Effect of polybrominated diphenyl ethers on sand-bentonite liner material.

Polybrominated diphenyl ethers (PBDEs) are present in biosolids and other solid wastes, as well as being detected in landfill leachates. As sand-bentonite mixtures are extensively used as basal liner materials for landfills, a sand-bentonite mixture was investigated by swelling tests and leaching column tests to determine whether it can effectively contain and/or immobilize PBDEs in landfill leachate. Leaching column tests were conducted with permeants consisting of biosolids' leachates diluted to 50% by volume and spiked with 50 μg/mL of a pentaBDE mixture solution. The results showed that the sand-bentonite retained up to 45-66% of the total PBDEs in the permeant; however, the concentration of PBDEs in the effluent increased continuously and reached a significant level during a 3-week period. PBDEs probably sorbed onto both fine and ultra-fine organic particles. During leaching, a compacted sand-bentonite admix could stop fine particles from passing, but at the same time, ultra-fine organic particles carried PBDEs through the barrier materials. The hydraulic conductivity, k, of the sand-bentonite was negatively affected by shrinkage of the clay interlayer caused by the permeant hydrophobicity. However, the hydraulic conductivity changed only to a limited extent, remaining at a magnitude of 10-9 cm/s, probably because the PBDE concentrations were low. Therefore, caution is needed when sand-bentonite is applied to landfill liners as a barrier for PBDEs.