Catalytic decolorization of methyl orange by the rectorite–sulfite system

Activation of sulfite by metal-organic framework-derived cobalt nanoparticles for organic pollutants removal

Sulfite (SO₃^2-) activation is one of the most potential sulfate-radical-based advanced oxidation processes, and the catalysts with high efficiency and low-cost are greatly desired. In this study, the cobalt nanoparticles embedded in nitrogen-doped graphite layers (Co@NC), were used to activate SO₃^2- for removal of Methyl Orange in aqueous solution. The Co@NC catalysts were synthesized via pyrolysis of Co²⁺-based metal-organic framework (Co-MOF), where CoO was firstly formed at 400℃ and then partially reduced to Co nanoparticles embedded in carbon layers at 800℃. The Co@NC catalysts were more active than other cobalt-based catalysts such as Co²⁺, Co₃O₄ and CoFe₂O₄, due to the synergistic effect of metallic Co and Co_xO_y. A series of chain reaction between Co species and dissolved oxygen was established, with the production and transformation of SO₃^•-, SO₅^2-, and subsequent active radicals SO₄^•- and HO•. In addition, HCO₃^- was found to play a key role in the reaction by complexing with Co species on the surface of the catalysts. The results provide a new promising strategy by using the Co@NC catalyst for SO₃^2- oxidation to promote organic pollutants degradation.

Energy efficient photocatalytic activation of peroxymonosulfate by g-C3N4 under 400 nm LED irradiation for degradation of Acid Orange 7

Photocatalytic activation of peroxymonosulfate (PMS) by graphitic carbon nitride (g-C₃N₄) is emerging as a sulfate radical anion based advanced oxidation process (S-AOP) for degradation of organic pollutants. Importantly, photocatalytic activation of PMS by g-C₃N₄ is energy intensive as light irradiation requires high electrical energy. Here, we studied activation of PMS by g-C₃N₄ under 400 nm light emitting diode (LED) irradiation (g-C₃N₄/PMS/400-LED system) for degradation of Acid Orange 7 (AO7). LED array having optical emission maximum around 400 nm (FWHM = 16 nm), with electrical input power of 1.54 W (14 V and 110 mA) was used for irradiation. Pseudo-first order rate constant (k_obs) value for degradation of AO7 by g-C₃N₄/PMS/400-LED system was determined to be 0.094 min^-1. O₂^·-, SO₄^·- were revealed by radical scavenging and ESR investigations. k_obs value in simulated ground and real tap water were determined to be 0.068 min^-1 and 0.063 min^-1, respectively. g-C₃N₄ was stable, and reused four times without any significant loss in its photocatalytic activity. Importantly, electrical energy per order (E_EO) for degradation of AO7 by g-C₃N₄/PMS/400-LED system was determined to be 24.51 kWh m^-3 order^-1. In contrast, the E_EO value for the degradation of AO7 by g-C₃N₄ activated PMS under visible light irradiation (>400 nm), using conventional xenon lamp, (g-C₃N₄/PMS/Vis-Xe system) was found to be very high as 2702 kWh m^-3 order^-1. Thus, the study highlights, LED irradiation source is promising for the development of energy efficient g-C₃N₄ photocatalytic activation of PMS for removal of organic pollutants.

Degradation of dye in wastewater by Homogeneous Fe(VI)/NaHSO3 system

The homogeneous Fe(VI)/Na₂SO₃ system has been proposed for highly efficient degradation of recalcitrant contaminants, in which sulfite could significantly enhance the transformation of organic substrate by Fe(VI). Also, the Fe(VI)/NaHSO₃ system could show high efficiency across a wide range of pH conditions. The degradation rates reached up to 80% within 2 min and 70% within 5 min in strongly acidic and alkaline conditions, respectively. Unexpectedly, a faster removal rate was obtained in Fe(VI)/NaHSO₃ system than that in Fe(VI)/Na₂SO₃ system for the degradation of methylene blue (MB). A reasonable dye degradation mechanism was proposed and verified by a series of experiments. The high oxidation potential of Fe(VI) and other species such as sulfate and hydroxyl radicals were responsible for the outstanding capabilities of Fe(VI)/NaHSO₃ system, which could significantly improve the treatment of organics in wastewater under a very wide range of pH conditions.

Photocatalytic degradation of organic pollutants in wastewater with g-C3N4/sulfite system under visible light irradiation

To develop low cost and high efficient sulfate radical (SO₄^-) based advanced oxidation processes (AOPs) for rapid remediation of contaminated waters is of great interest. In this study, a green and novel SO₄^- based AOPs, in situ visible light activation of sulfite by graphitic carbon nitride (g-C₃N₄), for the degradation of organic pollutants is reported. The g-C₃N₄+HSO₃^- + Vis system could achieve remarkably enhanced degradation of organic pollutants such as organic dyes and phenol in aqueous solution. The excellent reusability of the metal free catalyst was also observed during ten successive cycles. The efficiency of the system was dependent on the reaction conditions, which first increased and then decreased with the increase of HSO₃^- concentration and initial solution pH. The addition of HCO₃^- stimulated the pollutant degradation, but other water matrix components such as Cl^- and humic acid showed nearly no influence on the reaction. The mechanism investigations suggested that sulfite is oxidized in the system to sulfite radicals, which then react with dioxygen and superoxide radicals to form SO₅^- radicals and HSO₅^- respectively. SO₅^- radicals can be also reduced by sulfite or photoelectron to HSO₅^-. SO₄^- radicals were then produced from HSO₅^- reduction by photoelectron, and contributed to dye degradation in the system together with superoxide radicals. This study provides a novel new approach for efficient degradation of organic degradation via sulfite activation.

Cobalt nanoparticles encapsulated in nitrogen-rich carbon nanotubes as efficient catalysts for organic pollutants degradation via sulfite activation

The activation of sulfite by heterogeneous catalysts displays a great potential in the development of new sulfate radials based technologies for wastewater treatment. Herein, cobalt nanoparticles embedded in N-doped carbon nanotubes (Co@NC) were prepared by a simple pyrolysis method. Due to the synergistic effects of the cobalt nanoparticles and N-doped carbon nanotubes, the Co@NC catalyst intrinsically shows an outstanding efficiency, excellent reusability and high stability in the catalytic oxidation of methyl orange (MO) in the presence of sulfite and dioxygen. The structure and efficiency of the catalyst was significantly affected by the content of cobalt and pyrolysis temperature. Several quenching experiments and electron paramagnetic resonance were carried out to investigate the catalytic mechanism. It is found that hydroxyl and sulfate radicals worked together to degrade MO in the system. The formation and decomposition of peroxymonosulfate may be an important route of these reactive radicals production. The effect of different anions, bicarbonate concentration, initial solution pH and dye types on the performance of the catalyst was also studied. This study can open a new approach for design and preparation of encapsulated cobalt in carbon materials as effective catalysts for pollutants degradation via sulfite activation.

Catalytic degradation of Acid Orange 7 by manganese oxide octahedral molecular sieves with peroxymonosulfate under visible light irradiation

In this paper, the photodegradation of Acid Orange 7 (AO7) in aqueous solutions with peroxymonosulfate (PMS) was studied with manganese oxide octahedral molecular sieves (OMS-2) as the catalyst. The activities of different systems including OMS-2 under visible light irradiation (OMS-2/Vis), OMS-2/PMS and OMS-2/PMS/Vis were evaluated. It was found that the efficiency of OMS-2/PMS was much higher than that of OMS-2/Vis and could be further enhanced by visible light irradiation. The catalyst also exhibited stable performance for multiple runs. Results from ESR and XPS analyses suggested that the highly catalytic activity of the OMS-2/PMS/Vis system possible involved the activation of PMS to sulfate radicals meditated by the redox pair of Mn(IV)/Mn(III) and Mn(III)/Mn(II), while in the OMS-2/PMS system, only the redox reaction between Mn(IV)/Mn(III) occurred. Several operational parameters, such as dye concentration, catalyst load, PMS concentration and solution pH, affected the degradation of AO7.

Polyimides as metal-free catalysts for organic dye degradation in the presence peroxymonosulfate under visible light irradiation

Polyimide was demonstrated as a stable metal-free photocatalyst to activate peroxymonosulfate with visible light for the degradation of organic dyes.

Metal-free activation of peroxymonosulfate by g-C3N4 under visible light irradiation for the degradation of organic dyes

g-C₃N₄ was demonstrated as a stable metal-free photocatalyst to activate peroxymonosulfate under visible light irradiation for the degradation of organic dyes.