A complementary eco-friendly approach to heavy metal removal from wastewater/produced water streams through mineralization

Abstract

With the world's ever-increasing population, portable water needs have significantly increased. This has put enormous pressure on the limited supply of freshwater around the globe and has necessitated the development of water treatment technologies that would convert saline waters into potable water for domestic and industrial uses. As saline water is treated for usage, so is wastewater generated from domestic and industrial processes. More prominently, another wastewater source is produced water consequent to oil production. Wastewater and produced water are similar in that they are both high in pollutants (heavy metals), which in discharge streams need to be kept within a threshold. The fact that heavy and rare earth metals linked to the produced water cannot be completely removed by state-of-the-art technologies is even more concerning. Thus, a complementary technology must be developed for this reason. This study investigates the mineralization of water-polluting heavy metals. The approach adopted involves the exposure of contaminated water to atmospheric or captured CO2 and the use of additives to achieve the removal. To gain a deeper understanding of how these innovative efforts function, the study also looked into the mechanism used to eliminate the heavy metals. The findings show that heavy metals in produced water and industrial effluent can be precipitated and immobilized to remove them. More so, this is made possible by the infusion of heavy metals into the crystal structures of precipitates like aragonite, calcite, and halite. Furthermore, the mechanism of the heavy metal removal involves their dehydration, immobilization and precipitation. Additionally, DFT calculations were utilized to support the experimental results by shedding light on the heavy metal hydration's ground-state structures and how EDTA chelates the heavy metals and enhances the process kinetics. The study's findings show how this strategy complements the most advanced wastewater treatment method in terms of increasing its efficiency and water security and safety. More so, as this is a pioneering effort, the optimum concentration of chelating agent and route (atmospheric or captured CO2) must be determined on a case-to-case basis for field implementation. Furthermore, the limitation of this study is that the efficiency of heavy metal removal may differ based on the starting brine composition, thus, optimization must be conducted beforehand.