Effect of Dispersant on Transport of Nanoscale Iron Particles in Soils: Zeta Potential Measurements and Column Experiments

Nanoscale iron particles (NIPs) have been studied during the last few years for their potential for remediation of groundwater and soils contaminated by chlorinated hydrocarbons, pesticides, heavy metals, and some inorganic anions. However, the effectiveness of in situ remediation largely depends on quick and uniform distribution of NIPs into the contaminated subsurface zones. Previous studies have shown that transport of bare NIPs in subsurface soils is very limited and surface modification by dispersants such as polymers and surfactants is necessary. The objective of this study was to investigate eight different dispersants at different concentrations to determine their ability to modify the surface characteristics and increase the stability of the NIP suspension, thereby minimizing agglomeration and sedimentation of the NIPs. The studied dispersants and their tested concentration ranges [as % of NIP suspension, weight-to-weight ratio (w/w)] were as follows: aluminum lactate (Al-lactate; 2–15%), sodium lactate (Na-lactate; 6–12%), ethyl lactate (EL; 6–12%), aspartic acid (ASP; 2–8%), polyacrylic acid (PAA; 2–8%), 2-hydroxypropyl-$\beta$-cyclodextrin (1–4%), $\beta$-cyclodextrin ($\beta$-CD; 1–4%), and methyl-cyclodextrin (MCD; 1–4%). Zeta potential measurements and column experiments were performed on NIP-dispersant suspensions. Results showed that the zeta potential of bare NIPs was $41.7\pm 2.3\mathrm{mV}$. The influence of the dispersants was found to vary significantly depending on the chemical nature of the dispersant and the electrical charge of the ions in solution. Aluminum lactate released ${\mathrm{Al}}^{3+}$ into the solution, resulting in a reduction of the NIP zeta potential from $37.7\pm 1.8\mathrm{mV}$ at 2% concentration to $9.5\pm 0.7\mathrm{mV}$ at 15% concentration. Finally, cyclodextrins showed less influence on the zeta potential of NIPs with a slight increase (up to $49.3\pm 3.2\mathrm{mV}$). The results depict that a higher percentage of NIPs modified with aluminum lactate eluted from the soil. A 10% concentration of aluminum lactate exhibited the highest (93%) elution of the modified iron from the soil media. Aspartic acid and ethyl lactate showed less iron eluted (35–40%). The column tests demonstrated that aluminum or sodium lactate can increase the stability of the NIP suspensions and enhance their transport in subsurface soils. Furthermore, these dispersants are environmentally friendly, relatively inexpensive, and practical to use.