Soil Nanocolloid Properties and Metal Contaminant Transport Behavior.

Environmental nanoparticles found in soil systems and biosolid materials may serve as potential contaminant carriers to groundwater resources however, little information exists about their stability and contaminant transport behavior in natural environments. Nanocolloids (< 100 nm) and macrocolloids (100-2000 nm) fractionated from three Kentucky soils of differing mineralogy and one biosolid waste material were characterized using scanning/transmission electron microscopy (SEM-EDS/TEM), dynamic light scattering, X-ray diffraction, and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Sorption of Cu2+, Pb2+, AsO3-, and SeO4-2 contaminants were investigated using adsorption isotherms, and stability characteristics of their suspensions at different ionic strengths were determined with settling kinetics experiments and zeta potential measurements. Settling kinetics experiments indicated that nanocolloids are more stable in solution than macrocolloids with and without contaminants present. In spite of their higher surface area the nanocolloids did not show statistically greater metal sorption affinity than the macrocolloids. SEM-EDS suggested that this may be the combined effect of decreased availability of nanocolloid sorption sites due to aggregation of nanoparticles induced by higher organic carbon surface coating and nanoparticle attachment to macrocolloid surface sites, enhancing their sorption capacity. Inner sphere binding of metals was indicated through positive zeta potential shifts and decreased pH with increased metal loads, with a sorption preference for Pb>Cu>As/Se. SEM-EDS and ATR-FTIR analyses indicated contaminant sorption on edge and face sites of both size fractions, particularly higher for the cationic metal species. Overall, the comparisons of nanocolloid and macrocolloid particles suggested that while aggregation phenomena and surface effects may cause nanocolloids to act much like the larger size macrocolloid particles, their increased stability at varying ionic strength solutions may pose a greater contaminant transport risk in natural environments.