Sedimentation Kinetics of Hydrous Ferric Oxides in Ferruginous, Circumneutral Mine Water

Transport, transformation, and removal of iron inaqueous environments is primarily controlled by ferrous ironoxidation followed by aggregation and sedimentation of theresultant hydrous ferric oxides. The latter mechanisms areparticularly important for passive iron removal in mine watertreatment systems, yet the interrelation and underlying kinetics arepoorly understood. In this study, the sedimentation behavior ofnatural hydrous ferric oxides was systematically investigated underdifferent hydrogeochemical conditions via laboratory-based columnexperiments. The objective was to determine a robust modelapproach for the approximation of aggregation/sedimentationkinetics in engineered systems. The results showed thatsedimentation is governed by two interrelated regimes, a rapidsecond-order aggregation-driven step (r1) at high iron levels followed by a slowerfirst-order settling step (r2) at lower iron levels. Amixedfirst-/second-order model was found to adequately describe the process:kkFeFetdFedr2r12=[]+[]-[ ]withkr1= 9.4x10-3m3/g/h andkr2= 5.4x10-3h-1. Moreover, we were able to demonstrate that the removal of particulate hydrous ferric oxides at lowparticulate iron levels (<10 mg/L) may be reasonably well approximated by a simplifiedfirst-order relationship:kFetdFedsed=[]-[ ]withksed= 2.4 (+/- 0.4)x10-2h-1, which agrees well with incipient literature estimates. Only minor effects of pH, salinity, andmineralogy on kinetic parameters were observed. Hence, the results of this study may be broadly transferrable among different minesites.

Automated summary

This study systematically investigated the sedimentation behavior of hydrous ferric oxides in water and proposed a model approach to approximate the aggregation/sedimentation kinetics in engineered systems. The results show that the aggregation and settling processes are influenced by ferrous iron oxidation and the concentration of hydrous ferric oxides, with corresponding kinetic parameters determined. Additionally, the study found that the removal of hydrous ferric oxides at low iron levels can be well approximated by a simplified first-order relationship.