RESUMO
The sonochemical (850â¯kHz) reduction of Cr(VI) (0.3â¯mM, pH 2, reactor open to air) was analyzed in the presence of different additives. The effects on Cr(VI) reduction efficiency of added formic acid (FA, 10â¯mM), citric acid (Cit, 2â¯mM), ethylenediaminetetraacetic acid (EDTA, 1â¯mM), methanol (MeOH, 0.1â¯M), ethanol (EtOH, 0.1â¯M), 2-propanol (2-PrOH, 0.1â¯M), tert-butanol (t-BuOH, 0.1â¯M), phenol (PhOH, 2â¯mM) and sodium lauryl sulfate (SLS, 1â¯mM) have been evaluated in comparison with the system in the absence of additives. Complete Cr(VI) reduction was obtained only when using EDTA (at 120â¯min) and Cit (at 180â¯min). Cr(III) complexes with these compounds or with their degradation products were detected as final products. For EDTA, Cit, t-BuOH, FA and SLS, the Cr(VI) decay could be adjusted to a zero-order kinetics; in the cases of MeOH, EtOH and 2-PrOH, there was a deviation from the zero-order kinetics. The Cr(VI) conversion increased in the order SLS (very low)â¯<â¯no additiveâ¯â â¯MeOHâ¯â â¯EtOHâ¯â â¯2-PrOHâ¯<â¯FAâ¯<â¯t-BuOHâ¯<â¯PhOHâ¯<â¯Citâ¯<â¯EDTA. The role of EDTA and Cit in stabilizing intermediate Cr(V) peroxo compounds and enhancing their direct transformation into different Cr(III) species is considered a major factor in the acceleration of Cr(VI) reduction processes. Mechanistic pathways are proposed.