Colloid Mobilization and Arsenic(III) Transport in Soils: Effect of Ionic Strength.
Hua Zhang, Department of Agronomy & Environmental Management, LSU Agcenter, Louisiana State University, Baton Rouge, LA 70803-2110 and H. M. Selim, Department of Agronomy and Environmental Management, LSU Agcenter, Louisiana State University, Baton Rouge, LA 70803.
High concentrations of arsenic (As) in soils and aquifers have been observed worldwide. Moreover, elevated As concentration is a public health issue with potential to impact wetland, aquatic environment, and soils. It is often found that a significant portion of the mobile arsenic is present in the colloidal form, i.e., arsenic minerals or adsorbed on mineral surfaces. Studies have demonstrated that arsenic associated with colloidal iron oxides traveled much faster than dissolved arsenic. Mobilization and transport of colloidal particles were found to be highly dependent on several factors especially ionic strength. The objective of this study is to test the hypothesis that changing ionic strength will mobilize colloid particles (e.g., iron and aluminum oxides and hydroxides) which has high absorption capacity for arsenic, leading to colloid facilitated transport of highly toxic arsenic. Two surface soils, Olivier loam and Windsor sand, were used in miscible displacement saturated column experiments. After slowly saturated with 0.01M NaCl background solution, packed columns were supplied with 10 mg L-1 As(III) prepared in 0.01M NaCl solution, followed by leaching with deionized water. Flow interruptions were carried out during the As(III) pulse and deionized water leaching phase to evaluate non-equilibrium or kinetic controlled arsenic transport and colloid mobilization. Electrical conductivity (EC), turbidity, and pH of column effluent samples were monitored. Mineral composition of the mobilized colloids was analyzed with X-ray diffraction (XRD). Total concentrations of As, Fe, and Al in the effluent samples were measured by digesting the effluent samples with 16M HNO3, while the dissolved concentrations (<0.20 μm) were determined by filtration with 0.20 μm membrane filter. The colloidal As, Fe, and Al concentrations were calculated as the difference between total and dissolved concentrations. Our results indicate that a significant amount of colloidal particles was mobilized when the background solution (0.01 M NaCl) was displaced with deionized water. This was based on the high level of turbidity (>100 NTU) in the effuluent solution. XRD analysis of the effuluent suggests that the colloid generated from soils were largely amorphous material. The peak of colloid generation coincided with Fe and Al peak concentrations. This suggests the mobilization of colloidal Fe and Al oxides. Moreover, the transport of As(III) was greatly enhanced by the introduction of deionized water and the subsequent decrease in ionic strength. Colloidal arsenic was only observed after introducing of deionized water and accounted for a significant portion of the enhanced arsenic concentration in the effluent. Flow interruption during the leaching phase resulted in further mobilization of colloidal particles and increase in total Fe and Al concentrations. A major implication of this study is that changes in chemical composition of solutions in aquifers and vadose zones might result in colloid facilitated transport of contaminants such as arsenic. For instance, landfill leachate often contains high concentrations of heavy metals with high ionic strength. Displacement of landfill leachate by rainfall or irrigation water which typically has lower ionic strength could result in mobilization of arsenic associated with colloidal particles and potential contamination of surface or ground water.