Visible light-induced highly efficient organic pollutant degradation and concomitant CO₂ fixation using red lead

Abstract

Herein, red lead (Pb₃O₄) is firstly reported to be able to degrade organic pollutants efficiently and simultaneously fix CO₂ in aqueous solution under visible light irradiation. CO₂ bubbling accelerates both the organic contaminant decomposition and the CO₂ fixation. Pb₃O₄ was partially transformed to Pb₃(CO₃)₂(OH)₂ or PbCO₃ in the process. Thermal decomposition of the solid products can obtain high concentration of pure CO₂ and PbO. Pb₃O₄ can be regenerated easily by calcinations of PbO in O₂ atmosphere. This work provides a possible route to purify organic wastewater and fix CO₂ synchronously with visible light illumination.

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Both water scarcity in the land and CO₂ enrichment in the atmosphere are grave problems to be faced by the world in the 21^st century. For the former, the most important solution is the decontamination and reuse of wastewater.^1,2 Traditional treatment technologies, including bioprocessing degradation and chemical oxidation, can have difficulty meeting the actual needs due to either high cost or low efficiency.^3,4 For the latter, possible routes including adsorption elimination with metal organic frameworks,^5–7 zeolites,⁸ porous carbons,⁹ and chemical fixation by other compounds,^10–12 are still being studied. The existing commercial CO₂ capture technology,¹¹ such as amine solvents, oxyfuel combustion and calcium looping technologies, is expensive and energy intensive. In the last few decades, photocatalytic methods have received increasing attention due to the cleanness, low-cost, and environmentally benign nature of solar energy. However, there is not yet a photocatalyst that satisfies both the criteria of long-term stability and high efficiency.^13–16 Therefore, the development of a highly efficient photocatalytic or photochemical system used in visible light is still a focus of investigation in the eviromental field.

Pb₃O₄ is a material with interesting physicochemical characteristics, owing to its mixed valence of Pb ions and unique electronic structure.^17,18 Pb₃O₄ has been studied mainly as an anti-corrosive paint,¹⁹ as an electrode in batteries,^20,21 and as a selective oxidant.²² Pb₃O₄ has been rarely studied as a photocatalyst due to the toxicity of Pb,²³ although it is a semiconductor responding to visible light (band gap of ca. 2.1 eV).¹⁸ The present work shows that Pb₃O₄ is an excellent material for the photodegradation of organic pollutants and simultaneous fixation of CO₂ in aqueous solution under visible light irradiation.

The sample Pb₃O₄ was prepared by calcining Pb(NO₃)₂ in flowing oxygen (20 mL min⁻¹) at 753 K for 30 h. Fig. 1A is the XRD pattern of the as-synthesized sample. All the diffraction peaks can be perfectly indexed to the Pb₃O₄ phase (JCPDS No. 08-0019), indicating that pure Pb₃O₄ can be obtained by the facile thermal decomposition method.¹⁷ SEM and TEM images of the sample show that the prepared powder has an irregular morphology in the size range of ca. 100–800 nm (Fig. 1B and 1C). Further HRTEM analysis (Fig. 1C, inset) indicates that the Pb₃O₄ sample is a crystal with a d-spacing of 0.295 nm for the (110) plane. The BET surface area of the as-synthesized Pb₃O₄ powder was ca. 0.7 m² g⁻¹.

Fig. 1 XRD patterns (A), SEM image (B), TEM image (C), and UV-Vis diffusive reflectance spectrum (D) of the Pb₃O₄ samples. Inset in (C) is the HRTEM image of the Pb₃O₄ sample; Inset in (D) is the (αhv)²versus hv curve.

The UV-Vis DRS spectrum, as shown in Fig. 1D, suggests that the Pb₃O₄ sample can absorb solar energy with a wavelength shorter than ca. 580 nm. The optical band-gap energy of the sample was determined by the equation:²⁴

αhν = A(hν − E_g)^n/2 (1)

Where α, h, v and E_g are absorption coefficient, light frequency, proportionality constant, and band gap, respectively. In the equation, a semiconductor has a direct transition for n = 1 and an indirect transition for n = 4. Only in the case of n = 4 does the (αhv)²versus hv curve show linear behaviour near the band edge, indicating that Pb₃O₄ is an indirect transition semiconductor. By extrapolating the linear portion of the curve to the hv axis, the E_g of Pb₃O₄ was estimated to be 2.25 eV (inset in Fig. 1D), which is in good agreement with that reported previously in the literature.¹⁸ The position of the conduction band edge of Pb₃O₄ could be calculated using the following equation:²⁵

E_CB = X − E_C − 0.5E_g (2)

Where E_C is the energy of free electrons on the hydrogen scale, X is the electronegativity of the semiconductor, and E_g is the band-gap energy of the semiconductor. The edge of the conduction band (E_CB) of Pb₃O₄ is estimated at −0.08 V (vs NHE) and the edge of the valence band (E_VB) is at +2.17 V (vs NHE), which are close to the data determined by electrochemical measurements in the previous literature.²¹ This E_VB is more positive than the oxidation potentials of H₂O₂ and O₃ (1.35 and 2.07 V, respectively), suggesting that photogenerated holes of Pb₃O₄ have a stronger oxidation ability than H₂O₂ and O₃.

The photodegradation activity of Pb₃O₄ was evaluated by the degradation of several organic pollutants including Rhodamine B (RhB), Methyl Orange (MO) and salicylic acid (SA) under visible light irradiation (420 nm ≤ λ ≤ 800 nm). Fig. 2A shows the concentration of organic pollutants as a function of time with and without light irradiation. It is seen that the concentrations of all the pollutants have no significant change without irradiation. However, under visible light irradiation, both the RhB solution and the MO solution were discolored completely within 40 min, showing an efficient photooxidation process occurring over Pb₃O₄. As the organic dyes can adsorb visible light, leading to a self-sensitized reaction,²⁶ the SA solution without any absorption in the visible region was also tested. As shown in Fig. 2A, SA is decomposed efficiently within 100 min. Additionally, phenol can be photooxidized by Pb₃O₄ under visible light (Fig. S1, ESI†). These results indicate that the degradation of organic compounds is due to the excitation of Pb₃O₄ by visible light. We compared the performance of Pb₃O₄ with the well-known TiO₂ photocatalyst P25, and Pb₃O₄ exihibits a much better activity than P25, as shown in Table 1.

Fig. 2 (A) Variation of RhB, MO, and SA concentrations as a function of time with or without visible light illumination (420 nm ≤ λ ≤ 800 nm). (B) Cycling runs for the photodegradation of RhB over Pb₃O₄ under visible light irradiation. (C) Photodegradation curves of RhB over Pb₃O₄ under different reactive atmospheres and under visible light irradiation. The concentration of RhB solution here is 10 μmol L⁻¹, MO is 20 ppm, and SA is 500 μmol L⁻¹.

Table 1 Physical properties of P25 and Pb₃O₄ as well as their photocatalytic properties

Photocatalyst	Specific surface area (m^2 g^−1)	Particle size (nm)	Light source^a	Time required for decoloration (min)
a The light source: 500 W halogen lamp for P25, and 500 W halogen lamp equipped with an IR cut-off filter and a UV cut-off filter (420 nm ≤ λ ≤ 800 nm) for Pb3O4.
P25	55	∼21	UV+Vis	180
Pb3O4	0.7	100–800	Vis	40

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A durability test for Pb₃O₄ was performed by the degradation of RhB under visible light irradiation. As shown in Fig. 2B, the degradation activity of Pb₃O₄ has no remarkable decrease in the first seven cycles, but obvious deactivation occurs in the eighth cycle. To understand the cause of the decrease in activity, the sample after the cyclic tests was characterized by XRD. Some new diffraction peaks, attributed to a hydrocerussite Pb₃(CO₃)₂(OH)₂ phase, were observed for the sample (Fig. S2, ESI†). It is estimated from the XRD peak intensity that 50% of Pb₃O₄ is transformed into the Pb₃(CO₃)₂(OH)₂. A controlling experiment proved that Pb₃(CO₃)₂(OH)₂ is inert for degradation of organic pollutants under visible light. Therefore, the decrease in the activity is due to the transformation of Pb₃O₄ to Pb₃(CO₃)₂(OH)₂ during the photodegradation processes.

Chemically, Pb₃O₄ is also considered as plumbous plumbate with the chemical formula Pb₂[PbO₄]. Two lead atoms in Pb₃O₄ are present in the +2 oxidation state and one lead atom is in the +4 oxidation state, while all the lead atoms in Pb₃(CO₃)₂(OH)₂ are in the +2 oxidation state. This implies that Pb₃O₄ undergoes a reductive reaction to Pb₃(CO₃)₂(OH)₂ in the presence of CO₂ and H₂O during the degradation process. In order to obtain further insight into the reaction mechanism over Pb₃O₄, a series of additional experiments were carried out. (i) When suspending in pure water, Pb₃O₄ is unchanged in crystalline structure after visible light irradiation for 10 h, revealing that Pb₃O₄ is photo-stable in water solution; (ii) in the case of bubbling pure N₂ or O₂, the photodegradation of RhB over Pb₃O₄ was considerably inhibited (Fig. 2C); (iii) When CO₂ instead of N₂ or O₂ was bubbled, the photodegradation activity was enhanced remarkably (Fig. 2C). These results reveal that the presence of CO₂ is a key factor leading to efficient organic degradation, and also a key factor for the deactivation of Pb₃O₄ due to the formation of Pb₃(CO₃)₂(OH)₂. The different activity of Pb₃O₄ in N₂, O₂, air and CO₂ atmospheres may be in agreement with the difference in CO₂ concentration, which will be discussed in the following section.

Since the redox potential of the ·OH/OH⁻ couple (2.27 V vs. NHE at pH = 7)²⁷ is more positive than the E_VB of Pb₃O₄ (2.17 eV), the photogenerated hole cannot easily oxidize OH⁻ to ·OH. Therefore, the organic photooxidation on Pb₃O₄ occurs mainly via the direct hole oxidation mechanism.²⁸ This conjecture was well supported by the radical and hole trapping experiments (see ESI†). On the other hand, the redox potential of the O₂/O₂⁻ couple (−0.16 V vs. NHE at pH = 7)²⁷ is more negative than E_CB (−0.08 eV) of Pb₃O₄, indicating that the reduction of O₂ to the active oxygen species O₂⁻ was also not feasible. This is why the bubbling of O₂ did not promote pollutant photodegradation over Pb₃O₄. However, we cannot exclude the possibility of O₂ molecules capturing the photogenerated electrons. Accordingly, the possible mechanism of the photoreaction can be described as follows:

Pb₃O₄ + hv → h⁺ + e⁻ (3a)

Organics + H₂O + h⁺ → CO₂ + H⁺ (3b)

Pb₃O₄ + e⁻ + H⁺ → [PbO] + H₂O (3c)

[PbO] + CO₂ + H₂O → Pb₃(CO₃)₂(OH)₂ (3d)

O₂ + H⁺ + e⁻ →H₂O (3e)

Firstly, Pb₃O₄ is excited by visible light to form electrons and holes (3a). Then the organics are oxidized by the holes and mineralized to CO₂ and H⁺ (3b). At the same time, Pb₃O₄ is reduced by the photogenerated electrons to form [PbO] (3c). With CO₂ present in water, [PbO] would convert to Pb₃(CO₃)₂(OH)₂ (3d), and the partially photogenerated electrons could be consumed by O₂ (3e). In reactions 3c and 3d, the photogenerated electrons reduce Pb⁴⁺ to Pb²⁺, while CO₂ plays a crucial role in stabilizing Pb²⁺ by the formation of Pb₃(CO₃)₂(OH)₂. The cooperation of these two reactions leads to a high separation efficiency of the hole–electron pairs and thus efficient degradation of organics. This mechanism is also shown in Scheme 1.

Scheme 1 The photoreaction mechanism over Pb₃O₄ under visible light irradiation.

It is noted that in the above process, the CO₂ molecules produced by organics degradation were fixed to Pb₃(CO₃)₂(OH)₂. In this sense, the photoreaction over Pb₃O₄ can be a potential route to fix CO₂ and simultaneously purify organic wastewater with high efficiency.

In the absence of any gas bubbling, Pb₃O₄ needs 5 h to completely remove the RhB (100 μmol L⁻¹) in the first run and 7 h to remove ca. 90% RhB in the second run, under visible light illumination (Fig. 3A). However, with CO₂ bubbling, the organics are completely degraded in ca. 1 h, and 90% of RhB is decomposed in ca. 3 h in the second run (Fig. 3B). As expected, the photodegradation activity of Pb₃O₄ was greatly enhanced by CO₂ bubbling. Such a high removal efficiency under visible light illumination for the high concentration of organic contaminants is seldom seen in previous literature about photocatalytic degradation.^15,16,29,30

Fig. 3 (A) Photodegradation curves of RhB solution (100 μmol L⁻¹) over Pb₃O₄ without any gas bubbling. (B) Photodegradation curves of RhB solution (100 μmol L⁻¹) over Pb₃O₄ with CO₂ bubbling. All the processes were performed under visible light illumination (420 nm ≤ λ ≤ 800 nm). The pH of the solutions was kept at ca. 7 during reactions.

XRD analysis shows that Pb₃O₄ is partially transformed into Pb₃(CO₃)₂(OH)₂ without any gas bubbling, and to PbCO₃ with CO₂ bubbling after the second run (Fig. S4, ESI†). It shows that a measurable amount of CO₂ is fixed to Pb₃(CO₃)₂(OH)₂ or PbCO₃ along with the organic pollutants degradation under visible light illumination. The products contained Pb₃(CO₃)₂(OH)₂ or PbCO₃ can be decomposed thermally below 773 K to PbO and CO₂,^31,32 which was confirmed by TPD experiments (Fig. S5, ESI†). The amount of CO₂ released during the thermal decomposition of the solid products were calculated by the TPD plot of CO₂, as listed in Table 2. After treating the same amount of RhB, no matter whether the degradation proceeded in air or CO₂ bubbling, the amount of CO₂ released from the products was much the same. This result shows that the CO₂ fixed in the solid products is from that generated in the oxidation of RhB, although the adscititious CO₂ can accelerate the reaction.

Table 2 Amount of CO₂ released by the thermal decomposition of the solid products after the two runs of RhB photodegradation under different atmosphere over Pb₃O₄.

Atmosphere of the degradation process	CO2 amount released from the solid products (mg g^−1)^a
a The CO2 amounts were calculated by TPD plots of CO2 as presented in supporting imformation.
Non-bubbling (in air)	151
CO2 bubbling	147

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Pure phase Pb₃O₄ can be regenerated by calcination of the PbO in O₂ atmosphere at 753 K for 10 h, which was proved by the XRD of the sample. Perhaps by using Pb₃O₄ as a medium, we could constitute a reaction system to more efficiently remove organic pollutants and synchronously fix CO₂, in which Pb₃O₄ is able to recycle simply and with low-cost.

There is a problem to be mentioned. That is, there exists a Pb residue in the solution, which limits the application of Pb₃O₄. Therefore, the residual Pb(II) in solution after the purification processes was measured by ICP, as shown in Table 3. It is seen that the amount of Pb dissolved in solution is low, attaining Grade III of Environmental Quality Standards for Surface Water of China (GB3838-2002) (Grade I and II: ≤10 μg L⁻¹, Grade III and IV: ≤50 μg L⁻¹). This can be explained by the formation of Pb₃(CO₃)₂(OH)₂ and PbCO₃, whose K_sp values are 3.16 × 10⁻⁴⁶ and 7.9 × 10⁻¹⁴ respectively.^33,34

Table 3 The residual Pb(II) in solution after the photodegradation of organic pollutants in different experimental conditions.

Experimental conditions of photodegradation	Concentration of Pb (μg L^−1)^a in solution
a The concentration of Pb in solution was measured by ICP.
10 μmol L^−1 RhB, non-bubbling	20
100 μmol L^−1 RhB, CO2 bubbling	29

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In conclusion, Pb₃O₄ can be used for the photodegradation of organic pollutants under visible light irradiation and the reduced species produced in this process can fix CO₂ in aqueous solution. Additional CO₂ can greatly accelerate the two processes. The products of the reaction, Pb₃(CO₃)₂(OH)₂ or PbCO₃, can be decomposed thermally to obtain a high concentration of CO₂ and PbO. Pb₃O₄ can be regenerated easily by calcinations of the PbO in an O₂ atmosphere. The Pb residue in the solution is very low after the reaction. This is perhaps a new strategy toward highly efficient organic wastewater purification and simultaneous CO₂ fixation in mild conditions using sunlight.

Acknowledgements

This work was financially supported by the NSFC (Grants No. 21173044 and 21003021), and the National Basic Research Program of China (973 Program, No. 2012CB722607).

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Footnote

† Electronic Supplementary Information (ESI) available: See DOI: 10.1039/c2ra21660d

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