**4. Conclusions**

Herein, an extensive screening of the scientific literature related to the photocatalytic removal of various organic and inorganic hazardous compounds is presented. The review focuses on the photomineralization of a few relevant hazardous compounds into CO2 and other harmless products. Specifically, information is provided on the (i) photooxidation of primary alcohols and carboxylic acids in gaseous and liquid media (e.g., methanol, ethanol, oxalic acid) in gaseous and liquid media, (ii) photocatalytic removal of chlorinated and aromatic VOCs from indoor air and water (e.g., trichloroethylene, perchloroethylene, dichloroacetate anion, benzene, toluene, p-xylene, ethylene), (iii) photomineralization of phenol from wastewater, and (iv) efficient removal of nitrate and its conversion, as far as possible, to compounds that do not affect human life and the environment. The degradation of persistent pollutants is critically analyzed, highlighting the main factors affecting the overall process, such as ROS involvement. Particular attention has been paid to the reaction mechanisms established during the photocatalytic removal of inorganic pollutant NO3 − in connection with the selectivity to harmless nitrogen. The effects of working parameters (e.g., irradiation time, the procedure of operation) on performance are also discussed, along with the intrinsic properties of the applied materials (e.g., surface active sites and structure).

From a large amount of available data, it is clear that further research should be conducted in order to find efficient photocatalysts that are able to mineralize the pollutants into the non-toxic CO2. Only such photocatalysts can be considered for environmental remediation.

Pollutant elimination by only photocatalysis is a highly demanding task and difficult to carry out. Therefore, combining photocatalysis with other techniques is imperative. Photocatalysis as a depollution method has limited efficacy, but from the environmental point of view, it has great significance because the sun is a cheap and endless source. In fact, in nature, all the existent pollutants are exposed to solar irradiation. Thus, photocatalytic studies are highly relevant to the actual environment. Oxide materials (e.g., minerals) are slowly degrading pollutants under solar light, and the fate of intermediates is less known. Thus, the photocatalytic pollutant degradation mechanism in laboratory experiments is very important because it is relevant to the natural depollution processes.

**Author Contributions:** Conceptualization, M.P., C.A., R.-N.S., A.V., F.P. and I.B.; writing—original draft preparation, M.P., C.A., R.-N.S. and A.V.; writing—review and editing, I.B.; supervision, I.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The data that support the plots within this manuscript are available upon reasonable request from the authors.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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**Qiang Ren 1, Juming Liu 2, Qi Yang 1,\* and Wei Shen 3,\***


**Abstract:** Many organic pollutants are discharged into the environment, which results in the frequent detection of organic pollutants in surface water and underground water. Some of the organic pollutants can stay for a long time in the environment due to their recalcitrance. Advanced oxidation processes (AOPs) can effectively treat the recalcitrant organic compounds in water. Photocatalysis as one of the AOPs has attracted a lot of interest. BiOCl and g-C3N4 are nice photocatalysts. However, their catalytic activity should be further improved for industrial utilization. The construction of heterojunction between the two different components is deemed as an efficient strategy for developing a highly efficient photocatalyst. As a typical type-II heterojunction, g-C3N4/BiOCl heterojunctions showed better photocatalytic performance. To date, the g-C3N4/BiOCl composites were mainly studied in the field of water purification. The photoactivity of the pristine catalysts was greatly enhanced by the combination of the two materials. However, three kinds of proposed mechanisms were used to explain the improvement of the g-C3N4/BiOCl heterojunctions. But few researchers tried to explain why there were three different scenarios employed to explain the charge transfer. According to the articles reviewed, no direct evidence could indicate whether the band structures of the heterojunctions based on BiOCl and g-C3N4 were changed. Therefore, many more studies are needed to reveal the truth. Having a clearer understanding of the mechanism is beneficial for researchers to construct more efficient photocatalysts. This article is trying to start a new direction of research to inspire more researchers to prepare highly effective photocatalysts.

**Keywords:** BiOCl; C3N4; photocatalysis; mechanism
