Malaysian Journal of Analytical Sciences Vol 22 No 6 (2018): 965 - 972

Modified Leaching of ²³⁸U and ²³²Th from Water Leach Purification (WLP) Residue

(Modifikasi Larut Lesap Bagi ²³⁸U dan ²³²Th dari Residu Pemurnian Larut Lesap Air (WLP))

Nur Shahidah Abdul Rashid¹, Zetty Izzaty Saleh², Nurul Syiffa Mahzan², Siti Nur Ain Sulaiman², Khoo Kok Siong², Mohd. Idzat Idris², Syazwani Mohd Fadzil²*

¹Division of Advanced Nuclear Engineering,

Pohang University of Science and Technology, 790784 Pohang, Gyeongsangbuk, South Korea

²School of Applied Physics, Faculty of Science and Technology,

Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia

*Corresponding author: syazwanimf@ukm.edu.my

Received: 29 August 2017; Accepted: 29 October 2018

Abstract

In recent years, the importance of actinide element mobile fractions in risk assessment evaluation of contaminated sites has increased. The knowledge of the sorption kinetics of ²³⁸U and ²³²Th and the thermodynamics behind the sorption process on radioactive waste is important to understand their mobility. In this study, the leaching process of ²³⁸U and ²³²Th from water leach purification (WLP) residue of Lynas Advanced Materials Plant was investigated to obtain the optimum leaching conditions by combining synthetic precipitation leaching procedure and batch method for the simulation of acid rain and heavy flooding, correspondingly. The initial ²³⁸U and ²³²Th concentrations, and the concentration at varies pH and contact time of ²³⁸U and ²³²Th from the WLP residue were studied. Results show that the initial concentrations of ²³⁸U and ²³²Th in the WLP residue are 6.6 and 206.1 mg/kg, respectively. In general, the highest values of the concentration after the leaching process of ²³⁸U and ²³²Th are 0.363 and 8.288 mg/kg, correspondingly. These results show the maximum potential remobilisation of ²³⁸U and ²³²Th at pH 4 with the same contact time of 14 days. At a similar duration, the maximum percentages of leaching are 5.50% and 3.99% for ²³⁸U and ²³²Th, respectively. Moreover, at pH 7, the minimum leaching percentages of ²³⁸U and ²³²Th are 4.7% and 3.61%, correspondingly. Thus, remobilising ²³⁸U and ²³²Th shows that the rate of leaching is influenced by the pH of leachant used. The maximum concentrations of ²³⁸U and ²³²Th are obtained at low pH such as pH 4. At pH 7 and 8, the leached amounts of ²³⁸U and ²³²Th are minimum. Therefore, combining the SPLP and batch method is practical for estimating the leaching and remobilisation of ²³²Th and ²³⁸U from WLP residues. The combined method may be useful for monitoring and risk assessment in environmental studies.

Keywords: leaching, WLP residue, uranium, thorium

Abstrak

Dalam tahun-tahun kebelakangan ini, kepentingan pergerakan unsur aktinida menjadi perhatian semasa penilaian risiko tapak-tapak yang tercemar. Pengetahuan tentang penyerapan kinetik dan termodinamik di sebalik proses penyerapan sisa radioaktif adalah penting untuk memahami pergerakan ²³⁸U dan ²³²Th. Dalam kajian ini, proses kelarut lesapan ²³⁸U dan ²³²Th dari residu pemurniaan larut lesap air (WLP) dari Lynas Advanced Materials Plant telah dikaji untuk mendapatkan keadaan kelarut lesapan yang optimum menggunakan gabungan prosedur larut lesap pemendapan sintetik dan kaedah kelompok, masing-masing sebagai simulasi semasa keadaan hujan asid dan banjir. Kepekatan awal ²³⁸U dan ²³²Th, dan kepekatan ²³⁸U dan ²³²Th terhadap pH dan masa berbeza dari residu WLP turut dilihat. Keputusan menunjukkan kepekatan awal ²³⁸U dan ²³²Th dalam residu WLP masing-masing adalah 6.6 mg/kg dan 206.1 mg/kg. Secara amnya, nilai tertinggi kepekatan selepas proses larut lesap bagi ²³⁸U dan ²³²Th masing-masing adalah 0.363 mg/kg dan 8.288 mg/kg. Kedua-dua hasil menunjukkan bahawa potensi pergerakan maksimum adalah pada pH 4 bagi ²³⁸U dan ²³²Th dalam masa 14 hari. Dalam tempoh masa yang sama, peratusan kelarut lesapan maksimum dicatatkan sebanyak 5.50% dan 3.99% masing-masing bagi ²³⁸U dan ²³²Th. Selain itu, pada pH 7, peratusan larut lesap minimum bagi ²³⁸U dan ²³²Th masing-masing adalah 4.7% dan 3.61%. Oleh itu, pergerakan ²³⁸U dan ²³²Th menunjukkan ia dipengaruhi oleh pH pelarut yang digunakan. Kepekatan maksimum ²³⁸U dan ²³²Th diperoleh pada pH yang lebih rendah seperti pH 4. Pada nilai pH 7 dan 8 mencatatkan kepekatan larut lesap ²³⁸U dan ²³²Th yang minimum. Kesimpulannya, gabungan kaedah SPLP dan kaedah kelompok adalah praktikal untuk menganggar larut lesap dan pergerakan ²³⁸U dan ²³²Th dari residu WLP. Gabungan kaedah ini boleh digunakan sebagai tujuan pemantauan dan penilaian risiko terhadap kajian alam sekitar.

Kata kunci: larut lesap, residu WLP, uranium, torium

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