Enhanced degradation of herbicides in groundwater using sulfur-containing reductants and spinel zinc ferrite activated persulfate

Abstract

A challenge to successfully implementing an injection-based remedial treatment in aquifers is to ensure that the oxidative reaction is efficient and lasts long enough to contact the contaminated plume. Our objective was to determine the efficacy of zinc ferrite nanocomposites (ZnFe₂O₄) and sulfur-containing reductants (SCR) (i.e., dithionite; DTN and bisulfite; BS) to co-activate persulfate (S₂O₈^2-; PS) and treat herbicide-contaminated water. We also evaluated the ecotoxicity of the treated water. While both SCRs delivered excellent PS activation in a 1:0.4 ratio (PS:SCR), the reaction was relatively short-lived. By including ZnFe₂O₄ in the PS/BS or PS/DTN activations, herbicide degradation rates dramatically increased by factors of 2.5 to 11.3. This was due to the SO₄^- and OH reactive radical species that formed. Radical scavenging experiments and ZnFe₂O₄ XPS spectra results revealed that SO₄^- was the dominant reactive species that originated from S(IV)/PS activation in solution and from the Fe(II)/PS activation that occurred on the ZnFe₂O₄ surface. Based on liquid chromatography mass spectrometry (LC-MS), atrazine and alachlor degradation pathways are proposed that involve both dehydration and hydroxylation. In 1-D column experiments, five different treatment scenarios were run using ¹⁴C-labeled and unlabeled atrazine, and ³H₂O to quantify changes in breakthrough curves. Our results confirmed that ZnFe₂O₄ successfully prolonged the PS oxidative treatment despite the SCR being completely dissociated. Toxicity testing showed treated ¹⁴C-atrazine was more biodegradable than the parent compound in soil microcosms. Post-treatment water (25 %, v/v) also had less impact on both Zea Mays L. and Vigna radiata L. seedling growth, but more impact on root anatomies, while ≤4 % of the treated water started to exert cytotoxicity (<80 % viability) on ELT3 cell lines. Overall, the findings confirm that ZnFe₂O₄/SCR/PS reaction is efficient and relatively longer lasting in treating herbicide-contaminated groundwater.

Keywords: Ferrite nanocomposites; Groundwater remediation; Herbicide degradation; Mineralization; Persulfate activator; Sulfur-containing reductant.