Title : Adsorption of chromium(III) and manganese(II) on PVA-PEO-TEOS membranes doped with gold nanoparticles: Equilibrium and kinetic study, analytical application for simultaneous speciation of chromium and manganese
Trivalent chromium and divalent manganese adsorption was studied in a batch mode with PVA-PEO-TEOS membranes doped with starch-coated gold nanoparticles (Au-Starch NPs) at 298.15 K. Equilibrium and kinetic behaviours, as well as analytical application of the hybrid nanocomposite membranes as a sorbent for simultaneous chromium and manganese speciation were the focus of this work.
The employed organic polymers poly(vinyl alcohol) (PVA) and poly(ethylene oxide) 400 (PEO) are not toxic and biocompatible in nature; tetraethoxysilane (TEOS) was used as a precursor for the inorganic polymer silica in the hybrid polymer matrix. Solutions of PVA, PEO and pre-hydrolysed TEOS were mixed with Au-Starch NPs dispersions and cast hybrid nanocomposite membranes were obtained under drying at 50 °C. The physico-chemical characteristics of gold nanoparticles and hybrid membranes were studied by XRD, TEM, SEM and AFM observations, FTIR and UV-vis spectroscopy, DSC analysis. Gold nanoparticles were uniformly distributed in the PVA-PEO-TEOS membranes without any aggregation. Even more the presence of Au-Starch NPs in the hybrid membranes remarkably increased their mechanical stability, the most probably because of their crosslinking role.
The extraction efficiency of PVA-PEO-TEOS/Au-Starch NPs membranes toward environmentally relevant Cr and Mn species was studied at various pH values. Results obtained indicate that adsorption process attained equilibrium in 12 hours at optimal pH 9. Cr(III) and Mn(II) selectively sorbed on the membrane surface, while Cr(VI) and Mn(VII) remained in the solution. The adsorption behaviour of Cr(III) and Mn(II) was studied by isotherm (Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (DRK)) and kinetics (pseudo-first order, pseudo-second order, sigmoidal) models. The adsorption data were best fitted by Langmuir model, following sigmoidal kinetic model including three stages: inducement, precipitation and equilibrium periods.
A simple and sensitive analytical procedure was developed for separation and speciation of Cr and Mn species in aquatic environment using dispersive solid phase extraction of Cr(III) and Mn(II) by PVA-PEO-TEOS/Au-Starch NPs membranes prior to Cr(VI) and Mn(VII) determination in the supernatants. After sorption at optimal conditions, the effluate solution was easily decanted and Cr(VI), as well as Mn(VII), was measured using ETAAS; the membrane was washed twice with doubly distilled water, dissolved in aqua regia, diluted to desired volume and Cr(III) and Mn(II) measured again by ETAAS. The determination limits achieved for toxic species Cr(VI) and Mn(VII) is 0.05 μg/L, which fulfils the requirements for their monitoring in surface water bodies under the demand of Water Frame Directive. In addition, if necessary Cr(III) and Mn(II) might be determined in the same sample with even lower determination limit of 0.005 μg/L. Relative standard deviation for all determinations varied between 6 to 11% for Cr and Mn concentrations in the range 0.05-50 μg/L. The analytical method developed was validated for Cr(VI) by parallel analysis using standard method based on diphenyl carbazide. The accuracy of Mn(VII) quantification was confirmed by added found method. The developed analytical method was applied to the simultaneous speciation of chromium and manganese in river water samples from national monitoring points.
Acknowledgements: We acknowledge the Bulgarian Scientific Fund, Project, DN19/10, “Smart speciation”.
- Ecofriendly method for synthesis of gold nanoparticles and their incorporation in hybrid organic-inorganic polymer matrix.
- Selective sorption of Cr(III) and Mn(II) onto self-standing PVA-PEO-TEOS/Au-Starch NPs membranes with promising results regarding simultaneous speciation analysis of chromium and manganese.
- Simple analytical procedure for selective determination of Cr(III)/Cr(VI) and Mn(II)/Mn(VII) in surface waters usable in routine analytical practice.