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With the acceleration of industrialization, heavy metal pollution has significantly threatened environmental and human health. An innovative hydrate-based method was proposed in this study to treat wastewater containing heavy metal ions with high concentrations. The feasibility of this technology was confirmed by measuring ion concentration in purified water through hydrate dissociation. By forming R141b hydrate in simulated heavy metal solution under atmospheric pressure at 2 °C, the treatment effect on the wastewater was examined in terms of removal efficiency, water yield, and enrichment factor. Experiments explored how these treatment characters were influenced by changing the volume of hydrate former, post-treatment method, concentration, and species of ions. Considering the treatment effect and economy of the technology, we selected the optimal volume ratio was 4. We combined the solid-liquid separation as the post-treatment method of hydrate-based wastewater treatment and selected vacuum filtration combined with centrifugation as the optimal method, which increased the removal efficiency significantly. The treatment effect was determined by ion concentration, independent of the species of ions. The highest removal efficiency of copper ion reached 91.85%, up to 81.05% yield of water, with an enrichment factor of 2.686. The removal efficiency of different ions in complex wastewater was about 89%. Under the experimental conditions of this research, the limit value of copper ion concentration in the wastewater treated with the hydrate method reached 70 g/L. The wastewater treatment technology proposed in this study using hydrate may promote the development of treating wastewater containing high concentrations of heavy metals. [Display omitted] • A hydrate method is proposed to treat heavy metal sewage with high concentrations. • The limit value of ion concentration determines the removal efficiency. • The influence of concentration on treatment characteristics is addressed. • The suitable post-treatment combined with hydrates improve the removal efficiency. [ABSTRACT FROM AUTHOR]

Several trial productions have been conducted on marine natural gas hydrate reservoirs, demonstrating its enormous potential as an alternative energy source. Nonetheless, sustained and efficient production still remains challenging, with crucial issues on low gas production efficiency and blockages. The large amount of hydrate decomposition and resulting heat absorption can lead to prolonged subzero conditions close to the well; this can readily induce ice formation and blockage. This study aims to investigate the issue of ice blockages caused by insufficient energy supply during the recovery process. A systematic analysis of the energy consumption and ice blockage quantification at each stage of the traditional multi-stage direct depressurization and constant speed depressurization methods is conducted. A hybrid optimized scheme is proposed, which enhances energy utilization efficiency by appropriately utilizing reservoir sensible heat during the initial stage of production. Additionally, a multi-stage constant speed depressurization method is introduced in the later stages to mitigate production fluctuations and reduce blockage duration. Results showed a 69.5 % increase in energy efficiency and an average 67.1 % decline in gas blockage duration with comparable gas productivity. The proposed method could be a viable solution to mitigate challenges of icing blockages and fluctuating production during trial tests of marine hydrate reservoirs, achieving improved energy efficiency and sustainable production. • Ice blockage in hydrate reservoirs reduced production efficiency during direct and constant-speed depressurization methods. • Proposed a hybrid method to address gas production fluctuations caused by ice blockage through energy consumption analysis. • Multi-stage analysis revealed 70% energy efficiency improvement and 67% reduced ice blockage duration with the hybrid method. [ABSTRACT FROM AUTHOR]

• The micro-nano bubbles shorten the induction period of methane hydrate nucleation. • The nucleation rate of methane hydrate increased by 4.73 times. • Micro-nano bubbles of guest molecules with milder phase equilibrium conditions have more advantages in promoting methane hydrate nucleation. • The concentration of micro-nano bubbles affects the memory effect of hydrate, and the memory effect disappears when the concentration of micro bubbles decreases by 73.5%. Hydrate technology has significant application potential in the fields of phase-change cold storage, seawater desalination, gas separation, etc. The key factors influencing the development of hydrate technology are the reduction in the induction time and the promotion of nucleation and growth of hydrates. Micro-nano bubbles (MNBs) can significantly increase the nucleation rate of hydrates owing to their advantages of increasing the gas–liquid interface, enhancing mass transfer, and providing sites for nucleation. In this study, a visualization experimental system for in situ hydrate formation promoted by MNBs was developed. The induction time and the nucleation rate of methane hydrate under the action of MNBs, and the influence mechanism of different guest molecule bulk MNBs on the induction time of methane hydrate were studied. Furthermore, the effects of hydrate formation with injection of bulk MNBs and the reformation with dissociation solution were compared, and the mechanism of bulk MNBs on the memory effect of hydrates was analyzed. The results show that under the action of bulk MNBs (size 80–240 nm), the average induction time of hydrate nucleation is reduced by 80.9%, and the average nucleation rate is increased by 4.73 times. At the same time, by comparing the bulk MNBs of different guest molecules (CH 4 , N 2 , CO 2 , R134a), it was found that R134a has relatively mild phase equilibrium conditions and is easier to nucleate, which has an enhancement effect on the nucleation of methane hydrate. Finally, compared with the concentration changes of bulk MNBs in the dissociated solution, it was found that bulk MNBs play an important role in the hydrate memory effect. When the concentration of bulk MNBs in the dissociated solution decreases by 73.5%, the memory effect disappears, which quantitatively proves the mechanisms of bulk MNBs on memory effects. [ABSTRACT FROM AUTHOR]