Electrode preparation and physicochemical characterization. Three types of electrodes were prepared: electrodes containing a single-layer AT coating, a single-layer NAT coating, and a double-layer coating with a NAT layer on top of the base metal (i.e., Ti metal) AT coating (NAT/AT). Clean titanium (Ti) plates (2 cm × 3 cm) were etched in a 1:4 HF:HNO3 solution for 1 min before use. Deposition of the metal oxide coatings on the Ti plate was achieved by dip-coating in aqueous precursor solutions followed by calcination.
The precursor solution for AT contained 360 mM SnCl4⋅5H2O (98%, Aldrich) and 40 mM SbCl3 (>99.0%, Aldrich), while precursor solution for NAT contained 360 mM SnCl4⋅5H2O, 15 mM SbCl3, and 4 mM Ni(OCOCH3)⋅4H2O (98%, Aldrich). The Ti plate was dipped in the coating solution for 20 s, dried under room temperature, and annealed at 600°C for 10
min. This cycle was repeated until a desired mass loading level was attained, after which the plate was annealed at 600°C for 1.5 h. The mass loading level of the two single-layer electrodes was 1.3 mg/cm2. For the heterojunction anode, NAT(1.3)/AT(1.3 mg/cm2) was used for activity evaluations and accelerated lifetime tests. NAT(0.6)/AT(0.6 mg/cm2) was prepared exclusively for accelerated lifetime testing. IrO2 and BDD electrodes were purchased from Ecosan (China) and NeoCoat (Switzerland), respectively.
Physiochemical characterization of the prepared electrodes was obtained using a scanning electron microscope (SEM, Zeiss 1550 VP) equipped with an Oxford X-Max SDD X-ray energy dispersive spectrometer (EDS) system for elemental distribution.
Electrochemical characterization. All electrochemical tests were performed in an undivided electrolysis cell. An anode (6 cm2) was coupled to a stainless steel cathode (6 cm2) at a spacing of 5 mm. At the same spacing close to the anode, an Ag/AgCl reference electrode (BASI Inc.) was installed. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were characterized using a Biologic VSP-300 potentiostat. CVs were measured in 100 mM sodium perchlorate (NaClO4) solutions using a scan rate of 0.05 V/s.
The uncompensated resistance (Ru) in relevant electrolytes was determined by the impedance method at 100 kHz with a 20 mV sine wave. All anodic potentials (Ea) were corrected by the product of current (i) and Ru drop using equation Ea – iRu. EIS measurements were performed in aqueous solutions containing 350 mM K3Fe(CN)6, 350 mM K4Fe(CN)6, and 1.0 M KCl over the frequency range of 100 kHz to 0.1 Hz with a 10 mV sine wave.20 Perchloric acid (HClO4) or sodium hydroxide (NaOH) solutions were used to adjust pH.
Electrolysis and analytical methods. All tests were conducted at a constant current density of 10 mA/cm2 in 25 mL solutions (specific surface area = 24 m2/m3). Both NAT/AT and NAT produced O3 during electrolysis. The dissolved O3 was measured using the indigo method.21 Headspace O3 concentrations were determined using an ambient ozone monitor (Horiba APOA-360). For O3 generation in NaCl solutions, malonic acid (MA) was added to selectively quench reactive chlorine species.21 Chlorine evolution (CER) rate measurements were performed in 30 mM NaCl solutions. Chlorine concentrations were measured by DPD (N,N-diethyl-p-phenylenediamine) reagent (Hach DPD Method 10102). Current efficiencies (η) for free chlorine and O3 generation were calculated using the following equation:
η =nV𝐹 dC I dt
where n is the electrons required to form one mole of Cl2 from Cl- (n = 2) or O3 from O2- (n
= 6), V is the volume of electrolyte (25 mL), 𝐹 is Faraday constant (96,485 C/mol), and I is the current (A). The mass transfer rate constant for NAT/AT was determined using the limiting current technique.22
Benzoic acid (BA, 1 mM), nitrobenzene (NB, 1mM), phenol (Ph, 1 mM), and ibuprofen (IBP, 20 µM) were used as radical probes.23 The substrates were quantified using high- performance liquid chromatography coupled with a UV detector (HPLC-UV).
Carbamazepine, alone or in combination with four other common pharmaceutical products, was treated in both synthetic electrolyte solutions and human wastewater samples collected on campus. Parent pharmaceutical compound and transformation products were identified by an ultrahigh performance liquid chromatography system (Waters Acquity UPLC) coupled to a time-of-flight mass spectrometer (Waters Xevo GS-2 TOF). Metal (Sb and Ni) concentrations in electrolytes were quantified by inductively coupled plasma mass spectrometry (Thermo Scientific iCAP RQ ICP-MS). Details of analytical procedures are provided in SI Text 2.S1.
E. coli was cultured in Luris-Bertani (LB) broth (BD Difco) and grown at 37°C for 15 h.9 The bacterial suspension was centrifuged at 5000 rcf for 5 min and resuspended in 30 mM NaClO4 or 30 mM NaCl electrolyte solutions. The resulting microbial suspensions had concentrations of ∼109 CFU/mL determined by optical density at 600 nm (OD600).24 The suspensions were subsequently used as bacterial stock solutions. For electrolysis, the freshly- prepared stock was seeded into the electrolyte to an approximate concentration of 105-106 CFU/mL, a value typically observed in environmental waters. Samples taken during experiments were spread evenly onto LB agar (BD Difco) plates. The plates were then incubated at 37°C for 15 h, after which colonies were counted to obtain concentrations. MS2 was also seeded into the electrolyte to reach 105-106 PFU/mL. Sample concentrations were determined using the double agar layer method.25
Human wastewater was obtained from an electrochemical toilet prototype on the Caltech campus (Pasadena, CA). Prior to treatment, wastewater samples were filtered (0.45 μm) and spiked with pharmaceuticals, E. coli., and MS2.
Theoretical modeling. A kinetic model containing 102 elementary reactions obtained from literature was developed using the chemical kinetics software package, Kintecus 6.80.26 Unknown rate constants were estimated by fitting the model with experimental data (more details provided in SI Text 2.S2). The model was evaluated in both NaClO4 and NaCl electrolytes to investigate the influence of chlorine species on O3 and ⋅OH generation and subsequent transformations.