Lignin-based activated carbons as metal-free catalysts for the oxidative degradation of 4-nitrophenol in aqueous solution

Abstract

A wheat and hemp lignin, obtained from a soda pulping-precipitation process, was carbonized at 800 ◦C under N2 atmosphere. The resulting carbon material was thermally activated under oxidative atmosphere at four different temperatures (150, 200, 300 and 350 ◦C). The materials prepared at higher activation temperatures (300 and 350 ◦C) have proven their potential in the elimination of 4-nitrophenol (4-NP) from aqueous model solutions (5 g L−1) when using catalytic wet peroxide oxidation (CWPO). In these conditions we were able to remove around 70% of 4-NP after 24 h, with an efficient H2O2 decomposition, in experiments conducted at relatively mild operating conditions (atmospheric pressure, 50 ◦C, pH = 3, catalyst load = 2.5 g L−1 and [H2O2]0 = 17.8 g L−1). By increasing the working temperature to 80 ◦C, complete 4-NP removal was obtained within 48 h (against 93% 4-NP removal at 50 ◦C), with an efficiency of H2O2 consumption of 70% and a significant mineralization (61%). On the other hand, the materials prepared at lower activation temperatures (150 and 200 ◦C), with higher basicity, promote the faster but inefficient H2O2 decomposition, 4-NP removal being lower than 25% after 24 h at 50 ◦C in CWPO. This can be attributed to the formation of species other than HO• radicals during H2O2 decomposition, the recombination of the formed radicals into non-reactive species and a poor adsorption capacity.