Efficient ammonia removal and toxic chlorate control by using BiVO4/WO3 heterojunction photoanode in a self-driven PEC-chlorine system.

The efficient removal of ammonia is a difficult issue in wastewater treatment because ammonia is easily converted to nitrate instead of N2. The oxidation of ammonia by chlorine radical (Cl) is recognized as an effective method. However, the massive generation of toxic byproducts chlorate and nitrate pose great risk for its practical application due to the excessive oxidation capacity of hydroxyl radical. Herein, we propose a novel method to selectively generate Cl for efficient ammonia removal using BiVO4/WO3 photoanode in a self-driven photoelectrocatalytic (PEC) system. Cl was predominantly produced by regulating the valence band edge of WO3 though modifying BiVO4, which tuned the moderate oxidative force of hole to reduce OH generation and thereby inhibited the formation of chlorate and nitrate. The self-driven ammonia degradation was achieved by employing BiVO4/WO3 and Si photovoltaic cells as composite photoanodes to improve light-absorption and electron-hole separation, thus enhancing Cl production. These results showed that 10 mg L-1 of ammonia-N was completely removed (99.3 %) in 120 min with 80.1 % of total nitrogen removal. Toxic byproducts chlorate and nitrate were inhibited by 79.3 % and 31 %, respectively, compared to WO3. This work provides new insights to develop efficient, energy-saving and environment-friendly method for ammonia pollution treatment.

Self-Driven Photoelectrochemical Splitting of H2S for S and H2 Recovery and Simultaneous Electricity Generation.

A novel, facile self-driven photoelectrocatalytic (PEC) system was established for highly selective and efficient recovery of H2S and simultaneous electricity production. The key ideas were the self-bias function between a WO3 photoanode and a Si/PVC photocathode due to their mismatched Fermi levels and the special cyclic redox reaction mechanism of I-/I3-. Under solar light, the system facilitated the separation of holes in the photoanode and electrons in the photocathode, which then generated electricity. Cyclic redox reactions were produced in the photoanode region as follows: I- was transformed into I3- by photoholes or hydroxyl radicals, H2S was oxidized to S by I3-, and I3- was then reduced to I-. Meanwhile, H+ was efficiently converted to H2 in the photocathode region. In the system, H2S was uniquely oxidized to sulfur but not to polysulfide (Sxn-) because of the mild oxidation capacity of I3-. High recovery rates for S and H2 were obtained up to ∼1.04 mg h-1 cm-1 and ∼0.75 mL h-1 cm-1, respectively, suggesting that H2S was completely converted into H2 and S. In addition, the output power density of the system reached ∼0.11 mW cm-2. The proposed PEC-H2S system provides a self-sustaining, energy-saving method for simultaneous H2S treatment and energy recovery.

Highly selective transformation of ammonia nitrogen to N2 based on a novel solar-driven photoelectrocatalytic-chlorine radical reactions system.

A highly selective method for transforming ammonia nitrogen to N2 was proposed, based on a novel solar-driven photoelectrocatalytic-chlorine radical reactions (PEC-chlorine) system. The PEC-chlorine system was facilitated by a visible light response WO3 nanoplate array (NPA) electrode in an ammonia solution containing chloride ions (Cl-). Under illumination, photoholes from WO3 promote the oxidation of Cl- to chlorine radical (Cl). This radical can selectively transform ammonia nitrogen to N2 (79.9%) and NO3- (19.2%), similar to the breakpoint chlorination reaction. The ammonia nitrogen removal efficiency increased from 10.6% (PEC without Cl-) to 99.9% with the PEC-chlorine system within 90 min operation, which can be attributed to the cyclic reactions between Cl-/Cl and the reaction intermediates (NH2, NHCl, etc.) that expand the degradation reactions from the surface of the electrodes to the whole solution system. Moreover, Cl is the main radical species contributing to the transformation of ammonia nitrogen to N2, which is confirmed by the tBuOH capture experiment. Compared to conventional breakpoint chlorination, the PEC-chlorine system is a more economical and efficient means for ammonia nitrogen degradation because of the fast removal rate, no additional chlorine cost, and its use of clean energy (since it is solar-driven).