A novel wastewater treatment method: hydrate-based pollutant removal from wastewater

Treatment of industrial effluents such as the heavy metal-bearing wastewater before discharge is highly encouraged as one of the mitigation mechanisms to combat soil and water pollution. Among the available methods for treating the wastewater, coagulation-flocculation and chemical precipitation techniques relying majorly on the parameters of the wastewater are widely used. However, they are not suitable for treating wastewater with heaving metals due to several drawbacks. To this end, the development of more efficient wastewater treatment methods is highly desirable.

Recently, hydrate-based technology has attracted significant attention of researchers especially due to its ability to cause pore-water freshening and oxygen isotope fractional in deep seawater. This process utilizes an ion exclusion process to exclude dissolved sodium and chlorine ion from the hydrate structure. Generally, hydrates are crystalline solids comprising of water small molecules. This shows the possibility of excluding sodium and magnesium ions from the hydrate structure, which have been confirmed by molecular dynamics simulations. Unfortunately, the use of hydrate structure technology in the treatment of wastewater have not been fully explored.

To this note, Dalian University of Technology researchers: Dr. Hongsheng Dong (Postdoctoral fellow), Lunxiang Zhang, Zheng Ling, and Professor Jiafei Zhao and Yongchen Song developed an experiment to investigate the removal of heavy metal ions in wastewater via hydrate process. Fundamentally, the experimental system entailed various phases: hydrate formation, solid-liquid separation, and hydrate dissociation. An X-ray computed tomography was used to analyze the hydrate samples and to identify the controlling factors and their influence on the removal efficiency. On the other hand, the trapped solutions were removed by performing a total of five solid-liquid separation methods due to the random hydrate growth process. Mechanisms of ion exclusion, like that used in geological exploration, was determined based on the electrical conductivity measurements. Eventually, the solid-liquid separation methods were optimized to investigate the initial concentration effects of the removal characteristics. The research work is published in the journal, ACS Sustainable Chemistry and Engineering.

In terms of volume, the hydrate slurry was composed of 58.13% hydrate, 23.07% air, 16.03% aqueous solution and 2.77% R141b. The authors observed that the removal efficiency was controlled by the separation of the hydrates and trapped solutions from the hydrate slurry. This enabled a three-dimensional analysis of the hydrate samples. Hydrate formation provided a conducive environment for direct substance migration and none-guest molecule exclusion. Additionally, the effectiveness of the combined vacuum filtration and the centrifugal method was confirmed for relatively high concentration wastewater treatment. For instance, the removal efficiency of 96.63% was recorded.

In summary, the research team proposed a hydrate-based pollutant removal method from wastewater based on the concentration gradient and direct substance migration theory. As a proof of the concept, they examined the pollutant removal mechanism by determining the concentration near the hydrate front. Interestingly, this enabled the optimization of the heavy metal removal process. Altogether, hydrate-based technology is a promising technology for wastewater treatments. In particular, the provided information will enable further improvements that will permit low-energy and intensive treatments of wastewater with complex compositions.