Estimation of Polyethylene Oxide Polymer Train and Loop Densities on Contaminant Barrier Materials

Desiccation of exposed clayey materials in cover layers of waste containment systems can ultimately result in the development of cracks that can promote the infiltration of rainwater and/or snowmelt, and escape of gases from buried wastes. Clayey barriers can be stabilized against desiccation by introducing binders such as concentrated polymer liquids or aqueous solutions. In this research, a theoretical analysis of the morphological configuration of molecules of polyethylene oxide (PEO) on montmorillonite (a common barrier mineral) was performed. A methodology for estimating polymer train and loop densities using batch sorption measurements was developed and demonstrated using PEO sorption test data for sodium montmorillonite (specific surface area: 31.82±0.22 m²/g) at initial aqueous PEO (molecular weight 8,000,000) concentrations of 0, 0.5, 1.0, 2.0, and 4.0 g/L. From test data, the distribution coefficient of PEO was determined to be 0.1599 mL/g. The analyses indicate that the distribution of sorbed PEO molecules into trains and loops is very sensitive to PEO chain flexibility. For example, for a function of partition function (λ) value of 0.1, the numbers of PEO molecular trains on 1 g of sodium montmorillonite are estimated to be $3.4\times {10}^{20}$ and $4.7\times {10}^{20}$ for chain flexibilities of 0.2 and 0.8, respectively. The greater the chain flexibility, the greater the opportunity for polymer molecules to lie flat on clay particles. Within an initial PEO concentration range of 0–4.0 g/L, no significant dependence of sorbed polymer molecular configuration on initial concentration was found. Possibly, extension of sorbed polymer molecular loops into barrier soil pores narrows fluid flow channels and hence the desiccation rate. This research enhances understanding of the physico-chemical processes that underlie clay stabilization by aqueous polymers.