Acids released during anaerobic reductive dechlorination of chlorinated aliphatic hydrocarbons, such as trichloroethylene, may inhibit further biodegradative processes or leave the system in a more toxic state. Hydrogel polymers may provide a tunable localized environment to protect reductive microbes, however information on fundamental mass transport constants is lacking to better engineer these materials. Hydronium is unusual relative to other molecules in that it is transported in water through two mechanisms: 1) Brownian motion where the hydronium ions physically move in the solvent and 2) the Grotthuss mechanism where the charge of a proton is transferred to an adjacent coordinated water molecule without the physical movement of the proton. The objective of this project is to describe fundamental mass transfer constants (effective diffusivities) of hydronium within commonly used hydrogel materials and gain insights into potential transport mechanisms. Hydrogel blends of poly(vinyl alcohol) (PVA), sodium alginate, and chitosan were investigated with variations in cross-linking conditions and solvent ionic strength to determine how each influenced hydronium diffusivity. Crosslinking PVA with boric acid significantly increased De(H3O+, gel) by ~58% (p=4.5x10-4), while incorporation of 2% sodium alginate (SA) crosslinked with calcium and boric acid lowered De(H3O+, gel) by ~47% (p=0.010) compared to cryogel controls. Increasing ionic strength to 0.1 M with potassium chloride significantly increased De(H3O+, gel) by 188% in 10% PVA / 2% SA (p=1.8x10-5) hydrogels crosslinked with boric acid and calcium. In cryogels, increasing ionic strength up to 0.5 M significantly increased observed De(H3O+, gel) by ~100% in both 10% PVA (p=1.1x10-7) and 10% PVA / 2% SA (p=0.008). The free volume theory was used to predict hydronium transport in hydrogels. The free volume theory predicted the Brownian component of hydronium diffusivity in 10% PVA hydrogels to be significantly less than the measured value (p=1.2x10-4), suggesting both mechanisms are present in hydrogels and can be altered by hydrogel preparation and environment. Understanding hydronium mass transfer behavior improves the ability to engineer these materials for encapsulation of acid generating systems.

Diffusion dynamics of hydronium ions in poly(vinyl alcohol) hydrogels: Influence of crosslinking method, hydrogel blend, and solvent ionic strength

ACS Spring Meeting 2021

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