Synthesis & characterization of polymer-layered silicate nanocomposites and their evaluation as sorbents for lead and cadmium in water pollution remediation
Zeolites employ the principle of surface adsorption of toxic heavy metals in remediation of polluted water. However, at nanoscale, they tend to aggregate which reduces their effectiveness in adsorption. Incorporating suitable polymers like cellulose into their crystal lattice can improve their adsorption capacity by reducing the aggregation effect. The general objective of this study is to assess the capacity of fabricated cellulose-zeolite nanocomposites to remove lead and cadmium ions from spiked water samples. The composites were synthesized through melt blending and solution mixing. After mixing and annealing at 160oC, a composite lump was obtained which was pulverized to 150μm mesh-size. Scanning Electron Microscopy, Energy Dispersive X-Ray Spectroscopy, Fourrier Transform Infrared Spectroscopy and Powder X-Ray Diffraction Crystallography were used to assess the morphology, elemental composition, bonding and crystallinity of the nanocomposites respectively. Fourrier Transform Infrared Spectroscopy showed evidence of bonding between the residual silanol groups of the zeolite and hydroxyl groups of the cellulose chain. Scanning Electron Microscopy showed that porosity in the nanocomposites increased with increase in the zeolite loading. Powder X-Ray Diffraction Spectroscopy showed a highly crystalline spectrum for the pristine zeolite. However the zeolite peaks reduced with the increase in the ratio of cellulose. Also, with nanocomposite formation, there was a reduction of the 2 theta values to lower angles due to intercalation and an increase in the d-spacing. Cadmium and lead metal solutions were used to assess the viability of the composites in water remediation. It was noted that a maximum of 98.87% of Lead ions were adsorbed using 20% zeolite nanocomposite while 85.30% of Cadmium ions were adsorbed using 40% zeolite nanocomposites. The time-dependent adsorption experiments for both heavy metals in solution favoured a pseudo second order kinetic model. Contact time experiments were conducted for 5, 10, 20, 40 and 75 minutes. Lead ion uptake took place at an optimum pH range of 7. Maximum cadmium removal took place at a pH of 10 because of precipitation of its metal hydroxide. Thermodynamic parameters were determined for both metal ions over 298, 323 and 343K. The enthalpy of adsorption (ΔHAds) for Lead and Cadmium ions was 37.87kJ mol-1 and 41.43kJ mol-1, respectively. The adsorption entropic parameters (ΔSAds) for both Lead and Cadmium were 150.65Jmol-1K-1 and 142.40Jmol-1K-1, respectively. Regeneration studies for repeated adsorption by the nanocomposites were also carried out. Lead adsorption reduced from 91.47% for the first adsorption to 46.67% for the second adsorption while Cadmium adsorption reduced from 67.70% to 35.50%. The Langmuir Isotherm showed the best fit for both metals with R2 values of 0.985 and 0.996 for Lead and Cadmium ions respectively. The zeolite-cellulose acetate nanocomposites can be applied in metal removal from water and could be up-scaled and commercialized into membrane or granular based domestic water purification platforms.