SPINEL FERRITES AS NANO-ADSORBENTS OF HEAVY METAL CATIONS FROM WASTEWATER EFFLUENTS IN CHEMICAL PROCESS INDUSTRIES

Abstract

This thesis focuses on the application of spinel ferrites as nano-adsorbents for heavy metal cation from water. Three different spinel ferrites namely Li0.5Fe2O4, CuFe2O4 and Fe3O4 have been synthesized via sol-gel citrate nitrate autocombustion technique. Structural characterization was carried out using XRD, and FTIR. They indicated successful synthesis of nano-spinels as a single phase. FESEM, AFM, and HRTEM imaging techniques ascertained nanostructure of the synthesized spinel ferrites. Assessment of nanoparticle surface area was possible through BET technique which indicated a high enough surface area to serve as nano-adsorbents. Zeta potential measurement as a function of solution pH, indicated that isoelectric points i.e.pH_PZC, lie in the range 7.0-7.5. Spinel ferrite nanoparticles have positive surface charge distribution for pH < pH_PZC, while they have negatively charged surface for pH >pH_PZC. Studying magnetic and electrical properties of the synthesized spinel nano-ferrites revealed that they are soft ferromagnets with semiconducting-like behavior.

Using batch adsorption equilibrium experiments, the adsorptive behavior of the synthesized spinel ferrite nanoparticles, was investigated on prepared solutions of Pb2+, Cu2+, and Ni2+metal cations. The effect of various factors which can alter adsorption process have been studied. For the effect of contact time, adsorption initially proceeds rapidly up to 40 minutes then slowed down till an equilibrium was reached at about 60 minutes. For initial metal concentration, adsorption process increased with increasing metal cation concentration untill a plateau was reached due to saturation of active sites on nano-adsorbent surfaces. Increasing solution temperature increased the adsorption of Pb(II) while decreased the adsorption of Ni(II) and Cu(II). Thermodynamic analysis indicated that adsorption process was endothermic for Pb(II) and exothermic for Ni(II) and Cu(II). Variation of nano-adsorbent dosage resulted in a variation in quantity of adsorbed metal cation with a peak value at about 200 mg nano-adsorbent per 50 ml solution. Metal cation adsorption was also found to be affected by solution pH. Adsorption increased with increasing pH in the range 7-9. However, at lower or higher pH values, the adsorption decreased due to alteration in either metal speciation or nano-adsorbent surface charge distribution or both. Equilibrium adsorption data was analyzed using Langmuir, Freundlich isotherm models. It was found that Langmuir isotherm model best fits experimental data with large correlation coefficients. Kinetic modelling showed that experimental data fitted well with pseudo-second-order kinetic model. The intraparticle diffusion model showed two steps where intraparticle diffusion was not the only rate controlling step of adsorption process. A second phase of the study was conducted in which theoretical modelling of the sedimentation behavior of spinel ferrite nanoparticles in aqueous liquid dispersion, in presence of magnetic field gradient, has been analyzed. Various cases have been studied from which settling velocity of magnetic nano-adsorbents under the effect of external magnetic field, have been derived. Based on the obtained models, a suitable design for magnetic field actuated circular settling tank was proposed including detailed design of the associated magnetic circuit. This new equipment is recommended to be used as a tertiary wastewater treatment unit for removal of heavy metal cations from industrial wastewater.