Photochemical coupling reactions between Fe(III)/Fe(II), Cr(VI)/Cr(III) and polycarboxylates: inhibitory effect of Cr species

The roles of chromium species on photochemical cycling of iron and mineralization of polycarboxylates are examined in the presence of Cr(VI) or Cr(III) at pH 2.2−4.0. Under UV irradiation, Cr(III) altered the redox equilibrium of iron species, leading to the shift of the photosteady state toward Fe(II). After a longer time of illumination, total organic carbon (TOC) approached a steady state in the presence of Cr(III) or Cr(VI), whereas oxalate was thoroughly mineralized in the absence of Cr species. The TOC of steady state was closely related to the kind of polycarboxylates, Cr species dosages, pH and O2 atmosphere, but hardly affected by more addition of Fe(III). ESI-MS data indicates that several Cr−oxalate complexes formed in the photochemical reactions, which are responsible for protecting oxalate against further oxidation. A mechanism is proposed for the inhibitory effect of Cr species on oxidation of oxalate and Fe(II). The present study may provide a new insight into the dual environmental effects induced by Cr contaminants especially at heavily chromium-contaminated and dissolved organic matter (DOM)-rich sites

Effect of the incorporation of nitrogen to a carbon matrix on the selectivity and capacity for adsorption of dibenzothiophenes from model diesel fuel

Two synthetic, polymer-derived carbons were modified with urea to incorporate nitrogen surface functional groups. Then they were investigated as adsorbents of dibenzothiophene (DBT) and 4, 6-dimethyldibenzothiophene (DMDBT) from simulated diesel fuel under dynamic conditions with the total concentration of sulfur being 20 ppmw. The materials before and after adsorption were characterized using elemental analysis, XPS, adsorption of nitrogen, Potentiometric titration, and thermal analysis. The incorporation of nitrogen species caused a visible increase in the adsorption capacity. However, selectivity evaluated on the basis of the adsorption of naphthalene decreased. Whereas at low surface coverage the volume of pores smaller than 10 Å is important, with the progress of adsorption the surface chemistry gradually starts to play a more important role via either polar or acid/base interactions. The latter are important for the selectivity of adsorption when the aromatic hydrocarbons are present. Although polar interactions are weaker than the acid-base ones, the centers that they represent seem to be more favorable to attract DBT and DMDBT than arenes. There is an indication that nitrogen-containing groups contribute to chemical transformations of DBT and DMDBT/ oxidation and promote the involvement of oxygen from the surface groups in the reactive adsorption