Magnetic thianthrene-based covalent organic frameworks with high stability and reusability for enrichment of mercury species at trace level

Abstract

Covalent organic frameworks (COFs) with low chemical or thermal stability may lead to destruction of their molecular structure when used under harsh conditions and environments, thereby limiting their use in practical processes. A magnetic COFs nano-adsorbent (Fe₃O₄@COF-TF) was prepared by coating thianthrene-based COFs with high chemical and thermal stability onto Fe₃O₄ nanoparticles by a simple solvothermal method without post-modification to introduce the target functional groups. Based on the above nano-adsorbent, a simple and practical magnetic solid-phase extraction–high-performance liquid chromatography–inductively coupled plasma mass spectrometry (MSPE-HPLC-ICP-MS) method was developed for the enrichment and determination of trace mercury species, including Hg²⁺, methylmercury (MeHg), and ethylmercury (EtHg). The experimental results showed that Fe₃O₄@COF-TF could still recover mercury species quantitatively after acid and alkali soaking treatment or 35 consecutive adsorption-desorption cycles. This indicates Fe₃O₄@COF-TF had excellent durability, extraction stability and reusability. Key parameters affecting the enrichment of mercury species, including solution pH, eluent composition and volume, adsorption and desorption time and dissolved organic matter, were systematically optimized. Under optimized conditions, the limits of detections (LODs) for Hg²⁺, MeHg and EtHg were 0.50, 0.19 and 0.67 ng L⁻¹ based on 300 mL of sample, with enrichment factors (EFs) of 338, 356 and 329, respectively. The accuracy and reproducibility of the method were validated through spiked tests and determination of certified reference materials. The developed method with the advantages of simplicity, high anti-interference capability, high enrichment factor and high sensitivity was successfully applied to the enrichment and determination of trace mercury in groundwater, surface water, seawater, and other complex matrices.