Remediating 1,4-dioxane-contaminated water with slow-release persulfate and zerovalent iron

Abstract

1,4-dioxane is an emerging contaminant that was used as a corrosion inhibitor with chlorinated solvents. Metal-activated persulfate can degrade dioxane but reaction kinetics have typically been characterized by a rapid decrease during the first 30 min followed by either a slower decrease or no further change (i.e., plateau). Our objective was to identify the factors responsible for this plateau and then determine if slow-release formulations of sodium persulfate and Fe⁰ could provide a more sustainable degradation treatment. We accomplished this by conducting batch experiments where Fe⁰-activated persulfate was used to treat dioxane. Treatment variables included the timing at which the dioxane was added to the Fe⁰-persulfate reaction (T = 0 and 30 min) and including various products of the Fe⁰-persulfate reaction at T = 0 min (Fe²⁺, Fe³⁺, and SO₄^2-). Results showed that when dioxane was spiked into the reaction at 30 min, no degradation occurred; this is in stark contrast to the 60% decrease observed when added at T = 0 min. Adding Fe²⁺ at the onset (T = 0 min) also severely halted the reaction and caused a plateau. This indicates that excess ferrous iron produced from the Fe⁰-persulfate reaction scavenges sulfate radicals and prevents further dioxane degradation. By limiting the release of Fe⁰ in a slow-release wax formulation, degradation plateaus were avoided and 100% removal of dioxane observed. By using ¹⁴C-labeled dioxane, we show that ∼40% of the dioxane carbon is mineralized within 6 d. These data support the use of slow-release persulfate and zerovalent iron to treat dioxane-contaminated water.

Keywords: Chlorinated solvents; Dioxane; Persulfate; Slow-release oxidants; TCE.

Figure 1

Effects of dosages on dioxane degradation. A. One dose application demonstrating rapid degradation followed by plateau, B. Quarter dose applications; C. Eighth dose applications; D. Eight dose application of Fe⁰.

Figure 2

A. Effect of timing on dioxane degradation by persulfate and Fe⁰ (dioxane added at T = 0 vs. 30 min); B. Effect of adding Fe²⁺ at T = 0 min on dioxane degradation by persulfate and Fe⁰.

Figure 3

Dioxane degradation by slow-release persulfate and Fe⁰ candles during 1^st, 2^nd, and 3^rd cycle experiments. A. One persulfate candle paired with one Fe⁰ candle; B. Two persulfate candles paired with one Fe⁰ candle.

Figure 4

Effect of including SHMP in persulfate and Fe⁰ candle formulations. Batch reactors received one persulfate and one Fe⁰ candle.

Figure 5

Temporal changes in dioxane concentration and ¹⁴C activity following treatment with persulfate (PS) and Fe⁰ candles. Experimental units were kept in the dark during treatment.

Figure 6

Proposed degradation scheme of 1,4-dioxane by persulfate and Fe⁰ candles. Structures in boxes have been identified.