Description

A zinc chloride tank at an industrial site had leaked and released unknown quantities of ZnCl2 into the underground. Over the years, a significant, up to 500 m long contaminant plume developed in the groundwater. The sediment of the aquifer was dominated by medium sand with portions of fine sand. The zinc concentrations in the effluent plume were between 5 and 20 mg / l in the planned range of the in situ barrier. The groundwater velocity was around 0.7 m/day. Due to the active operation of the industrial site and the depth of the aquifer (groundwater level approx. 14 m below ground level), an excavation of the soil was not considered economical. ColFerroX technology was then tested in pilot scale in order to immobilize the pollutant.

Lab tests

Sediment and groundwater from the site were used in column experiments to show the mobility of the Colferrox™ particles in the sediment. The iron oxides were completely mobile during the injection phase. After one day in the sediment column, the Colferrox™ particles were stably bound to the sediment. Subsequently, groundwater containing heavy metals from the site (10 mg/L zinc) was pumped through the columns and the adsorption of the heavy metals was monitored. It was found that the sand itself had a relatively high content of iron oxides and clearly adsorbed heavy metals, but not enough to keep the pollutant level below the threshold values. By injecting the Colferrox™ particles, the retention of the heavy metals was strongly enhanced. The data showed an adsorption capacity under site conditions of 38 mg Zn/g for the Colferrox™ iron oxide nanoparticles. For the field application a conservative value of 30 mg Zn/g NP was considered. Thus, the potential efficacy of a Colferrox™ in situ barrier could be demonstrated.

Pilot application

A pilot application was carried out using two injection wells with a foreseen width of the in situ adsorption barrier of 9-10 meters. A total volume of ca. 20 m3 was injected into each well. Injection pressure remained below 1 bar throughout the injection. No iron oxide was found in the downstream monitoring well (4 meters from injection point), which confirms that all ColFerroX particles precipitated successfully within the intended radius of 3 meters from the injection points.

Results

The zinc concentrations in the groundwater were monitored over a period of 80 days. Upstream of the barrier, between 5 and 20 mg/L zinc was measured in the groundwater. Immediately after injection, the zinc concentration in the effluent of the barrier decreased to < 1 mg/L. Furthermore, the concentrations in the injection wells themselves decreased to < 1 mg/L in both wells. Independently, the upstream concentrations increased slightly during the period. The concentrations of dissolved iron were low at all levels, indicating that the Colferrox™ particles were no longer mobile after injection and were not displaced with the groundwater flow, but formed a stable barrier.