Context
On a former sulfuric acid production site, remnants of the production process have been used to raise the lower part of the site. This part of the site is located near the Nervion river and its mouth into the Atlantic Ocean. As a consequent, this part of the Nervion river and the groundwater on the site are influenced by the tidal action of the ocean. On average, twice a day, aerobic saline sea water is entering the Nervion river and the saturated zone of the site under investigation. Besides, a net horizontal groundwater flow of anoxic fresh water in the direction of the river is taking place and a vertical percolation of aerobic rainwater through the unsaturated zone recharges the phreatic groundwater. As a result, the pyrite ash both above and below the groundwater table is exposed continuously to changing physico-chemical conditions. Solubilization of high amounts of heavy metals present in this pyrite ash threatens the groundwater and river water quality and consequently causes a significant risk of spreading. In order to better understand the solubilization mechanisms under these changing conditions and to find the most suitable reagent(s) for this purpose, laboratory tests were carried out as a first approach. To confirm the hypotheses, the selected reagent and its optimal dose, a pilot test on site was designed and executed.
Reaction and reagents
The laboratory tests revealed that the heavy metals were mainly mobilized/solubilized by the combined effect of the high salinity (mainly chloride) of the river water, the high redox of the percolation rainwater and the lower pH of the groundwater. In extended batch test multiple precipitation/immobilization mechanisms were tested using different reagents. In order to further test the longevity of the treatment method, the best performing reagents were subjected to column tests. In order to simulate as accurately as possible the real conditions, these column tests were performed with both deionized water (simulating the percolation of rainwater) and 1 M KCl (simulating the influx of saline river water). These tests highlighted that a combination of increasing the pH, increasing the cation exchange capacity and solidification of the pyrite ash performed best to prevent the heavy metals of further solubilization and re-immobilize heavy metals already in solution.
Reagent
The comprehensive analysis of different types of reagents at different dosages and pH conditions revealed that aluminate was the reagent performing best in order to immobilize the heavy metals to acceptable levels according to current standards.
As the pyrite ash showed a significant heterogeneity, an additional field “titration” test was performed prior to the pilot test in order to fine-tune the aluminate dose to the specific situation of the pilot test zone. During the pilot test, the relationship between injection volume and radius of influence was accurately determined, allowing optimization of the injection grid design for an eventual full scale approach.
Location: Bilbao; Spain
Geology: Heterogeneous man-made geology of, silty sand mixed with pyrite ash
Pollutant(s): Heavy metal: maximum groundwater concentrations (µg/L) As: 6.600 / Cd: 61 / Cu: 2.500 / Pb: 1.800 /Ni : Mb : 330). Concentrations in soil (mg/kg dm) : As : 2.500 / Cd : 83 / Cu Pb: 19.000 / Zn : 29.000
Reaction: Immobilisation by precipitation, cation exchange and solidification
Reagent(s): Aluminate
Application type: Grid application (pilot test)
Surface/length: 100 m2
Number of points: 20
Depth interval: 1,5 – 4,5 m-bgl (unsaturated and saturated zone)
Dosage: 2.000 l/m