theChemistry

Introduction Researchers have studied the chemistry behind fireworks and how they can impact people’s lives by affecting ecosystems through air and water pollution. Fireworks are typically set off in outdoor areas and near rivers or lakes, in accordance with safety regulations. As a result, these waterfront areas become hotspots for the accumulation of postcombustion residues, including unburned fuels, oxidizers, metal salts, and packaging debris. Rather than dissipating rapidly, these residues persist and act as a delayed source of contamination. The effects of these residues on DOM were studied using optical and high-resolution mass spectrometry (HRMS). DOM is Dissolved Organic Matter found in all natural waters; its concentration is measured as an indicator of the health of a marine ecosystem. The residues release DOM enriched in low-molecular-weight sulfur-containing and aliphatic compounds, resulting in a decrease in the molecular weight, oxidation state, and unsaturation of natural waters. The extent of DOM changes induced by firecracker residues strongly depended on the initial water chemistry, with higher conductivity suppressing DOM release, whereas humic-rich waters promoted greater molecular redistribution.

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Techniques HRMS enables the resolution of thousands of individual molecular formulas, providing granular insights into the elemental composition, degree of unsaturation, and oxidation states of complex organic mixtures. Furthermore, coupling HRMS with optical spectroscopy and water chemistry provides a comprehensive toolkit to decode the transformation of DOM characteristics during the leaching process. 

Photo from Molecular-Level Perturbations of Dissolved Organic Matter Driven by Episodic Firecracker Residue Leaching | Environmental Science & Technology.

Conditions of experiments Leaching experiments were conducted using river water, lake water, and ultrapure water to assess the generality of the observed patterns. The leaching procedure was performed under dark conditions to avoid photochemical reactions and to specifically focus on the leaching processes, and was carried out in sealed containers to minimize gas exchange with the atmosphere.

Type of DOM In leaching experiments, DOM molecules are classified into three categories: released DOM, adsorbed DOM and persistent DOM.

• The released DOM represents the formulas that were absent in the original river or lake water samples but newly detected in the water after leaching, as well as those present both before and after leaching with a fold change (fc = peak intensity_after/peak intensity_before) greater than 2.0.
• The adsorbed DOM refers to the formulas that were present in the original water samples but disappeared after the leaching experiments, as well as those detected both before and after leaching with fc < 0.5.
• The persistent DOM represents the formulas that were simultaneously present in the water samples before and after leaching, with 0.5 ≤ fc ≤ 2.0.

Results

Photo from Molecular-Level Perturbations of Dissolved Organic Matter Driven by Episodic Firecracker Residue Leaching | Environmental Science & Technology. Water chemistry changes in Milli-Q, river, and lake waters before and after leaching of firecracker residues at 25 °C. (a) DOC, Dissolved Organic Carbon; (b) pH; (c) σ, electrical conductivity; (d) K⁺; (e) Mg²⁺; (f) Sb; (g) NO₃^–; (h) SO₄^2–; (i) a₃₀₀, the absorbance coefficient at 300 nm; (j)E2/E3, the absorbance ratio. The values represent the average increment of the parameter before and after leaching. P values were calculated using the Wilcoxon signed-rank test. *, p < 0.05; **, p < 0.01; ***, p < 0.001.

Specifically, DOC concentrations increased significantly in both river and lake water after leaching (p < 0.05, Figura 1a), indicating a substantial release of organic matter from firecracker residues. The results showed that the mass of DOC increased on average by 2.85 mg, 2.08 mg, and 3.55 mg per gram of firecracker residue in Milli-Q water, river water, and lake water, respectively, representing net changes that reflect the combined effects of release, adsorption, and potential transformation processes. DOC is dissolved organic carbon and refers to the portion of organic carbon that is found in a dissolved state in water or other liquids and that can pass through a filter membrane with very small pores, generally between 0.22 and 0.7 µm.

Previous studies have reported that DOC released from microplastics typically ranges from tens to hundreds of μg C g^–1 plastic day^–1 and is closely associated with photochemical aging processes. Microplastics require gradual cleavage of polymer chains to produce dissolved products, whereas firecracker residues contain preformed combustion products and additives that can be more readily released upon contact with water. Concurrently, the increase in pH following leaching may be attributed to the release of weakly alkaline components, as well as the potential consumption of H ions during interactions between the residues and the aqueous phase (Figure 1b). The increase in σ reflected the release of ionic species from the residues (Figure 1c). The concentration of K⁺ and Mg²⁺  increases in concentration after leaching which can be attributed to the presence of potassium nitrate and magnesium-based compounds commonly used as oxidizers and colorants in firecracker formulations (Figure 1d, 1e). Same fact for Sb, NO₃^–, SO₄^2– used in firecracker formulations.

Bibliographic sources For more info: Molecular-Level Perturbations of Dissolved Organic Matter Driven by Episodic Firecracker Residue Leaching | Environmental Science & Technology.

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