Rational design and synthesis of magnetic covalent organic frameworks for controlling the selectivity and enhancing the extraction efficiency of polycyclic aromatic hydrocarbons

A series of three-dimensional magnetic covalent organic frameworks were designed and synthesized via monomer selection, coating thickness optimization, and composite strategy transformation. Their structure properties including morphology, functional group, surface area, and pore size were characterized. The relationship between the structural properties and analytical performance was systematically investigated by density functional theory calculation and batch extraction experiments for polycyclic aromatic hydrocarbons. It is proven that the extractant modified by monomer 1,4-phthalaldehyde provides a hizgh affinity for high molecule weight polycyclic aromatic hydrocarbons and the right balance between extraction and elution efficiency. The relationship between coating thickness and mass transfer rate of polycyclic aromatic hydrocarbons was studied by accurate tuning of coating layers via layer-by-layer method. A mathematical model was derived and employed to determine that two coating layers were sufficient to provide the highest extraction efficiency with the shortest equilibrium time. The extractants synthesized by two different composite strategies (layer-by-layer and one-step) show opposite selectivity for polycyclic aromatic hydrocarbons. After optimization of the extraction conditions, dispersed solid-phase extraction coupled with gas chromatography-mass spectroscopy method was developed providing a wide linear range (5-500 ng L-1), good linearity (R-2 > 0.9923), high precision in intra-day (RSD% < 8.2%) and inter-day (RSD% < 12.3%) detection, and low detection limits (1.5-15.1 ng L-1). The method was applied to the simultaneous determination of 16 polycyclic aromatic hydrocarbons with acceptable recoveries, which were 87-109% for groundwater, 83-116% for East Lake water, and 82-116% for Yangtze River water samples.