In-situ zinc bioprecipitation by organic substrate injection in a high-flow, poorly reduced aquifer

• Zinc bioprecipitation is applicable in high flow, permeable aquifers.
• Zinc groundwater concentrations were reduced by 96–97%.
• Both emulsified vegetable oil and glycerol were effective substrates.
• Formed precipitates proved to be stable.
• In the long run (years), maintenance substrate additions may still be required.

We investigated if in-situ metal bioprecipitation (ISMP) is applicable to remediate a highly permeable zinc-contaminated aquifer at a metal-processing factory in Maasmechelen, Belgium. A large (more than 200 m long and 70 m wide) groundwater contamination plume has developed, with zinc concentrations in the range of 1–100 mg/L, whereas the legal Flemish clean-up standard is 0.5 mg/L. The estimated groundwater flow velocity is in the range 0.2–1 m/d. The groundwater is relatively oxidized, naturally low in DOC (< 1 mg/L) and relatively low in sulfate (40–50 mg/L). We conducted both laboratory feasibility tests as well as a long-term field pilot test in two sections of the plume. In the laboratory microcosm tests, zinc bioprecipitation (following addition of organic substrate and sulfate) removed more than 99% of the zinc from the water phase. Lactate, glycerol and vegetable oil were equally effective as substrates. 28-day anaerobic leaching tests indicated that the metal precipitates that were formed are stable, but they also suggested that substrate addition increases the solubility (leachability) of arsenic and manganese. In the field test, Zn concentrations were reduced by 2 to 3 orders of magnitude within the 232 day testing period and stayed low for the following 6 months in both pilot zones. In the field, no mobilization of arsenic occurred but manganese groundwater concentrations increased from 0.01–0.6 mg/L to 0.4–6.5 mg/L. Dissolved iron concentrations also increased markedly from below detection limits to concentrations as high as 67 mg/L. Zinc concentrations in groundwater were closely correlated to pH and redox potential (Eh): plotting y = [Zn] against x = pH/log(Eh), an exponential relationship was found:y=2⋅107⋅e−3,236x(R2≈0.7).y=2⋅107⋅e−3,236xR2≈0.7.