Ligand Exchange Adsorbents for Selective Phosphate and Total Ammonia Nitrogen Recovery from Wastewaters

ConspectusHuman interference in natural biogeochemical cycles has caused an unprecedented input of reactive phosphorus and nitrogen nutrients into the environment, contributing to perturbations of natural aqueous ecosystems (e.g., eutrophication). Furthermore, industrial phosphorus mining and Haber–Bosch ammonia production contribute significantly to global energy expenditures and greenhouse gas emissions. Existing wastewater treatment techniques, particularly those based on adsorption processes, have predominantly concentrated on nutrient removal, underutilizing the potential for the subsequent recovery of pure products. Recovering these nutrients from wastewaters (e.g., municipal, industrial, agricultural) can supplement mining and fertilizer production, leading to energy and emissions savings and contributing to a more circular resource economy. In addition, nutrient recovery provides economic incentives to expand the implementation of water treatment, which exhibits additional benefits, such as global public health and environmental remediation. Phosphate and total ammonia nitrogen (i.e., TAN, the sum of ammonia and ammonium) are emphasized in this Account because they comprise substantial portions of reactive phosphorus and nitrogen.Adsorption-based wastewater treatment processes are promising due to their simple construction and maintenance, scalability, and cost-effectiveness. However, adsorption of phosphate and TAN is generally attained through ion exchange (electrostatic attraction), which is a nonselective interaction. Additionally, acid or base used for adsorbent regeneration contributes most of the embedded energy and greenhouse gas emissions of the adsorption process. If adsorption could achieve high target nutrient selectivity and regenerability, then valuable phosphate and TAN products could be recovered efficiently and economically. Because phosphate and ammonia are uniquely strong nucleophiles among wastewater species, leveraging ligand exchange (coordinate covalent bonding) can enhance selectivity against competing ions. Because phosphate and ammonia have mild pH speciation equilibria that can interrupt coordinate covalent bonds, acid or base input for adsorbent regeneration can be minimized, mitigating the major contributors to energy, emissions, and cost.In this Account, we summarize our recent work on two ligand exchange adsorbents: (1) a ferric oxide-loaded poly(vinylbenzyl trimethylammonium) strong base anion exchange resin for selective phosphate recovery from municipal wastewater and (2) a zinc polyacrylate weak acid cation exchange resin for selective TAN recovery from hydrolyzed urine. To maximize adsorbent selectivity, capacity, and regenerability without eluting the immobilized ligand exchange electrophile (i.e., ferric oxide and divalent zinc) from the adsorbent, all interactions between the solutes, electrophile, and support polymer must be carefully controlled to favor the desired bonds. To optimize resource efficiency and material design, electrochemical systems, and supplemental characterization techniques are also discussed. Electrochemical pH buffering and adsorbent regeneration can eliminate external acid and base input and minimize external salt requirements, further lowering operational costs, energy, and emissions. Synchrotron methods can analyze adsorbent bonding with high precision to understand coordination environments and inform adsorbent structure improvements. Finally, we provide a perspective on future directions, including design for complete wastewater treatment trains, future adsorbent materials, and other valuable wastewater constituents. In summary, selective nutrient recovery from wastewaters will be essential for chemical manufacturing and pollution mitigation in a sustainable society.