Innovative hydrophobic/hydrophilic perfluoropolyether(PFPE)/polyvinylidene fluoride(PVDF)composite membrane for vacuum membrane distillation

Though membrane distillation(MD)has gained more and more attention in the field of desalination,the wetting phenomenon was still a non-negligible problem.In this work,a method combined dip-coating and UV in situ polymerization for preparing hydrophobic/hydrophilic perfluoropolyether(PFPE)/polyvinylidene fluoride composite membranes.This composite membrane consisted of a top thin hydrophobic coating layer and hydrophilic substrate membrane.In terms of anti-wetting properties,contact angle and liquid entry pressure of all composite membranes(except for those based on 0.45μm)exceeded 160°and 0.3 MPa,respectively.In particular,the desalination performance was tested in vacuum membrane distillation tests by feeding 3.5%(mass)saline solution(NaCl)at 60℃.The composite membranes with larger support pore size and lower PFPE content had higher membrane distillation flux.And for stability tests(testing the 0.22μm membrane coated by 5%(mass)PFPE),the highest MD flux29.08 kg·m^(-2)·h^(-1) and stable salt rejection(over 99.99%)during the period.Except that,the effects of coating material concentration and pore sizes of substrate membrane were also investigated for surface morphology and topography,porosity,mechanical strength and pore size characteristics.This work provided a simple and effective alternative to prepare excellent hydrophobic composite membranes for MD applications.

Hysteresis in sap flow and its controlling mechanisms for a deciduous broad-leaved tree species in a humid karst region

The critical zone(CZ) represents the intersection of the biosphere with the atmosphere, hydrosphere and lithosphere.Understanding the hydrological processes and human impact factors on the CZ is fundamental to sustainable water resources management for agroforestry. Transpiration(T) is an important component of terrestrial evapotranspiration(ET), and understanding the time lag(TL) between vegetation transpiration and meteorological factors can improve our knowledge of the mechanisms of vegetation adaptability to a changing environment. However, the controlling factors on the TL remain poorly understood. Therefore, the objective of this study is identifying the temporal dynamics of key controlling factors on the TL, using a typical deciduous broad-leaved tree species(Zenia insigins Chun) of CZ in subtropical humid karst regions. This species is used as an example to explore the characteristics of the TL between SF(sap flow) and hydro-meteorological forcing. Sap flow in these 6 trees was monitored using the thermal dissipation probes(TDP). Results showed that:(1) the peak of diurnal sap flow generally lagged behind PAR but preceded Ta(air temperature), RH(relative humidity) and VPD(vapor pressure deficit), with the mean TL of-67.4 min(PAR), 90.5 min(Ta), 91.6 min(RH) and 92.9 min(VPD), respectively;(2) TL had no significant relationships with the daily mean meteorological factors and soil moisture, but was highly(R^2>0.66) correlated to CRs(changing rates of meteorological factors) in the morning;(3) At seasonal scale, the sap flow rate and TL both were controlled by the seasonality of precipitation and temperature. Overall, the seasonality of the TL was caused by plants' high water loss and strongly active physiological response in hot seasons, leading to close stomata earlier than in cold seasons;(4) The reason why CRs proposed can explain the TL better than mean values of metrological factors is that the CRs considered the distribution and change processes of metrological factors in the daytime. This study may be helpful for understanding the physiological response of vegetation to climatic change, and may be useful for constructing models to simulate transpiration processes more accurately during a day.

Fingerprinting sediment sources in a typical karst catchment of southwest China

Due to the complex hydrogeological conditions in karst regions,it is difficult to measure sediment source contributions at a catchment scale directly.The objective of this study was to quantify the relative contributions of sediment sources and their temporal variability in a karst catchment in southwest China.Karst depressions can trap eroded sediment similar to a dam or reservoir and,thus,are representative and typical test beds for identifying sediment sources in karst regions.Three sediment cores were taken from a karst depression,58 soil samples from three potential sediment sources were collected,and 18 soil properties were analyzed.The relative contributions of cropland,forestland,and fissure or crack soils were calculated using a multivariate mixing model.The fingerprinting results demonstrated that the cropland was the main sediment source in karst catchment.Specifically,according to the mean sediment contributions of the three deposited sediment cores,the sediment contribution from cropland topsoil was 69.3%,varying from 46.9%to 92.3%,and forestland soil and crack soil accounted for 8.6%and 22.1%of the sediment yield,and varying from 2.8%to 16.5%and 4.8%-36.6%,respectively.This result indicated that great attention should be paid when using only a single core to quantify sediment provenance.Due to the deposited sediment was generally disturbed as cropland in the karst depression,the area that has not been disturbed in recent decades in depression was more appropriate to trace sediment sources in karst catchment.To the best of our knowledge,this study was the first to quantify the contributions of sediment sources in the karst catchment of southwest China.This study provides valuable information and a preliminary reference for applying a composite fingerprinting technique to quantify sediment sources in karst catchments.