Fenton's oxidation using iron-containing activated carbon as catalyst for degradation of p-nitrophenol in a continuous stirred tank reactor

Abstract

The degradation of a model compound (p-nitrophenol - PNP) was evaluated by a heterogeneous Fenton process using iron supported on N-doped activated carbon (Fe-ACM) as catalyst in a continuous stirred tank reactor - CSTR. The stability of the catalyst was verified during reutilization and long-term tests, without any iron loss from the catalyst. The presence of a hydroxyl radical scavenger in solution led to a decrease in the process efficiency, showing that PNP degradation and intermediates mineralization is essentially due to a radical mechanism via the hydroxyl species, but the contributes of hydrogen peroxide, and other radicals, should be taken into account. The effect of some relevant operating conditions of the heterogeneous Fenton process, namely the liquid hourly space velocity (LHSV), the residence time (τ) and the dose of oxidant fed in the removal of PNP, mineralization (in terms of reduction of total organic carbon - TOC) and consumption of the oxidant (H2O2) was also assessed by performing a parametric study. The best operating conditions found (pH = 3, T = 50 °C, LHSV = 6.25 min−1, τ = 120 min and [H2O2]feed = 0.75 g/L) enabled to achieve, at steady-stage, removals of PNP, TOC and COD (chemical oxygen demand) of 94, 83% and 86%, respectively, while a nontoxic effluent (0% of inhibition towards Vibrio fischeri) was generated with enhanced biodegradability (improvements of BOD – biological oxygen demand – from <1.0 to 47.5 mgO2/L and BOD:COD ratio from <0.001 to 0.41). The identification and quantification of reaction intermediates was also carried out, being found that the contribution of oxalic acid, in addition to PNP, to the remaining TOC was above 98%.