**1. Introduction**

The economic prosperity brought by industrialization is associated with the dramatic degradation of the environment (i.e., water and air pollution, loss of natural resources, climate change, etc.). The uncontrolled release of numerous hazardous contaminants, such as dyes, chemicals, heavy metals, organic solvents, petroleum products, and solid wastes, is strongly contaminating the environment.

Advanced oxidation processes (AOPs) have attracted considerable interest due to their significant potential for environmental remediation [1]. Among them, heterogeneous photocatalysis employing semiconductor materials and various light sources is a promising route for the removal of persistent pollutants to produce harmless end products. During the photocatalytic process in the presence of the light of suitable energy (with higher energy than the respective band gap of the material), an electron (e−) is excited from the valence band (VB) of a semiconductor to the conduction band (CB), generating a positive hole (h+) in the valence band. Photogeneration of charge carriers (e−/h+) initiates the photocatalytic degradation process. The valence band hole oxidize surface absorbed water molecules or OH<sup>−</sup> to produce hydroxyl radicals (•OH). The photoexcited electrons reduce oxygen molecules and produce hydroperoxyl radicals (HO2•) or superoxide radicals (•O2 −). During the photocatalytic process (Equations (1)–(4)), these reactive oxygen species

**Citation:** Pavel, M.; Anastasescu, C.; State, R.-N.; Vasile, A.; Papa, F.; Balint, I. Photocatalytic Degradation of Organic and Inorganic Pollutants to Harmless End Products: Assessment of Practical Application Potential for Water and Air Cleaning. *Catalysts* **2023**, *13*, 380. https:// doi.org/10.3390/catal13020380

Academic Editor: Ioannis Konstantinou

Received: 5 January 2023 Revised: 1 February 2023 Accepted: 7 February 2023 Published: 9 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

(ROS) and free electrons/holes react with the surface adsorbed molecules (e.g., organic, inorganic compounds) and convert the pollutants to unharmful products.

$$\text{Semiconductor} + \text{hv} \rightarrow \text{h}^+ + \text{e}^- \tag{1}$$

$$\text{e}^- + \text{h}^+ \rightarrow \text{energy} \tag{2}$$

$$\text{H}^+ + \text{H}\_2\text{O} \rightarrow \bullet \text{OH} + \text{H}^+ \tag{3}$$

$$\text{e}^- + \text{O}\_2 \rightarrow \text{\textbullet O}\_2^- \tag{4}$$

The efficiency of a photocatalytic reaction is mediated by the capability of the photocatalyst to generate longer-lived electrons and holes, leading to the formation of reactive free radicals. It also depends on the type of photocatalytic material and the operational parameters such as solution pH, irradiation time, and the presence of holes scavengers (sacrificial electron donors). Another factor affecting photocatalysis is the state of the material, e.g., powder form or its immobilization on support. Organic pollutants may contain a great diversity of elements, such as C, N, O, S, Cl, etc. During the successive degradation steps, each carbon atom requires at least four photo electrons (e−) and four photo holes (h+) in order to be mineralized into CO2. Consequently, it is clear that the high number of electrons and holes required for the mineralization of large organic molecules impede the efficiency of the depollution process. During the successive degradation steps, the photo charges break the large molecule down into smaller units via intricate redox processes. Consequently, photocatalytic technologies are easier for pollutants with one carbon atom in their molecule, such as formic acid, methanol, or formaldehyde. For larger organic molecules, the photocatalytic technologies should be associated with other methods, such as treatment with strong oxidants (e.g., ozone, persulfate ion, hydrogen peroxide) to raise the mineralization extent of the pollutant [2–5]. In depollution technologies, the fundamental problem is the extent to which the pollutant is photomineralized. Most papers analyze only the degradation of the target compounds into intermediates, which may be as harmful as the pollutant itself, without assessing the amount of carbon dioxide.

This work surveys the basic mechanisms involved in photocatalytic mineralization of the most common gaseous/liquid pollutants and a large area of efficient engineered materials used for their removal under light exposure. The novelty comes from the discrimination between the photocatalytic mechanisms/materials inducing the unknown or scarcely quantifiable intermediates and those leading to harmless end products such as CO2, Cl−, or N2. The manuscript is divided into four main subsections (Figure 1). The first subsection describes the photocatalytic removal of alcohols and carboxylic acids in gaseous and liquid phases. Methanol, ethanol, and oxalic acid were described as model molecules. The second subsection is related to the removal of volatile organic compounds (VOCs) from indoor air and wastewater. The class of the chlorinated VOCs is exemplified by trichloroethylene (TCE), perchloroethylene (PCE), and dichloroacetic anion (DCA) compounds, while the group of the aromatic VOCs is illustrated by benzene, toluene, xylene, and ethylene pollutants. The third subsection presents the photocatalytic mineralization of harmful aromatic compounds from wastewater, focusing on phenol model molecules. Finally, the fourth subsection refers to the photocatalytic removal of aqueous inorganic nitrogen-based compounds from wastewater concentrating on nitrate (NO3 −) reduction. All these target pollutants were chosen due to their widespread usage, toxicity, and environmental pollution.

Challenges and future directions of photocatalytic environmental remediation are discussed. The potential of novel emerging photocatalytic technologies transferable to industrial applications is also analyzed.

**Figure 1.** Schematic illustration of the manuscript structure and its objectives.

### **2. Photocatalytic Removal of Organic and Inorganic Pollutants**

Environmental pollution is a pervasive problem with consequences for human health, living organisms, and ecosystems. To solve this issue, the total transformation of the parent harmful compounds to eliminate their toxicity and persistence is necessary. In the following, recent advancements reported for the removal of recalcitrant organic and inorganic molecules are presented, taking a look at their reaction degradation mechanisms.
