Optical monitoring of adsorption of chromium (VI) molecular anions, molecules and clusters on carbon nanomaterials

Abstract

Search for efficient adsorbent materials for removal of hexavalent chromium compounds from environment is a topical technological problem. In this paper, the possibility of optical monitoring of the adsorption of chromium (VI) molecular compounds on the surface of carbon nanomaterials (CNM) is examined. Theoretical calculations of optical absorption spectra of CrO42- anions and molecular clusters of such anions with metals, n*K2CrO4 (n = 1, 2, 4) and MIICrO4 (MII = Zn, Cd, Pb) adsorbed on the surfaces of bare and doped carbon nanotubes and graphene are carried out by TD-DFT method. Results of calculations are compared with experimental data on optical transmission, diffuse reflection and photoluminescence of specially prepared samples containing chromate molecular compounds and particles adsorbed on CNM.It is shown that adsorption of CrO42- anions by CNM can be detected by the appearance of characteristic bands in diffuse reflectance spectra in 310–380 nm region. Nanoparticles of K2CrO4 crystal adsorbed on CNM reveal an additional luminescence band in 600–700 nm region, and it can be utilized in the monitoring of such adsorption. The effect of CNM surface on optical spectra of chromium (VI) molecular compounds is analyzed in terms of splitting of excited electronic states of quasi-tetrahedral CrO42- anions.