Practical Materials for Heavy Metal Ion Chelation: Polyethylenimines tailored onto The Surface of Porous Silica

INTRODUCTION As a result of heavy metal ion release from industrial wastewater, water pollution has become a serious problem. Waste streams contain solutions of metal ions, such as copper, nickel, cobalt and lead. Many reviews have discussed the removal of metal ions from aqueous solutions using approaches based upon adsorption, precipitation, solvent extraction and polymeric membranes. Recently, researches have reported the application of trimethoxysilylpropyldiethylenetriamine-functionalized mesoporous silica gels for the removal of copper ion from jet fuel. In contrast to the other methods, the process relied on coordination bonding to the metals. Thus the approach featured advantages over those used in other methods because of its good selectivity for metal ions and sludge-free operation. It followed that a similar polyamine approach could be applied to metal ion-containing aqueous solutions using silica gel coated with polyethylenimines. The principle enabling polyethylenimine to readily transform large-pore silica in passing along its surface is that the bond formed, albeit non-covalent, is practically permanent due to the size of polyethylenimine and its numerous possible surface interactions. This approach therefore described a cost-effective alternative in comparison to the use of an organosilane precursor and was tested in the scavenging of aqueous heavy metal ions. For comparative purposes, a standard silica gel scavenger material was synthesized using trimethoxysilylpropyldiethylenetriamine out of aqueous alcohol solution. Again for comparison, polyethylenimine was coated onto active carbon, however, its performance could not be readily assessed visually. Thus the synthesis was extended to the glutaraldehyde stage and used to immobilize catalase. Colorimetric ninhydrin analysis, FTIR, BET, CP-MAS NMR methods were used to characterize the prepared adsorbents. The feasibility of these materials for application to heavy metal-ion removal from aqueous solutions will be assessed. SUMMARY Silica gel was incubated in a dilute solution of polyethylenimine reagent and dried at elevated temperature. Following incubation, an apparently irreversible, non-covalent adsorption of the compound was verified. The hydrolysis products of trimethoxysilylpropyldiethylenetriamine reagent were also applied for comparison and afforded silica gel covalently attached onto the surface. Surface characterization using Brunauer-Emet-Teller analyses indicated that accessible areas were reduced following modification. Colorimetric analyses using ninhydrin also showed that surfaces were overlaid with amino groups. Carbon-13 CP-MAS NMR analyses identified repeating units of ethylenediamine upon the silica surface and silicon-29 CP-MAS analyses identified the surface atom originally from trimethoxysilylpropyldiethylenetriamine. NMR attempted on polyethylenimine-overlaid silica revealed that silicon-29 spectra differed significantly from the native silica in terms of line shape and peak intensity. Functionalized silica was used successfully to chelate metal ions such as chromium, copper, cobalt and nickel in aqueous media. Native surfaces in contrast displayed no ability to retain metal ions. While diethylenetriamine-based metal-ion scavengers have been reported, the polyethylenimine-based synthesis appears unprecedented. Both syntheses proved versatile, as active carbon surfaces were similarly tailored. Following glutaraldehyde activation, catalase enzyme was coupled thereon, affording to our knowledge the first covalently immobilized enzyme upon surface-tailored active carbon.