Chemical Compatibility of Polymer-Amended Bentonite–Sand Cutoff Wall Backfills Exposed to Copper Contaminants

Barriers are often applied to isolate contaminated sites to protect the surrounding ecological environment. However, some contaminants, such as heavy metals, could weaken the isolation effect or even lead to complete failure. In previous studies, it was verified that anionic polymers can reduce this impact. To further investigate the effectiveness of other types of polymers, a nonionic polymer, hydroxypropyl methylcellulose (HPMC), was adopted to prepare modified bentonite. Some experiments were conducted to investigate the engineering characteristics, such as swell index, compressibility, and permeability of the corresponding cutoff wall backfills. The results show that the addition of HPMC significantly enhances the compressibility and impermeability of the backfills. The chemical compatibility of the modified backfills against heavy metal contaminants was validated using Cu2+ as a representative. Cu2+ contaminants will reduce the compressibility of the backfills, and the difference in compression index of the backfills before and after modification is smaller with higher Cu2+ concentration. The hydraulic conductivity of the backfills with high HPMC dosage increases substantially with the increase of Cu2+ concentration, but they can still maintain greater impermeability than that of unmodified backfills. The mechanisms of the interaction between bentonite, HPMC and Cu2+ were explored by microscopic experiments (SEM and XRD). It was revealed that adsorption and bonding effects of HPMC are main improvement mechanisms for the chemical compatibility of barriers to heavy metal contaminants.