Simon J. Shankie MSc thesis abstract

This study assessed if a permeable reactive barrier (PRB) could be used to reduce sulphate and metal concentrations of Hogarth pit lake, a sulphite-toxic (up to 2,000 mg/L) pit lake at the former Steep Rock iron mine site in Atikokan, Ontario.  Both batch reactor and flow-through reactor experiments were performed to simulate a PRB at the bench-scale in order to assess the sulphate reducing capacity of different types of organic matter.

Batch reactor experiments were run using three different treatments to promote bacterial sulphate reduction in order to lower sulphate concentrations in water from the pit lake.  Treatment 1 contained organic matter, creek sediment (sulphate reducing bacteria source), carbonate rock (acid neutralizing agent) and glacial till (non-reactive medium).  Treatments 2 and 3 were similar to treatment 1, except that treatment 2 did not include creek sediment and treatment 3 contained molasses as a nutrient.  Treatment 1 with horse manure and wood chips as the organic source resulted in >99% reduction in sulphate concentration, combined with increases in pH and bicarbonate levels, reduced redox and decreased metal concentrations.  Bacterial sulphate reduction was also initiated with Treatment 2, although did not occur as quickly as treatment 1.  The results of treatment 3 with molasses showed that no sulphate reduction occurred in the batch reactors.  Based on these results, treatment 1 was selected for the flow-through experiment to simulate a PRB at a laboratory scale.

Flow-through reactor columns were run in duplicate and filled to create different reaction chambers that contained mixtures of treatment 1.  The most effective sulphate-reducing flow-through reactors consisted of two reaction chambers separated by silica sand, which resulted in an overall sulphate reduction average of 46% and 49%.  In comparison, all other flow-through reactors achieved a 39% reduction in sulphate concentrations.  Sulphate reducing bacteria activity was evident after three weeks with reductions in redox values and sulphate concentrations and increases in bicarbonate and pH levels.  Results of flow-through reactor 1, reduced sulphate concentrations to <300 mg/L between weeks 3 and 5, and had a gradual increase for the remainder of the experiment to around 1000 mg/L.  Results of flow through reactor 5, showed a decrease in sulphate concentration to <700 mg/L between weeks 3 and 8 before also increasing to around 1000 mg/L for the rest of the experiment.  All other reactors generally decreased to 900-1000 mg/L after 2 weeks and remained around 1000 mg/L between weeks 3 and 20.

Sulphate concentration in water from the adjacent Caland pit lake, has a sulphate concentration of <300 mg/L, and a previous study at the site concluded that a treatment system which consisted of a PRB flowing into a constructed wetland has the potential to reduce elevated sulphate levels in Hogarth pit lake.  However, the flow-through experiments show that the residence time is a limiting factor in the life span of a PRB.  Also, it is possible that sulphide precipitation is limited by the availability of divalent metals, in particular Fe²⁺.

A copy of the thesis can be downloaded here