Variation Characteristics of Different Fractions of Dissolved Organic Matter in Landfill Leachate during UV-TiO 2 Photocatalytic Degradation

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1 2013 Third International Conference on Intelligent System Design and Engineering Applications Variation Characteristics of Different Fractions of Dissolved Organic Matter in Landfill Leachate during UV-TiO 2 Photocatalytic Degradation Chen-zhong Jia a,, Jian-qiang Zhu b, and Qiao-Yan Qin a College of Chemical and Environmental Engineering, Yangtze University, Jingzhou , China b College of Agriculture, Yangtze University, Jingzhou , China Abstract Fractionation was performed to investigate the variation characteristics of dissolved organic matter (DOM) in leachate during UV-TiO2 photocatalytic treatment process. Experimental results showed that the leachate from Erfeishan landfill with low COD was relatively steady after long-time natural environmental evolution. DOM dominating the main organic component of the leachate was up to 90% of TOC. Biodegradation was comparatively poor due to low BOD/COD (0.09) as an index meaning that the main component was refractory organics in the leachate. DOM in the leachate was separated into six fractions: hydrophobic acid (HOA), hydrophobic base (HOB), hydrophobic neutral (HON), hydrophilic acid (HIA), hydrophilic base (HIB), and hydrophilic neutral (HIN). The content of different DOM fractions were in descending order as: HOA HON HIA HIN HOB HIB. The photocatalytic removal efficiency of COD was up to 60%, more than 74% for DOC, respectively. BOD/COD continued to increase from initial 0.09 to about 0.39 during the whole photocatalytic treatment process, which indicated that biodegradation ability of leachate was improved by photocatalysis. The DOM content was reduced sharply after photocatalytic treatment. However, the HIA fraction kept almost steady in all treatment samples, the HOA fraction changed slowly in 24 h, and changed obviously after 36 h treatment, the HIB fraction increased slowly in 36 h and decreased obviously after 36 h, and the HOB fraction decreased sharply in 6 h, increased from 6 h to 24 h, after 24h decreased,. The other two fractions (HON and HIN) decreased steadily during all treatment process. The predominant fraction in the raw leachate was HOA (30.5% of DOC), while the dominant fraction in the final effluent was HIA (69.4% of DOC). Keywords: Landfill leachate; Dissolved organic matter; Different fraction; Photocatalysis; Variation Characteristics. INTRODUCTION Dissolved organic matter (DOM) which is ubiquitous in the environment shows various properties in relation to its origin, and exhibits highly stable and microbially refractory properties. As a heterogeneous mixture of humic substances, hydrophilic acids, proteins, lipids, carbohydrates, carboxylic acids, amino acids, and hydrocarbons[1], DOM not only plays an important role in influencing microbial activity, degradation, transport and transformation, toxicity and bioavailability of trace metals and organic pollutants[2], but also can potentially be converted to toxic byproducts during treatment[3]. Landfill leachate is a kind of wastewater with a high content of DOM[4,5]. It is noted that 34-78% of COD was ascribed to the existence of DOM in leachate, while the DOM concentration (DOC) can be in the range from 800 mg/l to 20,000 mg/l or even higher[6]. Therefore, the removal of DOM would be significant in responsible of the pollutants removal in leachate. Usually, it is very difficult both to obtain satisfactory treatment efficiency and to reduce the cost for treatment of leachate by conventional treatment techniques[7]. In recent years, photocatalytic oxidation using TiO 2 and ultraviolet light (UV-TiO 2 ) has been proven to be an effective and inexpensive method to mineralize refractory organic compounds into less toxic compounds[8]. UV-TiO 2 involve generation of hydroxyl radical ( OH) with very high oxidation potential, which is able to oxidize a wide variety of toxic and refractory organic compounds to inorganics such as CO 2, H 2 O and mineral acids[9]. So, UV-TiO 2 photocatalytic treatment has been proposed as an alternative for mineralization of recalcitrant organics in landfill leachate. Nowadays, DOM has been a major concern in water, especially the variation characteristics of DOM during wastewater treatment process. It is noteworthy that most of the previous studies treated various DOM components as a whole in the investigation of DOM in treatment systems [10,11]. However, it is well known that DOM represents a large group of structurally complex organics with distinctly different characteristics. So, it is reasonable to expect that various DOM components may play different roles during treatment process[12].for the complexity of multi-components in landfill leachate, the fractionation and characterization method were usually recommended as a new and effective approach to differentiate, describe and analyze the chemical features of DOM and degradation of pollutants [13]. After fractionation, DOM can be isolated into much purer humic acid (HA), fulvic acid (FA) and hydrohydrophilic (HyI) fractions[14], or hydrophobic acid (HOA), hydrophobic base (HOB), hydrophobic neutral (HON), hydrophilic acid (HIA), hydrophilic base (HIB), and hydrophilic neutral (HIN) [15], for further analysis. Furthermore, by comparing DOM-fraction distribution in treatment process, we may be able to provide valuable information for evaluating the effect of the increasing discharge of wastewater-plant effluents on DOM distribution. Up to now, DOM fractionation analysis has rarely been applied to effluents during wastewater treatment process[14]. Similarly, the variation characteristics of leachate DOM fractions are still insufficient during the treatments [6,16]. In view of above, the main goal of this study was, (1) to characterize the content of DOM fractions in landfill leachate, (2) to reveal the variation characteristics of Corresponding author. Tel.: ; fax: ; jiachenzhong@163.com /12 $ IEEE DOI /ISDEA

2 different fractions during UV-TiO 2 photocatalytic treatment leachate. In particular, DOM was fractionated into six components on the basis of hydrophobicity and charge, and the photocatalytic transformation characteristic of each fractional DOM component was examined and compared. The knowledge on the characteristics and photocatalysis of DOM at the fractional level will greatly facilitate the process engineers to adapt more efficient control measures in photocatalytic treatment systems.. MATERIALS AND METHODS 2.1. Leachate samples and analytical methods Leachate samples were obtained from Erfeishan landfill located in Wuhan, a metropolitan in central China, which was in operation of 8 years. Leachate samples were collected in pre-cleaned brown glass amber bottles, and stored in the dark at 4 o C until analysis. Basic physicochemical parameters of the landfill leachate including COD Cr, BOD 5, ph, color, conductivity, ammonium nitrogen (NH + 4 -N), total solids (TS), and total dissolved solids (TDS) were measured according to standard methods[17]. The dissolved organic carbons (DOC) of leachate samples filtered with 0.45 μm microporous membrane (PL, Millipore) were determined by TOC analyzer (Liquitoc, Germany). 2.2 Description of the photocatalytic leachate treatment processes Photocatalytic experiments were carried out in a selfdesigned thermostated cylindrical Pyrex photocatalytic reactor with a total volume of about 1000 ml. All the reactor system at working was kept in darkness. The experimental procedures were described in details in previous studies[18]. According to our previous experiments when the ph of landfill leachate samples was adjusted to and nano-tio 2 dosage 2 g/l, the treatment effect was optimal DOM fractionation Preparative DOM fractionation based on compound hydrophobicity was conducted on leachate samples using the separation procedure developed by Leenheer with some modifications[19-21]. DOM was fractionated by three serial columns filled with nonionic Amberlite XAD- 8 (20-60 mesh), strong cation exchange resin ( mesh) and strong anion exchange resin ( mesh). Before adding into the columns, resins were cleaned according to the modified sequential Soxhlet extraction method[22]. Sample filtrates were fractionated into six fractions: HOB, HOA, HON, HIB, HIA, and HIN, based on their adsorption on a series of resin adsorbents. The different fractions were adjusted to neutral ph with NaOH or HCl solutions, diluted with ultrapure water befittingly and then subjected to DOC. The concentrations of different fractions were calculated as C concentration (DOC) measured by TOC analyzer (Liquitoc, Germany).. RESULTS AND DISCUSSION 3.1. Characteristics of the raw landfill leachate The general characteristics of the raw landfill leachate are presented in Table 1. The conventional parameters and the composition characteristics of landfill leachate can reflect its pollution features. From Table 1, it showed that the leachate from Erfeishan landfill with low COD cr was relatively steady after long-time natural environmental evolution. DOM dominating the main organic component of the leachate was up to 90% of TOC. Biodegradation was comparatively poor due to low BOD 5 /COD Cr (0.092) as an index meaning that the main component was refractory organics in this leachate. In addition, nutrition was very imbalance due to its low phosphorus and weak alkaline; ammonia nitrogen, TDS and color were high. The leachate possessed typical characteristics of mature landfill leachate and was not suitable for biological treatment. Table 1. Characteristics of landfill leachate from Erfeishan Parameter Value Parameter Value ph 8.24 COD Cr 2440 color (times) 2400 BOD Conductivity (ms/cm) 12.2 DOC 914 Alkalinity (CaCO 3) 2326 TOC 930 SS 164 NH 3-N 1359 TDS SO in mg/l except ph, color and conductivity; the BOD/COD ratio is Distribution of DOM fraction in landfill leachate The content of different fractions of DOM in landfill leachate are showed in Fig 1. The content of the five fractions including HOB, HOA, HIB, HIA and HIN was determined, and that of HON was calculated with the formula that is HON=DOC-HOB-HOA-HIB-HIA-HIN. Fig 1. Distribution of DOM fractions in landfill leachate Fig 1 showed that the content of different DOM fractions in the landfill leachate were in descending order as: HOA HON HIA HIN HOB HIB. HOA account for up to 30%, followed by HON as 21.4%, HIA and HIN were nearly 20%. Therefore, these four fractions dominated in DOM in the leachate. HIB and HOB were the relatively low levels respectively. Moreover, it can be seen that in the leachate, organic acid substances (HOA and HIA) content of more than 50%, nearly 40% of the content of organic neutral substances (HON and HIN), 1595

3 organic alkaline substances (HOB and HIB) was the lowest with only about 10%. The content order of different DOM fractions were according with the research results of Edzwald[23] and Wang[24]. In addition, hydrophobic components of DOM in the leachate account for 60% and was higher than hydrophilic components. It is generally believed that humus proportion in leachate will increase with the time extension, while the hydrophilic substances will decrease gradually[25], which explained that Erfeishan landfill leachate has become steady after the long-term natural environmental process Organic matter removal during photocatalytic treatment Treatment time (h) Table 2. Variation of organic matter during photocatalytic treatment process BOD 5 COD DOC BOD 5/COD raw The organic matter variations in the leachate in terms of COD, BOD 5, and DOC during photocatalytic treatment are summarized in Table 2. A total average reduction of COD (60%) and DOC (74%) could be achieved after the leachate was treated by UV-TiO 2 photocatalysis of 72 h. It is noted that BOD 5 increased remarkably, up to 400 mg/l after 36 h treatment, hereafter with a slight decrease in trend maintained more than 370 mg/l in 72 h. BOD/COD continued to increase from initial 0.09 to about 0.39 after 72 h degradation, which indicated that biodegradation ability of leachate was improved by photocatalysis[9]. This result explains that the photocatalytic processes can either break down or rearrange molecular structures of organic matters and convert the non-biodegradable organic matters to more easily biodegradable products, improving the efficiency and reducing the cost of further biological steps Removal characteristics of DOM fractions during photocatalytic treatment process The content variation characteristics of different DOM fractions during photocatalytic process are displayed in Fig 2. From Fig 2, it can be seen that except of HIA, the content of different DOM fraction in the leachate was reduced sharply after photocatalytic treatment, and the DOM polarity distributions in the photocatalytic effluents also changed significantly during treatment. However, the HIA fraction kept almost steady, its concentration was in mg/l during all treatment. The HOA fraction changed slowly in 24 h, and changed obviously after 36 h treatment. The HIB fraction increased slowly to 46.4 mg/l in 36 h, and decreased obviously after 36 h, reached to 16.2 mg/l ultimately. The HOB fraction decreased sharply in 6 h, increased from 6 h to 24h, after 24h decreased, reached to 8.7 mg/l ultimately. The other two fractions (HON and HIN) decreased steadily during all treatment process. It indicated that the neutral fractions could be preferentially removed by photocatalytic treatment. Fig 2. Variation of contents of different DOM fractions during photocatalytic process (a) hydrophobic substance; (b) hydrophilic substance Because of the hydrophilicity difference of leachate ingredients, the photocatalytic treatment of DOM fractions was also dissimilar. The DOC ratio distribution of six DOM fractions in the raw leachate and the effluents of photocatalytic treatment after different time are shown in Figure 3. The acidic fractions HOA and HIA dominated in the raw leachate, collectively accounting for more than 50% of the DOM, and the neutral fractions (HON and HIN) accounted for 21.39% and 17.04% respectively. In contrast, the basic fractions (HOB and HIB) were quite low (constituting 7.84% and 3.60%, respectively). The more acidic fractions than the neutral ones revealed the predominance of humic substances and carboxylic acids over the hydrocarbons and polysaccharides in leachate[24]. In addition, the hydrophobic fractions possessed larger DOC percentage than the hydrophilic fractions (hydrophobic: hydrophilic = 60%:40%). It is noted that the predominant fraction in the raw leachate was HOA (30.5% of DOC), while the dominant fraction in the final photocatalytic effluent was HIA (69.4% of DOC), which indicated that the neutral fractions could be preferentially removed by photocatalytic treatment. HIA fraction slightly changed during the whole photocatalytic treatment, which indicated that HIA was hardly degraded by photocatalysis and thus was one of the main fractions to restrict the photocatalytic efficiency. In addition, photocatalytic oxidation of hydrophobic and hydrophilic components there is no obvious difference. 1596

4 Fig 3. The distribution of DOM fractions in samples by photocatalytic treatment in different time (6h, 12h, 24h, 36h, 48h, 60h, 72h) (R refer to raw leachate) Č. ACKNOWLEDGMENTS CONCLUSIONS Photocatalytic technology has been proven to be effective, potentially economical and environmentally sound approaches for treatment of waster-water with persistent organic pollutants. Our results indicate that landfill leachate containing a variety of refractory organics and aromatics tends to be more recalcitrant. UV/TiO2 photocatalysis is very effective for the treatment of landfill leachate, including decolorization and mineralization. The landfill leachate becomes biodegradable in nature and could be treated by secondary biological treatment after UV/TiO2 photocatalytic treatment. The landfill leachate contained various groups of organic compounds and almost all of them could be degraded by photocatalysis, although some new compounds with lower molecular weights were formed in the photocatalytic effluent that were more biodegradable. The photocatalytic technique presented in this paper could serve as a cost-effective approach for treating landfill leachate. More work is needed to test the feasibility of photocatalytic treatment by utilizing sun light, and to further improve the efficiency of photocatalysis. This work was funded by National Natural Science Foundation of China (No and No ), Environmental Geology of Ministry of Education of China University of Geosciences (BGEGF200820), and the Scientific Research Fund for Dr from the Yangtze University. REFERENCES [1] Rosario-Ortiz, F. L., Snyder, S. A., Suffet, I. H.. Characterization of dissolved organic matter in drinking water sources impacted by multiple tributaries. Water Research, 2007, 41(18): [2] Brown, A., McKnight, D. M., Chin, Y.P., et al. Chemical characterization of dissolved organic material in Pony Lake, a saline coastal pond in Antarctica. Marine Chemistry, 2004, 89(1-4): [3] Narita, H., Zavala, M. A. L., Iwai, K., et al. Transformation and characterisation of dissolved organic matter during the thermophilic aerobic biodegradation of faeces. Water Research, 2005, 39(19):

5 [4] Huo S. L., Xi B. D., Yu H. C., et al. Characteristics of dissolved organic matter (DOM) in leachate with different landfill ages. Journal Environmental Sciences-China, 2008, 20(4), [5] Lu, F., Zhang, H., Chang, C.H., et al. Dissolved organic matter and estrogenic potential of landfill leachate. Chemosphere, 2008, 72(9), [6] Bu, L., Wang, K., Zhao, Q.-L., et al. Characterization of dissolved organic matter during landfill leachate treatment by sequencing batch reactor, aeration corrosive cell-fenton, and granular activated carbon in series. Journal of Hazardous Material, 2010, 179(1-3), [7] Ince, M., Senturk, E., Engin, G.O., et al. Further treatment of landfill leachate by nanofiltration and microfiltration-pac hybrid process. Desalination, 2010, 255(1-3), [8] Wiszniowski, J., Robert-, D., Surmacz, G., et al. Leachate detoxification by combination of biological and TiO2- photocatalytic processes. Water Science and Technology, 2006, 53(3), [9] de Morais, J.L., Zamora, P.P.. Use of advanced oxidation processes to improve the biodegradability of mature landfill leachates. Journal of Hazardous Material, 2005, 123(1-3), [10] Nagai, K., Aoki, S., Fuse, Y., et al. Fractionation of dissolved organic matter (DOM) as precursors of trihalomethane in Lake Biwa and Yodo rivers. Bunseki Kagaku, 2005, 54(9), [11] He, P.J., Xue, J.F., Shao, L.M., et al. Dissolved organic matter (DOM) in recycled leachate of bioreactor landfill. Water Research, 2006, 40(7), [12] Bu, L., Wang, K., Zhao, Q.L., et al. Characterization of dissolved organic matter during landfill leachate treatment by sequencing batch reactor, aeration corrosive cell-fenton, and granular activated carbon in series. Journal of Hazardous Material, 2010b, 179(1-3), [13] Müller, M.B., Frimmel, F.H.. A new concept for the fractionation of DOM as a basis for its combined chemical and biological characterization. Water Research, 2002, 36(10), [14] Huo, S.L., Xi, B.D., Yu, H.C., et al. Dissolved organic matter in leachate from different treatment processes. Water and Environment Journal, 2009, 23(1), [15] Swietlik, J., Sikorska, E.. Characterization of natural organic matter fractions by high pressure size-exclusion chromatography, specific UV absorbance and total luminescence spectroscopy. Polish Journal of Environmental Studies, 2006, 15 (1), [16] Zhang, L., Li, A.M., Lu, Y.F., et al. Characterization and removal of dissolved organic matter (DOM) from landfill leachate rejected by nanofiltration. Waste Management, 2009, 29(3), [17] Ministry of Environmental Protection of the People s Republic of China. Water and wastewater monitoring and analysis methods, 4th Edition. Bei Jing: China Environmental Science Press, (in Chinese) [18] Jia, C., Wang, Y., Zhang, C., et al. UV-TiO 2 Photocatalytic Degradation of Landfill Leachate. Water, Air, & Soil Pollution, 2011, 217(1): [19] Leenheer, J.A.. Comprehensive approach to preparative isolation and fractionation of dissolved organic carbon from natural waters and wastewaters. Environmental Science & Technology, 1981, 15(5): [20] Chefetz, B., Hadar, Y., Chen, Y.. Dissolved organic carbon fractions formed during composting of municipal solid waste: Properties and significance. Acta Hydrochimica Et Hydrobiologica,1998, 26(3), [21] Lou, Z.Y., Zhao, Y.C., Yuan, T., et al. Natural attenuation and characterization of contaminants composition in landfill leachate under different disposing ages. Science of the Total Environment, 2009, 407(10): [22] Lu, F., Chang, C.H., Lee, D.J., et al. Dissolved organic matter with multi-peak fluorophores in landfill leachate. Chemosphere, 2009, 74(4): [23] Edzwald, J.K., Tobiason, J.E.. Enhanced coagulation: US requirements and a broader view. Water Science and Technology, 1999, 40(9), [24] Wang, L.S., Hu, H.Y., Wang, C.. Effect of ammonia nitrogen and dissolved organic matter fractions on the genotoxicity of wastewater effluent during chlorine disinfection. Environmental Science & Technology, 2007, 41(1): [25] Fan, H.J., Shu, H.Y., Yang, H.S., et al. Characteristics of landfill leachates in central Taiwan. Science of the Total Environment, 2006, 361(1-3),