In order to solve the problem of group decision-making where the expert evaluation information is mixed with multiple attributes and the weights are unknown, a method for determining the weights of expert attributes based on the dissociation of attribute values and fuzzy entropy of each scheme is proposed firstly, and then the dynamic combination weights of different experts are obtained through linear weighting, and a new multi-attribute group decision-making method based on trapezoidal fuzzy number is proposed after combining them with Boolean values. Finally, the rationality and feasibility of this method are verifed by the actual case analysis.

Traditional damage detection methods typically involve the installation of a large number of sensors on bridges, which is costly, time-consuming, and labor-intensive, thereby limiting their application in practical engineering scenarios. This study proposes a damage identification method that integrates the Hilbert-Huang transform(HHT)and power spectral density(PSD)analysis for bridge damage detection. First, an acceleration sensor is installed on a test vehicle to record the vehicles acceleration response, which is then processed using HHT to obtain a three-dimensional time-frequency-amplitude spectrum. Subsequently, sudden changes in the amplitude spectrum are utilized to identify damage locations on the bridge. Finally, the damage severity α is evaluated using the PSD method. The- DOI:10.3969/j.issn.1000-1565.2025.05.002基于车辆响应HHT和PSD相结合的桥梁损伤识别方法方有亮^1,2,邢杰¹,张颖^1,2,刘雪婷¹,刘丹阳¹(1.河北大学 建筑工程学院,河北 保定 071002; 2.零碳能源建筑与计量技术教育部工程研究中心,河北 保定 071002)摘 要:传统的桥梁损伤检测方法通常在桥梁上布置大量的传感器,成本高,费时费力,限制了在实际工程中的应用.本文提出一种希尔伯特-黄变换(Hilbert-Huang transform,HHT)和功率谱密度(power spectral density,PSD)相结合的损伤识别方法,对桥梁进行损伤识别研究.首先,在测试车辆上布置1个加速度传感器,将车辆的加速度响应进行HHT变换,获取三维时频幅值谱;然后,利用该幅值谱的突变识别桥梁的损伤位置;最后,通过PSD识别桥梁的损伤程度α.结果表明,该方法不仅能够准确地识别桥梁的单损伤和多损伤位置,而且能够识别桥梁的损伤程度,识别精度较高,单损伤程度识别的最大误差为2.5%,多损伤程度识别的最大误差为2.6%.关键词:损伤识别;移动车辆响应;HHT;PSD中图分类号:U446 文献标志码:A 文章编号:1000-1565(2025)05-0458-08A bridge damage identification method based on HHT and PSD of vehicle responseFANG Youliang^1,2, XING Jie¹, ZHANG Ying^1,2, LIU Xueting¹, LIU Danyang¹(1. College of Civil Engineering and Architecture,Hebei University,Baoding 071002,China;2. Engineering Research Center of Zero-carbon Energy Buildings and Measurement Techniques, Ministry of Education, Baoding 071002,China)Abstract: Traditional damage detection methods typically involve the installation of a large number of sensors on bridges, which is costly, time-consuming, and labor-intensive, thereby limiting their application in practical engineering scenarios. This study proposes a damage identification method that integrates the Hilbert-Huang transform(HHT)and power spectral density(PSD)analysis for bridge damage detection. First, an acceleration sensor is installed on a test vehicle to record the vehicles acceleration response, which is then processed using HHT to obtain a three-dimensional time-frequency-amplitude spectrum. Subsequently, sudden changes in the amplitude spectrum are utilized to identify damage locations on the bridge. Finally, the damage severity α is evaluated using the PSD method. The- 收稿日期:2024-08-07;修回日期:2024-12-19 基金项目:河北省大型结构健康诊断与控制实验室开放基金项目(KLLSHMC2107);教育部春晖项目合作科研项目(HZKY20220256);河北大学实验室开放基金项目(sy202236) 第一作者:方有亮(1967—),男,河北大学教授,博士生导师,主要从事土木工程结构健康监测和结构振动分析与控制、结构灾害模拟研究.E-mail:fang_youliang@126.com 通信作者:张颖(1981—),女,河北大学副教授,主要从事桥梁结构健康监测、振动控制研究.E-mail:hbuzy@163.com 第5期方有亮等:基于车辆响应HHT和PSD相结合的桥梁损伤识别方法河北大学学报(自然科学版) 第45卷results demonstrate that the proposed method can accurately detect both single and multiple damage locations, as well as quantify the extent of damage. The maximum identification error for single damage severity is 2.5%, while that for multiple damage severities is 2.6%, indicating high identification accuracy.

The study on the diffusion characteristics of hydrogen-blended natural gas leakage is of vital significance for ensuring the safety of transportation through pipelines and for the effective promotion of this technology. This research introduced a novel leakage experimental system. The system was used to conduct experimental research on the leakage and diffusion behaviors of hydrogen-blended natural gas. The research shows that crack induced leakage has obvious directionality, whereas the circular hole did not have. Experiments were carried out under three pressure levels. The results showed that increase of the operation pressure led to an increased risk in the near field, while the far field was not affected significant. Boltzmann formula was used to fit the experimental data.The results of fitting showed that the combustible gas cloud in air have higher flammability and explosiveness than the operating medium. This research provides theoretical basis and data support for the safe and reliable operation of hydrogen-blended natural gas systems, and promotes the application of hydrogen energy.

To address the critical challenges of limited persulfate(PDS)diffusion and insufficient sustained supply of ferrous ions(Fe²⁺)during the remediation of low-permeability organic contaminated soils, this study developed a synergistic activation system based on a direct current(DC)electric field. By implementing a novel counter-directional delivery strategy-cathodic injection of PDS oxidant and anodic release of a ［Fe(Ⅱ)(C₆H₅O₇)］^- complex-the spatially oriented migration and dynamic coupling of redox-active components(PDS and Fe²⁺)were achieved, leading to substantial improvements in both transport efficiency and spatial homogeneity of reactive species. Following two-stage electrokinetic remediation, the- DOI:10.3969/j.issn.1000-1565.2025.05.004直流电场强化过硫酸盐与铁基缓释体系逆向输运协同修复氯代酚污染土壤杨旻昊¹,牟桉永²,赵冬梅³,徐宇程¹,任更波¹,马小东¹(1.河北工业大学 能源与环境工程学院,天津 300401;2. 中国石油大港油田公司 质量健康安全环保部,天津 300280;3. 大港油田原油运销公司,天津 300280)摘 要:针对低渗透性有机污染土壤修复过程中过硫酸盐(peroxydisulfate,PDS)扩散受限和亚铁离子(Fe²⁺)持续供应不足的技术瓶颈,构建了基于直流电场的逆向传输协同活化体系.通过阴极注入PDS氧化剂与阳极释放［Fe(Ⅱ)(C₆H₅O₇)］^-络合物的策略,实现了氧化-还原活性组分的空间定向迁移与动态匹配,显著提升了反应物质的迁移效能和空间分布均匀性,经二次修复后,目标污染物二氯酚的降解率提升至90.08%,土壤残留污染物质量分数(88.6 mg/kg)显著低于《土壤环境质量 建设用地土壤污染风险管控标准(试行)》(GB36600—2018)中第一类建设用地风险管控阈值(117 mg/kg).本研究提出的电化学逆向传质调控策略为有机污染土壤修复提供了新的技术路径,特别是在解决低渗透地层氧化剂传输与活化剂持续供给的协同优化方面具有重要应用价值.关键词:过硫酸盐;直流电;传质;二氯酚;土壤修复中图分类号:X53 文献标志码:A 文章编号:1000-1565(2025)05-0476-12Direct current electric field enhanced reverse transport of persulfate and iron-based sustained-release system for synergistic remediation of chlorophenol-contaminated soilYANG Minhao¹, MU Anyong², ZHAO Dongmei³, XU Yucheng¹, REN Gengbo¹, MA Xiaodong¹(1. School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin 300401, China; 2. Department of Quality, Health, Safety and Environmental Protection, PetroChina Dagang Oilfield Co., Tianjin 300280, China; 3. Crude Oil Distribution Company of Dagang Oilfield, Tianjin 300280, China)Abstract: To address the critical challenges of limited persulfate(PDS)diffusion and insufficient sustained supply of ferrous ions(Fe²⁺)during the remediation of low-permeability organic contaminated soils, this study developed a synergistic activation system based on a direct current(DC)electric field. By implementing a novel counter-directional delivery strategy-cathodic injection of PDS oxidant and anodic release of a ［Fe(Ⅱ)(C₆H₅O₇)］^- complex-the spatially oriented migration and dynamic coupling of redox-active components(PDS and Fe²⁺)were achieved, leading to substantial improvements in both transport efficiency and spatial homogeneity of reactive species. Following two-stage electrokinetic remediation, the- 收稿日期:2024-11-10;修回日期:2025-03-25 基金项目:国家重点研发计划项目(2020YFC1808603);国家自然科学基金项目(21876042);河北省重点研发计划项目(22374206D) 第一作者:杨旻昊(2001—),男,河北工业大学在读硕士研究生,主要从事有机污染土壤修复方向研究.E-mail:202331305015@stu.hebut.edu.cn 通信作者:马小东(1972—),男,河北工业大学教授,主要从事工业污染场地修复方向研究. E-mail:maxd@hebut.edu.cn第5期杨旻昊等:直流电场强化过硫酸盐与铁基缓释体系逆向输运协同修复氯代酚污染土壤河北大学学报(自然科学版) 第45卷degradation efficiency of dichlorophenol reached 90.08%, with residual concentrations(88.6 mg/kg)significantly below the risk control threshold(117 mg/kg)specified for Category I construction land in China’s Soil Environmental Quality Standard(GB36600—2018). The proposed electrochemically regulated counter-directional mass transfer strategy establishes a robust technical framework for organic soil remediation, particularly in overcoming the synergistic challenges of oxidant delivery and activator replenishment in low-permeability strata.

Microplastic pollution caused by traditional mulch residues seriously threatens the ecological safety of farmland, while biodegradable mulch film still has microplastic release and soil environmental risks in practical application. In order to evaluate the degradation characteristics of biodegradable films and their long-term impacts on soil environment, this study was conducted to analyze the degradation process of white, silver-black and black films under the conditions of surface soil exposure(0 cm)and deep burial(10 cm)using poly(butylene adipate-co-terephthalate)(PBAT)biodegradable films. The results showed that the degradation of mulch film was significantly higher in the deep soil layer than in the surface layer, while the effect of color on the degradation of mulch film was limited. The microplastics released from broken mulch could change the soil pore structure, resulting in a decrease in soil pH, an increase in soil electrical conductivity(EC), and a decrease in soil organic matter(SOM)content. The results provide a theoretical basis for the scientific application of degradable mulch film and the prevention and control of soil microplastic pollution.- DOI:10.3969/j.issn.1000-1565.2025.05.005PBAT可降解地膜多维度降解行为及其土壤微环境效应冯芳,梁存,黄佳,刘微(河北大学 化学与材料科学学院,河北 保定 071002)摘 要:传统地膜残留引发的微塑料污染严重威胁农田生态安全,而可降解地膜在实际应用中仍存在微塑料释放及土壤环境风险.为评估可降解地膜的降解特性及其对土壤环境的长期影响,本研究以聚己二酸/对苯二甲酸丁二酯可降解地膜为研究对象,分析白色、银黑色和黑色地膜在土壤表层暴露(0 cm)与深层掩埋(10 cm)条件下的降解过程.结果表明,地膜在深层土壤中的降解程度显著高于表层,而颜色对地膜降解的影响有限.地膜破碎后,释放的微塑料可以改变土壤孔隙结构,使土壤pH下降、土壤电导率升高,以及土壤有机质含量降低.本研究成果为可降解地膜的科学应用与土壤微塑料污染防控提供了理论依据.关键词:可降解地膜;表面特征;土壤理化性质;微塑料中图分类号:X53 文献标志码:A 文章编号:1000-1565(2025)05-0488-09PBAT degradable mulch film multidimensional degradation behavior and soil microenvironment effectFENG Fang,LIANG Cun,HUANG Jia,LIU Wei(College of Chemistry and Materials Science, Hebei University, Baoding 071002,China)Abstract: Microplastic pollution caused by traditional mulch residues seriously threatens the ecological safety of farmland, while biodegradable mulch film still has microplastic release and soil environmental risks in practical application. In order to evaluate the degradation characteristics of biodegradable films and their long-term impacts on soil environment, this study was conducted to analyze the degradation process of white, silver-black and black films under the conditions of surface soil exposure(0 cm)and deep burial(10 cm)using poly(butylene adipate-co-terephthalate)(PBAT)biodegradable films. The results showed that the degradation of mulch film was significantly higher in the deep soil layer than in the surface layer, while the effect of color on the degradation of mulch film was limited. The microplastics released from broken mulch could change the soil pore structure, resulting in a decrease in soil pH, an increase in soil electrical conductivity(EC), and a decrease in soil organic matter(SOM)content. The results provide a theoretical basis for the scientific application of degradable mulch film and the prevention and control of soil microplastic pollution.- 收稿日期:2025-01-25;修回日期:2025-03-05 基金项目:国家自然科学基金项目(42077369) 第一作者:冯芳(2001—),女,河北大学在读硕士研究生,主要从事可降解微塑料的光老化和土壤降解方向的研究. E-mail:flf0123ff@126.com 通信作者:刘微(1980—),女,河北大学教授,主要从事土壤污染防治及修复方向的研究.E-mail:uhlw80@126.com 第5期冯芳等:PBAT可降解地膜多维度降解行为及其土壤微环境效应河北大学学报(自然科学版) 第45卷

In this study, a colorimetric immunochromatography method based on colloidal gold was developed. Under optimal experimental conditions, the standard curve was established. The detection limits of this method for the three organophosphorus pesticides were 0.94, 0.32, 0.03 ng/mL, respectively, and the method was suitable for the preliminary screening of organophosphorus pesticide residues.The recovery of peach samples by standard addition showed that the recoveries rate of fenitrothion were 88%—113%, and the relative standard deviation were less than 10%. The recoveries rate of parathion methyl ranged were 107%—118%, and the relative standard deviation were less than 5%. The recoveries rate of parathion were 106%—113%, and the relative standard deviation were less than 5%.The experimental results showed that the method had high accuracy and reliability, and was not easily affected by the sample matrix, and could accurately detect organophosphorus pesticide residues in peach juice.

To achieve efficient and simultaneous treatment and resource utilization of excess sludge and aquaculture wastewater, a sediment microbial fuel cell with algal cathode-sediment microbial fuel cell(AC-SMFC)was constructed by combining algal-bacterial symbiosis technology with sediment microbial fuel cell. A bacterial cathode-sediment microbial fuel cell(BC-SMFC)with activated sludge inoculated on the cathode was used as a control. The power generation performance, removal efficiencies of carbon, nitrogen and phosphorus, and the reduction effect of excess sludge in AC-SMFC were investigated.Results showed that the cathode potential and maximum output power density of AC-SMFC were(283.5±15.3)mV and 91.18 mW/m², respectively. After 41 days of operation, the removal rates of chemical oxygen demand(COD), ammonia nitrogen, total nitrogen and total phosphorus in aquaculture wastewater by AC-SMFC reached- DOI:10.3969/j.issn.1000-1565.2025.05.007菌藻阴极SMFC同步处理剩余污泥及养殖废水效能研究丁陈玉,杜旺,李永琦,陈利争,张勇泉,王洪杰,李慧(河北大学 生态环境系,河北省湿地近自然修复技术重点实验室,河北 保定 071002)摘 要:为了实现剩余污泥和养殖废水的高效同步处理与资源化,将菌藻共生技术与沉积式微生物燃料电池结合构建了菌藻阴极型沉积式微生物燃料电池(algal cathode-sediment microbial fuel cell,AC-SMFC). 以阴极接种活性污泥的沉积式微生物燃料电池(baterial cathode-sediment microbial fuel cell,BC-SMFC)为对照,研究了AC-SMFC同步处理剩余污泥和养殖废水过程中的产电性能、阴极碳氮磷污染物去除效率及剩余污泥减量效果.结果表明,以剩余污泥和养殖废水分别作为底泥和上覆水时,AC-SMFC的最大输出功率密度和阴极电势分别为91.18 mW/m²和(283.5±15.3)mV.运行41 d后,AC-SMFC对养殖废水中化学需氧量、氨氮、总氮和总磷的去除率分别达到98.51%、81.63%、74.29%和65.84%,氮磷去除效果显著提升.AC-SMFC污泥减量效果显著,总化学需氧量和挥发性悬浮固体去除率分别为59.8%和68.5%.此外,AC-SMFC可促进松散结合胞外聚合物和紧密结合胞外聚合物中溶解性微生物产物、芳香族蛋白质和腐殖酸类物质的降解.关键词:养殖废水;降解;菌藻共生;微生物燃料电池;剩余污泥中图分类号:X703;TM911 文献标志码:A 文章编号:1000-1565(2025)05-0507-13Performance of the sediment microbial fuel cell with algal-bacterial cathode for simultaneous treatment of excess sludge and aquaculture wastewaterDING Chenyu, DU Wang, LI Yongqi, CHEN Lizheng, ZHANG Yongquan, WANG Hongjie, LI Hui(Hebei Key Laboratory of Close-to-Nature Restoration Technology of Wetlands, School of Eco-Environment, Hebei University, Baoding 071002, China)Abstract: To achieve efficient and simultaneous treatment and resource utilization of excess sludge and aquaculture wastewater, a sediment microbial fuel cell with algal cathode-sediment microbial fuel cell(AC-SMFC)was constructed by combining algal-bacterial symbiosis technology with sediment microbial fuel cell. A bacterial cathode-sediment microbial fuel cell(BC-SMFC)with activated sludge inoculated on the cathode was used as a control. The power generation performance, removal efficiencies of carbon, nitrogen and phosphorus, and the reduction effect of excess sludge in AC-SMFC were investigated.Results showed that the cathode potential and maximum output power density of AC-SMFC were(283.5±15.3)mV and 91.18 mW/m², respectively. After 41 days of operation, the removal rates of chemical oxygen demand(COD), ammonia nitrogen, total nitrogen and total phosphorus in aquaculture wastewater by AC-SMFC reached- 收稿日期:2024-12-10;修改稿日期:2025-01-09 基金项目:城市水资源与水环境国家重点实验室开放课题(HC202242);河北大学多学科交叉研究计划项目(DXK202106);河北省大学生创新创业训练计划项目(S202410075086);河北大学研究生创新项目(HBU2025SS023) 第一作者:丁陈玉(2001—),男,河北大学在读硕士研究生,主要从事生物电化学污水处理技术方向研究. E-mail:18655175391@163.com 通信作者:王洪杰(1972—),男,河北大学教授,博士生导师,主要从事生态修复技术原理与应用方向研究. E-mail:wanghj@hbu.edu.cn李慧(1987—),女,河北大学副教授,主要从事污水及污泥处理与资源化方向研究. E-mail:lihuihbu@hbu.edu.cn第5期丁陈玉等:菌藻阴极SMFC同步处理剩余污泥及养殖废水效能研究河北大学学报(自然科学版) 第45卷98.51%, 81.63%, 74.29% and 65.84%, respectively,indicating a significant improvement in nitrogen and phosphorus removal. AC-SMFC showed a significant sludge reduction effect, with the removal rates of total chemical oxygen demand(TCOD)and volatile suspended solids(VSS)reaching 59.8% and 68.5%, respectively. In addition, AC-SMFC could promote the degradation of soluble microbial products, aromatic proteins and humic substances in the loosely-bound extracellular polymeric substances(LB-EPS)and tightly-bound extracellular polymeric substances(TB-EPS).

To improve the recognition accuracy of nominal attribute instance differences and optimize the performance of classification algorithms, a hybrid distance measurement method based on conditional probability distribution is proposed, which comprehensively considers the attributes and category features of instances.Firstly, this method calculates the differences in conditional probability distributions among attributes and between attributes and categories. Secondly, a new hybrid distance metric is obtained by using mutual information to weight and combine the two differences. Finally, simulation experiments are performed on 20 UCI datasets based on K-nearest neighbor algorithm. Meanwhile, the distance measurement method is applied to the diagnosis and treatment of appendicitis in children. The results show that compared with the three measurement methods including overlap measurement, the distance measurement method proposed in this work significantly improves the accuracy of the classification algorithm and has good application prospects.

This paper investigates the robust control problem of unknown systems based on event driven observers. Unlike normal sampling control systems where the controller is periodically updated, in event driven systems, the controller is only updated when an event occurs, and the occurrence of an event is usually determined by whether some error signals meet certain predetermined conditions. For unknown systems with parameter perturbations, a method for solving the controller gain matrix and observer gain matrix is proposed. The results indicate that under event driven conditions, the proposed control method for parameter perturbed systems can still stabilize the system. Finally, the feasibility and effectiveness of the method proposed in this paper were validated through a simulation case.

The extraction of time-frequency ridge of rolling bearing is susceptible to noise and other interfering signals under variable speed conditions, leading to the extracted ridge to deviate from the true value thereby, and affecting the accuracy of fault diagnosis. A variable speed rolling bearing fault diagnosis method based on sliding window multi-time-frequency ridge extraction is proposed in this work. First, the high-resolution time-frequency representation of bearing fault signal is obtained by using the second-order synchrosqueezing short-time Fourier transform. Next, multiple feature ridges are extracted iteratively from the time-frequency map based on the sliding window, to effectively eliminate the noise and adjacent-component interference, and to accurately estimate the instantaneous rotational frequency and instantaneous fault-characteristic frequency of the bearing. Finally, the bearing fault type is identified by the ratio of instantaneous fault-characteristic frequency to instantaneous rotational frequency. Simulation and experimental analysis results of rolling bearing variable speed fault show that the proposed method has- DOI:10.3969/j.issn.1000-1565.2025.05.011基于滑动窗多时频脊线提取的滚动轴承变转速故障诊断郝紫阳¹,张晓宣¹,冯贺平²(1.河北大学 质量技术监督学院,河北 保定 071002;2.河北软件职业技术学院 智能工程系,河北 保定 071002)摘 要:变转速工况下滚动轴承时频脊线提取易受噪声等干扰信号影响,导致提取的时频脊线偏离真实值,从而影响故障诊断精度.本文提出一种基于滑动窗多时频脊线提取的滚动轴承变转速故障诊断方法.首先,利用二阶同步压缩短时傅里叶变换获取轴承故障信号高分辨率时频表示;然后,基于滑动窗从时频图中迭代提取多条特征脊线,有效排除噪声及相邻分量干扰,准确估计轴承的瞬时旋转频率及瞬时故障特征频率;最后,依据瞬时故障特征频率与瞬时旋转频率的比值判断轴承故障类型.滚动轴承变转速故障仿真及实验分析结果表明,本研究方法具有良好的噪声鲁棒性,其特征脊线提取效果优于经典的多时频曲线提取方法.关键词:时频脊线提取;变转速;滚动轴承;故障诊断中图分类号:TH133.3 文献标志码:A 文章编号:1000-1565(2025)05-0551-10Variable speed fault diagnosis of rolling bearings based on sliding-window multiple time-frequency ridge extractionHAO Ziyang¹, ZHANG Xiaoxuan¹, FENG Heping²(1. College of Quality and Technical Supervision, Hebei University,Baoding 071002, China;2. Intelligent Engineering Department, Hebei Software Institute, Baoding 071002, China)Abstract: The extraction of time-frequency ridge of rolling bearing is susceptible to noise and other interfering signals under variable speed conditions, leading to the extracted ridge to deviate from the true value thereby, and affecting the accuracy of fault diagnosis. A variable speed rolling bearing fault diagnosis method based on sliding window multi-time-frequency ridge extraction is proposed in this work. First, the high-resolution time-frequency representation of bearing fault signal is obtained by using the second-order synchrosqueezing short-time Fourier transform. Next, multiple feature ridges are extracted iteratively from the time-frequency map based on the sliding window, to effectively eliminate the noise and adjacent-component interference, and to accurately estimate the instantaneous rotational frequency and instantaneous fault-characteristic frequency of the bearing. Finally, the bearing fault type is identified by the ratio of instantaneous fault-characteristic frequency to instantaneous rotational frequency. Simulation and experimental analysis results of rolling bearing variable speed fault show that the proposed method has- 收稿日期:2025-03-11;修回日期:2025-07-11 基金项目:河北省自然科学基金项目(E2021201032) 第一作者:郝紫阳(1977—),男,河北大学讲师,主要从事机械装备状态监测与故障诊断研究.E-mail:haoziyang@hbu.edu.cn 通信作者:冯贺平(1979—),女,河北软件职业技术学院副教授,主要从事方程智能检测、智能控制研究.Email:fengheping@126.com第5期郝紫阳等:基于滑动窗多时频脊线提取的滚动轴承变转速故障诊断河北大学学报(自然科学版) 第45卷good noise robustness, and its feature ridge extraction effect is better than the classical multiple time-frequency curve extraction method.