A Novel Application of Bis (2-Hydroxyethyl) Terephthalate to Enhance Sand Bentonite Mixture for Landfills

Sand-bentonite mixtures (SBM) are usually compacted as clay covers and liners of the landfill. However, unavoidable factors such as heterogeneity in compaction result in variable densities that causes low strength, high volumetric shrinkage, and increased hydraulic conductivity of SBM; hence, additives are often required for enhancement. This article focuses on the applicability of Polythene terephthalate (PET)–based chemically recycled polymer, i.e., Bis (2-hydroxyl) terephthalate (BHET) to enhance SBM for covers and liners. The required BHET polymer content in SBM specimens prepared at three molding densities (1.55, 1.65, and $1.75\mathrm{g}/{\mathrm{cm}}^3$) was evaluated based on three criteria: low hydraulic conductivity ($k<{10}^{-7}\mathrm{cm}/\mathrm{s}$), high unconfined compressive strength ($q_u>200\mathrm{kPa}$), higher split tensile strength ($q_t$), and low volumetric shrinkage, $VS<4\%$. One-dimension (1D) consolidation, $q_u$, $q_t$ and $VS$ tests were performed by mixing 0%, 1%, 2%, and 3% of BHET with sand-bentonite ($75:25$ in weight ratio). BHET addition to SBM molded even at lowest density offered resistance to both compression ($C_c$) and swelling ($C_s$), thereby reducing the coefficient of consolidation ($C_v$) and the hydraulic conductivity ($k$). With an increase in molding density, the polymer addition showed an increment in $q_u$, $q_t$, and reduction in $VS$ values. A BHET content of 3% of the SBM was enough to meet all three criteria for covers and liners at all three molding densities. Scanning Electron Microscopy (SEM) revealed polymer links and films formation at the interparticle contact points of bentonites and SBM. Energy-dispersive X-ray spectroscopy (EDX) analysis confirms the elemental interaction of BHET and montmorillonite. Both SEM and EDX tests support the proposed binding mechanism and conceptual model for the BHET enhanced SBM.