Experimental evidence for ternary colloid-facilitated transport of Th(IV) with hematite (α-Fe₂O₃) colloids and Suwannee River fulvic acid

Previous field experiments have suggested colloid-facilitated transport via inorganic and organic colloids as the primary mechanism of enhanced actinide transport in the subsurface at former nuclear weapons facilities. In this work, research was guided by the hypothesis that humic substances can enhance tetravalent actinide (An(IV)) migration by coating and mobilizing natural colloids in environmental systems and increasing An(IV) sorption to colloids. This mechanism is expected to occur under relatively acidic conditions where organic matter can sorb and coat colloid surfaces and facilitate formation of ternary colloid-ligand-actinide complexes. The objective of this work was to examine Th transport through packed columns in the presence of hematite colloids and/or Suwannee River fulvic acid (SRFA). In the presence of SRFA, with or without hematite colloids, significant transport (>60% recovery within the effluent) of thorium occurred through quartz columns. It is notable that the SRFA contributed to increased transport of both Th and hematite colloids, while insignificant transport occurred in the absence of fulvic acid. Further, in the presence of a natural sandy sediment (as opposed to pure quartz), transport is negligible in the presence of SRFA due to interactions with natural, clay-sized sediment coatings. Moreover, this data shows that the transport of Th through quartz columns is enhanced in ternary Th-colloid-SRFA and binary Th-SRFA systems as compared to a system containing only Th. - Highlights: • Significant transport of Th-hematite-fulvic acid ternary colloids in quartz. • Significant transport of Th as pseudo colloids in laboratory experiments. • Significant mobilization of hematite colloids by fulvic acid in quartz sand. • Negligible colloid-facilitated transport of Th-hematite colloids in quartz. • Negligible colloid-facilitated transport of Th-hematite fulvic acid colloids in sediments.