(731g) Synthesis of Nitrogen-Containing Carbon Dots As Fe (III) Probe after Extracting Humic Acid from Compost of Sludge and Leaf

Sewage sludge is generated as a by-product during wastewater treatment and needs to be efficient and environmentally friendly disposal. In the past decades, the excess sludge in municipal wastewater treatment plants was mainly treated environmentally-unfriendly such as landfill, incineration. Therefore, recycling sludge into high value-added products, such as humic acid, arising great attention. Composting is an ideal aerobic transformation process for biowaste stabilization. Especially, humic acid contains conjugated double bonds and aromatic rings, the similar structure with natural biomass which were successfully used to hydrothermally synthesize carbon dots. Therefore, it’s promising to utilize humic acid as a precursor for hydrothermal oxidation to synthesize N-CDs.

In this work, blue fluorescent N-CDs was synthesized for the first time by the use of humic acid. The schematic illustration on the synthesis of fluorescent N-CDs is in Figure 1. Firstly, the humic acid was extracted from the compost of sludge and leaf for recycling solid waste. Then hydrothermal treatment of humic acid using hydrogen peroxide as an oxidant, blue fluorescent N-CDs was synthesized within short time. A series of characterization methods including TEM, XPS, FTIR and Raman analyses were used to conform the successful synthesis of N-CDs. As shown in Figure 2, the N-CDs has an average diameter of 1.88 nm and exhibits a well-defined spherical shape, and the distances between the lattice fringes are further characterized to be 0.218 nm and 0.355 nm, corresponding to the (010) in-plane lattice spacing and inter-layer distance of graphite[1]. Furthermore, Raman analysis of N-CDs revealed the existence of D and G bands at ~ 1390 cm−1 and ~ 1600 cm−1, respectively, confirming their architectures belonging to the carbon nanostructure family. Notably, the observed higher intensity of the G band than the D band verified the high quality of the synthesized products. Moreover, XPS measurements suggested that the synthesized N-CDs contains C, O and N elements with atom ratio of C:O:N = 67.56: 27.61: 4.82.

Nowadays, a large amount of iron ions-polluted wastewater is discharged into nature due to the widespread application and imperfect post-processing system in industrialized countries, leading to an increased accumulation of Fe³⁺ in the water system. Therefore, it’s an urgent task to probe and monitor the Fe³⁺ in the water system. Among the various method developed to probe Fe³⁺, fluorescence detection is a favourable method due to high sensitivity, selectivity and fast detection. The aqueous solution of the obtained N-CDs with excitation-dependent properties showed a strong blue emission with 8.8% quantum yield (QY) under the excitation of 340 nm, which indicates the N-CDs a promising sensing probes for Fe³⁺ in water. With the addition of Fe³⁺, the fluorescence intensity of the N-CDs gradually decreased. This research indicates an extremely sensitive method for detecting Fe³⁺ at a lower 1.9 nM level by using the synthesized N-CDs as fluorescent probes without a complicated procedure. The detectability is lower than the concentration of Fe³⁺ in the specified drinking water standard of 5.36mM[2]. Furthermore, it is lower than the limit concentration of Fe³⁺ in reverse osmosis influent, which assure the N-CDs can be used as a sensor to prevent the reverse osmose membrane from being contaminated by excess Fe³⁺. On the other hand, the practical feasibility of the synthesized N-CDs for probing Fe³⁺ in the actual sample of water has also been investigated. The reduction rate of fluorescence intensity of the N-CDs had a good linear relationship with the added water volume of the Youth Lake when the amount of lake water added was changed from 0 to 300 μL. The concentration of Fe³⁺ in the lake water was calculated according to the standard curve to be 20.4 mM, which is consistent with the result detected by ICP-MS, confirming the accuracy of the N-CDs method for detecting Fe³⁺ in actual environment.

Moreover, the mechanism of the N-CDs sensing technique was investigated in detail. It can be explained by the fluorescence static quenching effect and a ground-state complex formed in this process. The formation of the ground-state complex ensures the specificity of the N-CDs for the detection of Fe³⁺, that is, the N-CDs will have no quenching effect with other metal ions.

Acknowledgments

The authors are grateful to the financial support of National Natural Science Foundation of China (Nos. 21776203, 21576187 and 51478308).

References

[1]Tang, L , et al. Energy-level structure of nitrogen-doped graphene quantum dots. Journal of Materials Chemistry C 2013, 32: p. 4908-4915.

[2]Organization, World Health. “Guidelines for Drinking-water Quality 4th Ed”. (2011).