Volume Reduction of Municipal Solid Wastes Contaminated with Radioactive Cesium by Ferrocyanide Coprecipitation Technique
Abstract Municipal solid wastes (MSW) with elevated concentrations of radioactive cesium (rad-Cs hereafter) have been generated in some areas of Japan in the aftermath of the Fukushima Daiichi Nuclear Power Plant (F1 hereafter) accident. Both recycling and final disposal of the contaminated MSW have become a difficult problem in the affected areas, resulting in accumulation of treated residues in the treatment facilities.
The rad-Cs in MSW, especially fly ash, often showed a high leaching rate. Extraction of contaminated MSW with water or hot oxalic acid followed by selective removal of rad-Cs from the extract using ferrocyanide (Fer hereafter) coprecipitation technique could be an ultimate solution for waste volume reduction. The MSW extracts contain various metal components as well as chelating reagents like oxalic acid, and are often very saline. The composition of the extract varies widely depending on waste sources, applied treatment techniques, and rad-Cs extraction method etc. The applicability of the Fer coprecipitation technique had to be tested and validated before it could be applied for actual treatment.
In this work, we applied the Fer technique and observed removal of cesium (Cs) from water and oxalic acid extracts (all spiked with rad-Cs tracer or stable Cs) of various MSW samples collected from uncontaminated areas. Finally, the Fer technique was applied on site for removal of rad-Cs in the extracts of contaminated MSW. By modifying coprecipitation conditions according to solution matrix, Cs removal rates of higher than 95 % could be obtained.
Keywords Cesium • Ferrocyanide • Metal • Municipal solid waste • Oxalic acid • pH
Background and Objectives
The estimated sustainable life period of the existing final disposal sites for municipal solid wastes (MSW) in Japan was only 18 years as of the end of FY2008. Therefore, waste avoidance, waste volume reduction, and recycling of MSW have been a national policy. However, the Fukushima Daiichi Nuclear Power Plant (F1) accident has created an entirely new dimension in environmental pollution problems. Because waste incineration and water treatment are, by their nature, the processes that concentrate pollutants such as radioactive cesium (rad-Cs) in ashes and sludge, MSW containing high concentrations of rad-Cs are produced in some areas where high atmospheric deposition of rad-Cs occurred in the aftermath of the F1 accident. As a result, recycling of MSW as concrete material and compost has become difficult, and their reuse has been often prevented because of public opposition even when rad-Cs concentrations in the wastes are below the clearance level (100 Bq/kg). Most of the citizens in the affected area are in hard opposition to disposal of rad-Cs-containing wastes even if radioactivity of the wastes is below the governmental limit for their disposal in landfills with leachate collection systems (i.e., 8,000 Bq/kg of Cs-134 + Cs-137). As the result, treatment residues are now piling up in many treatment facilities in some area, which may eventually jeopardize the treatment itself and exert serious negative impacts to everyday life. For example, sewage facilities in Fukushima Prefecture stored 74,401 t of dewatered sludge, molten slug, and incinerator ashes as of May, 2014. Therefore, suitable technologies to reduce the volume of such wastes or to decontaminate rad-Cs at low cost are urgently required.
Private companies and agencies have been working on sludge volume reduction through drying combined with granule processing  with the purpose of alleviating storage problems at treatment facilities. High-temperature combustion of sludge with an additive for controlling basicity of incineration material also proved effective in condensing rad-Cs in fly ash. The cost of this technique, however, was high and would be justified only when a very strong social need for sludge volume reduction exists . Another tested technique in this regard is extraction of sewage by hot 0.1 M oxalic acid followed by recovery of the extracted rad-Cs by zeolite . The cost of the oxalic acid method is considered acceptable for largescale sewage treatment facilities, although waste volume reduction is dependent on the amount of zeolite necessary to remove Cs from the extract. The Cs distribution factor value (ml/g) reported for zeolite was a few thousand whereas the values for ferrocyanide (Fer) compounds determined by the in situ Fer coprecipitation method were between 104 and106 . Apparently, the use of the Fer coprecipitation technique for rad-Cs removal from waste extract is appropriate to maximize waste volume reduction.
On the other hand, there are concerns on the outcome of using Fer, especially
regarding the radiological risk of generating concentrated waste regarding rad-Cs and the chemical hazard from Fer compounds.
The concentration of rad-Cs in insoluble Fer precipitate [QCs (Bq/kg)] generated by adding 0.1 mM potassium ferrocyanide (the concentration used in most of our experiments) to the waste extract can be estimated as follows:
Here r is percentage of rad-Cs removed from the extract of MSW by Fer technique, E is percentage of rad-Cs extracted from MSW with water or oxalic acid, M is weight (kg) of MSW extracted by V (l) of the solvent, p is weight of Fer precipitate formed per unit volume of the extract (kg/l), and C0 is rad-Cs concentration (Bq/kg) in original MSW. Assuming that r, E, M, V, and p are 95 %, 90 %, 1 kg, 2.5 l, and 35 x 10-6 kg/l, respectively (the values typically encountered in our on-site tests), QCs (Bq/kg) is 9,771 C0, implying that rad-Cs concentration in the Fer precipitate can be about four orders of magnitude higher than that in the original MSW. The designated wastes with rad-Cs concentration >100,000 Bq/kg are going to be sent to the interim storage facility in Fukushima Prefecture and the waste volume reduction is going to be carried out at the interim storage site before final disposal. The wastes with rad-Cs concentration lower than 100,000 Bq/kg are going to be disposed in a leachate-controlled landfill constructed by the national government or a conventional municipal landfill. The amount of designated wastes stored in 12 prefectures is 140,343 t as of December 31, 2013 , but most are less than 100,000 Bq/kg in rad-Cs concentration. The amount of designated waste exceeding 100,000 Bq/kg is predicted to be 9,000 t with rad-Cs concentration varying between 120,000 and 540,000 Bq/kg depending on the origin of the waste . If the extraction of the waste followed by Fer coprecipitation was conducted for 9,000 t
of designated waste >100,000 Bq/kg, and r, E, M, V, and p values were the same as discussed early in this paragraph, 790 kg of insoluble Fer waste with rad-Cs concentration 1.2 x 109–5.3 x 109 Bq/kg (total amounts of rad-Cs, 9.2 x 1011 to 4.2 x 1012 Bq) can be generated. By comparison, the content of rad-Cs in a piece of vitrified high-level radioactive waste (weight, 500 kg) can be as high as 4.8 x 1015 Bq , that is, three orders of magnitude higher than that from 9,000 t of highly contaminated designated waste. With appropriate instrumentation and management, it is possible to handle the rad-Cs concentrated waste resulting from the volume reduction of designated waste relatively safely.
The chemical risk of using Fer compounds to concentrate rad-Cs also requires attention. Although reagents such as oxalic acid that may be used for the extraction of rad-Cs are biodegradable and the degradation products are nontoxic, Fer compounds contain a cyano group within their structure, and are potentially more hazardous. Chemical toxicity of Fer compounds, especially that of ferric ferrocyanide (Prussian blue, PB hereafter), in mammals has been studied extensively because PB is a decorporation drug to treat internal rad-Cs contamination for both humans and livestock animals . Based on laboratory animal studies, human male volunteer studies, and the experience of actual administration of PB to people contaminated with 137Cs, it was concluded that PB is basically nontoxic. The history of the use of Na-Fer, Ca-Fer, and K-Fer as food additives also indicates that the toxicity of Fer compounds is low. More important is the risk pertinent to the long-term decomposition of Fer and possibility of free cyanide leaching from waste materials. For example, large amounts of Fer compounds have been generated in the coal and petroleum gas purifier used in gas production industries. The used purifier (containing ferric ferrocyanide) was often abandoned around coal pyrolysis plants, etc., and has caused the pollution of soil and groundwater. The problem is widespread: 1,310, 234, and 1,100 to 3,000 sites are known in Germany, Netherlands, and the U.S., respectively . In these sites, groundwater contained cyanide complexes such as Fer rather than free and more toxic CNor HCN, probably because Fer was decomposed rapidly only when exposed to daylight and the decomposition of Fer in the dark underground was very slow . Laboratory experiments showed that Fer was eluted from soil at pH > 13 whereas ferricyanide (Fe(III)-CN complex) was easily eluted by freshwater . If Fer is to be used to concentrate rad-Cs, the resulting cyanide complex-containing waste should be managed properly by avoiding exposure to daylight and alkaline reagent. It is also possible to decompose Fer thermally or chemically (e.g., United States Environmental Protection Agency ) before the final disposal, depending on the cost allowed for the treatment.
Although the use of Fer coprecipitation technique has to be evaluated from the environmental safety considerations, it is also necessary to know if the technique is applicable to the actual MSW treatment at all. MSW waste extracts contain high concentrations of multiple transition metals (Fe, Mn, Cu, Zn, and Ni), alkali metal ions (Na and K), NH4+, alkaline earth ions (Ca and Mg), and anions (F-, Cl-, SO42-, NO3-, and PO43-). Mixtures of various kinds of insoluble Fer-metal precipitates can be formed in such solutions, and the substitution of alkali metals in the precipitate should also occur. Solubility of each Fer-metal compound as well as the reaction kinetics between Fer ion and each metal should influence the amount and chemical structure of Fer-metal precipitate thus formed. The types and concentrations of anions in the solution affect the efficiency of coagulation of colloidal Fer solid, and thus the solid–liquid separation. Therefore, the feasibility of the Fer coprecipitation technique has to be tested and validated before its application to the actual treatment.
The objective of this work is to identify the factors that are likely to govern Cs removal from MSW extracts by the Fer coprecipitation technique and to optimize coprecipitation conditions for Cs removal. As detailed information on chemical components in the extracts of rad-Cs contaminated wastes is hard to obtain, we first obtained and analyzed uncontaminated MSW extracts (i.e., do not contain rad-Cs from the F1 accident), applied Fer precipitation techniques to the uncontaminated MSW extracts, and then proceeded to rad-Cs-contaminated waste treatment.