Lead adsorption and transport in loess-amended soil-bentonite cut-off wall

Abstract

Soil-bentonite slurry-trench cutoff walls using backfill consisting on-site sandy or silty soil and bentonite are extensively used as engineered barriers for the containment of groundwater and soil pollution. Chinese loess has been shown to have large adsorption capacity regarding heavy metals. There is a great potential to use Chinese loess as amendments to improve the adsorption capacity of the wall. Batch and column tests were carried out to investigate the adsorption and transport of Pb(II) in loess modified soil-bentonite (LSB). Batch tests were conducted to study the effects of contact time, initial Pb(II) concentration, loess proportion in LSB, and pH on Pb(II) adsorption. The rate constant of pseudo-second-order kinetic adsorption model of LSB is two times greater than that of soil-bentonite (SB), which indicates that the adsorption rate of LSB is much faster than that of SB. The mean free energy of adsorption evaluated by D-R model is 15.56kJ/mol and 18.89kJ/mol for SB and LSB, respectively. This indicates the adsorption mechanisms of SB and LSB are mainly ion exchange and chemical reaction. The adsorption capacity of LSB increases linearly with the increase of the loess amount in LSB. The maximum adsorption amount of LSB containing 20% loess can be 2 times greater than that of SB. Results also indicate that Pb(II) is first adsorbed onto loess until the adsorption amount Pb(II) onto loess reaches its adsorption maximum. An equation considering this effect was proposed for prediction of the adsorption capacity of LSB. A set of column tests were carried out to evaluate the tortuosity, mechanical dispersion and retardation factor regarding lead transport in SB and LSB vertical engineered barrier. The retardation factor of LSB and SB are determined to be 38 and 15, respectively, by the column tests. The Kd values obtained from batch tests are significantly larger than those obtained from column tests due to the difference of soil-water ratio used for these two types of tests. Using the obtained parameter values, the breakthrough time of Pb(II) through LSB wall is evaluated to be about 2 times greater than that of the SB wall when the hydraulic head difference is <3m. This may be due to the increase of adsorption capacity of the soil-bentonite wall amended with the loess.