Comparative Experimental and Computational Studies of Hydroxyl and Sulfate Radical-Mediated Degradation of Simple and Complex Organic Substrates

Abstract

Persulfate (PS)-based advanced oxidation processes (AOPs) have been promoted as alternatives to H2O2-based AOPs. To gauge the potential of this technology, the PS/Fe­(II) and Fenton (H2O2/Fe­(II)) processes were comparatively evaluated using formate as a simple target compound and nanofiltration concentrate from a municipal wastewater treatment plant as a complex suite of contaminants with the aid of kinetic modeling. In terms of the short-term rate and extent of mineralization of formate and the nanofiltration concentrate, PS/Fe­(II) is less effective due to slow Fe­(II)/Fe­(III) cycling attributable to the scavenging of superoxide by PS. However, in the concentrate treatment, PS/Fe­(II) provided a sustained removal of total organic carbon (TOC), with ∼81% removed after 7 days with SO4 •– consistently produced via homolysis of the long-life PS. In comparison, H2O2/Fe­(II) exhibited limited TOC removal over ∼57% after 10 h due to the futile consumption of H2O2 by HO•. PS/Fe­(II) also offers better performance at transforming humic-like moieties to more biodegradable compounds as a result of chlorine radicals formed by the reaction of SO4 •– with the matrix constituents present in the concentrate. The application of PS/Fe­(II) is, however, subject to the limitations of slow oxidation of organic contaminants, release of sulfate, and formation of chlorinated byproducts.

Peroxymonosulfate Activation by Fe(III)–Picolinate Complexes for Efficient Water Treatment at Circumneutral pH: Fe(III)/Fe(IV) Cycle and Generation of Oxyl Radicals

“…Since Anipsitakis and Dionysiou first reported that Fe(II) added to a PMS solution could degrade 2,4-dichlorophenol, the Fe(II)/PMS system has received much attention. − The transformation of the target compound in Fe(II)/PMS systems typically exhibits a rapid stage lasting minutes, followed by a much slower stage. ,,, It has been suggested that the rapid stage proceeds via outersphere electron transfer from Fe(II) to PMS to give SO 4 •– or Fe IV =O (reactions or ), which dominates target removal with the final products of SO 4 2– and Fe(III). ,,,, Because the reactivity of Fe(III) toward PMS is too weak to sustain the reaction, ,,, further addition of Fe(II) is required to achieve complete degradation of the target compounds, which results ultimately in generation of excessive iron sludge when the effluent is returned to neutral pH. In addition, the working pH range is limited to the range of 3.0–3.5, the lower end of which is to maintain reactivity and the upper end is to avoid coprecipitation of Fe(III) and Fe(II). , Obviously, improving the reactivity of Fe(III) toward PMS would benefit the development of a sustainable iron-based system in engineering applications. Fe false( normalII false) + normalHSO 5 − → normalSO 4 • − + Fe false( normalIII false) + OH − ⁣ k 1 a = ( 2.1 − 3.74 ) × 10 4 M − 1 …”

“…PMS-based AOPs have several technical advantages over traditional H 2 O 2 -based AOPs. PMS in its stabilized form (as the complex, 2KHSO 5 ·KHSO 4 ·K 2 SO 4 ) is more convenient to transport and store and is more efficiently utilized in AOPs compared to H 2 O 2 . , PMS-based AOPs are less sensitive to dissolved water constituents (except for chloride) and have a broader working pH range. − Due in part to its asymmetric molecular structure, PMS is more easily activated to reactive oxygen species than is H 2 O 2 . , Numerous physicochemical means of PMS activation have been proposed, including those mediated by transition metal ions, metal oxides, carbonaceous materials, and energy sources (e.g., light, heat, and electricity). − Of the transition metals, iron is readily available, cheap, and environmentally benign, and therefore is the preferable activator in practice. ,, …”

A multiple Kirkendall strategy for converting nanosized zero-valent iron to highly active Fenton-like catalyst for organics degradation

“…Notably, a certain degree of decline (13.9%) was observed for SnZVI@PBC/PS system after further increasing PS amount to 8 mM ( Fig. 7 H ), which was due to that superfluous PS could quench the generated by generating with lower redox potential ( 73 ), and self-scavenging reactions among the excessive also existed ( 70 ).…”