360 nm, with an emission maximum at 435 nm. In other studies (Coble, 1996; He et al., 2011; Yu et al., 2010), C2 was identified as humic-like substances that exist in landfills. In this study, SAARB leachate from mature landfill leachate mainly comprised fulvic- and humic-like substances, inagreement with previous studies (Huo et al., 2008).

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Table 2 Fluorescent components in SAARB leachate. F.C.a

Ex

Em

Description

References

Fmax

C1 C2

245, 280 260,360

405 435

Fulvic-like substances humic-like substances

(Baker and Curry, 2004; He et al., 2011; Yu et al., 2010) (Coble, 1996; He et al., 2011; Yu et al., 2010)

1.666 1.955

a

F.C. represents fluorescent component.

Fig. 2. Effects of FeSO4 dosage on the (a) removal efficiencies of COD, UV254 , and CN and (b) removal efficiencies of C1 and C2; (c) variation of Fe2+ and Fe3+ concentration; and (d) residual H2 O2 concentrations at different initial FeSO4 dosages. Conditions: n(H2 O2 /FeSO4 ) = 8, pH0 = 4, and reaction time =60 min. Initial Fmax for C1 was approx. 1.67, and initial Fmax for C2 was approx. 1.95.

Fulvic- and humic-like substances are both complex in chemical constitution and are bio-refractory matter, which limit the treatment efficiency of SAARB. Therefore, AOPs should be further used for the treatment of SAARB leachate. Notably, protein-like components were not been detected in SAARB leachate originating from mature landfill leachate. The results implied that during the landfilling process the humification degree of landfill leachate was increased.

3.2. Operational parameter optimization The Fenton method was used in this study for the treatment of recalcitrant organic matter in SAARB leachate. The effects of FeSO4 dosage, n(H2 O2 /FeSO4 ) ratio, initial pH, and reaction time on Fenton treatment efficacy were studied. In this study, the effects of FeSO4 dosage (2–8 mmol/L), n(H2 O2 /FeSO4 ) ratio (2–14), initial pH (1–7), and reaction time (0–60 min) on the removal efficiencies of COD, UV254 , CN, C1, and C2 were also systematically investigated.

3.2.1. Effects of FeSO4 dosage The removal efficiencies for COD, UV254 , and CN significantly increased with increases in FeSO4 content (2–7 mmol/L) (Fig. 2[a]). In the Fenton process, the strong oxidative species, ·OH, are produced from H2 O2 by Fe2+ catalysis (Eq. 2). In a specific range of Fe2+ concentrations, more Fe2+ promotes the production of ·OH from H2 O2 , which also favors COD and UV254 removal. When the Fe2+ concentration ranged from 7 to 8 mmol/L, the removal results decreased by 1.92% (COD), 2.19% (UV254 ), and 2.19% (CN). Since excess Fe2+ can react with ·OH (Eq. 3), the treatment efficiencies decreased. The removal rates of C1 and C2 simultaneously increased with the increase in FeSO4 dosage (Fig. 2[b]). The molecular structure of fulvic-like substances is simpler than that of humic-like substances (Rajca and Bodzek, 2013); however, the removal rate of C2 was much higher than that of C1. In the Fenton process, the removal/degradation effect was mainly attributed to ·OH. Despite that C1 was simpler in structure than that of C2, a contrary result was obtained that C1 removal was both lower than that of C2, indicating that ·OH was more effective in degrading organics with higher humification degree and aromatic degree.

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Fig. 3. Effects of n(H2 O2 /FeSO4 ) ratio on the (a) removal efficiencies of COD, UV254 , and CN and (b) removal efficiencies of C1 and C2; (c) variation of Fe2+ and Fe3+ concentrations; and (d) residual H2 O2 concentrations at different n(H2 O2 /FeSO4 ) ratios. Conditions: [Fe2+ ]0 = 5 mmol/L, pH0 = 4, and reaction time =60 min. Initial Fmax for C1 was approx. 1.67, and initial Fmax for C2 was approx. 1.95.

This also indicates that the Fenton process was more efficient in the degradation of humic-like substances. Fe2+ + H2 O2 → Fe3+ + · OH + OH−

((2))

Fe2+ + · OH → Fe3+ + OH−

((3))

The Fe2+ concentration increased from 0.073 to 0.22 mg/L, and the Fe3+ concentration increased from 2.91 to 10.49 mg/L with increasing FeSO4 dosage (Fig. 2[c]). The Fe3+ concentration was much higher than that of Fe2+ . This result is mainly attributed to the oxidation of Fe2+ and the reaction of excess Fe2+ with ·OH (Eq. 3). Moreover, the sum of Fe2+ and Fe3+ concentrations was lower than that of the initial FeSO4 dosages, indicating that Fe2+ was first transformed to Fe3+ and then iron ions were removed as iron-based precipitates. Fig. 2(d) shows the residual H2 O2 concentration at different initial FeSO4 dosages. The residual H2 O2 concentration increased by 0.33 mmol/L as the FeSO4 dosage increased from 2 to 8 mmol/L. However, the variation and total concentration of residual H2 O2 were negligible. 3.2.2. Effects of n(H2 O2 /FeSO4 ) ratio Fig. 3(a) shows that the COD, UV254 , and CN removal rates increased by 51.89%, 11.75%, and 15.45%, respectively; the n(H2 O2 /FeSO4 ) ratio increased from 2 to 14. When the n(H2 O2 /FeSO4 ) ratio was > 10, each removal rate increased slowly. This is due to the insufficient amount of Fe2+ that reacted with H2 O2 . With increasing ratio, more H2 O2 produced more ·OH; therefore, the removal rates increased. Under conditions with high

n(H2 O2 /FeSO4 ) ratio, the Fe2+ catalyst content was constant; therefore, the catalytic effect had a certain limit (Kurniawan et al., 2006). The production of ·OH gradually became stable; hence, the removal rates of C1 and C2 changed little (Fig. 3[b]). The best removal rates of fulvic- and humic-like substances were achieved when the n(H2 O2 /FeSO4 ) ratio was 10. We can see that a higher and/or lower n(H2 O2 /FeSO4 ) ratio had effects on the removal of fulvic-like substances and humic-like substances. The Fe2+ concentration decreased from 0.23 to 0.12 mg/L, and the Fe3+ concentration increased from 6.62 to 8.08 mg/L (Fig. 3[c]). Overall, the Fe2+ concentration gradually decreased, while that of the Fe3+ one showed an opposite trend, which was caused by the Fe2+ oxidation by H2 O2 into Fe3+ . The residual H2 O2 concentration increased from 0.34 to 5.42 mmol/L with increasing n(H2 O2 /FeSO4 ) ratio (Fig. 3[d]). This resulted mainly from the increase in H2 O2 dosage. 3.2.3. Effects of initial pH The COD, UV254 , and CN removal rates increased as the pH increased from 1 to 5 (Fig. 4[a]). When the initial pH increased from 5 to 7, the removal rates dramatically decreased from 69.16% (COD), 73.99% (UV254 ), and 89.20% (CN) to 36.04%, 35.91%, and 68.50%, respectively. This can be explained by inhibition of OH production. High H+ concentration (initial pH was < 5) led to the formation of H3 O2 + , which increased H2 O2 stability (Kwon et al., 1999); thus, the removal rates increased with increasing initial pH. As the initial pH increased from 5 to 7, H2 O2 decomposed into O2 and H2 O (Zhang

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Fig. 4. Effects of initial pH on the (a) removal efficiencies of COD, UV254 , and CN and (b) removal efficiencies of C1 and C2; (c) variation of Fe2+ and Fe3+ concentrations; and (d) residual H2 O2 concentrations at different initial pH levels. Conditions: [Fe2+ ]0 = 5 mmol/L, n(H2 O2 /FeSO4 ) = 8, and reaction time =60 min. Initial Fmax for C1 was approx. 1.67, and initial Fmax for C2 was approx. 1.95.

et al., 2005), which also inhibited ·OH production. The removal rate of C1 was highly affected by the variation of the initial pH (Fig. 4[b]). Both C1 and C2 were greatly degraded (initial pH from 3 to 5), but C1 was hardly degraded when the initial pH levels were 1 and 7. We can conclude that the COD removal is greatly affected by C1 removal. Fig. 4(c) shows the variations of Fe2+ and Fe3+ concentrations with the increase in initial pH from 1 to 7. The Fe2+ concentration decreased from 0.37 to 0.01 mg/L, and the Fe3+ concentration decreased from 13.60 to 0.58 mg/L with increasing initial pH. This is because the precipitation of both Fe2+ and Fe3+ were influenced by the initial pH. Iron-based precipitants and colloids are produced with increasing pH (Cortez et al., 2011), which inhibits the Fenton process. Fig. 4(d) shows the residual H2 O2 concentration at different initial pH levels. The residual H2 O2 concentration first decreased from 8.13 to 3.98 mmol/L and finally increased to 8.70 mmol/L. This may be explained by the stability of H2 O2 at lower pH; thus, less H2 O2 participated in the reaction (Kwon et al., 1999). At higher pH, the reaction rate of the Fenton process was lower; therefore, the residual H2 O2 concentration was higher under strongly acidic and neutral conditions.

radical, thereby, destroying the humic-like substances. Meanwhile, the COD removal efficiency increased from 30.50% to 59.54% as the reaction time increased from 5 to 60 min. The gradual increase in COD removal efficiency suggested that the degradation of smaller organics was continuously proceed. Therefore, it was observed that the Fenton reaction tended to be gentle at reaction time of 50 min. On the other hand, the removal efficiency of C2 reached 80% in 10 min and changed slightly thereafter, while the removal efficiency of C1 was lower than that of C2 (Fig. 5[b]). These results suggest that the Fenton process can degrade aromatic substances in a short reaction time and is more efficient in the degradation of humic-like substances. The Fe2+ concentration decreased from 0.2 to 0.086 mg/L in the meanwhile, implying that more Fe2+ participated in the reaction. The Fe3+ concentration also decreased by 1.55 mg/L within 5 to 60 min (Fig. 5[c]). This result is mainly attributed to the participation of Fe3+ . the residual H2 O2 concentration decreased to 2.29 mg/L over the reaction time (Fig. 5[d]). This proves that more H2 O2 participated in the reaction at longer reaction time.

3.2.4. Effects of reaction time The removal efficiency of UV254 and CN increased in the first 10 min and then changed slightly (Fig. 5[a]), and the removal efficiency of C1 and C2 tended to balance at 10 min, showing a removal rule similar to that of UV254 and CN. The results indicated that CN and aromatic functional groups had a fast reaction rate with OH

Fig. 6 shows the UV–Vis spectra of SAARB leachate from the Fenton process with different reaction times. We observed no peak in the UV–Vis spectra probably because the SAARB leachate contains a variety of DOM (Han et al., 2011). The absorbance in the UV–Vis spectra is affected by unsaturated bonds and the aromatic structure of DOM in wastewater. It is apparent that the absorbance

3.3. UV–Vis analysis

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Fig. 5. Effects of reaction time on the (a) removal efficiencies of COD, UV254 , and CN and (b) removal efficiencies of C1 and C2; (c) variation of Fe2+ and Fe3+ concentrations with time; and (d) residual H2 O2 concentration at different reaction times. Conditions: [Fe2+ ]0 = 5 mmol/L, n(H2 O2 /FeSO4 ) = 8, and pH0 = 4. Initial Fmax for C1 was approx. 1.67, and initial Fmax for C2 was approx. 1.95.

Table 3 Specific absorbance wavelengths of SAARB leachate at different reaction times. Indexes

0 min

2 min

5 min

10 min

15 min

20 min

30 min

E254 E280 E240 /E420 E250 /E365 E300 /E400 A220–400 S275–295 S350–400

1.2533 0.2568 10.3799 3.9287 4.2793 128.6055 0.0114 0.0157

1.0291 0.1927 11.2409 4.4044 4.3947 102.4642 0.0121 0.0152

0.9698 0.1427 16.2833 5.3767 5.6107 92.2505 0.0129 0.018

0.8230 0.1136 17.0282 5.7773 5.7028 77.192 0.0134 0.0174

0.7057 0.0877 19.7860 6.4397 6.0847 64.8416 0.0146 0.0179

0.5301 0.0655 18.1091 6.6738 5.5328 48.4349 0.0157 0.0158

0.4912 0.0557 20.8000 7.1043 5.9976 44.5892 0.0162 0.0168

in the ultraviolet region gradually decreased, indicating that the unsaturated bonds of DOM were destroyed by ·OH (produced from H2 O2 by catalysis by Fe2+ and H+ ). Thus, the aromatic structure was destroyed. The specific absorbance at a certain wavelength in the UV–Vis spectra can reflect the DOM change. Here, the absorbance at a 254 nm (E254 ) represents the humic substance content. The absorbance at a 280 nm (E280 ) represents the aromaticity and hydrophobicity of DOM (Kang et al., 2002). E240 /E420 , E250 /E365 , and E300 /E400 ratios represent the humification degree of humic substances in wastewater (a higher E240 /E420 ratio indicates a lower aromatic degree of humic substances; a higher E250 /E365 ratio indicates a smaller molecular weight of organics (Wang et al., 2009); a higher E300 /E400 ratio indicates a smaller molecular weight and a lower condensation degree (Artinger et al.,

2000). A220–400 represents the variation of phenyl compounds (Wang et al., 2009); S275 and the (S275–295 )/(S350–400 ) ratio represent the molecular change of DOM (a higher value indicates a lower molecular weight). These indexes are shown in Table 3. E254 and E280 decreased with increasing reaction time, indicating that the humic substance content decreased, and that the aromatic degree and hydrophobicity gradually declined. E240 /E420 , E250 /E365 , and E300 /E400 ratios first slightly increased and finally were stable with increasing reaction time, suggesting that humification degree, molecular weight, and condensation degree declined. A220–400 decreased with increasing reaction time, indicating that the content of phenyl compounds decreased in the wastewater. S275–295 and the (S275–295 )/(S350–400 ) ratio both increased with increasing reaction time, suggesting that the molecular weight of DOM gradually decreased.

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poorer biodegradability than that of landfill leachate. The COD gradually decreased and BOD5 gradually increased with the Fenton process. At 60 min in the Fenton process, the biodegradability significantly improved from 0.09 to 0.36. These results are mainly attributed to the destruction of organic matter by the Fenton process; fulvic- and humic-like substances were degraded into smaller molecular organic matter. The decrement of aromatic degree and phenyl compound content increased with the BOD5 /COD ratio, favoring sequential biological treatment. 3.5. Fmax1 and Fmax2 /COD

Fig. 6. UV–Vis spectra of the Fenton process at different times. Conditions: [Fe2+ ]0 = 5 mmol/L, n(H2 O2 /FeSO4 ) = 8, and pH0 = 4.

To further investigate the removal effects of DOM in the SAARB leachate in the Fenton process, the amounts of C1 removed (Fmax1) and C2 removed (Fmax2) were plotted against the COD removed (COD) during the Fenton process (Fig. 8). Fmax1/COD and Fmax2/COD (the slope of the model line in Fig. 8[a] and [b]) quantitatively express the C1 and C2 reduction when 1 mg/L COD was removed. It can be observed that more C2 (k = 0.0035) was removed than C1 (k = 0.0021) when 1 mg/L COD was removed. In addition, this gap tended to increase with the reaction. In the Fenton process, organic matter was mainly degraded by hydroxyl radicals via hydrogen atom abstraction, addition to electron-rich sites, and electron transfer (Andreozzi et al., 1999; Pera-Titus et al., 2004). Fulvic-like substances have smaller molecular weight and are simpler in structure than humic-like substances; thus, C2 was more preferentially degraded by ·OH than was C1 (Fig. 8). In the latter stage of the Fenton process, the change in amounts of removed C1 and removed C2 was negligible. This may be explained by the lower organic concentration, which leads to the lack of target substances for hydroxyl radical and the side reaction between radicals. 4. Conclusion

Fig. 7. Biodegradability change of landfill leachate in the SAARB–Fenton process.

3.4. Improvement of biodegradability Fig. 7 shows the variation of biodegradability of leachate in the SAARB–Fenton process. It is apparent that SAARB leachate had

Two fluorescent components were identified with PARAFAC analysis coupled with 3D-EEM spectra: C1 [Ex/Em (nm), (245)280/405] was attributed to fulvic-like substances and C2 [(260)360/435] was attributed to humic-like substances. Under the optimum operational parameters of [Fe2+ ] of 5 mmol/L, n(H2 O2 /FeSO4 ) ratio of 8, initial pH of 4, and reaction time of 60 min, removal rates of C1 and C2 were 52.97% and 77.81%, respectively. C1 was more easily degraded than was C2 in the SAARB leachate in the Fenton process. UV–Vis analysis revealed that the molecular

Fig. 8. (a) Fmax1 and (b) Fmax2 vs. COD during the Fenton process treatment of SAARB leachate.

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