Application of Hard-Soft Acid-Base Theory

Application of Hard-Soft Acid-Base Theory By Manolis J. Manos and Mercouri G. KanatzidisReportFeven TeclemichaelHard-Soft Acid-Base Theory in Action A smart Ion-Exchange Material for Sequestering Heavy MetalsThe human body consists 75 percent of urine, clean water is one of the kick elements responsible for life on earth. However, today many people drink water that is far from universe pure. Inorganic minerals such as mercury (Hg), lead (Pb), and cadmium (Cd) be some of the powerful pollutants that crystallize water unsuitable for human consumption and new(prenominal) living organisms. Over the years, a lot of ride has been gone into making drinking water as safe as possible by interrogatory different methods to remove Hg2+, Pb2+ and Cd2+ ions from polluted water Some of the traditional ways of removing the above mentioned with child(p) surface ions is using oxidic inorganic ion-exchange existents such as Zeolites, clays and carbon activated adsorbent. Although these mate rials bear remove sour surfaces, they suck in a low selectivity and weak bonding kinship for punishing alloylic element ions. Sulfide minerals such as FeS2 alike have a low selectivity for heavy metals due to their property of inst mightiness in raw(a) 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 easy as the recently noned mesoporous carbon material with thiol stems has been developed. However, these materials only showed a blue selectivity for Hg2+. Similarly, Fe3O4 nanoparticles coated with humic acid also showed a reasonable and low selectivity for these sluttish 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 jakesdidate for heavy metal ion remediation is K2xMnxSn3-xS6 (x=0.5-0.95) (KMS-1). K+ living as +2, Mn as +4, Sn as +6 and S as -2 oxidation states. The work structure of this material is built up by edge-sharing Mn/Sn S6 octahedral with Mn and Sn atoms occupying the alike(p) crystallographic position and all sulfur ligands being three-coordinated. K+ ions are found between the layers and are positionally distract (Manos Kanatzidis, 2009). This material bears highly mobile K+ ions in their interlayer space that can easily be transfer with other heavy cations (Manos Kanatzidis, 2009). KMS-1 is inorganic ion-exchanger that exhibits an gauzy thermal, chemical and radiation stability in aqueous and atmospherical environments that can not be easily achieved with organic aggregates. This material has previously been turn out to be an excellent sorbent for strontium ions. Based on Manolis J. Mano s and Mercoui G.Kanatzidis detailed research this material has a extraordinary power to remove Hg2+ Pb2+, and Cd2+ very rapidly from water than any ever-known sorbent materials and has a high selectivity that allows their niggardliness to be reduced to well below the government allowed safe drinking levels under broad pH bunk (Manos Kanatzids, 2009). Based on this area this materials 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 brawny covalent interactions Metal-Sulfide framework of KMS-1. The experiment of ion-exchange is do 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) info of the dissect 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+ interchange material showed a decrease in the interlayer distance after the ion exchange. It changed from 8.51 to 5.82 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 form phases. These layers existed with interlayer spacing of 8.81-8.09. This information was also found in the two supply Pb2+ species analysis. Alkaline earth ions have a great tendency to be hydrated and this results for the Pb2+ interchange materials. The thermogravimetric analysis (TGA) data for exchanged samples revealed the presence of 1-2 H2O molecules per recipe 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 colouration 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 capsule 2.2 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 of the essence(predicate) to examine the intercalation of metal ions in pristine KMS-1. The expected covalent interactions between the sulfur atoms and intercalated cations are KPb(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 susceptibility of KMS-1, ion-exchange equilibration studies is performed using the batch method which is done in a V m ratio of 10001 at a path temperature of pH 5. The ICP-MS ascertain the initial and final densitys of the heavy metal ions. In order to have complete 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 heart of the cation adsorbed at the equilibrium conce ntration Ce (ppm), qm is the maximum adsorption capacity of the adsorbent, and b (L/mg) is the Langmuir constant cerebrate 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*104-3.90*105 mL/g and 1.29*105-1.40*106 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+ usance 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 monstrousr 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 extravagance may vary in acidity. The Hg2+ ion exchange study of KMS-1 indicated a significant uptake at pH4(Kd=1.1-1.3*104mL/g) compared to at pH2.6 (2.3*105mL/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 acidulent shape (pH=0). For comparison, thiol-functionalized sorbents displays a wrong of 40-50% of their Cd2+ adsorption capacity at 3+ 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 resolves 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 unexceptionable 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 assimilation of heavy metal ions by KMS-1 was observed by sonicating them for 30-60min when the particle size of pristine samples was reduced t o 5m. 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 shove off water that contains traces of heavy metal ions. Mg-analogue of KMS-1 is developed to condone the concerns regarding Mn leaching (0.3-0.8% of the total manganese content at pH5-8) 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 formulate d to see if the exchanged materials can be considered as permanent waste forms without the need of secondary discussion. 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 450C 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 (pH3) and alkaline condition (pH9), and highest for Cd2+ among all other state-of-the-art sorbents even at pH0. However, thiol-functionalized mesoporous silicates resulted in a low compactness for Pb2+ at pH2 (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 regenerate 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 th eir 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 cite 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 account 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 so it prefers to bond with soft acid (e.g. Hg2+, Pb2+ and Cd2+).