Decomposition of diclofenac in sewage from municipal wastewater treatment plant using ionizing radiation

All chemicals applied were of the highest purity grade available and were used as received. DCF (2-(2-(2,6-dichlorophenylamino) phenyl)acetic acid) was purchased from Sigma Aldrich, Poland. Acetonitrile (used as eluent in high-performance liquid chromatography, HPLC) was purchased from J. T. Baker (USA) and hydrogen peroxide from Stanlab Sp. J. (Poland). Stock solution of DCF was prepared in deionized water Milli-Q (Millipore).

Sewage sludge spiked with DCF to 50 mg L⁻¹ was irradiated both with gamma and electron beam (EB). Gamma parameters: Co⁶⁰ source Gamma Chamber with a dose rate of 2.5 kGy/h. The samples were irradiated in sealed 50-mL conical glass flasks. Dosimetry was carried out with the Fricke solution. The EB parameters were irradiation was carried out with an electron accelerator Elektronika (Russia) of beam energy 10 MeV, beam power 15 kW, and in the following operating conditions: pulse duration 4 μs, break between the pulses 2.5 ms, and dose delivered by pulse – 200 Gy. The dose rate was estimated at 20 kGy/s. Irradiated solutions were placed into polyethylene bags forming a layer of liquid on the conveyor (3–4 mm thick). DCF decomposition monitoring was carried out mainly with the use of HPLC analysis, according to our previous study [16].

Extraction procedure steps:

Sewage sludge spiked with DCF to 50 mg L⁻¹;

In the current regulations, it is important to develop analytical methods that will allow to determine the level of selected organic pollutants in sewage sludge at relatively low concentration and high accuracy. According to various estimates, sample preparation usually takes about 70–90% of the time spent on analysis. Therefore, a great deal of effort is focused on developing reliable procedures characterized by the simplicity of both operations and equipment involved in the process. Of the extraction methods, classical techniques are still relatively most commonly used: Soxhlet extraction and ultrasound extraction [24]. Most often the sewage sludge is freeze-dried or centrifuged before the extraction stage in order to remove as much water as possible. Then extraction is carried out and the analytes are transferred to the organic phase. In the first stage, a procedure for extraction of DCF from the sewage sludge was developed to enable its chromatographic determination. The initial DCF concentration was 50 mg L⁻¹. The concentration was selected in such a way as to be able to compare the obtained results with the results of earlier studies, which concerned the degradation of DCF in water under the influence of gamma radiation [16]. The data obtained for a series of eight repetitions allowed to assume that from an initial DCF concentration of 50 mg L⁻¹, about 45% is adsorbed on the sediment (Fig. 1). These values were used in further studies on the DCF degradation in the sewage sludge using radiation technology both gamma and electron beam. DCF concentration in both phases, sediments and solution over sediment, was carried out according to the previously developed analytical method [16].

Fig. 1

So far the studies carried out with initial DCF concentrations in the range of 20–50 mg L⁻¹ in aqueous solutions have shown that the decomposition using ionizing radiation takes place both by reaction with hydroxyl radical (OH) and hydrated electron (e⁻_aq) [17, 25]. The reaction with the ·OH radical occurs much faster, in the range of 9.0 [14] to 9.29 × 10⁹ M⁻¹ s⁻¹ [25], than the reaction with hydrated electron (1.53). Toxicity changes using the Microtox bioindication test with an increase in the dose of ionizing radiation were also examined [17, 25]. Our previous works have shown that for an aerated aqueous solution with initial DCF concentration of 50 mg L⁻¹, the total DCF degradation was observed at a dose of 4 kGy [16]. For the degradation process carried out for natural matrices, the presence of other so-called free radical scavengers, which compete with the substances for which the degradation process is carried out, may have a significant impact. They are primarily nitrates, carbonates, chlorides, or humic compounds. Their presence most often negatively affects the efficiency of the radiolytic decomposition of selected organic pollutants, although there are known cases in which a positive effect of the presence of these compounds has been observed [26]. Previous studies have shown that in the case of an aqueous DCF solution with an initial concentration of 50 mg L⁻¹, the presence of scavengers, that is, nitrates, carbonates, and humic acid at concentrations from 50 mg L⁻¹ to 100 mg L⁻¹ practically does not affect the efficiency of DCF decomposition in water [16]. On the other hand, Zhuan showed that for a DCF with an initial concentration of 30 mg L⁻¹, the gamma-ray yield at dose 0.8 kGy decreases from 80.8% to 62.9% in the presence of 30 mg L⁻¹ humic acid [27]. As the main reason for the decrease in decomposition efficiency, the authors mention the reactions of humic acid with hydroxyl radical. In our study, sludge spiked with DCF up to 50 mg L⁻¹ irradiated both the gamma and electron beam. Using the extraction procedure, degradation efficiency of DCF in the solution over the sediment and in the sediment was determined. The obtained results are presented in Fig. 2.

Fig. 2

Compared to previous experimental data on DCF decomposition in water in the presence of scavengers [16, 27], the efficiency of DCF decomposition in sewage sludge is much lower. When gamma radiation is used for doses below 5 kGy, the decomposition efficiency in the sediment is slightly lower than in the solution over the sediment. Both for the sediment and for solution over the sediment the maximum efficiency achieved for 5 kGy was only 50%. Significantly greater differences were observed in the case of electron beam application; for each of the applied doses the DCF decomposition efficiency in the sediment was lower than in the solution over the sediment. The maximum efficiency value obtained for a dose of 5 kGy was 30%. The results obtained may indicate that in the case of sludge, the thickness of the irradiated sludge layer is very important; perhaps centrifugation of the sludge before irradiation would allow for a higher degree of DCF decomposition in the sludge itself.

To improve the efficiency of DCF decomposition in the sewage sludge using ionizing radiation, the addition of hydrogen peroxide was used. It is well known that hydrogen peroxide reacts with hydrogen atom H (k (20°C) = 2.4 × 10¹⁰ M⁻¹s⁻¹) and with the solvated electron e⁻_aq (k (20°C) = 1.5 × 10⁷ M⁻¹s⁻¹), producing an additional portion of hydroxyl radicals [28], according to the following reaction: (1) eaq−+H2O2→ •OH+OH− {{\rm{e}}_{{\rm{aq}}}^ - } + {{\rm{H}}_2}{{\rm{O}}_2} \to {\;^\bullet}{\rm{OH}} + {\rm{O}}{{\rm{H}}^ - } (2) H++H2O2→ •OH+H2O {{\rm{H}}^ + } + {{\rm{H}}_2}{{\rm{O}}_2} \to {\;^\bullet}{\rm{OH}} + {{\rm{H}}_2}{\rm{O}}

However, the excess hydrogen peroxide in the solution can also act as a typical hydroxyl radical scavenger; the reaction takes place according to the rate k constant (20°C) = 3.1 × 10¹⁰ M⁻¹s⁻¹ as (3) 2 •OH+H2O2→2H2O+O2 {2}{\;^\bullet}{\rm{OH}} + {{\rm{H}}_2}{{\rm{O}}_2} \to 2{{\rm{H}}_2}{\rm{O}} + {{\rm{O}}_2}

It is very difficult to correctly determine the optimal amount of hydrogen peroxide needed to improve the efficiency of the radiolytic process, because it requires taking into account the numerous reactions of the generated radicals with intermediate products. In the gamma-radiation study, an addition of 840 μM hydrogen peroxide was applied. The initial DCF concentration was the same at 50 mg L⁻¹. It is a stoichiometric amount of hydrogen peroxide necessary to oxidize such an amount of DCF. Additionally, experiments were also performed in which the amount of added hydrogen peroxide was half of the stoichiometric amount and twice as much as the stoichiometric amount, which were 420 μM and 1.68 mM, respectively. In every case for DCF deposited in the solution over the sediment, no improvement in DCF decomposition efficiency was observed compared to irradiation without hydrogen peroxide support (Fig. 3).

Fig. 3

On the other hand, in the case of DCF deposited in the sediment, no DCF decomposition was observed in any case, but only for a dose of 5 kGy in the presence of 1.68 mM H₂O₂, 10% decomposition in relation to the initial amount was observed. Additionally, based on the obtained results, it can be concluded that the applied addition of hydrogen peroxide in quantities from 0.42 μM to 1.68 mM supports DCF adsorption on the sewage sludge, preventing its decomposition under the influence of gamma radiation.