The promoting effect of Fe on the immobilization of heavy metals in MSWI FA–lead–zinc tailings-based backfill materials under field application: Dynamic simulation leaching and immobilization mechanisms

• Published in: Chemical engineering journal (Lausanne, Switzerland : 1996) Switzerland : 1996), 2024-10, Vol.498, p.155734, Article 155734
• Main Authors: Zhao, Tong, Yang, Huifen, Zhang, Siqi, Wu, Zeping
• Format: Article
• Language: English
• Subjects: Backfill materials; Field application; Immobilization mechanism; MSWI FA; Simulated percolation
• ISSN: 1385-8947
• DOI: 10.1016/j.cej.2024.155734

[Display omitted] •The feasibility of filling MSWI FA-based backfill material in goafs had been proven.•The simulated dynamic filtration confirmed no leaching risk from MLBM over 100 years.•The solidification of Pb, Zn, Cr, As, Cl−, and SO42− was positively correlated with pH (>7.5).•Fe(OH)3(aq) and Fe(OH)4− played a role in the solidification of Pb, As, and Cr.•Pb and Zn were adsorbed by C–(A)–S–H gel in the forms of PbOH+ and ZnOH+. Currently, research on solid waste-based backfill materials containing municipal solid waste incineration fly ash (MSWI FA) was primarily concentrated at the laboratory stage, with insufficient studies on their leaching safety and heavy metal immobilization mechanisms under field application. This study demonstrated that MLBM (Binder weight ratio: MSWI FA: steel slag (SS): blast furnace slag (BFS): flue gas desulfurized gypsum (FGDG): coal fly ash (CFA) = 10 %:33.6 %:33.6 %:12.8 %:20 %; binder-to-lead–zinc tailings (LZT) ratio was 1:8; mass concentration was 60 %) exhibited excellent leaching safety in actual backfilling. The study found that MLBM posed no risk of heavy metal and anion leaching under various leaching standards. Furthermore, MLBM showed no risk of harmful element leaching within a simulated percolation period of 100 years. The release mechanisms of Pb, As, Zn, and Cr in MLBM were diffusion, diffusion, flushing, and dissolution, respectively. The immobilization efficiency of Pb, Zn, Cr, As, Cl−, and SO42− positively correlated with the pH value (pH>7.5). The immobilization mechanism included: 1) Formation of insoluble heavy metal compounds. In the MLBM system (The pH ranged from 7.89 to 8.87), Fe(III) (Fe(OH)3(aq) and Fe(OH)4−) contributed to the immobilization of Pb2+ and SO42− and oxidized As(III) (H2AsO3−) and Cr(III) (Cr(OH)3(aq)), forming As(V) (HAsO42−) and Cr(VI) (CrO42−), respectively. 2) Adsorption. The negatively charged surface of C–(A)–S–H gel could immobilize Zn2+ and Pb2+ through adsorption, and the ionic forms of Pb(II) and Zn(II) were PbOH+ and ZnOH+, respectively. This study provided theoretical support for the leaching safety and immobilization mechanisms of solid waste-based backfill materials containing MSWI FA in field applications.