Waterjet injection of powdered activated carbon for sediment remediation

Abstract

Purpose

In situ contaminated sediment remediation through the addition of activated carbon has been proven to be an effective remediation technique. An amendment delivery system was developed to accurately place and inject a powdered activated carbon slurry. The system was set up to deliver a series of discrete injections over a rectangular grid with the objective to deliver 3% carbon (C) by dry weight to an inundated saturated sediment at a maximum sediment depth of 30 cm.

Materials and methods

In situ conditions have been researched and small bench-scale models have been developed to provide valuable data for future field-scale models. Experiments were performed using kaolinite as a surrogate sediment because the color contrast between the clay and the C permitted the delivered C concentration to be quantified using a spectroradiometer. The experiments showed that a set of operational parameters for the injection system could be identified that provided relatively complete mixing of the C and clay at the desired depths. The experiments were repeated using soil samples contaminated with polyaromatic hydrocarbons (PAHs) and polydimethylsiloxane coated solid-phase microextraction fiber analyses to quantify pore water concentrations.

Results and discussion

Several different combinations of pressure, injection duration, and nozzle diameter were evaluated during the initial phase of the characterization of powdered activated C penetration in the surrogate sediment. Iterative approach tactics were conducted that concluded specifically placed, short-duration injections were necessary to deliver meaningful amounts of C in the test sediment. Analysis of these injections found that an average amended C concentration of 14% was achieved up to 26.7 cm deep in the surrogate sediment by a 9.5-s injection, whereas a 7.5-s injection at the same depth yielded an average concentration of 9.3%. The reduction in pore water PAHs concentrations through C injection was achieved in excess of 90% at all sampled locations within the injected perimeter.

Conclusions

Reduction in contaminant pore water concentrations within the soil/sediment appeared to be less dependent on the duration of the individual waterjet injections, and the effective depth of penetration appeared to be greater than that observed during the surrogate sediment experiments. The empirical nature of the waterjet work and the expected heterogeneity of contaminated soils/sediments suggest that it is appropriate to conduct site-specific bench-scale treatability testing prior to full-scale remediation using waterjet-delivered activated C.