Colloidal transport of heavy metals in low‐advective‐velocity environmental systems: Reactive transport model on biogeochemical and hydrodynamic impacts

In this study, the impact of colloid facilitated transport of heavy metals on the overall biogeochemical processes is demonstrated in example Lake Coeurd'Alene sediments. Release and transport of heavy metals (Pb and Zn) on initially sorbed colloidal Fe (hydr)oxide minerals are compared with immobile surfaces under various advective flow velocities. The reactive transport model integrates a coupled biotic reaction network with multiple terminal electron acceptors, including multicomponent diffusion and electrostatic double layer (EDL) treatment effects, illustrating the impact of colloidal transport under competing biogeochemical reaction dynamics for the first time to the authors’ knowledge. The model results illustrate the sensitivity of the results under low‐flow‐velocity conditions. Although enhanced Fe reduction prevails with immobile Fe (hydr)oxide mineral surfaces, the desorbed metal ions with aqueous sulfide complexes are rather “washed out” from the system along with advective transport of solutes, whereas the reductive dissolution of colloidal Fe (hydr)oxides from freshly coming colloidal surfaces results in the accumulation of metal and sulfide ions in the system. The results show that when the potential transport of sorbed contaminants with colloidal particles are ignored, the contaminant concentrations might be underestimated under low‐flow‐velocity conditions, especially around 10−8 or 10−9 m s−1, where the underestimation for the worst case scenario at the lowest bound of low‐flow‐velocity conditions may reach around 90% with depth. On the other hand, this impact may be less significant under cases of higher flow velocity, even around higher limits of low‐velocity environments around 10−7 m s−1, as well as in pure diffusive transport cases.