Photocatalytic removal of toxic metal ions from water using functionalized gC3N4

Abstract

Heavy metal ions in the environment are increasing geometrically, because the existing conventional wastewater treatment techniques are grossly deficient in the treatment of these toxic pollutants. Photocatalysis is the preferred method to eradicate the toxic effects of these heavy metal ions in water by converting the heavy metals in the toxic oxidation states to less-toxic and more useful oxidation states. Also, it is cost effective, environmentally friendly and perform maximally without generating secondary waste. However, a photocatalytic process requires photocatalysts that are thermally stable, insoluble and that can absorb in the visible region of the electromagnetic spectrum. Therefore, the synthesis of novel photocatalysts for the removal of toxic metal ions from water has inspired different studies. Bismuth-based ternary metal sulphides of the form M-Bi-S (where M = Ni, Ag and Cu) are good semiconductors with good structural and optical properties. Despite their good properties, utilization of bismuth-based ternary metal sulphides as photocatalysts is still very limited due to the fact that they are difficult to synthesize in their pure stoichiometry phase. In this study, the synthesis of AgBiS2 and Ni2Bi2S3 were achieved through a one-pot synthetic route. The synthesis of the non-stoichiometric phase of the copper bismuth sulphide (Cu3.21Bi4 .79S9) was first carried out through heat-up method by using copper(II)- and bismuth(III)- complexes of N-methyl-N-phenyldithiocarbamate complexes and oleylamine as the capping agent. The suitability of the dithiocarbamate complex as single source precursors were examined by the preparation of the respective binary sulphides from the complexes. Silver(I) sulphide (Ag2S) was prepared as a model binary sulphide from N-methyl-N-phenyl dithiocarbamate complex. The obtained ternary metal bismuth sulphides were incorporated into graphitic carbon nitride to form nanocomposites. Graphitic carbon nitride was not only utilized as a support material for these sulphides, but also as a semiconductor that is stable to heat with band gap energy of 2.70 eV. Apart from these bismuth-based ternary nanocomposites, metallic silver was also composited into graphitic carbon nitride in-situ. Metallic silver was used due to its higher surface plasmon resonance compared to other metals. The four functionalized graphitic carbon nitrides (Cu3.21Bi4.79S9/gC3N4, AgBiS2/gC3N4 Ni2Bi2S3/O-gC3N4 and Ag/gC3N4) were investigated for the photocatalytic reduction of heavy metal ions in water. As such, the graphitic carbon nitride functionalized with silver and copper bismuth sulphides were investigated for the photocatalytic reduction of Cr(VI). The graphitic carbon nitride functionalized with nickel bismuth sulphide and silver bismuth sulphide were investigated for ix the photocatalytic reduction of Ag(I) and Pb(II) respectively. The Ag(I), Pb(II) and Cr(VI) were chosen as a model for the monovalent, divalent and multivalent toxic metal ions. About 92.77% reduction of Cr(VI) was achieved at pH 2 using 10 mg of the Cu3.21Bi4.79S9/gC3N4 photocatalyst and 10 mg/L of the solution of Cr(VI) under the visible light irradiation. The pseudo-first order rate constant of photocatalysis was found to be 0.0393 min-1, which was 1.37 and 5.17 folds higher than that of gC3N4 and Cu3.21Bi4.79S9 respectively. The presence of bisphenol A and other heavy metal ions including Ag(I) and Pb(II) (as secondary pollutants) in the photocatalytic system reduced the rate of photocatalysis from 0.0393 min-1 to 0.0019 min-1 and 0.0039 min-1 respectively. The performance of photocatalytic reduction of Cr(VI) was 66.87% even after 2 h of visible light irradiation in the presence of Ag/gC3N4 photocatalyst. This implies that functionalization of graphitic carbon nitride with metallic silver is not as effective as functionalization with ternary copper bismuth sulphide. The use of 25 mg of Ni2Bi2S3/O-gC3N4 reduced about 93.08% (pseudo-first order rate constant of 0.0460 min-1) of Ag(I) within 1 h under the visible light. Mixed organic pollutants and persulfate were found to have inhibitory effects on the rate of photocatalytic reduction. In a bid to better understand the influence of other additives on the rate of photocatalytic reduction of Pb(II) using functionalized graphitic carbon nitride, AgBiS2/gC3N4 was used as photocatalyst for the reduction of Pb(II) in the presence different additives. The results revealed that the presence of easily-oxidizable organics has synergistic effects on the photocatalytic reduction of Pb(II), while persulfate displayed an inhibitive effect on Pb(II) reduction. The removal of Pb(II) in dye' s matrix was influenced by the type of dyes that were present in the water. The rate of Pb(II) reduction was reduced in the presence of methylene blue and methyl orange, but crystal violet displayed synergistic effects. Finally, the rate of degradation of dyes in the presence of Pb(II) was also investigated. The rate of photocatalytic reduction of Pb(II) decreased from 0.0045 min-1 to 0.0036 min-1 and 0.0016 min-1 in the matrix of methyl orange and methylene blue respectively. On the contrary, there was an increase in the rate of photocatalytic reduction of Pb(II) from 0.0045 to 0.0096 min-1 in the matrix of crystal violet. In general, the use of functionalized graphitic carbon nitride as the photocatalyst is a promising and sustainable alternative for the removal of toxic heavy metal ions from water.