Retention and Transport of Cellulosic Nanocrystals in Soils.

Cellulose nanocrystals (CNCs) have a large potential for environmental remediation due to their exceptional mechanical properties, large specific surface area and high aspect ratio. This study examines the mobility of CNCs (5-6 nm in thickness and 80-100 nm in length) in soils through a series of column experiments under saturated flow conditions in NaCl solution (5-50 mM). Results show that CNCs have high mobility in clean silica sand, with a small retardation of ~1-2 pore volumes compared with conservative tracer bromide. Increasing ionic strength retarded the transport of CNCs due to reduction of electrical repulsion between similarly charged CNCs and sand surface, but the reduction is small. Presence of positively charged iron oxide on sand surface caused a large retention of CNCs at various ionic strength levels due to strengthened electrical attraction. However, opposite to the ionic strength effect with clean sand, increasing ionic strength promotes transport of CNCs through iron oxide-coated sand, with maximum relative effluent concentrations (C/C₀) increasing from 0.6 at 5 mM to 0.95 at 50 mM. This result meets theoretical prediction. In addition, our data show that the transport of CNCs could be much retarded in natural soils. For instance, a three pore volumes of retardation relative to bromide is observed before a complete breakthrough occurs in NaCl solution (5 mM). This higher retention in soil than in clean sand suggests that certain soil components (e.g., organic substances and metal oxides) control the fate and transport of CNCs.