"Effects of pH Control Techniques on Transport of Zero-Valent Iron Nanoparticles in an Aquifer"Injection of nano zero valent iron (nZVI) suspensions into the subsurface is a new and promising method for in-situ remediation of contaminants that can treat not just a contaminant plume but also the source zone itself.
Although applications of injectable nZVI are already available in the market, the transport distance from the injection well is still limited. Additionally, because of their high reactivity, the corrosion that the nZVI particles undergo can limit their lifetime. While increasing the pH in the aquifer or in the nZVI suspension will reduce the anaerobic corrosion and thus increase the longevity of the nZVI, little is known to date on how the pH affects the particles transport. However, there is some evidence that a higher pH will reduce the agglomeration and deposition of nZVI particles, and then increase their transport.
The objective of this master thesis is to perform a 1D experimental study to evaluate the effect of increased pH on nZVI transport. Therefore, nZVI suspensions were injected into water saturated sand columns to see which the transport behavior of the particles is. To analyze the effect of an increased pH in the transport of nZVI, two strong bases were used: Calcium hydroxide, which was injected as a suspension or as a solution, and a sodium hydroxide solution. The bases were injected before, together with and after the nZVI suspension.
A better transport was obtained when calcium and sodium hydroxide suspensions and solutions were injected before and after the nZVI suspensions, even though only when a calcium hydroxide suspension was injected before the nZVI the effect was significant. It was clear that the transport distance of the nZVI particles was reduced when the nZVI was injected together with the bases, being the calcium hydroxide the base that produced a stronger effect because of its higher ionic strength.
The previous effect was caused by the already high pH values of the nZVI suspensions and by their high ionic strengths given by the bases that inhibited the repulsion forces between nZVI particles and between nZVI particles and sand grains, letting them sediment or get attached. It was proven, however, that the pH has an influence on the transport of nZVI, because when the pH of the nZVI suspension was decreased with HCl, the transport was reduced.
Finally, as a secondary objective, the feasibility of using calcium hydroxide particles for long term pH control was tested. It was demonstrated that it is feasible to inject calcium hydroxide particles into the aquifer; however the long term pH control depends on the aquifer hydraulics and chemistry.