Photocatalytic degradation of tylosin via ultraviolet-activated persulfate in aqueous solution

Abstract

Some of organic materials such as antibiotics are hazardous contaminants in the aquatic environment because of their adverse effects on aquatic life, environment, and humans. In this work, a batch reactor of ultraviolet (UV) light and peroxydisulfate were studied for the degradation of tylosin as a model antibiotic in water. The effect of different parameters such as UV irradiation, peroxydisulfate concentration, antibiotic concentration, and pH on removal of tylosin was investigated carefully. No significant destruction was observed using separate UV illumination. More than 90% removal occurred when peroxydisulfate was added to the solution in optimum levels of tylosin and peroxydisulfate concentration.

Background

Antibiotics play an important role due to their high consumption rates in both veterinary and human medicine. These materials such as tylosin (Table1) which is used in veterinary medicine to treat bacterial infections in a wide range of species at low concentrations in the environment the development of antibiotic resistant bacteria[1]. In fact, bacteria have been observed to transfer their resistance in laboratory settings as well as in the natural environment[2]. Furthermore, the presence of antibiotics in wastewaters has increased in recent years, and their abatement will be a challenged in the near future.

Table 1 Characteristics of tylosin

Antibiotic wastewater has high chemical oxygen demand (COD) and low biochemical oxygen demand (BOD); hence, biological processes are unsuitable for the wastewater treatment. Advanced oxidation processes (AOPs) have proved to be highly effective in the degradation of most of the pollutants in wastewaters[3]. AOPs are alternative techniques of destruction of many other organics in wastewater and effluents. These processes generally involve UV/H₂O₂, UV/O₃, UV/${\text{S}}_2{\text{O}}_8^{2-}$ or UV/Fenton's reagent for the oxidative degradation of contaminants[4–6]. Recently, there were numerous studies on UV/${\text{S}}_2{\text{O}}_8^{2-}$ application in wastewater treatment mainly because of high reactivity of UV/${\text{S}}_2{\text{O}}_8^{2-}$ process and high solubility of peroxydisulfate[4].

Peroxydisulfate is a strong oxidant (E₀ = 2.05 V) which can be purchased in the form of ammonium, sodium or potassium salt. It has been reported that the reaction rate constants of UV/${\text{S}}_2{\text{O}}_8^{2-}$ and UV/${\text{H}}_2{\text{O}}_2$ (the most common process) is the same[7]. Moreover, peroxydisulfate benefits UV/H₂O₂ and other similar approaches by the following reasons: (1) peroxydisulfate ions seem to be more promising because of the potential quenching effect of using H₂O₂ when the process is not well controlled (such as overdosing)[7]. (2) Since peroxydisulfate is a solid oxidant, it would be more capable for industrial uses in comparison to liquid oxidants such as H₂O₂. (3) Peroxydisulfate salts are much cheaper than other oxidants like hydrogen peroxide and ozone[8, 9].

In this study, degradation of tylosin as an antibiotic pollutant from contaminated water by UV/${\text{S}}_2{\text{O}}_8^{2-}$ process was studied. Moreover, effect of different parameters such as UV irradiation, peroxydisulfate concentration, antibiotic concentration, and pH was investigated.

Experimental

Tylosin was obtained from Razak Co. (Islamic Republic of Iran, P.O.B 13185-1671, Tehran, Iran) and used without further purification. Characteristics of this antibiotic are shown in Table1. The antibiotic is determined in an aqueous medium using a scanning UV–vis spectrophotometer (Shimadzu 160, Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan). Ammonium peroxydisulfate was obtained from Merck & Co., Inc. (Whitehouse Station, NJ, USA). Dilute solutions of sodium hydroxide and hydrochloric acid were used for pH adjustment. A 30-W UV lamp was manufactured by Philips (Koninklijke Philips Electronics N.V., St. Vali-asr Ave., Iran). The volume of sample was 100 ml, and all the temperature (25 ± 4°C). Peroxydisulfate concentration varied from 0.5 to 20 mM, while antibiotic concentration varied from 20 to 80 ppm. The effect of pH was investigated in the range of 2.4 to 9.8.

Results and Discussion

Effect of UV irradiation and peroxydisulfate on degradation of tylosin

Degradation of tylosin was investigated with UV irradiation only,${\text{S}}_2{\text{O}}_8^{2-}$ without UV radiation, and UV radiation plus${\text{S}}_2{\text{O}}_8^{2-}$ in concentrations of 2 mM of peroxydisulfate, the antibiotic concentration of 40 ppm and natural pH (pH = 6.7). In the first case without any peroxydisulfate, approximately, we had maximum degradation of 11%, while using peroxydisulfate without UV irradiation, we had no degradation. UV plus peroxydisulfate outweighs the two mentioned cases clearly by 80% removal (Figure1). This effective degradation is due to the formation of hydroxyl and sulfate radicals because of UV illumination. Reactions of peroxydisulfate were slow at normal temperature. Thus, as summarized in Equations 1 to 5, thermal or photochemical activated decomposition of${\text{S}}_2{\text{O}}_8^{2-}$ ion to${\text{SO}}_4^{·-}$ radical is proposed for acceleration of the process[10]:

$${\text{S}}_2{\text{O}}_8^{2-}+photonsorheat\to 2{\text{SO}}_4^{{·}_{-}}$$

(1)

$$S{O}_4^{·-}+R{H}_2\to S{O}_4^{2-}+H^{+}+R{H}^{·}$$

(2)

$${\text{RH}}^{·}+{\text{S}}_2{\text{O}}_8^{2-}\to \text{R}+{\text{SO}}_4^{2-}+{\text{H}}^{+}+{\text{SO}}_4^{·-}$$

(3)

$${\text{SO}}_4^{·-}+\text{RH}\to {\text{R}}^{·}+{\text{SO}}_4^{2-}+{\text{H}}^{+}$$

(4)

$${2\text{R}}^{·}\to \text{RR(dimer})$$

(5)

Figure 1

Effect of UV radiation and peroxyldisulfate in oxidative decolorization of tylosin. [Tylosin]₀ = 40 ppm, [${\text{S}}_2{\text{O}}_8^{2-}$]₀ = 2 mM.

where R is an organic reagent.

Once${\text{SO}}_4^{·-}$ is formed, it can produce a rapid attack on any oxidizable agent including organic contaminants (e.g., tylosin)[11]. Also, available oxidants in the solution and their corresponding intermediates are indicated in Equations 6 to 12:

$$\begin{array}{l}{\text{SO}}_4^{·-}+{\text{H}}_2\text{O}\to {\text{HSO}}_4^{-}\\ +{\text{OH}}^{·}(\text{k}=500\pm {60\text{s}}^{-1})\end{array}$$

(6)

$${\text{HSO}}_4^{-}\to {\text{H}}^{+}+{\text{SO}}_4^{2-}$$

(7)

$${\text{OH}}^{·}+{\text{S}}_2{\text{O}}_8^{2-}\to {\text{HSO}}_4^{-}+{\text{SO}}_4^{·-}+\frac{1}{2}{\text{O}}_2$$

(8)

$${\text{SO}}_4^{·-}+{\text{OH}}^{·}\to {\text{HSO}}_4^{-}+\frac{1}{2}{\text{O}}_2$$

(9)

$$\begin{array}{l}{2\text{OH}}^{·}\to {\text{H}}_2{\text{O}}_2\\ \left(\text{expect in alkaline solution}\right)\end{array}$$

(10)

$$\begin{array}{l}{\text{H}}_2{\text{O}}_2\to {\text{H}}_2\text{O}+\frac{1}{2}{\text{O}}_2\\ \left(\text{mostly in acidic solution}\right)\end{array}$$

(11)

$${\text{S}}_2{\text{O}}_8^{2-}+{\text{H}}_2{\text{O}}_2\to {2\text{H}}^{+}+{2\text{SO}}_4^{2-}+{\text{O}}_2$$

(12)

Both${\text{SO}}_4^{·-}$ and${\text{OH}}^{·}$ are possibly responsible for the destruction of organic contaminants, and either radical may predominate over the other depending on pH conditions.${\text{SO}}_4^{·-}$ and${\text{OH}}^{·}$ react with organic compounds mainly by three mechanisms: hydrogen abstraction, hydrogen addition, and electron transfer. Sulfate radicals exhibit a higher standard reduction potential than hydroxyl radicals at neutral pH, and both radicals exhibit similar reduction potentials under acidic conditions[8]. In general,${\text{SO}}_4^{·-}$ is more likely to participate in electron transfer reactions than is${\text{OH}}^{·}$ which is more likely to participate in hydrogen abstraction or addition reactions[12].

Effect of initial peroxydisulfate concentration

Initial${\text{S}}_2{\text{O}}_8^{2-}$ concentration has a promising effect on degradation of tylosin. Investigations were made by varying the concentration of${\text{S}}_2{\text{O}}_8^{2-}$ from 0.5 to 20 mM at fixed initial antibiotic concentration of 40 ppm, natural pH, and room temperature of 25°C ± 1°C. Studies revealed that increase in amount of${\text{S}}_2{\text{O}}_8^{2-}$ would enhance degradation of the tylosin respectively[13]. These observations can be explained by the fact that the increase in concentration of peroxydisulfate results in higher generation of hydroxyl and sulfate radicals (Figure2). It is likely because of excessive generation of hydroxyl radicals (Equations 1 and 6) would be recombined to form less reactive H₂O₂ (Equation 13), which is a known quencher of OH^· radical (Equation 14). Therefore, the destruction of tylosin was slightly slowed down at higher${\text{S}}_2{\text{O}}_8^{2-}$ dosages. However, such a recombination effect of the radical was likely not very effective due to the low steady-state concentrations of the radicals; higher decay rates of tylosin at higher${\text{S}}_2{\text{O}}_8^{2-}$ dosages are still expected[5, 13]:

$$\begin{array}{l}{2\text{OH}}^{·}\to {\text{H}}_2{\text{O}}_2\\ \left(\text{Only in acidic to neutral pH}\right)\end{array}$$

(13)

$${\text{OH}}^{·}+{\text{H}}_2{\text{O}}_2\to {\text{H}}_2\text{O}+{\text{HO}}_2^{·}$$

(14)

Figure 2

Effect of initial concentration of peroxydisulfate in oxidative degradation of tylosin. [Tylosin]₀ = 40 ppm.

Effect of initial tylosin concentration

The initial antibiotic concentration has a remarkable effect on photolytic degradation of tylosin. Tylosin concentration varied from 20 to 80 ppm at constant dosage of peroxydisulfate (2 mM). Degradation of 91% is observed in 20 ppm, while only 78% of degradation is achieved in 80 ppm concentration of tylosin (Figure3). As shown in Figure3, the higher concentration of tylosin was, the lower the degradation rate would be. One possible reason may be that the increase in antibiotic concentration decreases the ratio of hydroxyl radical to tylosin and percentage of degradation reduces in result.

Figure 3

Effect of initial concentration of tylosin in oxidative degradation of antibiotic after 30 min. [${\text{S}}_2{\text{O}}_8^{2-}$]₀ = 2 mM.

Effect of the initial pH

The effect of initial pH was investigated in the range of 2.4 to 9.8 in constant concentration of tylosin (40 ppm), peroxydisulfate (2 mM), and temperature (25°C ± 1°C; Figure4). pH 6.7 was found to be the most effective level of pH in decolorization of tylosin. However, effective photodegradation of tylosin was observed at all pH levels, which reveals the efficiency of this method in treatment of wastewater in different regions. The destruction performance increased from low to initial neutral pH (pH = 6.7) levels, but it started to reduce when basic pH was performed. The decreasing photodecay at pH ≥ 7 can be explained by the following reasons: (a) the instability of H₂O₂ at high pH level, (b) relatively higher amounts of${\text{OH}}^{·}$ and${\text{SO}}_4^{·-}$ were generated catalytically in alkaline conditions, which induced recombination of these two radicals (Equation 9), though this could be minor[14].

Figure 4

Effect of pH in oxidative decolorization of tylosin. [Tylosin]₀ = 20 ppm, [${\text{S}}_2{\text{O}}_8^{2-}$]₀ = 2 mM.

Conclusion

The application of peroxydisulfate along with UV irradiation as an advanced oxidation process at laboratory scale introduces an effectual and safe method for degradation of tylosin. Almost no antibiotic removal was achieved using peroxydisulfate alone. On the other hand, destruction percentage of 11% was obtained using UV irradiation alone. Finally, more than 97% of antibiotic content was removed using UV irradiation and peroxydisulfate simultaneously. Degradation rate of tylosin was dependent on the antibiotic concentration, peroxydisulfate concentration, and pH. Increase in antibiotic concentration would decrease the degradation as degradation of 91% was observed in 20 ppm, while only 78% of degradation was achieved in 80 ppm concentration of tylosin. Neutral pH (6.7) was monitored as the optimum pH; however, all levels of pH demonstrate satisfactory removal.