Abstract
The activated persulfate degradation of piroxicam, a non-steroidal anti-inflammatory drug (NSAID) belonging to oxicams, was investigated. Persulfate was activated with thermal energy or (UV-A and simulated solar) irradiation. Using 250 mg/L sodium persulfate at 40 °C degraded almost completely 0.5 mg/L of piroxicam in 30 min. Increasing piroxicam concentration from 0.5 to 4.5 mg/L decreased its removal. The observed kinetic constant was increased almost ten times from 0.077 to 0.755 min−1, when the temperature was increased from 40 to 60 °C, respectively. Process efficiency was enhanced at pH 5–7. At ambient conditions and 30 min of irradiation, 94.1% and 89.8% of 0.5 mg/L piroxicam was removed using UV-A LED or simulated solar radiation, respectively. Interestingly, the use of simulated sunlight was advantageous over UV-A light for both secondary effluent, and 20 mg/L of humic acid solution. Unlike other advanced oxidation processes, the presence of bicarbonate or chloride in the range 50–250 mg/L enhanced the degradation rate, while the presence of humic acid delayed the removal of piroxicam. The use of 0.5 and 10 g/L of methanol or tert-butanol as radical scavengers inhibited the reaction. The coupling of thermal and light activation methods in different aqueous matrices showed a high level of synergy. The synergy factor was calculated as 68.4% and 58.4% for thermal activation (40 °C) coupled with either solar light in 20 mg/L of humic acid or UV-A LED light in secondary effluent, respectively.
Highlights
In recent decades, the emergence of micropollutants in the water cycle has become a global issue of environmental concern
Unlike other advanced oxidation processes, the presence of bicarbonate or chloride in the range 50–250 mg/L enhanced the degradation rate, while the presence of humic acid delayed the removal of piroxicam
(i) ultrapure water (UPW): conductivity = 0.061 mS/cm, pH = 6 obtained from a purification system (EASY-pureRF-Barnstead/Thermolyne, Waltham, MA, USA); (ii) secondary treated effluent taken from the University of Patras treatment plant (WW): pH = 8, conductivity = 815 mS/cm, alkalinity = 182 mg/L, chemical oxygen demand = 21 mg/L, total organic carbon = 7 mg/L, total suspended solids = 1.8 mg/L, chloride = 79 mg/L, sulfate = 28 mg/L and nitrate = 5.8 mg/L
Summary
The emergence of micropollutants in the water cycle has become a global issue of environmental concern. Pharmaceutical compounds are detected mainly in urban wastewater [3], wastewater treatment plants (WWTPs) [4,5], hospital effluents and in the agricultural sector [6,7,8], while their concentration ranges from a few ng/L to several μg/L [9,10,11] Despite their low concentration, they are resistant to biological degradation because conventional WWTPs cannot achieve high rates of removal of micropollutants [12,13]. This work focused on the removal of piroxicam using heat- and (UV-A or simulated solar) light-activated persulfate with particular emphasis on the degree of synergy between the two methods to achieve higher drug degradation rates and to avoid disadvantages associated with the use of (i) homogeneous Fenton process (i.e., iron precipitation and the need for separation and neutralization) and/or (ii) UV-C lamps (low water transmittance, high energy demand and presence of mercury). The effect of several operating parameters such as SPS concentration, pH, temperature, type of irradiation, and the use of different water matrices was evaluated
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