Persistent organic pollutants in global surface soils:Distributions and fractionations

The distribution and fractionation of persistent organic pollutants(POPs)in different matrices refer to how these pollutants are dispersed and separated within various environmental compartments.This is a significant study area as it helps us understand the transport efficiencies and long-range transport potentials of POPs to enter remote areas,particularly polar regions.This study provides a comprehensive review of the progress in understanding the distribution and fractionation of POPs.We focus on the contributions of four intermedia processes(dry and wet depositions for gaseous and particulate POPs)and determine their transfer between air and soil.These processes are controlled by their partitioning between gaseous and particulate phases in the atmosphere.The distribution patterns and fractionations can be categorized into primary and secondary types.Equations are developed to quantificationally study the primary and secondary distributions and fractionations of POPs.The analysis results suggest that the transfer of low molecular weight(LMW)POPs from air to soil is mainly through gas diffusion and particle deposition,whereas high molecular weight(HMW)POPs are mainly via particle deposition.HMW-POPs tend to be trapped near the source,whereas LMW-POPs are more prone to undergo long-range atmospheric transport.This crucial distinction elucidates the primary reason behind their temperatureindependent primary fractionation.However,the secondary distribution and fractionation can only be observed along a temperature gradient,such as latitudinal or altitudinal transects.An animation is produced by a one-dimensional transport model to simulate conceptively the transport of CB-28 and CB-180,revealing the similarities and differences between the primary and secondary distributions and fractionations.We suggest that the decreasing temperature trend along latitudes is not the major reason for POPs to be fractionated into the polar ecosystems,but drives the longer-term accumulation of POPs in cold climates or polar cold trapping.

Noether Theorem for Fractional Singular Systems

Noether theorems for two fractional singular systems are discussed.One system involves mixed integer and Caputo fractional derivatives,and the other involves only Caputo fractional derivatives.Firstly,the fractional primary constraints and the fractional constrained Hamilton equations are given.Then,the fractional Noether theorems of the two fractional singular systems are established,including the fractional Noether identities,the fractional Noether quasi-identities and the fractional conserved quantities.Finally,the results obtained are illustrated by two examples.

Toward an understanding of dwell fatigue damage mechanism of bimodal Ti-6Al-4V alloys

Dwell fatigue effect is a long-standing problem threatening the long-term service reliability for fan blades and fan disks of an aircraft engine.To understand the basic mechanism of dwell fatigue damage,pure fatigue and 60 s dwell fatigue properties of bimodal Ti-6Al-4V alloys with different volume fractions of the primaryα(α_(p))phase were examined comparatively.The results showed that both pure fatigue and dwell fatigue life decreased with increasing the volume fraction of theα_(p)phase and the dwell fatigue life was lower than the pure fatigue one.The quasi-in-situ test results and the quantitative characterization of damage behaviors of the local microstructure units defined by theα_(p)-secondaryα(α_(s))combination reveal that theα_(s)phase close to theα_(p)phase with extensively slip activities was gradually damaged under dwell fatigue loading,while that under pure fatigue loading was undamaged,demonstrating that the dwell loading induced the damage of theα_(s)phase,and further reduced the fatigue life.A stress relaxation-based model is proposed to describe the physical mechanism on dwell fatigue damage of the bimodal Ti-6Al-4V alloy,i.e.the elastic deformation of theα_(s)phase caused by the strain incompatibility would be gradually transformed into plastic deformation during the dwell stage,and thus promotes fatigue damage.The model provides new insights into the microscopic process of stress/strain transfer between the soft and hard microstructure units under dwell fatigue loading.