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Application of Hard-Soft Acid-Base Theory

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By Manolis J. Manos and Mercouri G. Kanatzidis


Feven Teclemichael

"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<Pb<Hg which is reflected in the calculated band gap energies with the order of KMS-1>Pb(exchanged)>Hg(exchanged). The Cd2+ exchanged material band gap energy was measured to be 1.96ev; this result is consistent with its colour change from dark brown to orange-red. To assess the Hg2+, Pb2+and Cd2+ removal capacity of KMS-1, ion-exchange equilibration studies is performed using the batch method which is done in a V: m ratio of 1000:1 at a room temperature of pH 5. The ICP-MS determined the initial and final concentrations of the heavy metal ions. In order to have enough metal ions to saturate the exchange sites of K2xMnxSn3-xS6 (x=0.95) (the molar ratio M2+/KMS-1 was ~1), the initial concentration of Hg2+ and Pb2+ was much higher than Cd2+ since they can decompose to HgS or PbS unlike Cd2+. The Hg2+ and Pb2+ ion-exchange equilibrium data was fitted with the Langmuir isotherm model expressed as , where q (mg/g) is the amount of the cation adsorbed at the equilibrium concentration Ce (ppm), qm is the maximum adsorption capacity of the adsorbent, and b (L/mg) is the Langmuir constant related to the free energy of the adsorption. The maximum ion-exchange capacity qm of KMS-1 (x=0.95) was determined to be 377 mg/g and 319 mg/g, respectively. The affinity for the metal ions can be expressed in terms of the distribution coefficient Kd value. Kd coefficient describes the sorption/desorption propensity of a compound for a material. For Hg2+ and Pb2+ the Kd values were found in the range 3.50*10^4-3.90*10^5 mL/g and 1.29*10^5-1.40*10^6 mL/g, respectively. The equilibrium exchange data of Cd2+ was fitted with the Freundlich model: q= KfCe(1/n), where Kf is the Freundlich constant. The maximum capacity was calculated by averaging Cd2+ uptake values that corresponds to the saturation of the exchange sites of KMS-1 and it was found to be 329mg/g or 2.93mmol/g which is close to the theoretical value of 3.18mmol/g. The Kd value obtained for Cd2+ was 1.16 to 1.37*10^7mL/g which is larger compared to the initial concentration between 204.4 and 136.3ppm. The effect of pH on Hg2+ and Pb2+ adsorption was studied in the range of 2.6-9.4 and Cd2+ adsorption was tested in the pH range of 0-9, while taking into account that the pH of contaminated ground water and nuclear waste may vary in acidity. The Hg2+ ion exchange study of KMS-1 indicated a significant uptake at pH>4(Kd=1.1-1.3*10^4mL/g) compared to at pH~2.6 (2.3*10^5mL/g). For Pb2+ the maximum Kd value calculated for KMS-1 is at pH 3.7. The Kd value for Cd2+ revealed a remarkable affinity of KMS-1 under strong acidic condition (pH=0). For comparison, thiol-functionalized sorbents displays a loss of ~40-50% of their Cd2+ adsorption capacity at 3<pH<4. The effect of high background electrolyte on the absorption capacity of sulfide ligands in KMS-1 with ions like Na+ and Ca2+ was also examined for selectivity. The results showed a high selectivity for Hg2+ and Pb2+ even for Cd2+ since KMS-1 showed 86-88% removal of Cd2+ removal in the presence of 1M Na+ or Ca2+. Competitive-exchange Hg2+, Pb2+ and Cd2+ -Na+ experiment was performed in a very high or very low initial concentrations, these cations showed that KMS-1 has the ability of removing all 3 metal ions from solutions and shows a similar selectivity for solutions that contain a mixture of Hg2+, Pb2+ and Cd2+ in low initial concentrations. In final concentrations of the metal ions were found to be well below the acceptable levels for drinking water (Manos & Kanatzidis, 2009). To check how capable is KMS-1 to select heavy metal ions under realistic environment, it was tested with drinkable water that has a pH of 6.5 and was contaminated intentionally with high levels of Hg2+, Pb2+ and Cd2+ that has excess amount of Na+, Ca2+, Mg2+ than the heavy metal ions. The results showed that within 40 min KMS-1 lowered the concentration of Hg2+, Pb2+ and Cd2+ below their acceptable limits. A high absorption of heavy metal ions by KMS-1 was observed by sonicating them for 30-60min when the particle size of pristine samples was reduced to ≤5µm. This finding shows that pre-treated KMS-1 samples have a high ability to reduce the concentrations of heavy metal ions well below their acceptable levels after 2 min of solution/KMS-1 contact. These results prove that KMS-1 is highly capable of selecting and filtering contaminated waste water that contains traces of heavy metal ions. Mg-analogue of KMS-1 is developed to explain the concerns regarding Mn leaching (0.3-0.8% of the total manganese content at pH~5-8) during the use of KMS-1 to purify waste water. The analogue developed is denoted as KMS-2 (i.e. K2xMgxSn3-xS6 (x= 0.5-0.95)). The study indicated that KMS-1 and KMS-2 have not different capacity to purify heavy metal ions from water and have identical PXRD pattern. According to the study supporting information Mg2+ is non-toxic and a large level ppm of Mg2+ in water is acceptable. Since regeneration of exchanged materials is not possible under highly acidic environment of KMS-1 compounds, a test can be formulated to see if the exchanged materials can be considered as permanent waste forms without the need of secondary treatment. The study shows that first treatment results revealed no leaching of Pd2+ after its hydrothermal treatment at pH of ~7 or 4.8 for 24hrs. Similarly Hg2+ and Cd2+ only showed 0.05 and 0.09% of leaching, respectively. Whereas, the thermal treatment of Hg-laden samples for 60hrs at 450°C showed 93% of leaching which is almost all the Hg2+ content has been regenerated. This process can be used to recover mercury element. This study showed the high efficiency of KMS-1 to absorb heavy metal ions and proved that it is one of the only materials that has a high capacity for Hg2+, Pb2+ at acidic condition (pH~3) and alkaline condition (pH~9), and highest for Cd2+ among all other state-of-the-art sorbents even at pH≤0. However, thiol-functionalized mesoporous silicates resulted in a low absorption for Pb2+ at pH<5. If the efficiency in absorptivity of heavy metal ions by KMS-1, LiMoS2 (layered sulfide) and thiol-functionalized sorbents is compared, KMS-1 has the highest because it is stable in water and atmosphere, on the other hand LiMoS2 and thiol-functionalized have less absorption capacity because they have instability nature under aerobic conditions. KMS-1 is a sulfide layered metal that exhibits a high capacity and highly specific ion-exchanger for the removal of soft heavy metals by replacing K+ in between the metal sulfide layers of KMS-1. The driving force for heavy metal ion-exchange is the strong heavy metal ion-sulfur bonds in addition to the facile ion diffusion and access of all internal surfaces of layered metal sulfides. It is a low-cost promising material that can be used to purify waste water by reducing the concentration toxic heavy metal ion (i.e. Hg2+, Pb2+ and Cd2+) well below acceptable limits for drinking water.

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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