Soluble Graphene Nanosheets for the Sunlight-Induced Photodegradation of the Mixture of Dyes and its Environmental Assessment

Abstract

Currently, the air and water pollutions are presenting the most serious global concerns. Despite the well known tremendous efforts, it could be a promising sustainability if the black carbon (BC) soot can be utilized for the practical and sustainable applications. For this, the almost complete aqueous phase photodegradation of the three well-known organic pollutant dyes as crystal violet (CV); rhodamine B (RhB); methylene blue (MB) and their mixture (CV + RhB + MB), by using water-soluble graphene nanosheets (wsGNS) isolated from the BC soot under the influence of natural sunlight is described. The plausible mechanism behind the photocatalytic degradation of dyes and their mixture has been critically analyzed via the trapping of active species and structural analysis of photodegraded products. The impact of diverse interfering ions like Ca²⁺, Fe³⁺, SO₄^2-, HPO₄^2-, NO₃^-, and Cl^- on the photodegradation efficiency of wsGNS was also investigated. Importantly, the environmental assessment of the whole process has been evaluated towards the growth of wheat plants using the treated wastewater. The initial studies for the fifteen days confirmed that growth of wheat plants was almost the same in the photodegraded wastewater as being noticed in the control sample, while in case of dyes contaminated water it showed the retarded growth. Using the natural sunlight, the overall sustainability of the presented work holds the potential for the utilization of pollutant soot in real-practical applications related to the wastewater remediation and further the practical uses of treated water.

Figure 1

Schematic representation illustrating the application of the wsGNS for the photodegradation of the three different dyes and their mixture under the presence of sunlight. Further, the treated wastewater was being employed for growing the wheat plants.

Figure 2

(a,b) Low-resolution TEM images of wsGNS; (c,d) HRTEM image of wsGNS show existence of few-layered graphene sheets with surface defects. (e) A full survey scan XPS analysis of wsGNS along with its corresponding short scan; (f) C1s; and (g) O1s.

Figure 3

(a) Extent of plot of (C/C_ο) for the individual dyes (CV, RhB and MB) with wsGNS in dark and sunlight (with inset showing photocatalytic degradation of the respective dyes in the absence of wsGNS); with their (b) respective plot of ln(C_ο/C); (c) Extent of plot of (C/C_ο) (with inset showing photocatalytic degradation of the mixture of dyes in the absence of wsGNS) of a mixture of dyes with wsGNS in dark and sunlight; with their (d) respective plot of ln(C_ο/C).

Figure 4

Effect of scavengers t-BA, Na₂-EDTA, and p-BZQ for hydroxyl, holes, and superoxide radicals respectively on the degradation performance of (a) CV, (b) RhB, (c) MB and (d) on the mixture of dyes. (e) Schematic illustration showing degradation of different dyes using wsGNS under sunlight irradiation; (f) with their respective suggested pathways.

Figure 5

The chemical structure of (a) CV; (b) RhB; (c) MB; and (d) Mixture of dyes; including with their respective ¹H NMR spectra, before and after their photodegradation by wsGNS. Inset of all figure (a–d), shows the digital images of respective dye (a–c) and their mixture (d) before and after the photodegradation.

Figure 6

Effect of different interfering ions Ca²⁺, Fe³⁺, SO₄²⁻, HPO₄²⁻, NO₃⁻, and Cl⁻) (100 ppm) on the decolorization efficiency of (a) CV (b) RhB, and (c) MB separately by wsGNS with respect to control; (d–i) Effect of different interfering ions Ca²⁺, Fe³⁺, SO₄²⁻, HPO₄²⁻, NO₃⁻, and Cl⁻ (100 ppm) on the decolorization of mixture of dyes by wsGNS with respect to control.

Figure 7

The effect of dyes, their mixture verses treated wastewater on the wheat plants was tested for the 15 days of germination; seeds treated with (a) dyes water and their mixture (CV + RhB + MB); (b) control (as in DI water and in wsGNS solution); versus the (c) treated wastewater of dyes and their mixture.