Phosphate removal from aqueous solution by electrochemical coupling siderite packed column

Grinding siderite with ferric sulfate to generate an active ferrous source for Cr(VI) reduction

Activated siderite, endowed with excellent properties, was simply prepared by co-grinding with Fe sulfate to enhance its high reducing ability for Cr(VI). Batch experiments were conducted to investigate the main affecting parameters, such as material ratio, pH, temperature, etc. The removal of Cr(VI) by activated siderite was completed within 4 h of the reaction. The activated siderite maintained a high removal effect of Cr(VI) within a wide pH range (3-9). Various analytical methods, including XRD, SEM/EDS, XPS, etc., were employed to characterize the samples and discover variations before and after the reaction. The Fe (Ⅱ) in activated siderite becomes highly active, and it can even be released from the solid phase in the mildly acidic liquid phase to efficiently reduce Cr(VI) and mitigate its toxicity. These findings introduce an innovative approach for activating various minerals widely distributed in nature to promote the recovery of the ecological system.

Iron-containing nanominerals for sustainable phosphate management: A comprehensive review and future perspectives

Adsorption, which is a quick and effective method for phosphate management, can effectively address the crisis of phosphorus mineral resources and control eutrophication. Phosphate management systems typically use iron-containing nanominerals (ICNs) with large surface areas and high activity, as well as modified ICNs (mICNs). This paper comprehensively reviews phosphate management by ICNs and mICNs in different water environments. mICNs have a higher affinity for phosphates than ICNs. Phosphate adsorption on ICNs and mICNs occurs through mechanisms such as surface complexation, surface precipitation, electrostatic ligand exchange, and electrostatic attraction. Ionic strength influences phosphate adsorption by changing the surface potential and isoelectric point of ICNs and mICNs. Anions exhibit inhibitory effects on ICNs and mICNs in phosphate adsorption, while cations display a promoting effect. More importantly, high concentrations and molecular weights of natural organic matter can inhibit phosphate adsorption by ICNs and mICNs. Sodium hydroxide has high regeneration capability for ICNs and mICNs. Compared to ICNs with high crystallinity, those with low crystallinity are less likely to desorb. ICNs and mICNs can effectively manage municipal wastewater, eutrophic seawater, and eutrophic lakes. Adsorption of ICNs and mICNs saturated with phosphate can be used as fertilizers in agricultural production. Notably, mICNs and ICNs have positive and negative effects on microorganisms and aquatic organisms in soil. Finally, this study introduces the following: trends and prospects of machine learning-guided mICN design, novel methods for modified ICNs, mICN regeneration, development of mICNs with high adsorption capacity and selectivity for phosphate, investigation of competing ions in different water environments by mICNs, and trends and prospects of in-depth research on the adsorption mechanism of phosphate by weakly crystalline ferrihydrite. This comprehensive review can provide novel insights into the research on high-performance mICNs for phosphate management in the future.

Basket anode filled with CaCO3 particles: A membrane-free electrochemical system for boosting phosphate recovery and product purity

Phosphorus (P) is often regarded as the primary stimulant for eutrophication, while its importance as a crucial life element is also well acknowledged. Given its future scarcity, P recycling from waste streams is suggested and practiced. Electrochemically mediated precipitation (EMP) is a robust and chemical-free process for P removal and recovery, yet it requires further developments. The first generation of the CaCO₃-packed electrochemical precipitation column successfully solved the problem of H⁺-OH^- recombination, achieving enhanced P removal efficiency with less energy consumption but suffering from low Ca-phosphate purity in recovered products. Herein, a new concept of a basket-anode electrochemical system is proposed and validated to prevent direct H⁺-OH^- recombination and enhance product purity. The CaCO₃ pellets packed basket anode alleviates the OH^- depletion by CaCO₃-H⁺ interaction and provides extra Ca²⁺ for enhanced P removal. The novel structure of the basket anode, by its derived acidic anode region and alkaline cathode region, completely avoids the precipitation of Ca-phosphate on the packed CaCO₃ and greatly facilitates the collection of high-quality Ca-phosphate product. Our results suggest that almost 100% of the removed P was in high-purity, highly crystalline Ca-phosphate on the cathode. The recovered products contained significantly more P (13.5 wt%) than in the previous study (0.1 wt%) at similar energy consumptions (29.8 kWh/kg P). The applied current density, pellets size, and influent P concentration were critical for P removal performance, product purity, and power consumption. We further demonstrated the long-term stability of this novel system and its technical and economic feasibility in treating real stored urine. Our study provides new cell architectural designs to enhance the performance of EMP systems and may inspire innovations and developments in other electrochemical water treatment processes.

In-situ production of iron flocculation and reactive oxygen species by electrochemically decomposing siderite: An innovative Fe-EC route to remove trivalent arsenic

The removal of trivalent arsenic (As (III)) from water has received extensive attention from researchers. Iron electrocoagulation (Fe-EC) is an efficient technology for arsenic removal. However, electrode passivation hinders the development and application of Fe-EC. In this work, an innovative Fe-EC route was developed to remove As (III) through an electrochemical-siderite packed column (ESC). Ferrous ions were produced from siderite near the anode, and hydroxide was generated near the cathode during the electrochemical decomposition of siderite. As a result, an effect of Fe-EC-like was obtained. The results showed that an excellent removal performance of As (III) (>99%) was obtained by adjusting the parameters (As (III) concentration at 10 mg/L, pH at 7, Na₂SO₄ at 10 mM and the hydraulic retention time at 30 min) and the oxidation rate of As (III) reached 84.12%. The mechanism analysis indicated that As (III) was oxidized to As (Ⅴ) by the produced active oxide species and electrode, and then was removed by capturing on the iron oxide precipitates. As (III) was likely to be oxidized in two ways, one by the reactive oxygen species (possibly •OH, Fe(IV) and •O₂^- species), and another directly by the anode. The long-term effectiveness of arsenic removal demonstrated that ESC process based on the electrochemical-siderite packed column was an appropriate candidate for treating As (III) pollution.

New insight into the changes in metal-phosphonate complexes from the addition of CaCO3 to enhance ferric flocculation for efficient phosphonate removal

Due to the stable chelating effect of organic phosphonates in wastewater, phosphonates with increasing emission are difficult to be removed effectively by traditional ferric salt flocculation, which has posed tough challenges for reducing total phosphorus pollution in recent years. In this work, calcium carbonate (CaCO3) was introduced to work together with the widely investigated flocculant of ferric chloride (FeCl3) to realize an efficient removal of nitrilotrismethylenephosphonic acid (NTMP) at much lower dosage of FeCl3. With an aid of synergy effect from together use of CaCO3 and FeCl3, the remaining concentration as low as 0.16 mg-P/L, far below the sewage discharge limit (0.5 mg-P/L), was simply obtained with a significantly reduced Fe/P molar ratio at only 4, resulting from calcium source donor to form more stable Fe-Ca-P tridentate bridging complexes, high affinity towards ferric ions on CaCO3 surface and slow-release alkaline from CaCO3. A comparison among sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)2) and CaCO3 as additives, was carried out to highlight the advantages of using CaCO3 and clarify the mechanism for the greatly improved performance by a set of characterizations including XRD, FTIR, Zeta potential, XPS, SEM-EDS and TG analyses. The addition of CaCO3 in ferric flocculation resulted in further obvious advantages such as 75% shortened settling time and only one-third of sludge volume of the precipitant, beneficial to the sample handling in engineering application. The proposed new approach has been further confirmed to work efficiently on real phosphonate-containing wastewater. Discussion on the interaction between CaCO3 and ferric salts in phosphonate solutions shed new insights into the working mechanism of using CaCO3 for the treatment of phosphonates-containing wastewater.

Enhanced electrochemical precipitation of phosphorus in wastewater by the addition of drifting Corbicula shells

Phosphorus (P) is a finite and essential resource, and its linear movement from mines to waste streams may result in shortages. This has encouraged efforts to recover P from sewage systems for reuse. This study developed a new electrochemical P precipitation system for the subnatant of the sludge flotation thickening process, in which drifting Corbicula shells are added to provide a supply of calcium ions (Ca²⁺) to promote P precipitation. However, adding Corbicula shells to coexisting suspended solids (SS) and coagulant resulted in adsorption of the shells in the neutralized and hydrophobized floc clusters, which limited their electrochemical dissolution. Adding Corbicula shells after SS removal by flotation with electrochemically generated gases resulted in their successful electrochemical dissolution, which enhanced phosphate-P removal. Increasing the amount of Corbicula shells enhanced the phosphate-P removal to a point, after which further addition simply increased Ca²⁺. The consumption of H⁺ generated near the anode for the dissolution of Corbicula shells increased the pH of the bulk solution, which enabled P precipitation not only onto the cathode but also in the bulk solution. Analysis of chemical composition in the generated particles suggests that they can be used as a slow P-release fertilizer and soil conditioner.

Electrochemically mediated precipitation of phosphate minerals for phosphorus removal and recovery: Progress and perspective

Phosphorus (P) is an essential element for the growth and reproduction of organisms. Unfortunately, the natural P cycle has been broken by the overexploitation of P ores and the associated discharge of P into water bodies, which may trigger the eutrophication of water bodies in the short term and possible P shortage soon. Consequently, technologies emerged to recover P from wastewater to mitigate pollution and exploit secondary P resources. Electrochemically induced phosphate precipitation has the merit of achieving P recovery without dosing additional chemicals via creating a localized high pH environment near the cathode. We critically reviewed the development of electrochemically induced precipitation systems toward P removal and recovery over the past ten years. We summarized and discussed the effects of pH, current density, electrode configuration, and water matrix on the performance of electrochemical systems. Next to ortho P, we identified the potential and illustrated the mechanism of electrochemical P removal and recovery from non-ortho P compounds by combined anodic or anode-mediated oxidation and cathodic reduction (precipitation). Furthermore, we assessed the economic feasibility of electrochemical methods and concluded that they are more suitable for treating acidic P-rich waste streams. Despite promising potentials and significant progress in recent years, the application of electrochemical systems toward P recovery at a larger scale requires further research and development. Future work should focus on evaluating the system's performance under long-term operation, developing an automatic process for harvesting P deposits, and performing a detailed economic and life-cycle assessment.

Influences of electrode distance and electrolysis time on phosphorus precipitation and composition during electrolysis of anaerobic digestion effluent

Global demand for phosphorus (P) is increasing, which has led to concerns over future drought and has driven efforts to recover P from wastewater streams for reuse. In this study, platinum-coated titanium electrodes were applied to the electrochemical precipitation of P from anaerobic digestion effluent that was collected from a domestic wastewater treatment plant. The influence of the electrode distance on P removal and precipitation was investigated. In addition, the influence of the electrolysis time on the chemical structure and composition of the P precipitate was evaluated from the viewpoint of utilising the P precipitate as fertiliser. Regardless of the electrode distance (10, 5 and 1 mm), PO₄^3-, Ca²⁺ and Mg²⁺ were removed. The bulk solution pH increased during electrolysis because of the consumption of generated H⁺ as HCO₃^- transitioned to H₂CO₃ near the anode. A greater increase in the bulk solution pH was observed when the electrode distance was narrowed because of the enhanced H⁺ consumption. Narrowing the electrode distance reduced the energy consumption for P precipitation. The increase in the bulk solution pH with the narrowing electrode distance changed the dominant P precipitation pathway from onto the cathode to in the bulk solution. X-ray diffraction spectra of the precipitates showed that increasing the electrolysis time transformed amorphous P to hydroxyapatite and struvite. Most P existed in a citric acid-soluble form, which is recommended for use as a slow P release fertiliser. There were no significant changes in the citric acid-soluble P content of the precipitates with increasing electrolysis time. Therefore, increasing the electrolysis time has little influence on the suitability of the precipitate as a slow P release fertiliser.