In recent years, the emergence of antibiotics in the environment has received increasing attention [1
]. A large number of antibiotic residues are detected in the aquatic ecosystem due to the extensive production and use of antibiotics [3
], and some reports even pointed out that the antibiotic residues were also detected in tap water. The presence of antibiotics in water not only affects the water quality, but also causes potential adverse effects on humans and ecological systems [4
]. Although the concentration of antibiotics is very low in the environment (ng·L−1
), the antibiotics are hard to be degraded by microorganism due to the complex structure and the antibacterial nature [5
]. Hence, antibiotics are gradually enriched in the environment resulting in antibiotic pollution. When long-term exposure to an antibiotic-polluted environment, human health would be affected even at very low concentrations [6
]. Among all of the antibiotics, tetracycline (TC) is wildly used in human and veterinary medicine [7
]. TC has been detected in sewage water, surface water, groundwater, drinking water, and sludge due to its ineffective removal by conventional water treatment processes [8
At present, the conventional methods to treat antibiotic wastewater are physical methods (coagulant sedimentation, adsorption, and membrane separation) [11
], biological methods (activated sludge, biological contact oxidation, anaerobic sludge bed) [12
], advanced oxidation processes (AOPs) (ozone oxidation, fenton oxidation, photacatalytic oxidation, electrocatalytic oxidation) [13
], and some combination methods [16
]. However, the traditional physical and biological methods cannot remove TC effectively. Membrane separation technology (mainly Nanofiltration (NF) and Reverse Osmosis (RO)) could remove TC from water [19
], while membrane separation technology is based on the physical screening, which would inevitably result in membrane fouling and reduce the membrane flux. Although antibiotics could be removed by physical adsorption processes such as activated carbon [20
], they were just transferred to another medium, which required further disposal. Owing to the biological toxicity of TC, inhibiting the microbial activity, the biological treatment methods cannot decompose TC effectively either.
The traditional electrochemical treatment processes are not suitable for aqueous TC treatment due to the low efficiency and high energy consumption. Electrocatalytic oxidation is a novel AOPs method utilizing the catalyst to enhance the electrochemical reaction. The organic compounds are degraded by the hydroxyl radical (OH) and other active radicals generated by electrocatalysis, and no additional chemicals are needed [21
]. During the whole electrocatalytic oxidation process, no oxidation byproducts are generated. Therefore, the electrocatalytic oxidation of recalcitrant organic pollutants such as antibiotics have attracted increasing interest in recent years. Zhang et al.
designed the TC degradation experiment by anode oxidation with a Ti/RuO2
electrode and investigated the operating parameters such as electrical current density, initial pH, and antibiotic initial concentration on the TC oxidation effect [22
]. Further, Nihal Oturan systematically investigated the effect of different cathode materials (carbon-felt and stainless steel) and anode materials (Ti/RuO2
, Pt, and BDD) on the direct/indirect electro-oxidation of TC [23
However, compared with the membrane separation process, these usual electrocatalytic processes are operated intermittently, which limit the increase of total water flow and hinder the extensive application of electrocatalytic oxidation technology in water treatment field. Li et al.
introduced electrocatalytic oxidation into the membrane separation process and designed an electrocatalytic membrane reactor (ECMR) with a self-cleaning function and continuous operation for wastewater treatment [24
]. In their work, a tubular conductive membrane with nano-TiO2
loading as the anode and a stainless steel mesh as a cathode constituted an ECMR. Once the membrane anode is electrified, excitation of TiO2
creates electron-hole pairs. The obtained electrons and holes react with the H2
O and O2
to generate reactive intermediates such as ·OH, ·O2
, and H2
. These reactive intermediates can indirectly decompose the organic pollutants located on the surface or in the pores of the membrane into CO2
O, or other biodegradable products. Therefore, both the membrane separation and electrocatalytic oxidation function can be achieved through the electrocatalytic membrane technology. Subsequently, Yang used ECMR to treat 200 mg·L−1
oily wastewater. The results showed that the chemical oxygen demand (COD) removal rates were up to 87.4 and 100 with the liquid hourly space velocity of 7.2 h−1
and 21.6 h−1
, respectively [25
]. Wang and co-workers found the phenol removal rate and complete mineralization fraction were 99.96% and 72.4%, respectively, when treated by ECMR under the conditions of 10.0 mM phenolic wastewater, pH of 6, current density of 0.3 mA·cm−2
, and residence time of 5.2 min [26
]. Liu reported that using carbon nanotubes (CNT) as the anode, an electrochemical filter was fabricated, which could treat aqueous tetracycline effectively. The tetracycline oxidative flux was 0.025 ± 0.001 mol·h−1
at an initial tetracycline concentration of 0.2 mM, total cell potential of 2.5 V, and flow rate of 1.5 mL·min−1
]. All of these demonstrate that the electrocatalytic membrane is a highly efficient water treatment method.
The purpose of this work was to develop a novel process to treat aqueous TC by an electrocatalytic membrane. The electrocatalytic membrane was fabricated by a carbon membrane coated with nano-TiO2 via a sol-gel process. With the synergistic effect of electrocatalytic oxidation and membrane separation, the nano-TiO2/carbon composite membrane could remove aqueous TC efficiently. COD and TC concentration were employed as the detection index to illustrate the efficiency of TC degradation. The operating parameters, such as current density, temperature, residence time, initial TC concentration, and pH for the influence of TC degradation efficiency were analyzed systematically.