Evgenios Kokkinos

Membranes produced by 3D printing represent a groundbreaking technology for industrial and municipal wastewater treatment, enabling the fabrication of complex, customized architectures tailored to specific environmental challenges. Compared to conventional membranes fabricated via phase-inversion, 3D printing alternatives offer superior pore control and active catalytic functionality despite the higher initial costs. This review demonstrates how material selection and 3D printing methodologies establish improved structural and geometrical characteristics in the resulting membranes. Furthermore, it discusses the suitability of various membrane fabrication approaches relative to the specific demands of each application. Specifically, material selection—spanning polymers, ceramics, biopolymers, and nanocomposites—is discussed in light of printability requirements dictated by ink rheology, resolution, and post-processing across both light- and extrusion-based methods. Practical applications consider factors such as surface hydrophilicity tuning, pollutant affinity, structural durability, and the mitigation of microplastic release. In addition, leveraging biomimetic and minimal surface geometries can optimize hydrodynamics, reduce module size, and generate local fluid shear to suppress biofouling. A summary of reported case studies highlights the strong potential of 3D-printed membranes in applications such as membrane bioreactors, oil–water emulsion separations, organic dye degradation, and the removal of persistent micropollutants like PFAS. Finally, emerging trends—including 4D stimuli-responsive materials, AI-driven fluid dynamics, and upcycled green inks—are expected to guide 3D-printed membrane technology toward commercial scalability, particularly for decentralized water treatment solutions.

A major limitation for the wider use of membrane-based technologies is the presence of biofouling, which is related to the decline of permeate flux, as well as the associated energy and economic costs for the necessary cleaning. In this work, the interactions and compatibility of 28 common polymeric materials with 36 potential biofoulants (categorized in six groups) is examined, based on Hansen Solubility Parameters (HSPs). Also, a simple methodology is proposed for polymer screening and comparing the suitability of 28 polymers to be used as fabrication materials or coatings, aiming to produce membranes with lower biofouling potential. The methodology gives a score to each polymer based on its interaction with water and various foulants. The screening among the commonly used polymers showed that poly (vinyl alcohol) (PVOH) is a good selection for the manufacturing of membranes, or for effective surface coating to limit biofouling, when compared to the other candidate polymers. The case of PVOH material received the highest score (11.6), while other polymers ranked with lower scores (less than 10). Its physically cross-linked nature that arises from a strong self-association pattern may also be beneficial for biofouling mitigation, since it limits the available sites for interactions (e.g., through hydrogen bonds) with the potential foulant agents. Swelling experiments on the PVOH gels with real wastewater (produced after anaerobic digestion) support the predictions for lowering the biofouling potential.

The dimensions and the reduction capacity of Fe₃O₄ nanoparticles are considered to be the key parameters in achieving the successful, efficient removal of hexavalent chromium, aiming for drinking water purification. This research study focuses on the optimization of reaction parameters during the oxidative precipitation of FeSO₄ carried out in a microwave-heated plug-flow reactor, to realize the preparation of Fe₃O₄ nanoparticles with an increased reduction potential as reflected in the Fe²⁺/Fe³⁺ ratio by approximating the ideal value of 0.5. In particular, the coupling of synthesis with features that allow for control of the oxidation extent, and include the addition of a reducing agent, an increase in ageing temperature, and inhibition of aggregation, were tested as potential approaches to tune the reducing potential and overcome reported Cr(VI) capture efficiencies provided by Fe₃O₄ nanoparticles. The evaluation results showed that adding a reductant after nanoparticle formation inhibits spontaneoussurface oxidation, bringing an improvement in the Cr(VI) uptake capacity for a residual concentration equal to the new EU regulation limit, by around 40%, reaching a value of 2.15 mg/g. However, working at an ageing temperature of around 100 °C resulted in an even better performance with an uptake increase of 120% and a capacity value of 3.45 mg/g. Finally, adding nanoparticles in the form of a dispersion instead of a dried powder provides an extra 10% improvement as a consequence of limited aggregation.

Waste valorization involves reusing and recycling waste materials to create useful products such as materials, chemicals, fuels, or energy. The primary goal is the transition to a circular economy model while minimizing the impacts of hazardous waste. Adopting such policies appears to be a one-way path due to the continuous increase in the consumption of raw materials. According to recent projections, by 2050, 180 billion tonnes of materials will be consumed annually. Since natural resources cannot meet these requirements, new sources must be explored. Waste can serve as an alternative source and cover at least part of the needs that arise. In this work, good practices regarding waste valorization are presented. The case studies examined include the waste/by-products of ultrabasic rocks resulting in chromite and magnesite mining, as well as the tannery sludge produced after the corresponding wastewater treatment.

Developing an efficient recycling route for spent single-use medical devices is essential for recovering precious metals. The proposed complete hydrometallurgical route goes through the initial pyrolysis and acid digestion steps, expanding upon our previous relevant work in the field, followed by solvent extraction, stripping, and precipitation procedures. In this study, a complete hydrometallurgical process was developed for the recovery of gold, platinum, iridium, and tantalum, separating them from other metals, i.e., from iron, chromium, and nickel, also present in the examined medical devices, i.e., (i) diagnostic electrophysiology catheters, containing gold, (ii) diagnostic guide wires, containing platinum and iridium alloys, and (iii) self-expanding stents, containing tantalum. This study reports the experimental results of selecting an efficient extractant, stripping, and precipitation agent, along with the effects of key factors that influence each consecutive step of the process, i.e., agent concentration, aqueous to organic phase ratio, contact time, and pH, using simulated metal solutions and also applying the obtained optimal conditions to the treatment of real sample solutions. For the selective separation of gold, Aliquat 336 was used to extract it in the organic phase; it was then stripped using a thiourea solution and precipitated by utilizing an iron sulfate (II) solution and proper pH adjustment. The selective separation of platinum was achieved by using Aliquat 336 for the organic phase extraction and a perchlorate acid solution for stripping it back into the aqueous solution and adding a sodium bromate solution to precipitate it. Due to the similar chemical behavior, the selective recovery of iridium followed the same processes as that of platinum, and the separation between them was achieved through selective precipitation, as heating the solution and adjusting the pH value resulted in the selective precipitation of iridium. Lastly, the selective recovery of tantalum consists of extraction by using Alamine 336, then stripping it back to the aqueous phase by using sodium chloride, and precipitation by using potassium salt solution and proper pH adjustment. A total recovery of 88% for Au, 86% for Pt, 84% for Ir, and 80% for Ta was obtained, thus achieving a high uptake of precious metals from the examined real spent/waste samples.

Chromite is formed in nature in ophiolitic layers and ultrabasic rocks through fractional crystallization. The corresponding mining technologies separate the ore from these ultrabasic rocks, which are considered to be tailings for the process but may be valorized in other applications. The need to utilize this material is due to the large quantities of its production and the special management required to avoid possible secondary pollution. In the present work, the ultrabasic rocks of chromite mining were applied to acid mine drainage (AMD) neutralization. The aim was to increase the technological maturity of the method and promote circular economy principles and sustainability in the mining sector. Ultrabasic rocks were obtained from a chromite mining facility as reference material. Furthermore, an artificial AMD solution was synthesized and applied, aiming to simulate field conditions. According to the results, the sample was successfully utilized in AMD neutralization (pH 7), achieving rapid rates in the first 30 min and maximum efficiency (liquid to solid ratio equal to 8.3) at 24 h. However, the method presented a drawback since Mg was leached, even though the concentration of other typical metals contained in an AMD solution decreased.

Due to their unique properties, i.e., fluoroscopy response and inertness, noble metals and alloys are present in several widespread medical–technological products, such as catheters, guide-wires, and stents. Despite their value, these products serve as single-use consumables, following a fate of solid waste disposal and loss of their valuable metals. This work studies the development of a treatment methodology to recover noble metals such as Pt, Ir, Au, and Ta from certain commercial products commonly used for medical practices. In particular, a sequence of preliminary pyrolysis, aiming at polymer elimination, as well as an acid digestion step for selective metals dissolution, is suggested. Pyrolysis was capable of enriching samples with the targeted metals, though a small change in their oxidation states was observed. Still, acid digestion was fully able to successfully separate Au using a 50% v/v aqua regia solution for 30 min at room temperature and the Pt/Ir using concentrated aqua regia for 72 h under heating. Dissolution of Ta required a different leaching solution, i.e., a 50% v/v HF/H₂SO₄ mixture for 10 h under heating. According to the developed method, selective extraction of such noble metals in a concentrated slurry provides a high potential for the complete recovery and valorization of otherwise disposed medical wastes.

Chromite ore occurs mainly in ophiolitic mineral complexes and within ultrabasic rocks. As a result, the mining and enrichment processes applied to this ore lead to the production of large amounts of ultrabasic rocks, considered to be a waste stream. These wastes are mainly various structures of olivine and serpentine (i.e., hydrous structures of olivine). The ultrabasic rocks may be considered a by-product of this process, following the framework of the circular economy, but the presence of serpentine degrades their quality. In this work, it was proven that the thermal treatment of ultrabasic rock samples may eliminate the serpentine content.

Noble metals (Pt, Ir, Au, Ta, etc.) have found several applications in specific medical technology products since they present inertness and visibility when examined via fluoroscopy by an external observer/operator. These products are usually applied as single-use materials and, as a result, the corresponding waste streams may be quite rich in valuable metals. In this work, it was proven that noble metals can be recovered from such waste streams. Initially, pyrolysis may completely remove the plastic covering of medical products. In addition, selective dissolution of noble metals can be achieved by applying different inorganic acids or their mixtures.