Colloid-facilitated Transport of Heavy Metals in Lake Sediments

This work focuses on the development of a mechanistic Fe(hydr)oxide based (HFO) colloid-facilitated reactive transport model which identifies the impact of HFO colloids on the stability and mobility of heavy metals (Zn and Pb) in example subsurface benthic sediments of Lake Coeur d’Alene (LCdA), USA. Ferrihydrite colloids are considered as the major sorbing phases for heavy metals, where the metal adsorption is implemented by surface complexation using double layer modeling and with electrostatic double layer (EDL) implementation using the dual-domain diffusive mass transfer characteristics of the PHREEQC code. The transport of colloidal phases is implemented by using 4 different advective velocities of the solution and colloidal particles (3 x 10^-8 cm/s, 3 x 10^-7 cm/s, 9 x 10^-7 cm/s, and 3 x 10^-6 cm/s), which are within the range reported in the literature for similar porous systems with relevant ranges of Peclet numbers. The advective transport simulation results are also compared with pure diffusive transport of solutes (and HFO) in the system, where the effective diffusion coefficient of colloidal particles is determined by the surface characteristics of ferrihydrite mineral discussed in previous studies. The simulations compare the biogeochemical cycling of metals considering colloidal vs. immobile phases of Fe(hydr)oxide minerals in the lake sediments. The impact of colloidal HFO particles on reactive transport and sorption of heavy metals in a natural environment, integrating coupled biotic reaction network with multiple terminal electron acceptors is presented.

The simulation results show that the colloidal HFO runs indicate a significant difference in results when advective transport of solutes (and HFO) is considered, as opposed to pure diffusive transport of ions and colloidal particles in this system. The increase in flow velocity observed to result in an increase in the transport of heavy metals with depth with increases in heavy metal profiles, indicating the importance of colloidal transport in the presence of especially advective transport. Hence, the results of the study reveal that when the potential transport of sorbed contaminants with colloidal particles are ignored, the contaminant concentrations in aqueous environments might be underestimated.