In 2022, the last year of my B.Sc. in Industrial Chemistry at the University of Beira Interior, I had to choose a topic for my undergraduate thesis. I joined the Environmental Electrochemistry Laboratory, which is part of the Fiber Materials and Environmental Technologies (FibEnTech) research unit, to conduct my research on electrochemical phosphorus recovery from wastewater. I once read a book called “Sapiens – A Brief History of Humankind”, by Yuval Noah Harari, where I learned about the critical effect of the First Agricultural Revolution (10,000 – 8,000 BCE) on human population growth. Likewise, the beginning of fertilizer synthesis in the 20^th century was also relevant, since it allowed increasing agricultural production, so I thought that this was a nice topic. Indeed, fertilizers are very needed worldwide. Some of the key components are nitrogen and phosphorus: nitrogen is taken from atmospheric air, but phosphorus is sourced from phosphate rock, an abundant ore in Morocco. Beyond the well-known fact that phosphate rock mining contributes to water contamination by phosphorus and heavy metals, concerns about the eventual depletion of this non-renewable resource gained prominence in the late 20^th century, raising the question of when the reserves might be exhausted. Projections vary widely, with estimates spanning from 2043 to 2450. This prompted intensified interest in identifying alternative phosphorus sources, and, as a result, phosphorus recovery from waste streams emerged as a practical strategy for sustaining fertilizer production.

My first task was to learn about the topic, which I did by reading two excellent review articles: one from Yicheng Wang and co-authors published in Water Research (vol. 209, 2022, article 117891), the other from Yifan Ren and co-authors published in Environmental Functional Materials (vol. 1, 2022, pages 10 – 20). These papers were my first contact with phosphorus recovery topic and were extremely helpful for me. I also remember wondering how challenging it should be to write good review articles like those. Some months later, I successfully defended my thesis, and after that, I started the Ph.D. in Chemistry at the same university. I preferred to continue my work on phosphorus recovery, so it was time to conduct a literature review – more robust than that conducted during the undergraduate thesis – in order to well define the main objectives of my work. I followed my advisor’s suggestion of well organizing the review process, so it would be possible to publish a review article.

The aims of the review were to consolidate the current knowledge on phosphorus recovery and to identify research gaps – I should select which of them I would like to address during my Ph.D. We have found a wide range of technologies developed to enable the effective recovery of phosphorus from wastewater. Compared to biological processes, chemical and electrochemical methods offer several advantages, including higher recovery efficiencies, faster operation, lower sensitivity to environmental variability, and the possibility of implementation in modular configurations that require less space. Therefore, we focused the discussion on these approaches, with particular attention to how operational parameters influence process performance. We then select four main parameters: initial pH, critical ion concentration, magnesium source, and applied current or voltage. Regarding the research gaps, we have identified some points that, if addressed, could significantly advance phosphorus recovery technologies. Among them, two stand out as especially relevant to phosphorus precipitation: the application of electrochemical methods using liquid magnesium sources (such as seawater) and the development of affordable magnesium anodes.

Struvite (magnesium ammonium phosphate hexahydrate) is a slow-release fertilizer that can be obtained from phosphorus and nitrogen in wastewater. Because these nutrients are key drivers of eutrophication, their simultaneous removal is a major advantage of struvite-based recovery. However, struvite precipitation requires an adequate supply of magnesium, which is typically insufficient in wastewater. As a result, external magnesium dosing is needed, accounting for up to 75% of total recovery costs. Seawater offers a low-cost source of magnesium, making it an attractive option for coastal areas. Yet, its application generally requires adjusting the bulk pH to 9.0–9.5, the optimal range for struvite formation. An electrochemical approach could circumvent this need by increasing the pH locally at the cathode, thereby promoting struvite precipitation directly on its surface. The key question is whether raising the local pH electrochemically is more advantageous than chemically adjusting the bulk pH. Moreover, electrochemical precipitation often yields solids of higher purity due to localized formation, but it remains unclear whether this benefit persists when using seawater. Further investigation is required to address these uncertainties.

Another strategy for inducing struvite precipitation involves the use of magnesium anodes, which may consist of pure magnesium or magnesium alloys, most commonly magnesium–aluminum–zinc alloys. Although pure magnesium anodes offer certain performance advantages, their cost is prohibitive for large-scale phosphorus recovery. Magnesium alloys, in contrast, are cost-comparable to conventional magnesium salts. A promising avenue to further enhance circularity and reduce costs is the development of anodes derived from industrial byproducts, such as magnesite. While this approach has been explored using simulated wastewater, additional studies with real wastewater are needed to clarify its implications for solid purity and long-term operational performance.

A detailed discussion of operational parameters and research gaps can be found in the full open-access article. Some of the research gaps were included in my Ph.D. objectives, while some of them will certainly be investigated by other researchers. Sharing knowledge is the key for converting research ideas into scientific findings and then into real-world applications.