Simultaneous Removal of Ammonium and Nitrite in Aqueous Suspensions of Ferric Tannate Powder by Adsorption and Catalysis

Abstract

Ferric tannate (TA-Fe³⁺) has the potential to transform ammonium (NH₄⁺) and nitrite (NO₂⁻) into nitrogen gas (N₂) through adsorption and catalysis. Few reports have given detailed account of different adsorption behavior for NH₄⁺ and NO₂⁻, which is important to develop the potential of the material. TA-Fe³⁺ was synthesized here and its performance as an adsorbent/catalyst for the simultaneous removal of NH₄⁺ and NO₂⁻ from water was investigated. Results confirmed the adsorption and catalysis capability of TA-Fe³⁺ toward NH₄⁺ and NO₂⁻: (1) Following a 24 h adsorption and redox, the concentrations of NH₄⁺ and NO₂⁻ in the mixed solution decreased from initial values of 10.71 and 7.14 to 4.28 and 1.64 mmol/L, respectively, and 0.41 mmol/g of N₂ was produced with a maximal N₂ yield rate of 0.072 mmol/[g·h]. The N₂ yield was about 144 times that in the absence of TA-Fe³⁺; (2) intermediate products such as nitrous oxide (N₂O) and nitrate (NO₃⁻) were not detected; and (3) Raman spectrum analysis identified the site of Fe–O bond to be the center of adsorption and catalysis. Moreover, there were interesting findings: (1) TA-Fe³⁺ exhibited significantly distinct adsorption behavior toward NH₄⁺ and NO₂⁻. External diffusion and internal diffusion exerted a key influence on the adsorption toward NH₄⁺ and NO₂⁻, respectively; NH₄⁺ was adsorbed on TA-Fe³⁺ in the form of monolayer, and NO₂⁻ in the forms of both monolayer and multilayer; (2) TA-Fe³⁺ was easily regenerated with water; and (3) adsorption toward NO₂⁻ was the rate-determining step of the catalytic reaction. These findings will provide valuable enlightenment for the further work to reveal the adsorption and catalysis mechanisms of TA-Fe³⁺.