REMOVAL OF RADIOACTIVE AND HEAVY METAL IONS FROM WASTE WATER USING METAPURE

E. Tel Or and N. Cohen
The Hebrew University of Jerusalem
Faculty of Agriculture, Rehovot, ISRAEL
Tel. 972 - 8 - 9481262, Fax 972 - 8 -9467763

D. Ilzycer, I. Gilath, A. Mei-Marom and H. Zafrir
SOREQ
Nuclear Research Center, Yavne 81800, ISRAEL
Tel. 972 - 8 - 9434134, Fax 972 - 8 -9434403

ABSTRACT

A new biomass, Metapure, based on a dried plant is presented. It binds and concentrates radioactive and heavy metal ions from water solutions in a broad range of concentration (ppm to below ppt) and over a wide range of pH: 2-11. Metapure biomass is suitable for clean-up of nuclear industry waste water from radioactive and toxic metal ions. The biomass can be incinerated at temperature of around 500°C, to heavy metal enriched ashes, reducing drastically the weight of the waste for disposal up to 1/10.

Few examples of radionuclides accumulation by the biomass, from various solutions representing clean up problems, are presented.

INTRODUCTION

Metapure is a patented material based on a dried plant (biomass). It has remarkable properties of selective binding and concentration of radioactive and heavy metal ions removal from waste water in the ppm to below ppt concentrations and over a wide range of pH: 2-11. In real tests, Metapure was found to bind and concentrate metal ions in solution at ultra low concentrations, much below ppt.

The binding capacity for heavy metals is high, for example Metapure can bind lead (Pb) in a quantity equivalent to 10% of its dry weight. Metapure is a green product and does not release toxic components in the treated waste water. It is easily incinerated at low temperature (~ 300 - 500°C) to heavy metal enriched ashes, reducing the weight of the biomass waste for disposal up to 1/10. Metapure in the dry form is chemically stable and has an extensive shelf life.

Filtering system utilizing Metapure biomass is suitable for remediation of nuclear industry waste water from radioactive and toxic metal ions.

METHODOLOGY

Solutions containing metal ions with radioactive tracers such as Zr, Hf, Cs, Ru, Ce and Co were tested in the laboratory. The metal ions concentrations were in the ppm, ppb and ppt ranges. Similar experiments were performed with non-radioactive metalions such as Hg, Pb, Cd and others. The removal of most of the above ions by the biomass, was close to 100%.

The experimental laboratory set-up consisted mainly of a vessel for the feed solution, a peristaltic pump to flow the feed solution through a packed column with Metapure, a vessel for the effluent and a NaI nuclear detector for activity measurements (Fig.1).


Fig. 1. Experimental set up for uptake and distribution measurement of the radioactive tracer in the Metapure filter.

The laboratory column was 1.5 cm diameter and 15 cm length and contained 5 g of dry biomass. The flow rate experimented were from 2-120 ml/min. During the flow experiments through the biomass, the column was placed in front of the NaI detector for continuous real time measurement of radionuclides uptake in the column. The metal ion concentration was determined by activity measurements on feed and effluent solutions as well as the metal ion uptake on the Metapure column, allowing calculation of material balance.

The distribution of the metal ions on the Metapure column was determined by shielding the column with lead bricks leaving a slit toward the detector. The column was then slided along the slit and the activity recorded as a function of the column length.

The concentration of non-radioactive heavy metal ions was measured using ICP techniques. The metal ions concentrations in solution were measured directly. ICP analysis of metal ion accumulation on Metapure were performed after acid digestion of the biomass or ashing prior to acid digestion.

RESULTS

In order to demonstrate the efficiency of the biomass, results are presented for few examples of experiments.

  1. Metapure uptake for Cs-134 at 5 ppm and 100 ppb solution concentrations:

    One liter of Cs-134 feed solution, prepared with deionized water, was passed through a 5 g biomass column, at a flow rate of 4 ml/min, for each concentration. The Cs-134 uptake on the column was recorded at constant time intervals. Figure 2 a & b shows the accumulation of Cs-134 on the biomass as a function of the volume of the feed solution flowing through the column.
    The uptake is represented as the ratio of the column activity measurement to the activity of the feed solution. In both cases, the graphic representation of the uptake is linear and shows a 99.8% removal.


    Fig. 2. Biomass uptake for Cs-134 at 100 ppb (a) and 5 ppm (b) solution concentration.

  2. Removal of radio-isotopes from the IRR1 reactor pool cooling water at Soreq, comparison between Metapure and strong cation exchanger:

    In this section we show the binding capability of Metapure for a mixture of isotopes in a real solution, as well as a comparison of the Metapure and a strong cation exchanger uptake behaviors. The radionuclides present in the pool water are mainly activation products at very low concentrations, much below ppt. Two columns were prepared: one packed with 5 g Metapure and the second with 5 g cation exchanger, Dow HCR-SLE. Pool water reactor was flown through both columns in parallel and at the same time, ensuring that the same solution was passed through the columns. The flow rate was 9 l/hr.cm2 for the Metapure column and 5 l/hr.cm2 for the cation exchanger column. Duration of the experiment was 2 hr. The activity of a 200 ml pool water sample was measured as a reference. At the end of the experiment, the activity of the two columns were measured with a HPGe nuclear detector. The results are presented in the Table I.

    Table I Metapure and Dow HCR-SLE resin uptake of radionuclides from IRR1 reactor pool cooling water.

    As it can be seen, the resin filled with the light and short lived Na-24, while the Metapure had a distinctive preference for the heavier trace elements, in comparison to sodium. The Na removal yield was 100% for the resin and 5% only for the Metapure biomass. This shows the Metapure biomass selectivity.
  3. Removal of Cs and Sr from a simulant solution of radioactive waste water:

    A simulant solution was prepared according to the specifications and descriptions of the ORNL/TM-12903 report (1). This solution is simulating radioactive waste water containing Cs-137 and Sr-90. The composition of the simulant solution without the radioactive isotopes is shown in Table II.

    Table II Composition of Simulant Solution of Radioactive Waste Water

    The total cation concentration in the above solution is 60.3 mg/l. To this solution we have added 400 ppt Cs-134 and 200 ppb Sr.
    One liter of the feed solution was flown through a 5 g biomass column at a flow rate of 4.5 ml/min. The graphic representation of Cs-134 uptake to the volume of solution passed through the column shows a constant increase indicating that no saturation was obtained (Fig. 3). The removal for Cs was 90.4% and the uptake of Sr, calculated from ICP measurements was 63%.


    Fig. 3. Cs-134 uptake from Oak Ridge simulant waste water solution.

  4. Removal of Cs-134 from high salinity solution:

    A solution containing 2 ppb Cs-134 and 20g/l NaCl was prepared. One liter of this solution was passed through a 10 g Metapure column at 4 ml/min flow rate. From Fig. 4, we can see that the biomass binds very low concentration of radioactive cesium, even in very high salinity solution comparable to sea water. The Cs-134 removal from the solution reached 66%, which is 6-7% removal per gram of biomass. This confirms that Metapure biomass is rather inactive towards Na ions and has selective removal properties.


    Fig. 4. 2 ppb Cs-134 uptake from high salinity NaCl solution (20 g/l).

CONCLUSIONS

The efficiency of Metapure to remove trace quantities of radioactive or heavy metal ions was shown. The Metapure biomass has selective removal properties. Metapure biomass can be used efficiently for the clean up of nuclear industry waste water from radioactive and toxic metal ions.

REFERENCE

  1. D.T. BOSTICK, W.D. ARNOLD, P.A. TAYLOR, D.R. MCTAGGART, M.W. BURGESS and b. guo, Evaluation of Improved Techniques for the Removal of Sr-90 and Cs-137 from Process Wastewater and Groundwater: Chabazite Zeolite Baseline Study, ORNL/TM-12903, November 1994.