Electrolytic manipulation of persulfate reactivity by iron electrodes for trichloroethylene degradation in groundwater

Abstract

Activated persulfate oxidation is an effective in situ chemical oxidation process for groundwater remediation. However, reactivity of persulfate is difficult to manipulate or control in the subsurface causing activation before reaching the contaminated zone and leading to a loss of chemicals. Furthermore, mobilization of heavy metals by the process is a potential risk. An effective approach using iron electrodes is thus developed to manipulate the reactivity of persulfate in situ for trichloroethylene (TCE) degradation in groundwater and to limit heavy metals mobilization. TCE degradation is quantitatively accelerated or inhibited by adjusting the current applied to the iron electrode, following k1 = 0.00053·Iv + 0.059 (-122 A/m(3) ≤ Iv ≤ 244 A/m(3)) where k1 and Iv are the pseudo first-order rate constant (min(-1)) and volume normalized current (A/m(3)), respectively. Persulfate is mainly decomposed by Fe(2+) produced from the electrochemical and chemical corrosion of iron followed by the regeneration via Fe(3+) reduction on the cathode. SO4(•-) and ·OH cocontribute to TCE degradation, but ·OH contribution is more significant. Groundwater pH and oxidation-reduction potential can be restored to natural levels by the continuation of electrolysis after the disappearance of contaminants and persulfate, thus decreasing adverse impacts such as the mobility of heavy metals in the subsurface.

Figure 1

Effect of applied current through iron electrodes on (a) TCE degradation and (c) persulfate decomposition; correlation of (b) TCE degradation rate constants on current and (d) iron production rate constants on persulfate decomposition rate constants. Unless otherwise specified, the reaction conditions are based on 0.40 mM initial TCE concentration, 5 mM initial Na₂S₂O₈ concentration and initial pH of 5.6. Solution pH decreases to about 3 during treatment. The curves and lines in (a) and (c) refer to the fittings by pseudo first-order and pseudo zero-order kinetics for the data points in the same color, respectively.

Figure 2

Variations of (a) TCE and (b) persulfate concentrations in a divided electrolytic system. The reaction conditions are based on 25 mA, 0.40 mM initial TCE concentration, 5 mM initial Na₂S₂O₈ concentration and initial pH of 5.6. The curves in (a) refer to the fittings by pseudo first-order kinetics for the data points in the same color, respectively.

Figure 3

Electrolytic manipulation of (a) TCE degradation and (b) persulfate decomposition by periodically applying different currents. Unless otherwise specified, the reaction conditions are based on 0.79 mM initial TCE concentration, 5 mM initial Na₂S₂O₈ concentration and initial pH of 5.6. Note that a higher initial concentration of TCE is used to attain pronounced variation of degradation rates in each stage. Solution pH decreases to about 3 during treatment. k₁ and k₀ refers to the pseudo first-order rate consants of TCE degradation and pseudo zero-order rate constants of persulfate decomposition, respectively. The curves and lines in (a) and (b) refer to the fittings by pseudo first-order and pseudo zero-order kinetics for the data points in the same color, respectively.

Figure 4

(a) Effect of radical scavenging agents on TCE degradation. The reaction conditions are based on 5 mM initial Na₂S₂O₈ concentration, 0.40 mM TCE, +50 mA on the iron anode and initial pH of 5.6. The curves refer to the fittings by pseudo first-order kinetics for the data points in the same color. (b) ESR signals for iron anode activated persulfate oxidation. The reaction conditions are based on 5 mM initial Na₂S₂O₈ concentration, +50 mA on the iron anode and initial pH of 5.6. The samples were taken at 10 min. (c) Profile of TCE degradation. The reaction conditions are based on 5 mM initial Na₂S₂O₈ concentration, 1.58 mM TCE, +100 mA on the iron anode and initial pH of 5.6.

Figure 5

Variations of (a) pH/ORP and (b) TCE, persulfate and free Cu²⁺ concentrations during the course of treatment. The reaction conditions are based on 0.40 mM initial TCE concentration, 2 mM Na₂S₂O₈, 10 mg/L Cu²⁺, and +100 mA on the iron anode.