ISCR and Metals

601 views

Published on

Paper describing in-situ chemical reduction of metals as an additional benefit resulting from EHC injection for remediation of TCE in groundwater.

Published in: Technology, Health & Medicine
  • Be the first to comment

  • Be the first to like this

ISCR and Metals

  1. 1. Anticipating Secondary Concerns with In-situ Chemical Reduction: Data from an Integrated Pilot Study of EHC™ and KB-1™ James Peale (jpeale@mfainc.org), Erik Bakkom – Maul Foster & Alongi, Inc. Fayaz Lakhwala, Jim Mueller, Josephine Molin, Andrzej Przepiora – Adventus Americas ABSTRACT: Groundwater at a silicon wafer manufacturing facility in Portland, Oregon was impacted by TCE and its degradation products, cis-1,2-DCE and VC, up to maximum concentrations of 592, 90, and 5 mg/L, respectively. Groundwater discharges to a river approximately 450 ft downgradient of a suspected source area. An 11-month field pilot test was conducted in the source area and at the riverbank area. Permeable reactive barriers (PRBs) consisting of EHC™ and KB-1® were installed at depths ranging from approximately 50 to 130 feet below ground surface. At the riverbank, TCE and its degradation products were reduced to below regulatory screening levels or detection limits, and chlorinated VOC mass reduction of 99.99% was achieved. In the source area, TCE concentrations were reduced by approximately 99.5%, and overall concentrations of chlorinated VOCs (CVOCs) were reduced by between 46-99%. Significant concentrations of fully dechlorinated ethene were produced. Degradation rates were increased above background by 90-94%. The data confirmed that enhanced in-situ bioremediation (EIB) was successful, but concerns regarding the technology remain. First, mobilization of redox-sensitive metals (arsenic [As] and manganese [Mn]) due to increased reducing conditions was identified as a concern for receptors in the receiving surface water body. Increased downgradient concentrations of iron (Fe, due to the zero- valent iron component of EHC™) were also a concern. The pilot study data confirmed that in-situ chemical reduction (ISCR) using EHC™ does not increase concentrations of As and Mn downgradient of the PRB – instead, significant reductions were observed. As an unanticipated benefit, weak-acid dissociable cyanide concentrations were also significantly reduced. INTRODUCTION EHC™ is composed of controlled-release, complex organic carbon plus zero-valent iron (ZVI). When applied to the subsurface environment, EHC induces extremely low redox (Eh) conditions that facilitate the rapid and complete destruction of chlorinated organics via in situ chemical reduction reactions. KB-1 is a specialized inoculum containing Dehalococcoides ethenogenes (Dhc) and other microbes that enhance complete mineralization of certain compounds. Both EHC and KB-1 are established commercial products with documented success at remediating sites with CVOCs. Prior to this project, injection of EHC had not been attempted at depths corresponding to those presented at this site. Similarly, EHC and KB-1 had not been field tested at sites with TCE concentrations characteristic of the presence of non-aqueous phase liquid (NAPL). Details regarding the installation and monitoring of the field-scale pilot study were summarized previously (Peale, 2007). Figure 1 generally describes the layout of the riverbank pilot study area, which is the focus of this paper.
  2. 2. FIGURE 1: Riverbank pilot study area layout. The results confirmed that the combined approach was successful – regulatory screening levels were achieved at the riverbank, and TCE DNAPL (if present) was degraded in the source area. The CVOC results are summarized in Table 1. TABLE 1: Concentrations of CVOCs in groundwater – baseline and 18 months post-injection. Riverbank Area Concentration (ug/L) Percent Reduction Well Date TCE DCE VC CVOCs Total CVOC -- Regulatory Screening Level 3 70 2.4 WS-22-112 Jun-06 584 3,074 474 4,132 - (within PRB) Feb-08 ND ND ND ND 99.99% WS-11-125 May-06 22.9 10,557 2,490 13,069 - (Downgradient, with Feb-08 ND 80 16.4 96.4 99.3% MGP DNAPL) WS-20-112 Jun-06 1,100 10,067 1,610 12,777 - (Downgradient) Feb-08 ND 0.73 ND 0.73 99.99% Source Area Concentration (ug/L) Percent Reduction Well Date TCE DCE VC CVOCs Total CVOC WS-19-71 Jun-06 6,500 89,010 30 95,540 - (within PRB) Feb-08 ND 120 10,500 10,620 88.9% WS-19-101 Jun-06 92,900 39,497 22 132,419 - (within PRB) Feb-08 ND 94.3 156 250 99.8% WS-18-71 Jun-06 7,990 91,624 26 99,640 - (Downgradient) Feb-08 102 6,541 16,600 23,243 76.7% WS-18-101 Jun-06 198,000 34,133 41 232,174 - (Downgradient) Feb-08 2,920 97,315 24,900 125,135 46%
  3. 3. The analytical results confirmed the success of the combined approach for EIB, and may be discussed in more detail under separate cover. In short, the primary concern (will EIB achieve regulatory screening levels for CVOCs) has been addressed. This paper investigates the secondary effects of the amendment on groundwater geochemistry that could affect remediation decisions by the property owner and the regulatory agency. SITE DESCRIPTION Operations at the facility began in 1980, after the site had been developed by filling during the 1970s. Prior to development, portions of the property were used for waste disposal from a manufactured gas plant (MGP). The MGP waste included spent oxide material, which was incorporated into the fill, along with dredge and quarry spoils. Waste disposal resulted in elevated iron (average concentration ~ 40 mg/L) and CN (avg. ~ 0.6 mg/L). Elevated concentrations of Mn (avg. ~ 10 mg/L) and S (as SO4-2) (avg. 20 mg/L) in groundwater may also be related to the MGP legacy impacts. The current owner of the property is engaged in remediating the CVOC plume, which occurs within the larger plume of MGP-related impacts. Investigation of these legacy impacts by the successor to the MGP operator is underway, but not yet complete. EIB was evaluated for the source area and riverbank in a focused feasibility study (FFS). The FFS included evaluation of balancing factors, including effectiveness, long- term reliability, implementability, and implementation risk. Mobilization of redox- sensitive metals and potential discharge from groundwater to surface water were identified as implementation risks. Analytical data collected during the pilot study were used to evaluate potential implementation risks associated with mobilization of redox-sensitive metals. Mechanisms for precipitation of As, Mn, sulfur (S), and WAD CN were also identified. The results indicate that EIB does not result in undesirable secondary impacts to groundwater due to increased reducing conditions. DATA SUMMARY At the riverbank pilot study area, monitoring wells were installed 25 ft upgradient (WS-21-112), within (WS-22-112), and 20 ft downgradient of the PRB (WS-20-112). Monitoring wells were screened from 96-111 ft bgs. An existing monitoring well (WS- 11-125) was located 10 ft downgradient of the PRB, screened from 109-124 ft bgs. Pilot study sampling commenced immediately after installation of the PRB (baseline sampling event May/June 2006) and then on a monthly schedule. The wells were sampled for metals and cyanide (total and weak acid dissociable+free) throughout the pilot study (in addition to CVOCs). Figure 2 shows log molar (M) concentrations for Fe (dissolved), S (as sulfate), Mn (dissolved), As (dissolved), and CN (weak acid dissociable+free) from the riverbank wells. Concentrations of Fe, CN, and Mn decreased roughly two-fold, while sulfate was reduced to non-detect. Arsenic concentrations were much lower (approaching the method reporting limit of 1 ug/L) and did not change significantly.
  4. 4. -2.00 -2.50 Iron -3.00 -3.50 Sulfur (as SO4) Average Concentration (Log[M]) -4.00 Manganese -4.50 -5.00 -5.50 Cyanide -6.00 -6.50 Arsenic -7.00 -7.50 -8.00 Jun-06 Jul-06 Sep-06 Nov-06 Jan-07 Mar-07 May-07 Jul-07 Sep-07 Nov-07 Jan-08 Date FIGURE 2: Fe, S, Mn, As, and CN in groundwater. Averages from 3 wells. -75 12 11 ORP -125 10 9 -175 8 ORP (mV) pH 7 -225 6 pH 5 -275 4 3 -325 2 Apr-06 May-06 Jul-06 Sep-06 Nov-06 Jan-07 Mar-07 May-07 Jul-07 Sep-07 Nov-07 Date FIGURE 3: ORP and pH of groundwater from riverbank samples.
  5. 5. Field measurements of pH and redox potential (ORP) are shown on Figure 3. pH remained stable, between 6 and 8, with a slight increasing trend. ORP decreased significantly, from +152 mV (prior to injection; not shown) to -300 mV. Dissolved oxygen data (not shown) was generally below 0.5, confirming anaerobic conditions. The data show that the reducing conditions do not result in elevated concentrations of the redox-sensitive metals As and Mn. Also, elevated concentrations of iron (relative to site background, about 40 mg/L, or 10-3 M) do not persist after injection. In addition, concentrations of CN are significantly reduced. The monitoring data suggest that Fe, Mn, CN, and S are undergoing redox reactions that lead to their precipitation or adsorption. Further research was conducted to identify likely reactions for these species, as summarized below. Fe-CN Reactions The spent oxide waste is the most likely source of elevated iron and cyanide at MGP sites (Ghosh et al., 2004). Under the site conditions, the uncomplexed iron in solution is likely to occur as Fe2+. Iron forms strong acid-dissociable (SAD) CN complexes such as ferricyanide [Fe(CN)6]3-] and ferrocyanide [Fe(CN)6]4-], which can be precipitated under reducing conditions by reacting with additional iron (Young et al. 1995). The decrease in CN (present as HCN or weak complexes) can be attributed to the additional iron from the EHC PRB, which would have formed strong CN complexes and subsequent precipitates, via the following reactions (Meeusen et. Al, 1996): HCN(aq) + Fe2+ Fe(CN)xy- + H+ (where x and y depend on ORP) followed by Fe(CN) xy- + Fe2+ Fex[Fe(CN)y]z(s) (where (s) denotes solid) The decrease in CN can also be due to the slight increase in pH following injection. Fe-Mn-CN Reactions The average background concentration of dissolved Mn detected in the riverbank area is about 10 mg/L. The source of the Mn likely originated from leaching of spent oxide materials. Under the site conditions, Mn would likely be present in solution as Mn+2. Precipitation of Mn along with the ferricyanide complexes has been documented (Rennert, 2005) under reducing conditions, via the following reaction: Fe(CN) 64- + 2Mn2+ Mn2Fe(CN)6(s) Since the inital concentration of Mn2+ was much higher than that of CN (Figure 1), complexation by ferricyanide was one of the processes that controlled the observed losses in dissolved Mn2+ concentration. Other potential mechanisms were likely driven by the observed increases in pH [precipitation as Mn(OH)2(s)] and decreases in ORP [precipitation as MnS(s)]. Figure 4 shows the predominance areas for Mn2Fe(CN)6(s). Fe-S Reactions The source of the sulfate is likely related to the spent oxide materials which were used for H2S(g) scrubbing. The background total S concentration of 10 mg/L, assumed to be all sulfate under the background ORP, were reduced completely after the EHC amendment. Under reducing conditions, sulfate (SO42-) is reduced to sulfide (S2-). Precipitation of iron sulfides such as pyrite (and potentially mackinawite, pyrrhotite, or troilite) is the likely mechnism for the removal of S, via the following reactions: SO42- + 9H+ + 8e- ↔ HS- + 4H2O Fe2+ + HS- → FeS(s) + H+
  6. 6. Figure 4: Eh-pH Diagram for Mn-Fe-CN Species. Figure 5 – Eh-pH Diagram for Arsenopyrite (from Craw et al., 2003) Samples were not analyzed for sulfide during early monitoring. Later monitoring confirmed that sulfide was not present, suggesting that removal of S from the system
  7. 7. (potententially via formation of iron sulfide) may have occurred relatively quickly following injection since sulfate reduction is mediated by biological activity (Drever, 1997). As indicated above, MnS(s) is also a possible mineral phase of reduced sulfate. Fe-As-S Reactions Arsenic concentrations, detected at the site at about 1-3 ug/L, are representative of naturally-occuring background, and were orders of magnitude lower than the other ions. The data confirm that the reducing conditions created by the EHC injection did not mobilize arsenic. Any arsenic dissolved from the aquifer matrix (as arsenic-oxyhydroxides) would likely be precipitated as arsenopyrite, consistent with Figure 5. Concentrations of As remained very low, suggesting that either no As mobilization occurred, or that the reductive precipitation process above prevented mobilization of As present in the site aquifer material. Concentrations of other major cations (e.g., magnesium, aluminum) were relatively unchanged, suggesting that these ions are not reacting with the CN or S in solution. Concentrations of carbon species (i.e., carbonate, bicarbonate) are also relatively unchanged, but similar to iron (i.e., approximately 10-2.5 M) suggesting that removal of iron or manganese by formation of siderite (FeCO3) or rhodochrosite (MnCO3) is occurring. Review of an Eh-pH diagram for iron-carbonate species confirms the potential for these reactions (see Figure 6). FIGURE 6: Eh-pH Diagram for Siderite (FeCO3)(s).
  8. 8. CONCLUSIONS Data from a field pilot of EIB using EHC™ and KB-1® was very encouraging with respect to meeting potential remedial action objectives for TCE and its degradation products. EIB outscored other technologies (pump and treat, thermal, in-situ chemical oxidation) in technology screens and feasibility studies for the site. However, mobilization of Fe, As and Mn, with potential discharge to the adjacent river, was suggested by the regulatory agency as an implementation risk. The potential risk was anticipated in the pilot test design, and secondary data were collected, including metals and cyanide. The data from the pilot test confirm that the risk of mobilization and undesirable downgradient impacts is minimal. Concentrations of dissolved Fe and Mn decreased significantly in the EHC-amended zone. Surprisingly, concentrations of the relatively toxic form of cyanide were also reduced. Further evaluation of the data, along with literature research, helped identify likely reaction mechanisms for Fe, As, Mn, CN, and S in the groundwater. In addition to demonstrating the success of a combined approach for enhanced in-situ bioremediation, the results of this pilot study demonstrate that anticipated secondary impacts – mobilization of redox sensitive metals – are not a foregone conclusion following implementation of ISCR. The results also demonstrate the value of anticipating secondary concerns and collecting appropriate data throughout a pilot study, such that disputes regarding conclusions are avoided, and progress toward remediation is unimpeded. References Drever, J.I., 1997. The Geochemistry of Natural Waters. Surface and Groundwater Environments. 3rd Ed., Prentice Hall, NJ, pp. 436. Ghosh, R.S., Nakles, D.V. Murarka, I.P., and Neuhauser, E.F, 2004. Cyanide Speciation in Soil and Groundwater at Manufactured Gas Plant (MGP) Sites. Environmental Engineering Science, 21(6): 752-767. Meeussen, J.C.L., Keizer, M.G., and de Haan, F.A. M., 1992. Chemical Stability and Decomposition Rate of Iron Cyanide Complexes in Soil Solutions. Environ. Sci. Technol., 26, 511-516. Peale, J.G.D., Bakkom, E., Lakhwala, F., Mueller, J., Molin, J., Dmitrovic, E., Dennis, P. 2007. Integrated Use of EHC™ and KB-1 for Source Area Treatment and PRB Applications at a TCE Impacted Site. Paper H-12. In: A. Gavaskar and C. Silver (Eds)., In-Situ and On-Site Bioremediation – 2007. Battelle Press, Columbus, OH (CD format). Rennert, T., and Mansfeldt, T., 2005. Iron-cyanide complexes in soil under varying redox conditions: speciation, solubility and modeling. European Journal of Soil Science, 56, 527-536. Young, C.A., Cashin, S.P. & Jordan, T.S. 1995. Remediation technologies for the separation and destruction of aqueous cyanide species. Preprint Nº 96-149, SME, Littleton, CO.

×