Analysis of the stepwise immobilization of phosphorous and fluoride in PG leachate using Fe-doped C-S-H (Fe-C-S-H) prepared with gasification slag and the application of the solidified product

Abstract

Phosphogypsum leachate contains large amounts of phosphorus and fluoride impurities, resulting in serious pollution of soils and water bodies. This study synthesized calcium silicate hydrate (C-S-H) using SiO₂ and CaO, and Fe-doped calcium silicate hydrate (Fe-C-S-H) using fine coal gasification slag (FCGS) and CaO, both employed to immobilize phosphorus and fluoride impurities in phosphogypsum leachate. In this study, a stepwise precipitation method was applied to control the pH of the leachate system, with F^-, HPO₄^2-, and PO₄^3- in the leachate sequentially immobilized. In addition, the influences of C-S-H and Fe-C-S-H prepared with different calcium-to-silicon ratios on the immobilization efficiency of phosphorous and fluoride impurities were explored. Through XRD, FT-IR, zeta potential, SEM-EDS, and molecular dynamics, the mechanisms of phosphorus and fluoride immobilization by these materials were investigated. Furthermore, The immobilized leachate was further utilized as an admixture for phosphogypsum building materials (PBG) to produce enhanced PBG products, thus achieving the recycling utilization of phosphogypsum leachate. Results indicated that at a calcium-to-silicon ratio of 1.2, both C-S-H and Fe-C-S-H achieved optimal rates of immobilization for phosphorus and fluoride, with residual ion concentrations meeting Class V water discharge standards specified in Environmental Quality Standards for Surface Water (GB 3838–2002). In addition, F^-, HPO₄^2- and PO₄^3- in the phosphogypsum leachate spontaneously adsorbed onto the (001) crystal planes of C-S-H and Fe-C-S-H. PBG specimens incorporating the immobilized phosphogypsum leachate exhibited a 23.47 %–38.22 % increase in compressive strength. This study not only introduces a novel approach for FCGS utilization but also establishes an innovative method for immobilizing phosphorus and fluoride impurities in phosphogypsum leachate, thus promoting the circular utilization of phosphogypsum waste.