In-Situ Remediation of Heavy Metals Using Sulfur-Based Treatment Technologies
THE METALS PROBLEM: Highly toxic and highly soluble metals are contained in numerous waste streams including those from power, chemical, electronics facilities, general manufacturing plants, and mining facilities. Lead, chromium, arsenic, zinc, cadmium, copper, and mercury are the most common metal contaminants found on remediation project sites (US EPA, October, 2000). Cyanide is a common inorganic contaminant as well. Metal concentrations in excess of established health guidelines have been shown to create significant health risks to humans.
PAST REMEDIATION OPTIONS: In the past, conventional remediation of soil impacted by heavy metals has relied on excavation, which was expensive and disruptive. In addition, moving the soil only moved the problem, without treating the soil or reducing the long-term liability. For groundwater, pump and treat remediation relied on pumps to remove groundwater from the aquifer through a series of extraction wells or trenches. The extracted water was then treated above ground or disposed of off-site. Pump and treat methods fail to address the source of the contamination in the vadose zone. Although the construction of passive permeable treatment walls containing zero.
SULFUR-BASED TREATMENT TECHNOLOGY: Recently, sulfur-based metals treatment technologies have been the focus of an increasing number of research studies and commercial applications for treating metals contamination in soil and groundwater. The solubility of these metals is highly pH dependent. A reagent, such as calcium polysulfide (CaS4; brand name: Cascade®), precipitates the highly soluble metals (arsenic, lead, copper, zinc, cadmium, molybdenum, uranium) as less soluble, and non-toxic sulfides. As shown in Figure 1, metal sulfides are far less soluble than metal hydroxides. Metal hydroxides change solubility with changes in pH. Metal sulfides remain insoluble within a pH range of about 5 to 9.
Hexavalent chromium is reduced from Cr (VI) to trivalent chromium, Cr (III), and then precipitates as chromium hydroxide (US EPA, October, 2000). The cyanide ion, a common inorganic contaminant, reacts with calcium polysulfide to form thiocyanate, which is further degraded with excess calcium polysulfide or lime to ammonia, calcium carbonate and gypsum, eliminating the CNradical. Cascade® has a pH of 11.3 to 11.5, a specific gravity of 1.273 and is deep orange-red solution. Calcium polysulfide is water soluble and comes as 29% active ingredient solution. Polysulfide has been used to conduct in situ remediation of uranium, selenium, arsenic, copper, and chromium contamination.
REMEDIATION PHASES: The authors recommend a review of the existing physical and chemical data, including pH, permeability, lithology, and water depth, concentrations of metals, alkalinity, and other data and a simple bench test which can take a few days. Pilot scale tests are recommended to verify treatability. The in-situ pilot-scale or full-scale remediation can be performed shortly after the bench test results are available. DELIVERY SYSTEM: In-situ delivery is one of the key factors in successful remediation, since the treatment chemicals must fully contact and react withthe contaminant. High-pressure injection technology, also called jetting, uses a direct push method as well as a specialized lance system for the delivery of treatment chemicals has proved successful for metals remediation (Jacobs, 2001). Adjustments to pH of the treatment liquids can enhance the metals precipitation process.