The vadose zone is a very dynamic and active environment that directly affects natural attenuation and vapor intrusion of volatile organic compounds (VOCs). Therefore, it is important to understand the fate and transport of VOCs in the vadose zone. A column experiment combined with model study was conducted to investigate the influence of soil type, vadose zone thickness, and soil moisture content on benzene vapor transport and natural attenuation in the vadose zone. Vapor-phase biodegradation and volatilization to atmosphere for benzene are two main natural attenuation mechanism in the vadose zone. Our data showed that biodegradation in black soil is the main natural attenuation mechanism (82.8%) while volatilization is the main natural attenuation mechanism in quartz sand, floodplain soil, lateritic red earth and yellow earth (>71.9%). The R-UNSAT model-predicted soil gas concentration profile and flux were close with four soil column data except for yellow earth. Increasing the vadose zone thickness and soil moisture content significantly reduced the volatilization contribution while increased biodegradation contribution. The volatilization loss decreased from 89.3% to 45.8% when the vadose zone thickness increased from 30 cm to 150 cm. The volatilization loss decreased from 71.9% to 10.1% when the soil moisture content increased from 6.4% to 25.4%. Overall, this study provided valuable insights into clarifying the roles of soil type, moisture, and other environmental conditions in vadose zone natural attenuation mechanism and vapor concentration.
Luo Yan-Ling,Bai Rui-Xue,Xu Feng,Chen Ya-Shao,Li Hua,Dai Si-Si,Ma Wen-Bo
Novel multiwalled carbon nanotube graft polyethylene glycol -block -polystyrene copolymer nanohybrids (MWCNTs- g -PEG- b -PS) were prepared via atom transfer radical polymerization using carboxylated MWCNTs (MWCNTs–COOH) as a conductive carrier. The chemical structure and the compositions of graft copolymer nanohybrids were characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetry, and gas chromatography, and the morphologies and dispersion behavior were observed by ultraviolet visible spectrophotometer, scanning electron microscope, and transmission electron microscope etc. The MWCNTs- g -PEG- b -PS-grafted copolymer nanohybrids were fabricated into conductive composite thin film sensors to investigate their responsiveness to benzene vapor. The influence of the molecular weights or compositional ratios of PEG and PS blocks on the conductivity of the conducting polymer nanohybrids and responsibility towards benzene vapor was investigated. The conducting polymer nanohybrid film sensors were demonstrated to possess the characteristics of quick response and good restorability, repeatability, and stability.
Due to the large specific surface area and continuous pores in structures, metal–organic frameworks (MOFs) show great advantages in the adsorption of volatile organic compounds (VOCs). Photonic crystal (PC) sensors derived from MOFs are promising for the visual detection of VOC gases. However, they still have problems of low sensitivity and poor color saturation and tunability. Here, inspired by vapor-sensitive scales of Tmesisternus isabellae beetle and scattering light absorption of polydopamine, a porous one-dimensional PC sensor is constructed by combining ZIF-8 with TiO2@PDA nanoparticles. The PC sensor shows significant color changes under different concentrations of benzene vapor and reaches a detection limit of 0.8 g/m3. It has a response time of less than 1 s and maintains stable optical performance after 100 times of reuse. Moreover, ZIF-67 and ZIF-7 are both incorporated into the PCs for comparison; it reveals that ZIF-8 shows superior benzene detecting property. Additionally, the synergistic adsorption of VOCs in inner and outer holes of the ZIF-8 layer is demonstrated by real-time mass monitoring with quartz crystal microbalance with dissipation. This study provides a valuable reference for the fabrication of high-quality MOF-based PC sensors and sensing mechanism study between microscopic molecular adsorption and macroscopic performance.
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