Permeation behavior of organic matter in the compacted bentonite under saline condition

The permeation behavior of organic matter through a compacted bentonite was evaluated in low and high ionic strength condition (deionized distilled water (W) and artificial ocean water (SW). For simulating humic substances which exist in geological environment, poly(acrylic acid) (average molecular weight, MWave: 450,000) and poly(acrylic acid, sodium salt) (MWave: 15,000) were used. In this experiment, the organic matters which elute from bentonite were also investigated. Polyacrylic acid solutions in W or SW system permeated the compacted bentonite (Kunigel V1, 100%) at dry density 1.2 Mg·m-3. The permeation has been continued during maximum 140 days. The concentration of polyacrylic acid and the eluted organic matters from bentonite in the permeated solution were analyzed. In the experiment of W system, poly(acrylic acid, sodium salt)(MWave: 15,000) was detected in the permeated solution after 70 days. In that of SW system, it was detected after 52 days. Poly(acrylic acid)(MWave: 450,000) didn't permeate in the both of system. It indicates that organic matters less than MW: 450,000 aren't filtrated by bentonite. It is supposed that the organic compound which can change its conformation (molecular structure) to a chain-like structure, are not likely to be filtrated, even if its molecular weight is more than 15,000. The concentration of total organic carbon which elutes from bentonite decreased against experimental time. In low ionic strength condition (W system), the concentration of eluting organic carbon from bentonite was 7.4 - 12.5 mg·l-1. In high ionic strength condition (SW system) it was 4.2 - 6.5 mg·l-1. In the both of systems, almost same kinds of organic matters were eluted from bentonite to the permeate solution. Comparing with the molecular weight distribution of humic substances, the molecular weights of organic matters from bentonite is quite small. (author)