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.
Particle stability Site remediation Humic acid Calcium bridging Aggregation Transport Porous media Goethite Calcium dose Environmental effects Pollution
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This work was partly funded by H2020 EU project “Reground,” G.A. no. 641768. We are grateful to Dr. Rainer Meckenstock (University of Duisburg-Essen, Germany) for providing the goethite particles stock suspension. We thank Fabrizio Bianco and Dr. Adriano Fiorucci (Politecnico di Torino) for their chemical analyses on tap water.
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Conflict of interest
The authors declare that they have no competing interest.
ESM 1Electrophoretic mobility of humic acid-coated goethite nanoparticle suspensions as a function of ionic strength. Examples of raw data from aggregation experiments. Apparent aggregation rate as a function of slurry concentration in the presence of 0.7 mM CaCl2. Additional results of sedimentation experiments at varying concentration of CaCl2 and MgCl2. Pictures of sedimentation vials for different doses of calcium. Representative results of sedimentation experiments of suspensions from which most of the unadsorbed humic acid were removed by filtration. Breakthrough curves of humic acid-coated goethite nanoparticles (total solid content of 1.70 g/L) in silica sand in 10 mM NaCl. Pictures of the column during transport tests carried out in 1.5 mM CaCl2 and 10 g/L solid content. Pictures of sedimentation vials for different dilutions in tap water. Results of transport tests conducted with suspensions diluted with tap water (PDF). (PDF 1658 kb)
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