Transport of PBDE, MWCNTS, and BPA (Contaminants of Emerging Concern) Through Modern Landfill Liners
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Diffusion of three contaminants of emerging concern: (1) polybrominated diphenyl ether (PBDE), (2) multiwall carbon nanotubes (MWCNTs), and (3) bisphenol A (BPA) through high density polyethylene (HDPE) geomembrane (GMB) were investigated. Partitioning and diffusion coefficients, Sgf and Dg, and/or the permeation coefficient, Pg, were estimated depending on the contaminant behaviour. Laboratory experiments were performed at room temperature and a semi-analytical model was used to estimate the permeation parameters. Diffusion of BPA through a geosynthetic clay liner (GCL) was also examined. The experimentally inferred coefficients were used to examine the transport of these contaminants through a typical composite liner of a hypothetical landfill. Due to specific characteristics of PBDE with respect to HDPE, the conventional method for obtaining partitioning and diffusion coefficients did not work and a new method was developed to obtain these parameters. The PBDE partitioning coefficient (Sgf ~ 1.8×106) was four orders of magnitude higher and the diffusion coefficient was two orders of magnitude lower (Dg ~ 3.7×10-15 m2/s) than partitioning and diffusion coefficients of contaminants such as benzene and toluene usually considered in landfill design. Diffusion experiments performed on MWCNTs dispersed in an aqueous media have been running for about two years with no measurable partitioning of MWCNTs to the GMB or diffusion through the GMB. An upper bound permeation coefficient of MWCNTs through GMB (Pg ~ 5.6×10-15 m2/s) was estimated using the detection limit of MWCNTs in the receptor. BPA diffusion through HDPE GMB was investigated by using both BPA alone and in a mixture of phenolic compounds. The two experiments with different HDPE GMBs gave similar results with no measurable partitioning to the HDPE in four years of testing. Based on the diffusion tests, an upper bound permeation coefficient of BPA for an HDPE GMB (Pg ~ 2.9×10-15 m2/s) was estimated using the detection limit of BPA in the receptor. The permeation parameters of other phenolic compounds were also estimated and the partitioning coefficients ranged from 0.61 for 2-methylphenol (2-MP) to 31 for 2,3,4,6-Tetrachlorophenol and the diffusion coefficients ranged from 9.3×10-15 m2/s for pentachlorophenol (PCP) to 9.3×10-14 m2/s for 2-methylphenol (2-MP). The experimentally inferred BPA distribution and diffusion coefficients for a GCL at low stress (DGCL ~ 1.5×10-10 m2/s, Kd ~ 10-16 mL/g) are similar to reported values for other organic compounds in the literature. The GMB partitioning/diffusion/permeation coefficients of all three contaminants (PBDE, MWCNTs, and BPA) are all such that an HDPE GMB can be expected to be an excellent diffusive barrier for them in a municipal solid waste landfill. Modelling indicated that the impact for these contaminants of emerging concern will be controlled by holes on the geomembrane associated with wrinkles.