Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod The pH effect on the kinetics of 4-nitrophenol removal by CWPO with doped carbon black catalysts Jose L. Diaz de Tuesta a,b, ⁎ , Asuncion Quintanilla c , Jose A. Casas c , Sergio Morales-Torres d , Joaquim L. Faria b , Adrián M.T. Silva b , Helder T. Gomes a,b a Centro de Investigação de Montanha (CIMO), Instituto Politécnico de Bragança, Campus de Santa Apolónia, 5300-253 Bragança, Portugal b Laboratory of Separation and Reaction Engineering - Laboratory of Catalysis and Materials (LSRE-LCM), Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal c Departamento de Ingeniería Química, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, Ctra. de Colmenar km 15, 28049 Madrid, Spain d Carbon Materials Research Group, Department of Inorganic Chemistry, Faculty of Sciences, University of Granada, Campus Fuentenueva s/n, 18071 Granada, Spain ARTICLE INFO Keywords: Catalytic wet peroxide oxidation Doped carbon black Kinetic model pH effect Induction period Nitrophenol ABSTRACT P, B and N-doped carbon blacks prepared with H 3 PO 4 , urea and H 3 BO 3 were tested as catalysts in the wet peroxide oxidation of a concentrated 4-nitrophenol (4-NP) model solution (C 4-NP = 5 g·L -1 ). The highest cata- lytic activity was found for P-doped carbon black (complete removal of 4-NP after 4 h at 80 °C, C cat = 2.5 g·L -1 , = − C 17.8 g·L HO 2 2 1 and initial pH 3, whereas 44-19% removals were reached with the other catalysts). That was ascribed to the strongest acidity (pH PZC = 3.5) and hydrophilic character of the catalyst. Initial pH affected the oxidation, allowing to increase strongly the conversion of 4-NP with the P-doped catalyst decreasing the initial pH from 4 to 2 (4-NP removal from 20% to 99% after 8 h of reaction time at 50 °C, C cat = 2.5 g·L -1 and = − C 17.8 g·L HO 2 2 1 ). An autocatalytic-power-law kinetic model was developed to predict the observed in- duction period and the dependence on the pH of the 4-NP oxidation, H 2 O 2 consumption and pH evolution (k 4- NP = 2.2·10 -5 M -2 min -1 , = − − − k 4.0·10 M ·min HO 2 2 6 3 1 and = + − − − k 5.1·10 M min H 3 0.23 1 at 80 °C). 1. Introduction The complexity of industrial wastewaters requires the development of more efficient and economically viable treatment technologies. In particular, nitrophenols (NPs), whether mono-, di- or tri-nitrophenols, are contaminants commonly present in wastewaters of plastic, phar- maceutical, paper, pesticide, synthetic dyes, insecticides, herbicides and explosive industries, reaching high concentrations in these effluents [1,2]. NPs are well-known to be highly toxic, inhibitory and bio-re- fractory organic compounds [3,4]. The degradation rate of NPs is quite slow in wastewater treatment plants, and moderate in soil, and both acute and chronic effects have been reported for animals and humans [4]. The United States Environmental Protection Agency (USEPA) in- cluded those NPs in the list of priority pollutants [5] and reported maximum allowable concentrations in water of 20 ppb [6]. Advanced oxidation processes (AOP) are particularly useful tech- nologies to treat nondegradable compounds present in wastewaters in a wide range of initial concentrations (0.001–10 g·L -1 )[7–9], difficult to remove by the conventional biological processes [10–14]. In this context, different AOPs have been studied, such as ozonation [15,16], photo-degradation [17,18] and electrochemical oxidation [19,20]. All these techniques allow to remove completely 4-NP after 0.25–6 h de- pending on conditions tested. The main drawback of these processes is the expensive reagent and energy source necessities. Trapido and Kallas evaluated the degradation of 4-NP by different AOPs, such as Fenton, ozonation and photo-degradation, concluding that the Fenton reagent was found to be the most promising for the abatement of 4-NP [21]. Thenceforward, the removal of 4-nitrophenol (4-NP) by oxidation with hydrogen peroxide (H 2 O 2 ) has been studied applying Fenton and in- tensified Fenton treatments, such as photo-Fenton [3,22–24], micro- wave-assisted Fenton [25] and electro-Fenton [26–28] processes. The non-intensified Fenton process is able to totally remove 4-NP after 2 h (considering 1 mM of 4-NP at 25 °C, pH = 3, > 5 mM of H 2 O 2 and > 5 mg/L of Fe 2+ )[29]. However, the Fenton process requires the re- covery of the Fe catalyst at the end of treatment and a rigorous pH control to efficiently run the process. Within this context, catalytic wet peroxide oxidation (CWPO) appears more advantageous AOP [30,31], since run with a heterogeneous catalyst which can be recovered after https://doi.org/10.1016/j.cattod.2019.08.033 Received 31 January 2019; Received in revised form 10 August 2019; Accepted 27 August 2019 ⁎ Corresponding author at: Centro de Investigação de Montanha (CIMO), Instituto Politécnico de Bragança, Campus de Santa Apolónia, 5300-253 Bragança, Portugal. E-mail address: jl.diazdetuesta@ipb.pt (J.L. Diaz de Tuesta). Catalysis Today xxx (xxxx) xxx–xxx 0920-5861/ © 2019 Elsevier B.V. All rights reserved. Please cite this article as: Jose L. Diaz de Tuesta, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.08.033