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• Efficient two-step phase transition extracted valuable metals from gypsum. • Path-0.5 outpaced path-0, achieving 99.9 % Cr and 98.8 % Cd extraction. • Atom rearrangement in two-step transition triggered atom remove form the lattice. • Extraction efficiencies for trace Pb, Zn, Hg, and As were > 98.5 %, and 95.6 % for Sr. Retrieving valuable metals (VMs) from industrial waste is an attractive approach to reduce the continuous consumption of metallic resources and prevent the environmental contamination by heavy metals. However, metal extraction efficiencies are extremely low due to their entrapment in crystal lattices. Herein, a universal strategy for extracting VMs from gypsum (CaSO 4 ·2H 2 O), a common industrial waste, was reported using a two-step phase transition of dehydration–rehydration; metal extraction was achieved by designing an appropriate phase transition route and controlling the phase transformation kinetics. As the phase transition route to CaSO 4 ·0.5H 2 O (path-0.5) proceeded faster than that to CaSO 4 (path-0), higher extraction efficiencies were realized for Cr (99.9%) and Cd (98.8%) from gypsum. Mechanistic investigations indicated that during the selected path-0.5 dehydration, water molecule loss from gypsum initiated a direct solid-state transformation, triggering the first rearrangement of local atoms and excluding partially incorporated metals from the CaSO 4 ·0.5H 2 O lattice. The subsequent rehydration from CaSO 4 ·0.5H 2 O to CaSO 4 ·2H 2 O, with a phase transition rate faster than that of path-0, triggered the secondary atom rearrangement. The synergism between the two atomic rearrangements finally led to the complete release of trapped metals. Additionally, this strategy was used for retrieving other metals (e.g., Pb, Zn, Hg, As, and Sr) from gypsum, thereby potentially offering a new approach for designing an appropriate phase transition route for extracting VMs from other hydrous minerals. [ABSTRACT FROM AUTHOR]

Phosphogypsum (PG) is a common chemical by-product from wet process phosphoric acid production, accumulated stockpiles of 300 million tons in China. The challenge for the comprehensive utilization of PG is thorough removal of phosphorus and organic contaminants from PG, which inevitably entails the loss of overall whiteness and limits its utilization in building materials and other fields. This work investigated the speciation of phosphorus and organic contaminants in PG by FTIR spectroscopy, Raman spectroscopy, XRD, SEM-EDS and GC-MS. On this basis, by using K 2 S 2 O 8 as a mineralizer, PG was transformed into anhydrous calcium sulfate powder and the organic contaminants were fully decomposed (removal rate >80%, with residual TOC of 0.078%) under hydrothermal treatment. Under optimal hydrothermal conditions (0.3 M K 2 S 2 O 8 , heated at 120 °C for 12 h), the whiteness of product significantly improved from 40.9 to 97 and effective removal of phosphorus was achieved (more than 76%). The residual phosphorus in treated PG was 0.108%, which met the standard of PG as a building material in China GB/T-23456 2018 (P content < 0.2%). The mechanism study revealed that the phase transformation of calcium sulfate from dihydrate to anhydrous played a dominant role in the removal of phosphorus, H+ and SO 4 2- from K 2 S 2 O 8 decomposition facilitated the dissolution-recrystallization process of calcium sulfate, allowing the phosphorus in the lattice to escape. In addition, by thermal activation, K 2 S 2 O 8 generated free radicals of SO 4 •- and OH• dissipated the organic contaminants in PG. This work provides theoretical and practical bases for the harmless disposal and resource recycling of industrial by-product gypsum, and it is of great significance to build the "zero-waste city". • A feasible method was developed to efficiently remove phosphorus and organic contaminants from phosphogypsum. • The whiteness of product significantly improved from 40.9 to 97, with residual phosphorus of 0.108%. • The phase transformation of gypsum and oxidizing K 2 S 2 O 8 play an important role in the removal. [ABSTRACT FROM AUTHOR]

• A feasible and universal method for recycling gypsum waste was proposed. • 99.8% of Cr(VI) was extracted from gypsum waste under hydrothermal treatment with H+. • H+ accelerated the growth of gypsum crystal thus facilitated the separation of Cr(VI). • H+ decreased the phase transformation temperature ensuring the detoxification. Heavy metal-containing gypsum is a widespread hazardous waste. In this work, H+ was found to be the most essential factor of the mineralizers in hydrothermal treatment to completely (≥99.8%) extract Cr(VI) from gypsum waste to the supernatant, where the significant growth (from several μm to several hundreds of μm) and perfection of the gypsum crystals were observed. Moreover, with increasing concentration of H+, the crystal growth (undergoing Ostwald ripening process) was accelerated and the phase transformation temperature of gypsum was decreased from 110℃ (at 0.2 mol/L of HCl) to 100℃ (at 0.3 mol/L of HCl), which are favorable to enhance Cr(VI) extraction efficiency. Pilot experiments further certified this method to be practicable even in ton-scale. This work proposes a practicable and universal method to completely extract Cr(VI) from gypsum waste, and would also inspire the recycle of gypsum waste containing other heavy metals, such as As, Pb, Cd, and Hg. [ABSTRACT FROM AUTHOR]