It is a valid path to realize the zero discharge of coal chemical wastewater by using the fractional crystallization method to recycle the miscellaneous salt in high-salinity wastewater. In this study, the thermodynamics and nucleation kinetics of sodium chloride (NaCl) and sodium sulfate (Na₂SO₄) crystallization in coal chemical wastewater were systematically studied. Through analyses of solubility, metastable zone width, and induction period, it was found that the impurity dimethoxymethane would increase the solid–liquid interface energy and critical crystal size during the nucleation of Na₂SO₄. Ternary phase diagrams of the pseudo-ternary Na₂SO₄-NaCl-H₂O systems in simulated wastewater were plotted in the temperature range of 303.15 to 333.15 K, indicating that a co-ionization effect existed between NaCl and Na₂SO₄, and NaCl had a strong salting out effect on Na₂SO₄. Finally, the nucleation rate and growth rate of Na₂SO₄ crystals under simulated wastewater conditions were determined by the intermittent dynamic method, and the crystallization kinetic models of Na₂SO₄ were established. The crystallization nucleation of Na₂SO₄ crystals was found to be secondary nucleation controlled by surface reactions. The basic theoretical research of crystallization in this study is expected to fundamentally promote the application of fractional crystallization to realize the resource utilization of high-salinity wastewater in the coal chemical industry.

Mercury ions (Hg²⁺), a heavy metal contaminant of strong biotoxicity, pose a serious threat to ecosystems and human health in aquatic environments. Developing highly sensitive and specific detection methods is therefore of great importance. This study presents a novel label-free fluorescent biosensor for Hg²⁺ by ingeniously coupling target-induced aptamer switching with rolling circle amplification (RCA). Upon Hg²⁺ binding, the conformational change releases a sequestered primer to initiate RCA, generating G-quadruplex-rich DNA products that produce a strong “turn-on” signal with N-methylmesoporphyrin IX (NMM). Under optimized conditions, the assay exhibits excellent linearity from 10 to 1000 nM with a detection limit of 3.2 nM, along with high selectivity over competing metal ions. Validation using spiked environmental water samples yielded accurate and reproducible recoveries in the range of 93.8% to 106.0%. With its operational simplicity, high sensitivity, and robust performance in complex matrices, this label-free strategy offers a reliable and promising platform for detecting Hg²⁺ in environmental waters.