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

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.