Thermal activation significantly improves the organic pollutant removal rate of low-grade manganese ore in a peroxymonosulfate system

Low-cost, eco-friendly and effective catalysts are essential for activating peroxymonosulfate (PMS) to purify water. Hence, we investigated using thermal activation natural low-grade manganese ore (CNMO) as an effective catalyst to activate PMS for the removal of Acid Orange 7 (AO7), a harmful azo dye. CNMO exhibited a more effective activation ability than either the pure component substances alone or natural manganese ore (NMO), owing to its increased charge transfer, pore size and acidic sites. The activation mechanism of PMS was elucidated, and the degradation of AO7 was noted to have been caused by singlet oxygen (1O2), and increased electron transfer. Moreover, the outstanding degradation of AO7 in actual water indicated that the CNMO/PMS system was highly resistant to surrounding organic and inorganic compounds, and the CNMO exhibited extraordinarily high stability and recyclability. Thus, this study provides not only a new choice of PMS activator that offers low cost, and excellent and stable performance, but also a novel direction for the efficient utilization of low-grade manganese ore.


Text 1. Effects of operational parameters 1. Effect of initial concentration
As shown in Figure S9(a), when the AO7 initial concentrations (C 0 ) were 20 mg/L and 40 mg/L, the AO7 removal efficiencies in the CNMO/PMS system were similar, reaching 99.1%-99.2%, with the fastest reaction rate (k obs ), 0.1056 min -1 , obtained when C 0 was 20 mg/L (Figure S5(a)).
However, with the increase of C 0 , the AO7 removal efficiency and k obs in the CNMO/PMS system decreased significantly. This could be mainly attributed to the fact that, as the initial concentration of AO7 increased, the ROS provided by a certain amount of PMS became insufficient, thus reducing degradation efficiency 1 .

Effects of catalyst dosage
More active sites can decompose more PMS into ROS in a given time, which is conducive to promoting the degradation of pollutants. In this study, when the catalyst dosage (M CNMO ) increased from 0.5 g/L to 1.5 g/L, the k obs and removal efficiency increased obviously (Figure S5(b) and Figure S9(b)), reaching 2.09 times and 1.31 times of 0.5 g/L, respectively. The rate of ROS generation accelerated with the increased CNMO, possibly suggesting the reason for the remarkable degradation effect on AO7 2 . However, AO7 removal efficiency did not change significantly when M CNMO was greater than 1.5 g/L, possibly limited by the PMS dosage (M PMS ) of the CNMO/PMS system.

Effect of PMS dosage
The effect of M PMS (0.08-0.46 mM) was subsequently investigated to ascertain the most suitable amount of oxidant for AO7 degradation in the CNMO/PMS system. The AO7 removal efficiency and k obs increased with the increase in M PMS (Figure S9(c)). When the M PMS was 0.46 mM, AO7 removal efficiency reached 98.11%, with k obs at 0.1134 min -1 , indicating that PMS dosage was the limiting factor in the removal of AO7. Previous studies have reported that more PMS can enhance mass transfer between CNMO and PMS, resulting in a greater concentration of ROS for the removal of AO7 3, 4 .

Effect of initial pH
Generally, the influence of different initial pH levels on the catalytic system cannot be ignored. Figure S7(d), the CNMO/PMS system maintained a high AO7 removal rate (> 97.50%) under both strong acid (pH = 2) and strong base (pH = 9) conditions. Even at pH = 11, the removal rate of the CNMO/PMS system was still able to reach 88.67%. The results indicate that CNMO has excellent PMS activation performance within a wide initial pH range (pH = 2-11), thereby deeming it a good prospect for practical application.

Effect of reaction temperature
The effect of reaction temperature on AO7 degradation in the CNMO/PMS system is shown in Figure S7(e) and Figure S5(e). As the temperature increased from 15 °C to 45 ℃, the removal rate and k obs increased, with the k obs significantly enhanced from 0.0665 min -1 to 0.1431 min -1 , thus demonstrating that reaction temperature exerts a notable influence on AO7 degradation in the CNMO/PMS system. In addition, as shown in the plot of ln(k obs ) versus 1000/T (Figure S7(f) when the E a of AO7 in the system was calculated using the Arrhenius equation (Eq. (2)) it was found to be 20.62 kJ mol -1 , which is lower than previously reported values 1, 2 and demonstrated the ease of activation of PMS by CNMO.

Text 2. Measurement of oxidative intermediate species
In order to further understand the AO7 degradation pathway, the intermediates in the AO7 degradation process were analyzed by GCMS-QP2020NX, with eight intermediates detected (Table   S4). First, the 1 O 2 generated in the CNMO/PMS system attacked the -N=N-of AO7 and broke the bond, thereby forming 1-amino-2-naphthol and 4-aminobenzenesulfonic acid. Thereafter, the 1-amino-2-naphthol was oxidized to form P1/P2/P3 (Pathway I), while 1-amino-2-naphthol was oxidized to P4-P6 and, finally, to P7 and P8 (Pathway II). Finally, these intermediates of AO7 degradation could further react with ROS and convert into CO 2 and H 2 O ( Figure S10).