Exploring the optimal conditions for the infiltration and immobilization of colloidal activated carbon in sandy aquifers for subsurface groundwater treatment

Abstract

Due to the growing world population and the resulting increase in the amount and diversity of micro pollutants in surface and groundwaters, conventional water treatment methods are more and more unable to provide sufficient pollutant removal. Against this background, it is important that water treatment technologies are improved or new advanced technological developments are developed to keep water treatment effective and sustainable. Near well subsurface groundwater treatment is a recent idea to enhance water treatment in an effective and economical way. With this technique, the reactivity of the sediment around a pumping well is enhanced with a reactive material to create a reactive zone around the pumping well. A suitable reactive material for this purpose is activated carbon colloids (ACC) since these colloids are small enough (<10 μm) to be transported through a porous system, it’s effective against a wide range of micro pollutants and it is chemically stable. In their natural state, these colloids tend to coagulate. To inject ACC’s into the aquifer, stabilization of the suspension is essential. However, the mobile suspension has to immobilize in the subsurface so that it is not returned during subsequent abstraction. In this study, we aimed to further explore the possibility of ACC as an agent for near well subsurface treatment. Batch suspension tests were conducted to determine the ACC stability on different variables like ACC types, pH, ionic strength, carboxymethyl cellulose (CMC) concentration and mixing intensity. This was followed by column experiments where the different suspensions and their performance in a saturated porous was analyzed. A high suspension stability was obtained in high pH (10.6-11) deionized water and with addition of CMC. A high mixing intensity also increases suspension stability significantly. The creation of a reactive zone over the full length of the column was obtained using high pH deionized water and CMC as stabilizers. However, when CMC was used as a stabilizer, a larger decrease in hydraulic conductivity (K) of the sand was observed. The decrease in K was also larger when the flow velocity was reduced. In the remaining experiments where high pH deionized water as a stabilizer was used, the decrease in K was minimal. These results suggest that pH deionized water is a promising method for the stabilization of ACC’s in suspension, intended for infiltration in the subsurface with the goal of creating a reactive zone.