Mineralization and degradation of 4-Nitrophenol using homogeneous Fenton oxidation process

Minz Sudha; Gupta Renu; Garg Sangeeta

doi:10.4491/eer.2019.145

Environ Eng Res > Volume 26(3); 2021 > Article

Sudha, Renu, and Sangeeta: Mineralization and degradation of 4-Nitrophenol using homogeneous Fenton oxidation process

Research

Environmental Engineering Research 2021; 26(3): 190145.

Published online: June 2, 2020

DOI: https://doi.org/10.4491/eer.2019.145

Mineralization and degradation of 4-Nitrophenol using homogeneous Fenton oxidation process

Minz Sudha, Gupta Renu^†

, Garg Sangeeta

Department of Chemical Engineering, Dr. B. R. Ambedkar National Institute of Technology, Jalandhar-144011, Punjab, India

^†Corresponding author: Email: bansalr@nitj.ac.in, Tel: +08283022833

Received April 11, 2019 Accepted May 27, 2020

(open-access):

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The mineralization analysis of wastewater is important for the growth of flora and fauna. The mineralization along with the degradation of 4-NP has been studied using homogeneous Fenton oxidation process. The mineralization of 4-NP has been reported in terms of chemical oxygen demand (COD) removal % and total organic carbon (TOC) removal %. The main parameters that affect the mineralization and degradation of 4-NP i.e. initial pH (1 – 5), reaction time (0 – 60 min), the concentration of hydrogen peroxide (1 – 5 mM) and concentration of ferrous ions (0.05 – 0.5 mM) have been optimized. In the present study, optimum reaction conditions for mineralization and degradation of 4-NP are observed at pH 3, 4 mM of H₂O₂ concentration, 0.2 mM of Fe²⁺ concentration and in 40 min of reaction time. At the optimum condition, COD removal and TOC removal were 50% and 60.3%, respectively along with 93.6% of 4-NP degradation. At optimized conditions, intermediate products obtained after homogeneous degradation of 4-NP were identified based on LC-MS spectra analysis. The degradation of 4-NP followed, second order reaction rate. The present study showed the higher degradation and mineralization of 4-NP as compared to available studies on 4-NP.

Keywords: Chemical oxygen demand, Degradation, 4-Nitrophenol, Mineralization, Total organic carbon

1. Introduction

4-Nitrophenol (4-NP) is a hazardous, bio-refractory organic compound and is widely used for the production of pesticides, herbicides, insecticides and synthetic dyes [1]. It is highly stable and soluble in water and persists in soil and ground water for a longer period of time [2]. The discharge of 4-NP from various industries to water streams is of main concern due to its toxicity and suspected carcinogenicity. The development of efficient methods for removal of 4-NP, is becoming an important field of study for researchers.

Several treatment methods such as chemical, physical and biological have been employed for the treatment of 4-NP from waste-water [3]. Physical and biological treatment techniques are not very satisfactory as they transfer the pollutants from one phase to another, and the equipment required for these processes is also very costly [4]. Recently advanced oxidation processes (AOPs) showed considerable potential for the removal of 4-NP from industrial effluents [5–9]. In AOPs, the hydroxyl radicals (^•OH) are generated in solution and are responsible for the oxidation and mineralization of the organic pollutants to water and carbon dioxide [10–13]. Among various AOPs, the Fenton oxidation process has been used extensively, as reagents (ferrous sulphate heptahydrate and hydrogen peroxide) used are easy to handle and environmental friendly [14]. The main advantage of this method is that the reaction takes place at ambient temperature, atmospheric pressure and short reaction period, which makes the process less expensive [15].

Various studies available on degradation of organic pollutants using a homogeneous Fenton oxidation process are presented in Table 1.

It is observed from Table 1, that the homogeneous Fenton oxidation process is still very active for the degradation of various organic pollutants. Further, it is observed that most of the studies are available on the degradation/ decolourization of organic pollutants, however, very few studies are reported on mineralization (TOC and COD removal) of the organic pollutants. The studies of Fathi and Keshmirizadeh [4], Ertugay and Acar [16], Giri and Golder [17] and Barbusinski [18] have analyzed COD removal % for various organic pollutants. Giri and Golder [17] has reported TOC removal (37.9%) for ciprofloxacin. The degradation studies alone do not reveal the conversion of toxic organic pollutants in simpler organic compounds. Thus mineralization of organic pollutants (TOC removal % and COD removal %) is also important along with decolorization/degradation studies and there is a need to study the mineralization of organic pollutants along with the degradation of organic pollutants.

The present study has been focused on mineralization of 4-NP along with its degradation using homogeneous Fenton oxidation process. Various parameters like initial pH, reaction time, concentration of hydrogen peroxide and ferrous ions that affect the degradation and mineralization (COD and TOC removal %) of 4-NP have been optimized. Further the studies have been extended to identify the degradation products of 4-NP using LC-MS spectra technique. The results of mineralization and degradation of 4-NP have also been compared with the available studies.

2. Material and Methods

2.1. Materials

4-NP (w/w, 98% purity), ferrous sulphate heptahydrate (FeSO₄.7H₂O) (w/w, purity>99%), sulphuric acid (98% purity), mercuric sulphate (purity 99%), silver sulphate (purity > 99%), were obtained from Loba Chemie Pvt. Ltd. (Mumbai, India). Hydrogen peroxide (30% w/w) was purchased from Ranbaxy Fine Chemicals Limited (Gujarat, India). Sodium hydroxide (NaOH, 97% purity), ferrous ammonium sulphate (FAS) (99% purity) was taken from s.d. fine Chemical Limited (Mumbai, India). Potassium dichromate (K₂Cr₂O₇, purity > 99%) was obtained from Qualigens Fine Chemicals (Mumbai, India). All the solutions were prepared using double distilled water.

2.2. Experimental Procedure

Batch experiments for mineralization and degradation of 4-NP were performed by taking 0.05 mM ferrous sulphate solution into a 250 mL Erlenmeyer glass flask containing 100 mL of 4-NP (100 mg/L) solution. The pH of the solution was adjusted by the addition of a few drops of H₂SO₄ (0.1 N) and NaOH (0.1 N) and was measured using a pH meter (pH 700 Eutech). After the pH adjustment, 1 mM H₂O₂ was added quickly to the solution with continuous stirring at 30°C. With the addition of H₂O₂ solution, the reaction started and the samples were taken out from the flask periodically and the reaction was stopped by adding two drops of 1 M NaOH. Thereafter, the solution was filtered and the filtrate was analyzed for mineralization (TOC and COD removal %) and percentage degradation. The above experiments were repeated to study the effect of various parameters such as: pH (1 – 5), reaction time (0 – 60 min), concentration of ferrous ions (0.05 – 0.5 mM) and concentration of H₂O₂ (1 – 5 mM). All the experiments were repeated three times to get reproducibility of ± 5%.

2.3. Analytical Methods

The 4-NP degradation (%) after the reaction was analyzed using UV/VIS spectrophotometer at λ_max 318 nm (Shimadzu double beam spectrophotometer Model: UV-VIS 2600/2700). The degradation efficiency was calculated as:

(1)

Degradation efficiency (%) = (1 - \frac{C_{t}}{C_{0}}) \times 100

Where, C₀ is the initial concentration, C_t is the concentration at any time t.

The mineralization studies, chemical oxygen demand (removal %) and total organic carbon (removal %) were also analyzed after reaction. The COD of the samples before and after the reaction, was analyzed with MAC COD digester (Model: COD-439, Karnal, India) using a standard APHA method [28]. The COD efficiency of 4-NP was calculated as:

(2)

COD removal (%) = (1 - \frac{{COD}_{t}}{{COD}_{0}}) \times 100

Where, COD_t concentration at any time t, and COD₀ is initial concentration at time zero.

The TOC analysis of the samples was done using a TOC analyser (Model: ASI-V and TOC-V CPN, Shimadzu, Japan) before and after the degradation of 4-NP. The non-dispersive infrared method was adopted for the detection of TOC. The TOC efficiency was calculated as:

(3)

TOC removal (%) = (1 - \frac{{TOC}_{t}}{{TOC}_{0}}) \times 100

Where, TOC_t concentration at any time t, and TOC₀ is initial concentration at time zero.

Liquid chromatography–mass spectroscopy (LC–MS) analysis of the reacted 4-NP samples was made to identify the intermediate products over the mass range of 40–400 amu. (model: Q-TOF micro waters, Waters, USA). The reacted sample was analyzed for determination of intermediate products using negative mode electrospray ionization (ESI) technique.

3. Results and Discussion

3.1. Effect of pH

The pH value of solution influenced the generation of hydroxyl radicals and thus the oxidation efficiency of 4-NP. It is known that the Fenton’s reaction is active in acidic medium. Thus, the effect of pH on the degradation of 4-NP was carried out by varying the pH in the range 1 to 5 at the initial reaction conditions of H₂O₂ concentration of 2 mM, Fe²⁺ concentration of 0.2 mM at 30°C and 40 min of reaction time. The effect of pH on oxidation of 4-NP i.e. degradation efficiency, COD removal (%) and TOC removal (%) is presented in Fig. 1.

It is observed that the degradation (%) of 4-NP increased from 28% to 84% with an increase in pH from 1 to 3 and thereafter it decreased to 65% with an increase in pH from 3 to 5. Similarly, the COD and TOC removal (%) of 4-NP has increased from 16% to 42% and 23% to 53%, respectively with increase in pH from 1 to 3 and it decreased to 32% and 40%, respectively with further increase in pH from 3 to 5. The maximum degradation of 84 % was obtained for 4-NP at pH 3. The COD removal (%) and TOC removal (%) for 4-NP, at 3 pH were 42% and 53% respectively. Therefore, the optimum pH for the degradation along with mineralization of 4-NP was 3. Below pH 3, the degradation (%) of 4-NP was low because very less ^•OH radicals were produced as hydrogen peroxide was not decomposed by Fe²⁺ ions due to the stable oxygen concentration [29]. At very low pH, H⁺ ion acts as ^•OH radical scavengers according to Eq. (4) [30].

(4)

{HO}^{•} + H^{+} + e^{-} \to H_{2} O

Inhibition of ^•OH radical generation at low pH is due to the formation of complex species [Fe(H₂O)6]²⁺ which reacts very slowly with hydrogen peroxide. In addition, the H₂O₂ gets solvated in the presence of high concentration of H⁺ ions at low pH to form stable oxonium ion [H₃O₂]⁺. The oxonium ion makes H₂O₂ electrophilic to enhance its stability, thus reducing the reactivity with Fe²⁺ ions [31]. At pH 4 and 5, due to the formation of ferrous and ferric oxy-hydroxides complexes, lesser amount of the ^•OH radicals were generated which might have resulted in lower degradation and mineralization of 4-NP [11]. De Luis et al. [32] study has also observed maximum degradation of phenols (ortho, meta and para-cresol) at pH 3 using a homogeneous Fenton oxidation process. Meric et al. [22] achieved COD removal of 78.6% for Reactive Black 5 (RB5) dye at pH 3.

3.2. Effect of Reaction Time

The degradation of 4-NP was analyzed by varying the reaction time from 4 min to 1 h and is shown in Fig. 2. The reaction time needed for the degradation of 4-NP will depend on various parameters like pH, initial concentration of 4-NP, concentrations of ferrous ions and H₂O₂. For the initial reaction of 100 mg/L of 4-NP, 2 mM of H₂O₂, 0.2 mM of Fe²⁺ concentration and at pH 3, it is observed that the degradation of 4-NP increased linearly up to 8 min of reaction time and then followed the non-linear increase up to 40 min. The degradation of 4-NP was increased from 35% to 84% with increase in reaction time from 4 min to 40 min and thereafter no further degradation of 4-NP has been noticed. Therefore, the optimum reaction time for the degradation of 4-NP was observed to be 40 min for further studies.

3.3. Effect of H₂O₂ Concentration

The dosage of H₂O₂ is an important parameter as it affects the generation of OH radicals. The H₂O₂ concentration was varied from 1 mM to 5 mM to study the mineralization and degradation of 4-NP. The effect of H₂O₂ on the degradation, COD removal and TOC removal of 4-NP is shown in Fig. 3. It is observed that the degradation, COD removal and TOC removal of 4-NP increased with increase in concentration of H₂O₂ from 1 mM to 4 mM and thereafter decreased. The H₂O₂ concentration of 4 mM, degradation, COD removal (%) and TOC removal (%) of 4-NP were obtained to be 93.6, 50% and 60.3%, respectively. Increased concentration of H₂O₂ from 1 mM to 4 mM have resulted in more ^•OH radicals, which might have increased the degradation and mineralization of 4-NP. Further at higher H₂O₂ dosage, i.e. 5 mM the decrease in degradation, COD removal and TOC removal of 4-NP might be due to the scavenging of generated ^•OH radicals i.e. the ^•OH radicals may react with H₂O₂ to generate HO₂ ^• radicals and which are less reactive than ^•OH radicals [18, 33].

(5)

H_{2} O_{2} + {HO}^{•} \to {HO}^{•}_{2} + H_{2} O

Also the unused portion of H₂O₂ during the Fenton process contributed to COD and hence the excess amount was not recommended [34, 35].

3.4. Effect of Fe²⁺ Ions Concentration

It is utmost important to analyze the optimum loading of Fe²⁺ ions to mineralize and degrade 4-NP, because at low Fe²⁺ ion concentrations, the degradation and mineralization of 4-NP are less due to insufficient amount of Fe²⁺ ions. But higher quantities of Fe²⁺ ions lead to an unutilized quantity of iron salt, which further contributes to increase the total dissolved solids content in the solution [36, 37]. The range of Fe²⁺ ion concentration was selected by performing initial experiments. The concentration of Fe²⁺ ions was varied from 0.05 mM to 0.5 mM and keeping the rest of the parameters constant and the results are shown in Fig. 4. It was found that the degradation and mineralization (TOC and COD removal) rate increased with an increase in the Fe²⁺ ion concentration up to 0.2 mM of aqueous solution and after that the degradation and mineralization rate decreased (Fig. 4). The maximum degradation of 4-NP (93.6%) was observed at Fe²⁺ ion concentration of 0.2 mM.

The mineralization i.e. COD removal and TOC removal increased from 30% to 50% and 50.4% to 60.3%, respectively, with increase in Fe²⁺ ion concentration from 0.05 mM to 0.2 mM. The higher dosage of Fe²⁺ ions (>0.2mM) may result to scavenging effect with ^•OH radicals present in solution as per the following reaction [22]:

(6)

{Fe}^{2 +} + {HO}^{•} \to {Fe}^{3 +} + {OH}^{-}

(7)

H_{2} O_{2} + {HO}^{-} \to {HO}^{•}_{2} + H_{2} O^{+}

3.5. Reaction Mechanism Showing the Degradation Pathways of 4-NP

The ^•OH radicals were generated during the reaction between Fe²⁺ ions and hydrogen peroxide. The ^•OH radicals reacted with 4-NP compound and led to the formation of intermediate products. The reaction products of the degraded 4-NP were identified using LC-MS spectra at a retention time of 3.542 min (Fig. 5). 3,4-dihydroxynitrobenzene, 1,2,4 trihydroxylbenzene, 1,2-dihydroxyaminophenol, 4-aminophenol, Hydroquinone, p-benzoquinone, 3,4,5-tridydroxynitrobenzene, 1,2,6-trihydroxy-4-aminophenol and acetic acid were identified as the some of the intermediate products by the following reactions:

(8)

\begin{array}{l} C_{6} H_{5} {NO}_{3} + {HO}^{•} \to C_{6} H_{5} {NO}_{4} + H^{+} \\ 4 -Nitrophenenol 3, 4 -dihydroxynitrobenzene \end{array}

(9)

C_{6} H_{5} {NO}_{4} + {HO}^{•} \to C_{6} H_{6} {NO}_{3} + {NO}_{2} 1, 2, 4 -trihydroxylbenzene

(10)

C_{6} H_{5} {NO}_{3} + 2 H^{+} \to C_{6} H_{7} NO + O_{2} + e^{-} 4 -aminophenol

(11)

C_{6} H_{5} {NO}_{4} + H^{+} \to C_{6} H_{7} {NO}_{2} + e^{-} 1, 2 -dihydroxyaminophenol

(12)

C_{6} H_{7} NO + {HO}^{•} \to C_{6} H_{8} NO + e^{-} 4 -aminophenol

(13)

C_{6} H_{8} NO + {HO}^{•} \to C_{6} H_{6} {NO}_{2} + {NH}_{3} Hydroquinone

(14)

C_{6} H_{6} O_{2} \leftrightarrow C_{6} {HNO}_{2} + 2 H^{+} 1, 4 -benzoquinone

(15)

C_{6} H_{5} {NO}_{4} + {HO}^{•} \to C_{6} H_{5} {NO}_{5} + H^{+} (3, 4, 5 -tridydroxynitrobenzene)

(16)

C_{6} H_{5} {NO}_{5} + 2 H^{+} \to C_{6} H_{7} {NO}_{3} + O_{2} 1, 2, 6 -trihydroxy- 4 -aminophenol

(17)

C_{6} H_{4} O_{2} + 2 {HO}^{•} \to C_{6} H_{4} O_{4} + 2 H^{+}

(18)

C_{2} H_{4} O_{2} + 8 {HO}^{•} \to 2 {CO}_{2} + 2 H_{2} O Acetic acid

Details of identified intermediate products formed after the degradation of 4-NP are given in Table 2.

3,4-dihydroxynitrobenzene, 1,2,4-trihydroxylbenzene, 3,4,5 trihydroxynitrobenzene, hydroquinone, p-benzoquinone, CO₂ and H₂O were identified as the intermediate products by Ziang et al. [38]. Daneshvar et al. [39] study has identified 3,4-dihydroxynitrobenzene, 1,2,4 trihydroxylbenzene, hydroquinone as the intermediate products using photo-oxidative degradation of 4-NP. Jiang et al. [40] carried out electrochemical degradation of 4-NP and resulted in the formation of hydroquinone, p-benzoquinone, 4-aminophenol as the intermediate products. Sun and Lemley [41], Zhao et al. [42] and Minz et al. [43] studies have reported the formation of 3,4-dihydroxynitrobenzene, 1,2,4-trihydroxylbenzene, hydroquinone, p-benzoquinone, as the intermediate products after degradation of 4-NP. Some of the residual fragments against molecular weights 76.96, 188.99, 217.0, 225.15, 255.21, 281.22, 283.25, 299.19 and 327.28 remained unidentified may be due to the complexity of their formation in the solution phase.

3.6. Kinetic Study

The kinetics of the reaction helps to study the factors which influence the rate of reaction like reactant concentration, oxidant concentration and composition of the reaction mixture [44] and provides useful information about the mechanism and rate of chemical reaction. Kinetic study for the degradation of 4-NP was done as a function of time at optimum reaction conditions, i.e. initial 4-NP concentration 100 mg/L, [H₂O₂]: 4 mM, [Fe²⁺]: 0.2 mM and pH 3. The degradation data of 4-NP was analyzed for 1^st order and 2^nd order rate expressions as:

First order kinetics

(17)

ln \frac{C_{0}}{C_{t}} = kt

Second order kinetics

(18)

\frac{1}{C_{t}} = \frac{1}{C_{o}} + k^{'} t

Where, C₀ is the initial concentration of 4-NP and C_t is the concentration of 4-NP at any time t, k and k′ represents the rate constants of first and second order reactions respectively.

Kinetic rate constant for the degradation of 4-NP for 1^st order and 2^nd order rate expression are 0.0829 min⁻¹ and 0.0037 L mg⁻¹ min⁻¹. The higher value of the regression coefficient (0.974) for 2^nd order reaction as compared to 1^st order reaction (0.853) shows that the degradation of 4-NP obeys 2^nd order reaction rate with rate constant of 0.0037 L mg⁻¹ min⁻¹. Ghosh et al. [26] also observed second order rate for the degradation of 4-chlorophenol (4-CP) and nitrobenzene using Fenton reagent. The degradation of 2,6-dimethyl aniline using ^•OH radicals also showed that the data was well fitted to second order kinetic model [45].

3.7. Comparison with Earlier Studies

A comparison of homogeneous Fenton process for the degradation of 4-NP for the present and available reported studies has been made and is shown in Table 3. Ma et al. [21] achieved 99% degradation of 4-NP using 5 mM of H₂O₂ concentration, 5 mg/L of Fe²⁺ at pH 3 in 2 h. Also, the dissolved organic carbon (DOC) removal efficiency was 30.6%, i.e. only one-third of 4-NP was mineralized. Another study on the degradation of nitrophenols: 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), 2,4-dinitrophenol (DNP) and 2,4,6-trinitrophenol (TNP), the maximum mineralization (DOC) for mono-nitrophenols was 32% [23]. However, in the present study, 93.6% degradation of 4-NP was achieved with 50% COD removal and 60.3% TOC removal efficiency in the short reaction period of 40 min. The mineralization efficiency achieved is higher than the previous studies.

4. Conclusions

The mineralization studies on 4-NP along with degradation have been made using homogeneous Fenton’s oxidation.

The present study revealed that homogeneous Fenton’s oxidation process can be used for the mineralization and degradation of 4-NP. The optimum reaction conditions for 4-NP degradation and mineralization are observed at pH 3, H₂O₂ concentration of 4 mM, Fe²⁺ concentration of 0.2 mM in 40 min of reaction time. At optimal condition, 93.6% of 4-NP were degraded within 40 min with COD removal (%) and TOC removal (%) of 50% and 60.3%, respectively. The kinetic study revealed the degradation of 4-NP by second order kinetics with rate constant of 0.0037 L mg⁻¹ min⁻¹ and correlation coefficient 0.974. The higher mineralization efficiency of 4-NP (50% COD removal and 60.3% TOC removal) has been observed in the present study as compared to the previous reported studies. Based upon the LC-MS spectra analysis 3,4-dihydroxynitrobenzene, 1,2-dihydroxyaminophenol, 1,2,6-trihydroxy-4-aminophenol, 3,4,5-trihydroxynitrobenze, 4-aminophenol, hydroquinone, p-benzoquinone, acetic acid etc. have been identified as the intermediate products. The present study indicated that the Fenton process was a feasible method to treat acidic wastewaters. This method could also be used in combination with other AOPs for better results.

Acknowledgment

The authors thankfully acknowledge the facilities extended by sophisticated analytical instrumentation facility of Panjab University, Chandigarh, India and Dr. B. R. Ambedkar National Institute of Technology, Jalandhar, India.

Notes

Author Contributions

S.M. (Ph.D student) conducted all the experimental work under the joint supervision of Dr. R.G. (Associate Professor) and Dr. S.G. (Associate Professor).

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Fig. 1

Effect of pH on the degradation of 100 mg/L of 4-NP at reaction conditions: [H₂O₂] = 2 mM, [Fe²⁺] = 0.2 mM and time = 40 min.

Fig. 2

Effect of reaction time on the degradation of 4-NP; Conditions: [4-NP] = 100 mg/L, [H₂O₂] = 2 mM, [Fe²⁺] = 0.2 mM, pH = 3 and time 60 min.

Fig. 3

Effect of H₂O₂ concentration on the degradation of 4-NP, COD removal and TOC removal; Conditions: [4-NP] = 100 mg/L, [Fe²⁺] = 0.2 mM, pH = 3, time = 40 min.

Fig. 4

Effect of ferrous ions concentration on the degradation of 4-NP, COD removal and TOC removal; Conditions: [4-NP] = 100 mg/L, [H₂O₂] = 4 mM, pH = 3, time 40 min.

Fig. 5

LC-MS spectra analysis after degradation of 4-NP at optimized reaction conditions: 4 mM of H₂O₂ concentration, 0.2 mM of Fe²⁺ concentration, 40 min reaction time.

Table 1

Degradation of Organic Pollutants Using Homogeneous Fenton Oxidation Process

Reference	Organic pollutants	Operating parameters/conditions	Observations
Barbeni et al., 1987 [19]	2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 3,4-dichlorophenol and 2,4,5-trichlorophenol.	C₀ = 3×10⁻⁴ M, T = 25°C	- Increase in concentration of Fe²⁺ ions enhanced decomposition rate of chlorophenols. - Chlorinated aliphatic species were formed as intermediates.
Kwon et al., 1999 [20]	p-chlorophenol	C₀ = 2.0 mM, [H₂O₂] = 3 – 12 mM, [Fe²⁺] = 0.1 – 1.0 mM, pH = 1–5	- The optimum reaction conditions were: 6 mM H₂O₂, 0.3 mM Fe²⁺ and pH 3. - The decomposition of p-chlorophenol was high in pH range 2 – 4. - The initial reaction rate was high and followed first order reaction rate with initial concentration of Fe²⁺ and H₂O₂.
Ma et al., 2000 [21]	4-Nitrophenol	C₀ = 1 mM, [H₂O₂] = 1 – 10 mM, [Fe²⁺] = 1 – 10 mg/L, pH = 3	- 99% decomposition of 4-NP was achieved at 5 mM H₂O₂ and 5 mg/L Fe²⁺ in 2 h. - The dissolved organic carbon removal efficiency was 30.6%. - 4-NP degradation obeyed pseudo first order reaction rate.
Meriç et al., 2004 [22]	Reactive Black 5	C₀ = 100 and 200 mg/L, [FeSO₄]/H₂O₂ = 0.5 to 0.2, pH = 2.5 – 4, T = 30 – 60 °C	- For 100 mg/L of reactive black 5 dye, the color removal and COD removal were 99% and 71% with 100 mg/L of FeSO₄, 400 mg/L of H₂O₂, pH 3 and 40°C. - For 200 mg/L of reactive black 5, the color removal and COD removal were 99% and 84% with 200 mg/L of FeSO₄, 1000 mg/L of H₂O₂, pH 3 and 40°C.
Barbusinski, 2005 [18]	Industrial wastewater having maleic acid anhydride (MAA), 2-ethylhexyl alcohol (2-EHA) n-butanal, urea-formaldehyde resin adhesive (UFRA), pesticide-containing wastewater (PCW)	[H₂O₂] = 2 – 10 g/dm³, [Fe²⁺/H₂O₂] = 0.2 – 0.75, pH = 2 – 5, t = 2 – 24 h	- At optimized conditions of 5 g/dm³ H₂O₂, molar ratio of Fe²⁺/H₂O₂: 0.33 and pH 3, the toxicity removal% for MAA, 2-EHA, UFRA and PCW were 47%, 51%, 12% and 100%, respectively. - COD removal for MAA, 2-EHA, UFRA and PCW were 87.8%, 86.3%, 88.6% and 71.7%, respectively. - Only for MAA, 2-EHA and UFRA, the toxicity was reduced to non-toxic level.
Kavitha and Palanivelu, 2005 [23]	2-nitrophenol (2-NP), 4-nitrophenol (4-NP), 2,4-dinitrophenol (DNP), 2,4,6-trinitrophenol (TNP)	C₀ for [2-NP/4-NP] = 1.43, [DNP] = 1.08 mM and [TNP] = 0.87 mM, pH 3	- For mono-NP, at the optimum conditions of 17.3 mM H₂O₂ and 0.45 mM Fe²⁺, the mineralization efficiency was 32%. - For DNP, at the optimum conditions of 12.9 mM H₂O₂ and 0.36 mM Fe²⁺, the mineralization efficiency was 25%. - Whereas for TNP at 10.4 mM of H₂O₂ and 0.36 mM of Fe²⁺, it was 21%. - Degradation of NPs followed pseudo-first-order reaction.
Chang et al., 2008 [10]	p-nitrophenol (PNP)	C₀ = 0.1 – 0.4 mM, [H₂O₂] = 2 – 20 mM, [Fe²⁺] = 0.1 – 0.8 mM, pH = 3	- The optimum conditions obtained for the degradation of PNP were 0.1 mM Fe²⁺, 4 mM H₂O₂ and pH 3. - For low concentration of PNP, the reaction rate was first order whereas at higher concentrations of PNP, it was zeroth order.
La Nafie, 2010 [24]	Spiked soil having polycyclic aromatic hydrocarbons	C₀ = 200 each PAHs, pH = 3, H₂O₂/Fe²⁺ = 10:1	- Polycyclic aromatic hydrocarbons containing naphthalene, anthracene, fluoranthene, pyrene and benzo(a)pyrene in spiked soil were degraded to 96%.
Khamaruddin et al., 2011 [25]	Diisopropanolamine	C₀ = 3 g/L, T = 30°C – 60°C, pH = 2 – 4, [H₂O₂/Fe²⁺] = 95	- Degradation efficiency of diisopropanolamine increased with increase in temperature and at 60°C, it was 85%. - The optimum pH for diisopropanolamine degradation was 2.5. - The degradation of diisopropanolamine followed pseudo first-order reaction rate.
Ghosh et al., 2012 [26]	p-Chlorophenol (pCP) and Nitrobenzene (NB)	C_0-pCP = [3.889 – 7.778] × 10⁻³ gmol/l, [H₂O₂]/[pCP] = 5.65 – 7.55, [H₂O₂]/[Fe²⁺] = 8.2 C_0-NB = [3.25 – 4.87] × 10⁻³ gmol/l, [H₂O₂]/[NB] = 3.6 – 10.8, [H₂O₂]/[Fe^z+] = 8.2	- The degradation of p-chlorophenol was 99.29% at [H₂O₂]/[pCP] of 7.55 whereas for nitrobenzene, it was 96.32% at [H₂O₂]/[pCP] of 3.60. - The degradation of p-chlorophenol and nitrobenzene followed second order reaction rate with constants 2.144 × 10⁶/M-s and 1.102 × 10⁶/M-s, respectively.
El Haddad et al., 2014 [27]	Reactive Yellow 84 dye	C₀ = 60 mg/L, H₂O₂ = 50 – 500 mg/L, Fe²⁺ = 10 – 100 mg/L, pH = 2 – 10, T = 20°C – 50°C	- 85% decolorization efficiency for reactive yellow 84 was obtained at the optimum conditions of 25 mg/L Fe²⁺, 250 mg/L H₂O₂, pH 3, 20°C in 20 min. - Decolorization of reactive yellow 84 followed pseudo second order reaction rate. - The activation energy for the Fenton reaction was 16.78 kJ/mol.
Giri and Golder, 2014 [17]	Ciprofloxacin	C₀ = 15 mg/L, [Fe²⁺/H₂O₂] = 0.125, time = 45 min, T = 25°C and pH 3.5	- The degradation, COD and TOC removal for ciprofloxacin were 74%, 47.1% and 37.9%, respectively. - Thirteen intermediate products have been reported by mass spectra analysis. - The degradation of ciprofloxacin followed 2^nd order reaction rate with rate constant of 3.13 × 10³ M⁻¹ s⁻¹.
Fathi and Keshmirizadeh, 2015 [4]	Acid Red 52 dye, Basic Red 18 dye	C_0-AR52 and C_0-BR18 = 100 – 500 mg/L, [H₂O₂] = 1 – 3.5 g/L, [Fe²⁺] = 4 – 8 mg/L, pH = 3 – 9.	- The optimum conditions were: pH 3, 8 mg/L of Fe²⁺, 1.5 g/L of H₂O₂, 25°C and 18 min in reaction time. - The color and COD removal for acid red 52 dye were 99.5% and 92%, whereas for basic red 18 dye, it were 98.5% and 85%, respectively.
Ertugay and Acar, 2017 [16]	Direct Blue 71 azo dye	C₀ = 25 – 150 mg/L, pH = 2.0 – 6.0, [Fe²⁺] = 0 – 5 mg/L, [H₂O₂] = 25 – 150 mg/L, T = 20°C – 60°C	- At optimum conditions: pH 3.0, 3 mg/L Fe²⁺, 125 mg/L H₂O₂ and 20°C, the decolorization and COD removal of direct blue 71 were 100% and 49%, respectively.

Table 2

The Various Intermediate Products Formed After The Degradation of 4-NP

Intermediate name	Chemical formula	Molecular weight
4-nitrobenzene-1,2-diol (3,4-dihydroxynitrobenzene)	C₆H₅NO₄	155
Benzene-1,2,4-triol (1,2,4 trihydroxylbenzene)	C₆H₆O₃	126
4-aminobenzene-1,2-diol (1,2-dihydroxyaminophenol)	C₆H₇NO₂	126
4-aminophenol	C₆H₇NO	109
Hydroquinone	C₆H₆O₂	110
p-benzoquinone 1,4-benzoquinone	C₆H₄O₂	108
5-nitrobenzene-1,2,3-triol (3,4,5-tridydroxynitrobenzene)	C₆H₅NO₅	171
5-aminobenzene-1,2,3-triol (1,2,6-trihydroxy-4-aminophenol)	C₆H₇NO₃	141
Acetic acid	C₂H₄O₂	60

Table 3

Comparison of Present Study for The Degradation of 4-NP with Previous Studies

Reference	Pollutant	Initial operating conditions	Degradation (%)	COD removal (%)	TOC removal (%)	DOC removal (%)	Kinetic study
Ma et al., 2000 [18]	4-Nitrophenol	[4-NP]₀ = 1 mM, [H₂O₂] = 1 – 10 mM, [Fe²⁺] = 1 – 10 mg/L, pH = 3	99	-	-	30.6	4-NP degradation profiles fitted well into a pseudo first-order kinetic equation.
Kavitha and Palanivelu, 2005 [20]	2-nitrophenol (2-NP), 4-nitrophenol (4-NP), 2,4-dinitrophenol (DNP), 2,4,6-trinitrophenol (TNP)	[2-NP/4-NP]₀ = 1.43 mM, [H₂O₂] = 17.3 mM, [Fe²⁺] = 0.45 mM, [DNP] = 1.08 mM, [TNP] = 0.87 mM, pH = 3	-	-	-	32	Degradation of NPs obeys a pseudo-first-order kinetics. Treatment efficiency for the NPs followed the sequence: NP > DNP > TNP.
Present Study	4-NP	[4-NP]₀ = 0.72 mM, [H₂O₂] = 1 – 5 mM, [Fe²⁺] = 0.05 – 0.5 mM, pH = 3	93.6	50	60.3	-	The degradation of 4-NP followed second order kinetics.

Mineralization and degradation of 4-Nitrophenol using homogeneous Fenton oxidation process

Abstract

1. Introduction

2. Material and Methods

2.1. Materials

2.2. Experimental Procedure

2.3. Analytical Methods

(1)

(2)

(3)

3. Results and Discussion

3.1. Effect of pH

(4)

3.2. Effect of Reaction Time

3.3. Effect of H2O2 Concentration

(5)

3.4. Effect of Fe2+ Ions Concentration

(6)

(7)

3.5. Reaction Mechanism Showing the Degradation Pathways of 4-NP

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

(16)

(17)

(18)

3.6. Kinetic Study

(17)

(18)

3.7. Comparison with Earlier Studies

4. Conclusions

Acknowledgment

Notes

References

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Table 1

Table 2

Table 3

3.3. Effect of H₂O₂ Concentration

3.4. Effect of Fe²⁺ Ions Concentration