Research article
Effects of biochar and wood pellets amendments added to landfill cover soil on microbial methane oxidation: A laboratory column study

https://doi.org/10.1016/j.jenvman.2017.01.068Get rights and content

Highlights

  • Mixed soil-biochar layers performed better than thin biochar layers in column tests.

  • Thin biochar layers affected moisture distribution and infiltration in soil columns.

  • A highly active methanotrophic population was observed within the biochar layer.

  • Soils with uncharred wood pellets achieved similar oxidation rates as biochar.

  • Oxidation rates were primarily governed by soil moisture and soil type.

Abstract

Alternate landfill covers designed to enhance microbial methane (CH4) oxidation and reduce the negative impacts of landfill gas emissions on global climate have recently been proposed and investigated. In this study, the use of biochar as a soil amendment is examined in order to assess the feasibility and effectiveness for enhanced CH4 removal in landfill covers when incorporated under high compaction conditions and relatively low soil moisture. Four different cover configurations were tested in large soil columns for ∼510 days and potential CH4 oxidation rates were determined following long-term incubation in small batch assays. Cover designs tested include: a thin biochar layer at 15–18 cm; 2% mixed soil-biochar layer at 20–40 cm; 2% mixed soil-uncharred wood pellets at 20–40 cm; and soil obtained from intermediate cover at an active landfill site. The placement of a thin biochar layer in the cover significantly impacted moisture distribution and infiltration, which in turn affected CH4 oxidation potential with depth. An increase in CH4 removal rates was observed among all columns over the 500 day incubation period, with steady-state CH4 removal efficiencies ranging from ∼60 to 90% in the final stages of incubation (inlet load ∼80 g CH4 m−2 d−1). The thin biochar layer had the lowest average removal efficiency as a result of reduced moisture availability below the biochar layer. The addition of 2% biochar to soil yielded similar CH4 oxidation rates in terminal assays as the 2% uncharred wood pellet amendment. CH4 oxidation rates in terminal assays were positively correlated with soil moisture, which was affected by the materials’ water holding capacity. The high water holding capacity of biochar led to higher oxidation rates within the thin biochar layer, supporting the initial hypothesis that biochar may confer more favorable physical conditions for methanotrophy. Ultimate performance was apparently affected by soil type and CH4 exposure history, with the highest oxidation rates observed in the unamended field soil with higher initial methanotrophic activity.

Introduction

Methane (CH4) generated from landfills is a serious concern for both society and the environment due to its high global warming impact (∼25 times greater than CO2 over a 100-year time period; IPCC, 2013). Biocovers have been identified as a key mitigation strategy to target low-level methane emissions that would not otherwise be captured by landfill gas (LFG) extraction systems and have thus been the subject of recent research investigating alternative landfill covers for enhanced methane removal via microbial oxidation (Sadasivam and Reddy, 2014). The majority of these alternate landfill covers involve the amendment of cover soils with organic-rich materials (e.g. compost) either over large areas (i.e. biocovers) or in targeted zones of high gas emissions (i.e. biowindows/biofilters), requiring significant changes to current cover construction and monitoring practices. A number of studies have investigated the use of organic amendments to landfill cover soils as a means of promoting microbial activity and biotic methane oxidation (e.g. Barlaz et al., 2004, Bogner et al., 2010, Scheutz et al., 2011), though our laboratory is the first to investigate the use of charred organic matter (biochar) as a possible soil amendment in biocovers and biofilters (Yaghoubi, 2011, Reddy et al., 2014, Sadasivam and Reddy, 2015a, Sadasivam and Reddy, 2015b).

Biochar is produced via the pyrolysis or gasification of organic matter in environments with low or zero oxygen (Reddy et al., 2014). This process yields three products – biogas, bio-oil and biochar - the first two of which are commonly used for sustainable energy generation (Laird, 2008). The solid product (biochar) contains a more stable form of carbon than the initial feedstock, prompting interest for its use as a sink for atmospheric CO2 (Spokas and Reicosky, 2009). Because biochar is composed of more recalcitrant forms of carbon than alternative cover amendments, such as compost, biochar may be more suitable for long-term applications due to lower rates of biodegradation of the cover material. Biochar also has a number of favorable characteristics – such as a high internal porosity, specific surface area, and high water-holding capacity – that also contribute to improving soil cover conditions for microbial CH4 oxidation (Reddy et al., 2014, Yargicoglu et al., 2015). Studies investigating biochar application to agricultural and natural soils have also reported positive impacts on soil fertility, including increases in soil pH, nutrient retention and availability, and cation exchange capacity (e.g. Glaser et al., 2002, Chan et al., 2007, Mukherjee et al., 2014). Thus biochar amendment may also be suitable for use in vegetated final covers on landfills.

Biochar has a long lifetime in soils, with some reports of biochar having a half-life in soil on the order of several hundred days to years (Steinbeiss et al., 2009; Bolan et al., 2012). Biochars are able to store soil carbon effectively over long timescales, and thus have recently been investigated as soil amendments to enhance carbon sequestration (e.g. Spokas and Reicosky, 2009) in addition to their use in landfill cover soils (Reddy et al., 2014, Sadasivam and Reddy, 2015a, Sadasivam and Reddy, 2015b). Other alternative covers evaluated in previous studies have utilized materials that would require special design considerations not compatible with current practice (e.g. compost biocover; Barlaz et al., 2004). As a result, many of these designs may not be easily incorporated into current landfill cover configurations, potentially limiting their widespread use. In addition, the methane oxidation removal of these materials in laboratory studies may not adequately reflect their expected performance when constructed in the field, generally with relatively high degrees of compaction.

The present study investigates biochar-amended soil covers under field-relevant conditions using biochar amendment applied either to small sections of the cover depth or to the entire depth profile. In order to investigate the performance of biochar-amended soils as alternate landfill covers, two sets of large soil column tests were conducted in conjunction with a field study performed at an active landfill site in northeast Illinois (Yargicoglu, 2016). This paper compares the results obtained from monitoring four columns under simulated landfill cover conditions. One column (C5) was constructed with biochar applied only at 15–18 cm depth in order to compare the performance of biochar applied in concentrated layers versus homogenous soil-biochar mixtures applied over larger depth zones (i.e. 20–40 cm depth). The second column (C6) was constructed with 2% biochar amended to 20–40 cm depth; fine ash was retained in the biochar before adding to soil in order to evaluate the impact of higher ash content on the methane removal efficiency over time. A third column (C7) was constructed in the same manner as C6 using the raw, uncharred wood pellet feedstock used by the vendor to produce the pelleted biochar investigated in this study in order to examine the impact of charring biomass on its effectiveness as a soil cover amendment. The fourth column (C8) was an unamended soil (no biochar added) utilizing sieved soil obtained from an intermediate cover area at the site of related field tests (Yargicoglu, 2016) in order to establish the expected performance of unamended cover soil and facilitate comparisons between performance in field trials and laboratory tests.

Specific objectives of these column tests were to: (1) evaluate biochar-amended cover performance under high compaction conditions at an initial moisture content lower than the optimal moisture content (OMC) in terms of standard geotechnical engineering practice; (2) determine the performance of unamended soil cover using sieved field soil from an active intermediate landfill cover in order to assess variability among field and laboratory studies due to differences in soil type; (3) compare performance of biochar-amended soil with soil amended with uncharred pinewood pellets (i.e. raw feedstock) to investigate the impact of charring organic amendments prior to adding them to cover soils. It was hypothesized that the high water holding capacity and internal porosity of biochar would promote the growth of methane-oxidizing bacteria more readily than wood pellet amendment or unamended soil.

Section snippets

Column setup and design configurations tested

A column test program employing acrylic columns (1 m height; 18.4 cm ID) was performed to simulate landfill cover conditions and evaluate the performance of each cover under controlled CH4 input using synthetic landfill gas (60% CH4, 40% CO2). Atmospheric air was passed through an in-line filter and humidification tank and fed into the headspace of each column in order to provide sufficient O2(g) necessary for CH4 oxidation to proceed. Inlet and outlet flow rates were monitored continuously

Physical properties of soil and biochar-amended soil columns

The initial physical properties of the cover substrates are summarized in Table 2. All columns were compacted to bulk densities of ∼1.5–∼1.6 g/cm3, resulting in initial estimated air-filled porosities of ∼30–33%. Initial moisture content was set to ∼30% of the materials’ water holding capacities, or ∼6.5% gravimetric water content in Columns 5, 6 and 7. A higher initial gravimetric water content was achieved in C8 due to its higher water holding capacity (∼53% dry wt. basis), compared to only

Effect of cover properties on methane removal efficiency

Prior work has established the importance of soil cover textural properties on the gas diffusivity and overall aeration of cover soils, which governs the extent to which methane can be oxidized to CO2 as it travels upward through the cover profile (Gebert et al., 2011c, Scheutz et al., 2011). Adequate diffusion of atmospheric oxygen into the interior pores of soil is critical to maintaining high oxidation rates, as O2 is required for the microbial oxidation of CH4 to proceed. It is well

Conclusions

A series of column tests were performed to assess the performance of several types of biochar-amended soil covers under conditions designed to replicate typical field and construction conditions. From these tests the long term performance of soil columns in terms of methane removal efficiency was assessed to compare the effects of different amendment types and environmental conditions imposed. The results of this work indicate that the design configuration and amendment type are both

Acknowledgement

This research project was funded by the U.S. National Science Foundation (Grant CMMI #1200799), which is gratefully acknowledged. The authors are thankful to J. Bogner, K. Spokas, B.Y. Sadasivam, E. Schmidt, D. Mecha, D. Perzan, K. Saad, and several graduate and undergraduate research assistants for their advice and assistance during this project.

References (40)

  • I. Rachor et al.

    Assessment of the methane oxidation capacity of compacted soils intended for use as landfill cover materials

    Waste. Manag.

    (2011)
  • B.Y. Sadasivam et al.

    Adsorption and transport of methane in biochars derived from waste wood

    Waste. Manag.

    (2015)
  • B.Y. Sadasivam et al.

    Adsorption and transport of methane in landfill cover soil amended with waste-wood biochars

    J. Environ. Manag.

    (2015)
  • C. Scheutz et al.

    Evaluation of respiration in compost landfill biocovers intended for methane oxidation

    Waste. Manag.

    (2011)
  • K.A. Spokas et al.

    Limits and dynamics of methane oxidation in landfill cover soils

    Waste. Manag.

    (2011)
  • S. Steinbeiss et al.

    Effect of biochar amendment on soil carbon balance and soil microbial activity

    Soil. Biol. Biochem.

    (2009)
  • E.N. Yargicoglu et al.

    Physical and chemical characterization of waste wood derived biochars

    Waste. Manag.

    (2015)
  • ASTM

    Standard Test Method for pH of Soils

    (2007)
  • ASTM

    Standard Test Method for Particle-size Analysis of Soils

    (2007)
  • ASTM

    Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass

    (2010)
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