Soil chemistry and mineralogy changes induced by calcium peroxide injection

Michael J. Kirby, Shaw Environmental, Inc. Abstract:

A large-scale pilot study was conducted at a phosphoric acid plant in Kansas, involving injection of calcium peroxide and iron sulfate heptahydrate to remediate arsenic in groundwater.  The goal of the project was to change the arsenic in groundwater from the more mobile As (III) to the less mobile As (V) species, which has a stronger affinity for soils than As (III).  Groundwater was monitored on a quarterly basis, and soil samples were collected one month and six months after the injection was completed. Twenty-eight soil samples from six different locations in the source area were analyzed for arsenic, arsenic species, iron, calcium, orthophosphate, and total organic carbon.  Porosity and permeability were measured to determine if the injectate would block natural flow paths in the soil.  Arsenic concentrations decreased in most of the soil samples and arsenic species tended to favor As (V) with time.  Orthophosphate, calcium, and iron concentrations decreased in most samples, and total organic carbon decreased in all samples.  Permeability and porosity did not significantly change during the process. Powder X-ray diffraction and scanning electron microscope analyses were also performed on each sample to determine the type and abundance of minerals capable of hosting arsenic.

Potassium iron phosphate decahydrate (KFe₃P₆O₂₀-10H₂O), or KFP, an anthropogenic mineral believed to have formed from the excess phosphate in the source area, a former arsenic sludge pond, was present in a number of the samples.  KFP formed in a number of sample locations during the pilot study.  Arsenic concentrations in samples containing KFP was higher than the arsenic concentrations in samples which did not contain KFP. Changes in the soil chemistry paralleled changes observed in overall groundwater chemistry at the site as a whole, both inside and outside the injection area.