2025 AIChE Annual Meeting

(44d) Phosphate Recovery from Wastewater Via Electrochemically-Regenerated Sorbents

Authors

Neha Sharma, Stanford University
Ashley Martinez, Stanford University
Edward Apraku, Stanford University
William Tarpeh, Stanford University
Phosphate is a key component of agricultural fertilizers, but conventional supply is limited due to the depletion of finite mineral resources. Additionally, excess phosphate from agricultural runoff causes eutrophication and harmful algae blooms in many freshwater lakes and reservoirs, with significant environmental and economic consequences. Adsorbents offer a promising solution for recovering phosphate from wastewater as value-added products (e.g., fertilizers), and adsorption-based technologies can establish a circular phosphate economy by closing the loop from pollutants to products. However, high phosphate selectivity is needed to make adsorption economically and environmentally viable compared to conventional treatment processes. Hybrid anion exchange (HAIX) resins (polymeric ion-exchange resins infused with hydrated iron oxide nanoparticles) have a high affinity for phosphate under neutral pH conditions, facilitating effective removal even in dilute phosphate solutions like wastewaters. However, the resins require a strong base for regeneration, and the embodied emissions of industrial strong bases significantly increases the carbon footprint of the process. Conversely, electrochemically generated base could be employed as a low-carbon alternative for pH swings and resin regeneration, but knowledge gaps remain regarding the (1) mechanisms of phosphate selectivity, (2) influence of competing ions, and (3) optimization and assessment of electrochemical regeneration. These gaps hinder needed efforts to model adsorption dynamics and design scalable adsorption processes.

In this work, we demonstrated a hybrid electrochemical-ion exchange (IX) progress for phosphate recovery from wastewaters of increasing complexity. To understand how material properties influence phosphate distribution and speciation, we collected comprehensive information on iron speciation in resins with radiography and X-ray absorption spectroscopy. We complemented aqueous-phase analysis (of adsorption solution) with direct solid-phase measurements (of adsorbent, via micro-X-ray fluorescence and micro-X-ray near edge structure spectroscopy) to determine phosphate distribution and speciation on HAIX post-adsorption. Integrating synchrotron techniques with established aqueous characterization illuminated where and how phosphate binds (and desorbs) in the resins, enabling us to differentiate between phosphate bound to iron oxide sites versus phosphate bound to basic amino functional groups. Phosphorus distribution closely aligned with iron distribution, indicating selective adsorption of phosphate to the iron oxide nanoparticle sites. We used this integrated approach to evaluate the effects of several process parameters (adsorbent dose, phosphate concentration, presence of competing anions and organic matter, and electrochemical regeneration parameters) on phosphate recovery and selectivity. Finally, we compared the selectivity, recovery, and energy demand of the process to traditional regeneration methods. Ultimately, the results support the potential of adsorption technologies for recovering phosphate from complex wastewater streams and advance understanding of molecular-scale adsorption interactions and speciation within HAIX. The findings can enable tailored adsorbent design and system optimization and assessment for phosphate recovery technologies.