Exothermic laws applicable to the degradation of o-phenylenediamine in wastewater via a Fe3+/H2O2 homogeneous quasi-Fenton system

We studied the exothermic laws of Fe3+/H2O2 homogeneous quasi-Fenton degradation of o-phenylenediamine in waste water, and analyzed the effects of [H2O2] and [Fe3+], initial reaction temperature, and other factors on the solution temperature elevation (Δt), temperature elevation duration (T), and chemical oxygen demand degradation rate (η) during the degradation of the target pollutant. Our study found that [H2O2] is a major factor affecting Δt, while [Fe3+] and t0 are the main factors influencing the exothermic reaction rate. For the conditions wherein [H2O2] is 0.2 mol L−1, [Fe3+] is 10 mmol L−1, pH = 7.8, initial reaction temperature is 30 °C, and reaction duration is 30 min, Δt of 200 mL of 0.04 mol L−1o-phenylenediamine is 7.2 °C and η is 93.45%. The exothermic reaction between the free radicals (·OH and ) and o-phenylenediamine and the exothermic reaction due to auto-consumption of free radicals are the main reasons for the increased temperature of the solution.

To reduce the environmental damage caused by fuel combustion, comprehensive energy utilization technology represented by heat pump has gained increasing attention. Low-input, high-grade electrical energy can be used utilized, via heat pumping technology, to upgrade low-grade thermal energy from urban wastewater to high-grade thermal energy that can be used during winter. The unique thermal energy collection method of this technology can effectively avoid generating harmful substances such as sulfur dioxide, nitrous oxide, heavy metals, and dust that occur during the combustion of fossil fuels; 1-3 as such, it is a green heat supply technology that should be promoted. To ensure that the overall wastewater treatment process is not affected, existing wastewater heat pumps usually control the wastewater temperature to within 5 C during thermal energy extraction. Therefore, studying treatment techniques in which increases in water temperature approaches or exceeds 5 C during wastewater degradation has important signicance for increasing the thermal energy output of the wastewater heat pump and utilizing the energy efficiently during wastewater treatment.
Aniline chemicals such as o-phenylenediamine are raw materials for the manufacture of chemicals such as dyes, pesticides, rubber, and paints, and when the sewage is discharged into the environment, it may cause potential harm to aquatic organisms and human health. It is difficult to effectively remove it by water treatment technology using traditional physical and chemical means or biodegradation. In recent years, advanced oxidation water treatment technology, which is mainly composed of strong oxidizing free radicals, has been rapidly developed. During the Fenton advanced oxidation process, the $OH ions produced by Fenton's reagent have an extremely strong oxidation ability, which is only weaker than F 2 among known oxidation reagents and can effectively degrade organic matter in wastewater. In addition, the entire process is simple and is oen used to treat wastewater containing undegradable organic matter. [4][5][6] However, conventional homogeneous Fenton systems require pH levels to be controlled below 3; thus, high volumes of acid are required for the acidication as a part of the wastewater treatment. Therefore, aer degradation is complete, correspondingly high quantities of alkalis are required to neutralize the treated wastewater, which greatly increases the cost of the process. [7][8][9] At present, research on Fenton technology mainly focuses on overcoming its own shortcomings such as narrow pH range. [10][11][12] Deng et al. 13 employed liquid-phase precipitation to prepare an iron vanadate catalyst and expanded the pH range for the degradation to 3.0-8.0. Sun et al. 14 prepared an iron vanadate Fenton catalyst under neutral conditions using hydrothermal methods and tested it with relatively few samples. This resulted in better dispersion of nanorods and a higher ability of oxidative degradation at a pH range of 1.0-9.0. These heterogeneous Fenton systems employed a solid catalyst to promote H 2 O 2 degradation to produce $OH while maintaining the strong oxidation capability of a homogeneous Fenton system, expanding the pH range suitable for the reaction, and decreasing wastewater treatment costs. However, studies on the heat generation law of Fenton's reagents for degrading organic wastewater have not been reported.
In recent years, there has been a high volume of research and applications using Fenton or quasi-Fenton advanced oxidation technology to treat hard-to-degrade organic wastewater that primarily produces $OH. There are several thousands of studies on this, but there is still no report of work on the exothermic law of the Fe 3+ /H 2 O 2 homogeneous Fenton systems during wastewater treatment. To realize the comprehensive utilization of energy treatment of wastewater treatment, H 2 O 2 and Fe 3+ were added at various concentrations to wastewater with o-phenylenediamine being the target pollutant. o-Phenylenediamine concentration, inuence of initial reaction temperature on the increase in solution temperature, duration for which the temperature remained elevated, and chemical oxygen demand (COD) degradation rate were determined. We also summarized the exothermic law of the Fe 3+ /H 2 O 2 quasi-Fenton system during degradation of wastewater containing o-phenylenediamine and carried out a preliminary investigation of exothermic mechanisms. This study used a digital display thermostatic water bath (HH-S, Tianjin Saidelisi Experimental Analyzer Factory), a pH meter (PHS-3C, Yoke Instrument Co., Ltd), a temperature and humidity recorder (TH22R-XX, Hua Han Wei Co., Ltd), a COD analyzer (JHR-2, Genstar Electronic Technology Co., Ltd), and a laboratory stirrer (JJ-1, Changzhou Boyuan Instrument Plant). Fig. 1 shows the experimental setup. The reactions were carried out in a 500 mL round-bottom ask containing a stirring rod and a thermometer. The stirring rod was connected to a stirrer through the mouth of the ask, and the thermometer was connected to the temperature and humidity recorder with wires. The outer wall of the round-bottom ask was completely covered with a thermal insulation layer.

Experiment process
A certain amount of o-phenylenediamine was weighed, and a certain concentration of o-phenylenediamine simulated wastewater was prepared in a volumetric ask with deionized water. Potassium dichromate method was used to determine the COD of the solution. The pH of the simulated wastewater was adjusted using sulfuric acid and sodium hydroxide. Before the start of the degradation reaction, the volumetric ask containing the o-phenylenediamine solution was placed in a constant temperature water bath, and the solution was warmed to above 5 C above the initial set reaction temperature. During the experiment, a measuring cylinder was used to add 200 mL of heated o-phenylenediamine solution to the roundbottom ask (Fig. 1), aer which the stirrer was turned on (600 rpm). Aer being cooled naturally to the initial reaction temperature, H 2 O 2 solution was added to the o-phenylenediamine solution rst; Fe 2 (SO 4 ) 3 solid powder was added to ophenylenediamine solution 2 min later while the temperature and humidity recorders were started simultaneously. This was taken to be the start of the reaction. The reaction temperature and reaction duration were recorded once every minute. During the reaction, the increase in solution temperature Dt was calculated using eqn (1), while the duration of the increase in solution temperature T was calculated from the reaction start point to the point at which the temperature began to drop.
Aer the reaction began, samples were collected at 10 min intervals and a 102 moderate speed qualitative lter paper was used to lter the sample. The COD of the ltrate was measured using potassium dichromate. The level of COD reduction in the solution, DQ, was calculated using eqn (2) below: The COD degradation rate of the solution, h, was calculated using eqn (3) as follows: where Dt is the temperature difference before and aer o-phenylenediamine degradation, t 0 and t are the temperatures of the o-phenylenediamine solution before and aer the reaction, respectively, Q 0 is the initial COD of the o-phenylenediamine solution, Q is the COD value aer o-phenylenediamine degradation, and h is the COD degradation rate of the o-phenylenediamine solution.

Effects of the concentration of hydrogen peroxide added
To study the effect of the concentration of H 2 O 2 on Dt, T and h, the reaction conditions were as follows: the concentration of ophenylenediamine solution was 0.04 mol L À1 , volume was 200 mL, initial reaction temperature was 30 C, pH ¼ 7.8, concentration of Fe 3+ was 10 mmol L À1 , and reaction time was 30 min. According to the Fenton mechanism, 15 reactions depicted by eqn (4)-(9) occur simultaneously in the Fenton system. Fig. 3 shows that when the concentration of the H 2 O 2 added increased from 0.05 mol L À1 to 0.2 mol L À1 , the h of o-phenylenediamine increased from 61.13% to 93.45%. The value of h rst increased, before decreasing to 85.89%, as the concentration of H 2 O 2 added increased to 0.4 mol L À1 . 16,17 This occurred because when the concentration of H 2 O 2 added was relatively low, almost all the $OH and HO 2 produced during the reactions in eqn (4) and (5) also reacted with o-phenylenediamine as shown in eqn (6). 18 At this point, the degradation rate increased as the concentration of H 2 O 2 added increased. When the concentration of H 2 O 2 added reached a certain value, $OH and HO 2 free radicals underwent auto-consumption as per eqn (7)- (9). This resulted in ineffective decomposition of some of the H 2 O 2 , 19,20 and consequent decrease in the degradation rate.

Effects of the concentration of Fe 3+ added
To study the effect of the concentration of Fe 3+ on Dt, T and h, the reaction conditions were as follows: the concentration of ophenylenediamine solution was 0.04 mol L À1 , volume was 200 mL, initial reaction temperature was 30 C, pH was 7.8, concentration of H 2 O 2 was 0.2 mol L À1 , and the reaction time was 30 min.    Fig. 5 shows that when the concentration of Fe 3+ added increased from 5 mmol L À1 to 10 mmol L À1 , h increased from 89.5% to 93.45%. Subsequently, when the concentration of Fe 3+ added increased to 30 mmol L À1 , h decreased to 82.67%. This showed that h rst increased before decreasing, when the concentration of Fe 3+ added increased. 21 This was mainly due to the fact that when Fe 3+ concentration was low, the reaction rate was slow; this inhibited the synthesis of $OH and HO 2 ; 22,23 thereby affecting the reaction between the free radicals and ophenylenediamine. As the concentration of Fe 3+ added increased, the reaction rate and the number of free radicals produced per unit of H 2 O 2 added increased, causing a subsequent rise in h. When the concentration of Fe 3+ added was too high, the reaction rate of the processes in eqn (4)- (6), which are favorable to organic matter degradation, and the ones in eqn (7)-(9) that cause autoconsumption, increased simultaneously. This resulted in a rapid decrease in the reaction heat release duration T. When the autoconsumption reaction rate exceeds the organic matter degradation reaction rate, the degradation rate decreases. 18

Effects of o-phenylenediamine concentration
To study the effect of the initial concentration of o-phenylenediamine on Dt, T and h, the reaction conditions were as follows: the volume of o-phenylenediamine solution was 200 mL, initial reaction temperature was 30 C, pH was 7.8, concentration of H 2 O 2 was 0.2 mol L À1 , concentration of Fe 3+ was 10 mmol L À1 , and reaction time was 40 min. Fig. 6 and 7 depict the Dt/T curves and h curves of the o-phenylenediamine solution with the initial concentration of o-phenylenediamine respectively. Fig. 6 and 7 show the Dt/T and h curves of how o-phenylenediamine concentrations changed from initial levels when the initial reaction temperature was 30 C, concentration of H 2 O 2 added was 0.2 mol L À1 , concentration of Fe 3+ added was 10 mmol L À1 , and duration of the reaction was 40 min. As the COD values for o-phenylenediamine solutions with different concentrations were different, DQ was used instead of h to more accurately reect the degradation of these o-phenylenediamine solutions in the H 2 O 2 /Fe 3+ system. Fig. 6 and 7, respectively, show that when [C 6 H 8 N 2 ] is 0.01 mol L À1 , Dt is 6.6 C, T is 6.2 min, and DQ is 1352.73 mg L À1 ; and when [C 6 H 8 N 2 ] is 0.06 mol L À1 , Dt is 10.3 C, T is 34 min, and DQ is 9956.62 mg L À1 , the values of Dt, T, and DQ increase as the initial concentration of o-phenylenediamine increases. This is because when the initial concentration of o-phenylenediamine is increased, there is an increased probability of collisions between o-phenylenediamine molecules and $OH and HO 2 ; and the number of molecules participating in the reaction and the heat released by the reaction increases, causing Dt and DQ to increase. When the number of reactions between o-phenylenediamine and $OH and HO 2 increases, phase changes alleviate the occurrence of side reactions by the free radicals and increase the effective utilization rate of $OH and HO 2 : 24 The rate of at which $OH and HO 2 are generated in the solution limits the reaction rate between the free radicals   and o-phenylenediamine molecules. When the initial concentration of H 2 O 2 and Fe 3+ added to the solution are xed, the initial reaction rate between $OH and HO 2 and ophenylenediamine is relatively stable. The greater the number of o-phenylenediamine molecules in the solution, the longer the time required for the reaction; this is evidenced as an increase in T as the initial concentration of ophenylenediamine increases.

Effects of initial reaction temperature
To determine the effect of initial reaction temperature on Dt, T and h, the reaction conditions were as follows: the concentration of o-phenylenediamine solution was 0.04 mol L À1 , volume was 200 mL, pH was 7.8, concentration of H 2 O 2 was 0.2 mol L À1 , Fe 3+ was added at a concentration of 10 mmol L À1 , and the reaction time was 60 min. Fig. 8 shows that Dt increases as the initial reaction temperature increases and that T decreases rapidly as the initial reaction temperature increases. When the initial reaction temperature is 10 C, T is 63 min; when the initial reaction temperature is 60 C, T is 1 min. This occurs mainly because the number of activated molecules in the solution increases when initial reaction temperature increases, thereby accelerating the reaction rate. Fig. 9 shows that when t ¼ 10 C, h is 94.81% and when t ¼ 60 C, h is 90.18%, the h curve shows a decreasing trend when initial reaction temperature increases. This is mainly because the rates of auto-consumption caused by the processes described in eqn (7) and (8)

Environmental heat analysis
In Fig. 11, curve a shows the variation in temperature when the room temperature is 20.5 C and the initial temperature of 200 mL deionized water is 37.3 C in the experimental setup; curve b is the corresponding temperature reduction curve. Curve a shows that even though thermal insulation measures were adopted as much as possible in the experimental setup given in Fig. 1, the problem of heat dissipation to the surrounding environment still exists. In the experimental setup, the temperature of the deionized water gradually decreases as time increases; it reached 26.6 C from the initial temperature of 37.3 C in 120 min, a temperature reduction of 10.7 C. The curve shows that the differences in temperature reduction gradually increase as the incubation time increases; the differences in temperature reduction for 10, 20, 30, and 120 min were 1.6 C, 3 C, 4.2 C, and 10.7 C, respectively. This shows that heat dissipation from the system to the surrounding environment is an important factor affecting temperature elevation data. When reaction time is longer, revisions in heat dissipation values are required for the temperature elevation data.   added is 0.2 mol L À1 , the concentration of Fe 3+ added is 10 mmol L À1 , and the reaction duration is 120 min. Curve b is the measured Dt curve and curve a is the Dt curve aer compensating for heat dissipation from the system to the surrounding environment. Curve a shows that during o-phenylenediamine degradation in the Fe 3+ /H 2 O 2 quasi-Fenton system, the process by which the temperature of the solution increases can be divided into rapid, stable, and lag phases of temperature increase. In the rst 10 min aer the start of the reaction, Dt and h increase rapidly as time progresses. When T is 10 min, Dt is 8.7 C and h is 89.62%. This is because during the early stages of the reaction the concentrations of o-phenylenediamine, H 2 O 2 , and Fe 3+ in the solution are relatively high; therefore, the cyclic reaction between Fe 3+ , Fe 2+ , and H 2 O 2 is faster and the $OH and HO 2 produced reacts with o-phenylenediamine. During o-phenylenediamine degradation, large amounts of heat are released simultaneously, causing Dt and h to increase rapidly. This is the rapid phase of the temperature increase. As the reaction progresses, the concentration of ophenylenediamine and H 2 O 2 in the solution decreases rapidly, the reaction becomes weaker, and t and h show a slight increasing trend. When T is 20 min, the increases in t and h reach a critical point, curve a shows a decline in increasing trend, an inection point appears on curve b and starts to decrease while h approaches its maximum. This shows that aer 20 min of reaction, the speed of the heat release from the reaction system starts decreasing below the rate of heat dissipation to the surrounding environment. This is the stable phase of the temperature increase. Following that, the H 2 O 2 in the solution is almost depleted as the reaction time increases; the reaction becomes extremely weak and the heat released by the system itself approaches 0. This stage is the lag phase of the temperature increase.

pH change analysis
It was found that Fe 3+ is the main factor affecting the pH value of o-phenylenediamine solution, while H 2 O 2 has little effect. Change of pH in the reaction process was studied under the following reaction conditions: concentration of o-phenylenediamine solution ¼ 0.04 mol L À1 , initial reaction temperature ¼ 30 C, pH ¼ 7.8, concentration of H 2 O 2 ¼ 0.2 mol L À1 . pH change of an o-phenylenediamine solution aer the addition of H 2 O 2 and Fe 3+ is shown in Fig. 13. pH of the o-phenylenediamine solution rapidly decreased aer the addition of H 2 O 2 and Fe 3+ . When Fe 3+ was 5 mmol L À1 , 10 mmol L À1 and 20 mmol L À1 , the pH of the o-phenylenediamine solution was 4.8, 3.2 and 1.8, respectively; this can be attributed to the hydrolysis reaction of Fe 3+ in the solution. As shown in eqn (10), the higher the Fe 3+ concentration, the more H + produced in the solution, resulting in lower pH of the solution.
Fe 3+ + 3H 2 O 2 / Fe(OH) 3 + 3H + The above results show that under conditions in which the concentrations of o-phenylenediamine and Fe 3+ added are identical, Dt rapidly increases as the concentration of H 2 O 2   added increases. This shows that the concentration of H 2 O 2 added is a main factor affecting the heat released by the Fe 3+ / H 2 O 2 quasi-Fenton system. Moreover, under the same conditions, Dt rapidly increases as the concentration of Fe 3+ added increases. This shows that the concentration of Fe 3+ added or the initial reaction temperature of the solution are the main factors affecting the heat released by the Fe 3+ /H 2 O 2 quasi-Fenton system. The effects of the concentration of Fe 3+ added on the exothermic reaction rate is mainly due to the acceleration of the cyclic reaction between Fe 3+ , Fe 2+ , and H 2 O 2 that occurs when the concentration of Fe 3+ added increases 25,26 and more $OH and HO 2 are produced in the solution. This accelerates the degradation of organic matter. The initial reaction temperature affects reaction rate is because the number of activated molecules in the solution increases as temperature increases.

Conclusions and outlook
The study of the exothermic laws during H 2 O 2 /Fe 3+ homogeneous Fenton degradation of o-phenylenediamine wastewater allowed us to draw the following conclusions: (1) o-Phenylenediamine degradation by the H 2 O 2 /Fe 3+ homogeneous Fenton system is accompanied by an increase in the temperature of the solution, and the concentration of H 2 O 2 added is the main factor affecting solution temperature elevation. The increase in temperature of the solution occurs as the concentration of H 2 O 2 increases. However, the concentration of Fe 3+ added and the initial reaction temperature of the solution are the main factors affecting the exothermic reaction rate of the Fe 3+ /H 2 O 2 quasi-Fenton system. The greater the concentration of Fe 3+ added to the system, or the higher the initial reaction temperature, the faster the exothermic reaction rate. When [H 2 O 2 ] is 0.2 mol L À1 , Fe 3+ is 10 mmol L À1 , initial reaction temperature is 30 C, reaction duration is 30 min, Dt is 7.2 C, and h is 93.45% when the concentration of 200 mL o-phenylenediamine is 0.04 mol L À1 .
(2) The heat produced during the degradation of o-phenylenediamine by the Fe 3+ /H 2 O 2 quasi-Fenton system originates mainly from the exothermic reaction between the $OH and HO 2 and o-phenylenediamine molecules, its intermediate products, and the exothermic reaction when active groups undergo autoconsumption.
This paper summarizes the exothermic laws applicable when wastewater containing o-phenylenediamine is degraded by a Fe 3+ /H 2 O 2 homogeneous quasi-Fenton system, and has positive signicance for reducing wastewater treatment costs and improving energy utilization. However, there is still insufficient research on the amount and proportion of heat released in the various stages. Future studies will examine the heat released during these stages from the mechanism by which o-phenylenediamine is degraded by $OH and HO 2 :

Conflicts of interest
There are no conicts to declare.