Effective and Selective Removal of Phosphate from Wastewater Using Guanidinium-Functionalized Polyelectrolyte-Modified Electrodes in Capacitive Deionization

Abstract

The potential phosphorus shortage along with phosphorus pollution has drawn global attention, calling for effective technologies for phosphorus removal and recovery from wastewater. Capacitive deionization (CDI) is a promising technology for salt removal and nutrient recovery. However, their performances suffer from limited ion storage capacity and low selectivity, e.g., phosphate removal was lower than the other salts. Herein, guanidinium-functionalized polyelectrolyte-coated carbon nanotube (Gu-PAH/CNT) electrodes were prepared through the self-assembly technique. The Gu-PAH/CNT electrodes selectively adsorbed phosphate ions and prevented the repulsion of coions due to the combined effect of the ion-selective polyelectrolyte layer and the NH protons of Gu groups, resulting in strong electrostatic and hydrogen-bond interactions with phosphate ions. The Gu-PAH/CNT anode was coupled with a COOH-CNT cathode for selective capture of phosphate in a CDI device. The phosphate ion adsorption capacity of Gu-PAH/CNT electrodes is up to 23−30 mg PO 4 3− g −1 in a phosphate solution with a wide pH range and mixtures (NaH 2 PO 4 /NaCl, NaH 2 PO 4 /Na 2 SO 4 , and NaH 2 PO 4 /NaNO 3 ) at 1.2 V, presenting superior electroadsorption capacity, phosphate selectivity, and stable regeneration property. These encouraging results demonstrate that functionalized polyelectrolyte-modified electrode materials act as powerful platforms for effective and selective removal and recovery of specific contaminants from wastewater.

Selective Phosphorus Removal from Wastewater Using Graphene Aerogel Loaded with Ferrocene-Polyaniline: Synergetic Adsorption and Electrochemically Mediated Oxidation

“…The Fc-PANI 1:3 @NGA achieved a phosphite ion adsorption capacity of 24 mg HPO 3 2– g –1 . In comparison, the adsorption capacity of this cell for the phosphate ion was 29 mg PO 4 3– g –1 (Figure S5), significantly higher than that of the reported CDI/MCDI systems under similar conditions (∼2.4–25 mg g –1 ). − Figures a and S5 show that the adsorption equilibrium time of the Fc-PANI 1:3 @NGA to phosphate was longer than that of phosphite, indicating that the Fc-PANI 1:3 @NGA had a faster adsorption rate to phosphite, because the electronegativity and mobility of phosphite (HPO 3 2– ) might be greater than that of phosphate (coexistence of H 2 PO 4 – and HPO 4 2– ) at pH 6.5. Notably, during the desorption stage, the release amount of phosphite ion was obviously less than the adsorption capacity, and a significant fraction (49.5%) of phosphite was present as phosphate in the solution, suggesting that phosphite has been oxidized to phosphate at applied voltages of 1.2 and −1.2 V. For phosphate, applying a voltage of −1.2 V in this cell for 30 min led to the complete release of the adsorbed phosphate ions from the Fc-PANI 1:3 @NGA.…”

“…The selectivity of these anions by the Fc-PANI 1:3 @NGA electrode decreases in the following order: phosphite/phosphate > SO 4 2– > NO 3 – > Cl – . The selectivity of phosphite/phosphate over chloride (infinite), phosphite/phosphate over sulfate (6.8/8.5), and phosphite/phosphate over nitrate (10.86/11.4) was higher than that of the reported CDI/MCDI cells and the previously developed Gu-PAH/CNT electrode (P/Cl selectivity of 0.8–4, P/SO 4 selectivity of ∼1.5, P/NO 3 selectivity of ∼2.5). − Continuous adsorption–desorption cycle experiments were performed using the voltages of 1.6 and −1.6 V for phosphite and the voltages of 1.2 and −1.2 V for phosphate. The results show that the adsorption capacities of phosphite and phosphate ions slightly decreased after 10 cycles, which might be explained by a small amount of ions entrapped/bonding in the Fc-PANI 1:3 @NGA or by a decrease of the available active surface due to the detachment of a small portion of the poorly bonded Fc-PANI 1:3 @NGA (Figure d).…”

Phosphorus Recovery from Eutrophic Lake Water Using Al/Fe Micro-Electrolysis

“…Iron/aluminum (Fe/Al) micro-electrolysis can generate Fe/Al ions and positively charged colloidal hydroxides, which can precipitate, adsorb, or flocculate dissolved and particulate matters. − With its advantages of low cost, high efficiency, and easy operation, Fe/Al micro-electrolysis has been widely applied to concentrate and harvest high-value resources from wastewater. − For instance, Fe micro-electrolysis was used to recover rare-earth elements from wastewater . In eutrophic lake water, P exists in both dissolved and particulate forms after uptaking by microalgae or absorption by particles .…”

Highly Selective Recovery of Phosphorus from Wastewater via Capacitive Deionization Enabled by Ferrocene-polyaniline-Functionalized Carbon Nanotube Electrodes

“…The discharge of phosphorus is the main cause of eutrophication of water bodies, and the removal and recovery of phosphorus govern the key to controlling the rampant pollution of water and achieving a sustainable use of phosphorus resources. , Numerous technologies based on physical, chemical, or biological processes have been proposed and developed to remove or recover phosphorus from wastewater, − among which capacitive deionization (CDI) has drawn increasing attention as an emerging energy-efficient and chemical-free technology for phosphorus recovery. − In a typical CDI process, the charged species, under the applied electrical field, are captured in the electric double layers formed at the electrode surface, which can be released afterward by short-circuiting or reversing the voltage …”