Electrolysis-assisted UV/sulfite oxidation for water treatment with automatic adjustments of solution pH and dissolved oxygen

Abstract

Sulfite as precursor to generate sulfate radical (SO₄ ^•-) for water treatment has gained attention. Here we report a metal-free and highly efficient electro/UV/sulfite process to produce SO₄ ^•- for water treatment. UV/sulfite reaction induces sulfite radical (SO₃ ^•-), which transforms into SO₄ ^•- in the presence of oxygen generated by water electrolysis. Electro/UV/sulfite process generates a steady-state SO₄ ^•- concentration of 0.2 to 1.1 × 10^-12 M in our tests. Solution pH affects sulfite species distribution, and higher pH mediates improved yield of steady-state SO₄ ^•- concentration. Effect of sulfite concentration exhibits a bell-shaped pattern toward SO₄ ^•- production due to self-scavenging. The oxidation capability of electro/UV/sulfite process is manifested by removing representative micropollutants (i.e., ibuprofen, salicylic acid, and bisphenol A) and Escherichia coli model pathogen, in both synthetic and natural water matrices. This novel electro/UV/sulfite process has obvious advantages, since it bypasses metal ion catalysts, supplies reaction with electrolytically generated nascent oxygen, and overcomes the acidic pH requirement, that are challenging to traditional metal/sulfite processes. Considering the features of environmental friendliness and low cost, the proposed electro/UV/sulfite process should lead to successful applications in the future.

Keywords: UV/sulfite; decontamination; disinfection; electrolysis; sulfate radical.

Fig. 1.

Electrolysis promoted UV/sulfite process to produce sulfate radical. (a) UV/sulfite process induced SO₃^•− formation, and (b) electrolytic oxygenation (c, d) transformed SO₃^•− into SO₄^•−. Conditions: 5 μM benzoic acid, 1 mM sulfite, 10 mM Na₂SO₄, 10 mM phosphate buffer at pH 7, 200 mA, UV. 50 mM 5,5-dimethyl-1-pyrrolidine-N-oxide was used during (a) electron spin resonance assay; 5 mM tert-butanol or ethanol was used during (d) radical-scavenging assay. Data were fitted with pseudo first-order reaction kinetics.

Fig. 2.

Electro/sulfite process generated sulfate radical, as (a) visualized in electron spin resonance assay and (b) demonstrated by benzoic acid removals. Conditions: 5 μM benzoic acid, 1 mM sulfite, 10 mM Na₂SO₄, 10 mM phosphate buffer at pH 7, 200 mA.

Fig. 3.

Schematic mechanism of sulfate radical proliferation during electro/UV/sulfite process. Primarily, UV induced sulfite to produce SO₃^•−, which then transformed into SO₄^•− under electrolytic oxygenation. Moreover, electrolysis of sulfite solution also produced SO₄^•− through direct one-electron transfer from sulfite molecule to anode surface.

Fig. 4.

Steady-state sulfate radical concentrations mediated by electro/UV/sulfite process under varying (a) pH and (b) sulfite concentration. Typical conditions: 0.2 mM benzoic acid as capturing agent, 1 mM sulfite, 10 mM Na₂SO₄, 10 mM phosphate buffer at pH 7, 200 mA, UV. 200 mM tert-butanol and ethanol were used as scavengers. (a) different pH buffers were used, that is, 10 mM acetate (pH 4–5), phosphate (pH 6–8), and borate (pH 9–10). (b) sulfite concentration varied between 0.25 and 2 mM, under 10 mM phosphate buffer at pH 7.

Fig. 5.

Comparisons of benzoic acid degradation in terms of (a) kinetics and (b) removals after reaction for 30 min by electro/UV/sulfite and metal/sulfite processes. Conditions: electro/UV/sulfite process − 5 μM benzoic acid, 1 mM sulfite, 10 mM Na₂SO₄, 200 mA, UV; metal/sulfite processes − 5 μM benzoic acid, 1 mM sulfite, 0.1 mM metal ions (i.e., Cu(II), Co(II), Fe(II), or Mn(II)). Reactions were carried out in 10 mM MOPS buffer at pH 7.

Fig. 6.

Electro/UV/sulfite process efficiently removed (a-c) various organic compounds and (d) E. coli microbe, together with automatic adjustments of (e) dissolved oxygen and (f) solution pH. Conditions: 1 mM sulfite, 10 mM Na₂SO₄, 200 mA, UV. 5 μM ibuprofen, salicylic acid, and bisphenol A were used as micropollutants; 10⁶ CFU/mL E. coli cell in exponential-phase was used as model pathogen. 5 mM tert-butanol and ethanol were used for (a-c), and 100 mM tert-butanol and thiourea were used for (d). Solution was neutral before sulfite addition, and it turned alkaline (~ pH 8.6) after sulfite addition. Data in panels (a-c) were fitted with pseudo first-order reaction kinetics.

Fig. 7.

Performance of electro/UV/sulfite process in natural water matrices. Electro/UV/sulfite process was used to treat (a) ibuprofen, (b) salicylic acid, (c) bisphenol A, and (d) E. coli, accompanied with automatic adjustments of (e) dissolved oxygen and (f) solution pH. Conditions: 1 mM sulfite, 200 mA, UV. 5 μM ibuprofen, salicylic acid, and bisphenol A were used as micropollutants; 10⁶ CFU/mL E. coli cell in exponential-phase was used as model pathogen. Note: MIT groundwater, Pozo mita groundwater in our Superfund site in Puerto Rico; JAM surface water, Jamaica pond in local Boston, MA.