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 [8]. Based on laboratory animal studies,
human male volunteer studies, and the experience of actual administration of PB
to people contaminated with
137
Cs, 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 [9]. In these sites, groundwater contained cyanide
complexes such as Fer rather than free and more toxic CN
À or HCN, probably
because Fer was decomposed rapidly only when exposed to daylight and the
decomposition of Fer in the dark underground was very slow [10]. Laboratory
experiments showed that Fer was eluted from soil at pH > 13 whereas ferricyanide
(Fe(III)-CN complex) was easily eluted by freshwater [11]. 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 [12]) 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), NH 4
+
, alkaline earth ions (Ca and Mg), and anions (F
À , Cl
À ,
SO 4
2À
, NO 3
À
, and PO 4
3À
). 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
332
Y. Fujikawa et al.
because PB is a decorporation drug to treat internal rad-Cs contamination for
both humans and livestock animals [8]. Based on laboratory animal studies,
human male volunteer studies, and the experience of actual administration of PB
to people contaminated with
137
Cs, 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 [9]. In these sites, groundwater contained cyanide
complexes such as Fer rather than free and more toxic CN
À or HCN, probably
because Fer was decomposed rapidly only when exposed to daylight and the
decomposition of Fer in the dark underground was very slow [10]. Laboratory
experiments showed that Fer was eluted from soil at pH > 13 whereas ferricyanide
(Fe(III)-CN complex) was easily eluted by freshwater [11]. 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 [12]) 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), NH 4
+
, alkaline earth ions (Ca and Mg), and anions (F
À , Cl
À ,
SO 4
2À
, NO 3
À
, and PO 4
3À
). 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
332
Y. Fujikawa et al.