Photolytic and photocatalytic removal of a mixture of four veterinary antibiotics

The removal of a mixture of four veterinary antibiotics (VA) – tetracycline (TET), ciprofloxacin (CIP), sulfadiazine (SDZ) and sulfamethoxazole (SMX) – via photo-degradation (UVC) and photocatalysis with TiO2 (UVC/TiO2) was investigated in a batch reactor under different initial concentrations (20, 100, 500 and 1000 μg/L per antibiotic). Ultra-high performance liquid chromatography coupled to a mass spectrometry (UHPLC-MS/MS) was used to determine the removal of these veterinary antibiotics. The removal of all antibiotics via photolysis was around 98–99% after 100 h for TET, 122 h for CIP, 212 h for SDZ and 240 h for SMX. Nevertheless, the removal of all antibiotics via photocatalysis was around 99–100% after 4.2 h for TET, 3.5 h for CIP, 7.1 h for SDZ and 16.5 h for SMX. The photolysis for the four veterinary antibiotics followed a first-order irreversible kinetic model. The photocatalysis of TET, CIP and SDZ followed a Langmuir-Hinshelwood kinetic model, and adsorption was considered the limiting step. SMX followed a first-order irreversible kinetic model. The photolytic degradation rate constant (k1) was 0.00073 min−1 for TET, 0.00055 min−1 for CIP, 0.00031 min−1 for SDZ and 0.00027 min−1 for SMX. While for photocatalysis k1 was 0.0044 min−1 for SMX; kL-H was 0.0284 min−1 for TET, 0.0379 min−1 for CIP and 0.0141 min−1 for SDZ. The VA degradation was enhanced by the use of a catalyst. Additionally, electrical energy per order (EEO) was assessed to estimate the electrical energy efficiency of each process. EEO values for photolysis were 339.06 kWh/m3/order for TET, 449.84 kWh/m3/order for CIP, 795.31 kWh/m3/order for SDZ and 897.71 kWh/m3/order for SMX. On the other hand, EEO values for photocatalysis were 14.96 kWh/m3/order for TET, 12.07 kWh/m3/order for CIP, 20.39 kWh/m3/order for SDZ and 62.10 kWh/m3/order for SMX. The energy consumption for photocatalysis was considerably lower than for photolysis. This study determined an overall degradation rate constant for a wide range of TET, CIP, SDZ and SMX concentrations. Furthermore, when working with a pH of 8 (a typical pH from wastewater from livestock farms) and a VA mixture whose concentrations resemble the characteristics of real water samples, that photolysis and photocatalysis are potential processes for wastewater treatment with low energy consumption.

本研究于间歇式反应器中，针对单种抗生素初始浓度分别为20、100、500、1000 μg/L的四种兽医抗生素（veterinary antibiotics, VA）混合体系——四环素（tetracycline, TET）、环丙沙星（ciprofloxacin, CIP）、磺胺嘧啶（sulfadiazine, SDZ）与磺胺甲恶唑（sulfamethoxazole, SMX）——的去除效果展开探究，分别采用UVC光降解与二氧化钛光催化（UVC/TiO2）两种工艺。采用超高效液相色谱-串联质谱（ultra-high performance liquid chromatography coupled to mass spectrometry, UHPLC-MS/MS）对目标抗生素的去除率进行定量测定。仅通过UVC光解处理时，四环素、环丙沙星、磺胺嘧啶、磺胺甲恶唑分别在反应进行100 h、122 h、212 h、240 h后，其去除率可达98%~99%；而采用UVC/TiO2光催化工艺时，上述四种抗生素仅需4.2 h、3.5 h、7.1 h、16.5 h即可实现99%~100%的去除率。四种抗生素的光解过程均遵循一级不可逆动力学模型；四环素、环丙沙星与磺胺嘧啶的光催化过程符合朗缪尔-欣谢尔伍德（Langmuir-Hinshelwood）动力学模型，且吸附为反应限速步骤，磺胺甲恶唑的光催化过程则遵循一级不可逆动力学模型。光解过程的一级速率常数（k1）分别为：四环素0.00073 min⁻¹、环丙沙星0.00055 min⁻¹、磺胺嘧啶0.00031 min⁻¹、磺胺甲恶唑0.00027 min⁻¹。光催化体系中，磺胺甲恶唑的一级速率常数k1为0.0044 min⁻¹；四环素、环丙沙星、磺胺嘧啶的朗缪尔-欣谢尔伍德速率常数kL-H则分别为0.0284 min⁻¹、0.0379 min⁻¹、0.0141 min⁻¹。添加催化剂可显著提升抗生素的降解效能，本研究同时通过单位对数级电能消耗量（electrical energy per order, EEO）评估两种工艺的电能利用效率：光解工艺的EEO值分别为四环素339.06 kWh/m³/order、环丙沙星449.84 kWh/m³/order、磺胺嘧啶795.31 kWh/m³/order、磺胺甲恶唑897.71 kWh/m³/order；光催化工艺的EEO值则分别为四环素14.96 kWh/m³/order、环丙沙星12.07 kWh/m³/order、磺胺嘧啶20.39 kWh/m³/order、磺胺甲恶唑62.10 kWh/m³/order。相较而言，光催化工艺的能耗远低于光解工艺。本研究明确了四环素、环丙沙星、磺胺嘧啶与磺胺甲恶唑在宽浓度范围内的整体降解速率常数；此外，当反应体系pH为8（畜禽养殖场废水的典型pH值）且混合抗生素浓度贴合实际水体样品特征时，UVC光解与UVC/TiO2光催化均为具备低能耗潜力的废水处理技术。