Unexpected high indirect impacts of riverine organic matter to coastal deoxygenation

Unraveling the pyridinic nitrogen vacancy in carbon nitride for photo-self Fenton-like purification of organic contaminants

This work reports a carbon nitride with pyridinic nitrogen-vacancy (N2CV-CN), which purifies organic contaminants via an in-situ photo-self Fenton-like reaction. Experiments and calculations demonstrated that the nitrogen-vacancy induces lone-paired (LP) and symmetry-unpaired electrons, promoting the formation of low-energy LP-π hybridized orbitals and helping to overcome the pairing energy required for oxygen to accept electrons. Furthermore, the nitrogen-vacancy accelerates film and intra-particle diffusion rates of organic contaminants on N2CV-CN, creating beneficial conditions for reactive oxide species to mineralize organic contaminants. Under sunlight and atmospheric oxygen, a photo-self Fenton-like reaction involving proton-coupled electron transfer occurred on the surface of N2CV-CN. Furthermore, by integrating photocatalysis with flocculation, about 99.1 % suspended substance, 45.5 % chemical oxygen demand, and 38.4 % biological oxygen demand were reduced from polluted river-water. Constructing N2CV-CN and understanding its crucial role offer theoretical and methodological insights into the in-situ purification of contaminated water bodies.

Promoting effect of raft-raised scallop culture on the formation of coastal hypoxia

The coastal waters around the Yangma Island are an important mariculture area of raft-raised scallop and bottom-seeded sea cucumber in the North Yellow Sea, China. Large-scale hypoxia in the bottom water of this area has caused the death of a large number of sea cucumbers and heavy economic losses. To find out the formation mechanism of hypoxia, the data obtained in each August during 2015-2018 were analyzed. Compared with the non-hypoxic year (2018), the temperature, trophic index (TRIX) and dissolved organic carbon (DOC) in the bottom water were relatively higher, and the water column was stratified causing by continuous high air temperature and low wind speed meteorological conditions in the hypoxic years (2015-2017). These sites with the coexistence of thermocline and halocline, and the thickness of thermocline >2.5 m and its upper boundary >7.0 m deep, were prone to hypoxia. Spatially, the hypoxic place was highly consistent with the scallop cultivating places, and the DOC, TRIX, NH₄⁺/NO₃^- and apparent oxygen utilization (AOU) at the culture sites were higher, indicating that organic matter and nutrients released by scallops may lead to local oxygen depletion. In addition, the bottom water of the culture sites had higher salinity, but lower turbidity and temperature, indicating that the slowed water exchange caused by scallop culture was a dynamic factor of hypoxia. All sites with AOU >4 mg/L at the bottom had hypoxia occurrence, even if there was no thermocline. In other words, stratification promoted the formation of hypoxia in coastal bottom water, but it was not indispensable. The raft-raised scallop culture could promote the formation of coastal hypoxia, which should arouse the attention for other coastal areas with intensive bivalve production.