Water pollution is one of the most important problems in the field of environmental protection in the whole world, and organic pollution is a critical one for wastewater pollution problems. How to solve the problem effectively has triggered a common concern in the area of environmental protection nowadays. Around this problem, scientists have carried out a lot of research; due to the advantages of high efficiency, a lack of secondary pollution, and low cost, photocatalytic technology has attracted more and more attention. In the past, MnO₂ was seldom used in the field of water pollution treatment due to its easy agglomeration and low catalytic activity at low temperatures. With the development of carbon materials, it was found that the composite of carbon materials and MnO₂ could overcome the above defects, and the composite had good photocatalytic performance, and the research on the photocatalytic performance of MnO₂-carbon materials has gradually become a research hotspot in recent years. This review covers recent progress on MnO₂-carbon materials for photocatalytic water treatment. We focus on the preparation methods of MnO₂ and different kinds of carbon material composites and the application of composite materials in the removal of phenolic compounds, antibiotics, organic dyes, and heavy metal ions in water. Finally, we present our perspective on the challenges and future research directions of MnO₂-carbon materials in the field of environmental applications.

Nitrogen pollution in the oligotrophic Chaihe Reservoir leads to reconsideration of whether nitrogen control is necessary. Does this mean that eutrophication control has the potential to create “cleaner lakes that are dirtier lakes?” The Chaihe Reservoir is a typical case of a water body with high nitrogen and low algal populations. The relationship between the chlorophyll a (Chla) and nutrients (total nitrogen [TN] and total phosphorus [TP]) in the reservoir was analyzed using the filtering trajectory method (FTM). The results showed that the Chaihe Reservoir recovered to an oligotrophic state after the TP concentration greatly decreased, but it faces a new dilemma of nitrogen pollution. This provides further empirical support for the role of phosphorus in controlling the nitrogen cycle and fate in freshwater ecosystems. The contour plot and Chla-TN/TP diagram indicated that a higher TN/TP contributed to a reduction in Chla. The thresholds for TP and TN in the reservoir were proposed using the double probability method (DPM). It is recommended to remove nitrogen at the end of a drinking water supply system due to the nitrogen pollution of the Chaihe Reservoir. The TN threshold for buffering control provides a quantitative basis for implementing a nutrient management plan.