Insight into electrochemical degradation of Cartap (in Padan 95SP) by boron-doped diamond electrode: kinetic and effect of water matrices

Abstract

In this work, the kinetic electrochemical degradation of Cartap (CT) (in Padan 95 SP) at boron-doped diamond (BDD) electrode was investigated. This study indicated that the degradation of CT underwent both direct and indirect oxidations. Water matrices can either accelerate or inhibit the removal efficiency of CT: adding 15 mM Cl^- improved k_CT from 0.039 min^-1 to 0.054 min^-1 (increased by 38%), while k_CT decreased by 61.5% and 64% when increasing the concentration of HCO₃^- and humic acid (HA) to 15 mM and 15 mg L^-1, respectively. CT degradation was inhibited in the presence of methanol (MeOH) and tert-butanol (TBA) due to the scavenging effect of those chemicals toward reactive species. The contribution of reactive oxidants was calculated as: DET (direct electron transfer) accounted for 15%; •OH accounted for 61.5%; SO₄^•- accounted for 12.8%; ROS (the other reactive oxygen species) accounted for 8.5%. The transformation pathways of major reactive species were established.

Keywords: BDD electrode; Cartap degradation; cyclic voltammetry; kinetic; linear sweep voltammetry; mechanism.

Figure 1

Elemental analysis for BDD electrode. a) SEM image of BDD; b) EDX for detecting elements; c) XRD patterns of the BDD electrode; d) Detected elements on BDD.

Figure 2

EIS spectrum of BDD. Inert: EIS spectrum of Pt and Ti.

Figure 3

CVs of BDD in the presence of CT and Na₂SO₄. a) CVs of BDD in 300 mg L⁻¹ CT at different scan rates. b, c) The comparison of CVs between blank solution and CT solution. 0.05 M Na₂SO₄ was used as supporting electrolyte.

Figure 4

Effect of HCO₃⁻, Cl⁻, and HA on CT degradation in electrochemical process. (a, c, e) The first-order kinetics of CT in the presence of HCO₃⁻, Cl⁻, and HA, respectively. (b, d, f) Relative degradation of CT in the presence of HCO₃⁻, Cl⁻, and HA, respectively. Experimental conditions: pH = 3, current density j = 40 mA cm⁻², [CT] = 40 μM, [Na₂SO₄] = 0.05 M.

Figure 5

a) The first-order rate constant (k_CT) of CT degradation in the presence of TBA and MeOH; b) Relative degradation of CT in the presence of TBA and MeOH. Experimental conditions: pH = 3, current density j = 40 mA cm⁻², [CT] = 40 μM, [TBA] = [MeOH] = 100 mM, [Na₂SO₄] = 0.05 M.

Figure 6

Reaction kinetics of CT in the presence of probes. Experimental conditions: pH = 3, current density j = 40 mA cm⁻², [CT] = 40 μM, [Na₂SO₄] = 0.05 M.

Figure 7

The electrochemical oxidation of CT and the transformation of radicals.