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Hydroxyl radical (OH) scavenging in young and mature landfill leachates
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Hydroxyl radical (OH) scavenging in young and mature landfill leachates Niloufar M. Ghazi a, Andres A. Lastra b, Michael J. Watts c,* a
Department of Civil & Environmental Engineering, Florida State University, 2525 Pottsdamer Street, Tallahassee, FL 32310, USA b Florida Department of Environmental Protection, Domestic Wastewater Section, 2600 Blair Stone Rd., Tallahassee, FL 32399, USA c Garver, 2049 E Joyce Blvd., Suite 400, Fayetteville, AR 72703, USA
article info
abstract
Article history:
The final discharge point for collected landfill leachates is frequently the local municipal
Received 19 September 2013
wastewater treatment facility. The salinity, color, nutrient, and anthropogenic organics
Received in revised form
contamination of leachates often necessitate some form of pre-treatment. When advanced
18 February 2014
oxidation processes (AOPs) are considered for pre-treatment, the unique composition of
Accepted 3 March 2014
dissolved organic matter (DOM) and the relatively high concentrations of some inorganic
Available online 13 March 2014
solutes in leachate will inhibit treatment efficiency. The most important benchmark for design of AOPs is the expected steady-state production of free radical (OH). Without a
Keywords:
quantitative assessment of total OH consumption in high-strength waste water, like a
Landfill
landfill leachate, efficient AOP treatment is uncertain. For this reason, two landfill leach-
Leachate
ates, distinct in color, DOM, and age of landfill, were characterized for OH-scavenging
Ozone
using an established competition kinetics method. After stripping the samples of inorganic
Hydrogen peroxide
carbon, the DOM in leachate from mature (stabilized) landfill was found to react with OH
Hydroxyl radical
at a rate of 9.76 108 M1s1. However, DOM in leachate from newer landfill was observed
Dissolved organic matter
to scavenge available OH at a faster rate (8.28 109 M1s1). The combination of fast rate of reaction with OH and abundance of DOM in the sampled leachate severely limited the contribution of OH to degradation of an O3- and OH-labile organic probe compound (bisphenol-a) in oxidized mature leachate (fOH ¼ 0.03). Substantial dosing of both O3 and H2O2 (>70 mg/L and >24 mg/L, respectively) may be required to see at least 1-log-removal (>90%) of an OH-selective leachate contaminant (i.e., parachlorobenzoic acid) in a mature landfill leachate. ª 2014 Elsevier Ltd. All rights reserved.
Abbreviations: AOP, Advanced Oxidation Process; DOM, Dissolved Organic Matter; POTW, Publicly-Owned Treatment Works; MSW, Municipal Solid Waste; COD, Chemical Oxygen Demand; BOD, Biochemical Oxygen Demand; LDOM, Leachate Dissolved Organic Matter; pCBA, parachlorobenzoic acid; TOC, Total Organic Carbon; BPA, bisphenol-a. * Corresponding author. Tel.: þ1 479 527 9100; fax: þ1 479 527 9101. E-mail addresses: [email protected] (N.M. Ghazi), [email protected] (A.A. Lastra), [email protected] (M.J. Watts). http://dx.doi.org/10.1016/j.watres.2014.03.001 0043-1354/ª 2014 Elsevier Ltd. All rights reserved.
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1.
Introduction
In 2010, approximately 54% of the municipal solid waste (MSW) generated in the U.S. was disposed in landfills (USEPA, 2011). When water (precipitation, ‘wetting’ water, and water in waste) infiltrates the layers of a landfill, it is contaminated by a variety of organic and inorganic pollutants. Previous studies have demonstrated that 1 ton of MSW disposed in landfill will produce 0.2 m3 of leachate (Kurniawan and Lo, 2009). A leachate collection system often consists of impermeable liners, pipes, pumps, and centralized storage. Leachate characteristics are dependent on the type of waste buried in the landfill, location, and local climate. In general, landfill leachates are characterized by high COD, low biodegradability, high turbidity, and poor UV transmittance. Often the age of landfill plays an important role in the quality of collected leachate. The pH of young (or ‘recent’ landfill; generally, <2e3 years from cell construction) landfill leachate is often more acidic than older landfill leachate, and COD concentrations in young leachate can be greater than 10,000 mg/L (vs. a typical COD concentration of 4000 mg/L or less in mature landfill leachate) (Renou et al., 2008). Finally, dissolved organic matter composition changes significantly over the first 20 years of landfill operation. Leachate DOM is substantially more bio-available in the first 2e3 years of operation (BOD/COD w 0.5e1), with the recalcitrant DOM fraction becoming more prevalent with age (BOD/COD < 0.1 in stabilized landfill leachates) (Foo, 2009). Over the first 12 years of landfill operation, reductions in the hydrophobic acid fraction, by w20%, have been reported for DOM extracts from a MSW landfill leachate (Ziyang et al., 2009). Due to common characteristics of landfill leachate, such as low UV transmittance and high concentration of organic and inorganic nitrogen, on-site pre-treatment of landfill leachate can be beneficial before transfer to municipal POTW. Zhao et al. studied the impact of a mixture of wastewater and biologically treated leachate (95:5) on wastewater disinfection with germicidal UV (Zhao et al., 2012). They concluded that when leachate represents 5% or more of the total treatment volume at a small POTW, UV254 transmittance is reduced to 65e80%. Various physical-chemical-biological treatment methods can be applied for on-site treatment of leachate, and sound process selection would be based on regional and local considerations. Advanced oxidation technologies (AOTs or AOPs) represent one option for on-site pre-treatment of leachate ‘color’ and priority, anthropogenic contaminants. AOPs generate the short-lived OH, a non-selective, strong oxidant which alters organic compounds via H-abstraction or OH-substitution at near-diffusion-limited rates of reaction. UV/H2O2, O3, and O3/H2O2 are the most frequently applied processes for generating OH in secondary or tertiary wastewater effluents. Several variations of AOP have been studied for landfill leachate treatment. A review of investigations into treatment of leachate with the UV advanced oxidation process, UVH2O2, found that overcoming the relatively poor UV transmittance of landfill leachate required g/L doses of H2O2 (to achieve measurable COD removal) (Wang et al., 2003). Therefore, O3-based AOPs are the most frequently tested. O3-
based AOPs have been extensively applied on the laboratorybench for successful COD and BOD oxidation in leachates (Cortez et al., 2011; Monje-Ramirez and Vela´squez, 2004; Rivas et al., 2003). In a custom-built, continuous-O3 batch reactor Tizaoui et al. (2007) observed as much as 45e50% oxidation of initial COD in leachate with 40 min of operation (Tizaoui et al., 2007). Optimum process efficiency (O3 þ 2 g/L H2O2) was achieved at a cost of $2.30 USD/kg of COD removed. However, it is often the case that full-scale applications of AOP are not designed for extensive organics mineralization, but for the targeted removal of specific contaminants of concern. Aqueous O3 alters the molecular structure of the organic compounds in leachate and oxidizes them into smaller, more biodegradable products which can be removed in biological treatment systems. O3 AOPs can also reduce the retention time required for further biological treatment (Montalvao et al., 2005). During ozonation, target compounds are either oxidized directly by O3, or indirectly by OH. Previous research has reported the OH and O3 scavenging rate constants of a wide-array of dissolved substances, both organic and inorganic (Buxton et al., 1988b; von Gunten, 2003). These rate constants show that O3 is extremely reactive with electronrich organic moieties (ERMs) such as phenol, polycyclic aromatics and activated aromatic compounds. On the other hand, OH, has the ability to react with the majority of solutes in water, with the exception of chlorinated organics (Lee and von Gunten, 2012). In many natural and waste waters, O3 can decompose to OH without the addition of H O . These chain reactions are as 2 2 follow: O3 þ OH /HO 2 þ O2
k ¼ 70 M1 s1
(1)
HO 2 þ O3 /O3 þ HO2
k ¼ 2:8 106 M1 s1
(2)
O3 can also react with DOM to produce OH: O3 þ DOM/OH þ by products However, treatment of leachate with O3 differs from wastewater and drinking water due to the fact that the concentration of dissolved organic matter (DOM) in leachate is significantly higher than in wastewater and drinking water, and is likely more anthropogenic in origin. Therefore, compared to wastewater treatment, a higher consumption of O3, a lower production of OH, and a slower AOP reaction kinetic is expected during an ozonation process for leachate treatment. Limited research exists on the exact impact of leachate DOM concentrations on indirect O3 oxidation (OH) kinetics. In this study, bench-scale O3 oxidation experiments were conducted to observe the oxidation of anthropogenic organic landfill contaminants with increasing levels of dissolved O3 in two municipal landfill leachates. A significant goal of the work was to quantitatively assess the background OH scavenging rate of ‘mature’ and ‘young’ landfill leachates, compare oxidation efficiencies for treatment of trace organic micro-pollutants with non-selective OH and relatively selective O3, and provide insight into the feasibility and efficiency of selected advanced oxidation technologies for leachate pretreatment.
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2.
Experimental
2.1.
Reagents
In this study nitrobenzene (C6H5NO2, 99.9%, CAS No. 98-95-3) was purchased from Fisher Scientific, NJ, USA. Parachlorobenzoic acid (C7H5ClO2, 99%, CAS No. 74-11-3, ACROS Organics, NJ, USA) was used as received. Bisphenol-a (C15H16O2, 99%, CAS No. 80-05-7), tert-butanol (C4H10O, 99.5%, CAS No. 75-65-0), and dichloromethane (CH2Cl2, 99.5%, CAS No. 75-09-2) were purchased from SIGMA-ALDRICH Chemistry, MO, USA. Hydrogen peroxide (H2O2, 30e32%) and phosphoric acid (H3PO4, 85%, CAS No. 7664-38-2) were supplied by Macron Chemicals Company (PA, USA) and used as received. HPLC-grade methanol (CH3OH, 99.8%, CAS No. 67-561) was purchased from Alfa Aesar Company, MA, USA. Deionized, organics-free water was used for all dilutions, and was produced from serial filtration of GAC-filtered ‘tap’ water using 2 ion exchange beds (Thermo Scientific Barnstead D8901 and D8902), and a final polishing column for oxygen and organics removal (Thermo Scientific Barnstead D8903).
2.2.
Leachate sampling
Leachate samples were collected from two municipal landfills in North Florida, delivered to the laboratory the same day and kept in cold storage until use. The two landfills were different in age; the 1st landfill has been operating for more than 25 years, whereas the 2nd landfill has been operating for 5 years and is still receiving municipal solid waste from densely populated urban and suburban communities. In both cases, a centralized collection system serving multiple lined cells was tapped. The leachate samples were considered a representative mix of leachate from both closed and open cells. The collected leachate samples were significantly different with respect to total organic carbon concentrations, odor, color and pH. The young leachate sample was significantly darker in color than the mature sample with a TOC concentration of 1681 mg/L and pH of 6.7. The mature leachate sample had a TOC concentration of 116 mg/L, pH of 7.1, and 60.6 mg/L as CaCO3 alkalinity.
2.3.
Ozone generation and dosing
Ozone was generated using an AZCOZON Industries plasmaarc generator (model no. RMU16-04, Langley, B.C, Canada), capable of producing up to 4 g/hr of O3 on compressed O2 with glass/quartz electrodes. To produce saturated ozone stock solutions, O3 was diffused into 250 mL of pH 6.1, 10 mM phosphate buffered solution in a borosilicate glass, gas washing column (250 mL capacity) with a fine-bubble diffuser. The pH 6.1 buffer was kept refrigerated until use. While generating O3 stock solutions, the gas washing column was placed in an ice bath to maintain a temperature of 6 C or less. The ozone concentration in the stock solution was determined using the Standard Method 4500-O3 B Indigo Colorimetric Method (Eaton and Franson, 2005). All solution absorbance measurements were made using an Agilent Technologies spectrophotometer (Cary 60 UVevis). The
maximum measured ozone stock solution concentration was 33 mg/L O3. Aliquots of ozone stock solution were transferred to both leachate and water samples by ozone-demand-free glassware to achieve the selected ozone doses for testing. The instantaneous ozone demand of the mature leachate was measured as 8.9 mg/L and all of the applied ozone doses were below this instantaneous ozone demand.
2.4.
UV/H2O2 competition kinetics
Using a quasi-collimated beam apparatus with dual apertures, a UV fluence of 500 mJ/cm2 was applied to each sample. The collimated beam was generated using 4 germicidal-UV lamps (15 W each), emitting primarily l ¼ 253.7 nm (G15T8, General Electric Co., Cleveland, OH). The incident light intensity for each sample was measured by an International Light radiometer (model ILT 1400-A, Peabody, MA). By following a wellestablished method (Bolton and Linden, 2003), the solution absorbance at l ¼ 254 nm, exposure time, and incident irradiance were used to calculate the applied UV fluence to each irradiated solution. Each irradiated sample consisted of parachlorobenzoic acid (pCBA, Acros Organics, CAS No. 74-11-3), a measured leachate aliquot, H2O2 (20 mg/L, Macron Chemicals, 29e32% H2O2, CAS No. 7722-84-1) and tert-butanol (0e900 mM, SigmaeAldrich, CAS No. 75-65-0), dissolved in deionized, organicsfree water with 8 mM phosphate buffer. In order to isolate the bulk dissolved organic matter in each sampled leachate, the samples were air sparged following dilute acid addition. The pH of each sample was lowered to <3 before sparging. Purified, compressed air was added to each sample via diffuse bubbling for 5 min. Finally, using potassium hydroxide, the pH of the samples was returned to the initial, measured pH of 7. By evaluating the degradation of pCBA (as the selective OH probe compound) in the presence of varying concentration of tert-butanol (as the competing OH scavenger) in each sample, the unknown scavenging rate of LDOM was determined (further discussion below). This method has been previously validated for determining unknown kOH,S in natural and wastewater matrices (Katsoyiannis et al., 2011).
2.5.
Analytical
A Teledyne Tekmar (Manson, Ohio) Phoenix 8000 UVPersulfate TOC analyzer was used to measure the concentration of TOC in the samples. The UV absorbance of the samples was measured by an Agilent Technologies spectrophotometer (model: Cary 60 UVevis). High-pressure liquid chromatography, operating in reverse-phase was used to measure the concentration of pCBA and BPA. A 10 cm C-18 column (ODS-2 Hypersil, 4.6 mm dia., 5 mm particle size, Thermo Scientific) produced analyte separation, along with a 45:55 10 mM phosphate buffer:methanol isocratic mobile phase. The operating flow rate was maintained at 0.4 mL/min, and 200 bar. A single-channel UV detector (ESA, Inc., 528 UVevis detector) measured analyte absorbance at l ¼ 234 nm. A Hewlett Packard 5890 series II gas chromatograph equipped with a Hewlett Packard 5971 series mass detector was employed to measure the concentrations of nitrobenzene in each sample before and after each UV irradiation. The GC
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separation was performed by using a TR-35MS column (Thermo Scientific). The column had a length of 30 m, diameter of 0.25 mm and a film thickness of 0.25 mm. Helium was used as the carrier gas. Before injecting the sample, the injector port was heated to 300 C, while the detector was heated to 280 C. The oven column temperature was gradually heated from 100 C (Initial time: 1min) to 350 C (Final time: 1min) at the rate of 15 C min1. The total run time was 18.67 min per sample. The MS detector was operated in single ion monitoring (SIM) mode for all samples being analyzed; 77, and 123 were the predominant m/z values for quantification of nitrobenzene. Nitrobenzene external standards for calibration were prepared in dichloromethane. A liquideliquid extraction technique with dichloromethane was developed to extract aqueous nitrobenzene for GCeMS analysis. In this technique 10 mL of dichloromethane was mixed with 30 mL of the sample. The 40 mL mixture was shaken for 10 min so that a complete phase change for nitrobenzene was assured. A borosilicate glass syringe was used to extract 1e2 mL of the organic phase.
3.
Results and discussion
3.1.
Observed advanced oxidation in ozonated waters
The extent of oxidation of selected probe compounds via O3 and O3/H2O2 were compared in the sampled landfill leachates, unmodified prior to exposure to direct and indirect ozone oxidation. Bisphenol-a, a plastic monomer that has been detected world-wide in municipal landfill leachate (Wintgens et al., 2006), is a water soluble organic compound with significant OH and O3 scavenging rates. As Fig. 1 illustrates, the addition of H2O2 results in an enhanced degradation of BPA in organics-free, lab-grade water (negligible background scavenging). The rate of reaction for BPA and OH (kOH, BPA ¼ 1.02 1010 M1s1) (Rosenfeldt and Linden, 2004), relative to molecular ozone (kO3,
151
4 1 1 BPA ¼ 1.68 10 M s ) (Deborde et al., 2005), indicates the potential for enhanced oxidation with the addition of H2O2 to solution prior to ozonation. Because of the selectivity of pCBA for OH(kOH,pCBA ¼ 5 109 M1s1) and its significantly slower O3 scavenging rate (kO3,pCBA ¼ 0.15 M1s1) (David Yao and Haag, 1991), pCBA was added to selected systems to observe the extent of direct ozone oxidation of BPA. As shown in Fig. 1, the addition of pCBA (5 mg/L) significantly reduces the availability of OH for oxidation of BPA, and indicates the significance of OH to total contaminant oxidation even when the solution is treated with O3 alone (vs. O3/H2O2). At an initial O3 dose of 4 mg/L, a 60% and 50% decrease in initial BPA was observed for O3/H2O2 and O3 treatment of BPA spiked in organics-free water. The same ozone dose and initial BPA concentration in mature landfill leachate saw only a 10% and 20% decrease in BPA with O3/H2O2 and O3 treatments, respectively. The results indicate that applying O3 alone is more efficient for the degradation of BPA in mature leachate than sequential H2O2 and O3. It is likely that O3 is rapidly consumed by the present DOM in leachate. Therefore, ∙OH formation is limited by the decomposition of O3, rather than the reaction of O3 and H2O2. In comparison with lab-grade water exposures, the rate of BPA oxidation in leachate was inhibited by the presence of background scavengers of OH and O3. Applying sequential H2O2 and O3 does not increase the impact on total oxidation of organic probe compounds in this sampled leachate.
3.2.
OH scavenging by landfill leachate DOM (LDOM)
As previously discussed, mature landfill leachates contain a relatively high concentration of refractory organic compounds. Dissolved organic matter in most waste and natural waters presents the most significant obstacle to targeted advanced oxidation. To assess the impact of leachate total organic carbon (TOC) on an OH-probe compound’s oxidation kinetics, dilute solutions of nitrobenzene, a nitro-aromatic industrial solvent, and mature leachate (pre-filtered with
Fig. 1 e Observed oxidation of bisphenol-a (BPA) in both organics-free, lab-grade water and mature landfill leachate (pH [ 7.5) with increasing doses of O3 (mg/L), and in the presence of H2O2 (2:1 [H2O2]:[O3]) and para-chlorobenzoic acid (pCBA, 5 mg/L). Dilute BPA solutions (2 mg/L) in lab-grade water were buffered to pH 7. The unmodified leachate sample was spiked with BPA prior to exposure.
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Fig. 2 e Observed pseudo-first-order oxidation rate of nitrobenzene (k0 ), and relative rate of nitrobenzene decay (k0 /k0 0) in DI water with increasing TOC (from mature landfill leachate).
0.45 mm membrane) were exposed to OH via indirect photolysis (UV/H2O2). Aliquots of mature landfill leachate were spiked into organics-free, laboratory water to achieve varying concentrations of TOC (10e50 mM). Oxidizing solutions of varying TOC concentration was intended to elucidate the significance of mature leachate organic matter on free OH in AOP treated leachate. The initial nitrobenzene concentration was 5 mg/L. Fig. 2 shows the inverse relationship between the concentration of TOC from mature landfill leachate and the rate of oxidation of nitrobenzene (pseudo-first-order k0 , s1). Increasing the concentration of TOC from the mature leachate significantly reduced the oxidation of nitrobenzene due to the competitive scavenging of available OH. The total OH scavenging in the mature landfill leachate was significant, requiring only 0.6 mg/L of TOC from leachate to reduce the rate of nitrobenzene advanced oxidation by 65%. These results indicate that LDOM may have a much larger contribution to total OH-scavenging in leachate influence waters, than has
been previously observed in AOP treated waters dominated by naturally-sourced organic matter. Competition kinetics with UV/H2O2 was performed to quantitatively assess the OH scavenging rate of LDOM. After applying the previously described UV/H2O2-based method for each sample, it was observed that with a lower concentration of tert butanol as the OH scavenger, a greater degradation of pCBA was achieved. In order to assess the impact of UV photolysis on the degradation of pCBA, the same UV fluence of 500 mJ/cm2 was applied to a sample which did not contain H2O2. The results indicated that the direct photolysis of pCBA from UV light was negligible; consequently OH was considered the only effective pathway for the degradation of pCBA in the irradiated samples. According to the pseudo-first-order rate law, the observed rate of pCBA degradation will be a function of the concentration of OH and the measured rate of reaction between pCBA and OH. kpCBA;obs ¼ kOH;pCBA ½OHss
(3)
Fig. 3 e Observed pseudo-first-order decay rates of para-chlorobenzoic acid (pCBA) with increasing doses of tert-butanol (mM), and in the presence of H2O2 (20 mg/L) in dilute solutions of young and mature sampled landfill leachates (TOC [ 3 mg/ L) at pH 7. Each sample was exposed to a UV fluence of 500 mJ/cm2.
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where [OH]ss is the steady-state concentration of OH radical. The validity of the steady-state assumption has been previously established for predicting observed rates of indirect photolysis by OH (Crittenden et al., 1999; Glaze et al., 1995). The steady-state concentration of OH (M) can be approximated in the UV/H2O2 treated samples as the ratio of the rate of OH formation to the rate of its simultaneous consumption.
½OHss ¼
Table 1 e Measured second-order rates of OH scavenging by LDOM (kOH,LDOM) in each sampled landfill leachate. Leachate sample Mature Young
3.3. Relative contribution of O3 and OH to observed contaminant oxidation in leachate As was seen in Fig. 1, applying O3 in the presence of H2O2 (O3/ H2O2) in organics-free, lab-grade water increases the OH concentration in solution. However, in more complicated water matrices such as municipal wastewater, H2O2 does not
aOH kOH;LDOM ½LDOM þ kOH;tBuOH ½t BuOH þ kOH;pCBA ½pCBA þ kOH;H2 O2 ½H2 O2
where aOH is the formation rate of OH in UV-irradiated solution (M s1), kOH,t-BuOH ¼ 6 108 M1s1 (Buxton et al., 1988a), and kOH,H2O2 ¼ 2.7 107 M1s1 (Christensen et al., 1982). If 1/kpCBA,obs is plotted as a function of tert-butanol (tBuOH) concentration (see Fig. 3), linear regression can be used to solve for the unknown rate constant, kOH,LDOM, for each leachate sample, as previously demonstrated (Katsoyiannis et al., 2011). Comparing the calculated kOH,LDOM to the previously published OH scavenging rates for natural and waste waters, it can be seen that the measured kOH,LDOM of each leachate sample (Table 1) are significantly higher than the kOH,DOM reported for other water matrices. Katsoyiannis et al. (2011) reported a value of 2e2.7 104 L mgC1 s1 for three different Swiss surface waters (Katsoyiannis et al., 2011). In the same research, Katsoyiannis et al. (2011) obtained a higher kOH,DOM of 3.5 104 L mgC1 s1 for a wastewater sample. Brezonik and Fulkerson-Brekken (1998) obtained a range of 1.53e3.07 104 L mgC1 s1 for the five different North American surface water samples they studied (Brezonik and Fulkerson-Brekken, 1998). It is clear that both the relative concentration of DOM in leachate, and its unique composition (likely both autochthonous and anthropogenic in origin) are major impediments to efficient AOP treatment of landfill leachates. It has been previously suggested that during leachate stabilization in a landfill cell, microbial processes result in extensive degradation of humic, fulvic, and proteinlike structures (Yunus et al., 2011). Through fluorescence spectral analysis and LDOM fractionation, the LDOM from young landfill has been reported to be predominantly proteinlike, while older landfill produced leachate with a humic-and fulvic-like dominated composition (Huo et al., 2008). Proteins with a greater degree of CeH bonding present an increased opportunity for H-abstraction by OH, and their reduced presence in mature landfill leachate may contribute to the order-of-magnitude difference in observed kOH between the two sampled leachates.
kOH,LDOM (M1s1)
kOH,LDOM (L mgC-1s1)
9.75 0.54 108 8.28 0.19 109
8.13 0.45 104 6.90 0.15 105
153
(4)
necessarily enhance the OH exposure (Buffle et al., 2006). For the sampled leachates, it can be anticipated that the fractional contribution of OH to the overall oxidation rate of dissolved organic probe compounds will be quite small. In solutions where the oxidation of the probe compound (with a known O3 and OH scavenging rates) and a reference compound (with a known, higher OH scavenging rate than the pollutant compound and a negligible O3 scavenging rate) is occurring simultaneously, the following equation can be used to estimate the contribution of OH reactions in the oxidation of the probe compound (Huber et al., 2005). f OH ¼
½RCt ln ½RC 0 ½Mt ln ½M
kOH;M kOH;RC
(5)
0
where, M is the target compound and RC is the reference compound. In order to provide a useful estimation of the contribution of OH in the oxidation of BPA in a mature landfill leachate, a continuous ozone exposure in a gas washing column containing mature leachate was performed (250 mL). In this experiment BPA and pCBA were selected for the probe and reference compounds, respectively. It was observed that BPA was completely oxidized after 1.5 min, whereas the concentration of pCBA was measurable even after 10 min of continuous ozonation of the leachate sample. By calculating fOH, it was determined that the contribution of OH to BPA oxidation was significantly lower in the mature leachate (fOH ¼ 0.03) than in organics-free, lab-grade water (fOH ¼ 0.15). The young leachate sample was not tested, as it was anticipated that the order-of-magnitude greater kOH,LDOM (Table 1) would result in negligible fOH. The amount of O3 it would require to produce significant OH in the mature landfill leachate sample can be assessed by, again, applying a steady-state approximation for the production of $OH with continuous dosing of H2O2 to leachate with known residual O3 ([O3]res). ½OHss ¼
2k1 ½H2 O2 0 10ðpHpK;H2 O2 Þ ½O3 res k11 HCO 3 0 þ k13 ½LDOM
(6)
In this equation, k1 is the rate constant of eq. (2) (2.8 106 M1s1). The concentration of bicarbonate ion was found to be 1.2 M in the sampled mature landfill leachate, and k11 is the known second-order rate constant for the reaction of bicarbonate with OH (8.5 106 M1s1). The observed rate of OH scavenging in the mature leachate sample was selected
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Fig. 4 e The predicted formation of OH in the sampled mature landfill leachate and the corresponding rate of OH-oxidation of an organic probe compound (kOH [ 5 3 109 ML1sL1) with increasing O3 and H2O2 doses.
for k13. With an assumed molar ratio of [H2O2]:[O3] of 0.5, [OH]SS was calculated for increasing doses of O3 in the sampled mature landfill leachate. As can be seen in Fig. 4, as much as 70 mg/l of O3 would be required to see oxidation of 90% of relatively OH-selective, organic micro-pollutants having kOH w5 109 M1s1 (e.g., pCBA).
4.
Conclusions
This study assessed the OH scavenging rate of leachate dissolved organic matter and its potential impact on selection of ozonation processes for pre-treatment of landfill leachate. Leachate, in general is characterized by high TOC, and the concentration of LDOM is in an inverse relationship with the age of the landfill. The OH scavenging rates of LDOM in mature and young landfill leachates (FL, USA) were determined to be 9.76 108 M1s1 and 8.28 109 M1s1, respectively. These observed OH scavenging rates of LDOM were significantly greater than previously reported kOH,DOM in municipal wastewater and surface waters. Although O3 based AOPs can be considered as a potential solution for color, COD, and anthropogenic organic pollutants in landfill leachate, due to the observed kOH,LDOM it can be concluded that landfill leachate pre-oxidation is more effective with application of O3 (without H2O2), instead of an O3/H2O2 AOP. Relatively high concentrations of O3 and H2O2 are needed to produce sufficient OH for targeting OH-selective organic micropollutants in stabilized, mature landfill leachate (greater than 40 mg/L of residual O3 and 14 mg/L of H2O2 required to see more than 50% oxidation of the organic probe compound). Since OH scavenging rates in leachate from ‘young’ landfill cells are expected to be an order-of-magnitude higher, applying O3/H2O2 for advanced oxidation could be prohibitively expensive for many landfills treating leachate of any age.
Acknowledgments The authors would like to acknowledge the financial support for this research provided by the Hinkley Center for Solid and Hazardous Waste Research (University of Florida, Gainesville, FL, #0932017).
references
Bolton, J.R., Linden, K.G., 2003. Standardization of methods for fluence (UV dose) determination in bench-scale UV experiments. J. Environ. Eng. 129 (3), 209. Brezonik, P.L., Fulkerson-Brekken, J., 1998. Nitrate-induced photolysis in natural waters: controls on concentrations of hydroxyl radical photo-intermediates by natural scavenging agents. Environ. Sci. Technol. 32 (19), 3004e3010. Buffle, M.-O., Schumacher, J., Meylan, S., Jekel, M., von Gunten, U., 2006. Ozonation and advanced oxidation of wastewater: effect of O3 dose, pH, DOM and HO-Scavengers on ozone decomposition and HO generation. Ozone: Sci. Eng. 28 (4), 247e259. Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.B., 1988a. J. Phys. Chem. Ref. Data 17 (2), 513. Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.P., 1988b. J. Phys. Chem. Ref. Data 17, 513. Christensen, H., Sehested, K., Corfitzen, H., 1982. Reactions of hydroxyl radicals with hydrogen peroxide at ambient and elevated temperatures. J. Phys. Chem. 86 (9), 1588e1590. Cortez, S., Teixeira, P., Oliveira, R., Mota, M., 2011. Evaluation of Fenton and ozone-based advanced oxidation processes as mature landfill leachate pre-treatments. J. Environ. Manag. 92 (3), 749e755. Crittenden, J.C., Hu, S., Hand, D.W., Green, S.A., 1999. A kinetic model for H2O2/UV process in a completely mixed batch reactor. Water Res. 33 (10), 2315e2328. David Yao, C.C., Haag, W.R., 1991. Rate constants for direct reactions of ozone with several drinking water contaminants. Water Res. 25 (7), 761e773. Deborde, M., Rabouan, S., Duguet, J.-P., Legube, B., 2005. Kinetics of aqueous ozone-induced oxidation of some endocrine disruptors. Environ. Sci. Technol. 39 (16), 6086e6092.
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Eaton, A., Franson, M., 2005. Standard Methods for the Examination of Water & Wastewater. Amer Public Health Assn. Foo, K.Y., 2009. An overview of landfill leachate treatment via activated carbon adsorption process. J. Hazard. Mater. 171 (1e3), 54e60. Glaze, W., Lay, Y., Kang, J., 1995. Advanced oxidation processes. A kinetic model for the oxidation of 1, 2-dibromo-3chloropropane in water by the combination of hydrogen peroxide and UV radiation. Ind. Eng. Chem. Res. 34 (7), 2314e2323. ¨ bel, A., Joss, A., Hermann, N., LO ¨ ffler, D., Huber, M.M., GO McArdell, C.S., Ried, A., Siegrist, H., Ternes, T.A., von Gunten, U., 2005. Oxidation of pharmaceuticals during ozonation of municipal wastewater effluents: a pilot study. Environ. Sci. Technol. 39 (11), 4290e4299. Huo, S., Xi, B., Yu, H., He, L., Fan, S., Liu, H., 2008. Characteristics of dissolved organic matter (DOM) in leachate with different landfill ages. J. Environ. Sci. 20 (4), 492e498. Katsoyiannis, I.A., Canonica, S., von Gunten, U., 2011. Efficiency and energy requirements for the transformation of organic micropollutants by ozone, O3/H2O2 and UV/H2O2. Water Res. 45 (13), 3811e3822. Kurniawan, T.A., Lo, W.-h, 2009. Removal of refractory compounds from stabilized landfill leachate using an integrated H2O2 oxidation and granular activated carbon (GAC) adsorption treatment. Water Res. 43 (16), 4079e4091. Lee, Y., von Gunten, U., 2012. Quantitative structureeactivity relationships (QSARs) for the transformation of organic micropollutants during oxidative water treatment. Water Res. 46 (19), 6177e6195. Monje-Ramirez, I., Vela´squez, M.T.O.d, 2004. Removal and transformation of recalcitrant organic matter from stabilized saline landfill leachates by coagulationeozonation coupling processes. Water Res. 38 (9), 2359e2367. Montalvao, A.F., Silva, A.C., Dezotti, M., 2005. Ozonation of a landfill leachate: evaluation of toxicity removal and biodegradability improvement. J. Hazard. Mater. 117 (2e3), 235e242.
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Renou, S., Givaudan, J.G., Poulain, S., Dirassouyan, F., Moulin, P., 2008. Landfill leachate treatment: review and opportunity. J. Hazard. Mater. 150 (3), 468e493. Rivas, F.J., Beltra´n, F., Gimeno, O., Acedo, B., Carvalho, F., 2003. Stabilized leachates: ozone-activated carbon treatment and kinetics. Water Res. 37 (20), 4823e4834. Rosenfeldt, E.J., Linden, K.G., 2004. Degradation of endocrine disrupting chemicals bisphenol A, ethinyl estradiol, and estradiol during UV photolysis and advanced oxidation processes. Environ. Sci. Technol. 38 (20), 5476e5483. Tizaoui, C., Bouselmi, L., Mansouri, L., Ghrabi, A., 2007. Landfill leachate treatment with ozone and ozone/hydrogen peroxide systems. J. Hazard. Mater. 140 (1e2), 316e324. USEPA, 2011. Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and Figures for 2010 Agency. U.S.E.P, Washington, DC. von Gunten, U., 2003. Ozonation of drinking water: Part I. Oxidation kinetics and product formation. Water Res. 37, 1443e1467. Wang, F., Smith, D.W., El-Din, M.G., 2003. Application of advanced oxidation methods for landfill leachate treatment e a review. J. Environ. Eng. Sci. 2 (6), 413e427. Wintgens, T., Lyko, S., Spalding, G., Melin, T., 2006. Behaviour of bulk and trace organics in a full scale membrane bioreactor for landfill leachate treatment. Proc. Water Environ. Fed., 4781e4790. Yunus, A., Smallman, D., Stringfellow, A., Beaven, R., Powrie, W., 2011. Leachate dissolved organic matter characterization using spectroscopic methods. J. Water Reuse Desalination 1 (2), 78e87. Zhao, R., Novak, J.T., Goldsmith, C.D., 2012. Evaluation of on-site biological treatment for landfill leachates and its impact: a size distribution study. Water Res. 46 (12), 3837e3848. Ziyang, L., Youcai, Z., Tao, Y., Yu, S., Huili, C., Nanwen, Z., Renhua, H., 2009. Natural attenuation and characterization of contaminants composition in landfill leachate under different disposing ages. Sci. Total Environ. 407 (10), 3385e3391.
Available online at www.sciencedirect.com
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Hydroxyl radical (OH) scavenging in young and mature landfill leachates Niloufar M. Ghazi a, Andres A. Lastra b, Michael J. Watts c,* a
Department of Civil & Environmental Engineering, Florida State University, 2525 Pottsdamer Street, Tallahassee, FL 32310, USA b Florida Department of Environmental Protection, Domestic Wastewater Section, 2600 Blair Stone Rd., Tallahassee, FL 32399, USA c Garver, 2049 E Joyce Blvd., Suite 400, Fayetteville, AR 72703, USA
article info
abstract
Article history:
The final discharge point for collected landfill leachates is frequently the local municipal
Received 19 September 2013
wastewater treatment facility. The salinity, color, nutrient, and anthropogenic organics
Received in revised form
contamination of leachates often necessitate some form of pre-treatment. When advanced
18 February 2014
oxidation processes (AOPs) are considered for pre-treatment, the unique composition of
Accepted 3 March 2014
dissolved organic matter (DOM) and the relatively high concentrations of some inorganic
Available online 13 March 2014
solutes in leachate will inhibit treatment efficiency. The most important benchmark for design of AOPs is the expected steady-state production of free radical (OH). Without a
Keywords:
quantitative assessment of total OH consumption in high-strength waste water, like a
Landfill
landfill leachate, efficient AOP treatment is uncertain. For this reason, two landfill leach-
Leachate
ates, distinct in color, DOM, and age of landfill, were characterized for OH-scavenging
Ozone
using an established competition kinetics method. After stripping the samples of inorganic
Hydrogen peroxide
carbon, the DOM in leachate from mature (stabilized) landfill was found to react with OH
Hydroxyl radical
at a rate of 9.76 108 M1s1. However, DOM in leachate from newer landfill was observed
Dissolved organic matter
to scavenge available OH at a faster rate (8.28 109 M1s1). The combination of fast rate of reaction with OH and abundance of DOM in the sampled leachate severely limited the contribution of OH to degradation of an O3- and OH-labile organic probe compound (bisphenol-a) in oxidized mature leachate (fOH ¼ 0.03). Substantial dosing of both O3 and H2O2 (>70 mg/L and >24 mg/L, respectively) may be required to see at least 1-log-removal (>90%) of an OH-selective leachate contaminant (i.e., parachlorobenzoic acid) in a mature landfill leachate. ª 2014 Elsevier Ltd. All rights reserved.
Abbreviations: AOP, Advanced Oxidation Process; DOM, Dissolved Organic Matter; POTW, Publicly-Owned Treatment Works; MSW, Municipal Solid Waste; COD, Chemical Oxygen Demand; BOD, Biochemical Oxygen Demand; LDOM, Leachate Dissolved Organic Matter; pCBA, parachlorobenzoic acid; TOC, Total Organic Carbon; BPA, bisphenol-a. * Corresponding author. Tel.: þ1 479 527 9100; fax: þ1 479 527 9101. E-mail addresses: [email protected] (N.M. Ghazi), [email protected] (A.A. Lastra), [email protected] (M.J. Watts). http://dx.doi.org/10.1016/j.watres.2014.03.001 0043-1354/ª 2014 Elsevier Ltd. All rights reserved.
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1.
Introduction
In 2010, approximately 54% of the municipal solid waste (MSW) generated in the U.S. was disposed in landfills (USEPA, 2011). When water (precipitation, ‘wetting’ water, and water in waste) infiltrates the layers of a landfill, it is contaminated by a variety of organic and inorganic pollutants. Previous studies have demonstrated that 1 ton of MSW disposed in landfill will produce 0.2 m3 of leachate (Kurniawan and Lo, 2009). A leachate collection system often consists of impermeable liners, pipes, pumps, and centralized storage. Leachate characteristics are dependent on the type of waste buried in the landfill, location, and local climate. In general, landfill leachates are characterized by high COD, low biodegradability, high turbidity, and poor UV transmittance. Often the age of landfill plays an important role in the quality of collected leachate. The pH of young (or ‘recent’ landfill; generally, <2e3 years from cell construction) landfill leachate is often more acidic than older landfill leachate, and COD concentrations in young leachate can be greater than 10,000 mg/L (vs. a typical COD concentration of 4000 mg/L or less in mature landfill leachate) (Renou et al., 2008). Finally, dissolved organic matter composition changes significantly over the first 20 years of landfill operation. Leachate DOM is substantially more bio-available in the first 2e3 years of operation (BOD/COD w 0.5e1), with the recalcitrant DOM fraction becoming more prevalent with age (BOD/COD < 0.1 in stabilized landfill leachates) (Foo, 2009). Over the first 12 years of landfill operation, reductions in the hydrophobic acid fraction, by w20%, have been reported for DOM extracts from a MSW landfill leachate (Ziyang et al., 2009). Due to common characteristics of landfill leachate, such as low UV transmittance and high concentration of organic and inorganic nitrogen, on-site pre-treatment of landfill leachate can be beneficial before transfer to municipal POTW. Zhao et al. studied the impact of a mixture of wastewater and biologically treated leachate (95:5) on wastewater disinfection with germicidal UV (Zhao et al., 2012). They concluded that when leachate represents 5% or more of the total treatment volume at a small POTW, UV254 transmittance is reduced to 65e80%. Various physical-chemical-biological treatment methods can be applied for on-site treatment of leachate, and sound process selection would be based on regional and local considerations. Advanced oxidation technologies (AOTs or AOPs) represent one option for on-site pre-treatment of leachate ‘color’ and priority, anthropogenic contaminants. AOPs generate the short-lived OH, a non-selective, strong oxidant which alters organic compounds via H-abstraction or OH-substitution at near-diffusion-limited rates of reaction. UV/H2O2, O3, and O3/H2O2 are the most frequently applied processes for generating OH in secondary or tertiary wastewater effluents. Several variations of AOP have been studied for landfill leachate treatment. A review of investigations into treatment of leachate with the UV advanced oxidation process, UVH2O2, found that overcoming the relatively poor UV transmittance of landfill leachate required g/L doses of H2O2 (to achieve measurable COD removal) (Wang et al., 2003). Therefore, O3-based AOPs are the most frequently tested. O3-
based AOPs have been extensively applied on the laboratorybench for successful COD and BOD oxidation in leachates (Cortez et al., 2011; Monje-Ramirez and Vela´squez, 2004; Rivas et al., 2003). In a custom-built, continuous-O3 batch reactor Tizaoui et al. (2007) observed as much as 45e50% oxidation of initial COD in leachate with 40 min of operation (Tizaoui et al., 2007). Optimum process efficiency (O3 þ 2 g/L H2O2) was achieved at a cost of $2.30 USD/kg of COD removed. However, it is often the case that full-scale applications of AOP are not designed for extensive organics mineralization, but for the targeted removal of specific contaminants of concern. Aqueous O3 alters the molecular structure of the organic compounds in leachate and oxidizes them into smaller, more biodegradable products which can be removed in biological treatment systems. O3 AOPs can also reduce the retention time required for further biological treatment (Montalvao et al., 2005). During ozonation, target compounds are either oxidized directly by O3, or indirectly by OH. Previous research has reported the OH and O3 scavenging rate constants of a wide-array of dissolved substances, both organic and inorganic (Buxton et al., 1988b; von Gunten, 2003). These rate constants show that O3 is extremely reactive with electronrich organic moieties (ERMs) such as phenol, polycyclic aromatics and activated aromatic compounds. On the other hand, OH, has the ability to react with the majority of solutes in water, with the exception of chlorinated organics (Lee and von Gunten, 2012). In many natural and waste waters, O3 can decompose to OH without the addition of H O . These chain reactions are as 2 2 follow: O3 þ OH /HO 2 þ O2
k ¼ 70 M1 s1
(1)
HO 2 þ O3 /O3 þ HO2
k ¼ 2:8 106 M1 s1
(2)
O3 can also react with DOM to produce OH: O3 þ DOM/OH þ by products However, treatment of leachate with O3 differs from wastewater and drinking water due to the fact that the concentration of dissolved organic matter (DOM) in leachate is significantly higher than in wastewater and drinking water, and is likely more anthropogenic in origin. Therefore, compared to wastewater treatment, a higher consumption of O3, a lower production of OH, and a slower AOP reaction kinetic is expected during an ozonation process for leachate treatment. Limited research exists on the exact impact of leachate DOM concentrations on indirect O3 oxidation (OH) kinetics. In this study, bench-scale O3 oxidation experiments were conducted to observe the oxidation of anthropogenic organic landfill contaminants with increasing levels of dissolved O3 in two municipal landfill leachates. A significant goal of the work was to quantitatively assess the background OH scavenging rate of ‘mature’ and ‘young’ landfill leachates, compare oxidation efficiencies for treatment of trace organic micro-pollutants with non-selective OH and relatively selective O3, and provide insight into the feasibility and efficiency of selected advanced oxidation technologies for leachate pretreatment.
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2.
Experimental
2.1.
Reagents
In this study nitrobenzene (C6H5NO2, 99.9%, CAS No. 98-95-3) was purchased from Fisher Scientific, NJ, USA. Parachlorobenzoic acid (C7H5ClO2, 99%, CAS No. 74-11-3, ACROS Organics, NJ, USA) was used as received. Bisphenol-a (C15H16O2, 99%, CAS No. 80-05-7), tert-butanol (C4H10O, 99.5%, CAS No. 75-65-0), and dichloromethane (CH2Cl2, 99.5%, CAS No. 75-09-2) were purchased from SIGMA-ALDRICH Chemistry, MO, USA. Hydrogen peroxide (H2O2, 30e32%) and phosphoric acid (H3PO4, 85%, CAS No. 7664-38-2) were supplied by Macron Chemicals Company (PA, USA) and used as received. HPLC-grade methanol (CH3OH, 99.8%, CAS No. 67-561) was purchased from Alfa Aesar Company, MA, USA. Deionized, organics-free water was used for all dilutions, and was produced from serial filtration of GAC-filtered ‘tap’ water using 2 ion exchange beds (Thermo Scientific Barnstead D8901 and D8902), and a final polishing column for oxygen and organics removal (Thermo Scientific Barnstead D8903).
2.2.
Leachate sampling
Leachate samples were collected from two municipal landfills in North Florida, delivered to the laboratory the same day and kept in cold storage until use. The two landfills were different in age; the 1st landfill has been operating for more than 25 years, whereas the 2nd landfill has been operating for 5 years and is still receiving municipal solid waste from densely populated urban and suburban communities. In both cases, a centralized collection system serving multiple lined cells was tapped. The leachate samples were considered a representative mix of leachate from both closed and open cells. The collected leachate samples were significantly different with respect to total organic carbon concentrations, odor, color and pH. The young leachate sample was significantly darker in color than the mature sample with a TOC concentration of 1681 mg/L and pH of 6.7. The mature leachate sample had a TOC concentration of 116 mg/L, pH of 7.1, and 60.6 mg/L as CaCO3 alkalinity.
2.3.
Ozone generation and dosing
Ozone was generated using an AZCOZON Industries plasmaarc generator (model no. RMU16-04, Langley, B.C, Canada), capable of producing up to 4 g/hr of O3 on compressed O2 with glass/quartz electrodes. To produce saturated ozone stock solutions, O3 was diffused into 250 mL of pH 6.1, 10 mM phosphate buffered solution in a borosilicate glass, gas washing column (250 mL capacity) with a fine-bubble diffuser. The pH 6.1 buffer was kept refrigerated until use. While generating O3 stock solutions, the gas washing column was placed in an ice bath to maintain a temperature of 6 C or less. The ozone concentration in the stock solution was determined using the Standard Method 4500-O3 B Indigo Colorimetric Method (Eaton and Franson, 2005). All solution absorbance measurements were made using an Agilent Technologies spectrophotometer (Cary 60 UVevis). The
maximum measured ozone stock solution concentration was 33 mg/L O3. Aliquots of ozone stock solution were transferred to both leachate and water samples by ozone-demand-free glassware to achieve the selected ozone doses for testing. The instantaneous ozone demand of the mature leachate was measured as 8.9 mg/L and all of the applied ozone doses were below this instantaneous ozone demand.
2.4.
UV/H2O2 competition kinetics
Using a quasi-collimated beam apparatus with dual apertures, a UV fluence of 500 mJ/cm2 was applied to each sample. The collimated beam was generated using 4 germicidal-UV lamps (15 W each), emitting primarily l ¼ 253.7 nm (G15T8, General Electric Co., Cleveland, OH). The incident light intensity for each sample was measured by an International Light radiometer (model ILT 1400-A, Peabody, MA). By following a wellestablished method (Bolton and Linden, 2003), the solution absorbance at l ¼ 254 nm, exposure time, and incident irradiance were used to calculate the applied UV fluence to each irradiated solution. Each irradiated sample consisted of parachlorobenzoic acid (pCBA, Acros Organics, CAS No. 74-11-3), a measured leachate aliquot, H2O2 (20 mg/L, Macron Chemicals, 29e32% H2O2, CAS No. 7722-84-1) and tert-butanol (0e900 mM, SigmaeAldrich, CAS No. 75-65-0), dissolved in deionized, organicsfree water with 8 mM phosphate buffer. In order to isolate the bulk dissolved organic matter in each sampled leachate, the samples were air sparged following dilute acid addition. The pH of each sample was lowered to <3 before sparging. Purified, compressed air was added to each sample via diffuse bubbling for 5 min. Finally, using potassium hydroxide, the pH of the samples was returned to the initial, measured pH of 7. By evaluating the degradation of pCBA (as the selective OH probe compound) in the presence of varying concentration of tert-butanol (as the competing OH scavenger) in each sample, the unknown scavenging rate of LDOM was determined (further discussion below). This method has been previously validated for determining unknown kOH,S in natural and wastewater matrices (Katsoyiannis et al., 2011).
2.5.
Analytical
A Teledyne Tekmar (Manson, Ohio) Phoenix 8000 UVPersulfate TOC analyzer was used to measure the concentration of TOC in the samples. The UV absorbance of the samples was measured by an Agilent Technologies spectrophotometer (model: Cary 60 UVevis). High-pressure liquid chromatography, operating in reverse-phase was used to measure the concentration of pCBA and BPA. A 10 cm C-18 column (ODS-2 Hypersil, 4.6 mm dia., 5 mm particle size, Thermo Scientific) produced analyte separation, along with a 45:55 10 mM phosphate buffer:methanol isocratic mobile phase. The operating flow rate was maintained at 0.4 mL/min, and 200 bar. A single-channel UV detector (ESA, Inc., 528 UVevis detector) measured analyte absorbance at l ¼ 234 nm. A Hewlett Packard 5890 series II gas chromatograph equipped with a Hewlett Packard 5971 series mass detector was employed to measure the concentrations of nitrobenzene in each sample before and after each UV irradiation. The GC
w a t e r r e s e a r c h 5 6 ( 2 0 1 4 ) 1 4 8 e1 5 5
separation was performed by using a TR-35MS column (Thermo Scientific). The column had a length of 30 m, diameter of 0.25 mm and a film thickness of 0.25 mm. Helium was used as the carrier gas. Before injecting the sample, the injector port was heated to 300 C, while the detector was heated to 280 C. The oven column temperature was gradually heated from 100 C (Initial time: 1min) to 350 C (Final time: 1min) at the rate of 15 C min1. The total run time was 18.67 min per sample. The MS detector was operated in single ion monitoring (SIM) mode for all samples being analyzed; 77, and 123 were the predominant m/z values for quantification of nitrobenzene. Nitrobenzene external standards for calibration were prepared in dichloromethane. A liquideliquid extraction technique with dichloromethane was developed to extract aqueous nitrobenzene for GCeMS analysis. In this technique 10 mL of dichloromethane was mixed with 30 mL of the sample. The 40 mL mixture was shaken for 10 min so that a complete phase change for nitrobenzene was assured. A borosilicate glass syringe was used to extract 1e2 mL of the organic phase.
3.
Results and discussion
3.1.
Observed advanced oxidation in ozonated waters
The extent of oxidation of selected probe compounds via O3 and O3/H2O2 were compared in the sampled landfill leachates, unmodified prior to exposure to direct and indirect ozone oxidation. Bisphenol-a, a plastic monomer that has been detected world-wide in municipal landfill leachate (Wintgens et al., 2006), is a water soluble organic compound with significant OH and O3 scavenging rates. As Fig. 1 illustrates, the addition of H2O2 results in an enhanced degradation of BPA in organics-free, lab-grade water (negligible background scavenging). The rate of reaction for BPA and OH (kOH, BPA ¼ 1.02 1010 M1s1) (Rosenfeldt and Linden, 2004), relative to molecular ozone (kO3,
151
4 1 1 BPA ¼ 1.68 10 M s ) (Deborde et al., 2005), indicates the potential for enhanced oxidation with the addition of H2O2 to solution prior to ozonation. Because of the selectivity of pCBA for OH(kOH,pCBA ¼ 5 109 M1s1) and its significantly slower O3 scavenging rate (kO3,pCBA ¼ 0.15 M1s1) (David Yao and Haag, 1991), pCBA was added to selected systems to observe the extent of direct ozone oxidation of BPA. As shown in Fig. 1, the addition of pCBA (5 mg/L) significantly reduces the availability of OH for oxidation of BPA, and indicates the significance of OH to total contaminant oxidation even when the solution is treated with O3 alone (vs. O3/H2O2). At an initial O3 dose of 4 mg/L, a 60% and 50% decrease in initial BPA was observed for O3/H2O2 and O3 treatment of BPA spiked in organics-free water. The same ozone dose and initial BPA concentration in mature landfill leachate saw only a 10% and 20% decrease in BPA with O3/H2O2 and O3 treatments, respectively. The results indicate that applying O3 alone is more efficient for the degradation of BPA in mature leachate than sequential H2O2 and O3. It is likely that O3 is rapidly consumed by the present DOM in leachate. Therefore, ∙OH formation is limited by the decomposition of O3, rather than the reaction of O3 and H2O2. In comparison with lab-grade water exposures, the rate of BPA oxidation in leachate was inhibited by the presence of background scavengers of OH and O3. Applying sequential H2O2 and O3 does not increase the impact on total oxidation of organic probe compounds in this sampled leachate.
3.2.
OH scavenging by landfill leachate DOM (LDOM)
As previously discussed, mature landfill leachates contain a relatively high concentration of refractory organic compounds. Dissolved organic matter in most waste and natural waters presents the most significant obstacle to targeted advanced oxidation. To assess the impact of leachate total organic carbon (TOC) on an OH-probe compound’s oxidation kinetics, dilute solutions of nitrobenzene, a nitro-aromatic industrial solvent, and mature leachate (pre-filtered with
Fig. 1 e Observed oxidation of bisphenol-a (BPA) in both organics-free, lab-grade water and mature landfill leachate (pH [ 7.5) with increasing doses of O3 (mg/L), and in the presence of H2O2 (2:1 [H2O2]:[O3]) and para-chlorobenzoic acid (pCBA, 5 mg/L). Dilute BPA solutions (2 mg/L) in lab-grade water were buffered to pH 7. The unmodified leachate sample was spiked with BPA prior to exposure.
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Fig. 2 e Observed pseudo-first-order oxidation rate of nitrobenzene (k0 ), and relative rate of nitrobenzene decay (k0 /k0 0) in DI water with increasing TOC (from mature landfill leachate).
0.45 mm membrane) were exposed to OH via indirect photolysis (UV/H2O2). Aliquots of mature landfill leachate were spiked into organics-free, laboratory water to achieve varying concentrations of TOC (10e50 mM). Oxidizing solutions of varying TOC concentration was intended to elucidate the significance of mature leachate organic matter on free OH in AOP treated leachate. The initial nitrobenzene concentration was 5 mg/L. Fig. 2 shows the inverse relationship between the concentration of TOC from mature landfill leachate and the rate of oxidation of nitrobenzene (pseudo-first-order k0 , s1). Increasing the concentration of TOC from the mature leachate significantly reduced the oxidation of nitrobenzene due to the competitive scavenging of available OH. The total OH scavenging in the mature landfill leachate was significant, requiring only 0.6 mg/L of TOC from leachate to reduce the rate of nitrobenzene advanced oxidation by 65%. These results indicate that LDOM may have a much larger contribution to total OH-scavenging in leachate influence waters, than has
been previously observed in AOP treated waters dominated by naturally-sourced organic matter. Competition kinetics with UV/H2O2 was performed to quantitatively assess the OH scavenging rate of LDOM. After applying the previously described UV/H2O2-based method for each sample, it was observed that with a lower concentration of tert butanol as the OH scavenger, a greater degradation of pCBA was achieved. In order to assess the impact of UV photolysis on the degradation of pCBA, the same UV fluence of 500 mJ/cm2 was applied to a sample which did not contain H2O2. The results indicated that the direct photolysis of pCBA from UV light was negligible; consequently OH was considered the only effective pathway for the degradation of pCBA in the irradiated samples. According to the pseudo-first-order rate law, the observed rate of pCBA degradation will be a function of the concentration of OH and the measured rate of reaction between pCBA and OH. kpCBA;obs ¼ kOH;pCBA ½OHss
(3)
Fig. 3 e Observed pseudo-first-order decay rates of para-chlorobenzoic acid (pCBA) with increasing doses of tert-butanol (mM), and in the presence of H2O2 (20 mg/L) in dilute solutions of young and mature sampled landfill leachates (TOC [ 3 mg/ L) at pH 7. Each sample was exposed to a UV fluence of 500 mJ/cm2.
w a t e r r e s e a r c h 5 6 ( 2 0 1 4 ) 1 4 8 e1 5 5
where [OH]ss is the steady-state concentration of OH radical. The validity of the steady-state assumption has been previously established for predicting observed rates of indirect photolysis by OH (Crittenden et al., 1999; Glaze et al., 1995). The steady-state concentration of OH (M) can be approximated in the UV/H2O2 treated samples as the ratio of the rate of OH formation to the rate of its simultaneous consumption.
½OHss ¼
Table 1 e Measured second-order rates of OH scavenging by LDOM (kOH,LDOM) in each sampled landfill leachate. Leachate sample Mature Young
3.3. Relative contribution of O3 and OH to observed contaminant oxidation in leachate As was seen in Fig. 1, applying O3 in the presence of H2O2 (O3/ H2O2) in organics-free, lab-grade water increases the OH concentration in solution. However, in more complicated water matrices such as municipal wastewater, H2O2 does not
aOH kOH;LDOM ½LDOM þ kOH;tBuOH ½t BuOH þ kOH;pCBA ½pCBA þ kOH;H2 O2 ½H2 O2
where aOH is the formation rate of OH in UV-irradiated solution (M s1), kOH,t-BuOH ¼ 6 108 M1s1 (Buxton et al., 1988a), and kOH,H2O2 ¼ 2.7 107 M1s1 (Christensen et al., 1982). If 1/kpCBA,obs is plotted as a function of tert-butanol (tBuOH) concentration (see Fig. 3), linear regression can be used to solve for the unknown rate constant, kOH,LDOM, for each leachate sample, as previously demonstrated (Katsoyiannis et al., 2011). Comparing the calculated kOH,LDOM to the previously published OH scavenging rates for natural and waste waters, it can be seen that the measured kOH,LDOM of each leachate sample (Table 1) are significantly higher than the kOH,DOM reported for other water matrices. Katsoyiannis et al. (2011) reported a value of 2e2.7 104 L mgC1 s1 for three different Swiss surface waters (Katsoyiannis et al., 2011). In the same research, Katsoyiannis et al. (2011) obtained a higher kOH,DOM of 3.5 104 L mgC1 s1 for a wastewater sample. Brezonik and Fulkerson-Brekken (1998) obtained a range of 1.53e3.07 104 L mgC1 s1 for the five different North American surface water samples they studied (Brezonik and Fulkerson-Brekken, 1998). It is clear that both the relative concentration of DOM in leachate, and its unique composition (likely both autochthonous and anthropogenic in origin) are major impediments to efficient AOP treatment of landfill leachates. It has been previously suggested that during leachate stabilization in a landfill cell, microbial processes result in extensive degradation of humic, fulvic, and proteinlike structures (Yunus et al., 2011). Through fluorescence spectral analysis and LDOM fractionation, the LDOM from young landfill has been reported to be predominantly proteinlike, while older landfill produced leachate with a humic-and fulvic-like dominated composition (Huo et al., 2008). Proteins with a greater degree of CeH bonding present an increased opportunity for H-abstraction by OH, and their reduced presence in mature landfill leachate may contribute to the order-of-magnitude difference in observed kOH between the two sampled leachates.
kOH,LDOM (M1s1)
kOH,LDOM (L mgC-1s1)
9.75 0.54 108 8.28 0.19 109
8.13 0.45 104 6.90 0.15 105
153
(4)
necessarily enhance the OH exposure (Buffle et al., 2006). For the sampled leachates, it can be anticipated that the fractional contribution of OH to the overall oxidation rate of dissolved organic probe compounds will be quite small. In solutions where the oxidation of the probe compound (with a known O3 and OH scavenging rates) and a reference compound (with a known, higher OH scavenging rate than the pollutant compound and a negligible O3 scavenging rate) is occurring simultaneously, the following equation can be used to estimate the contribution of OH reactions in the oxidation of the probe compound (Huber et al., 2005). f OH ¼
½RCt ln ½RC 0 ½Mt ln ½M
kOH;M kOH;RC
(5)
0
where, M is the target compound and RC is the reference compound. In order to provide a useful estimation of the contribution of OH in the oxidation of BPA in a mature landfill leachate, a continuous ozone exposure in a gas washing column containing mature leachate was performed (250 mL). In this experiment BPA and pCBA were selected for the probe and reference compounds, respectively. It was observed that BPA was completely oxidized after 1.5 min, whereas the concentration of pCBA was measurable even after 10 min of continuous ozonation of the leachate sample. By calculating fOH, it was determined that the contribution of OH to BPA oxidation was significantly lower in the mature leachate (fOH ¼ 0.03) than in organics-free, lab-grade water (fOH ¼ 0.15). The young leachate sample was not tested, as it was anticipated that the order-of-magnitude greater kOH,LDOM (Table 1) would result in negligible fOH. The amount of O3 it would require to produce significant OH in the mature landfill leachate sample can be assessed by, again, applying a steady-state approximation for the production of $OH with continuous dosing of H2O2 to leachate with known residual O3 ([O3]res). ½OHss ¼
2k1 ½H2 O2 0 10ðpHpK;H2 O2 Þ ½O3 res k11 HCO 3 0 þ k13 ½LDOM
(6)
In this equation, k1 is the rate constant of eq. (2) (2.8 106 M1s1). The concentration of bicarbonate ion was found to be 1.2 M in the sampled mature landfill leachate, and k11 is the known second-order rate constant for the reaction of bicarbonate with OH (8.5 106 M1s1). The observed rate of OH scavenging in the mature leachate sample was selected
154
w a t e r r e s e a r c h 5 6 ( 2 0 1 4 ) 1 4 8 e1 5 5
Fig. 4 e The predicted formation of OH in the sampled mature landfill leachate and the corresponding rate of OH-oxidation of an organic probe compound (kOH [ 5 3 109 ML1sL1) with increasing O3 and H2O2 doses.
for k13. With an assumed molar ratio of [H2O2]:[O3] of 0.5, [OH]SS was calculated for increasing doses of O3 in the sampled mature landfill leachate. As can be seen in Fig. 4, as much as 70 mg/l of O3 would be required to see oxidation of 90% of relatively OH-selective, organic micro-pollutants having kOH w5 109 M1s1 (e.g., pCBA).
4.
Conclusions
This study assessed the OH scavenging rate of leachate dissolved organic matter and its potential impact on selection of ozonation processes for pre-treatment of landfill leachate. Leachate, in general is characterized by high TOC, and the concentration of LDOM is in an inverse relationship with the age of the landfill. The OH scavenging rates of LDOM in mature and young landfill leachates (FL, USA) were determined to be 9.76 108 M1s1 and 8.28 109 M1s1, respectively. These observed OH scavenging rates of LDOM were significantly greater than previously reported kOH,DOM in municipal wastewater and surface waters. Although O3 based AOPs can be considered as a potential solution for color, COD, and anthropogenic organic pollutants in landfill leachate, due to the observed kOH,LDOM it can be concluded that landfill leachate pre-oxidation is more effective with application of O3 (without H2O2), instead of an O3/H2O2 AOP. Relatively high concentrations of O3 and H2O2 are needed to produce sufficient OH for targeting OH-selective organic micropollutants in stabilized, mature landfill leachate (greater than 40 mg/L of residual O3 and 14 mg/L of H2O2 required to see more than 50% oxidation of the organic probe compound). Since OH scavenging rates in leachate from ‘young’ landfill cells are expected to be an order-of-magnitude higher, applying O3/H2O2 for advanced oxidation could be prohibitively expensive for many landfills treating leachate of any age.
Acknowledgments The authors would like to acknowledge the financial support for this research provided by the Hinkley Center for Solid and Hazardous Waste Research (University of Florida, Gainesville, FL, #0932017).
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