Impact of biodegradation on the barrier performance of polymer-modified bentonite

Polymer modification shows potential for improving the performance of clays in geotechnical applications. This includes its use as a filler in clay liners under challenging geotechnical and environmental conditions (e.g., exposure to aggressive leachates and intensive drying), as well as applications in soil stabilisation and as supporting fluids for mechanised tunneling and diaphragm wall construction. However, due to the organic nature of most commonly used polymers, they may serve as a substrate for microbial metabolism. The limited understanding of polymer biodegradation and its influence on bentonite behaviour raises concerns regarding the long-term stability of polymer-enhanced clays. This study investigates potential biological degradation of cationic (PAA + ) and anionic (PAA - ) polyacrylamide-modified bentonite under saturated conditions. Bentonite MX80 was modified via solution intercalation and subjected to 32 weeks of permeation testing. The outflow was analysed chemically and microbiologically. Following permeameter testing, the hydraulic conductivity, rheological behaviour and microbial community composition of the soil material were determined, and initial and post-test microbial communities were compared. Based on the analysis a conceptual degradation model was developed and discussed. Polymer degradation was found to depend on the presence and persistence of specific polymer-degrading microorganisms, with Pseudomonas species identified as key contributors. Although PAA + modified MX80 exhibited higher microbial degradation activity, the long-term performance of polymer-modified bentonite was governed primarily by the charge-dependent polymer-clay binding mechanism rather than by the extent of polymer loss. Surface-bound PAA + showed greater robustness than bridging-dominated PAA - systems, as confirmed by hydraulic and rheological analyses.