Story

Project - Pilot Study Injection of EHC^®-M for Lead

Former Battery Recycling Facility - Brazil 

Lead Consultant - SGW Services, Sao Paulo, Brazil

Summary

EHC^®-M has been shown to effectively immobilize lead (Pb) from groundwater at a former battery recycling facility in Brazil. Following the injection of EHC-M performed in November 2007, subsequent performance monitoring conducted in January, February, March and June 2008 showed  Pb below the detection limit of 10 ppb, which represents a >97% decrease in soluble Pb. The treatment also involved adjusting the pH to around 8 at the upper portion of the aquifer.

The Challenge

The site groundwater is impacted with lead up to a maximum concentration of ca 600 ppb. The impacts have migrated from the former battery recycling facility and the main groundwater plume measure an estimated  250 m long x 150 m wide x 15 m deep (from 15 to 30 m bgs).  There is also a smaller plume to the north measuring 50 m long x 50 m wide x 15 m deep (from 15 to 30 m bgs). The goal is to immobilize the lead in situ to limit the migration of groundwater plume. This will be achieved via the injection of EHC-M, which will promote precipitation of soluble lead as stable sulfides. Baseline conditions are relatively oxic with a DO of 2 to 6 ppm and ORP generally in the range of 80 to 400 mV.  Shallow groundwater is very acidic with a pH as low as 2.5 and a key portion of the treatment include adjusting the pH to around 8. Geology is fine sand / sandy clay and the linear groundwater flow is estimated at 3 m/year.      

EHC-M for In Situ Immobilization of Lead

Metallic lead is only stable at very low redox potentials and from slightly acidic to extremely basic pH conditions (Figure 2). As a result, Pb is often converted into other oxidized forms (e.g., hydrocerussite [Pb₃(CO₃)₂(OH)₂], cerussite [PbCO₃], PbCO₃, PbSO₄ and trace amounts of massicot [PbO]). The mineral phase of hydrocerussite is stable between pH 7.7 and 10, while cerussite is stable between pH 6 and 7.7. However, the stability ranges of Pb₃(CO₃)₂(OH)₂ and PbCO₃ will differ due to variations in thermodynamic constants and total soluble Pb concentration.

Figure 2: Eh-pH diagram equilibrium/stability diagram for solids and dominant solute species in a system of Pb + CO₂ + H₂O at 25°C and 0.101 MPa pressure (ionic strength [I] = 5 x 10^-3, CO₂ = 1 x 10^-3 mol L^-1, Pb = 1 x 10^-6 mol L^-1).  The influence of ZVI and S are not shown.

EHC^®-M combines controlled-release carbon, ZVI and a slow-release source of sulfide ion. Following placement of EHC-M into the subsurface, a combination of biological and chemical reactions will serve to establish very low Eh conditions and elevated levels of reduced iron and sulfide, under which conditions reduced Pb(s) will strongly bind as carbonates and Fe-oxides. However, lead and all of its corrosion products are soluble in acidic environments.  To complement the influence of EHC-M and facilitate the biological component of its mode of action, finely pulverized limestone can also be added to the subsurface in order to basify the aquifer, ideally to a pH above 7.  This increase in pH will greatly reduce the solubility of hydrocerussite and cerussite, which have minimum solubility's near pH 9. A raise in aquifer pH will also improve conditions for microbiological activity. The addition of limestone will also increase the total carbonate concentration resulting in increased capacity to limit lead mobility. In the presence of phosphate, lead will also be immobilized as pyromorphite (Pb₅(PO₄)₃OH) or chloropyromorphite (Pb₅(PO₄) ₃Cl), both of which are extremely insoluble in water.  

Field-Scale Pilot Study

A pilot-scale field effort was undertaken to: i) validate EHC-M performance under field conditions, and ii) assess the effectiveness of the construction methodology (i.e., direct injection of EHC-M/dolomite slurry). A mixture of EHC-M, dolomite and water was injected into 12 injection points spaced 3 m on center using direct push technology (Figure 3).  The pilot study area measure an estimated 12 m long x 9 m wide x 10 m deep and is located directly upgradient of monitoring well PB-01 (Figure 4).

The following dosing of EHC-M and pulverized dolomite was injected into the pilot treatment area:

 A total of 1,000 kg of EHC-M was injected from ca 17 to 27 m bgs, which resulted in an application rate of approximately 0.05% to soil mass (1000 kg / (1,080 m3 x 1800 kg/m3)). 

 A total of approximately 5,000 kg dolomite was injected into the same area to increase the pH; the application of dolomite was limited to the upper 5 ft of the treatment zone and dosed in accordance with the results from pH titration testing. This resulted in an application rate of approximately 0.5% to soil mass in the targeted depth interval. 

The Result

Subsequent performance monitoring conducted in January, February, March and June 2008 showed Pb below the detection limit of 10 ppb at the pilot study well PB-01, which represents a >97% decrease in soluble Pb from the November data of 306 ppb (Figure 5). Subsequent monitoring has also confirmed that the injections have been successful in establishing reducing conditions and a significant increase in the aquifer pH. The redox potential has remained around -100 mV since January 2008, compared to a baseline value of 300 mV measured in November 2007. The pH has increased from a baseline value of around 4 to a maximum of 13 measured in January 2008; the pH has since decreased to just below 10 measured in the latest sampling round conducted in June 2008.  Continued monitoring is expected to show that in situ chemical reduction using the EHC-M technology offers a safe, effective and cost-efficient remedial solution for similarly impacted environments.

The Cost 

The material cost of using EHC-M was $4.50/m³. 

Visit: SGW Services, Sao Paulo, Brazil