Investigation on the Determination of Dechlorane Plus Compounds in Complex Matrix Sludge Using Microwave-Assisted Extraction Coupled with GC-MS/MS and Evaluation of Treatment Efficiency

Significance: Dechlorane plus (DP) is synthesized through the diels-alder addition reaction. It was first produced in the United States in the 1950s，and its products are diverse. As a highly chlorinated flame retardant，it is widely used^［1-2］. Due to its tendency to enter the environment during production，use，and other processes，DP has been frequently detected in environmental samples around the world^［4-5］. It exhibits characteristics of persistent organic pollutants (POPs)，affecting the health of organisms^［8-9］，and has been included in China's list of key controlled emerging pollutants.

　　Sewage treatment plants are important sources of pollutants. Dechlorane plus compounds tends to accumulate in sludge，and the existing disposal methods carry ecological risks^［12］. However，there are relatively few studies on dechlorane plus compounds in the sludge of sewage treatment plants. Accurately determining its content is of great significance. Nevertheless，the complex sludge matrix poses challenges to the analysis. Most of the existing detection methods are designed for soil and sediment. For example，Soxhlet extraction is time-consuming and requires a large amount of solvent^［14］. Although QuEChERS has its advantages，it has problems such as large variations in recovery rates (59.6%-115.9%)^［16-17］. At present，gas chromatography is primarily employed to separate dechlorane plus compounds. Common detection methods include GC-MS and GC-HRMS^［19-20］. However，the former is prone to matrix interference，while the latter has high operation requirements and comes at a high cost^［21］. GC-MS/MS，on the other hand，boasts high sensitivity and strong specificity，showing distinct advantages in multi-component and ultra-trace analysis^［24-25］.

　　Due to the special matrix of the sludge in sewage treatment plants，the existing detection methods are difficult to meet the requirements. This research intends to use microwave-assisted extraction combined with adsorbents for preliminary purification，then strengthen the purification effect with solid-phase extraction columns，and finally determine the content by GC-MS/MS. The aim is to establish an accurate，easy-to-operate method suitable for batch detection，providing a basis for the study of the environmental fate of dechlorane plus compounds.

Methods: In April and June 2024，sludge samples were collected from 11 urban sewage treatment plants in China，including samples not contaminated by dechlorane plus compounds. The activated sludge process was adopted in sewage treatment plants numbered 1-5，while the Anaerobic-Anoxic-Oxic (A²/O) process was used in plants numbered 6-11. The samples were collected in pre-cleaned brown glass containers. Three parallel samples and blank samples were taken for each sample. The samples were stored refrigerated at around 4℃，and extraction was completed within 14 days after collection.

　　Accurately weigh 10.00g of fresh sludge sample. Then mix it successively with 3.00 grams of copper powder，an appropriate amount of anhydrous sodium sulfate，0.50 grams of graphitized carbon black (GCB)，and 0.50g of primary-secondary amine sorbent (PSA)，and grind them evenly. Then add the purification internal standard solution. Next，perform extraction using acetone-n-hexane (1:1，V/V) under conditions of 1500W power and 120℃ temperature. After the extraction is completed，separate and concentrate the extraction solution. Then，purify it through GCB/PSA solid-phase extraction. Add the injection internal standard，and then analyze it by GC-MS/MS.

　　For gas chromatography，the injection port temperature is set to 260℃ with splitless injection; the injection volume is 1.0μL. The chromatographic column used is a DB-5 (15m×0.25mm，0.1μm)，and the column flow rate is 1.0mL/min. The temperature program is as follows: initial temperature of 120℃，held for 2min; then increased to 220℃ at a rate of 20℃/min and held for 1min; and finally increased to 310℃ at a rate of 10℃/min and held for 2min. For mass spectrometry，the ion source is an EI source，with an ion source temperature of 240℃，ionization energy of 70eV，interface temperature of 280℃，and solvent delay time of 4.5minutes. Data acquisition is performed using multiple reaction monitoring (MRM) mode. The chromatograms and mass spectrometry condition parameters for data collection are shown in Fig.1 and Table 1.

Data and Results: (1) This study focuses on the detection of dechlorane plus compounds in sludge. A comparative experiment of direct extraction and extraction after drying was conducted on the same fresh water-containing sample. It was found that the recovery rate of the wet sample was slightly higher (Fig.2). Therefore，direct extraction of fresh sludge was adopted to improve efficiency.

　　In the optimization of sample extraction and purification，extraction experiments with different organic reagents were performed，and acetone-n-hexane (1:1，V/V) was determined as the optimal extraction solvent，with a recovery rate of 98.6%-108.7% (Fig.3). The effects of different temperatures and powers on extraction efficiency were studied，as shown in Fig.4 and Fig.5 respectively，and finally 120°C and 1500W were selected as the optimal conditions，with recovery rates of 92.1%-110.4% and 89.2%-107.3%，respectively. For purification，it was determined that adding 3.00g of copper powder to 10.00g of sludge effectively removes sulfur. Simultaneously adding 0.50g of graphitized carbon black (GCB) and 0.50g of primary secondary amine (PSA) in sequence can effectively remove interferences such as pigments and lipids. The GCB/PSA solid-phase extraction column is used for further purification，with a recovery rate of 92.6%-105.3% (Fig6).

　　The standard solution test showed that dechlorane plus compounds exhibited good linearity (r≥0.998)，with a limit of detection ranging from 0.017 to 0.040ng/g (Table 3). The average recoveries were 79.8%-99.5%，86.2%-104.8% and 91.2%-106.1% at low，medium and high concentrations，respectively (Table 4)，with relative standard deviations of less than 7%，meeting the analysis requirements.

　　Compared to methods used in the literature for determining dechlorane plus compounds in soil and sediment，this method faces the challenge of a more complex sludge matrix but offers several advantages. It can detect a wider range of compounds，and its recovery rates are comparable to those achieved by pressurized liquid extraction technology and superior to the traditional Soxhlet extraction method. In terms of detection limits，this method has a clear advantage over GC-MS，although it is not as sensitive as GC-HRMS. Microwave-assisted extraction processes 44 samples in just 45min. Additionally，direct extraction of fresh sludge avoids issues associated with drying. The use of GC-MS/MS technology effectively reduces interference from co-extracted compounds and minimizes the matrix effect，providing an efficient，accurate，and practical detection method for analyzing sludge samples from sewage treatment plants.

(2) DP has two conformational isomers，syn-DP and anti-DP. The ratio of its isomers is described by fractional abundance. The contents of dechlorane plus compounds in the sludge of 11 sewage treatment plants were measured according to the methods. Dechlorane plus compounds were detected in the sludge of each sewage treatment plant. The total content ranges from 31.4 to 195.6ng/g，which is similar to the values reported in the literature^[37]. The quality control in the experiment was strict，ensuring reliable results.

　　The distribution of dechlorane plus compounds in the sludge varies. Concentrations may be related to local usage patterns. As shown in Fig.7，anti-DP is the primary contributing monomer，while the content of Mirex is low. The dechlorination products of DPs have high concentrations and require attention. The concentration of dechlorane plus compounds in the sludge of Sewage Treatment Plant 1 is particularly high，possibly due to the presence of an electronic waste dismantling base nearby^[33]. The average f_syn value in this study is 0.27，which is lower than that of DP products^[39]. This may be due to the strong adsorption of anti-DP by the sludge or the preferential degradation of syn-DP.

　　As shown in Fig.8，Different water treatment processes affect dechlorane plus compounds differently. Compared with the water treatment by the activated sludge process，the concentration of dechlorane compounds in the A²/O process is significantly reduced (p＜0.05)，and its average concentration drops from 135.7ng/g in the activated sludge process to 75.2ng/g. Comparing the f_syn values obtained from different water treatment processes (Fig.8b)，when the A²/O process is adopted，the f_syn value increases significantly (p＜0.05)，rising from 0.26 to 0.29.