Disilicate-Assisted Iron Electrolysis for Sequential Fenton-Oxidation and Coagulation of Aqueous Contaminants

Abstract

Sodium disilicate (SD), an inorganic and environmentally friendly ligand, is introduced into the conventional iron electrolysis system to achieve an oxidizing Fenton process to degrade organic pollutants. Electrolytic ferrous ions, which are complexed by the disilicate ions, can chemically reduce dioxygen molecules via consecutive reduction steps, producing H₂O₂ for the Fenton-oxidation of organics. At the near-neutral pH (from 6 to 8), the disilicate-Fe(II) complexes possess strong reducing capabilities; therefore, a near-neutral pH rather than an acid condition is preferable for the disilicate-assisted iron electrolysis (DAIE) process. Following the DAIE process, the different complexing capacities of disilicate for ferrous/ferric ions and calcium ions can be used to break the disilicate-iron complexes. The addition of CaO or CaCl₂ can precipitate ferrous/ferric ions, disilicates and possibly heavy metals in the wastewater. Compared to previously reported organic and phosphorus ligands, SD is a low-cost inorganic agent that does not lead to secondary pollution, and would not compete with the target organic pollutants for •OH; therefore, it would greatly expand the application fields of the O₂ activation process. The combination of DAIE and CaO treatments is envisioned to be a versatile and affordable method for treating wastewater with complicated pollutants (e.g., mixtures of biorefractory organics and heavy metals).

Figure 1.

(a) Degradation of 2,4-DCP via iron electrolysis processes in different electrolytes. (b) The degradation and mineralization of 2,4-DCP in a disilicate-assisted iron electrolysis (DAIE) system with a 20 mA current, and the changes in the pH and Cl⁻ concentration over the electrolysis. (c) Dissolved H₂O₂, Fe(II) and Fe(III) species in the DAIE system. (d) Degradation of 2,4-DCP in a divided cell with disilicate electrolyte. Unless elsewise noted, the conditions of the electrolysis were [2,4-DCP]_ini = 20 mg/L, I = 10 mA, pH_ini 7.5, [Na₂SO₄] = 5 mM, [Na₂SiO₃] = 5 mM and [SD] = 5 mM. The experiments shown in panels (a)–(c) were carried out in the electrolytic cell with a single chamber (SI Figure S1a), while the one shown in panel (d) was conducted with a divided electrolytic cell (SI Figure S1b).

Figure 2.

(a) Degradation of 2,4-DCP with disilicate—Fe(II) complexes bubbled with O₂ or N₂. [2,4-DCP]_ini = 20 mg/L, pH_ini 7.5, [SD] = 10 mM, and [Fe(II)]_ini = 5 mM. (b) Effects of scavengers on the degradation of 2,4-DCP; pH_ini 7.5, I =10 mA, [2,4-DCP]_ini = 20 mg/L, [SD] = 5 mM, [BQ] = 0.01M, [TBA] = 0.1 M, and [MeOH] = 0.5M. (c) EPR signals in the Fe(II)/SD aqueous solution. (d) CV of the ferrous ions in the presence of different amounts of SD (50 mV/s scan rate, 2 mM ferrous iron).

Figure 3.

(a) Degradation of 2,4-DCP at different initial pH values for [2,4-DCP] = 20 mg/L, I = 10 mA and [SD] = 5 mM. (b) The oxidation potentials of ferrous iron in the presence of disilicate at different pH for [SD] = 5 mM and [Fe(II)] = 2 mM; the blue dash line indicates the oxidation potential of free ferrous ions at pH 4. Effects of (c) the electrical current, (d) chloride anions and (e) bicarbonate anions on the degradation of 2,4-DCP for [2,4-DCP]_ini = 20 mg/L, pH_ini 7.5, and [SD] = 5 mM. The current was 10 mA if not otherwise specified.

Figure 4.

(a) Degradation of 2,4-DCP and the variation of pH over a DAIE process lasting for 420 min for [2,4-DCP]_ini = 100 mg/L, I = 20 mA, pH_ini 7.5 and [SD] = 5 mM. The insets 1, 2, and 3 show the reaction solutions at 0, 180, and 420 min, respectively. (b) UV–vis spectra of the reaction solutions before and after the addition of 200 mg/L CaO. The inset shows the concentrations of dissolved iron in the solution before and after adding CaO. (c) Removal of the wastewater containing 2,4-DCP, AsO₃³⁻ and Cu²⁺ with a combination of DAIE and the addition of CaO for [2,4-DCP]_ini = 20 mg/L, [Cu]_ini = [As]_ini = 10 mg/L, I = 20 mA, pH_ini 7.5, and [SD] = 5 mM. (d) Removal of disilicate via the addition of CaO. The solutions for CaO treatment were obtained after the DAIE treatment ([SD] = 3.2 mM).

Scheme 1.

Illustration of the Mechanism for Combining Disilicate-Assisted Iron Electrolysis (DAIE) and CaO/CaCl₂ Treatment for Sequential Fenton- Oxidation and Coagulation