Promotor: Prof. Erik Smolders
INTRODUCTION
Contamination of groundwater with arsenic (As) poses serious health risks to people all over the world through use as drinking or irrigation water but the current remediation techniques are costly and labour-intensive. Injection of nanoparticulate (smaller than 100 nm) iron oxides or oxyhydroxides (FeOx) may offer a more affordable solution because As has a tendency to adsorb on FeOx surfaces. The benefits of nanoparticles (NPs) are: (i) they are easy to inject in the contaminated groundwater, and (ii) their small size causes them to have a higher adsorption capacity.
Figure 1: Set-up of a reactive zone for the remediation of contaminated groundwater. Adapted from (Crane & Scott 2014).
After injection, the FeOx nanoparticles aggregate due to the high ionic strength of groundwater. This creates a reactive zone within the groundwater layer where the As adsorbs on the solid phase. Since the contaminant is left behind, the water flowing out of the reactive zone will be depleted in As. Ideally, the groundwater concentration is reduced far enough to comply to the WHO guideline of 10 μg L-1.
EXPERIMENTAL WORK
The efficiency of As removal by goethite (α-FeOOH) NPs was investigated through two experiments.The first was a batch adsorption experiment in equilibrium conditions, for NPs in suspension and NPs aggregated on sand to mimic the aquifer matrix. The adsorption maxima of both materials were similar: 68 mg As g-1 Fe for the particles in suspension and 65 mg As g-1 Fe for the particles aggregated on the sand. Therefore, it was concluded that aggregation did not influence the adsorption capacity of the NPs, under the applied conditions (neutral pH and 5 mM CaCl2 to mimic the ionic strength and composition of groundwater).
Secondly, the transport of As through a sand column containing immobilized NPs was investigated by conducting column experiments at two Darcy flow rates (0.08 cm min-1 and 0.008 cm min-1) and two injected As concentrations (1 mg L-1 and 5 mg L-1). The adsorption maxima of the sand in the columns were lower than those from the batch experiments and ranged between 35 and 59 mg As g-1 Fe. The parameters from the batch experiment were used to model the breakthrough curves of the columns under equilibrium conditions and this model did not fit the experimental data. This observation indicated that the transport of As in the columns occurred under non-equilibrium conditions. Flow interruption experiments also supported this hypothesis.
CONCLUSION
This work indicated that goethite NPs may be a suitable material for the remediation of As contaminated groundwater. The transport of As through a matrix containing these NPs did not occur under equilibrium conditions at the investigated flow rates. This may indicate the need for non-equilibrium or kinetic models to predict the interaction between the particles and the contaminant (As). However, groundwater flow rates are often reported lower than the flow rates that were investigated. As transport may occur under equilibrium conditions in the real application, which simplifies the modelling required to predict the effectiveness.
REFERENCES
Crane, R.A. & Scott, T.B., 2014. The Removal of Uranium onto Nanoscale Zero-Valent Iron Particles in Anoxic Batch Systems. Journal of Nanomaterials, pp.1–9.