- Remotox is a water-soluble compound that has been shown to be a cost effective for in-situ stabilization of metals. The common name for Remotox is calcium polysulfide, (CaS5) with a molecular weight of 199.8 gram per mole. Remotox has a pH of 11 to 11.5, a specific gravity of 1.27 and is deep orange-red color in solution. It is water soluble and comes as 29% (Wt. % CaS5) aqueous solution. Remotox is commercially available in pails, drums, totes, and tankers.
While effective treatment can be achieved over a wide pH range, best results are typically obtained within pH 7-10. Adjustments to pH of the treatment liquids can enhance the metals precipitation process. Application and dosage is dependent on site-specific conditions. Bench and pilot-scale testing is recommended to verify treatability.
Detailed Product Description
Remotox is a water-soluble compound that has been shown to be a cost effective for in-situ stabilization of metals. Cation metals such as arsenic, lead, cadmium, and copper are precipitated as non-toxic sulfides. Oxidized metals such as hexavalent chrome (Cr+6) are reduced and then precipitated as chromium hydroxide.
When mixed with water, Remotox dissociates into bisulfide, aqueous hydrogen sulfide and other sulfur anions, which react to the heavy metals to form less soluble, and non-toxic metal sulfides. Hexavalent chromium (Cr(VI) is reduced to trivalent chromium (Cr(III), which then precipitates as chromium hydroxide.
Remotox is routinely used to precipitate metals in contaminated aquifers and wastewater treatment systems. When injected into the ground it causes precipitation of cations as sulfides (FeS, ZnS, PbS, CdS, and CuS) and chromium, unlike the other metals listed, precipitates as hydroxide.
Arsenic: Acid medium only forms various arsenic sulfides; pH>7; arsenic-sulfur compounds are soluble; pH<7 the compounds are insoluble.
Lead: Wide range of pH (4-9) forms lead sulfide (PbS).
Copper: Optimal pH 5-7 forms copper sulfide (CuS).
Zinc: Wide range of pH (4-9) forms zinc sulfide (ZnS).
Cadmium: Wide range of pH (4-9) forms cadmium sulfide (CdS).
Molybdenum: Wide range of pH (4-9) forms molybdenum disulfide (MoS2).
Uranium: Wide range of pH (4-9) forms uranium disulfide (US2).
Cyanide: Chemical conversion produces thiocyanate. Thiocyanate can be bio-treated, or it can be treated with lime, producing calcium carbonate, gypsum and ammonia.
Chromium (Cr VI): can be treated with calcium polysulfide, and the Cr(VI) is reduced to Cr(III), which is then precipitated as chromium hydroxide. Hexavalent chromium is reduced to trivalent chromium by calcium thiosulfate and hydrogen sulfide. The sulfide of trivalent chromium is unstable in water or soil moisture and precipitates as chromium hydroxide (Cr(OH)3) which is stable across a wide range of naturally occurring geochemical conditions.
Cr6+ + 3/2 CaS203 + 3/2 H20 -» Cr3+ + 3/2 CaS04 + 3/2S + 3H+
Cr6+ + 3/2 H2S -> Cr3+ + 3/2 5 + 3//+
Hexavalent chromium may also be reduced indirectly by reaction with ferrous iron produced by interaction (reduction) of ferric iron (naturally present in soils) with calcium polysulfide.
Methods of Application
The in-situ reduction and fixation approach targets soils containing hexavalent chromium and other oxidized metal contaminates. Affected soils can also be treated as a precautionary measure to provide a treatment barrier to address vertical migration of hexavalent chromium. Remotox, a reductant, was selected for in situ reduction and fixation of hexavalent chromium to trivalent chromium. Remotox can be delivered to the affected subsurface soils by infiltration and/or direct injection. Remotox (CaSx) reacts with oxygen (02) and carbon dioxide (C02) to form calcium thiosulfate (CaS203), hydrogen sulfide (H2S), sulfur (S) and calcium carbonate (CaC03) as shown below:
CaSx + 3/2 02 -» CaS203 + (x – 2)5
CaSx + C02 -» CaC03 + H2S + (x – 1)5
The primary application of Remotox involves injecting a compound slurry into direct-push borings (e.g., Geoprobe). The solution can be varied by depth and location to insure contact with the contaminate plume.
Remotox can be mixed directly with contaminated soil and then using the mixture as backfill or hauling it to a disposal site. Remotox can also be placed into drilled boreholes, to create a reactive barrier, or in the bottom of an excavation which has penetrated the saturated zone.