Short communication
Correlation between iron mobilization and emergence of benzene in leachate at old landfills

https://doi.org/10.1016/j.jenvman.2013.02.014Get rights and content

Abstract

Leachate monitoring data from a closed municipal solid waste (MSW) landfill with negligible gas production showed increasing iron, alkalinity and benzene levels over time. Due to significant rain water infiltration, concentration data alone was not adequate to analyze the possible correlations between the leachate quality parameters monitored. When quantities (kg/d) present in leachate rather than concentrations (mg/L) of benzene, bicarbonate and iron were analyzed; there were significant correlations between benzene and iron as well as biocarbonate and iron quantities. Increasing iron quantity in leachate was accompanied by increasing quantities of benzene and bicarbonate which are indicative of transformation reactions of polymeric organic compounds present in MSW. The results indicate that biotransformation mechanisms in landfills are similar to those in anaerobic sediments due to lack of electron acceptors with higher energy yields.

Highlights

► Iron and benzene levels in leachate had similar patterns. ► Concentrations (mg/L) in leachate can be affected by rainwater infiltration. ► Correlations should be based on quantity (kg/d) not concentration (mg/L). ► Correlation between benzene and iron quantities in leachate was significant. ► Correlation between biocarbonate and iron quantities in leachate was significant.

Introduction

Energetics of substrate utilization are governed by the redox potentials of the electron donors with the possible electron acceptors changing from the most preferred to the least preferred as O2 > NO3 > Mn+4 > Fe+3 > SO42 > CO2. The oxidation–reduction potential (Eh) and pH affect the solubility and fate of iron in anaerobic environments (Charlatcka and Cambier, 2000) as well as cation exchange capacity (CEC) of the waste, dynamics of CEC changes during MSW decomposition, and the presence of more oxidized functional groups on the organic matter in MSW and leachate (Aulin et al., 1997; Martensson et al., 1999). It has been reported that energetics of substrate utilization under anaerobic conditions are more complicated and the reaction mechanisms which were thought as impossible could be accomplished by the syntrophic organisms (Strous and Jetten, 2004; Alperin and Hoehler, 2009).

The intrinsic biotransformations at old landfills can affect the groundwater chemistry (i.e., concentrations of organic matter, VOCs, nitrate, iron, sulfate) both temporally and spatially (Christenson and Cozzarelli, 2003). Significant research exists on anaerobic conversion of organic matter in relatively young landfills. However, at mature landfills, electron transfer mechanisms are more complicated due to the changes and shifts in availability of potential electron acceptors over time. Mechanisms of these reactions are poorly understood and some of the organisms that can carry out these transformations have been identified in relatively recent years (Boetius et al., 2000; Kniemeyer et al., 2007).

Iron chemistry in landfill environment involves complex reaction sequences depending on the changing conditions for thermodynamically favorable reactions. Because the energy yields of electron acceptors depend on pH; a redox reaction can be thermodynamically favorable in one environment but not in another (Burgin et al., 2011). In addition, increase in partial pressure of hydrogen in the landfill environment reduces the free energy of reactions that require hydrogen. Hence, hydrogen consuming reactions can occur as hydrogen gas forms.

Decomposition reactions in landfills continue even in the absence of gas generation (Kawai et al., 2012). Fig. 1 presents the shifts in major decomposition reaction mechanisms over time. Iron plays an important role as it can serve as both at oxidizing and reducing capacities. Dynamics of iron transformation in landfills initially depends on microbially catalyzed oxidation to Fe+3 followed by reduction to Fe+2 (Weber et al., 2006). Leachate monitoring studies at old landfills with negligible gas generation show increasing levels of iron; accompanied with increase in alkalinity (indicative of CO2 formation) and VOCs (indicative of transformation of polymeric compounds) (Tansel and Sizirici, 2011; Sizirici and Tansel, 2010; Kim et al., 2006; Klett et al., 2006; Reineke, 2001). At a landfill site, after closure, Statom et al. (2004) reported that iron levels in leachate first fluctuated and then significantly increased over time. Reduction of Fe+3 with carboxylic acids is a critical step for carbon flow in anaerobic systems. Studies with anaerobic sediments show that organic matter can be completely mineralized with Fe+3 serving as the electron acceptor (Lovley and Phillips, 1986; Coates et al., 1995). It has been observed that during MSW decomposition, carboxylic acids (e.g., acetate) can act as chelators resulting in dissolution of iron (Calmano et al., 1993; Bozkurt et al., 1997; Kjeldsen et al., 2002).

The sequence of redox reactions driven by electron rich organic matter in decreasing energy yield is as follows (Sylvia et al., 2004):O2NO3MnO2Fe2O3SO42

During the succession of anaerobic oxidation processes, the redox potential, Eh, decreases due to formation of reduced products (i.e., hydrogen). The approximate redox potentials for disappearance of oxygen, disappearance of nitrate, appearance of manganese ions, appearance of ferrous iron ions, and disappearance of sulfate ions are +330, +220, +200, +120, −150 mV, respectively. The general schematic of oxidation–reduction processes that take place in sediments are provided by Sylvia et al. (2004).

The purpose of this study was to identify the correlations between benzene, bicarbonate and iron quantities in leachate based on monitoring data from a closed landfill in Florida. Both the leachate quantity and leachate quality data were analyzed to establish the effect rain water infiltration on leachate dilution. The leachate monitoring data for benzene, bicarbonate and iron were analyzed in terms of concentration and quantities present in leachate. Correlations between benzene and iron as well as bicarbonate and iron quantities were developed.

Section snippets

Methodology

The leachate monitoring data at a closed landfill in South Florida, USA were evaluated. The landfill had been closed since 1987. The site consists of a Class I landfill (inert fill materials) and Class III landfill (putrescible fill materials) areas. The partially lined landfill and has a leachate collection system which allows monitoring of both the quality and quantity of leachate. The landfill cover consists of a two-foot compacted limestone layer which is covered with six-inch soil to

Results

Selected leachate parameters monitored in leachate were analyzed both in terms of concentration (mg/L) and quantity (kg/d). Iron in leachate is mainly in Fe+2 form due to its high solubility in comparison to Fe+3. Although fraction benzene formed could volatilize and carried with landfill gas, a proportional fraction will be dissolved in leachate. Fig. 2 compares the monthly leachate quantity with monthly rainfall data. The monthly leachate quantity data exhibited a similar pattern with monthly

Conclusions

Emergence of Fe+3 as electron acceptor at old landfills can activate biodegradation of persistent MSW fractions (i.e., plastics, cellulose) after depletion of readily available electron acceptors with higher energy yields. Leachate monitoring data at a closed landfill showed that increasing iron quantity in leachate was accompanied by increasing quantities of benzene and bicarbonate which are indicative of occurrence of transformation reactions of polymeric organic compounds in MSW. The results

Acknowledgments

Partial support for this work was provided by Hinkley Center for Solid and Hazardous Waste Management of University of Florida. The data were provided by Mr. Richard Meyers and Sermin Unsal of Broward County Solid Waste and Recycling Services.

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