1.Department of Land, Environment, and Infrastructure Engineering (DIATI),Politecnico di Torino,Torino,Italy
Abstract:
Nanosized colloids of iron oxide adsorb heavy metals, enhance the biodegradation of contaminants, and represent a promising technology to clean up contaminated aquifers. Goethite particles for aquifer reclamation were recently synthesized with a coating of humic acids to reduce aggregation. This study investigates the stability and the mobility in porous media of this material as a function of aqueous chemistry, and it identifies the best practices to maximize the efficacy of the related remediation. Humic acid-coated nanogoethite (hydrodynamic diameter ～90 nm) displays high stability in solutions of NaCl, consistent with effective electrosteric stabilization. However, particle aggregation is fast when calcium is present and, to a lesser extent, also in the presence of magnesium. This result is rationalized with complexation phenomena related to the interaction of divalent cations with humic acid, inducing rapid flocculation and sedimentation of the suspensions. The calcium dose, i.e., the amount of calcium ions with respect to solids in the dispersion, is the parameter governing stability. Therefore, more concentrated slurries may be more stable and mobile in the subsurface than dispersions of low particle concentration. Particle concentration during field injection should be thus chosen based on concentration and proportion of divalent cations in groundwater.
Graphical abstract Goethite nanoparticles are used in contaminated site remediation. The particles are stable in monovalent ion solutions due to an adsorbed layer of humic acids. Above a threshold dose of divalent cations, particles aggregate and sediment. High particle/calcium ratios increase colloidal stability. Stability in suspension and transport in porous media correlate well. Delivery into subsurface can be improved by either increasing particle concentration or reducing divalent cation content in the carrier fluid.
