The Daylight Solution:
Ultraviolet Radiation and the Remediation of Cyanide-Containing Water and Acid-Mine Drainage

Courtney A. Young, Ph.D.
Department of Metallurgical Engineering, Montana Tech

This work is being performed under the Mine Waste Technology Program, funded by the U.S. Environmental Protection Agency (EPA) and jointly administered by the EPA and the U.S. Department of Energy, Contract Number DE-AC22-96EW96405.

Remediation of industrial discharge waters has been considered paramount at several locations in Montana and throughout the United States. The U.S. Environmental Protection Agency (EPA) lists cyanide-containing waters and acid-mine drainage (AMD) among the most important remediation issues. Discharge waters from municipal and industrial operations transport many dissolved chemicals into the environment. As a result, these chemicals may cause various heavy metals to leach or cause aquatic life to sicken or die. Numerous technological approaches are being investigated as possible solutions to these environmental problems.

Montana Tech has been investigating the benefits of a naturally-occurring processCsunlightCas a response to these remediation issues. Specifically, research underway seeks to characterize the effects of ultraviolet (UV) radiation, referred to as photolysis, on cyanide destruction and AMD remediation. For the past five years, Tech students from the Mine Waste Technology Program and the Undergraduate Research Program have been studying the effect of photolysis on these systems to determine if they warrant further study for field implementation. Graduate and undergraduate students participating in the study are Steve Cashin (M.S. 1996), Yu Chuan Tai (M.S. 1997), Kip Slaybaugh (B.S. 1997), Marlo Pruss (B.S. 1997), Josh Knutson (B.S. 1998), Brent Mikelson (B.S. 1998), Marty Bennett (M.S. 1999), Matt Griffith (B.S. 1999), and Kevin Ritari (B.S. 1999).

Photolytic-Process Remediation

In photolysis, solutions are irradiated with UV rays to promote electron transfer reactions between chemical species. It is prerequisite that some chemical constituent in the system absorbs the UV radiation. If the constituent being remediated is the absorbent, the photolytic process is a direct method. Typically, however, photolysis is accomplished by indirect methods, in which the absorbent transfers the photo-energy to the constituent being remediated. For example, the semi-conductor TiO₂ (anatase), absorbs UV radiation and promotes electrons across the band gap to create excited electrons (e^-) in the conductance band, and holes (h⁺) in the valence band:

The process in Figure 1 shows that oxidation reactions (electron releasing) can be induced by holes, and reduction reactions (electron consuming) can be induced by the excited electrons. These processes can create reduction/oxidation (redox) environments in waste water that can effectively remove the contaminant chemical species. Similarly, dissolved constituents can become excited when absorbing UV radiation to create radicals:

 Depending on the constituent and chemistry of the system, the radical can induce redox reactions as well. The best example of this is the production of hydroxyl radicals (OH*) from hydrogen peroxide
(). If the radical is destroyed during the redox reaction, as it is with hydrogen peroxide, the process is referred to as homogeneous photolysis; otherwise, it is termed homogeneous photocatalysis.

Cyanide Investigations  

Some results obtained by Cashin, Mikelson, Pruss, and Slaybaugh appear in Table 1. The results compare the remediation of several cyanide species using hydrogen peroxide and anatase in the absence (dark) and presence (light) of UV radiation. Because hydrogen peroxide is a strong oxidant, some of the cyanide species, which include free cyanide () and the weak-acid dissociables (WADs) of , , and , were remediated in the dark. This use of in darkness, known as the Degussa Process, is clearly effective at remediating the free and WAD complexes of cyanide, which bear the greatest toxicological concerns. Hydrogen peroxide (or bleach) is therefore employed at sites where cyanide is used. In the presence of UV rays, an additional cyanide complex is remediated, , which is a strong-acid dissociable (SAD). Thus, in the case of gold complexes, the production of hydroxyl radicals (OH*) provides stronger oxidation conditions and allows for cyanide destruction (see reaction 2).

Experiments conducted with anatase in the dark showed no remediation. However, as soon as cyanide was exposed to the UV rays, some level of remediation was observed for all of the species, including the other two SADs, and . These results are significant because they dispute research conducted throughout the past decade showing that hydroxyl radicals were produced by hydroxide () reacting with holes. It now appears that the heterogeneous photocatalytic mechanism additionally involves a direct reaction of cyanide with holes. Although this mechanism helps account for the remediation of all the cyanide species, it is the sole mechanism for the remediation of the stronger SADs.

Acid-Mine Drainage Investigations

Using preliminary results obtained earlier by Cashin on Berkeley Pit Lake water, Tai developed a process for its remediation while simultaneously and selectively recovering the valuable metals, specifically copper and zinc. This process required seven stages:

1) homogeneous photolysis with hydrogen peroxide to precipitate iron presumably as Schwertmannite,, and simultaneously remove arsenic by adsorption on the iron precipitate,

2) sulfide addition to precipitate copper,

3) increased sulfide addition to precipitate zinc and cadmium,

4) potassium permanganate addition to precipitate manganese,

5) sodium hydroxide addition to precipitate aluminum,

6) lime addition to precipitate sulfate as gypsum and any of the remaining metals as oxides/hydroxides, and

7) sulfate reduction to sulfide, which is recycled back to steps 2 and 3.

Each stage (except for the seventh) was investigated in the absence and presence of UV radiation. Eventually, it was shown that only the first stage required the use of UV rays. After each of the stages where precipitates are produced, filtering was needed for solid/liquid separation. Furthermore, as one stage proceeded to the next, the pH gradually increased to neutrality. In effect, the water met discharge requirements for all constituents (see Table 2).

In a study relevant to examining natural remediation processes, Bennett built a 12-ft-high, 6-in.- diameter column to hold approximately 18 gallons of Berkeley Pit Lake water and analyzed various parameters as a function of depth: temperature, pH, potential (E_H), dissolved oxygen (DO), specific conductivity, and turbidity. Over 10 experiments were conducted, each lasting at least three weeks. Samples were taken at the conclusion of each test to measure metal concentrations as a function of depth as well. Depth profiles were determined similar to those for the Berkeley Pit Lake. However, the parameters changed, depending on the atmosphere (air versus ) maintained above the column, the column’s exposure to UV radiation, and the wavelength of UV radiation. The depth profiles for pH, E_H, and DO indicated that two ferrous-to-ferric oxidation reactions were taking place:

(3)

(4)

These reactions occur in the absence (3) and presence (4) of UV radiation. Once the ferric cation () formed, it would precipitate out as Schwertmannite as suggested in Tai’s previous work. Interestingly, specific conductivity and turbidity profiles were more constant in the presence of UV radiation than in its absence. To explain this phenomenon, it was proposed that the photo-energy was being transferred to all positions in the column like an extension cord. This explanation was credible because water, being a strong UV absorbent, only allows UV radiation to penetrate a few inches. For this to happen, the absorbent must be present in significant amounts. Iron, sulfate, and possibly organic compounds can serve this purpose. Future investigations are continuing to test this hypothesis and also to see if such processes can be improved.

It was concluded that nature is indeed using photolytic processes to remediate the Berkeley Pit Lake water. The investigators expect that new and perhaps better remediation technologies for AMD can be developed that take advantage of naturally-occurring remediation from sunlight.