Synergistic 2,4-D Degradation by Fenton Reagent Obtained from Glow Discharge Plasma Formed H₂O₂ and DBD Pretreated Fe-Rich Sludge Catalyst

Abstract

In this study, Fe-rich sludge was pretreated by dielectric barrier discharge (DBD) and used as a Fenton catalyst for the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous solution, where hydrogen peroxide was in situ supplied from a glow discharge plasma (GDP) in contact with the solution. Synergistic mechanism of the two plasmas during 2,4-D degradation was explored. The Fe-rich sludge pretreated by H₂-DBD could significantly accelerate the 2,4-D degradation as it converted Fe(III) in the sludge to the active Fe(II). 2,4-D can be completely removed in the presence of H₂-DBD pretreated Fe-rich sludge with 25 min of GDP at pH₀ 3.0 and catalyst dosage of 1.0 g/L. Quenching experimental studies showed that both ·OH (oxidative degradation) and ·H (reductive dechlorination) were responsible for 2,4-D degradation. Reducing radicals (such as ·R and·H) formed during GDP can continuously reduce the Fe³⁺ back to Fe²⁺, which further promoted the degradation and dechlorination of 2,4-D. The results of three consecutive experiments indicated the stability and reusability of catalyst. This study provides a potential method for the recycle of Fe-rich sludge as a novel and cost-effective catalyst for 2,4-D removal from aqueous environments, which is significant for the terminal disposal of Fe-rich sludge.

Graphical Abstract

Appendix

See Tables 3, 4, 5, 6 and Figs. 7, 8, 9, 10, 11, 12, 13, 14.

$${\text{H}}_{{2}} {\text{O}}_{{2}} {\text{ + Fe}}^{{2 + }} \to {\text{Fe}}^{{3 + }} { + } \cdot {\text{OH + OH}}^{{{ - }{\kern 1pt} }} {\kern 1pt} \quad {\text{k}}_{{1}} {\text{ = 76M}}^{ - 1} \cdot {\text{s}}^{{ - {1}}}$$

(27)

$$H_{2} O_{2} + Fe^{3 + } \to Fe^{2 + } + HO_{2} \cdot + H^{ + } \quad k_{2} = 0.01M^{ - 1} \cdot s^{ - 1}$$

(28)

$$\equiv Fe(II) + H_{2} O_{2} \to \equiv Fe(III) + \cdot OH + OH^{{^{\_} }}$$

(29)

$$\equiv Fe(III) + H_{2} O_{2} \to \equiv Fe(II) + \cdot HO_{2} + H^{ + }$$

(30)

$$\equiv Fe(III) + \cdot HO_{2} \to \equiv Fe(II) + O_{2} + H^{ + }$$

(31)

$$\equiv {\text{Fe(II,III) + H}}_{{2}} {\text{O}} \to \equiv {\text{Fe(II,III)OH + H}}^{ + }$$

(32)

$$\equiv Fe(II,III)OH + H + \to \equiv Fe(II,III)OH_{2}$$

(33)

Table 2 EDS elemental analysis of Hyd-catalyst

Table 3 XPS analysis of oxygen content of different catalysts

Table 4 Kinetic analysis of 2,4-D degradation by Hyd-catalyst recycling.(pH₀ = 3.0; C₀(2,4-D) = 100 mg/L; C(Hyd-catalyst) = 1.0 g/L)

Table 5 Kinetic Analysis of the Effect of Hyd-catalyst Amount on GDP Degradation 2,4-D. (pH₀ = 3.0; C₀(2,4-D) = 100 mg/L)

Table 6 Kinetic analysis of the effect of pH₀ on 2,4-D degradation.(C₀(2,4-D) = 100 mg/L; C(Hyd-catalyst) = 1.0 g/L)

Fig. 7

XPS analysis of the DBD pretreated Fe-rich sludge as catalyst: a Air-catalyst; b Hyd-catalyst; c Raw-catalyst

Fig. 8

Emission spectra of GDP and Hyd-catalyst in 2,4-D solution. (pH₀ = 3.0; C₀ (2,4-D) = 100 mg/L; C(Hyd-catalyst) = 1.0 g/L)

Fig. 9

Reusability of the Hyd-catalyst as GDP catalyst for 2,4-D degradation. (pH₀ = 3.0; C₀(2,4-D) = 100 mg/L; C(Hyd-catalyst) = 1.0 g/L)

Fig. 10

Effect of ·H radical scavengers on degradation of 2,4-D. (pH₀ = 3.0; C₀(2,4-D) = 100 mg/L; C(Hyd-catalyst) = 1.0 g/L)

Fig. 11

Change in pH during GDP + Hyd-catalyst. (pH₀ = 3.0; C₀(2,4-D) = 100 mg/L; C(Hyd-catalyst) = 1.0 g/L)

Fig. 12

The scanning electron microscope images of Raw-catalyst

Fig. 13

The scanning electron microscope images of Hyd-catalyst

Fig. 14

EDS elemental analysis of Hyd-catalyst