Impacts of presence of lead contamination on settling behavior and microstructure of clayey soil - calcium bentonite blends

The impact of heavy metal contamination on settling behavior and microstructure of kaolin-bentonite blends is evaluated via a series of sedimentation tests. The bentonite content in the blends varies from 0% (i.e., pure kaolin) to 15% by dry weight. Concentrated lead nitrate (Pb(NO3)2) solution with target lead (Pb) concentrations ranging from 0 (i.e., distilled deionized water) to 60 mmol/L is used as a representative heavy metal contamination. The blend subjected to the sedimentation test is prepared by both the conventional method (Method 1) and a new method (Method 2), in which Method 2 allows for sufficient chemical interaction between clay particles and concentrated Pb(NO3)2 solution. The results demonstrate that the sample preparation method, bentonite content, and Pb concentration considerably affects the settling behavior. A threshold Pb concentration (approximately 0.7 mmol/L to 2 mmol/L) is found to differentiate the settling behavior of the blends. The blend displays an accumulation sedimentation when the Pb concentration is lower than the threshold, otherwise it shows a flocculation/aggregation sedimentation. The final sediment volume (SVF), obtained at the termination of the sedimentation test (80 days), increases with increasing bentonite content. In addition, SVF increases with increasing Pb concentration when the blend shows the accumulation sedimentation; whereas opposite tendency of the SVF - Pb concentration relationship is found in the flocculation/aggregation sedimentation. A power function is derived for assessing the relationship between SVF and liquid limit of the blends tested in this study and soils reported in previous studies. In addition, the chemical compatibility, in terms of hydraulic conductivity and compression index, can be preliminarily assessed through combination of sediment volume ratio and liquid limit ratio. Finally, the micro-mechanisms controlling the change in settling behavior with lead concentration is addressed by analyzing microstructures of the sediments accumulated at the termination of sedimentation tests via scanning electron microscopy.