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“Hard-Soft Acid-Base Theory in Action: A New Ion-Exchange Material for Sequestering Heavy Metals”
The human body consists 75 percent of water, clean water is one of the prime elements responsible for life on earth. However, today many people drink water that is far from being pure. Inorganic minerals such as mercury (Hg), lead (Pb), and cadmium (Cd) are some of the powerful pollutants that make water unsuitable for human consumption and other living organisms. Over the years, a lot of effort has been gone into making drinking water as safe as possible by testing different methods to remove Hg2+, Pb2+ and Cd2+ ions from polluted water Some of the traditional ways of removing the above mentioned heavy metal ions is using oxidic inorganic ion-exchange materials such as Zeolites, clays and carbon activated adsorbent. Although these materials can remove heavy metals, they have a low selectivity and weak bonding affinity for heavy metal ions. Sulfide minerals such as FeS2 also have a low selectivity for heavy metals due to their property of instability in natural environment (i.e. when exposed to air and water it gets oxidized). To overcome these problems novel sorbents such as resins, organoceramics and mesoporous silicates as well as the recently noted mesoporous carbon material with thiol groups has been developed. However, these materials only showed a high selectivity for Hg2+. Similarly, Fe3O4 nanoparticles coated with humic acid also showed a reasonable but low selectivity for these soft heavy metals. On the other hand, unlike iron -based sulfides sulfide-based ion exchangers have a higher ability to remove heave metals ions regarding their functional group and surface property. This is due to their higher affinity of their soft basic framework for soft Lewis acids (e.g. Hg2+, Cd2+,Pb2+).
One of sulfide-based material that has been found to be a high candidate for heavy metal ion remediation is K2xMnxSn3-xS6 (x=0.5-0.95) (KMS-1). K+ existing as +2, Mn as +4, Sn as +6 and S as -2 oxidation states. The layer structure of this material is built up by edge-sharing “Mn/Sn S6 octahedral with Mn and Sn atoms occupying the same crystallographic position and all sulfur ligands being three-coordinated. K+ ions are found between the layers and are positionally disordered (Manos & Kanatzidis, 2009). This material contains highly mobile K+ ions in their interlayer space that can easily be exchanged with other heavy cations (Manos & Kanatzidis, 2009). KMS-1 is inorganic ion-exchanger that exhibits an excellent thermal, chemical and radiation stability in aqueous and atmospheric environments that can not be easily achieved with organic compounds. This material has previously been proved to be an excellent sorbent for strontium ions. Based on Manolis J. Manos and Mercoui G.Kanatzidis detailed research this material has a extraordinary capacity to remove Hg2+ Pb2+, and Cd2+ very rapidly from water than any ever-known sorbent materials and has a high selectivity that allows their concentration to be reduced to well below the government allowed safe drinking levels under broad pH range (Manos & Kanatzids, 2009). Based on this study this material’s structure allows a rapid ion-exchange kinetics of the intercalated K+ ions with soft Lewis acids and binds to these soft heavy metal ions through a strong covalent interactions Metal-Sulfide framework of KMS-1. The experiment of ion-exchange is done by isolating a filtered polycrystalline material from the mixture of A(NO3)2.yH2O (0.07mmol) (A=Hg, Pb, Cd) with 20ml of water and a solid KMS-1 90.07mmol, 40mg). The filtrates were analyzed for their heavy metal content by using a coupled plasma-mass spectroscopy (ICP-MS). The energy-dispersive spectroscopy (EDS) data of the study has confirmed the removal of K+ ions as well as the binding of the heavy metal ions. Two analyses were done to see how the interlayer spacing changed and to obtain information about the structural change after metal ion exchange material. These are the Power X-ray diffraction (PXRD) measurement and the Pair distribution function (PDF) analysis. PXRD data of Hg2+ exchanged material showed a decrease in the interlayer distance after the ion exchange. It changed from 8.51º® to 5.82º® this is because of the smaller size of Hg2+ compared to K+ as well as due to the strong covalent bond formed between Hg-S. This analysis also revealed the presence of two layered phases. These layers existed with interlayer spacing of 8.81º®-8.09º®. This information was also found in the two hydrated Pb2+ species analysis. Alkaline earth ions have a great tendency to be hydrated and this results for the Pb2+ exchanged materials. The Thermogravimetric analysis (TGA) data for exchanged samples revealed the presence of 1-2 H2O molecules per formula unit. The process of Cd2+ exchange was different than Hg2+ and Pb2+ processes. Hg2+ and Pb2+ exchanged only with K+ ions where as Cd+2 exchanged not only with k+ but with Mn2+ ions of the layers as well. The EDS data of KMS-1 showed no detection of Mn even using ICP Mn ion was not identified. The molar ratio of Cd2+: KMS-1 in the exchanged material was found to be ~2 with a formula of Cd1.8Sn2.1S6 and no sign of Mn2+ ion. Cd2+ exchange also yielded in a colour change from dark-brown to orange-red. The TGA data of Cd2+ exchanged material revealed the presence of partially hydrated Cd2+ cation ~1-1.5 water molecules per formula unit and the PXRD indicated the consistency of interlayer contraction ~2.2º® relative to KMS-1 strong Cd-S bonding interactions in the interlayer space (Manos & Kanatzidis, 2009).
Solid state near infrared-ultraviolet-visible (NIR-UV-Vis) spectroscopic studies was important to examine the intercalation of metal ions in pristine KMS-1. The expected covalent interactions between the sulfur atoms and intercalated cations are K
HSAB theory elaborates that soft acids prefer bonding with soft bases, and the adduct of the result tends to form a covalent bond. Equivalently hard acids prefer bonding with hard bases, and their adducts form a stronger bond called ionic interactions (electrostatics attraction). This study provides a practical application of HSAB theory concepts. It proved that HSAB theory can be useful to identify compounds that can potentially be used in predicting toxicant-target interactions and the bonding mode can be determined using the principle. The main purpose of the study was to explore or discover a material that can reduce or remove major water pollutants such as Hg2+, Pb2+ and Cd2+.This study experiment reported a sulfide layered metal material that can rapidly remove toxic heavy metals from water called KMS-1. As per HSAB rule sulfur is considered to be a much softer base element therefore it prefers to bond with soft acid (e.g. Hg2+, Pb2+ and Cd2+).
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