Selective Recovery of Phosphorus from Synthetic Urine Using Flow-Electrode Capacitive Deionization (FCDI)-Based Technology

Abstract

Extraction of high-purity phosphate (P) from source-separated urine has attracted growing interest, given its potential economic and environmental benefits. In this study, we present an innovative strategy for selectively separating P from synthetic urine containing a high concentration of Cl − , simply by adjusting the charging and discharging processes of a flowelectrode capacitive deionization (FCDI) unit. During the charging process, both P and Cl − will be transported to the anode chamber and be adsorbed by the charged carbon particles. The inevitable Faradaic reactions induced the generation of H + and led to the conversion of charged P ions into uncharged H 3 PO 4 and spontaneous desorption into the electrolyte. When the electrode polarity is reversed, constantly charged species like Cl − would be mostly pushed back into the spacer chamber, whereas neutral H 3 PO 4 was expected to be selectively trapped in the anode chamber due to sluggish pH variations (particularly when using a higher carbon content), forming a P-rich solution. Under the optimal operating conditions (i.e., carbon content of 5 wt %, charging and discharging current densities of 10 and −15 A/m 2 , respectively, and charging and discharging current times of 120 and 30 min, respectively), a stable recovery efficiency (164 mg/L per cycle) and selectivity (ρ > 2, compared to Cl − ) of P were attained at a relatively low level of electric energy consumption. Results demonstrated that FCDI could be a promising technology for efficient P removal and recovery from source-separated urine without the consumption of additional chemicals.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

“…FCDI is sometimes used to concentrate inorganic nutrients and organic molecules for later recovery. ,,− Bian et al introduced FCDI as an alternative to a complex and costly nutrient removal system for the extraction and up-concentration of inorganic nutrients, such as NH 4 + , NO 3 – , and PO 4 3– , from wastewater . Although the system achieved a high recovery of NH 4 + (89–99%), NO 3 – (83–99%), and PO 4 3– (49–91%), it required post-treatment to further separate inorganic nutrients from desalinated salts (e.g., NaCl).…”

Membrane-Current Collector-Based Flow-Electrode Capacitive Deionization System: A Novel Stack Configuration for Scale-Up Desalination

“…Comparison of the desalination performance of the gradient FCDI system (a) with the reported literature (b–i). ,,,,,,,,, Note that the summaries of the literature data used in constructing these figures are provided in Table S1 in Supporting Information.…”

“…In most FCDI systems, graphite current collectors contribute to the weight and volume of FCDI cells, influencing the amplification of FCDI in practical applications. In comparison, the integrated membrane and current collector assembly can not only simplify the device configuration but also improve the 5,14,20,24,25,27,29,32,42,43 Note that the summaries of the literature data used in constructing these figures are provided in Table S1 in Supporting Information. modularity and scalability of the FCDI systems.…”

“…It was recently demonstrated by Rommerskirchen et al that a reduction in the energy demand by > 70% is achieved by stacking three cell pairs into the FCDI system. However, the planar dimension in most FCDI cells is limited to the centimeter level. − Thus, enormous effort has been applied to overcome such disadvantages. Yang et al reported that a parallel-type FCDI stack configuration equipped with five units is feasible for use in scaling water productivity.…”

Flow Electrode Capacitive Deionization (FCDI): Recent Developments, Environmental Applications, and Future Perspectives

“…There is a critical need to enhance the selectivity toward these useful species during the resource recovery process . By developing innovative electrodes and membranes, , manipulating the operational modes, , and controlling the in situ Faradaic reactions appropriately, , FCDI can achieve the removal and recovery of target ions with a higher selectivity compared to conventional CDI systems (Figure a). For instance, an extraordinary NH 4 + /Na + selectivity factor of 31 was observed, even when the initial concentration of NH 4 + was only one-third of that of the competing Na + , with this high selectivity obtained on use of potassium dititanate (K 2 Ti 2 O 5 ) based flow-electrodes operated under SC mode .…”