Soil and Biosolid Nano- and Macro-Colloid Mediated Transport of Se, As, Cu and Pb Contaminants Through Undisturbed Soil Monoliths.

Very little effort has been invested in comparing the contaminant transport behavior of natural nanocolloid and their corresponding macrocolloid fractions.  Nanocolloids are expected to sorb and transport greater quantities of contaminants than macrocolloids due to enhanced stability and surface reactivity.  This study characterized stability and transportability of nano- (<100 nm) and macro-colloids (100-2000 nm) derived from three Kentucky soils and one biosolid waste through undisturbed soil monoliths in the presence of As, Cu, Pb and Se contaminants.  Stability studies evaluated suspended colloid concentrations after 48 hours in the absence and presence of 2 and 10 mg L^-1 contaminants.  Metal sorption coefficients (Kf) for each colloid were derived from Freundlich isotherms.  Leaching experiments involved infusion of 50 mg L^-1 colloid suspensions spiked with 2 mg L^-1 mixed contaminants through duplicate 18x30 cm soil monoliths with -5 cm boundary tensions over four continuous pore volumes.  Unsaturated flow conditions were maintained using Soil Measurement Systems baseplates attached to a Tension Infiltrometer (upper boundary) and a marriote device (lower boundary).  Colloid-metal suspensions were applied at 5.57 mm h^-1, representing a 10-day rainstorm with a 2-year frequency of reoccurrence.  Colloid-free solutions spiked with contaminants acted as controls.  Stability studies showed significantly higher dispersivity for nanocolloids than macrocolloids in the absence and presence of 2 and 10 mg L^-1 contaminants.  Kf values did not indicate significant sorption differences between sizes.  However, nanocolloids were eluted at significantly higher concentrations than corresponding macrocolloids.  Contaminant elutions varied by colloid and contaminant, with the following sequences: soil-colloids>bio-colloids and Se>As>Pb>Cu.  Nanocolloid elutions were likely enhanced from higher OC surface coatings, surface charge, and mobility in the higher pH column matrix.  Our findings demonstrate that nanocolloids may exhibit significantly higher mobility and contaminant transport potential over great distances in subsoil environments than their corresponding macrocolloids.