Effect of Si/Al molar ratio and curing temperatures on the immobilization of radioactive borate waste in metakaolin-based geopolymer waste form

Byoungkwan Kim; Jaehyuk Kang; Younglim Shin; Tae-Min Yeo; Jong Heo; Wooyong Um

doi:10.1016/j.jhazmat.2023.131884

Effect of Si/Al molar ratio and curing temperatures on the immobilization of radioactive borate waste in metakaolin-based geopolymer waste form

J Hazard Mater. 2023 Sep 15:458:131884. doi: 10.1016/j.jhazmat.2023.131884. Epub 2023 Jun 17.

Authors

Byoungkwan Kim¹, Jaehyuk Kang², Younglim Shin¹, Tae-Min Yeo³, Jong Heo¹, Wooyong Um⁴

Affiliations

¹ Division of Advanced Nuclear Engineering (DANE), Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea.
² Division of Advanced Nuclear Engineering (DANE), Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea; Department of Electrical and Energy Engineering, Jeju National University, 102, Jejudaehak-ro, Jeju-si, Jeju Special Self-Governing Province, 63243, Republic of Korea.
³ Graduate Institute of Ferrous & Energy Materials Technology (GIFT), Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea.
⁴ Division of Advanced Nuclear Engineering (DANE), Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea; Nuclear Environmental Technology Institute (NETI), Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea; Division of Environmental Science and Engineering (DESE), Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea. Electronic address: wooyongum@postech.ac.kr.

PMID: 37348372
DOI: 10.1016/j.jhazmat.2023.131884

Abstract

Immobilization of radioactive borate waste (RBW) using a geopolymer with a high Si/Al ratio has been challenging because boron-silicon networks lower the compressive strength and delay the setting time. In this study, metakaolin-based geopolymer waste form to immobilize simulant RBW was fabricated using different Si/Al ratios (1.0-1.4) and curing temperatures (26 and 60 ℃). The 7-day compressive strength results revealed that a certain amount of silicon and an elevated curing temperature are required to achieve high compressive strength and waste loading. Following waste acceptance criteria tests, all geopolymers exhibited compressive strengths higher than 3.445 MPa. The leachability index of boron was higher than 6.0, and the leaching mechanism was identified as diffusion. No significant structural changes in the geopolymer were observed after thermal cycling and gamma irradiation tests. The physically bound or unincorporated RBW was leached out of the geopolymer during water immersion and leaching tests; however, boron, which was chemically connected with silicon, was present as an inert phase together with a geopolymer binder. Consequently, immobilizing RBW using a geopolymer with a low Si/Al ratio (1.4) is beneficial in terms of RBW loading and structural durability.

Keywords: Borate waste; Geopolymer waste form; Immobilization; Metakaolin-based geopolymer; Si/Al ratio; Waste acceptance criteria.