Rhodamine B (RhB) as a widely used organic dye often appears in industrial wastewater, posing a serious threat to human health. In response to this problem, an advanced persulfate oxidation technology can effectively remove organic pollutants by virtue of its highly activated free radicals, having a great potential for application in the field of wastewater treatment due to its significant advantages of easy operation and wide pH range. In the Fenton system, transition group metals show superior performance, especially the cobalt(II)/potassium peroxymonosulfate (PMS) system, whose activation performance is particularly outstanding, and can efficiently degrade harmful substances in wastewater. However, the direct utilization of substances derived from Prussian blue analogs results in severe leaching of metal ions. Therefore, in this study, we prepared Fe₃O₄/CoS/g-C₃N₄ composite catalysts by a hydrothermal method for the degradation of rhodamine B by photo-Fenton reaction. The effects of their structure, morphology and catalytic conditions on the performance were investigated. In addition, the preparation conditions and reaction parameters were also optimized.

An appropriate amount of Fe₃O₄/CoS/g-C₃N₄ catalyst (with a mass concentration of 35-65 mg/L) was added into 100 mL RhB solutions at different mass concentrations (i.e., 10-30 mg/L). Under dark conditions, the mixed suspension was stirred in a magnetic stirrer for 30 min to achieve the equilibrium of adsorption and desorption. After equilibrium, an xenon lamp was turned on and a specified amount of PMS (with a mass concentration of 60-90 mg/L) was added. During the reaction, the sample was taken once per minute. The supernatant was obtained by centrifugation. The absorbance of RhB was determined by an ultraviolet-visible spectrophotometer. After the reaction was completed, the catalyst collected using a magnet was rinsed with deionized water for several times, and dried for cycle experiments.

In the PMS+Vis system, the degradation efficiencies of RhB by CoS and Fe₃O₄/CoS within 2 minutes are 87.7% and 88.6%, respectively. In the PMS+Vis system, the degradation efficiencies of RhB by Fe₃O₄/CoS/g-C₃N₄ within 1 min and 2 min are 90.3% and 96.7%, respectively, which are higher than those of RhB in the single PMS system (i.e., 83.4% and 94.7%). Since the degradation efficiencies of Fe₃O₄/CoS/g-C₃N₄ and Fe₃O₄/CoS/g-C₃N₄-2 for RhB are similar, and the content of transition metal elements in Fe₃O₄/CoS/g-C₃N₄-2 is relatively small, 100 mg g-C₃N₄ can be used as an optimal addition amount to avoid the secondary pollution to the environment caused by transition metal ion clusters. Under different catalytic conditions, the degradation efficiency of RhB increases with the increase of PMS as the addition amount of PMS increases from 60 mg/L to 75 mg/L. The degradation efficiency of RhB remains unchanged when the addition amount of PMS increases from 75 mg/L to 90 mg/L. This is because Fe₃O₄/CoS/g-C₃N₄-2 catalyst is completely consumed at the addition amount of PMS of 75 mg/L. The excessive PMS has no more catalyst to activate it when the addition amount of PMS is 90 mg/L. From the perspectives of cost saving and environmental protection, a subsequent selection of 75 mg/L is the optimum addition amount of PMS. The degradation efficiency of RhB firstly increases and then remains unchanged as the addition amount of the catalyst increases. The catalytic degradation efficiency of RhB does not increase when the addition amount of the catalyst exceeds 50 mg/L. This is mainly because RhB all is degraded at the addition amount of the catalyst of 50 mg/L. There is no more RhB for degradation after further increasing the amount of the catalyst. Therefore, 50 mg/L is the optimal addition amount of the catalyst. The results clearly show that the degradation effect of RhB is dominant in the pH value range from 5 to 9, showing a good stability. However, the degradation efficiency of RhB decreases significantly when the pH value decreases to 3. This phenomenon is mainly attributed to the increase in the concentration of H⁺ in the reaction system in an acidic environment. At a high H⁺ concentration, the key active species in the reaction process, such as SO₄^•- and OH⁻, may be consumed, resulting in a decrease in the number of active species. The reduction of active species affects negatively the efficiency of the degradation process, thereby leading to a decrease in the degradation efficiency of RhB. The degradation efficiency of RhB also decreases at the pH value of the solution of 11. This is because under strongly alkaline conditions, OH⁻ reacts with the active species SO₄^•- in the degradation process to generate ·OH, and the oxidation capacity of ·OH is much lower than that of SO₄^•−, thus causing the decreased degradation efficiency of RhB.

The results indicated that this catalyst outperformed pure CoS in terms of both stability and degradation efficiency. A significant advantage of introducing Fe₃O₄ was a capability for rapid recovery via magnet and reuse, thus greatly enhancing the efficiency and economic viability of the catalyst. Moreover, CoS derived from Co-Co PBA had an adaptability in a broad pH value range, thereby improving the catalyst's stability and applicability under diverse environmental conditions. In summary, Fe₃O₄/CoS/g-C₃N₄ catalyst had an outstanding performance in degradation efficiency, environmental friendliness, cycle stability, and broad applicability, specially for the treatment of RhB.