EFFECT OF ELECTROLYTE ANIONS ON THE SORPTION PROPERTIES OF MANAGANESE DIOXIDE

Abstract

Manganese oxides are important scavengers of trace metals like Pb, Cu, Co, Ni, Zn, Cr and other contaminants in natural environments because of their ubiquitous presence in clays, soils and sediments. They have high surface area and strong affinity for many elements, thus their surfaces mediates the fate and transport of metallic ions and their compounds in many natural systems. Although the concentration of manganese oxides in soil and sediments is less than the oxides of iron and aluminum, yet, their effective surface charges, enable them to effectively alter the distribution of trace metals in soils, sediments and natural water systems by adsorption/ion exchange mechanism. Being a model adsorbent many researchers have focused on the sorption properties of manganese oxide not only due to its importance in soil and sediments, but also due to its applications in too many industrial processes. Manganese oxides exist in many geological forms in nature, such as birnessite, pyrolusite, cryptomelane and ramsdellite. Among them only pyrolusite and ramsdellite are the most stable forms having true modifications of MnO 2 whereas all others are nonstoichiometric and may cover a relatively large range of compositions. Therefore, the present study reports the characterization of pyrolusite (- MnO 2 ) along with its sorption properties for Cd, Pb, Co and Cu as affected by the anions of different electrolytes and most importantly phosphate which is thought to be an important nutrient in soil for plants and greatly affects the chemical reactions of metal cations and their complexes on mineral surfaces. Therefore, investigations involving the characterization and sorption properties of Manganese oxide (pyrolusite) for heavy metal cations like Cd, Pb, Cu and Co, become particularly important from environmental as well industrial point of view. Manganese oxide (Pyrolusite), purchased from Merck, has been characterized for Surface Area, Point of Zero Charge ( pHpzc), XRD, TG-DTA, TEM, SEM, EDX, XPS, Particle Size Measurement and FTIR analyses. ii Its BET Surface area is 83.5 m 2 /g withaverage pore width and micropore volume of 142.2 Å and 0.07 cm 3 respectively, which indicate the mesoporous nature of the solid. The point of zero charge of solid is 8.8, which decreases with phosphate treatment to 6.2 by increasing the concentration of phosphate from 0.001 to 0.1M, due to the formation of inner sphere complexes at the surface of the solid. The XRD analysis shows that the solid is crystalline in nature. TEM and SEM images also confirm the solid to be crystalline having nanorod-like structure with an average width of 0.64±0.2 μm and particle length between 0.91 to 2.960 μm suggesting the particle size diversity of the sample. TG-DTA analyses reveal that the solid is stable in the temperature range of 30-600 o C, while above 600 o C, MnO 2 changes into Mn 2 O 3 . Dissolution study of manganese dioxide in the presence of different electrolyte anions suggests that the solid is stable in the pH range 4-7. Its dissolution is maximum at pH3 and decreases with increasing the pH of the aqueous system. Further, at each pH value, dissolution of the solid is less in the presence of phosphate as compared to nitrate and sulphate anions, indicating the hydrolytic stability that phosphate anions impart to the solid via surface complexation reactions. Metal ions sorption studies onto manganese oxide, as a function of pH, temperature and phosphate concentration, suggest that sorption of all the metal ions increases with increasing pH, temperature and phosphate anions treatment as compared to nitrate. This trend of metal ions sorption is due to the fact that phosphate anion shifts the pH edges to lower pH values and hence, sorption of Cu and Pb in phosphate starts even at the lowest pH value of 3. Similarly, increase in temperature also increases the sorption capacity of the solid by creating new sites and increasing the mobility of the ions at the solid-liquid interface. It has been observed that the sorption of metals in nitrate follow the order; Cu2 + >Co 2+ > Pb 2+ >Cd 2+ , which changes into Pb 2+ > Cu 2+ >Co 2+ > Cd 2+ in the presence of phosphate. Langmuir equation shows the appropriate applicability to describe the sorption data and the constant X m increases with increase in pH and temperature showing the endothermic nature iiiof the sorption process. Similarly, the sorption of each metal cation in the presence of phosphate increases at each pH value which suggests that phosphate anions facilitate the sorption of these cations at each pH unit. From the Langmuir’s binding energy constant, the respective thermodynamic parameters including H o ,S o and G o have been derived. The values of ΔH o for Cu 2+ and Cd 2+ ions sorption are negative at lower pH and become positive at pH5 and 6 in the presence of phosphate anions. This shift from negative to positive values points toward the change in sorption mechanism from ligand-like complexes at low pH values to metal-like complexes or metal phosphate precipitation at higher pH values. Similarly, the negative values of ΔG o indicate the spontaneous nature of the sorption reactions. Desorption studies have also been conducted for Pb 2+ , Cu 2+ , and Cd 2+ ions in the presence of different electrolytes in the range 293-323 K. The desorption of metal ions in nitrate and sulphate has been observed to follow the order; Co 2+ >Cd 2+ >Cu 2+ >Pb 2+ . However, the desorption of these metal in phosphate are very low due to the stability of lead phosphate precipitates formed at the MnO 2 surface. In the present study the sorption kinetics of Cd 2+ has also been evaluated at pH 6 in the temperature range 293 -323K. This kinetic data suggest that sorption of Cd 2+ ion increases with contact time and temperature and the system attained equilibrium within 60 min in the presence of nitrate. However, equilibrium time is shifted to 90 min in the presence of phosphate anions. The rate constant k and initial sorption rate h calculated from pseudo second order kinetics model increase with increasing temperature and phosphate treatment. The thermodynamic activation parameters such as activation energy Ea, H ‡ , G ‡ and S ‡ show that the sorption process is endothermic and nonspontaneous, with a decreased free energy of activation, being 15.95 kJ.mol -1 in nitrate and 8.76 kJ.mol -1 in phosphate anions. These low ivvalues of activation energies in both the cases suggest diffusionally controlled uptake of the metal ions by ion exchange or ligand like mechanism. Microscopic and spectroscopic analyses reveal the formation of a new phase in the form of metal phosphate precipitates at higher pH values while formation of ligand like complexes at low pH values. SEM and TEM images demonstrate the appearance of new homogenous solid phase along with the nonorod like structure of manganese oxide particles while EDX spectra shows some additional peaks for metal ions and phosphate after metal ions sorption. FTIR studies shows some changes in the frequencies and intensities of the –OH group vibrations after phosphate anions sorption. The appearance of peaks at 1740 and 2904 cm -1 are far mono and dibasic orthophosphate respectively. The manganese oxide after metal ions sorption in the presence of phosphate also shows a decrease in the intensities of the bands at 740 and 1116 cm -1 while the broad band at 1315 cm -1 disappears completely pointing toward the formation of ligand like metal complexes at the surface of the solid. These observation provide a strong evidence that the local environment of -OH groups present at the surface of manganese oxide changes with the amount of metal ions incorporation and thus are responsible for metal ions uptake from solution. The XPS shows that the positions of Mn2p and O1s of the manganese oxide remain the same after metal ions sorption. However, various photoelectron peaks after metal ions incorporation at different binding energy appears, like 138.3 eV and 143.8 eV for lead, and 137 eV and 142.1 eV for phosphate, which confirm the mechanism of the ligand like metal complexes and formation of different types of lead phosphate precipitates on the surface of manganese oxide.