585343
research-article2015
WMR0010.1177/0734242X15585343Waste Management & ResearchIlyas et al.
Original Article
Residual organic matter and microbial respiration in bottom ash: Effects on metal leaching and eco-toxicity
Waste Management & Research 1–7 © The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X15585343 wmr.sagepub.com
A Ilyas1, KM Persson2 and M Persson2
Abstract A common assumption regarding the residual organic matter, in bottom ash, is that it does not represent a significant pool of organic carbon and, beyond metal-ion complexation process, it is of little consequence to evolution of ash/leachate chemistry. This article evaluates the effect of residual organic matter and associated microbial respiratory processes on leaching of toxic metals (i.e. arsenic, copper, chromium, molybdenum, nickel, lead, antimony and zinc), eco-toxicity of ash leachates. Microbial respiration was quantified with help of a respirometric test equipment OXITOP control system. The effect of microbial respiration on metal/residual organic matter leaching and eco-toxicity was quantified with the help of batch leaching tests and an eco-toxicity assay – Daphnia magna. In general, the microbial respiration process decreased the leachate pH and eco-toxicity, indicating modification of bioavailability of metal species. Furthermore, the leaching of critical metals, such as copper and chromium, decreased after the respiration in both ash types (fresh and weathered). It was concluded that microbial respiration, if harnessed properly, could enhance the stability of fresh bottom ash and may promote its reuse. Keywords Residual organic matter, microbial respiration, leaching, eco-toxicity, bottom ash
Introduction Bottom ash (BA) is a by-product of the municipal solid waste incineration process, which aims to convert organic content of solid wastes into thermal and electrical energy. However, owing to incomplete thermal oxidation of organic matter during the incineration, [some residual] organic matter is still present in BA (Stefano Ferrari and Baccini 2002). A common assumption is that this residual organic matter (ROM) in BA does not represent a significant pool of organic carbon. Beyond the metal ion complexation process, e.g. Arickx et al. (2010), it is of little consequence to evolution of ash/leachate chemistry. Previously, it has been shown that the ROM can be oxidised by microbes during respiration (Rendek et al., 2006b; Zhang et al., 2004). The ROM oxidation provides the energy for microbial growth and releases carbon dioxide (Wang et al., 2003). The CO2 released during the respiration can also react with BA, further supplementing the carbonation treatment. Moreover, the ROM has been shown to support the development of microbial biofilms, which could potentially trap the toxic metals (Aouad et al., 2008; Johnson et al., 2007) and/or immobilise metals (Lack et al., 2002). In essence, the microbial respiration plays a decisive role in stabilising the organic matter (Kalbitz et al., 2003). In the absence of it, leaching of the ROM can occur, which through complexation also can enhance metal leaching (Zomeren and Comans, 2009). Another interesting aspect of microbial respiration is its
effects on redox and minerals. For example, the excessive respiration process, by consuming oxygen, can alter the mobility and speciation of redox sensitive metals such as arsenic (Aaltonnen, 2008; Oremland and Stolz, 2005). By using secondary minerals, i.e. ferri-hydrites, for their respiration, the microbes can dislodge metals bound on mineral surfaces (Stolz et al., 2006). However, despite the dynamic nature of ROM and the diversity of impacts, it has not been linked to the evolution of BA’s chemistry, i.e. effects on metal leaching and eco-toxicity. Therefore, there is a need to further elucidate the role of ROM and microbial respiration in the stabilisation of BA. In previous studies, the ROM has been measured as a lumped parameter, expressed as a total organic carbon (TOC), which does not distinguish between inorganic and organic or labile fractions. This differentiation is important since assuming that all of 1Department
of Hydraulic and Environmental Engineering, Norwegian University of Science and Technology, Trondheim, Norway 2Division of Water Resources Engineering, Lund University, Lund, Sweden Corresponding author: A Ilyas, Department of Hydraulic and Environmental Engineering, Norwegian University of Science and Technology, Trondheim N-9471, Norway. Email: [email protected]
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the measured ROM is labile can lead to over estimation of its role or impacts (Kumpiene et al., 2011). Therefore, quantifying the respiration, along with the TOC, can be an indirect way of assessing the portion of labile (biodegradable) fraction of the ROM. Since, microbial respiration is sensitive to factors, i.e. moisture and temperature (Christ and David, 1996), and/or metal concentrations (Nwachukwu and Pulford, 2011). It can also be used as an indicator of the BA’s stability and environmental impact. In this study, we investigated the impact of aerobic respiration on the metal leaching as well as eco-toxicity of BA. The objectives of the study were threefold: (1) to quantify ROM as TOC; (2) to measure the oxygen consumption without addition of external substrate; (3) to evaluate the impacts of respiration on metal mobility, and acute toxicity of leachate.
Materials and methods BA sampling The sampling was done at an incineration plant in southern Sweden. Two composite samples of BA were collected from both fresh and 1.5 year old ash heap. To achieve a representative sample, BA heaps were sampled at three locations (top, middle and bottom), with the help of a mechanical shovel, and a composite sample of roughly 40 kg each was created. The composite samples were then reduced to approximately 5 kg through a quartering producer. After mechanical sieving, the particle size 11), but it increased in concentration between 8–11, perhaps owing to dissolution of ettringite (Cornelis et al., 2006). Although, in a later study, Cornelis, et al. (2012) attributed the antimony increase to dissolution of romeite (a calcium–antimonite mineral) instead of ettringite [Ca6Al2(SO4)3(OH)12·26H2O] within this range of pH. While, in the case of arsenic, the consumption of oxygen by respiration during the test could have affected its behaviour. In case of groundwater, it has been shown that arsenic tended to increase in low oxygen conditions, especially when organic matter was present. Further, arsenic is known to complex with organic acids (Bauer and Blodau, 2006), which are released during oxidation of ROM. The leaching macro-ions, i.e. Ca2+, Cl- and SO42- also decreased after the respiration in the fresh ash (Figure 5). This decrease could be owing to precipitation of calcium minerals, which are an important part of the weathering process (Piantone et al., 2004; Speiser et al., 2000). From the results, it is obvious that the microbial respiration was the driver behind the reduction in leachable TOC (Figure 2), and the decrease of pH. The decrease in the critical metals (copper, chromium, lead, etc.) and macro-ions, after the respiration, indicates that promoting the respiratory activities would help the stabilisation of ash. Figure 4 presents the effects of respiration on metal leaching from the weathered ash, which highlights the contrast with the
1
As
Cr
Cu
Mo
Ni
Pb
Sb
Zn
Figure 4. Effects of respiration on metal leaching from weathered ash samples. 2500 2000 Before mg/kg
As
A er
1500 1000 500 0 Ca Fresh
SO4 Fresh
Ca Weathered SO4 Weathered
Figure 5. Leaching of calcium and SO42- before and after the respiration.
behaviour of fresh BA. Metals such as nickel, antimony, lead and zinc increased in their concentrations, while metals such as arsenic, chromium, copper and molybdenum decreased in their concentrations. As discussed earlier, the reduction in metal leaching seems to be related to the reduction leachable TOC after the respiration tests. One plausible explanation is that weathered ash is already stable with formation of secondary mineral phases, such as calcite and aluminium/iron-oxides, etc. The respiration is disrupting this equilibrium by changing redox conditions and adding humic acids to the solution. Hence, it is not surprising that metal concentrations increase along with an increase in calcium and sulphates. As far as arsenic is concerned, the decrease in its concentration could be related to an increase in calcium concentration. Arsenic is known to precipitate with free calcium ions. Sadiq (1997) has reported that adding calcium ions to a metal-containing solution can lead to formation of calcium–arsenic precipitates and decrease in arsenic concentrations. The metals, such as lead, can complex with ions such as Cland SO42-, which precipitate with the aging process (Speiser et al., 2000). Thus, in the weathered ash, dissolution of carbonates and metal salts can explain the increased leaching of these metals (nickel, zinc and lead). While antimony, as in fresh ash, seems more affected by the consumption of oxygen and/or increased SO42- concentration, which increased in weathered ash (Figure5).
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Table 1. Changes in sample pH, O2 consumption and EC50 owing to the respiratory process. The labels B1 and A1 refer to fresh ash before and after the leaching tests, while B2 and A2 weathered ash before and after the leaching tests. Sample
pH
O2(mg/Kg dry BA)
LC/EC50
B1 A1 B2 A2
11.64 ± 0.05 10.74 ± 0.49 8.82 ± 0.18 8.40 ± 0.05
0 0.87 ± 0.087 0 0.73 ± 0.058
6.59 ± 0.01 16.77 ± 0.01 113.86 ± 0.05 414.11 ± 0.05
Table 2. Pearson correlation between EC50 values and metal ion concentrations before and after the respiration tests. Fresh bottom ash EC50 P-val
Cd –0.953 0.003
Co –0.828 0.042
Cu –0.886 0.019
Mo –0.961 0.002
Si 0.967 0.002
Zn –0.908 0.012
Cu –0.736 0.095
Pb 0.803 0.054
Mo –0.76 0.079
Si 0.966 0.002
Zn 0.855 0.03
Weathered bottom ash EC50 P-val
Co 0.779 0.068
Bold entries indicate a strong and significant correlation between the variables.
Suer et al.,(2009) reported from leaching tests on 12-year BA that antimony was controlled by available SO4 concentrations. In real life, one can expect the effect of pH change to be stronger, since in the experiment CO2 was removed from the system and only a small percentage may have reacted. This was done to accurately quantify the oxygen consumption, which would not be possible had CO2 been allowed to freely react with the ash. The consequence of microbial respiration would differ depending the age of the BA. For weathered BA, the production and dissolution of CO2 would generate acidity in pore water, further supplementing the acidity from proton released during the respiration. Weathered ash is already stabilised and its capacity to take up more CO2 may be limited, therefore, dissolution of secondary minerals, owing to acidity, can lead to enhanced leaching of metals. In contrast, in fresh ash, the production and dissolution of CO2 in pore water would facilitate its reaction with alkaline components owing to a high pH. This would lead to a reduction of pH and enhanced formation of secondary mineral phases and reduced leaching of metals. This conclusion about the effect of microbial respiration and the ROM on the weathered ash is line with Ilyas et al. (2014), who showed that the addition of organic matter and increased respiration lead to increased leaching metals in landfill covers.
Respiration and eco-toxicity Table 1 shows the pH values of samples along with EC50 (in %). The sample number B1 and A1 represent the fresh ash, while B2 and A2 represent the weathered ash, before and after the degradation, respectively. The EC50 value for the fresh ash leachate was 6.59, which increased after the respiration to 16.77. For the weathered ash, high EC50 (113 and 414) values indicate a nontoxic leachate even before the respiration tests. However, the
impact of respiration was similar to the fresh ash as EC50 increased after the respiration. The results show that the respiration reduced the toxicity of ash leachate. The co-relation between the pH change and the EC50 value was high (–0.835). At a higher pH, increased activity of OH- is known to cause toxicity to Daphnia (Seco et al., 2003). These results show that for the fresh ash the EC50 value was pH dependent and any modification in pH would lead to change in its toxicity. Since the change in pH comes after the respiration, it seems reasonable to conclude that there is a relationship between the respiration and the change in toxicity of ash leachate. In weathered ash, the total concentrations of four metals (nickel, lead, antimony and zinc) increased, but the EC50 of ash leachate decreased, which perhaps indicates the modification of metal speciation to less toxic forms. This is not unusual since microbes are known to modify the metal species during their respiratory and metabolic activities (Gadd, 2000). Another possible explanation is the compensating effect of other ions, such as calcium and magnesium. In both cases, the release of calcium and magnesium increased after the respiration. The old ash, perhaps owing to calcite dissolution, had higher concentrations of Ca than fresh ash even before the respiration tests. When it comes to metals, given the number of metal ions and other competing ions present in the leachate, it is rather difficult to ascertain exactly which metal ion is responsible for the observed toxicity. Pearson correlation points out a strong correlation (both +ve and –ve) between several metal ions and EC50 values of fresh and weathered BA (Table 2). This result points out that when it comes to the toxicity of a complex liquid, such as leachate it is more than one more parameter that can cause a toxic effect. In general, one can observe a reduction in toxicity after the respiration. The process of respiration seems to reduce the pH and metal concentrations, both of which could have contributed to this reduction in EC50 values.
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Conclusions The results of the current study provided interesting insights into the respiration process and its effect on leaching and eco-toxicity of the ashes. The results show that 1.5 years of natural weathering on an incinerator site did not diminish the pool of biodegradable ROM, which shows the inefficiency of this process. More importantly, the results point out that despite being enriched in toxic metals, BA still allows the microbial respiratory processes, even at a very high pH. Contrary to the current perception that ROM is of no biological consequence in the ashes, the results show that it not only supports respiratory activities, but also exerts control over leaching of the organic carbon, metal mobility and ecotoxicological impact of ash leachates. The respiration reduced the pH and eco-toxicity in both ashes, which indicates the toxicity modification or treatment potential of respiration. For weathered ash, the leaching of metals (nickel, lead, antimony and zinc) will probably increase, which can be problematic in reuse situations, i.e. landfill covers where additional organic matter input is a strong possibility. Finally, the results indicate that the respiration can have variable effects on BA depending on its age. For fresh BA, the respiration can provide a route to ‘bio-carbonation’, but for the weathered BA it may not be a desirable activity. This biogenic CO2, much like atmospheric, can also react with alkaline components of fresh BA during the carbonation reactions. However, unlike the atmospheric CO2, the biogenic CO2 would be better distributed throughout the ash heaps, because microbial respiration would occur inside the heaps. Thus, the microbial respiration represents an intrinsic and potentially efficient pathway to carbonation of the ashes. However, based on this limited set of data, one can only highlight the macroscopic effects of the ROM and associated respiratory process. For bio-carbonation, the effects of process parameters, such as moisture, temperature, substrate, respiration rates and ash volume, need to be investigated further.
Acknowledgements Dr Martijn van Praagh of CEC at Lund University in Sweden is acknowledged for providing the OXITOP setup used in the study.
Declaration of conflicting interests The authors declare that there is no conflict of interest.
Funding This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
References Aaltonnen E (2008) Prokaryotic Arsenic Resistance – Studies in Bacillus Subtilis. PhD Thesis, Department of Cell and Organism Biology, Lund University, Lund, Sweden, p. 66. Aouad G, Crovisier JL, Damidot D, et al. (2008) Interactions between municipal solid waste incinerator bottom ash and bacteria (Pseudomonas aeruginosa). Science of the Total Environment 393: 385–393. Arickx S, De Borger V, Van Gerven T and Vandecasteele C (2010) Effect of carbonation on the leaching of organic carbon and of copper from MSWI bottom ash. Waste Management 30: 1296–1302.
Arickx S, Van Gerven T, Boydens E, et al. (2008) Speciation of Cu in MSWI bottom ash and its relation to Cu leaching. Applied Geochemistry 23: 3642–3650. Bauer M and Blodau C (2006) Mobilization of arsenic by dissolved organic matter from iron oxides, soils and sediments. Science of the Total Environment 354: 179–190. Bayard R, Pestre C and Gourdon R (2009) Aerobic microbial activity in fresh and aged bottom ashes from municipal solid waste incineration (MSWI). International Biodeterioration & Biodegradation 63: 739–746. Bendz D, Tüchsen PL and Christensen TH (2007) The dissolution kinetics of major elements in municipal solid waste incineration bottom ash particles. Journal of Contaminant Hydrology 94: 178–194. Binner E (2003) Assessment of MBP waste-misinterpretations of respiration activity. In: Cossu R (ed.) Proceedings Sardinia 2003, 9th international waste management and landfill symposium, CISA, Environmental Sanitary Engineering Centre, Italy, S. Margherita di Pula, Cagliari, Italy, 6–10 October 2003. Binner E, Lechner P, Widerin M and Zach A (1997) Laboratory test methods characterizing the biological reactivity of waste. In: Cossu R (ed.) Sardinia 1997, 6th international waste management and landfill symposium, CISA, Italy, 13–17 October 1997. BMLFUW (2001) Richtlinie für die mechanisch biologische Behandlung von Abfällen [Guidelines for the Mechanical Biological Pretreatment of Solid Wastes]. Notice No. 2001/423/A, Federal Republic of Austria. Chimenos JM, Fernández AI, Nadal R and Espiell F (2000) Short-term natural weathering of MSWI bottom ash. Journal of Hazardous Materials 79: 287–299. Christ MJ and David MB (1996) Temperature and moisture effects on the production of dissolved organic carbon in a Spodosol. Soil Biology and Biochemistry 28: 1191–1199. Cornelis G, Gerven TV and Vandecasteele C (2012) Antimony leaching from MSWI bottom ash: Modelling of the effect of pH and carbonation. Waste Management 32: 278–286. Cornelis G, Van Gerven T and Vandecasteele C (2006) Antimony leaching from uncarbonated and carbonated MSWI bottom ash. Journal of Hazardous Materials 137: 1284–1292. Dijkstra JJ, van der Sloot HA and Comans RNJ (2006) The leaching of major and trace elements from MSWI bottom ash as a function of pH and time. Applied Geochemistry 21: 335–351. Dugenest S, Combrisson J, Casabianca H and Grenier-Loustalot MF (1999) Municipal solid waste incineration bottom ash: Characterization and kinetic studies of organic matter. Environmental Science Technology 33: 1110–1115. Freyssinet P, Piantone P, Azaroual M, et al. (2002) Chemical changes and leachate mass balance of municipal solid waste bottom ash submitted to weathering. Waste Management 22: 159–172. Gadd GM (2000) Bioremedial potential of microbial mechanisms of metal mobilization and immobilization. Current Opinion in Biotechnology 11: 271–279. Ilyas A, Lovat E and Persson K (2014) Effects of substrate induced respiration on the stability of bottom ash in landfill cover environment. Waste & Management Research 32: 1241–1246. Jin Q and Bethke CM (2007) The thermodynamics and kinetics of microbial metabolism. American Journal of Science 307: 643–677. Johnson CA and Furrer G (2002) Influence of biodegradation processes on the duration of CaCO3 as a pH buffer in municipal solid waste incinerator bottom ash. Environmental Science & Technology 36: 215–220. Johnson K, Szymanowski J, Borrok D, et al. (2007) Proton and metal adsorption onto bacterial consortia: Stability constants for metal–bacterial surface complexes. Chemical Geology 239: 13–26. Kalbitz K, Schwesig D, Schmerwitz J, et al. (2003) Changes in properties of soil-derived dissolved organic matter induced by biodegradation. Soil Biology & Biochemistry 35: 1129–1142. Kumpiene J, Robinson R, Brännvall E, et al. (2011) Carbon speciation in ash, residual waste and contaminated soil by thermal and chemical analyses. Waste Management 31: 18–25. Lack JG, Chaudhuri SK, Kelly SD, et al. (2002) Immobilization of radionuclides and heavy metals through anaerobic bio-oxidation of Fe(II). Applied and Environmental Microbiology 68: 2704–2710. Nwachukwu OI and Pulford ID (2011) Microbial respiration as an indication of metal toxicity in contaminated organic materials and soil. Journal of Hazardous Materials 185: 1140–1147.
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Ilyas et al. Olsson S, van Schaik JWJ, Gustafsson JP, et al. (2007) Copper(II) binding to dissolved organic matter fractions in municipal solid waste incinerator bottom ash leachate. Environmental Science & Technology 41: 4286–4291. Oremland RS and Stolz JF (2005) Arsenic, microbes and contaminated aquifers. Trends in Microbiology 13: 45–49. Piantone P, Bodenan F and Chatelet-Snidaro L (2004) Mineralogical study of secondary mineral phases from weathered MSWI bottom ash: Implications for the modelling and trapping of heavy metals. Applied Geochemistry 19: 1891–1904. Rendek E, Ducom G and Germain P (2006a) Influence of organic matter on municipal solid waste incinerator bottom ash carbonation. Chemosphere 64: 1212–1218. Rendek E, Ducom G and Germain P (2006b) Carbon dioxide sequestration in municipal solid waste incinerator (MSWI) bottom ash. Journal of Hazardous Materials B128: 73–79. Sadiq M (1997) Arsenic chemistry in soils: An overview of thermodynamic predictions and field observations. Water, Air, and Soil Pollution 93: 117–136. Saffarzadeh A, Shimaoka T, Wei Y, et al. (2011) Impacts of natural weathering on the transformation/neoformation processes in landfilled MSWI bottom ash: A geoenvironmental perspective. Waste Management 31: 2440–2454. Seco JI, Fernández-Pereira C and Vale J (2003) A study of the leachate toxicity of metal-containing solid wastes using Daphnia magna. Ecotoxicology and Environmental Safety 56: 339–350. Speiser C, Baumann T and Niessner R (2000) Morphological and chemical characterization of calcium-hydrate phases formed in alteration processes
of deposited municipal solid waste incinerator bottom ash. Environmental Science & Technology 34: 5030–5037. Stefano Ferrari HB and Baccini P (2002) Chemical speciation of carbon in municipal solid waste incinerator residues. Waste Management 22: 303– 314. Stolz JF, Basu P, Santini JM and Oremland RS (2006) Arsenic and selenium in microbial metabolism. Annual Review Microbiology 60: 107–130. Suer P, Bendz D and Toomväli C (2009) Ageing under field conditions – results from a long-term lysimeter study. In: WASCON 2009, proceedings of international conference on environmental and technical implications of construction with alternative materials, Lyon, France, 1–4 June 2009. Wang WJ, Dalal RC, Moody PW and Smith CJ (2003) Relationships of soil respiration to microbial biomass, substrate availability and clay content. Soil Biology and Biochemistry 35: 273–284. Wei Y, Shimaoka T, Saffarzadeh A and Takahashi F (2011) Mineralogical characterization of municipal solid waste incineration bottom ash with an emphasis on heavy metal-bearing phases. Journal of Hazardous Materials 187: 534–543. You GS, Ahn JW, Han GC and Cho HC (2006) Neutralizing capacity of bottom ash from municipal solid waste incineration of different particle size. Korean Journal of Chemical Engineering 23: 237–240. Zhang S, Herbell JD and Haake BG (2004) Biodegradable organic matter in municipal solid waste incineration bottom ash. Waste Management 24: 673–679. Zomeren Av and Comans RNJ (2009) Carbon speciation in municipal solid waste incinerator (MSWI) bottom ash in relation to facilitated metal leaching. Waste Management 29: 2059–2064.
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research-article2015
WMR0010.1177/0734242X15585343Waste Management & ResearchIlyas et al.
Original Article
Residual organic matter and microbial respiration in bottom ash: Effects on metal leaching and eco-toxicity
Waste Management & Research 1–7 © The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X15585343 wmr.sagepub.com
A Ilyas1, KM Persson2 and M Persson2
Abstract A common assumption regarding the residual organic matter, in bottom ash, is that it does not represent a significant pool of organic carbon and, beyond metal-ion complexation process, it is of little consequence to evolution of ash/leachate chemistry. This article evaluates the effect of residual organic matter and associated microbial respiratory processes on leaching of toxic metals (i.e. arsenic, copper, chromium, molybdenum, nickel, lead, antimony and zinc), eco-toxicity of ash leachates. Microbial respiration was quantified with help of a respirometric test equipment OXITOP control system. The effect of microbial respiration on metal/residual organic matter leaching and eco-toxicity was quantified with the help of batch leaching tests and an eco-toxicity assay – Daphnia magna. In general, the microbial respiration process decreased the leachate pH and eco-toxicity, indicating modification of bioavailability of metal species. Furthermore, the leaching of critical metals, such as copper and chromium, decreased after the respiration in both ash types (fresh and weathered). It was concluded that microbial respiration, if harnessed properly, could enhance the stability of fresh bottom ash and may promote its reuse. Keywords Residual organic matter, microbial respiration, leaching, eco-toxicity, bottom ash
Introduction Bottom ash (BA) is a by-product of the municipal solid waste incineration process, which aims to convert organic content of solid wastes into thermal and electrical energy. However, owing to incomplete thermal oxidation of organic matter during the incineration, [some residual] organic matter is still present in BA (Stefano Ferrari and Baccini 2002). A common assumption is that this residual organic matter (ROM) in BA does not represent a significant pool of organic carbon. Beyond the metal ion complexation process, e.g. Arickx et al. (2010), it is of little consequence to evolution of ash/leachate chemistry. Previously, it has been shown that the ROM can be oxidised by microbes during respiration (Rendek et al., 2006b; Zhang et al., 2004). The ROM oxidation provides the energy for microbial growth and releases carbon dioxide (Wang et al., 2003). The CO2 released during the respiration can also react with BA, further supplementing the carbonation treatment. Moreover, the ROM has been shown to support the development of microbial biofilms, which could potentially trap the toxic metals (Aouad et al., 2008; Johnson et al., 2007) and/or immobilise metals (Lack et al., 2002). In essence, the microbial respiration plays a decisive role in stabilising the organic matter (Kalbitz et al., 2003). In the absence of it, leaching of the ROM can occur, which through complexation also can enhance metal leaching (Zomeren and Comans, 2009). Another interesting aspect of microbial respiration is its
effects on redox and minerals. For example, the excessive respiration process, by consuming oxygen, can alter the mobility and speciation of redox sensitive metals such as arsenic (Aaltonnen, 2008; Oremland and Stolz, 2005). By using secondary minerals, i.e. ferri-hydrites, for their respiration, the microbes can dislodge metals bound on mineral surfaces (Stolz et al., 2006). However, despite the dynamic nature of ROM and the diversity of impacts, it has not been linked to the evolution of BA’s chemistry, i.e. effects on metal leaching and eco-toxicity. Therefore, there is a need to further elucidate the role of ROM and microbial respiration in the stabilisation of BA. In previous studies, the ROM has been measured as a lumped parameter, expressed as a total organic carbon (TOC), which does not distinguish between inorganic and organic or labile fractions. This differentiation is important since assuming that all of 1Department
of Hydraulic and Environmental Engineering, Norwegian University of Science and Technology, Trondheim, Norway 2Division of Water Resources Engineering, Lund University, Lund, Sweden Corresponding author: A Ilyas, Department of Hydraulic and Environmental Engineering, Norwegian University of Science and Technology, Trondheim N-9471, Norway. Email: [email protected]
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the measured ROM is labile can lead to over estimation of its role or impacts (Kumpiene et al., 2011). Therefore, quantifying the respiration, along with the TOC, can be an indirect way of assessing the portion of labile (biodegradable) fraction of the ROM. Since, microbial respiration is sensitive to factors, i.e. moisture and temperature (Christ and David, 1996), and/or metal concentrations (Nwachukwu and Pulford, 2011). It can also be used as an indicator of the BA’s stability and environmental impact. In this study, we investigated the impact of aerobic respiration on the metal leaching as well as eco-toxicity of BA. The objectives of the study were threefold: (1) to quantify ROM as TOC; (2) to measure the oxygen consumption without addition of external substrate; (3) to evaluate the impacts of respiration on metal mobility, and acute toxicity of leachate.
Materials and methods BA sampling The sampling was done at an incineration plant in southern Sweden. Two composite samples of BA were collected from both fresh and 1.5 year old ash heap. To achieve a representative sample, BA heaps were sampled at three locations (top, middle and bottom), with the help of a mechanical shovel, and a composite sample of roughly 40 kg each was created. The composite samples were then reduced to approximately 5 kg through a quartering producer. After mechanical sieving, the particle size 11), but it increased in concentration between 8–11, perhaps owing to dissolution of ettringite (Cornelis et al., 2006). Although, in a later study, Cornelis, et al. (2012) attributed the antimony increase to dissolution of romeite (a calcium–antimonite mineral) instead of ettringite [Ca6Al2(SO4)3(OH)12·26H2O] within this range of pH. While, in the case of arsenic, the consumption of oxygen by respiration during the test could have affected its behaviour. In case of groundwater, it has been shown that arsenic tended to increase in low oxygen conditions, especially when organic matter was present. Further, arsenic is known to complex with organic acids (Bauer and Blodau, 2006), which are released during oxidation of ROM. The leaching macro-ions, i.e. Ca2+, Cl- and SO42- also decreased after the respiration in the fresh ash (Figure 5). This decrease could be owing to precipitation of calcium minerals, which are an important part of the weathering process (Piantone et al., 2004; Speiser et al., 2000). From the results, it is obvious that the microbial respiration was the driver behind the reduction in leachable TOC (Figure 2), and the decrease of pH. The decrease in the critical metals (copper, chromium, lead, etc.) and macro-ions, after the respiration, indicates that promoting the respiratory activities would help the stabilisation of ash. Figure 4 presents the effects of respiration on metal leaching from the weathered ash, which highlights the contrast with the
1
As
Cr
Cu
Mo
Ni
Pb
Sb
Zn
Figure 4. Effects of respiration on metal leaching from weathered ash samples. 2500 2000 Before mg/kg
As
A er
1500 1000 500 0 Ca Fresh
SO4 Fresh
Ca Weathered SO4 Weathered
Figure 5. Leaching of calcium and SO42- before and after the respiration.
behaviour of fresh BA. Metals such as nickel, antimony, lead and zinc increased in their concentrations, while metals such as arsenic, chromium, copper and molybdenum decreased in their concentrations. As discussed earlier, the reduction in metal leaching seems to be related to the reduction leachable TOC after the respiration tests. One plausible explanation is that weathered ash is already stable with formation of secondary mineral phases, such as calcite and aluminium/iron-oxides, etc. The respiration is disrupting this equilibrium by changing redox conditions and adding humic acids to the solution. Hence, it is not surprising that metal concentrations increase along with an increase in calcium and sulphates. As far as arsenic is concerned, the decrease in its concentration could be related to an increase in calcium concentration. Arsenic is known to precipitate with free calcium ions. Sadiq (1997) has reported that adding calcium ions to a metal-containing solution can lead to formation of calcium–arsenic precipitates and decrease in arsenic concentrations. The metals, such as lead, can complex with ions such as Cland SO42-, which precipitate with the aging process (Speiser et al., 2000). Thus, in the weathered ash, dissolution of carbonates and metal salts can explain the increased leaching of these metals (nickel, zinc and lead). While antimony, as in fresh ash, seems more affected by the consumption of oxygen and/or increased SO42- concentration, which increased in weathered ash (Figure5).
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Table 1. Changes in sample pH, O2 consumption and EC50 owing to the respiratory process. The labels B1 and A1 refer to fresh ash before and after the leaching tests, while B2 and A2 weathered ash before and after the leaching tests. Sample
pH
O2(mg/Kg dry BA)
LC/EC50
B1 A1 B2 A2
11.64 ± 0.05 10.74 ± 0.49 8.82 ± 0.18 8.40 ± 0.05
0 0.87 ± 0.087 0 0.73 ± 0.058
6.59 ± 0.01 16.77 ± 0.01 113.86 ± 0.05 414.11 ± 0.05
Table 2. Pearson correlation between EC50 values and metal ion concentrations before and after the respiration tests. Fresh bottom ash EC50 P-val
Cd –0.953 0.003
Co –0.828 0.042
Cu –0.886 0.019
Mo –0.961 0.002
Si 0.967 0.002
Zn –0.908 0.012
Cu –0.736 0.095
Pb 0.803 0.054
Mo –0.76 0.079
Si 0.966 0.002
Zn 0.855 0.03
Weathered bottom ash EC50 P-val
Co 0.779 0.068
Bold entries indicate a strong and significant correlation between the variables.
Suer et al.,(2009) reported from leaching tests on 12-year BA that antimony was controlled by available SO4 concentrations. In real life, one can expect the effect of pH change to be stronger, since in the experiment CO2 was removed from the system and only a small percentage may have reacted. This was done to accurately quantify the oxygen consumption, which would not be possible had CO2 been allowed to freely react with the ash. The consequence of microbial respiration would differ depending the age of the BA. For weathered BA, the production and dissolution of CO2 would generate acidity in pore water, further supplementing the acidity from proton released during the respiration. Weathered ash is already stabilised and its capacity to take up more CO2 may be limited, therefore, dissolution of secondary minerals, owing to acidity, can lead to enhanced leaching of metals. In contrast, in fresh ash, the production and dissolution of CO2 in pore water would facilitate its reaction with alkaline components owing to a high pH. This would lead to a reduction of pH and enhanced formation of secondary mineral phases and reduced leaching of metals. This conclusion about the effect of microbial respiration and the ROM on the weathered ash is line with Ilyas et al. (2014), who showed that the addition of organic matter and increased respiration lead to increased leaching metals in landfill covers.
Respiration and eco-toxicity Table 1 shows the pH values of samples along with EC50 (in %). The sample number B1 and A1 represent the fresh ash, while B2 and A2 represent the weathered ash, before and after the degradation, respectively. The EC50 value for the fresh ash leachate was 6.59, which increased after the respiration to 16.77. For the weathered ash, high EC50 (113 and 414) values indicate a nontoxic leachate even before the respiration tests. However, the
impact of respiration was similar to the fresh ash as EC50 increased after the respiration. The results show that the respiration reduced the toxicity of ash leachate. The co-relation between the pH change and the EC50 value was high (–0.835). At a higher pH, increased activity of OH- is known to cause toxicity to Daphnia (Seco et al., 2003). These results show that for the fresh ash the EC50 value was pH dependent and any modification in pH would lead to change in its toxicity. Since the change in pH comes after the respiration, it seems reasonable to conclude that there is a relationship between the respiration and the change in toxicity of ash leachate. In weathered ash, the total concentrations of four metals (nickel, lead, antimony and zinc) increased, but the EC50 of ash leachate decreased, which perhaps indicates the modification of metal speciation to less toxic forms. This is not unusual since microbes are known to modify the metal species during their respiratory and metabolic activities (Gadd, 2000). Another possible explanation is the compensating effect of other ions, such as calcium and magnesium. In both cases, the release of calcium and magnesium increased after the respiration. The old ash, perhaps owing to calcite dissolution, had higher concentrations of Ca than fresh ash even before the respiration tests. When it comes to metals, given the number of metal ions and other competing ions present in the leachate, it is rather difficult to ascertain exactly which metal ion is responsible for the observed toxicity. Pearson correlation points out a strong correlation (both +ve and –ve) between several metal ions and EC50 values of fresh and weathered BA (Table 2). This result points out that when it comes to the toxicity of a complex liquid, such as leachate it is more than one more parameter that can cause a toxic effect. In general, one can observe a reduction in toxicity after the respiration. The process of respiration seems to reduce the pH and metal concentrations, both of which could have contributed to this reduction in EC50 values.
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Waste Management & Research
Conclusions The results of the current study provided interesting insights into the respiration process and its effect on leaching and eco-toxicity of the ashes. The results show that 1.5 years of natural weathering on an incinerator site did not diminish the pool of biodegradable ROM, which shows the inefficiency of this process. More importantly, the results point out that despite being enriched in toxic metals, BA still allows the microbial respiratory processes, even at a very high pH. Contrary to the current perception that ROM is of no biological consequence in the ashes, the results show that it not only supports respiratory activities, but also exerts control over leaching of the organic carbon, metal mobility and ecotoxicological impact of ash leachates. The respiration reduced the pH and eco-toxicity in both ashes, which indicates the toxicity modification or treatment potential of respiration. For weathered ash, the leaching of metals (nickel, lead, antimony and zinc) will probably increase, which can be problematic in reuse situations, i.e. landfill covers where additional organic matter input is a strong possibility. Finally, the results indicate that the respiration can have variable effects on BA depending on its age. For fresh BA, the respiration can provide a route to ‘bio-carbonation’, but for the weathered BA it may not be a desirable activity. This biogenic CO2, much like atmospheric, can also react with alkaline components of fresh BA during the carbonation reactions. However, unlike the atmospheric CO2, the biogenic CO2 would be better distributed throughout the ash heaps, because microbial respiration would occur inside the heaps. Thus, the microbial respiration represents an intrinsic and potentially efficient pathway to carbonation of the ashes. However, based on this limited set of data, one can only highlight the macroscopic effects of the ROM and associated respiratory process. For bio-carbonation, the effects of process parameters, such as moisture, temperature, substrate, respiration rates and ash volume, need to be investigated further.
Acknowledgements Dr Martijn van Praagh of CEC at Lund University in Sweden is acknowledged for providing the OXITOP setup used in the study.
Declaration of conflicting interests The authors declare that there is no conflict of interest.
Funding This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
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