Final Report: Removal of Mercury and Other Heavy Metals of Industrial and Contaminated Site Waste Waters by Organic Chelation, Coprecipitation, and High-Efficiency Particulate Removal

EPA Contract Number: 68D00241
Title: Removal of Mercury and Other Heavy Metals of Industrial and Contaminated Site Waste Waters by Organic Chelation, Coprecipitation, and High-Efficiency Particulate Removal
Investigators: Bloom, Nicolas S. , Hensman, Carl E.
Small Business: Frontier Geosciences Inc.
EPA Contact: Richards, April
Phase: I
Project Period: September 1, 2000 through March 1, 2001
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2000) RFA Text |  Recipients Lists
Research Category: Watersheds , SBIR - Water and Wastewater , Small Business Innovation Research (SBIR)

Description:

In its inorganic form, mercury is not bioaccumulative; however, in the ambient environment, mercury is readily converted to the neurotoxin methyl mercury. Recent heightened concern regarding methylation and bioaccumulation of mercury in the aquatic environment has led to a proposed lowering of discharge limits. For example, discharge limits as low as 1.7 ng/L have been proposed for the Great Lakes Watershed, while several states are considering requirements that Hg be discharged at levels no greater than those in the ambient receiving waters, typically 0.5-5 ng/L.

The effluents of many industrial processes, as well as surface and groundwater from historically polluted sites, often contain unacceptably high levels of mercury and other toxic trace metals. In these situations, most toxic trace metals exist initially in cationic form. Significant portions of the metals remain in the wastewater, due to complexation with dissolved or colloidal ligands. A treatment that will strip the metals from the natural ligand/metal complex and supply a removal vehicle is required.

Frontier Geosciences, Inc., has identified a thiol-based chelating agent, "Magic Compound X," or "MCX." MCX not only exhaustively complexes most dissolved toxic trace metals of concern, including Ag, Cd, Cu, Hg, Pb, Se(IV), As(III), Zn, and Tl, but also in most cases, it has the ability to strip them from indigenous, competitive metal-binding ligands such as ethylene diaminetetraacetic acid (EDTA). The MCX/metal complex is insoluble in aqueous phase, and so precipitates efficiently in solution. MCX complexes Hg greater than 99.9 percent from pH 1-12, and greater than 99.999 percent from pH 3-6. The following studies were conducted at pH 4-6 as a compromise condition to optimize removal of as many metals as possible in one treatment (see Table 1).

Table 1. Percentage removal of trace metals using MCX as a chelating agent at pH 5.

In its inorganic form, mercury is not bioaccumulative; however, in the ambient environment, mercury is readily converted to the neurotoxin methyl mercury. Recent heightened concern regarding methylation and bioaccumulation of mercury in the aquatic environment has led to a proposed lowering of discharge limits. For example, discharge limits as low as 1.7 ng/L have been proposed for the Great Lakes Watershed, while several states are considering requirements that Hg be discharged at levels no greater than those in the ambient receiving waters, typically 0.5-5 ng/L.

The effluents of many industrial processes, as well as surface and groundwater from historically polluted sites, often contain unacceptably high levels of mercury and other toxic trace metals. In these situations, most toxic trace metals exist initially in cationic form. Significant portions of the metals remain in the wastewater, due to complexation with dissolved or colloidal ligands. A treatment that will strip the metals from the natural ligand/metal complex and supply a removal vehicle is required.

Frontier Geosciences, Inc., has identified a thiol-based chelating agent, "Magic Compound X," or "MCX." MCX not only exhaustively complexes most dissolved toxic trace metals of concern, including Ag, Cd, Cu, Hg, Pb, Se(IV), As(III), Zn, and Tl, but also in most cases, it has the ability to strip them from indigenous, competitive metal-binding ligands such as ethylene diaminetetraacetic acid (EDTA). The MCX/metal complex is insoluble in aqueous phase, and so precipitates efficiently in solution. MCX complexes Hg greater than 99.9 percent from pH 1-12, and greater than 99.999 percent from pH 3-6. The following studies were conducted at pH 4-6 as a compromise condition to optimize removal of as many metals as possible in one treatment (see Table 1).

Table 1. Percentage removal of trace metals using MCX as a chelating agent at pH 5.

Hg V Fe Co Ni Cu Zn As(III) Ag Cd Pb
>99.999 99 >95 >99.9 >95 >99 96 >99 99 >99 >99

*The method used to analyze Hg had higher resolution than ICP/MS, used to analyze other metals.

Satisfactory complexation and precipitation of the target metals of interest occurred very rapidly, with greater than 99 percent of metal removal occurring within the first minute. This allows easy integration of this technology with existing continuous-flow treatment plant equipment, while maintaining a short duty-cycle of the treatment process.

MCX was tested in a controlled environment of competing metal-binding ligands. Fulvic/humic acids, ammonium, citrate, NTA, sulphide, iodide, and cyanide showed little significant effect on the ability of MCX to extract Hg, Cu, Se(IV), Ag, Cd, Tl, and Pb from the aqueous samples. There was a reduction of 55 percent in the removal efficiency of Pb in the presence of high levels of EDTA. NTA also reduced the removal efficiency of V, Zn, and Mo. Interestingly, for most metals, sulphide as a ligand either improved the removal efficiency, or left it unchanged.

Valence state of As and Se has a significant effect on their removal efficiency. In both cases, the lower valence state (Se[IV] and As[III]) was removed to better than 99 percent by MCX, at pH 4.5. Unfortunately, less than 5 percent of the higher oxidation s

Hg V Fe Co Ni Cu Zn As(III) Ag Cd Pb
>99.999 99 >95 >99.9 >95 >99 96 >99 99 >99 >99

*The method used to analyze Hg had higher resolution than ICP/MS, used to analyze other metals.

Satisfactory complexation and precipitation of the target metals of interest occurred very rapidly, with greater than 99 percent of metal removal occurring within the first minute. This allows easy integration of this technology with existing continuous-flow treatment plant equipment, while maintaining a short duty-cycle of the treatment process.

MCX was tested in a controlled environment of competing metal-binding ligands. Fulvic/humic acids, ammonium, citrate, NTA, sulphide, iodide, and cyanide showed little significant effect on the ability of MCX to extract Hg, Cu, Se(IV), Ag, Cd, Tl, and Pb from the aqueous samples. There was a reduction of 55 percent in the removal efficiency of Pb in the presence of high levels of EDTA. NTA also reduced the removal efficiency of V, Zn, and Mo. Interestingly, for most metals, sulphide as a ligand either improved the removal efficiency, or left it unchanged.

Valence state of As and Se has a significant effect on their removal efficiency. In both cases, the lower valence state (Se[IV] and As[III]) was removed to better than 99 percent by MCX, at pH 4.5. Unfortunately, less than 5 percent of the higher oxidation states (Se[VI] and As[V]) were removed under these conditions. Further research will be required to enable the modification of redox conditions if this method is to be commercially applicable to As- and Se-containing waste streams.

In dilute solutions, the natural particle size distribution of the MCX/metal precipitate tends toward the colloidal range. In a process environment, small-pore filtration may not be feasible. Therefore, a higher concentration of carrier metal may be needed to stimulate larger particle growth and faster settling. Most industrial wastewater contains a metal of sufficient concentration to act as a carrier. However, if this is not the case, an innocuous metal such as Co or Fe can be added. The best extraction technique for the MCX/metal precipitate, other than filtering, is settling. Filtering demonstrated the best performance for separation, but small-pore filtration of millions of liters within a treatment facility could prove impractical. A short burst of mixing of the treated stream aids in the aggregation of the precipitate material. Then allowing the material to settle for a period of time gives an acceptable removal rate of precipitate.

At high levels of carrier concentrations, there is degradation in removal efficiency in certain metals. This indicates that the binding coefficient (Kf) is important, and that any metal with a lower Kf than the carrier/MCX complex will not show full removal if competing with an excess of carrier metal. However, in all the situations, no degradation in Hg removal was recorded. This suggests that the binding coefficient of Hg/MCX is larger than that of the other metal complexes present, or that Hg has faster MCX binding kinetics than most other metals present. Therefore, the amount of MCX required to remove a specific metal, or range of metals, is determined not only by the total metals concentration, but also by the specific metals present itates (Se[VI] and As[V]) were removed under these conditions. Further research will be required to enable the modification of redox conditions if this method is to be commercially applicable to As- and Se-containing waste streams.

In dilute solutions, the natural particle size distribution of the MCX/metal precipitate tends toward the colloidal range. In a process environment, small-pore filtration may not be feasible. Therefore, a higher concentration of carrier metal may be needed to stimulate larger particle growth and faster settling. Most industrial wastewater contains a metal of sufficient concentration to act as a carrier. However, if this is not the case, an innocuous metal such as Co or Fe can be added. The best extraction technique for the MCX/metal precipitate, other than filtering, is settling. Filtering demonstrated the best performance for separation, but small-pore filtration of millions of liters within a treatment facility could prove impractical. A short burst of mixing of the treated stream aids in the aggregation of the precipitate material. Then allowing the material to settle for a period of time gives an acceptable removal rate of precipitate.

At high levels of carrier concentrations, there is degradation in removal efficiency in certain metals. This indicates that the binding coefficient (Kf) is important, and that any metal with a lower Kf than the carrier/MCX complex will not show full removal if competing with an excess of carrier metal. However, in all the situations, no degradation in Hg removal was recorded. This suggests that the binding coefficient of Hg/MCX is larger than that of the other metal complexes present, or that Hg has faster MCX binding kinetics than most other metals present. Therefore, the amount of MCX required to remove a specific metal, or range of metals, is determined not only by the total metals concentration, but also by the specific metals present in the sample. For example, in the case of Hg in the simulated samples, MCX only needed to be one-tenth of the total metal concentration; however, this would most likely not be the case for a different matrix.

Initial investigations into the toxicology of MCX suggest that it is not of acute environmental concern. However, an accelerated study may be required concerning the subtle and/or long-term effects of MCX on environmental systems.

The chelation efficiency, and therefore the removal efficiency, of any metal depends on the speciation of that metal. In the target applications for this technique, dissolved Hg is expected to be found as mono-methyl Hg (MMHg), soluble Hg2+, and metallic Hg0. The MCX technology removes greater than 99.999 percent of Hg2+, greater than 99 percent of Hg0, and 95 percent of MMHg.n the sample. For example, in the case of Hg in the simulated samples, MCX only needed to be one-tenth of the total metal concentration; however, this would most likely not be the case for a different matrix.

Initial investigations into the toxicology of MCX suggest that it is not of acute environmental concern. However, an accelerated study may be required concerning the subtle and/or long-term effects of MCX on environmental systems.

The chelation efficiency, and therefore the removal efficiency, of any metal depends on the speciation of that metal. In the target applications for this technique, dissolved Hg is expected to be found as mono-methyl Hg (MMHg), soluble Hg2+, and metallic Hg0. The MCX technology removes greater than 99.999 percent of Hg2+, greater than 99 percent of Hg0, and 95 percent of MMHg.

Summary/Accomplishments (Outputs/Outcomes):

In "real-world" situations, MCX performs admirably. Natural gas-produced water and laboratory sediment digestion waste required presettling, or neutralization and settling, respectively, followed by a simple MCX addition and filtration. In the first case, this brought the Hg level down from 9,569 mg/L to 0.035 mg/L, 285-fold lower than the discharge limit. In the second case, this brought the Hg level down from 6,214 mg/L to 16 mg/L, 12.5-fold lower than the discharge limit. More difficult samples required oxidization pretreatments. The groundwater from a chloralkali plant had hypochlorite added to convert strongly chelated or colloidal Hg into Hg2+, allowing complexation with MCX. This reduced the Hg level from 18.43 mg/L to 0.661 mg/L, 4.5-fold lower than the discharge limit. In the industrial chemical-waste landfill sample, organic components bound the Hg too strongly for the MCX to form a complex at any time. Therefore, potassium permanganate was required to break down the organic material first, freeing the Hg for complexation. This allowed reduction of the initial Hg level from 345.1 mg/L to 0.322 mg/L, within the anticipated discharge limit.

A toxicity characterization leachability protocol (TCLP) study was performed on the MCX/metal sludge, which was created while treating the chloralkali plant groundwater and the laboratory sediment digestion waste. In both samples, insignificant amounts of Hg (0.005 percent), Ag (0.8 percent), and Cu (0.05 percent) were detected. Even for the highest levels (initial Hg in the solids of > 0.5 percent by weight), the Hg leached was 13-fold lower than the TCLP limit of 200 µg/L. Arsenic and zinc were considerably more soluble under TCLP (10-70 percent), meaning that additional stabilization for these metals might be necessary.

Conclusions:

Given enough chemical knowledge, any wastewater can be treated and brought within even the most stringent discharge specifications. MCX acts as a gross removal reagent of Hg and other toxic trace metals such as Cu, Ag, and Pb. The sample may need to be pretreated to remove interfering organic components prior to MCX addition. It also may be possible and cost effective to remove the bulk of the target metals with neutralization and/or filtration, followed by a lower dose of MCX to "polish" the treatment. Another cost-effective measure would be to use high indigenous metal concentrations, such as the As or Pb in the wastewater examples, to act as a coprecipitate and trace metal/MCX complex carrier.

Supplemental Keywords:

wastewater treatment, metals recovery, mercury, chelation, coprecipitation, engineering, chemistry., RFA, Scientific Discipline, Toxics, INTERNATIONAL COOPERATION, Water, Waste, Sustainable Industry/Business, National Recommended Water Quality, particulate matter, Chemical Engineering, cleaner production/pollution prevention, Remediation, Wastewater, Environmental Chemistry, Sustainable Environment, Chemistry, HAPS, Technology for Sustainable Environment, Hazardous Waste, Hazardous, 33/50, Engineering, Chemistry, & Physics, Environmental Engineering, Mercury, particulates, Silver, hazardous waste treatment, contaminated sites, industrial wastewater, chelation, cadmium & cadmium compounds, Lead Compounds, formation in spark ignited engines, advanced treatment technologies, contaminated waste sites, cyanide, lead & lead compounds, lead, fine particulates, metal recovery, organic chelation, mercury & mercury compounds, Mercury Compounds, copper, contaminant management, contaminated groundwater, contaminant removal, water treatment, nickel & nickel compounds, Nickel Compounds, cadmium, heavy metal contamination, heavy metals, Cadmium Compounds, nickel, metal removal, metals

SBIR Phase II:

Removal of Mercury and Other Heavy Metals of Industrial and Contaminated Site Waste Waters by Organic Chelation, Coprecipitation and High-Efficiency Particulate Removal  | 2000 Progress Report  | Final Report