(521c) Selective Recovery of Ammonia Nitrogen from Wastewaters with Transition Metal-Loaded Polymeric Cation Exchange Adsorbents

The presence of nitrogen compounds, such as ammonia and ammonium (total ammonia nitrogen, or TAN), in wastewater is concerning due to their contribution to eutrophication, which has a wide range of negative environmental effects. Typically, TAN is removed from wastewater using microbial nitrification-denitrification that converts TAN into nitrogen gas (N₂). This process reverses the Haber-Bosch process, which consumes roughly 1% of global energy to convert N₂ into ammonia. A more sustainable and economical approach would be to separate TAN from wastewater directly and use it for value-added products. Ion exchange is a well-established technology for removing ions from water, but due to low ion selectivity, resulting eluent streams currently require additional costly steps to further separate ions before value-added products can be made.

Therefore, we have modified commercial polymeric cation-exchange adsorbents by simple ion-exchange for selective ammonia removal and recovery. Since metal cations like Cu²⁺ and Zn²⁺ are known to form inner-sphere complexes with ammonia, we hypothesized that metal ligand exchange would drive selectivity towards ammonia in the presence of other common wastewater cations like Na⁺, K⁺, Mg²⁺, and Ca²⁺. However, because some anions (e.g. phosphate) and organics can also form complexes, we utilized the Donnan-Exclusion Effect of cation-exchange resins to repel them.

We loaded each metal into three resins with three different functional groups: acrylic acid (AA), iminodiacetic acid (IDA), and bis-picolylamine (BPLA). The goal was for metal to occupy all functional sites and limit ion exchange with nontarget cations, since heavy metal cations bind strongly to these groups. We then performed batch adsorption experiments at pH 9.5, based on recent reports of pH 9-10 as optimal for metal-ammine complexation. A pH of 9.5 is also similar to the pH of hydrolyzed urine, one of the most concentrated and ubiquitous ammonia-rich waste streams. Finally, we used batch sulfuric acid treatment to regenerate resins and recover ammonia ligands as ammonium sulfate.

Compared to commercial resins, metal-ligand exchange adsorbents exhibited higher ammonia removal capacity (8 meq/g) and recovery selectivity (N/K⁺ concentration factor of 5) in binary equimolar solutions of TAN and K⁺. During acid regeneration, we found an optimal pH range that caused under 0.7% transition metal elution through H⁺ ion-exchange while recovering 60-70% of adsorbed ammonia. However, in urine-level cation solutions, divalent cation exchange increased transition metal elution and reduced ammonia adsorption. Considering optimal ammonia concentrations (200-300 meq/L) and pH (9-10) for metal-ligand exchange, we identified hydrolyzed urine as a promising candidate for selective TAN recovery. Ultimately, metal-ligand exchange adsorbents can advance nitrogen-selective separations from wastewaters.