Advanced oxidation processes that use sulfate radicals are an emerging technology for treating wastewater.36,37 In this method, PS, SO52- or S2O82- or PMS, HSO5– can be activated to form sulfate radicals (SO4·-) by various means, such as UV radiation, sonication, heat, alkaline pH, or transition metal ions.38–40 Oxone, which is the triple salt KHSO5·0.5 KHSO4·0.5 K2SO4, is a more stable.41 The sulfate radical-based AOPs are more effective and potent than ·OH-based AOPs; this is derived from more stability of SO4·-, compared to ·OH, during degradation reactions so that it has exhibited an outstanding oxidation ability in an extensive range of pH (2-8).42,43

·OH exhibits a redox potential range of 1.89-2.8 V, which classifies it as a highly effective oxidant. SO4·- is produced from PS, which has a standard redox potential of 2.01 V, exceeding that of PMS (1.81 V).44,45

When compared, it can be observed that SO4·- possesses a redox potential that is either similar or greater (ranging from 1.81-3.1 V) than that of other substances, depending on its method of activation.46,47 SO4·- is produced from PS, which has a standard redox potential of 2.01 V, exceeding that of PMS (1.81 V).48 The removal efficiency of Acid Orange 7 dye by heat activation follows the order of PS > PMS > H2O2. However, it is PS > H2O2 > PMS after employing UV for activation.48 Achieving a removal efficiency of 95.3% and 58.4% for 25 mg/L furfural was possible under sonication activation using PS and PMS, respectively.49 The order of SMX removal efficiency by UV activation was found to be different from that of other factors, with PMS showing the highest efficiency followed by PS and H2O2.42 This suggests that the contribution of the entire oxidation system on changing oxidation potential of PS and PMS.50

Sonication is an effective method for eliminating resistant pollutants. It also triggers the production of SO4·- through the reactions of PS and PMS with ·OH, which are generated in situ during sonication. These reactions can be represented by Eqs (1) to (6).51–53

Thus, the degradation of antibiotics has been ascribed to producing SO4·- and ·OH resulting from the PS or PMS activation during sonication.54

Antibiotics PS (mM) Frequency (kHz) % Removal PS % Removal sono % Removal sono/PS References SMX 2 20 4.8 6.7 91.3 21 TC 200 20 20.1 4.5 55.5 22 TC 5 35 57.3 26.9 88.5 23 SFZ 4 20 4.9 37.3 62.4 24 TC 4.4 40 7.5 2.5 18.5 25 CMH 4.8 20 8.8 9.7 45.2 26 SMX 1 40 11.1 1.6 47.2 27 TC 2 20 54.3 8.6 99.6 28 Table 1.
Summary of sonication systems for degrading antibiotics. Antibiotics pH C0 (mg/L) Time (min) Reaction rate % Removal References AMX 2 10 60 0.221 95.4 7 LFX 4 25 60 0.172 76.3 8 AMX 3 25 75 0.144 77.9 9 TC 3 25 60 0.085 79.3 13 SMX 5 50 60 0.074 84.3 16 TC 3 20 75 0.096 91.2 17 TC 2 25 75 0.141 62.4 18 CIP 3 50 45 0.116 99.6 19 Table 2.
Results of various reported studies on the effect of different parameters.

Figure 1:
The SDZ degradation mechanism in sono/Fe0/PS system.

Figure 2:
Suggested TC degradation pathway in a sono/S2O82- system.

After that, chemical bonds of antibiotics, e.g., S-N, S-C, and N-C in SMZ, or N-methyl, hydroxyl, and amino groups in TC, are cleaved through oxidation with ·OH and SO4·-.30

The TC sonocatalytic degradation in the Fe3O4/PS system has been observed to exhibit an increase in the removal efficiency of TC with a rise in PS concentration (20 to 200 mM). Nevertheless, the removal efficiency of TC begins to decrease when the PS concentration exceeds 200 mM. This is attributed to excessive production of sulfate anions by PS instead of active SO4·-, hence diminishing the effectiveness of the process.52 Speculations suggest that the formation of SO4·- could be potentially mitigated by excess PS that may act as a scavenger, thereby impeding the generation of ·OH. It is noteworthy that the supply of 200 mM PS is deemed sufficient to facilitate generation.53 The solution pH level has a significant impact on several factors, including the dissociation of antibiotics, the adsorption of antibiotics onto catalysts, and the leaching of metals and their oxides. These factors ultimately affect the antibiotic sonocatalytic degradation in the presence of PS or PMS.35

Achieving efficient degradation of SDZ (95.7%-98.4%) in the sono/PS system is possible at pH values from 3.0 to 7.0. However, at pH=10, the system’s efficiency was significantly diminished by 35.7%. In lower pH ranges, Fe0 is more prone to corrosion and results in the formation of soluble Fe2+. Conversely, in alkaline conditions, the soluble iron ions precipitate and passivate the Fe0 surface, leading to low production of oOH and SO4·-.36 Moreover, for neutral or alkaline pH, the generated SO4·- not only undergoes a reaction with H2O and OH– but also causes a reduction in oOH reactivity.36 In addition, during the antibiotic degradation at pH ranges of 3.0-7.0, a gradual diminution in the pH of the solution could be detectable, which is attributed to generating carboxyl acid products and the decomposing PS. At the end of a degradation process, a decrease in pH value from 10 to 6.5.29

The research of Pan et al. has revealed that the sono/ premagnetized-Fe0/PS system experiences a more rapid decrease in pH as reaction time increases when compared to other systems. This swift decrease in pH facilitates the faster generation of Fe2+ and consequently, the more generation of SO4·-, which results in a highly effective degradation of SMZ. Additionally, it also leads to remarkable synergistic effects in the mentioned system, further aiding in the degradation of SMZ.55

The efficiency of the Sono/PS system for TC degradation is strongly influenced by the initial value of pH. The degradation rates of TC (in the absence of any buffer) at pH 4, 7, and 10 were found to be 77.4%, 62.5%, and 88.5%, respectively, after 120 min.35 At different pH levels, TC (pKa of 3.3, 7.7, and 9.7) behaves differently due to its amphoteric nature. At pH = 4, TC molecules are mostly neutral or have positive charges while at pH=9, they have negative charges. TC molecules with a negative charge are highly reactive and can attract species such as oOH, due to the increased electric density on the ring system. This leads to speeding up in TC degradation. At pH>10, alkaline-activated PS is the main source of SO4·-, O2·- and ·OH. Moreover, the reaction between SO4·- and OH– under alkaline conditions is an effective way to generate ·OH.35 Thus, an increase in pH leads to improvement in decomposing PS to produce ·OH and SO4·-, as described in Equations no. 7-9.55–57

The effectiveness of Sono/S2O82- or Sono/Oxone processes in degrading antibiotics was improved significantly due to the rise in temperature, which increased the cavitational activity and chemical reactions. As depicted in equation no.10, SO4·- can be produced by activating PS through heating.58

At pH of 4, SO4·- is the primary agent that leads to the degradation of TC, whereas at pH of 7, the degradation of TC is caused by both SO4·- and ·OH. Consequently, the degradation rate of TC at a pH of 7 is reduced, which is ascribed to the competition between SO4·- and ·OH and TC.31