Treatment of ammonia-embodied wastewater by a transition-metal-based photochemical catalysis strategy

Inspired by the self-purification process and a low nitrogen content of the ocean, and the fact that the driving-force behind ecological cycle is solar irradiation, a novel photochemical strategy was designed to spontaneously remove inorganic ammonia nitrogen from wastewater with solar irradiation. This strategy is based on the principles of green chemistry and energy efficiency, and meanwhile the prevention from the introduction of accompanying pollution. In our strategy, a photo-Fe (or Mn)-O₂ system was built to remove ammonia-nitrogen from its aqueous solution. The results show that with full band solar irradiation at a range of 10-30 mW cm^-2, in weak alkaline condition, more than 90% of ammonia-nitrogen can be effectively removed from NH₄Cl aqueous solution by the new strategy, with a residual concentration as low as 2 mg L^-1. Mn(III) was proved to be a better catalyst than Fe(III). The catalytic mechanism of N-removal is the generation of ^•OH during the process of the photoreduction of transition metal hydroxides. DFT theory had been applied to help explaining the mechanism. Different from general knowledge, in our strategy, an alkaline environment, where the generation rate of radicals was relatively slow and comparable to oxidation rate of transition metal ions, can guarantee the stability and persistency of the catalytic reaction. No NOx was produced in this strategy. This new strategy provides a new possibility of cost-efficient and environmental-friendly wastewater treatment, and has certain meaning of understanding how self-purification works in nature.