Background
Phosphogypsum is a by-product of the production of phosphoric acid, a necessary component of commercial phosphate fertilizers. The production of phosphoric acid involves the reaction of sulfuric acid and phosphate rock. The resulting solution is a combination of gypsum (CaSO4), hydrogen fluoride (HF) and phosphoric acid, hence the name phosphogypsum (PG) (Rutherford et al. 1994).
Ca5(PO4)3 + 5 H2SO4 + 2 H2O → 3 H3PO4 + 5 CaSO4 · 2 H2O
Source: www.epa.gov
Once the gypsum material has been filtered from the solution, it is mixed with water to form a slurry which can be pumped into settling ponds (wet stacking). These settling ponds grow as the gypsum settles out forming large stacks, referred to as phosphogypsum stacks (Wissa 2002). The stacks can grow to an immense scale
depending on the engineered capacity and can occupy vast amounts of land.
Global estimates of world production of PG are approximately 180 million tonnes (Abril et al. 2009). In Canada, there are PG stacks in Ontario, Quebec, New Brunswick, British Columbia and Alberta, although the largest and most numerous amounts are located in Alberta (Thorne 1990). Fort Saskatchewan alone is home to 5 million tonnes of PG material, produced between 1965 and 1991 (Hallin 2009).
depending on the engineered capacity and can occupy vast amounts of land.
Global estimates of world production of PG are approximately 180 million tonnes (Abril et al. 2009). In Canada, there are PG stacks in Ontario, Quebec, New Brunswick, British Columbia and Alberta, although the largest and most numerous amounts are located in Alberta (Thorne 1990). Fort Saskatchewan alone is home to 5 million tonnes of PG material, produced between 1965 and 1991 (Hallin 2009).
Environmental Impacts of Phosphogypsum Stacks
Source: www.altenergymag.com
The complexity of the chemical process of obtaining phosphoric acid from phosphate rock is not as straightforward as the chemical equation listed above has suggested. In reality there are numerous impurities integrated into the composition of the phosphate source material including radium, uranium, arsenic, barium, cadmium, chromium, lead, mercury, selenium and phytotoxic fluoride (Rutherford et al. 1995). These trace elements accumulate in the PG byproduct and the water held within its pore space. The presence of these toxic metals and radionuclides have limited the use of the PG material and as a result, the PG must be isolated until such a time that further uses can be explored (Degirmenci et al. 2007, Rutherford et al. 1994).
The leading concern regarding PG stacks includes contamination of the environment by multiple integrated vectors such as PG leachate to groundwater, wind and water dispersion to surrounding land and radionuclide emissions from the stack (SENES 1987; Thorne 1990). Studies have demonstrated that concentration and mobility of most trace elements within the PG pore water decrease with time as the stack drains (Rutherford et al. 1995). However, the introduction of precipitation into the PG stack can facilitate the mobility of these trace elements as well as dissolve the gypsum material (SENES 1987).
The leading concern regarding PG stacks includes contamination of the environment by multiple integrated vectors such as PG leachate to groundwater, wind and water dispersion to surrounding land and radionuclide emissions from the stack (SENES 1987; Thorne 1990). Studies have demonstrated that concentration and mobility of most trace elements within the PG pore water decrease with time as the stack drains (Rutherford et al. 1995). However, the introduction of precipitation into the PG stack can facilitate the mobility of these trace elements as well as dissolve the gypsum material (SENES 1987).
Regulatory Framework
Source: www.ualberta.ca
Currently, there is little information available regarding reclamation of PG stacks in Canada. Most of the information that has been produced regarding this topic is from locations with humid climates such as Florida, North Carolina, and Tennessee. At present, there is no formal regulation regarding reclamation protocols for PG stack closure in Alberta, but Alberta Environment has recommended a minimum of 1 meter of topsoil as a suitable cap (Alberta Environment 2008). The use of such a large amount of soil is potentially expensive and requires removal of good topsoil from surrounding farmland. The results from the proposed research will be used to inform hydrological models that will be used to quantify potential environmental risks associated with the PG stacks.
Research Objectives
Source: www.ualberta.ca
This research will investigate the responses of various capping depths of topsoil to naturally occurring hydrological events. Specifically, the objectives include:
• Quantification of the soil water balance in the topsoil cap,
• Evaluation of the potential drainage of water from the topsoil cap into the underlying phosphogypsum,
• Influences of various plant species on soil water balance in topsoil cap.
• Quantification of the soil water balance in the topsoil cap,
• Evaluation of the potential drainage of water from the topsoil cap into the underlying phosphogypsum,
• Influences of various plant species on soil water balance in topsoil cap.
Expected Results
• Minimal capping depth will increase the probability of interaction between infiltrating water and PG, resulting in an increase in migration of water into and through the PG material.
• Medicago sativa (Alfalfa) will absorb more soil water then the four grass species, thus limiting the movement of soil water into the soil profile.
• Medicago sativa (Alfalfa) will absorb more soil water then the four grass species, thus limiting the movement of soil water into the soil profile.
Disclaimer: All datasets, events and parameters have been manipulated and/or randomly generated.