Mechanisms of photochemical release of dissolved organic matter and iron from resuspended sediments

Photochemical reactions can alter the transformation of sedimentary organic matter into dissolved organic matter(DOM)and affect its ultimate fate in water ecosystems.In the present study,the photorelease of DOM and Fe from resuspended lake sediments was investigated under different O_(2)and NO_(3)-concentration conditions to study the mechanisms of DOM and Fe photorelease.The amount of photoreleased Fe,which ranged from 0.22 to 0.70μmol/L,was significantly linearly correlated with the amount of photoreleased DOM.O_(2)and NO_(3)-could promote the photochemical release of DOM and Fe,especially during the initial 4 h irradiation.In general,the order of the photorelease rates of DOM and Fe under different conditions was as follows:NO_(3)-/aerobic>aerobic≈NO_(3)-/anaerobic>anaerobic.The photorelease rates of DOM and Fe were higher for the initial 4 hr irradiation than these for the subsequent 8 hr irradiation.The photorelease of DOM and Fe is thought to proceed via direct photodissolution and indirect processes.The relative contributions of indirect processes(>60%)was much greater than that of direct photodissolution(<40%).The photoproduced H 2 O_(2)under aerobic and anaerobic conditions indicated that hydroxyl radicals(·OH)are involved in the photorelease of DOM.Using·OH scavengers,it was found that 38.7%,53.7%,and 77.6%of photoreleased DOM was attributed to·OH under anaerobic,aerobic,and NO_(3)-/aerobic conditions,respectively.Our findings provide insights for understanding the mechanisms and the important role of·OH in the DOM and Fe photorelease from resuspended sediments.

Numerical simulation of phosphorus release from resuspended sediment

One of the main issues in environmental hydraulics is pollutant release from sediments.For instance,the strong affinity between phosphorus and sediment permits most of the phosphorus to be adsorbed on the surface of the sediment particles in rivers or lakes.Post sediment resuspension,phosphorus is desorbed from the sediment to the overlying water.The release of phosphorus from the resuspended sediment is an important process in the secondary pollution of water.Herein,a coupled mechanical model of the overlying water,sediment,and pollutant was established based on the experimentally gathered data.Two types of sediment with different adsorption and desorption characteristics were selected to simulate the process of sediment resuspension and phosphorus release under different hydrodynamic conditions.The simulation results were subsequently used to analyze the relationship between the flow field characteristics and phosphorus concentration,from which the relationships between velocity,particle volume fraction,turbulent kinetic energy,total phosphorus concentration,desorbed phosphorus concentration,and time were elucidated.Based on the results,phosphorus is rapidly released into the overlying water from the resuspended sediment,and it reaches a peak value in a short duration.Unlike the release process of non-adsorption pollutants,hydrodynamic conditions and sediment properties play a crucial role in the phosphorus release process.The turbulent kinetic energy rapidly increases with the flow velocity,whereas the desorbed phosphorus concentration exhibits a certain relationship with the particle volume fraction and turbulent kinetic energy.In particular,the turbulent kinetic energy increases the desorbed phosphorus concentration per unit time.Additionally,the time taken by the total phosphorus concentration to attain its peak value is closely related to the characteristics of the flow field,whereas the amount of phosphorus is closely related to sediment properties.Post sediment resuspension,the release of phosphorus shows the characteristics of a centralized and massive release,which suggests that the total phosphorus concentration in the overlying water would change in a short duration and cause secondary pollution in the water environment.