# Electrochemical Removal of Chromium (VI) from Wastewater

^{*}

## Abstract

**:**

_{2}SO

_{4}, current density, and reaction temperature. The reduction efficiency was up to 86.45% at an H

_{2}SO

_{4}concentration of 100 g/L, reaction temperature of 70 °C, current density at 50 A/m

^{2}, reaction time at 180 min, and stirring rate of 500 rpm. The reduction process of chromium (VI) followed a pseudo-first-order equation, and the reduction rate constant could be expressed as Kobs = k [H

_{2}SO

_{4}]

^{1}·[j]

^{4}·exp

^{−4170/RT}.

## 1. Introduction

^{2+}and the free electron, while the current intensity had little effect on the reduction process. In this paper, electrochemical technology was applied to reduce chromium (VI) in an acidic medium. The mechanisms and parameters affecting the reaction—including the concentration of H

_{2}SO

_{4}, reaction time, reaction temperature, and current density—were investigated; the kinetic model was also simulated.

## 2. Materials and Methods

#### 2.1. Materials

_{2}Cr

_{2}O

_{7}) and sulfate acid (H

_{2}SO

_{4}), and were purchased from Kelong Co., Ltd., Chengdu, China. All solutions were prepared with deionized water with a resistivity greater than 18 MΩ/cm (HMC-WS10).

#### 2.2. Experimental Procedure

_{2}Cr

_{2}O

_{7}in distilled water, and the acidic medium was prepared by adding different volumes of H

_{2}SO

_{4}, then the current supplied by a DC power supply was applied as the solution was heated to a predetermined temperature. During the experiments, the samples were collected at different intervals (5 min), and analyzed for the residual concentration of chromium (VI) in the solution [10]. The electrode used in the experiments was a plate-like dimensionally stable anode (Baoji Zhiming Special Metal Co., Ltd., Shanxi, China) with a surface area of 1 cm

^{2}(1 × 1 cm). A cathode with an identical surface area was fixed at a distance of 2 cm [21].

_{1}and C

_{2}are the concentrations of chromium in the solution before and after the experiment, in g/L; and V

_{1}and V

_{2}are the volumes before and after the experiment, in L.

#### 2.3. Kinetics Model

_{obs}is the reaction rate constant which depends on fluid flow and reaction temperature conditions; and c

_{0}is the initial concentration of chromium (VI) in the wastewater, in g/L.

## 3. Results and Discussions

#### 3.1. Electro-Reduction of Chromium (VI)

#### 3.1.1. Reaction Mechanism

^{2+}, by the oxidation of a steel electrode by a DC power supply:

^{2+}and chromium (VI). The reactions are described in Equations (5) and (6), taking the pH of the wastewater into account.

^{−}) supplied by the DC power supply. The reaction that occurred between the e

^{−}and chromium (VI) is shown in Equations (7) to (10). The Gibbs free energy of the equations at different reaction temperatures was calculated, and the results are shown in Figure 1. The negative of △G indicated that the reduction reaction was feasible in thermodynamics [28].

#### 3.1.2. Effect of Concentration of H_{2}SO_{4}

_{2}SO

_{4}on the reduction efficiency was investigated, while other conditions—the initial concentration of Cr (VI) of 1.000 g/L, current density of 50 A/m

^{2}, reaction temperature of 70 °C, and stirring rate of 500 rpm—were kept constant. The concentration of H

_{2}SO

_{4}was set as 20 g/L, 40 g/L, 60 g/L, 80 g/L, and 100 g/L.

_{2}O

_{7}

^{2−}was more easily reduced in acidic conditions than in neutral/alkaline conditions. This can be seen in Figure 1, where the ΔG was smallest according to Equations (7), (9), and (10) [8]. The chemical equilibrium of Equation (8) was destroyed and the reaction proceeded to the generation of Cr

_{2}O

_{7}

^{2−}with the increasing acid concentration. Following this, the molar fraction of the Cr

_{2}O

_{7}

^{2−}was drastically increased, which was beneficial for the reduction of chromium (VI). Therefore, a concentration of 100 g/L was selected for further experiments.

#### 3.1.3. Effect of Reaction Temperature

^{2}, concentration of H

_{2}SO

_{4}of 100 g/L, and stirring rate of 500 rpm—were kept constant. The results shown in Figure 3 indicate that the reduction of chromium (VI) could be easily achieved at a higher reaction temperature (≥50 °C), which was partially consistent with recent studies [8]. A higher temperature would decrease the diffusion resistance and favor the contact of the free electron and chromium (VI), resulting in a high reduction efficiency. Thus, a reaction temperature of 70 °C was chosen to be the optimum.

#### 3.1.4. Effect of Current Intensity

_{2}SO

_{4}of 100 g/L, reaction temperature of 70 °C, and stirring rate of 500 rpm—were kept constant. It could be seen that the reduction efficiency increased when current density was increased, even though it was not increased by very much. The reduction efficiency was up to 86.45% at a current density of 50 A/m

^{2}. In other words, a higher current density could achieve a high reduction efficiency.

#### 3.2. Kinetic Model

^{2}) were all close to 1, which indicates that the kinetic model followed a pseudo-first-order model equation. The results shown in Figure 5a indicate that the reduction rate constant (K

_{obs}) increased when the concentration of H

_{2}SO

_{4}([H

_{2}SO

_{4}]) was increased. Figure 5d displays the relationship between K

_{obs}and [H

_{2}SO

_{4}]. The results show that K

_{obs}changed linearly with [H

_{2}SO

_{4}]—the relationship between them could be expressed as Equation (11). The effect of current density ([j]) was a little complicated, with the K

_{obs}shown to be multipower with [j] following Equation (12). The relationship between K

_{obs}and the reaction temperature (T) could be expressed with the Arrhenius equation (Equation (13)) and the specific apparent activation energy could be calculated. The results shown in Figure 5f show that the simulated Arrhenius equation and the Ea was calculated as 4.74 KJ/mol, so the Arrhenius equation could be changed to make Equation (14):

_{2}SO

_{4}, current density, and reaction temperature. The reduction rate constant could be express as Equation (15).

## 4. Conclusions

_{2}SO

_{4}concentration of 100 g/L, reaction temperature of 70 °C, current density at 50 A/m

^{2}, reaction time of 180 min, and stirring rate of 500 rpm.

_{2}SO

_{4}, current density, and reaction temperature. The reduction rate constant of chromium (VI) to chromium (III) could be expressed as Kobs = k [H

_{2}SO

_{4}]

^{1}·[j]

^{4}·exp

^{−4170/RT}.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 5.**Pseudo-first-order models for the reduction of chromium (VI). (

**a**): Effect of H

_{2}SO

_{4}concentration; (

**b**): Effect of current density; (

**c**): Effect of reaction temperature; (

**d**): Plot of K

_{obs}as a function of H

_{2}SO

_{4}concentration; (

**e**): Plot of K

_{obs}as a function of current density; (

**f**): Plot of K

_{obs}as a function of reaction temperature concentration.

**Table 1.**Constants and correlation coefficients of pseudo-first order for reduction of chromium (VI).

K_{obs} | R^{2} | |
---|---|---|

[H_{2}SO_{4}] | ||

100 g/L | 0.01276 | 0.9985 |

80 g/L | 0.01077 | 0.9993 |

60 g/L | 0.00712 | 0.9978 |

40 g/L | 0.00712 | 0.9849 |

20 g/L | 0.00608 | 0.9964 |

Current density | ||

50 A/m^{2} | 0.01276 | 0.9985 |

40 A/m^{2} | 0.01163 | 0.9987 |

30 A/m^{2} | 0.01312 | 0.9975 |

20 A/m^{2} | 0.01249 | 0.9836 |

10 A/m^{2} | 0.01191 | 0.9946 |

Reaction temperature | ||

70 °C | 0.01276 | 0.9985 |

60 °C | 0.01242 | 0.9980 |

50 °C | 0.01238 | 0.9983 |

40 °C | 0.01115 | 0.9967 |

30 °C | 0.01027 | 0.0017 |

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**MDPI and ACS Style**

Peng, H.; Leng, Y.; Guo, J.
Electrochemical Removal of Chromium (VI) from Wastewater. *Appl. Sci.* **2019**, *9*, 1156.
https://doi.org/10.3390/app9061156

**AMA Style**

Peng H, Leng Y, Guo J.
Electrochemical Removal of Chromium (VI) from Wastewater. *Applied Sciences*. 2019; 9(6):1156.
https://doi.org/10.3390/app9061156

**Chicago/Turabian Style**

Peng, Hao, Yumeng Leng, and Jing Guo.
2019. "Electrochemical Removal of Chromium (VI) from Wastewater" *Applied Sciences* 9, no. 6: 1156.
https://doi.org/10.3390/app9061156