Many technologies have been proposed to oxidize chromium, such as roasting-water leaching technology and hydrometallurgical methods such as pressure oxidative leaching coupled with oxygen, ozone, permanganate and ferrate, but the problems associated with the high temperature, low overall resource utilization efficiency, high energy consumption, and the environmental pollution, still remain unsolved. This paper focuses on the oxidation process of chromium (III) with hydrogen peroxide (H2O2) in an alkaline medium. The effect of parameters including dosage of H2O2, dosage of NaOH, reaction time, reaction temperature and stirring rate on the oxidation efficiency of chromium were investigated. The oxidation efficiency was significantly affected by the dosage of H2O2 and NaOH, reaction time and reaction temperature took second place; last was the stirring rate. Oxidation efficiency was nearly 100% under the optimal conditions: volume ratio of H2O2 to mass of Cr2(SO4)3 of 2.4 mL/g, mass ratio of NaOH to Cr2(SO4)3 0.6 g/g, reaction time of 90 min, reaction temperature of 90 °C and stirring rate of 500 rpm.

Chromium is an important strategic resource widely used in chemical manufacturing, electroplating, leather tanning, metal corrosion inhibition, and pigment production (Gupta et al. 2011; Jin et al. 2013; Steffy et al. 2013; Wessel & Dronskowski 2013; Saleh & Al-Absi 2017; Saleh et al. 2017a; Saleh et al. 2017b; Xin et al. 2017; Yufen et al. 2017; Peng et al. 2018a; Peng et al. 2018b; Saleh 2018; Saleh et al. 2018; Abubakar et al. 2019). Chromium mainly exists as chromium (III) (Cr (III)) and chromium (VI) (Cr (VI)) which are stable and show different characteristics (Gupta et al. 2011; Saleh et al. 2016; Ali et al. 2017; Adio et al. 2019). Chromium (VI) is emitted by some industrial activities like petroleum refining, battery production and electroplating, etc. (Xu et al. 2004; Peng et al. 2015; Peng et al. 2016; Qing et al. 2017). And it mainly exists as dichromate (Cr2O72−) in acidic solution and chromate (HCrO4, CrO42−) in alkaline solution.

Many hydrometallurgical processes have been proposed to recover chromium (Shiyuan et al. 2016; Yuan et al. 2016; Hongrui et al. 2017; Qing et al. 2017; Spanka et al. 2018). Conventionally, sodium roasting-water leaching technology, calcium roasting-acid leaching (Xue et al. 2017b), and hydrometallurgical methods such as pressure oxidative leaching coupled with oxygen, ozone, permanganate and ferrate have been developed (Zhang et al. 2014; Li et al. 2015; Kim et al. 2016; Jing et al. 2017; Xue et al. 2017a), but the problems associated with the high temperature salt roasting technologies, including the low overall resource utilization efficiency, the high energy consumption, and the environmental pollution (toxic gas, and waste water), still remain unsolved. Hydrogen peroxide (H2O2) is widely used in the treatment of wastewater as an oxidant like the Fenton-like method (Li et al. 2018; Wang & Liu 2018; Xu et al. 2018; Zhao et al. 2018). H2O2 reacts with transition metal ions to generate ·OH according to Equations (1) and (2), which can be treated as the mechanism of Fenton-like reaction (Rahim Pouran et al. 2016; Wang et al. 2016).
formula
(1)
formula
(2)
formula
(3)
·OH is a powerful oxidizing reagent with high oxidation potential and it exhibits a faster rate of oxidation reaction (Gogate & Pandit 2004). ·OH attacks the organic pollutant molecules, leading to their degradation.

In this paper, H2O2 acted as the oxidant with the direct advanced oxidation process applied to oxidize chromium (III) to chromium (VI). The effect of dosage of H2O2, dosage of NaOH, reaction time, reaction temperature and stirring rate on the oxidation efficiency of chromium were preferentially examined.

Materials

Chromium sulfate (Cr2(SO4)3), hydrogen peroxide (H2O2), and sodium hydroxide (NaOH) of analytical grade were purchased from Kelong Co., Ltd, Chengdu, China, and used as received without purification. Deionized water used in the experiments was produced by a water purification system (HMC-WS10).

Apparatus and procedures

All experiments were performed in a glass beaker with a thermostatic mixing water bath pot. A predetermined amount of chromium sulfate and deionized water was added to the beaker to produce a homogeneous solution under constant stirring. The solution was heated to a predetermined temperature. Next, the sodium hydroxide was added to the reactor. Finally, H2O2 was added to the medium manually. After the required reaction time, the concentration of chromium (III) and chromium (VI) was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, PerkinElmer Optima 6300DV). The oxidation efficiency (η) of chromium was calculated as Equation (4):
formula
(4)
where, C1 and C2 are the concentration of chromium in the solution before and after the experiment; V1 and V2 are the volume before and after the experiment, mL.

Technology principle

The main reaction during the oxidation process was between chromium sulfate, hydrogen peroxide and sodium hydroxide, which reacted as Equation (5) (Dalun & Jianhua 2002). The of Equation (5) at different temperatures could be calculated with , and at 298 K, shown in Equations (6)–(8) (Dalun & Jianhua 2002).
formula
(5)
formula
(6)
formula
(7)
formula
(8)
Equation (9) was obtained by merging Equations (6)–(8).
formula
(9)
The specific heat capacity, Cp, was calculated as Equation (10)
formula
(10)
And also, ΔCp was calculated as Equation (11).
formula
(11)
Then was calculated using Equation (12).
formula
(12)
Integrate:
formula
(13)
The , , a, b, c and d in Equation (13) could be obtained from the reference (Dalun & Jianhua 2002). The calculation result is shown in Figure 1.
Figure 1

Relationship between ΔG and temperature of oxidation of chromium (III).

Figure 1

Relationship between ΔG and temperature of oxidation of chromium (III).

Close modal

The results shown in Figure 1 indicate that the oxidation of Cr3+ with hydrogen peroxide is feasible in thermodynamics as the ΔG was negative (Xiancai et al. 2005).

During the oxidation process, ·OH, O22− and O2 were produced by heating the H2O2 solution. These were responsible for the oxidation of chromium (III) according to Equations (14)–(17).
formula
(14)
formula
(15)
formula
(16)
formula
(17)

Effect of volume ratio of H2O2 to mass of Cr2(SO4)3

The effect of volume ratio of H2O2 to mass of Cr2(SO4)3 (VR) on the oxidation efficiency of chromium was preferentially examined under the following conditions: reaction temperature of 90 °C, reaction time of 90 min, mass ratio (MR) of NaOH to Cr2(SO4)3 of 0.6 g/g and stirring rate of 500 rpm. The oxidation efficiency and the concentration of Cr (III) and Cr (VI) in the solution are summarized in Figure 2.

Figure 2

Effect of VR on oxidation efficiency of chromium (reaction temperature 90 °C, reaction time of 90 min, mass ratio of NaOH to Cr2(SO4)3 of 0.6 g/g, stirring rate of 500 rpm, VR was set as 0.2 (V(H2O2) = 1 mL), 0.8 (V(H2O2) = 4 mL), 1.4 (V(H2O2) = 7 mL), 1.8 (V(H2O2) = 9 mL) and 2.4 (V(H2O2) = 12 mL)).

Figure 2

Effect of VR on oxidation efficiency of chromium (reaction temperature 90 °C, reaction time of 90 min, mass ratio of NaOH to Cr2(SO4)3 of 0.6 g/g, stirring rate of 500 rpm, VR was set as 0.2 (V(H2O2) = 1 mL), 0.8 (V(H2O2) = 4 mL), 1.4 (V(H2O2) = 7 mL), 1.8 (V(H2O2) = 9 mL) and 2.4 (V(H2O2) = 12 mL)).

Close modal

The results in Figure 2 show that the Cr3+ could be oxidized to CrO42− effectively in an alkaline medium and the dosage of H2O2 had a significant effect on the oxidation of Cr3+. The oxidation efficiency was linearly increasing with the increase of VR. The chromium in the solution was Cr3+ in the original solution and the solution was dark green in color. The color of the solution and formation of chromium changed with the addition of NaOH and H2O2. When VR = 0.2 mL/g, chromium mainly existed as Cr(OH)3 in the alkaline medium and the color of the solution was green. As the VR increased, the reaction occurred in the solution according to Equation (14) and Cr3+ was oxidized to CrO42−; the solution was changed to yellow. When the VR = 2.4 mL/g, the chromium in the solution almost all existed as CrO42− and the oxidation efficiency was nearly 100%.

Effect of mass ratio of NaOH to Cr2(SO4)3

The Cr3+ in the acid medium was hard to oxidize except if the oxidant was persulfate salt. While it was easy for it to be oxidized in an alkaline medium (Banwen & Yuji 1998), the dosage of NaOH had a big influence on the oxidation efficiency of Cr3+. The effect of dosage of NaOH on the oxidation efficiency and the concentration of Cr (III) and Cr (VI) in the solution was preferentially studied under the following conditions: VR of 2.4 mL/g, reaction temperature of 90 °C, reaction time of 90 min and stirring rate of 500 rpm. During the oxidation process, NaOH was not only an important reactant but also it provided an alkaline medium. The results shown in Figure 3 indicate that the dosage of NaOH significantly affected the oxidation process. The oxidation efficiency was only 36.92% at MR = 0.2 and then increased to 99.67% as MR increased to 0.6.

Figure 3

Effect of dosage of NaOH on oxidation efficiency of chromium (VR of 2.4 mL/g, reaction temperature of 90 °C, reaction time of 90 min and stirring rate of 500 rpm; MR was set as 0.2 (m(NaOH) = 1.0 g), 0.4 (m(NaOH) = 2.0 g), 0.6 (m(NaOH) = 3.0 g), 0.8 (m(NaOH) = 4.0 g) and 1.0 (m(NaOH) = 5.0 g)).

Figure 3

Effect of dosage of NaOH on oxidation efficiency of chromium (VR of 2.4 mL/g, reaction temperature of 90 °C, reaction time of 90 min and stirring rate of 500 rpm; MR was set as 0.2 (m(NaOH) = 1.0 g), 0.4 (m(NaOH) = 2.0 g), 0.6 (m(NaOH) = 3.0 g), 0.8 (m(NaOH) = 4.0 g) and 1.0 (m(NaOH) = 5.0 g)).

Close modal

Effect of reaction temperature

The reaction temperature (T) was another important parameter that had a significant influence on the oxidation process of chromium, both thermodynamic and kinetic. Figure 4 summarizes the effect of reaction temperature on the percentage of Cr (III) and Cr (VI) in a reactant solution under the standard conditions: VR of 2.4 mL/g, reaction time of 90 min, MR of 0.6 g/g and stirring rate of 500 rpm.

Figure 4

Effect of reaction temperature on oxidation efficiency of chromium (VR of 2.4 mL/g, reaction time of 90 min, MR of 0.6 g/g and stirring rate of 500 rpm).

Figure 4

Effect of reaction temperature on oxidation efficiency of chromium (VR of 2.4 mL/g, reaction time of 90 min, MR of 0.6 g/g and stirring rate of 500 rpm).

Close modal

It could be concluded that reaction temperature was an important parameter during the oxidation process according to the reaction equations, and was consistent with the results shown in Figure 4. The oxidation efficiency was about 70% at 30 °C and then increased linearly with the increase of reaction temperature. The results were consistent with the results shown in Figure 1, in which the oxidation of Cr3+ happened easily at low temperature. In a high temperature condition, reactants like Cr3+, ·OH, O22− and O2 had high reactivity and the reaction happened more easily. Therefore, high reaction temperature was an essential condition during the oxidation process.

Effect of reaction time

The effect of reaction time on the oxidation efficiency and the concentration of Cr (III) and Cr (VI) in the solution were preferentially examined under the following conditions: VR of 2.4 mL/g, stirring rate of 500 rpm, reaction temperature of 90 °C, and MR 0.6 g/g. The oxidation efficiency and the concentration of Cr (III) and Cr (VI) in the solution are summarized in Figure 5. It could be concluded that the reaction time has little influence on the oxidation efficiency of Cr3+, as the oxidation efficiency was about 80% at 30 min according to the results showed in Figure 5. Extending the reaction time could accelerate the oxidation reaction and improve the oxidation efficiency as it was nearly 100% at 90 min.

Figure 5

Effect of reaction time on oxidation efficiency of chromium (VR of 2.4 mL/g, reaction temperature of 90 °C, stirring rate of 500 rpm, and MR 0.6 g/g).

Figure 5

Effect of reaction time on oxidation efficiency of chromium (VR of 2.4 mL/g, reaction temperature of 90 °C, stirring rate of 500 rpm, and MR 0.6 g/g).

Close modal

Effect of stirring rate

The mixing performance was significantly affected by the stirring rate and stirring blade in the chemical process (Liu et al. 2014). The effect of stirring rate on the oxidation efficiency and the concentration of Cr (III) and Cr (VI) in the solution were preferentially examined under the following conditions: VR of 2.4 mL/g, reaction temperature of 90 °C, and MR 0.6 g/g and reaction time of 90 min. The results shown in Figure 6 suggest that stirring rate had little effect on the reaction. In other words, the oxidation reaction between Cr3+ and H2O2 in an alkaline medium was easily reacted and Cr3+ was easily oxidized to CrO42− by H2O2.

Figure 6

Effect of stirring rate on oxidation efficiency of chromium (VR of 2.4 mL/g, reaction temperature of 90 °C, MR 0.6 g/g and reaction time of 90 min).

Figure 6

Effect of stirring rate on oxidation efficiency of chromium (VR of 2.4 mL/g, reaction temperature of 90 °C, MR 0.6 g/g and reaction time of 90 min).

Close modal

Oxidation kinetics

The oxidation kinetic of chromium by H2O2 in an alkaline medium could be expressed as Equation (18):
formula
(18)
where, Ccr is the concentration of Cr(VI); k is the reaction constant; Pc is the concentration of other reagents except Cr(VI), which was a constant as the concentration of other reagents were much larger.
Equation (18) was simplified as:
formula
(19)
Integrate:
formula
(20)
formula
(21)
The oxidation efficiency of chromium at different temperatures is fitted in Figure 7, and the value of K was obtained, where K is the reaction rate constant corresponding to the slopes of the straight lines. Then the specific apparent activation energy could be calculated based on the Arrhenius equations (Equation (22)), the result are shown in Figure 8.
formula
(22)
where Ea is the apparent activation energy, A is the pre-exponential factor, and R is the molar gas constant.
Figure 7

Plot of oxidation kinetics of chromium at various reaction temperatures.

Figure 7

Plot of oxidation kinetics of chromium at various reaction temperatures.

Close modal
Figure 8

Natural logarithm of oxidation constant versus reciprocal temperature of chromium.

Figure 8

Natural logarithm of oxidation constant versus reciprocal temperature of chromium.

Close modal

The apparent activation energy of chromium oxidation is calculated to be 16.29 kJ/mol, the value was small and that indicated that chromium was easily oxidized by H2O2 in an alkaline medium.

In this paper, the oxidation process of Cr3+ with H2O2 in an alkaline medium was investigated. The results can be summarized as follows:

  • (1)

    Chromium in low valence (Cr3+) is easily oxidized to high valence (CrO42−) with H2O2 in an alkaline medium. It is an environmentally-friendly technology showing substantial advantages in terms of energy efficiency, overall resource utilization efficiency and environmental pollution.

  • (2)

    The dosage of H2O2 and NaOH had significant influence on the oxidation efficiency of chromium; reaction time and reaction temperature took second place; and last was the stirring rate.

  • (3)

    The oxidation efficiency was nearly 100% under the optimal conditions: volume ratio of H2O2 to mass of Cr2(SO4)3 of 2.4 mL/g, mass ratio of NaOH to Cr2(SO4)3 0.6 g/g, reaction time of 90 min, reaction temperature of 90 °C and stirring rate of 500 rpm.

This work was supported by Chongqing Science and Technology Commission (CN) (No. cstc2018jcyjAX0018), Talent Introduction Project of Yangtze Normal University (No. 2017KYQD117) and National Natural Science Foundation of China (No. 21576033 and No. 21636004).

The authors declare no conflicts of interest.

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