Removal of Radium-226 from Radium-Contaminated Soil Using Humic Acid by Column Leaching Method

REMOVAL OF RADIUM-226 FROM RADIUM-CONTAMINATED SOIL USING HUMIC ACID BY COLUMN LEACHING METHOD

(PENYINGKIRAN RADIUM-226 DARIPADA TANIH TERCEMAR RADIUM MENGGUNAKAN ASID HUMIK MELALUI KAEDAH LARUT-LESAP TURUS)

a,bEsther Phillip*, bMuhamad Samudi Yasir, aMuhamat Omar, aMohd Zaidi Ibrahim, aZalina Laili

“Malaysian Nuclear Agency, Bangi, 43000 Kajang, Selangor Darul Ehsan; bUniversiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan.

*Corresponding author: [email protected]

ABSTRACT

In this study, evaluation o f radium-226 removal from radium-contaminated soil using humic acid extracted from peat soil by column leaching method was carried out. Humic acid of concentration 100 ppm and pH 7 was leached through a column packed with radium-contaminated soil and leachates collected were analysed with gamma spectrometer to determine the leached radium-226. Results obtained indicated low removal o f radium-226 between 1 — 4 %. Meanwhile, leaching profile revealed that radium-226 was bound to soil components with three different strength, thus resulting in three phases o f radium-226 removal. It was estimated that the total removal of radium-226 from 10 g radium-contaminated soil sample studied could be achieved using approximately 31500 — 31850 mL HA solutions with leaching rate o f 1 mL/min.

Keywords: Radium-226, soil, humic acid, column, leaching

ABSTRAK

Dalam kajian ini, penilaian penyingkiran radium-226 menggunakan asid hunikyang diekstrak daripada tanih gambut melalui kaedah larut-lesap turus telah dijalankan. Asid humik berkepekatan 100 ppm dan pH 7 dilalukan pada turus yang dipadatkan dengan tanih tercemar radium dan bahan larut-lesap yang dikumpulkan dianalisis menggunakan alat spektrometer gama bagi menentukan radium-226 yang terlarut-lesap. Hasil kajian menunjukkan penyingkiran radium-226 yang rendah antara 1 — 4 %. Sementara itu, profil larut-lesap menunjukkan bahawa radium-226 terjerap pada komponen tanih dengan tiga kekuatan berbeza, justeru mewujudkan tiga fasa dalam penyingkiran radium-226. Walau bagaimanapun, dijangkakan penyingkiran keseluruhan radium-226 daripada 10 g sampel tanih tercemar radium yang dikaji boleh dicapai dengan menggunakan 31500 — 31850 mL larutan asid humik pada kadar larut-lesap 1 mL/min.

Kata kunci: Radium-226, tanih, asid humik, turus, larut-lesap

INTRODUCTION

Contamination of soil with radionuclides could arise due to various activities that involved the processing and utilization of radioactive materials. In early days, application of radioactive material such as radium (Ra) in dial painting activities had resulted in the contamination of soil with Ra (Murphy et al., 1980; Esposito and Clark, 1987; Robinet et al., 1987; Ladsous, 1996; Edwards, 1997; MDCH, 1997; US NCR, 2010; Phillip et al., 2012). Upon decommissioning of the radioluminescence dial painting facilities, the radium-contaminated soil could be excavated from the sites and transferred to radioactive waste storage facilities for further treatment before deciding on the final disposal option.

Treatment of Ra-contaminated soil that involved the removal of Ra from soil by various methods including extraction, desorption and leaching had been widely investigated (Frissel and Koster, 1990). Most of the studies adopted batch method in investigating the removal of Ra from soil (Rusanova, 1962; Rusanova, 1964; Taskaev et al., 1977, Taskaev, 1978; Williams, 1978; Megumi, 1979; Marple, 1980; Arkhipov et al., 1981; Cooper et al., 1981; Kalin and Sharma, 1982; Ibrahim et al., 1982; Oliviera et al., 1984; Teixeira and Franca, 1983; Vos et al., 1981; Landa, 1984; Phillip et al., 2012). Nevertheless, Nathwani and Phillips (1978) had investigated the leaching of Ra from contaminated soil by column method.

In this study, humic acid (HA) extracted from Malaysian peat soil was used as removal agent due to its ability to remove radium-226 (226Ra) from Ra-contaminated soil by batch washing method (Phillip et al., 2012). Batch washing of Ra-contaminated soil using HA yielded higher removed 226Ra compared to when distilled water was used (Phillip et al., 2011). Interaction of Ra with organic material in particular HA had earlier been investigated by Hansen and Huntington (2002), Ibrahim et al. (2009) and Laili et al. (2010). Oxygen-containing functional groups such as COOH and phenolic-OH in the humic molecules interacted well with metal ions (Stevenson, 1994) via numerous mechanisms including adsorption, complexation and ion exchange (Broekaert et al., 1990).

In this paper, evaluation on the removal of 226Ra from Ra-contaminated soil using HA by column leaching method would be presented. 2

MATERIALS and METHODS

Materials

HA used was extracted from peat soil obtained from the east coast of Peninsular Malaysia using a simplified acid-base extraction method described by Swift (1996) with minor modifications as performed by Ibrahim et al. (2008; 2009). Ra-contaminated soil was collected from a former compass dial painting facility site in Malaysia. The Ra-contaminated soil sample used was prepared by air- drying the collected soil at room temperature (27°C) in a fume hood prior to manual grinding. The ground soil was then sieved using a 2 mm sieve and the sieved soil sample was finally stored in a sealed container for experimental purposes. Ra-226 standard solutions were obtained from Isotope Products Laboratories (an Eckert & Ziegler Company). Other reagents and chemicals used were of analytical grade from R&M Chemicals or Sigma Aldrich.

Column leaching

HA solutions of concentration 100 ppm and pH 7 were used as leachants. A glass column with a 2 cm internal diameter and 25 cm length was packed with 10 g Ra-contaminated soil that was earlier mixed with 10 g clean sand to increase permeability. 100 mL HA solutions were introduced into the column and left overnight to allow for even distribution of HA solutions. HA solutions were then continuously fed into the column and passed through the column at a flow rate of 1 mL/min. Leachates were collected every 350 mL. The collected leachates were then left for at least 21 days before the determination of 226Ra radioactivity concentration using a gamma spectrometry system. In this study, the determination of 226Ra activity concentration was based upon indirect measurement from a line of a 226Ra daughter isotope; the 609 keV line of 214Bi. The 609 keV line of 214Bi was selected due to the absence of interference from natural radionuclide as well as the high yield of gamma ray produced. Schematic diagram of the leaching column was shown in Fig. 1.

Fig. 1 Schematic diagram of column leaching

RESULTS and DISCUSSION 3

In column leaching studies, variation of experimental data could be due to varying degrees of channeling in the soil column (Enell et al., 2004). The variation in experimental data thus caused poorer reproducibility in column leaching method compared to batch method (Jackson et al., 1984). However, experimental data obtained in this column leaching study was found to be stable and reproducible as the removal of 226Ra from the three different columns studied was almost similar. Removal of 226Ra obtained for three studied soil columns (C1, C2 and C3) was shown in Fig. 2. Leaching profile shown as removal of 226Ra over fractions of collected leachate in Fig. 2 showed that three varying phases of 226Ra removal were present thus indicating that 226Ra was adsorbed to soil components with three different strength. 226Ra species that were weakly bound to soil components were leached earlier compared to those that were strongly bound. Soil sampling and pre-treatment such as drying and sieving could destroy soil structures (Jean-Soro et al., 2012) thus allowing leaching to progress readily especially for metal ions that were weakly bound to soil components. Nevertheless, the low removal of 226Ra observed in this study implied that 226Ra was strongly adsorbed to soil components. In addition, long interaction time of about 10 - 40 years in the course of the dial painting activities allowed various interactions between 226Ra and soil to happen. Besides that, it was also revealed that

Fraction (350 mL leachate each)

—• —Removal of radium-226, % (C1) —■—Removal of radium-226, % (C2) ♦ Removal of radium-226, % (C3)

Fig. 2 Removal of radium-226 (226Ra) obtained for three studied soil columns; C1, C2 and C3 (run at identical conditions)

Generally, the removal of 226Ra by column leaching was very low. The cumulative removal of 226Ra obtained as shown in Fig. 3 revealed that approximately 24 % 226Ra was removed after leaching with 5250 mL of HA solutions at leaching rate 1 mL/min. It was anticipated that approximately 31500 - 31850 mL HA solutions were required to achieve total removal of 226Ra from 10 g Ra- contaminated soil sample studied as long as the removal of 226Ra was maintained around 1 % throughout the leaching process. Continuous batch washing using HA (to be reported in other publication) revealed that 226Ra contained in the contaminated soil studied could be removed completely thus suggesting that it could also be removed by column leaching method but in a much longer period.

m ui ida

i ^ em 226 e £ti al lu m u C Fraction (350 mL leachate each)

■Cumulative removal of radium-226, % (C1) Cumulative removal of radium-226, % (C2) Cumulative removal of radium-226, % (C3)

Fig. 3 Cumulative removal of radium-226 (226Ra) obtained for three studied soil columns; C1, C2 and C3 (run at identical conditions) 4

CONCLUSION2 26 The removal of 226Ra from the Ra-contaminated soil studied using HA by column leaching method was low and the removal trend showed three varying phases of 226Ra removal with decreasing removal rate as leaching progressed.

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