The single-atom catalyst (SAC) activated persulfate process has emerged as a highly efficient technology for eliminating refractory organic compounds in aqueous environments. This review delves into the intricacies of utilizing SACs for the effective removal of various contaminants in water. The common supports and the preparation procedures of SACs are summarized at first. The synthesis methods of SACs (i.e., wet chemical method, one-pot hydrothermal method, and high-temperature pyrolysis method) are also described. Then, a comprehensive overview of the diverse reaction mechanisms in SAC-activated persulfate systems is presented, including a radical oxidation process via sulfate or hydroxyl radicals and superoxide radicals, or a nonradical process via single oxygen, surface active complex, and high-valent metal-oxo species oxidation. The impact of key factors such as peroxides concentration, SAC dosage, reaction pH, inorganic anions, organic matter, operando stability, and real water is also delved. The removal of various pollutants (i.e., azo dyes, phenolic compounds, pharmaceuticals, and bacteria) by this process is further summarized. Finally, the challenges and perspectives in the field of water treatment utilizing SACs are discussed.

The development of appropriate and effective periodate (PI) activation technology is currently a popular research area. This study presents a novel efficient photocatalytic activation approach of PI for pollutant degradation based on carbon nitride (g-C₃N₄) and visible light (Vis). The results show that the system can remove 92.3% of acid orange 7 (AO7) within 60 min under the g-C₃N₄/PI/Vis reaction system. The degradation rate constant (k_obs) reached 4.08 × 10⁻² min⁻¹, which is 4.21, 5.16 times, and 51.3 times higher than that of the g-C₃N₄/Vis system (9.7 × 10⁻³ min⁻¹), PI/Vis system (7.9 × 10⁻³ min⁻¹) and the g-C₃N₄/PI system (7.96 × 10⁻⁴ min⁻¹), respectively. Clearly, the addition of PI significantly enhances the degradation efficiency of AO7 in the system. Additionally, under the same reaction conditions, the presence of PI showed excellent oxidation capacity in the photoactivation process compared with other common oxidants, such as peroxymonosulfate, peroxydisulfate, and H₂O₂. Moreover, the g-C₃N₄/PI/Vis system showed excellent removal of AO7 across a wide range of pH levels and in the presence of various anions. Electron paramagnetic resonance (EPR) and quenching experiments suggested that the superoxide anions (^•O₂⁻) and singlet oxygen (¹O₂) dominated in the oxidation of pollutants in the g-C₃N₄/PI/Vis system. In addition, the catalyst showed relative stability during cyclic testing, although a slight reduction in degradation efficiency was observed. In brief, the g-C₃N₄/PI/Vis system is highly efficient and environmentally friendly, with significant application potential in wastewater treatment.

In this work, a method of enhanced packed-bed microbial oxidation–neutralization has been employed to treat Fe²⁺-rich acid mine drainage. The method features the use of a large number of immobile Acidithiobacillus ferrooxidans (A. ferrooxidans) in a bioreactor to promote the oxidation of Fe²⁺ to Fe³⁺. Results show that when the influent Fe²⁺ concentration is about 900 mg/L and the Fe²⁺ oxidation efficiency tends to 100%, the maximum oxidation rate of Fe²⁺ in the bio-ceramsite, bio-volcanic stone, and bio-activated carbon packed columns are 301 mg/(L·h), 234 mg/(L·h), and 139 mg/(L·h), respectively. Compared with the direct neutralization method, the enhanced microbial oxidation–neutralization method has several advantages. Firstly, it oxidizes Fe²⁺ to Fe³⁺, directly neutralizing the acid mine drainage at low pH and reducing the consumption of neutralizer. Secondly, more economical CaCO₃ can be used as neutralizer. Thirdly, it produces precipitates with high solid content (5.50%), good settling performance (SV₃₀ = 4%), and small volume, and the capillary suction time (CST) is 8.9 s, which is easy to dehydrate.

In this study, peroxymonosulfate (PMS) was activated by cheap and readily available natural iron ore to remove Acid Orange 7 (AO7) in water with the assistance of hydroxylamine (HA). Results show that the presence of HA could accelerate the Fe(II)/Fe(III) cycle on the ore surface, promoting the activation of PMS to generate reactive oxidative species. The effects of ore dosage, PMS dosage, HA dosage and initial pH on the degradation of AO7 were investigated in the HA/Ore/PMS system. Under the optimal conditions, the removal of AO7 could reach 93.1% during 30 min, which was 41.4% higher than the ore/PMS system. The AO7 removal increased with the increase of HA, PMS and ore dosage, but was unaffected by the initial solution pH. Based on radical scavenging experiments and EPR tests, the dominant reactive species in the HA/Ore/PMS system were revealed to be the sulfate radical (SO₄^•−), singlet oxygen (¹O₂), superoxide radical (O₂^•−) and hydroxyl radical (^•OH), which were responsible for the AO7 degradation. Furthermore, the possible reaction mechanism of PMS activation was proposed. This study provides an efficient technique for the removal of azo dye organic contaminant in water, which has great practical significance.

Highly efficient removal of phosphate from water bodies is of great importance to controlling eutrophication. Herein, the lanthanum-based perovskites (i.e., LaMnO3, LaFeO3) were synthesized and employed for phosphorus adsorption. Adsorption experiments showed that the phosphate removal by LaMnO3 was well fitted with the pseudo second-order kinetic model, with a multi-stage adsorption process. The LaMnO3 exhibited a maximum adsorption capacity of 51.3 mg/g for phosphate calculated using the Langmuir model, which was 2.1 times higher than that of LaFeO3 under neutral conditions. Characterization and DFT calculation results further revealed that the La element on the surface of perovskite was the main adsorption site via inner-sphere complex and/or electrostatic interactions, whereas the B-site metal (i.e. Mn element) could trigger the surface electronic structure modulation such as the reduction of surface bonding barrier, for further improving the phosphate adsorption. Additionally, the LaMnO3 adsorbent exhibited commendable performance in treating natural water with low phosphorus concentration and good regenerative capability. This work provides insight into the development of novel perovskite-type adsorbents for efficiently removing phosphorus in environmental remediation.

Advanced oxidation process based on heterogeneous activation of peroxymonosulfate (PMS) has received significant attention in wastewater remediation. Herein, a facile and effective electrochemical method was introduced in a tungsten sulfide (WS₂)-activated PMS process for the removal of a typical azo dye Acid Orange 7 (AO7) in aqueous solution. It was found that the electrochemical activation could remarkably promote the removal of organic pollutants by coupling with WS₂/PMS system. The elimination of AO7 in the electro-assisted WS₂-activated PMS (E/WS₂/PMS) system achieved 95.8% of AO7 removal in 30 min, with the optimal conditions of 1.0 g/L WS₂, 1.0 mM PMS, current density of 1.0 mA/cm² and initial pH of 6.5. Based on quenching experiments and EPR techniques, mechanistic studies confirmed that hydroxyl radical (^•OH) and singlet oxygen (¹O₂) are the primary reactive oxygen species for the oxidation of pollutants. In addition, the influences of pH, WS₂ dosage, PMS concentration, current density, common anions and humic acid on the AO7 removal are also investigated in detail. Furthermore, the system exhibited resistance to aqueous matrices, verifying the accepted applicability in real water (i.e., Yangtze River water and Shahu Lake water). In summary, this study demonstrates a green system for the effective removal of contaminants in water, holding significant implications for practical application.