CHEMICAL REDUCTION



Heavy Metals Treatment

ISOTEC offers in-situ heavy metals treatment via two technologies – calcium polysulfide (CaSx or CAPS) and ISOTEC Fe catalyst (AB24). Additional reagents utilized include FerroBlack®, sodium metabisulfite and sodium thiosulfate. In-situ chemical reduction treatment is a comparatively economical and rapid alternative to pump-and-treat. Furthermore, complex subsurface mixes of heavy metals and organic contaminants (e.g. VOCs, SVOCs) can be treated simultaneously using a combination of ISOTEC's proprietary technologies.

Comparative laboratory bench-scale studies are designed and conducted at our research facility to identify the most suitable reagent and optimal reagent dosing.   Based on treatability study results, a site-specific treatment program is designed and implemented.  Depending on the size of your site, most treatment programs are completed in a matter of days to a few weeks.  As always, ISOTEC’s two decades of injection experience and impeccable safety and performance record ensures the best possible outcome for your site.

Heavy Metals Treatment

 

Calcium Polysulfide

Calcium polysulfide (CaSx) is a strongly reducing lime-sulfur solution that is applied for in-situ heavy metal treatment via site-specific injection pathways (e.g. temporary direct push points, injection wells, infiltration trenches, etc.). CaSx is an effective stabilizer over a wide range of pH values for many regulated heavy metals including chromium, lead, cadmium, copper and zinc. CaSx can also be employed to reduce mercury contamination to low levels. The chemical reaction is often instantaneous upon contact and the chemical has shown no interference in promoting ISCO reactions (i.e. using modified Fenton's reagent) if other organic contaminants are also present.

Most heavy metals precipitate as insoluble metal sulfides (FeS, PbS, CuS, ZnS & CdS).  For instance, excess calcium precipitates as calcium carbonate or calcium sulfate, cyanide, if present, converts to thiocyanate, and  hexavalent chromium is reduced to trivalent chromium, as shown in the chemical reaction below.  The trivalent chromium precipitates as chromium hydroxide [Cr(OH)3].

Cr+6 + 1.5CaS2O3 + 1.5H2O → Cr+3 + CaSO4 + 1.5S + 3H+

Where:

Cr+6 = Hexavalent Chromium
CaS2O3 = Calcium Polysulfide
H2O = Water
Cr+3 = Trivalent Chromium
CaSO4 = Calcium Sulfate
S = Sulfur
H+ = Hydrogen Ion

 


Fe(II)-C Reagent (AB24)

Our AB24 reagent is a chelated ferrous-iron complex designed to enhance in-situ distribution and for hexavalent chromium (Cr+6) reduction. This technology is compatible, and can be used in combination with ISCO technologies (e.g. modified Fenton's reagent, activated sodium persulfate), especially when organic contaminants are also present at the site. The AB24 reagent mitigates rapid iron precipitation under circum-neutral pH conditions by utilizing proprietary chelating agents to formulate the Fe(II)-chelate complex. AB24 ensures that iron remains in solution under native pH conditions and mitigates the sorption effects of native aquifer material. As a result, the AB24 reagent has excellent mobility characteristics, is able to treat large distances away from the point of injection, and can overcome the oxidizing conditions generated during ISCO. The chemical reaction associated with Cr+6 reduction using the AB24 reagent is provided below.

Fe+2 → Fe+3e-
Cr+6 + 3e- → Cr+3

Where:
Fe+2 = Ferrous Ion
Fe+3 = Ferric Ion
Cr+6 = Hexavalent Chromium
Cr+3 = Trivalent Chromium
e- = Electrons

The process consists of an oxidation-reduction reaction that involves exchange of electrons between chromium and iron; thereby, changing the oxidation state of ferrous to ferric and that of hexavalent chromium to trivalent chromium.  The ferric iron precipitates as ferric hydroxide while trivalent chromium precipitates as chromium hydroxide [Cr(OH)3] or is coagulated by ferric hydroxide precipitate.