Dispersive Solid Phase Micro-Extraction Used for Monitoring Water Samples

Scientists from Universidad Autónoma del Estado de Hidalgo in Hidalgo, Mexico recently tested a new dispersive solid phase micro-extraction (DSPME) technique for removing naphthol isomers from water samples. Their findings were published in the Journal of Chromatography A (1).

Detail of sampling beaker used for water samples collecting | Image Credit: © Couperfield - stock.adobe.com

Polycyclic aromatic hydrocarbons (PAHs) are a diverse set of chemical molecules that consist of two or more hydrophobic fused aromatic rings. PAHs naturally occur in coal, crude oil, and gasoline, though they can also form when coal, oil, gas, wood, garbage, and tobacco are burned (2). However, they can also form from anthropogenic sources and involve emissions from industrial activities, such as garbage incineration or steel production. Sixteen PAHs, including acenaphthene, benzo[ghi]perylene, chrysene, acenaphthylene, benz[a]anthracene, benzo[b]fluoranthene, anthracene, benzo[k]fluoranthene, benzo[a]pyrene, fluoranthene, indenol[1,2,3-cd]pyrene, phenanthrene, dibenz[a,h]anthracene, fluorene, pyrene, and naphthalene, have been classified as priority contaminants by the U.S. Environmental Protection Agency (EPA); this stems from their potential human exposure, toxicity, and common occurrence at hazardous waste sites.

Naphthalene (Np) is a prevalent PAH that serves as a precursor in the synthesis of naphthol (NAP) production, such as α-naphthol (α-NAP) and β-naphthol (β-NAP), which is employed in the production of dyes, pesticides, and pharmaceutical products. These compounds can be widely applied and are subsequently released into the environment, where they can be distributed through air, soil, and water. Recent studies show that α-NAP and β-NAP are persistent in drinking water and industrial wastewater. They have lethal dose (LD₅₀) values of 3000 mg/kg and 4200 mg/kg of α-NAP and β-NAP, respectively, and exhibit comparable toxicity to Np, which is classified as a persistent organic pollutant with notable mutagenic, teratogenic, and potentially carcinogenic effects on humans (1).

In this study, a method based on using layered double hydroxides (LDH) coupled to DSPME was used to remove α-NAP and β-NAP isomers from water samples. LDHs are metal hydroxides arranged in sheets with hydroxide ions that have a formula A²⁺B³⁺(OH)₅ (3). Three different LDHs, MgAl-LDH, NiAl-LDH, and CoAl-LDH, were used to study how the interlayer anion and molar ratio affected the removal performance (1).

The critical factors in the DSPME procedure (pH, LDH amount, and contact time) were optimized via the univariate method under optimal conditions: pH 4–8; LDH amount of 5mg; contact time of 2.5min. The method was found to be applicable in real sample waters, removing NAP isomers, even in ultra-trace concentrations. When using a large volume sample stacking (LVSS-CE) technique, limits of detection (LODs) of 5.52 µg/L and 6.36 µg/L were found for α-naphthol and β-naphthol, respectively (1).

With their findings, the scientists identified trends that can help further our understanding of how organic pollutants are adsorbed into NAPs. The optimal adsorbent, MgAl/Cl^--LDH, held a molar ratio of 3:1 while efficiently removing and fully eluting NAPs. This method could help remove NAP in tap water under optimal conditions, making it suitable for future use in preconcentration systems. Overall, the DSPME methodology has sown itself as simple, rapid, requiring low solvent consumption, and is cost-effective (1).

References

(1) Aurelio-Soria, D.; Rodriguez, J. A.; Paez-Hernandez, M. E.; et al. Development of a Dispersive Solid Phase Microextraction Method Based on the Application of MgAl, NiAl, and CoAl-Layered Double Hydroxides for the Efficient Removal of α- and β-Naphthol Isomers from Water Samples by Capillary Electrophoresis. J. Chromatogr. A 2024, 1731, 465174. DOI: 10.1016/j.chroma.2024.465174

(2) Polycyclic Aromatic Hydrocarbons (PAHs). Centers for Disease Control and Prevention (CDC) 2009. https://www.epa.gov/sites/default/files/2014-03/documents/pahs_factsheet_cdc_2013.pdf (accessed 2024-8-20)

(3) Layered Double Hydroxides. Elsevier B.V. 2024. https://www.sciencedirect.com/topics/materials-science/layered-double-hydroxides (accessed 2024-8-20)