Membrane Distillation Coupled with Peroxymonosulfate Advanced Oxidation for Antibiotic Removal and Wastewater Treatment

<p dir="ltr">Antimicrobial wastewater poses significant risks such as genotoxicity, ecotoxicity, and antimicrobial resistance (AMR). Conventional treatment approaches have demonstrated limited success in removing antibiotics from wastewater. Therefore, this study was aimed at integrating thermally activated peroxymonosulfate based sulfate radical advanced oxidation with direct contact membrane distillation (PMS-MD) for the efficient removal of antibiotics from wastewater. The other aim of the study was to compare the antibiotic removal performance of the PMS-MD process with that of standalone UV-activated PMS (PMS/UV), standalone heat-activated PMS (PMS/Heat), and conventional powdered activated carbon (PAC) adsorption, with the PAC method serving as a benchmark. Tetracycline (TC) and ciprofloxacin (CP) served as model antibiotics considering their widespread use, recalcitrance and adverse impacts on the environment and human health.</p><p dir="ltr">PMS/UV and PMS/Heat processes were performed in batch mode under various reaction conditions. At higher antibiotic concentrations (50 mg L-1), neither TC nor CP was fully broken down by PMS/UV; however, TC removal of >97 % was achieved at 5 mg L-1 TC, after 120 minutes of UVC irradiation with 5 mM PMS. While there was no discernible improvement in CP removal (9.6 % to 30.6 %), the TC removal efficiency increased from 21.5 % to 75.3 % when the PMS concentration was changed from 1 mM to 7.5 mM at a 50 mg L-1 TC and CP. PMS/Heat demonstrated comparatively better performance in TC and CP removal. Antibiotic degradation was significantly influenced by activation temperature, showing a 28.9 % increment in TC removal and a 104.9 % increment in CP removal when the temperature was increased from 40 °C to 80 °C. CP degradation performance varied significantly under different antibiotic concentrations, with >80 % removal efficiencies observed at concentrations below 25 mg L-1. PMS/Heat performed comparatively better in both high and low concentrations of TC. Antibiotic removal improved significantly with increased PMS concentration, with TC and CP achieving nearly 100 % and 70.9 % removal at 7.5 mM PMS, respectively. </p><p dir="ltr">Due to its higher resistance to degradation compared to TC in both PMS/UV and PMS/Heat systems, CP was selected for the PMS-MD studies. After establishing system stability of the MD system at a temperature range of 40-80 °C, antibiotic rejection tests (70 °C; 300 ml min-1 flow rate; 1 L feed; 300 ml DI in permeate tank) were performed using CP (50 mg L-1) in a NaCl solution (2000 mg L-1) as the feed to examine the effects of CP on membrane wetting and fouling. The polytetrafluoroethylene (PTFE) membrane demonstrated excellent antibiotic rejection (100 % rejection) with no significant fouling or wetting. When 5 mM PMS was incorporated with 25 mg L-1 CP in the feed (70 °C; 300 ml min-1 flow rate; 1 L feed; 300 ml DI in permeate tank), 93.7 % CP degradation resulted, and the degradation efficiency decreased considerably when the PMS concentration was reduced below 5 mM. When the initial CP concentration in the feed tank was reduced to 5 mg L-1, >99 % CP removal resulted in the feed side in PMS-MD. Moreover, the PMS-MD system showed faster CP degradation rates compared to the standalone PMS/Heat process, for a range of antibiotic concentrations (5-50 mg L-1). In order to assess the effectiveness of PMS-MD in antimicrobial wastewater treatment under actual circumstances, CP (5 mg L-1) spiked municipal wastewater was utilised as the feed. The PMS-MD treatment demonstrated excellent CP removal performance in real wastewater matrix with >99 % removal within 30 minutes as confirmed by high performance liquid chromatograph (HPLC) and liquid chromatography-mass spectrometry (LC-MS). </p><p dir="ltr">The PMS-MD process achieved 70% water recovery within 6.5 hours while removing over 99% of CP from the system. In comparison, the standalone MD process required 10.5 hours to reach the same level of water recovery with significant water flux reduction due to significant membrane fouling. CP was not detected on the permeate side in both systems with UV-visible and HPLC analyses. Moreover, the permeate of PMS-MD exhibited a total organic carbon (TOC) level below 5 mg L-1. Scanning electron microscopy (SEM) and laser microscopy confirmed that PMS-MD was able to alleviate membrane fouling in real wastewater treatment. In comparison to the benchmarking experiments with PAC adsorption, the PMS-MD system demonstrated improved reaction kinetics by removing CP at a faster rate than PAC adsorption. Nonetheless, both technologies demonstrated high CP removal performance by achieving >99 % in both synthetic wastewater and CP-spiked municipal wastewater. Moreover, the PMS-MD system performed significantly effectively and efficiently compared to standalone PMS/UV and PMS/Heat in CP removal. The outcomes of this study show that the suggested integrated system is more effective than standalone methods at eliminating CP and reducing related environmental hazards. Additionally, it establishes the foundation for creating expandable methods to treat antimicrobial wastewater, greatly contributing to water purification efforts and global environmental health initiatives.</p>