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[MatSQ Column] #013. Application of Computational Science to the Removal of Heavy Metal Ions from Water

2021-04-26 09:43:31

1) Water Pollution

Water is an essential element for human survival. Inversely, pollution sources in drinking water can easily flow into the human body, causing deaths or fatal diseases. Therefore, poor access to safe drinking water has been recognized as an urgent issue worldwide, especially by many countries suffering from drinking water contamination and water scarcity. According to statements by the United Nations Environment Programme (UNEP) in 2016, more than 300 million people drink contaminated drinking water, while 3.4 million die every year from waterborne diseases. Several water pollutants include nutrients, organic matter, oil, metals, and pesticides, some of which occur naturally. However, despite these multiple naturally occurring sources, the primary source of pollution stems from human activities (industrial effluent, domestic sewage, and agricultural/livestock wastewater). The rate at which these human sources produce pollution is steadily on the rise. [Figure 1]


[Figure 1] Various water pollutants: industrial wastewater, domestic sewage, agricultural and livestock wastewater



2) Adsorptive Removal of Heavy Metal Ions

Among sources of water pollution, heavy metals, such as arsenic, chromium, and lead, are easily detected in industrial wastewater and natural environments, such as mines, groundwater, and rivers or streams. Even trace amounts of heavy metals in drinking water can result in lethal and permanent damage to the human body. Thus, drinking water sources should be managed more thoroughly. Because heavy metals are not metabolized by the human body once entering the body, continued exposure at low concentrations can give rise to various diseases or disorders. Most of the toxic heavy metals are usually present as complex ions with sizes of up to 1 nm in natural waters; thus, applying general filtration methods is not appropriate for the removal of heavy metals. Instead, methods by means of chemical reactions with heavy metal ions, such as ion exchange, adsorption, precipitation, and biological treatment, are utilized. Above all, adsorption is the most widely available technique because by-products are not created, and post-treatment is relatively simple. As the key to this method, the adsorbent needs to meet requirements, such as economic efficiency, environmental friendliness, and the ability to adsorb and remove heavy metal ions, to ensure the effectiveness of this method. In addition, it is highly likely that heavy metal ions already adsorbed can be desorbed depending on changes in the surrounding conditions, including temperature, acidity (pH), interactions with other ions, and more, consequently causing reverse environmental contamination. Thus, it is also important to understand and control the desorption conditions. Careful consideration should be given to interactions between heavy metal ions and adsorbents, and the impact of the surrounding environments to develop adsorbents that satisfy these requirements.

The adsorption mechanism of toxic gases is known relatively accurately considering both the surface of the adsorbent and toxic gases (adsorbates) in the air mostly feature stabilized forms. However, molecules in water strongly interact with each other, thus existing as diverse molecules or ions (or complex ions). The adsorbent’s surface structure also changes depending on underwater conditions, which, in turn, transforms the adsorption mechanism and the zeta potential of the adsorbent. For example, chromium has different complex ions according to pH values [Figure 2] and is adsorbed onto carbon materials through several steps [Figure 3]. As mentioned above, numerous factors act on liquid-phase adsorption, making it difficult to fully understand the adsorption mechanism despite continuous advances in experimental measurement and analytical techniques.


[Figure 2] Various structures of complex ions according to the underwater environment


[Figure 3] Multi-stage adsorption reaction of Cr and addition of functional groups of adsorbent by interaction with the solution



3) Computational Science

Over the past decades, computational science has achieved rapid growth because of the well-established theory of computation and the evolution of computational power. Most of all, density functional theory (DFT) is a method intended to calculate the electronic structure and, accordingly, understand material properties (optical properties, vibration, structure, conductivities, etc.) and material interactions based on quantum mechanics. Thus, it ensures highly accurate simulation results, for this method is particularly effective in investigating diverse nanoscale natural phenomena; therefore, its applications had steadily expanded since the 1980s, when the research began in earnest [Figure 4]. DFT techniques have recently revealed remarkable outcomes in energy storage and catalytic reaction simulations, in addition to their earlier achievements in physics, such as predicting physical properties or identifying the causes. Despite those successful applications, DFT has the limitation of calculating only thousands of atoms because of the rapidly increasing computing costs commensurate with the size of advanced computing systems.


[Figure 4] Increasing trend of papers published with the keyword “DFT”



4) Combining Experimentation and Computational Science

As mentioned above, the liquid-phase adsorption mechanism involves diverse reactions, so it is challenging to understand the effects of each reaction on the final adsorption step. In particular, toxic ions, such as heavy metal ions, are so reactive and unstable that these ions further increase the complexity of the adsorption mechanism. Hence, the application of DFT to independent analysis of each reaction in the overall adsorption mechanism may be effective. Note that the DFT method can only deal with minimal space, so calculations should be performed after considering experimental analysis results or situational models. Conversely, the interpretation of experimental analysis or measurement involves uncertainty, which requires DFT-based measurement and analysis to be conducted to acquire reasonable analysis results. Therefore, joint research between experimental measurement/analysis and computational science must be carried out while mutually complementing each other to ascertain complicated liquid-phase chemical reactions, such as the adsorptive removal of heavy metal ions, as shown in Figure 5.


[Figure 5] Computation-experimental joint research through mutual complementation




Ph.D. Keunsu Choi

UNIST, Research Professor


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