On-Site Production of Ferrate for Water and Wastewater Purification

Ferrate is the most powerful, multipurpose, environmentally friendly water and wastewater purification chemicals known. In a single dose, super-charged iron(VI) replaces multiple treatment chemicals because it can simultaneously disinfect, oxidize, and coagulate to remove pathogens and contaminants from water. Ferrate destroys human pathogens, including viruses, spores, bacteria, and protozoa. It can remove or inactivate toxic metals, pesticides, drugs, hormones, and industrial toxins found in drinking water, wastewater, ballast water, municipal and industrial effluents, tainted soils, and groundwater. Unlike chlorine and ozone, which produce harmful by-products, ferrate’s by-product is an environmentally friendly insoluble iron(III) species. Ferrate’s efficacy and utility have been thoroughly documented in over 440 scientific and peer-reviewed publications.

Figure 1 – Ferrator ferrate generation system.

Ferrates can be synthesized in several ways, utilizing an oxidation process to convert metallic, ferrous, or ferric iron to a higher valence. If a pure ferrate compound is desired, then a separation process is required to isolate it from the other reaction by-products. The oxidation step can be easily performed in a dry state utilizing heat with an oxidant, or by using electricity or chemical oxidants in aqueous solution. It is important to note that while this first step in ferrate synthesis (oxidation) is simple and inexpensive, the second step (separation) is very time-consuming and expensive. Therefore, the synthesis of a dry or powder high-purity ferrate product, with sufficient stability to be shipped and handled, is prohibitively expensive. Past attempts at commercial synthesis of a pure ferrate, e.g., K2FeO4, failed because the average cost of that product was approximately $60 per pound of ferrate.

The need to generate ferrate on site, which requires only the oxidation step of ferric iron to ferrate+6, led to the design and development of the Ferrator© (Figure 1) (Ferrate Treatment Technologies, LLC [FTT], Orlando, FL). When employed, the Ferrator reduces the chemical cost of a pound of ferrate to approximately $5 and makes its use in industrial water purification a reality.

Generation of ferrate using aqueous chemical oxidation of ferric iron

The Ferrator utilizes the most efficient and cost-effective way to generate ferrate: aqueous chemical oxidation of ferric iron utilizing chlorine in a caustic medium. Specifically, a ferric salt, either ferric chloride or ferric sulfate, is oxidized to FeO42– by hypochlorite (either calcium or sodium) in the presence of sodium hydroxide, as shown in Eq. (1). The feedstocks are inexpensive and readily available at most treatment plants in their commercial forms (12% bleach, 40% ferric chloride or sulfate, and 50% caustic):

2FeCl3 + 3NaOCl + 10NaOH → 2Na2FeO4 + 9NaCl + 5H2O                               (1)

When added to aqueous systems, ferrate(VI) is a powerful oxidant that readily decomposes to ferric iron [Fe(OH)3] and oxygen according to:

2FeO42– + 5H2O → 2Fe(OH)3 + 1.5O2 + 4OH—                                                   (2)

Ferrate dose and treatment efficiency

Once ferrate has been generated on site, careful consideration must be given to its dose and application. Because ferrate has multitreatment capacity, it is difficult to generalize optimum doses for each application. For example, when ferrate has been used for disinfection of clean systems such as drinking water, or highly treated wastewater effluent, the doses are usually in the range of 1–5 mg per liter (as FeO42–). If ferrate is to be used principally as an oxidant of organics, the dose can vary between 2 and 15 mg per liter. In applications in which ferrate has been utilized for coprecipitation of heavy metals, doses as low as 0.5 mg per liter have proven effective. Industrial wastes, especially those with a high total organic carbon (TOC) component, can require upwards of a 50 mg/L dose.

A recent pilot study was run at a wastewater reclamation facility to demonstrate disinfection of total coliforms and enterococci, without the formation of halogenated methane compounds, and the removal of phosphorus to allow surface water discharge. The effluent was denitrified and had very low suspended solids. The TOC was approximately 4 mg/L, and background coliform and enterococci counts were 103 most probable number (MPN). The required ferrate dose to achieve the treatment goal was between 1 and 2 mg/L. The Ct values determined were 21 for total coliforms and 3.5 for enterococci.

Water purification by removing color and metal

Ferrate has been utilized extensively for the removal of color in drinking water and wastewater effluents, and has also been a primary focus of research for the removal of metal. Color in water (and wastewater) may be caused by many different organic compounds, and removal of them solely by oxidation is problematic. Ferrate removes organics both by oxidation (usually to CO2) and co-precipitation on ferric hydroxide surfaces generated by ferrate decomposition. As a result of this multidimensional treatment capacity, ferrate can remove color at a very high efficiency.

Figure 2 shows a summation of color removal treatability tests utilizing ferrate(VI). The data in this figure reflect a summary from over 80 individual samples of raw water, domestic wastewater, and industrial waste from around the world. Obviously, the nature of the organics in these samples were highly variable, and the initial color levels varied from >3000 CU to 20 CU. In addition, the initial water quality (pH, alkalinity, TOC, total suspended solids [TSS], and total dissolved solids [TDS]) was highly variable among the samples. Regardless of this variability, however, it can be seen that ferrate removed color in a very predictable manner in terms of required dose.

Figure 2 – Ferrate(VI) removal of true color from water and wastewater samples.

For the removal of heavy metals, ferrate has been demonstrated to be an efficient treatment and purification system. Recent large-scale pilot tests have been run to demonstrate ferrate’s efficacy at removing metals from solution. Results of extensive testing at all scales have shown that ferrate is effective at removing essentially all heavy metals and most radionuclides. Potts and Churchwell1 demonstrated that ferrate added at a low dose could reduce gross alpha activity from 37,000 pCi/L to 40 pCi/L at a Department of Energy (DOE) facility in the U.S. Subsequently, the DOE installed a ferrate treatment system to replace its existing lime softening process.

Extensive work has been done (and published) describing ferrate’s ability to oxidize arsenic(III) to arsenic(V) and remove it from solution via co-precipitation on the ferric hydroxide generated from ferrate decomposition. For example, Lee et al.2 showed that ferrate applied at 2 mg/L (as Fe) was able to reduce arsenic from an initial concentration of 517 μg/L to <50 μg/L in river water from Bangladesh.

Additionally, significant interest has been generated about toxic metals in waste streams from mines. Treatability studies at several mines have demonstrated ferrate’s ability to remove various metals from mining effluents, and thereby produce a waste stream that is suitable for discharge to the environment. One example of this is shown in Figure 3. In this case, mining wastewater was being discharged to a lake in Peru, and toxic levels of both zinc and lead were harming the environment. It can be seen that ferrate, when applied in very small doses (less than 0.5 mg/L), was able to remove both lead and zinc to below detection levels. Full-scale ferrate systems are now being designed for installation at these mine sites. In addition, published research, as well as large-scale treatment applications, has clearly shown that ferrate is more effective at removing heavy metals than common coagulants such as ferric iron and aluminum.

Figure 3 – Removal of Pb and Zn by ferrate from mine wastewater.

Mobile, cost-effective solution for the industrial use of ferrates

With the development of the mobile Ferrator, ferrate can now be manufactured in bulk quantities for broad industrial water purification use. Ferrators have a small footprint and can easily retrofit into existing plant infrastructure and are scaleable to accommodate any size treatment flow. Constructed from proven industrial components, the fully automated system utilizes commodity feedstocks that are available at most treatment plants. The programmable logic controller (PLC) operates the ferrate synthesis process and provides remote operation and 24-hr off-site monitoring available via Internet or cellular modem. The systems can be shipped anywhere in the world and, depending on dose requirements, the smallest Ferrator can treat up to 10 million gallons of drinking water per day. Pilot Ferrators can be resold into treatment plants serving small to medium-sized U.S. cities. When flow rates change, turbidity increases, or future regulatory changes require a more powerful treatment, the Ferrator can be programmed locally or remotely to simply “dial-up” the dose without incurring additional infrastructure costs.

Environmental restoration, a key by-product of ferrate water purification

Ferrate treatment will be used for wastewater disinfection and restoration of approximately 30,000 acres of wetlands in New Orleans. Hurricane Katrina destroyed the East Bank Wastewater Treatment Facility, and the Sewerage and Water Board of New Orleans conducted stakeholder meetings and contracted for engineering analyses resulting in selection of ferrate treatment. The reasons for selection included: lowest cost, ferrate’s by-products are environmentally beneficial to the restored wetlands, and ferrate reduces the hormonal activity of endocrine-disrupting compounds within the wastewater. The project will begin as a pilot demonstration and will ultimately be expanded to treat the full 100 million-gallon-per-day capacity of the plant.

References

  1. Potts, M.E.; Churchwell, D.R. Wat. Environ. Res.1994, 66, 107–9.
  2. Lee, J.; Um, I.H. et al. Environ. Sci. Technol.2003, 37, 5750–6.

Additional reading

  • Chick, H. J. Hygiene1908, 8, 92–157.
  • Jiang, J.Q.; Panagoulopoulos, A. et al. Environ. Management2006, 79, 215–20.
  • Neveux, N.; Aubertin, N. et al. In: Chemical Water and Wastewater Treatment III; Klute, R.; Hahn, H.H., Eds.; Berlin Heidelberg: Springer, 1994; pp 95–103.
  • Sharma, V.K. Environ. Sci. Technol.2010, 44, 5148–52.
  • Watson, H.E. J. Hygiene1908, 8, 536–42.

Thomas D. Waite, Ph.D., PE, is Chief Technical Officer, Ferrate Treatment Technologies LLC, 300 Sunport Ln., Orlando, FL 32809, U.S.A.; tel.: 407-857-5721; e-mail: sales@ferrate.biz.

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