Copper(II)-Doped Carbon Dots as Catalyst for Ozone Degradation of Textile Dyes

Abstract

A catalytic ozonation advanced oxidation process (AOP) with a copper(II)-doped carbon dot as catalyst, Cu-CD (using L-cysteine and polyethylene glycol (PEG) as precursors and passivation agents), was developed for textile wastewater treatment (T = 25 °C and pH = 7). Four dyes were analyzed—Methyl Orange (MO), Orange II sodium salt (O-II), Reactive Black 5 (RB-5) and Remazol Brilliant Blue R (RBB-R), as well as a real effluent from the dying and printing industry. The Cu-CD, with marked catalytic ozonation properties, was successfully synthesized by one-pot hydrothermal procedure with a size of 4.0 nm, a charge of −3.7 mV and a fluorescent quantum yield of 31%. The discoloration of the aqueous dye solutions followed an apparent first-order kinetics with the following rate constants (kap in min−1): MO, 0.210; O-II, 0.133; RB-5, 0.177; RBB-R, 0.086. In the presence of Cu-CD, the following apparent first-order rate constants were obtained (kapc in min−1) with the corresponding increase in the rate constant without catalyst (%Inc): MO, 1.184 (464%); O-II, 1.002 (653%); RB-5, 0.709 (301%); RBB-R, 0.230 (167%). The presence of sodium chloride (at a concentration of 50 g/L) resulted in a marked increase of the discoloration rate of the dye solution due to generation of other radicals, such as chlorine and chlorine oxide, resulting from the reaction of ozone and chloride. Taking into consideration that the real textile effluent under research has a high carbonate concentration (>356 mg/L), which inhibits ozone decomposition, the discoloration first-order rate constants without and with Cu-CD (kap = 0.0097 min−1 and kapc = 0.012 min−1 (%Inc = 24%), respectively) were relatively small. Apparently, the Cu-CD, the surface of which is covered by a soft and highly hydrated caramelized PEG coating, accelerates the ozone decomposition and dye adsorption, increasing its degradation.

UV-Based Advanced Oxidation Processes of Remazol Brilliant Blue R Dye Catalyzed by Carbon Dots

“…3. Polyethylene glycol (PEG) has been used in CD synthesis to increase the water solubility and to coat the CD with a soft surface [5]. Indeed, PEG undergoes caramelization, leading to the production of PEGylated nanoparticles [7,[34][35][36][37].…”

“…Polyethylene glycol with an average molecular weight of 200 (PEG) (Sigma-Aldrich, St. Louis, MO, USA, P3015), L-cysteine (Sigma-Aldrich, St. Louis, MO, USA, W326305), titanium(IV) isopropoxide (Aldrich, St. Louis, MO, USA, 87560), ethylenediamine (Aldrich, St. Louis, MO, USA, E2,626-6), titanium dioxide (Sigma-Aldrich, St. Louis, MO, USA, anatase nanopowder < 25 nm, 637254), hydrogen peroxide (Chem-Lab, Zedelgem, Belgium, CL00.2308), and sodium persulfate (Sigma-Aldrich, St. Louis, MO, USA, 216232) were used. The dyes used in this work were similar to those described in reference [5]. The chemical structure of the anthraquinone dye RBB-R is shown in Figure 1.…”

“…The following solution was prepared for hydrothermal treatment: 0.400 g of L-cysteine was dissolved in 30 mL of water; after, only for the Tip-CD sample, 6.00 mL of PEG was used, and 0.60 mL of ethylenediamine was added and homogenized; lastly, 1.00 mL of titanium(IV) isopropoxide was added to the previous solution. The resulting mixture was placed in a Parr Series 4700 Pressure Vessel and treated hydrothermally for 5 h at 200 • C. The purification procedure was similar to that described in Reference [5].…”

“…The increase of fast-fashion trends complicates the environmental impact of textile business, and the lack of garment eco-design inhibits efficient circular economy strategies [3]. About 80% of the global emissions of this industry are discharged into environmental water, and, although textile effluents are constituted by a huge number of chemical substances, natural fibers, and microplastics, of greater concern are the large amount of non-biodegradable organic compounds, especially textile dyes [4,5]. The treatment of these substances in wastewaters requires advanced oxidation processes (AOPs) [6].…”

“…To promote the decomposition of an oxidant molecule such as H 2 O 2 and S 2 O 8 2− , an antioxidant is necessary. We have been using L-cysteine as a carbon precursor in CD synthesis [5]. Moreover, due to the existence of SH functional groups, it has antioxidant properties [30]; 2.…”