Polymer-modified bentonites with low hydraulic conductivity and improved chemical compatibility as barriers for Cu2+ containment

Containment and remediation of copper mine contamination with high concentration of heavy metals (Cu, Pb, Zn) and low pH (2-5) demand barriers with low hydraulic conductivity (k) and good chemical compatibility. To prepare modified barriers containing copper erosion by replacing Wyoming bentonite with local bentonite and screen out economic dosage, the k of 12 polymer-modified bentonites with sodium polyacrylate (PAA) or sodium-carboxymethyl cellulose (CMC) at dosages from 2 to 10%, was measured by modified fluid loss (MFL) test. The k of unmodified bentonites at 690 kPa top pressure ranged from 1.8 x 10(-11) m/s to 1.9 x 10(-11) m/s, which were reduced down to 4.6 x 10(-12) m/s by 5% PAA or to 5.5x10(-12) m/s by 5% CMC. The SEM & EDX-substantiated pore-filling effect of PAA, exfoliation of bentonite plates by PAA and CMC, and the XRD & FTIR-substantiated intercalation of CMC contributed to the decrement of available flow space and the increment of tortuosity of flow channel, and eventually k reduction. Excessive polymer (> 5%) might prop open interaggregate voids, increase pore connectivity and cause slight k increment up to 9.8 x 10(-12) m/s and 6.5 x 10(-12) m/s for 10% PAA- and 10% CMC-modified bentonites, respectively. Acidic condition (pH = 3) slightly increased the k of unmodified, 5% PAA- and 5% CMC-modified bentonite by less than 42%. At Cu2+ concentrations ([Cu2+]) of 0.38-10 mM (26 times the field concentration, 3.6 times the maximum concentration in the investigation), the k of 5% CMC-modified bentonite remained at 10(-12) m/s, demonstrating good chemical compatibility. This was attributed to the intercalated CMC chains, which increased interlayer repulsion, enhanced osmotic swelling of bentonite plates, and reduced cation exchange reaction with Cu2+ at bentonite plate surfaces. When [Cu2+] reached 15 mM, k of 5% CMC-modified bentonite further increased to 6.0x10(-11) m/s. This paper filled the gap of creating low permeable polymer-modified bentonite for permeants with acidic and high heavy metal content condition, which is typical near copper mines. The outcome of this research results in practically usable barrier materials, as used in a geosynthetic clay liner (GCL) or similar applications without the need of importing high-purity bentonite (e.g., Wyoming bentonite).