Composition and Transport Behavior of Soil Nanocolloids in Natural Porous Media.

Water dispersible mineral and organic colloids were fractionated from selected subsoil horizons with variable mineralogical composition and biosolid wastes commonly applied to land surfaces. Colloid suspensions spiked with Cu, Zn, Pb, atrazine, and metolachlor were infused into undisturbed soil monoliths at a rate of 0.7 cm/hr, sustaining unsaturated flow conditions. Leaching outputs were monitored continuously with respect to eluent volume, and colloid, metal, and herbicide concentrations. The mean diameter of the eluted particles ranged from 50 to 120 nm for the mineral colloids and 76 to 134 nm for the biocolloids, suggesting an overwhelming majority within the nano-size range.  Colloid composition, size, and surface properties were the primary parameters controlling their stability, mobility, and co-transportability of metals and herbicides. Decreases in mean nanocolloid diameter, and increases in surface charge, surface area, and electrophoretic mobility drastically enhanced nanocolloid stability and transport through undisturbed soil monoliths, even at relatively high ionic strength levels, and significantly (up to 11-fold) increased metal and herbicide transport in subsurface soil environments. Substantial increases of both total metal and soluble metal loads in the presence of nanocolloids suggested the influence of strongly interactive chemical and physical processes between nanocolloid surfaces and soil matrices in controlling the co-transport behavior. The findings emphasize the importance of investing more advanced technology resources toward improved characterizations of environmental nanocolloids so that we obtain a better understanding of their role and behavior under natural conditions as potential pollutants or remediators.