Colloid-enhanced desorption of zinc in soil monoliths

The desorption and co-transport of metals by ex situ water dispersible soil colloids moving through macropores was evaluated in a leaching experiment utilizing Zn-saturated soil monoliths. The monoliths were created by hydraulically driving steel pipe sections (50 cm diameter x 50 cm length) into a Loradale silt loam soil (fine, silty, mixed, mesic Typic Argiudolls). Each monolith was saturated in excess of the soil cation exchange capacity (CEC) with ZnCl₂, then leached at a constant flux with a suspension of water-dispersible colloids fractionated from either the Bt or O horizon of a Beasley (fine, montmorillonitic, mesic Typic Hapludalfs) or Rayne (fine-loamy, mixed, mesic, Typic Hapludults) soil, respectively. Eluents from the monoliths were collected and analysed periodically for colloid and Zn concentration in the soluble and sorbed phase. Colloid and Zn concentration in the eluents varied greatly with respect to colloid type. The Beasley colloid, which had a small particle diameter (< 800 nm) and high sorptive affinity for Zn, showed considerable migrating ability through the soil matrix and enhanced Zn desorption by up to 400 times over that exhibited by a distilled water flushing solution. Eluted Beasley colloids were saturated with Zn at concentrations greater than 50% of the soil's CEC. Enhanced Zn desorption by colloids generated from the organic horizon of the Rayne soil was insignificant because the large colloid size (> 1200 nm) limited transport through the soil matrix. Mineralogical analysis of the eluted colloids revealed that in situ colloid generation was negligible, thus confirming that nearly all of the desorbed Zn fraction was due to the ex situ applied colloid.