Effect of Cu2+ Concentration on Adsorption - Sorption Mechanisms, Modes, Critical Concentration Edge, and Spontaneity of Octahedral [Cu (H2O) 6]2+ on γ Alumina

Abstract

Aims: To apply a fast, & save method for water treatments by using physical removal of toxic metal ions such as Cu2+ by adsorption at solid insoluble, non toxic, reusable adsorbent as γ Alumina. to indicate the effects of Cu2+ concentrations ranges on the adsorption modes, mechanisms,  maximum efficiency, and the thermodynamic parameters indicating spontaneity of the processes. As we have to get strong adsorption to avoid desorption of ions to solution but the low apparent adsorption spontaneity was contradicted (not inconsistent) with the produced high strength of adsorption) encourage us to determined the true free energy change for each adsorption mode by studying the adsorption mechanism and energy relations of each adsorption mode. Place and Duration of Study: Benghazi University, Chemistry Department, Benghazi, Libya between Sep 2009 and Dec 2011. Methodology:  Solutions of concentrations from, 1.5 to 15.5 x10-3M of Cu2+ which resembling that of industrial, agriculture, and waste water. Cu2+ form cupric water octahedral complex ion, (CWOCI) [Cu(H2O)6]2+ was prepared by dissolving in buffer solution to be maintained at pH = 4.5. The  batch technique of adsorption test was followed in testing 50 ml of each test solution which was simultaneously shacked with 0.5 gram adsorbent ( γ Al2O3 ) for 60 minutes with shacking rate of 300 cycle per minute using water thermostat shaker at 30ºC. The amount of Cu2+ adsorbed was determined by measuring the difference of its concentration before, and after adsorption by measuring the optical density of the blue [Cu (NH3)4)]2+ at pH=11 formed by adding NH4OH to Cu2+ solution, using spectroscopic method  with a beam of wave length  580nm. Results: The results were applicable to Langmuir, and Ismaeel adsorption isotherms all over this concentration range but it show a deflection of Langmuir line, and deflection with splitting of Ismaeel line at the same equilibrium concentration edge (CE)= 3.3 x 10-3mole  which divided  the isotherms into two low, and higher concentrations ranges. The same divisions were occurred also to the lines of the relations of the solution concentration versus adsorption rate, the surface coverage fraction θ, the adsorption efficiency % and spontaneity indicating two modes and mechanisms of adsorption before and after CE. Conclusion: We have to avoid applying adsorption processes on concentrations higher than the CE, by using dilution with pure water to just lower the concentration than CE or by increasing the mass of adsorbent. To gain maximum (adsorption- sorption), Spontaneity, Efficiency, Rate and Surface coverage %. Applying Ismaeel adsorption isotherm, enable to know the adsorption modes and activation energy used to release and replace the water molecules already adsorbed on adsorbent surface, and determining the true value of adsorption free energy change which identify the adsorption type as, chemisorptions confirmed by the high adsorption spontaneity as ∆G true reached more negative than -296 KJM-1. The length of the long axes of (CWOCI) was exactly calculated from experimental results, which was difficult to be determined by (XAFS) technique.