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Coupled heat-activated persulfate – Electrolysis for the abatement of organic matter and total nitrogen from landfill leachate
Waste Management 97 (2019) 47–51
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Coupled heat-activated persulfate – Electrolysis for the abatement of organic matter and total nitrogen from landfill leachate Jefferson E. Silveira ⇑, Juan A. Zazo, Jose A. Casas Chemical Engineering, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 12 April 2019 Revised 5 July 2019 Accepted 27 July 2019 Available online 1 August 2019 Keywords: Heat-activation Landfill leachate Sulfate radical Ammonium removal Electrolysis
a b s t r a c t This work analyzes the viability of a coupled heat-activated persulfate (PS) and electro-oxidation treatment toabatetheorganic matter and nitrogen from ahigh polluted landfill leachate (5500 mg L1 TOC; 5849 mg L1 TN, pH: 8.4). These characteristics makes PS as a suitable oxidant to deal with the recalcitrant organic matter. Under the optimal conditions (70 °C and 60% of the stoichiometric amount of PS), around 60% of the initial organic load was mineralized. On the contrary, the nitrogen removal was below 20%. A subsequent electrolytic stage using Ti/IrO2–TaO2 anode at 175 mA cm2 and 0.42 M NaCl during 60 min, led to overall organic matter and nitrogen removal above 85% and 90%, respectively, with energy requirement of 38 kWh per kg of nitrogen removed. In this sense, the combined process achieves a significant reduction in terms of energy consumption, up to one fifth in relation to sole electrolysis. These results confirm the feasibility of this combined process to treat landfill leachate. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The disposal of municipal solid waste into sanitary landfills provides one of the more economical alternatives. However, it entails several environmental problems such as the generation of leachates. These, in absence of appropriate collection systems, can reach surface and ground waters, polluting them. The nature of this secondary waste, characterized by high concentrations of ammonia and organic compounds refractory to conventional biological oxidation (Silveira et al., 2014), limits the conventional treatment technologies. Nowadays, the most common leachate strategies include coagulation-flocculation, precipitation, biological treatment, adsorption on activated carbon or chemical oxidation coupled with membrane technologies such as reverse osmosis (Abbas et al., 2009). Nonetheless, these solutions involve some drawbacks, including membrane fouling, low efficiency for color removal and expensive investment and maintenance cost (Fernandes et al., 2017; Renou et al., 2008). The use of advanced oxidation processes (AOPs) based on hydroxyl (HO) and sulfate (SO 4 ) radicals have been proposed to assist the current technologies. Within the first group, photo and electro-Fenton (Primo et al., 2008; Sruthi et al., 2018), electrochemical treatment (Li et al., 2016), ozone and sonication (Asaithambi et al., 2017) have been extensively studied and used for landfill leachate treatment. Nonetheless, the pH and high alka⇑ Corresponding author. E-mail address: [email protected] (J.E. Silveira). https://doi.org/10.1016/j.wasman.2019.07.037 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
linity of the leachate hinder the on-site application of these processes (Zhao et al., 2010). In a previous work we proposed induced SO 4 generation from persulfate (PS) activation to mitigate those short comings (Silveira et al., 2018), achieving significant organic matter removal. SO 4 generation can be carried out by PS decomposition using transition metals such as iron, (Liu et al., 2018; Wu et al., 2018), ultraviolet light (Hassan et al., 2016), ultrasounds (Yang et al., 2015), or heat-activation (Deng and Ezyske, 2011). This later becomes very attractive in those landfills with biogas exploitation to obtain power (Zuberi and Ali, 2015). More recently, electrochemical activation of sulfate and PS have been studied for wastewater treatment (Chen et al., 2018; Silveira et al., 2017). Despite AOPs based on SO 4 radicals can be considered a good alternative to remove the organic matter (Wang and Wang, 2018), there is greatly limited towards nitrogen removal (Abu Amr et al., 2013; Deng and Zhao, 2015). In this sense, electrolysis has been widely use to remove ammonium from landfill leachate through an indirect oxidation by chlorine/hypochlorite formed by chloride electro-activation (Li and Liu, 2009; Moraes and Bertazzoli, 2005). For instance Chiangi et al. (1995) removed about 2.6g L1 of ammonium using 150 mA cm2 and 0.21 M additional chloride with a Sn-Pd-Ru oxide anode after 240 min. It must be noted that the removal of ammonia is predominant in the competition between NH+4 and COD oxidation (Fernandes et al., 2015). Nonetheless, the formation of toxic chlorinated intermediates and the operational costs suppose a great drawback towards the application of this process (Mandal et al., 2017).
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Therefore, based on the aforementioned concerns, this work outlines a new approach to treat landfill leachates by coupling heat-activated PS to remove organic matter and in situ electrogenerated active chlorine using Ti/IrO2–TaO2 anode to remove the nitrogen content. Thus, the proposed treatment would minimize the formation of chlorinated organic compounds as well as the energy requirements, since electrolysis is applied in a second stage to remove N. 2. Material and methods 2.1. Reactants All chemicals used were purchased from Sigma-Aldrich, USA, except for persulfate, which was obtained from Panreac (Spain). 2.2. Typical reaction procedure The heat-persulfate system set-up consisted of a magnetically stirred sealed glass jacketed batch reactor (500 mL useful volume). The concentration of PS was varied between 20% (0.19 M) and 100% (0.94 M) of the theoretic stoichiometrical amount needed to mineralize COD (12 g PS/g COD). The temperature was varied in the range 30–90 °C, and the initial pH amid 3 and 12, without further control. Samples were withdrawn at regular time intervals and placed in an ice bath to quench the reaction. Then, the samples were passed through a 0.45 mm filter and immediately analyzed to follow the progress of the reaction. The supernatant phase and the sludge formed during this first treatment were removed before the electrochemical oxidation. The subsequent electro-oxidation step, used for nitrogen removal, was carried out at 25 °C in a magnetically stirred jacketed electrochemical reactor using Ti/IrO2–TaO2 (Ir/Ta weight ratio 70/30) as anode and stainless steel as cathode. The gap between electrodes was 5 mm. The role of current density and NaCl concentration was assessed within the range 25–250 mA cm2 and 0.14– 0.55 M, respectively. 2.3. Analytical procedure The leachate samples were collected in a municipal landfill sited in Madrid (Spain). The main parameters of this effluent are shown in Table 1. The analytic procedure to quantify total organic carbon (TOC), total nitrogen (TN), COD, residual persulfate and ammonium was described elsewhere (APHA, 2012). The identification of the aromatic by-products was performed following the procedure described by Zhang et al. (2011).
Fig. 1. Time evolution of TOC removal with heat-activated PS at different temperatures. (Experimental conditions: [TOC]0 = 5500 mg L1, [PS] = 100% stoichiometric dose, pH0 = 8.4).
temperature above 50 °C are necessary to promote PS decomposition into SO. 4 (Eq (1)). Additional oxidants such as hydroxyl radicals (HO) and peroxymonosulfate (HSO 5 ) can be generated in the reaction media, as shown in Eqs. (2) and (3) (Waldemer et al., 2007).
S2 O8 2 + heat ! 2SO4
ð1Þ
SO4 + H2 O ¡ SO4 2 + Hþ + HO
ð2Þ
SO4 + HO ! HSO5
ð3Þ
TOC conversion reached 82% after 5 h when temperature was fixed at 70 °C, and increased up to 94% at 90 °C, with PS conversions of 84 and 98%, respectively (Fig. S1). Almost complete color reduction was achieved working at 70 °C after 5 h (Fig.S2). In this oxidation step, a residual sludge was formed. Elemental analysis of this residue (210 mg L1 dry weight) for the 70 °C run shows amounts of C (25%), N (5.5%), S (11%) and H (3.4%). Besides trace metals concentrations (Fe, Ti, Al, Mn, Ni and Cu) were also detected. Fig. 2 shows the effect of the initial PS dose upon TOC time-evolution at 70 °C. As can be observed, the dosage of PS had
3. Results and discussion 3.1. Effect of operational parameters on heat activated persulfate Fig. 1 shows the mineralization profiles as a function of reaction temperature using the stoichiometric PS dose. As can be observed Table 1 Landfill leachate composition. Parameters
Values
COD, mg L1 TOC, mg L1 TN, mg L1 Absorbance, a.u. at 465 nm pH Chloride, mg L1 Sulfate, mg L1 Conductivity, mScm1
15,000 5500 5849 4.8 8.4 8030 1120 37.2
Fig. 2. Time evolution of TOC removal at 70 °C as a function PS dose. (Experimental Conditions: [TOC]0 = 5500 mg L1, T: 70 °C, and pH0 = 8.4).
J.E. Silveira et al. / Waste Management 97 (2019) 47–51
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a positive effect upon the TOC removal that increased around 68% as PS concentration increased from 20% to 100% of the stoichiometric dose. In terms of PS yield (Ɛ), defined as the amount of TOC removed per unit weight of PS fed (Silveira et al., 2017), the highest value was obtained using 60% PS at 70 °C. For this, the experimental value (Ɛexp: 29.9) was close to the theoretical one (Ɛtheoretical: 30.5). Therefore, these were considered as the optimum experimental conditions. The slight difference between both values can be justified by the amount of carbon in the generated sludge. In this sense, the elemental analysis of this solid revealed the presence of 52 mg of C (approximately 1% of the initial TOC). The initial pH value has influence upon the PS activation (Fig. S3). In acid solution, PS hydrolysis leads to HSO 5 , which can be further hydrolyzed into H2O2 and HSO 4 (Eqs. (4) and (5)) (Xu et al., 2015), whereas in alkaline conditions, sulfate anion radical can generate HO by reaction with OH (Eq. (6)). Maximum mineralization was reached at initial pH 3 and 6. Nonetheless, it should be noted that pH steeply decreases as the oxidation occurs, with final pH values around 3 in all the performed experiments.
S2 O8 2 + H2 O ! HSO5 + HSO4
ð4Þ
HSO5 + 2H2 O ! H2 O2 + HSO4
ð5Þ
SO4 + OH ! HO + SO4 2 SO 4
ð6Þ
The contribution of and HO was confirmed by using EtOH and TBA as scavengers (Silveira et al., 2017). The results indicated that SO 4 was the predominant radical species on the absorbance removal in heat-activated persulfate (Fig. S4). Fig. 3. Effect of current density (a) and Cl concentration (b) upon TN removal by electrolysis after heat-activated PS pretreatment.
3.2. Nitrogen removal by electro oxidation as a post-treatment The previous results confirmed the feasibility of heat-activated PS to reduce the organic matter in landfill leachate. Nevertheless, the nitrogen reductionto N2was lower than 20%. NOX generation was also studied, being negligible in the PS-oxidation (Fig. S5). The subsequent experiments assess the effect of electrolysis to reduce the TN content. Briefly, the applied current leads to chlorine/hypochlorite generation by anodic oxidation of chloride (Eqs. (7)–(9)), usually found in landfill leachate. In presence organic matter and nitrogen, both substrates compete for the radicals (Eqs. (10) and (11)), reducing the overall efficiency of the process.
2Cl ! Cl2 + 2e
ð7Þ
Cl2 + H2 O ! Cl + HOCl + Hþ
ð8Þ
HOCl ! OCl + Hþ
ð9Þ
Organics + OCl ! intermediates ! CO2 + Cl + H2 O
ð10Þ
2NH4 þ + 3HOCl ! N2 + 3Cl + 5Hþ + 3H2 O
ð11Þ
Fig. 3 shows the effect of applied current and NaCl concentration upon the TN reduction using the PS pre-treated effluent. As described in literature (Scialdone et al., 2009), a higher chlorine/ hypochlorite generation by chloride anodic oxidation is achieved when increasing the applied current. In this sense, the TN removal degree was enhanced from 6.5 to 38% when elevating the current density from 25 to 250 mA cm2 (Fig. 3a). Still, as previously described, the electrolyte concentration is the most influencing parameter for ammonia abatement during electrochemical treatment of landfill leachate (Silveira et al., 2014). Thus, the addition of NaCl (0.41–0.55 M) as supporting electrolyte increases the treat-
Fig. 4. Removal of TN by coupled heat-activated PS and electrolysis. Experimental Conditions: [PS] = 60% T = 70 °C, [NaCl] = 0.42 M at 175 mA cm2. (Red dots corresponds to the TN evolution by sole electrolysis). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ment efficiency by approximately 80% with respect to blank control without Cl addition (Fig. 3b), removing more than 4 g of N within 60 min at 175 mA cm2. Fig. 4 shows the NH+4 evolution along the coupled treatment. For the sake of comparison the NH+4 evolution with sole electrolysis and for heat-activated PS is included, as ammonia can be oxidized
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Fig. 5. GC–MS analysis of raw landfill leachate and the effluents after electrolysis, heat-activated PS and coupled process. (Experimental conditions: [PS] = 60% T = 70 °C, [NaCl] = 0.41 M at 175 mA cm2).
to NH2 by SO 4 (Yang et al., 2017). In a sequence of chlorine mediated oxidations, ammonia is transformed to NH2Cl, NHCl2, NOH and then to N2 (Moraes and Bertazzoli, 2005). The analysis of the gas phase during electrolysis dismissed the NOx formation so it can be concluded that ammonia is oxidized by active chlorine and converted to N2 (Li and Liu, 2009; Silveira et al., 2014). These results are significantly better than those achieved when using electrolysis as a sole treatment due to competition between TN and COD. Moreover, the PS pretreatment avoids the passivation of the electrode surface and the formation of foams during the electrolytic stage. These results endorse the proposed treatment as a viable alternative for the treatment of landfill leachate, achieving high removals of both organic matter and total nitrogen. Fig. 5 gathers the GC–MS analysis of the effluent after the coupled treatment. For the sake of comparison, the results obtained by single steps heat-activated PS and electrolysis are also included. As can be observed, the amount of organic pollutants is remarkably reduced after the coupled process. The residual organic matter corresponds to trace of low-molecular-weight organics, including phenols (2,4-Di-tert-butylphenol), PAHS (naphthalene), esters (benzyl carbazate), and phthalates (Bis (2-ethylhexyl) phthalate). In terms of energy consumption (EC) (Salazar et al., 2012), the combined process decreased 5 times the needs per kg of TN removed compared with the sole electrolysis (Fig. S6). Chiang et al. also observed a decrease on EC from 159 to 99 kWhm3 during single electrochemical treatment and electrolysis combined with coagulation pretreatment, respectively (Chiang et al., 2001). Besides, electrolyte addition also had a significant effect upon EC. Hence, applying a current density of 175 mA cm2 during 60 min, EC varied from about 74 kWhkg1 without external NaCl addition to 38 kWhkg1 in the presence of 0.42 M NaCl. These results are in agreement with those obtained by Pérez et al. (2012) which also
reported an EC reduction close to 40% as Cl dose increased from 5 to 20 g L1 during electrochemical NH+4 removal using BDD electrodes. The average current efficiency (ACE) was calculated for the conversion of NH+4 to N2 (Chen et al., 2007). As can be observed in Fig. S7, the ACE gradually increased about 40% when the NaCl increased from 0.14 to 0.56 M. 4. Conclusions Coupled heat-activated PS and electrolysis treatment emerges as a feasible alternative to abate both organic matter and nitrogen from landfill leachates. The first stage entails an inexpensive and non-selective oxidation process able to overcome the drawbacks associated to the color and turbidity of this effluent. The subsequent electrolytic treatment is a well-known alternative to remove the nitrogen content that was not significantly reduced by the previous treatment. Both processes allow tailoring the operational conditions to achieve the desired organic matter and nitrogen reduction. The main advantage of the coupled treatment lies on the optimization of the energy requirements during electrolysis that is focuses on the TN removal. Besides the pre-treatment with heatactivated PS avoids the passivation of the electrode surface and the formation of foams during the subsequent electrolysis. Acknowledgements This work has been supported by MINECO and Comunidad Autónoma de Madrid through projects CTM2016-76454-R and P2018/EMT-4341, respectively. Jefferson E. Silveira acknowledges the support from CAPES: Science Without Borders Program, Ministry of Education Brazil, under grant BEX-1046/13-6.
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Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.07.037.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Coupled heat-activated persulfate – Electrolysis for the abatement of organic matter and total nitrogen from landfill leachate Jefferson E. Silveira ⇑, Juan A. Zazo, Jose A. Casas Chemical Engineering, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 12 April 2019 Revised 5 July 2019 Accepted 27 July 2019 Available online 1 August 2019 Keywords: Heat-activation Landfill leachate Sulfate radical Ammonium removal Electrolysis
a b s t r a c t This work analyzes the viability of a coupled heat-activated persulfate (PS) and electro-oxidation treatment toabatetheorganic matter and nitrogen from ahigh polluted landfill leachate (5500 mg L1 TOC; 5849 mg L1 TN, pH: 8.4). These characteristics makes PS as a suitable oxidant to deal with the recalcitrant organic matter. Under the optimal conditions (70 °C and 60% of the stoichiometric amount of PS), around 60% of the initial organic load was mineralized. On the contrary, the nitrogen removal was below 20%. A subsequent electrolytic stage using Ti/IrO2–TaO2 anode at 175 mA cm2 and 0.42 M NaCl during 60 min, led to overall organic matter and nitrogen removal above 85% and 90%, respectively, with energy requirement of 38 kWh per kg of nitrogen removed. In this sense, the combined process achieves a significant reduction in terms of energy consumption, up to one fifth in relation to sole electrolysis. These results confirm the feasibility of this combined process to treat landfill leachate. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The disposal of municipal solid waste into sanitary landfills provides one of the more economical alternatives. However, it entails several environmental problems such as the generation of leachates. These, in absence of appropriate collection systems, can reach surface and ground waters, polluting them. The nature of this secondary waste, characterized by high concentrations of ammonia and organic compounds refractory to conventional biological oxidation (Silveira et al., 2014), limits the conventional treatment technologies. Nowadays, the most common leachate strategies include coagulation-flocculation, precipitation, biological treatment, adsorption on activated carbon or chemical oxidation coupled with membrane technologies such as reverse osmosis (Abbas et al., 2009). Nonetheless, these solutions involve some drawbacks, including membrane fouling, low efficiency for color removal and expensive investment and maintenance cost (Fernandes et al., 2017; Renou et al., 2008). The use of advanced oxidation processes (AOPs) based on hydroxyl (HO) and sulfate (SO 4 ) radicals have been proposed to assist the current technologies. Within the first group, photo and electro-Fenton (Primo et al., 2008; Sruthi et al., 2018), electrochemical treatment (Li et al., 2016), ozone and sonication (Asaithambi et al., 2017) have been extensively studied and used for landfill leachate treatment. Nonetheless, the pH and high alka⇑ Corresponding author. E-mail address: [email protected] (J.E. Silveira). https://doi.org/10.1016/j.wasman.2019.07.037 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
linity of the leachate hinder the on-site application of these processes (Zhao et al., 2010). In a previous work we proposed induced SO 4 generation from persulfate (PS) activation to mitigate those short comings (Silveira et al., 2018), achieving significant organic matter removal. SO 4 generation can be carried out by PS decomposition using transition metals such as iron, (Liu et al., 2018; Wu et al., 2018), ultraviolet light (Hassan et al., 2016), ultrasounds (Yang et al., 2015), or heat-activation (Deng and Ezyske, 2011). This later becomes very attractive in those landfills with biogas exploitation to obtain power (Zuberi and Ali, 2015). More recently, electrochemical activation of sulfate and PS have been studied for wastewater treatment (Chen et al., 2018; Silveira et al., 2017). Despite AOPs based on SO 4 radicals can be considered a good alternative to remove the organic matter (Wang and Wang, 2018), there is greatly limited towards nitrogen removal (Abu Amr et al., 2013; Deng and Zhao, 2015). In this sense, electrolysis has been widely use to remove ammonium from landfill leachate through an indirect oxidation by chlorine/hypochlorite formed by chloride electro-activation (Li and Liu, 2009; Moraes and Bertazzoli, 2005). For instance Chiangi et al. (1995) removed about 2.6g L1 of ammonium using 150 mA cm2 and 0.21 M additional chloride with a Sn-Pd-Ru oxide anode after 240 min. It must be noted that the removal of ammonia is predominant in the competition between NH+4 and COD oxidation (Fernandes et al., 2015). Nonetheless, the formation of toxic chlorinated intermediates and the operational costs suppose a great drawback towards the application of this process (Mandal et al., 2017).
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Therefore, based on the aforementioned concerns, this work outlines a new approach to treat landfill leachates by coupling heat-activated PS to remove organic matter and in situ electrogenerated active chlorine using Ti/IrO2–TaO2 anode to remove the nitrogen content. Thus, the proposed treatment would minimize the formation of chlorinated organic compounds as well as the energy requirements, since electrolysis is applied in a second stage to remove N. 2. Material and methods 2.1. Reactants All chemicals used were purchased from Sigma-Aldrich, USA, except for persulfate, which was obtained from Panreac (Spain). 2.2. Typical reaction procedure The heat-persulfate system set-up consisted of a magnetically stirred sealed glass jacketed batch reactor (500 mL useful volume). The concentration of PS was varied between 20% (0.19 M) and 100% (0.94 M) of the theoretic stoichiometrical amount needed to mineralize COD (12 g PS/g COD). The temperature was varied in the range 30–90 °C, and the initial pH amid 3 and 12, without further control. Samples were withdrawn at regular time intervals and placed in an ice bath to quench the reaction. Then, the samples were passed through a 0.45 mm filter and immediately analyzed to follow the progress of the reaction. The supernatant phase and the sludge formed during this first treatment were removed before the electrochemical oxidation. The subsequent electro-oxidation step, used for nitrogen removal, was carried out at 25 °C in a magnetically stirred jacketed electrochemical reactor using Ti/IrO2–TaO2 (Ir/Ta weight ratio 70/30) as anode and stainless steel as cathode. The gap between electrodes was 5 mm. The role of current density and NaCl concentration was assessed within the range 25–250 mA cm2 and 0.14– 0.55 M, respectively. 2.3. Analytical procedure The leachate samples were collected in a municipal landfill sited in Madrid (Spain). The main parameters of this effluent are shown in Table 1. The analytic procedure to quantify total organic carbon (TOC), total nitrogen (TN), COD, residual persulfate and ammonium was described elsewhere (APHA, 2012). The identification of the aromatic by-products was performed following the procedure described by Zhang et al. (2011).
Fig. 1. Time evolution of TOC removal with heat-activated PS at different temperatures. (Experimental conditions: [TOC]0 = 5500 mg L1, [PS] = 100% stoichiometric dose, pH0 = 8.4).
temperature above 50 °C are necessary to promote PS decomposition into SO. 4 (Eq (1)). Additional oxidants such as hydroxyl radicals (HO) and peroxymonosulfate (HSO 5 ) can be generated in the reaction media, as shown in Eqs. (2) and (3) (Waldemer et al., 2007).
S2 O8 2 + heat ! 2SO4
ð1Þ
SO4 + H2 O ¡ SO4 2 + Hþ + HO
ð2Þ
SO4 + HO ! HSO5
ð3Þ
TOC conversion reached 82% after 5 h when temperature was fixed at 70 °C, and increased up to 94% at 90 °C, with PS conversions of 84 and 98%, respectively (Fig. S1). Almost complete color reduction was achieved working at 70 °C after 5 h (Fig.S2). In this oxidation step, a residual sludge was formed. Elemental analysis of this residue (210 mg L1 dry weight) for the 70 °C run shows amounts of C (25%), N (5.5%), S (11%) and H (3.4%). Besides trace metals concentrations (Fe, Ti, Al, Mn, Ni and Cu) were also detected. Fig. 2 shows the effect of the initial PS dose upon TOC time-evolution at 70 °C. As can be observed, the dosage of PS had
3. Results and discussion 3.1. Effect of operational parameters on heat activated persulfate Fig. 1 shows the mineralization profiles as a function of reaction temperature using the stoichiometric PS dose. As can be observed Table 1 Landfill leachate composition. Parameters
Values
COD, mg L1 TOC, mg L1 TN, mg L1 Absorbance, a.u. at 465 nm pH Chloride, mg L1 Sulfate, mg L1 Conductivity, mScm1
15,000 5500 5849 4.8 8.4 8030 1120 37.2
Fig. 2. Time evolution of TOC removal at 70 °C as a function PS dose. (Experimental Conditions: [TOC]0 = 5500 mg L1, T: 70 °C, and pH0 = 8.4).
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a positive effect upon the TOC removal that increased around 68% as PS concentration increased from 20% to 100% of the stoichiometric dose. In terms of PS yield (Ɛ), defined as the amount of TOC removed per unit weight of PS fed (Silveira et al., 2017), the highest value was obtained using 60% PS at 70 °C. For this, the experimental value (Ɛexp: 29.9) was close to the theoretical one (Ɛtheoretical: 30.5). Therefore, these were considered as the optimum experimental conditions. The slight difference between both values can be justified by the amount of carbon in the generated sludge. In this sense, the elemental analysis of this solid revealed the presence of 52 mg of C (approximately 1% of the initial TOC). The initial pH value has influence upon the PS activation (Fig. S3). In acid solution, PS hydrolysis leads to HSO 5 , which can be further hydrolyzed into H2O2 and HSO 4 (Eqs. (4) and (5)) (Xu et al., 2015), whereas in alkaline conditions, sulfate anion radical can generate HO by reaction with OH (Eq. (6)). Maximum mineralization was reached at initial pH 3 and 6. Nonetheless, it should be noted that pH steeply decreases as the oxidation occurs, with final pH values around 3 in all the performed experiments.
S2 O8 2 + H2 O ! HSO5 + HSO4
ð4Þ
HSO5 + 2H2 O ! H2 O2 + HSO4
ð5Þ
SO4 + OH ! HO + SO4 2 SO 4
ð6Þ
The contribution of and HO was confirmed by using EtOH and TBA as scavengers (Silveira et al., 2017). The results indicated that SO 4 was the predominant radical species on the absorbance removal in heat-activated persulfate (Fig. S4). Fig. 3. Effect of current density (a) and Cl concentration (b) upon TN removal by electrolysis after heat-activated PS pretreatment.
3.2. Nitrogen removal by electro oxidation as a post-treatment The previous results confirmed the feasibility of heat-activated PS to reduce the organic matter in landfill leachate. Nevertheless, the nitrogen reductionto N2was lower than 20%. NOX generation was also studied, being negligible in the PS-oxidation (Fig. S5). The subsequent experiments assess the effect of electrolysis to reduce the TN content. Briefly, the applied current leads to chlorine/hypochlorite generation by anodic oxidation of chloride (Eqs. (7)–(9)), usually found in landfill leachate. In presence organic matter and nitrogen, both substrates compete for the radicals (Eqs. (10) and (11)), reducing the overall efficiency of the process.
2Cl ! Cl2 + 2e
ð7Þ
Cl2 + H2 O ! Cl + HOCl + Hþ
ð8Þ
HOCl ! OCl + Hþ
ð9Þ
Organics + OCl ! intermediates ! CO2 + Cl + H2 O
ð10Þ
2NH4 þ + 3HOCl ! N2 + 3Cl + 5Hþ + 3H2 O
ð11Þ
Fig. 3 shows the effect of applied current and NaCl concentration upon the TN reduction using the PS pre-treated effluent. As described in literature (Scialdone et al., 2009), a higher chlorine/ hypochlorite generation by chloride anodic oxidation is achieved when increasing the applied current. In this sense, the TN removal degree was enhanced from 6.5 to 38% when elevating the current density from 25 to 250 mA cm2 (Fig. 3a). Still, as previously described, the electrolyte concentration is the most influencing parameter for ammonia abatement during electrochemical treatment of landfill leachate (Silveira et al., 2014). Thus, the addition of NaCl (0.41–0.55 M) as supporting electrolyte increases the treat-
Fig. 4. Removal of TN by coupled heat-activated PS and electrolysis. Experimental Conditions: [PS] = 60% T = 70 °C, [NaCl] = 0.42 M at 175 mA cm2. (Red dots corresponds to the TN evolution by sole electrolysis). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ment efficiency by approximately 80% with respect to blank control without Cl addition (Fig. 3b), removing more than 4 g of N within 60 min at 175 mA cm2. Fig. 4 shows the NH+4 evolution along the coupled treatment. For the sake of comparison the NH+4 evolution with sole electrolysis and for heat-activated PS is included, as ammonia can be oxidized
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Fig. 5. GC–MS analysis of raw landfill leachate and the effluents after electrolysis, heat-activated PS and coupled process. (Experimental conditions: [PS] = 60% T = 70 °C, [NaCl] = 0.41 M at 175 mA cm2).
to NH2 by SO 4 (Yang et al., 2017). In a sequence of chlorine mediated oxidations, ammonia is transformed to NH2Cl, NHCl2, NOH and then to N2 (Moraes and Bertazzoli, 2005). The analysis of the gas phase during electrolysis dismissed the NOx formation so it can be concluded that ammonia is oxidized by active chlorine and converted to N2 (Li and Liu, 2009; Silveira et al., 2014). These results are significantly better than those achieved when using electrolysis as a sole treatment due to competition between TN and COD. Moreover, the PS pretreatment avoids the passivation of the electrode surface and the formation of foams during the electrolytic stage. These results endorse the proposed treatment as a viable alternative for the treatment of landfill leachate, achieving high removals of both organic matter and total nitrogen. Fig. 5 gathers the GC–MS analysis of the effluent after the coupled treatment. For the sake of comparison, the results obtained by single steps heat-activated PS and electrolysis are also included. As can be observed, the amount of organic pollutants is remarkably reduced after the coupled process. The residual organic matter corresponds to trace of low-molecular-weight organics, including phenols (2,4-Di-tert-butylphenol), PAHS (naphthalene), esters (benzyl carbazate), and phthalates (Bis (2-ethylhexyl) phthalate). In terms of energy consumption (EC) (Salazar et al., 2012), the combined process decreased 5 times the needs per kg of TN removed compared with the sole electrolysis (Fig. S6). Chiang et al. also observed a decrease on EC from 159 to 99 kWhm3 during single electrochemical treatment and electrolysis combined with coagulation pretreatment, respectively (Chiang et al., 2001). Besides, electrolyte addition also had a significant effect upon EC. Hence, applying a current density of 175 mA cm2 during 60 min, EC varied from about 74 kWhkg1 without external NaCl addition to 38 kWhkg1 in the presence of 0.42 M NaCl. These results are in agreement with those obtained by Pérez et al. (2012) which also
reported an EC reduction close to 40% as Cl dose increased from 5 to 20 g L1 during electrochemical NH+4 removal using BDD electrodes. The average current efficiency (ACE) was calculated for the conversion of NH+4 to N2 (Chen et al., 2007). As can be observed in Fig. S7, the ACE gradually increased about 40% when the NaCl increased from 0.14 to 0.56 M. 4. Conclusions Coupled heat-activated PS and electrolysis treatment emerges as a feasible alternative to abate both organic matter and nitrogen from landfill leachates. The first stage entails an inexpensive and non-selective oxidation process able to overcome the drawbacks associated to the color and turbidity of this effluent. The subsequent electrolytic treatment is a well-known alternative to remove the nitrogen content that was not significantly reduced by the previous treatment. Both processes allow tailoring the operational conditions to achieve the desired organic matter and nitrogen reduction. The main advantage of the coupled treatment lies on the optimization of the energy requirements during electrolysis that is focuses on the TN removal. Besides the pre-treatment with heatactivated PS avoids the passivation of the electrode surface and the formation of foams during the subsequent electrolysis. Acknowledgements This work has been supported by MINECO and Comunidad Autónoma de Madrid through projects CTM2016-76454-R and P2018/EMT-4341, respectively. Jefferson E. Silveira acknowledges the support from CAPES: Science Without Borders Program, Ministry of Education Brazil, under grant BEX-1046/13-6.
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Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.07.037.
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