Estimation of transport parameters of phenolic compounds and inorganic contaminants through composite landfill liners using one-dimensional mass transport model

Abstract

One-dimensional (1D) advection–dispersion transport modeling was conducted as a conceptual approach for the estimation of the transport parameters of fourteen different phenolic compounds (phenol, 2-CP, 2-MP, 3-MP, 4-MP, 2-NP, 4-NP, 2,4-DNP, 2,4-DCP, 2,6-DCP, 2,4,5-TCP, 2,4,6-TCP, 2,3,4,6-TeCP, PCP) and three different inorganic contaminants (Cu, Zn, Fe) migrating downward through the several liner systems. Four identical pilot-scale landfill reactors (0.25 m³) with different composite liners (R1: 0.10 + 0.10 m of compacted clay liner (CCL), L_e = 0.20 m, k_e = 1 × 10⁻⁸ m/s, R2: 0.002-m-thick damaged high-density polyethylene (HDPE) geomembrane overlying 0.10 + 0.10 m of CCL, L_e = 0.20 m, k_e = 1 × 10⁻⁸ m/s, R3: 0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick bentonite layer encapsulated between 0.10 + 0.10 m CCL, L_e = 0.22 m, k_e = 1 × 10⁻⁸ m/s, R4: 0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick zeolite layer encapsulated between 0.10 + 0.10 m CCL, L_e = 0.22 m, k_e = 4.24 × 10⁻⁷ m/s) were simultaneously run for a period of about 540 days to investigate the nature of diffusive and advective transport of the selected organic and inorganic contaminants. The results of 1D transport model showed that the highest molecular diffusion coefficients, ranging from 4.77 × 10⁻¹⁰ to 10.67 × 10⁻¹⁰ m²/s, were estimated for phenol (R4), 2-MP (R1), 2,4-DNP (R2), 2,4-DCP (R1), 2,6-DCP (R2), 2,4,5-TCP (R2) and 2,3,4,6-TeCP (R1). For all reactors, dispersion coefficients of Cu, ranging from 3.47 × 10⁻⁶ m²/s to 5.37 × 10⁻² m²/s, was determined to be higher than others obtained for Zn and Fe. Average molecular diffusion coefficients of phenolic compounds were estimated to be about 5.64 × 10⁻¹⁰ m²/s, 5.37 × 10⁻¹⁰ m²/s, 2.69 × 10⁻¹⁰ m²/s and 3.29 × 10⁻¹⁰ m²/s for R1, R2, R3 and R4 systems, respectively. The findings of this study clearly indicated that about 35–50% of transport of phenolic compounds to the groundwater is believed to be prevented with the use of zeolite and bentonite materials in landfill liner systems.

Introduction

Leachates from municipal solid waste (MSW) landfills and various discarded products contain a wide mixture of chemical pollutants and constitute a potential risk to the quality of receiving water bodies, such as surface water or groundwater (Paxéus, 2000, Christensen et al., 2001, Baun et al., 2004, Oman and Junestedt, 2008). A number of chemicals from MSW leachates are released during the lifetime of the landfill and result in emission of various volatile organic compounds (VOCs) and toxic inorganic pollutants. For this reason, MSW landfills have been identified as one of the major threats to groundwater resources (US EPA, 1984). Therefore, the impact of landfill leachate on the surface and groundwater has given rise to a number of studies in recent years (Fatta et al., 1999, Looser et al., 1999, Abu-Rukah and Al-Kofahi, 2001, Saarela, 2003, Longe and Enekwechi, 2007, Rowe, 2005).

Pollutants in MSW landfill leachate can be divided into four groups: dissolved organic matter; inorganic macro components; heavy metals; and xenobiotic organic compounds (Kjeldsen et al., 2002). Researchers have reported from full-scale landfills, test cells, and laboratory studies that average heavy metal concentrations of landfill leachate are fairly low and heavy metals in landfill leachate at present are not a major concern (Revans et al., 1999, Kjeldsen and Christophersen, 2001). However, the issue of xenobiotic organic compounds (XOCs) in landfill leachates have been addressed in a number of studies (Christensen et al., 2001, Kjeldsen et al., 2002, Baun et al., 2004, Slack et al., 2005, Oman and Junestedt, 2008, Bejerg et al., 2009).

The XOCs include a variety of aromatic hydrocarbons, phenols, chlorinated aliphatics, pesticides, and plastizers. Among them, phenol is the precursor to the synthesis of many organic compounds and is of high concern because of potential toxicity (Boopathy, 1997). Phenol and substituted phenols are common transformation products of several pesticides. Many substituted phenols, including chlorophenols, nitrophenols, and cresols, have been designated as priority pollutants by the US Environmental Protection Agency (Boyd et al., 1983).

The liner system is one of the most important elements of a modern engineered landfill. There are two pathways for contaminant transport through composite liners: advection and diffusion of inorganic and organic solutes through defects in the geomembrane and subsequently through the soil liner; and diffusion of organic solutes through the intact geomembrane and subsequently through the soil liner (Edil, 2003, Rowe, 2005). Due to its high strength, impermeability, and resistance to chemicals, the high density polyethylene (HDPE) geomembranes are the most widely used components of a modern liner system in solid waste landfills. However, many studies have shown that geomembranes are essentially impervious to diffusion of inorganic contaminants but organic compounds can readily penetrate through geomembranes in a short period of time (Sangam and Rowe, 2001, Joo et al., 2005).

The other significant component of a modern liner system is soil liner generally comprising clay material. Early concerns regarding contaminant transport through clay liners focused on advective transport (e.g. contaminants migrating along with the flow of water through the clay) but recently researchers have concluded that diffusive transport (contaminant migration driven by the difference in concentration between the upper and lower sides of the liner) is often the dominant mode of contaminant transport through well-built liner systems (Kim et al., 2001, Foose et al., 2002, Kalbe et al., 2002, Edil, 2003) including compacted clay liners (Toupiol et al., 2002, Willingham et al., 2004, Bezza and Ghomari, 2008), geosynthetic clay liners (Malusis and Shackelford, 2004, Rowe et al., 2005) and, composite liners (Foose et al., 2002, Kalbe et al., 2002, Edil, 2003).

Although several studies on emission and impact of various VOCs and toxic inorganic pollutants have been published more and more in recent years (Rowe et al., 2000, Lake and Rowe, 2004, El-Zein and Rowe, 2008, McWatters and Rowe, 2009), there are almost no systematic papers in the literature specifically devoted to a study of the estimation of transport parameters of various phenolic compounds through composite landfill liners using one-dimensional mass transport model. Therefore, clarification of the place of the present subject in the scheme of MSW landfills can be considered as a particular field of investigation for the leachate management. For this reason, the present study aims at fulfilling the gap in this field by particularly focusing upon the transport mechanisms of various phenolic compounds. In this study, fourteen different phenolic compounds (phenol, 2-chlorophenol (2-CP), 2-metilphenol (2-MP), 3-metilphenol (3-MP), 4-metilphenol (4-MP), 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), 2,4-dinitrophenol (2,4-DNP), 2,4-dichloropenol (2,4-DCP), 2,6-dichloropenol (2,6-DCP), 2,4,5-trichlorophenol (2,4,5-TCP), 2,4,6-trichlorophenol (2,4,6-TCP), 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP), pentachlorophenol (PCP)) and also three different inorganic contaminants (Cu, Zn, Fe) were selected to represent organic and inorganic compounds of the leachate constituents.

Considering the above-mentioned facts, the specific objectives of this study were (1) to determine the transport parameters (molecular diffusion and dispersion coefficients) of the selected inorganic and organic compounds migrating downward through the clay and composite liners; (2) to assess the applicability of simple diffusion models for the preliminary comparison of the performance of different liner systems; and (3) to evaluate the importance of organic and inorganic contaminant transport by means of landfill leachate management and groundwater quality.