Environmental engineering parameters affecting the removal of hexavalent chromium and nitroaromatic compounds from water by granular activated carbon

Cr(VI) reduction rate by Fe(II) was closely related to the solution pH. Much more rapid reduction was observed when NaOH was added than when the pH of solutions was initially adjusted. This may be due to existence of local short-term conditions of high pH. The amount of Cr(VI) removed by GACs decreased significantly, as the solution pH increased from 4 to 7.5. Removal of DO from the experimental systems resulted in much greater removal of Cr(VI) from solution, possibly as a result of reduction to Cr(III). However, pretreatment of the two GACs with reductants (Fe(II) or dithionite) did not improve Cr(VI) removal. The carbons regenerated by 0.01M K₂HPO ₄ Solution extraction followed by 0.02N H₂SO₄ acid wash, showed larger performance, primarily because of the favorable adsorption of Cr(VI) at lower pH values and the reduction of Cr(VI) to Cr(III).; The impacts of dissolved oxygen (DO), background dissolved organic matter (BOM) and pretreatment of GAC on the sorption of phenolics (phenol, o-nitrophenol (ONP), and 2,4-dinitrophenol (DNP)) and 2,4-dinitrophenol (DNT) from AGW were investigated. The sorption of these compounds was somewhat affected by the presence of dissolved oxygen (DO), particularly phenol, DNP, and DNT. The extraction efficiencies of phenol sorbed onto GAC showed similar results compared to previous studies, but those of ONP and DNP showed a trend that decreased with increasing contact time from the GACs sorbed under both oxic and anoxic conditions, indicating that there were some chemical reactions besides oxidative coupling occurring on GAC surfaces in the presence/absence of DO. Pretreatment of GAC with a reductant deteriorated the sorption of phenol but improved sorption of ONP and DNP except DNP at pH 7.5 under oxic conditions. Preloaded GAC with BOM showed a decreased sorption capacity for phenol and DNT, increased sorption for ONP at pH 4 in a certain range of equilibrium concentration, and increased sorption for DNP at pH 4 under both oxic and anoxic conditions and at pH 7.5 under anoxic condition. The changed sorption capacities for ONP and DNP under certain conditions imply that there were some chemical reactions besides oxidative coupling on GAC surfaces.