Resin-Mediated pH Control of Metal-Loaded Ligand Exchangers for Selective Nitrogen Recovery from Wastewaters

Abstract

Highly selective separation materials that recover total ammonia nitrogen (i.e., ammonia plus ammonium, or TAN) from wastewaters as a pure product can supplement energy-intensive ammonia production and incentivize pollution mitigation. We recently demonstrated that commercial acrylate cation exchange polymer resins loaded with transition metal cations, or metal-loaded ligand exchangers, can recover TAN from wastewater with high selectivity (TAN/K + equilibrium selectivity of 10.1) via metal−ammine bond formation. However, the TAN adsorption efficiency required further improvement (35%), and the optimal concentration and pH ranges were limited by both low ammonia fractions and an insufficiently strong resin carboxylate−metal bond that caused metal elution. To overcome these deficiencies, we used a zinc− acrylate ligand exchange resin and a tertiary amine acrylic weak base resin (pH buffer resin) together to achieve resin-mediated pH control for optimal adsorption conditions. The high buffer capacity around pH 9 facilitated gains in the adsorbed TAN per ligand resin mass that enhanced the TAN adsorption efficiency to greater than 90%, and constrained zinc elution (below 0.01% up to 1 M TAN) because of decreased ammonia competition for zinc−carboxylate bonds. During TAN recovery, resin-mediated pH buffering facilitated recovery of greater than 99% of adsorbed TAN with 0.2% zinc elution, holding the pH low enough to favor ammonium but high enough to prevent carboxylate protonation. For selective ion separation, solid phase buffers outperform aqueous buffers because the initial solution pH, the buffering capacity, and the ion purity can be independently controlled. Finally, because preserving the resin−zinc bond is crucial to sustained ligand exchange performance, the properties of an ideal ligand resin functional group were investigated to improve the properties beyond those of carboxylate. Ultimately, ligand exchange adsorbents combined with solid pH buffers can advance the selective recovery of nitrogen and potentially other solutes from wastewaters.

Removal of PFAS from groundwater using weak‐base anion exchange resins

“…In addition, the high buffering capacity of WB‐AERs is beneficial to increase the pH of acidic influents such as semiconductor, chromium‐impacted, and laundry wastewaters (Deng et al, 2010; Du et al, 2015). WB‐AERs were shown to outperform liquid‐phase buffers and were previously implemented for nitrogen recovery, by favoring the protonation of ammonium ions (Clark et al, 2022), water softening applications, and demineralizing process in water treatment (Harland, 1994).…”

“…Several other properties of WB‐AERs were overlooked in the literature but have valuable implications for water treatment. Compared to conventional SB‐AERs, WB‐AERs have greater capacity, are more resistant to oxidative attacks (Harland, 1994), less susceptible to organic fouling (Dixit et al, 2021), and possess high buffering potential (Clark et al, 2022; Zhang et al, 2014).…”

A Unit Process Approach to Nontarget Screening of Organic Contaminants during Urine Treatment

“…77 Rational design of urine treatment trains is a critically needed step to leverage process-specific innovations toward informed implementation. As new unit processes are developed to expand the portfolio of urinederived products (e.g., high-purity ammonia fertilizers, 78,79 carbonate-based bricks, 80 microbially derived proteins 81 ), adapting the enrichment ratio framework can guide their development and integration into multifunctional treatment trains. Future work on urine treatment will likely identify specific transformation products of interest, as is common in conventional wastewater treatment processes.…”

Reimagining Excreta as a Resource: Recovering Nitrogen from Urine in Nairobi, Kenya

“…In parallel with toilet-level efforts to recover urine, ElectroSan researchers at Stanford are investigating more selective adsorbents to improve the lifetime, cost- (Clark & Tarpeh, 2020;Clark et al, 2022). Electrochemical stripping has also been further developed with predictive models for removal and recovery and energy estimations that show that electrochemical stripping is on par with other nitrogen removal technologies (Liu et al, 2020).…”