Comparison of various advanced oxidation processes for the degradation of 4-chloro-2 nitrophenol

Abstract

In the present study an attempt is made efficiently to degrade USEPA listed 4-chloro-2-nitrophenol (4C-2-NP), widely available in bulk drug and pesticide wastes using various advanced oxidation processes (AOPs). A comparative assessment using various AOPs (UV, H₂O_2, UV/H₂O₂, Fenton, UV/Fenton and UV/TiO₂) was attempted after initial optimization studies, viz., varying pH, peroxide concentration, iron concentration, and TiO₂ loading. The degradation of the study compound was estimated using chemical oxygen demand (COD) reduction and compound reduction using spectrophotometric methods and further validated with high performance liquid chromatography (HPLC). The degradation trends followed the order: UV/Fenton > UV/TiO₂ > UV/H₂O₂ > Fenton > H₂O₂ > UV_. It can be inferred from the studies that UV/Fenton was the most effective in partial mineralization of 4C-2-NP. However, lower costs were obtained with H₂O₂. Kinetic constants were evaluated using first order equations to determine the rate constant K.

Introduction

Conventional wastewater treatment technologies have limitations hence demanding advanced research to tackle complex wastewater treatment. One of the promising technologies could be the use of single chemical oxidants, or the more effective destruction by the use of advanced oxidation processes (AOPs) [1]. Industrial use of phenol and its derivatives over the past decades has led to severe environmental pollution. The total waste generation comprising these hazardous substances in the southern state of Andhra Pradesh, India is estimated as 6884 t per month (TPM). Out of this around 190.3 TPM constitute phenolic wastes disposed mainly by petrochemicals, pharmaceuticals and polymer industries [2]. 4-Chloro-2-nitrophenol (4C-2-NP), which was chosen as the model compound in the present study, is recalcitrant and persistent towards biodegradation and is constituent intermediate of many industrial effluents. Well-acclimated organisms also showed ineffectiveness in treating wastewaters containing low levels of 4C-2-NP concentration due to their low biodegradability and inhibitory nature in wastewater. The generation of 4C-2-NP containing aqueous wastes during formulation, distribution and field application of pesticides is often unavoidable. Therefore chemical oxidation methods involving advanced oxidation processes appear to be a viable alternative to destroy such compounds compared to the existing methods. Advanced oxidation process offers a highly reactive, non-specific oxidant namely hydroxyl radical (OH), capable of destroying wide range of organic pollutants in water and wastewater [3].

• UV: Involves the interaction of light with molecules-in addition to water – to bring about their dissociation in to fragments with the following mechanistic pathway [4].

  Compound + hν → intermediates

  Intermediates + hν → CO₂ + H₂O + Cl⁻

• UV/H₂O₂: Based on formation of OH* radicals by means of the photolysis of hydrogen peroxide and the subsequent propagation reactions. Requires a relatively high dose of H₂O₂ and/or a much longer UV exposure time.

  H₂O₂ + hν → 2OH*

• Fenton process: Production of OH radicals by Fenton reagent occurs by means of addition of H₂O₂ to Fe²⁺ salts [5]. Iron can be considered as the real catalyst. However, it cannot lead to complete mineralization of organic compounds [6].

  Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + OH*

  Fe³⁺ + H₂O₂ → H⁺ + FeOOH²⁺

  FeOOH²⁺ → HO₂* + Fe²⁺

• Photo-Fenton: An extension of Fenton process, using UV–vis light irradiation at wavelengths higher than 300 nm. The photolysis of Fe³⁺ complexes allows Fe²⁺ regeneration and the occurrence of Fenton reactions due to the presence of H₂O₂ (Eq. (3)).

  Fe(OH)²⁺ → Fe²⁺ + HO₂*

• Photo catalysis: Employs an artificial UV light and a semiconductor-like TiO₂ or ZnO. It is slow compared to other AOPs and in some cases leads to incomplete mineralization of organics [7]. Absorption of the radiation with the formation of electron-hole pairs initiates the reaction and the reducing power of formed electrons allows metal reduction with the formation of the super oxide radical ion O₂*⁻ whereas remaining holes are capable of oxidizing adsorbed H₂O or HO⁻ to reactive HO radicals.

  TiO₂ → e + h⁺

  TiO₂(h⁺) + H₂O_ad → TiO₂ + HO_ad* + H⁺

  TiO₂(h⁺) + HO_ad⁻ → TiO₂ + HO_ad*

In order to evaluate 4C-2-NP degradation capabilities using selected AOPs, a methodical study is attempted using UV, H₂O₂, UV/H₂O₂, Fenton, UV/Fenton and UV/TiO₂ and the efficacy of the treatment is presented in terms of COD removal and compound degradation using instrumental methods. Further degradation rate kinetics along with cost evaluation was performed for various AOPs studied.