Theme: 08. Metals

Topic Area: 8c. Precipitation and Stabilization of Metals

Formatted Title:

Laboratory and Pilot Testing for Precipitation of Chromium and Nickel in Groundwater

Background/Objectives:

Metals can be precipitated out of groundwater by altering geochemical conditions to make the metals less soluble or by causing the metals to undergo a chemical reaction that results in an insoluble product. Hexavalent chromium is soluble in water, however hexavalent chromium can be removed from groundwater by reducing it to trivalent chromium which is not soluble and precipitates out of the water. Reducing agents that contain sulfur such as calcium polysulfide, sodium dithionite, or sodium bisulfite can be used to reduce hexavalent chromium to trivalent chromium.

Other metals such as nickel can be precipitated by reacting them with sulfide to produce metal sulfides which are insoluble. Sulfur-containing reducing agents can also be used to form these metal sulfides.

Hexavalent chromium and nickel were present in groundwater in separate but overlapping plumes at a former automobile industry site in Michigan. These plumes had been contained by being enclosed within a slurry wall and a groundwater pump and treat system operated to maintain an inward gradient and prevent the migration of impacted groundwater. In order to stop operation of the groundwater pump and treat system, in situ treatment of the chromium and nickel in groundwater was required.

Approach/Activities:

A laboratory treatability study was performed to determine the reagents and doses of sulfur-containing reducing agents required to remove chromium and nickel from the groundwater. Several reducing agents were screened at different doses to evaluate the effectiveness at removing chromium and nickel from site groundwater. Biological treatment was also tested. Pilot studies were then performed to test these treatments in the field. After a successful pilot test, soil cores were removed from the pilot study area so that the reversibility of the precipitation of the chromium and nickel could be assessed. Low pH and highly oxidizing conditions were tested to determine whether the precipitated metals could be re-mobilized.

Results/Lessons Learned:

The laboratory study determined that the best removal of chromium without affecting the groundwater pH was obtained with sodium dithionite. Good removal of nickel was observed with sodium sulfide. The results of the first field pilot study showed that the sodium dithionite was consumed before moving downgradient therefore a higher dose was tested in the subsequent study. Biological precipitation of chromium was found to be possible, however at high concentrations of chromium, toxicity was observed and biological treatment did not occur. Although good treatment of chromium was observed, nickel concentrations were not reduced even by the higher dose of sodium dithionite. Since sodium sulfide performed well in the laboratory study for the precipitation of nickel, sodium dithionite, sodium sulfide and a mixture of sodium dithionite and sodium sulfide were tested in the next study and were successful in precipitating the nickel.

Since the pilot study showed that metals were precipitated by the injections, it was necessary to determine whether the metals could be resolubilized. Soil cores were collected from the treated areas. The reversibility testing performed on the soil cores showed that precipitated chromium could not be mobilized by low pH but could be mobilized by high doses of a chemical oxidant. Precipitated nickel could not be mobilized by a chemical oxidant but could be mobilized by extremely low pH (less than pH 3). As these conditions were not likely to occur naturally in the field, the in situ treatment was approved and a full-scale implementation has been performed.

Alex Cox

Affiliation

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Laboratory and Pilot Testing for Precipitation of Chromium and Nickel in Groundwater