Unsaturated Transport and Targeted Binding of Pluronic-Coated Nanoparticles: Lysimeter Experiments

Abstract

Efficient delivery of engineered nanoparticles (NPs) to a non-aqueous-phase liquid (NAPL) target zone located above the water table requires an understanding of their transport and binding characteristics. In this investigation, NPs coated with a tunable amphiphilic copolymer were employed in a series of experiments using a 1.4-m-long lysimeter. A crude oil zone was emplaced in the lysimeter to evaluate NP binding capabilities to a representative NAPL. Experimental observations were supported by a reactive transport model. NPs coated with a polymer concentration that promotes enhanced binding to crude oil were successfully delivered and retained in the NAPL zone at concentrations about three times higher than elsewhere in the lysimeter and consistent with the distribution of total petroleum hydrocarbons. Model simulations were able to reproduce the observed asymmetrical NP breakthrough curves and retention profile. The estimated attachment rate coefficient was two orders of magnitude higher for the NAPL zone than elsewhere, supporting the observed preferential binding to the crude oil. Depth-dependent straining was used in the model to capture the NP retention observed near the top of the lysimeter, presumably due to film straining caused by the increased capillary pressure. In addition to the reversible attachment and straining mechanisms, model simulations also indicated that loss of aqueous NP mass was required deeper in the lysimeter to provide a reasonable fit to the observed NP data. Due to increased contact with sediments, the polymer structure that coats the NPs may be slowly removed, leading to aggregation and reduced mobility because of physical trapping. Findings of this study revealed that despite the demonstrated targeted binding capability of these NPs in unsaturated systems, their delivery to a target NAPL zone distant from an injection location may be a challenge due to possible aggregation over longer travel distances and thus an important design consideration. Soil and groundwater contamination by petroleum hydrocarbons is a global issue due to the harm it causes to the environment and human health. Hence, developing novel in situ technologies is necessary due to limitations associated with ex situ remediation. One emerging technology called nanoremediation involves injecting nanosize particles into the impacted area to interact with contaminants effectively due to their small size and large surface area. By modifying their surface, particles can transport through soil and stick to specific contaminants, which is called targeted delivery. To better understand this technology, it should be studied under conditions similar to those in the field. Hence, we investigated targeted delivery of engineered nanoparticles in a large 1.4-m-long column packed with natural soil, representative of the unsaturated zone above a shallow aquifer. Our findings demonstrated preferential nanoparticle attachment to the crude oil–impacted sediment which provides the foundation to advance the use of these nanoparticles as a treatment agent and increases treatment efficiency. However, the noted increase in nanoparticle retention over longer travel distances is suggestive of their stability issues in unsaturated porous media, which may pose a limitation for the delivery of these NPs to a target zone distal from the injection location.