Various industrial surface materials are tested for their photocatalytic self-cleaning activity by performing the ISO 27448:2009 method. The samples are pre-activated by UV irradiation, fouled with oleic acid and irradiated by UV light. The degradation of oleic acid over time is monitored by taking water contact angle measurements using a contact angle goniometer. The foulant, oleic acid, is an organic acid that makes the surface more hydrophobic. The water contact angle will thus decrease over time as the photocatalytic material degrades the oleic acid. In this study, we argue that the use of this method is strongly limited to specific types of surface materials, i.e., only those that are hydrophilic and smooth in nature. For more hydrophobic materials, the difference in the water contact angles of a clean surface and a fouled surface is not measurable. Therefore, the photocatalytic self-cleaning activity cannot be established experimentally. Another type of material that cannot be tested by this standard are rough surfaces. For rough surfaces, the water contact angle cannot be measured accurately using a contact angle goniometer as prescribed by the standard. Because of these limitations, many potentially interesting industrial substrates cannot be evaluated. Smooth samples that were treated with an in-house developed hydrophilic titania thin film (PCT/EP2018/079983) showed a great photocatalytic self-cleaning performance according to the ISO standard. Apart from discussing the pros and cons of the current ISO standard, we also stress how to carefully interpret the results and suggest alternative testing solutions.

The subsurface is increasingly being recognized as a multifunctional resource supporting key activities in the transition towards a sustainable society (e.g. geothermal energy, CO ₂ storage and groundwater extraction). In existing literature, there remains a lack of integrative, cross-disciplinary research addressing the interactions and trade-offs between these activities. Addressing this gap is essential for sustainably managing the subsurface. This paper introduces the DIAMONDS Elevator, a conceptual framework designed to support such collaboration by organizing subsurface decision making across interconnected levels and facilitating knowledge exchange between disciplines. The framework is developed within the DIAMONDS research initiative and is applied to a case study on the multifunctional use of the subsurface in Flanders, focusing on interactions between aquifer thermal energy storage and groundwater extraction. Through this case study, we demonstrate how the DIAMONDS Elevator can be used to structure interdisciplinary analyses and decision-making processes, illustrated in detail from the perspective of an economics researcher. In addition, we position integrative methods and tools, including principles, criteria and indicators, within the Elevator framework. This paper shows how structured interdisciplinary approaches can enhance transparency, support scenario development and inform more sustainable subsurface governance. The DIAMONDS Elevator, as a visual guideline, intends to support interdisciplinary collaboration for sustainable subsurface management.

Geosystem services (GS) play an important role in the energy and climate transition. Aquifer thermal energy storage, geothermal energy, (seasonal) gas storage and even storage of nuclear waste are all activities derived from GS that can help humanity move towards climate neutrality. As with all ecosystem services, GS need to be used sustainably and fairly. Overuse of GS to speed towards climate neutrality could exhaust these essential services and place a debt on the future.The interdisciplinary DIAMONDS project: Dynamic Integrated Assessment Methods fOr the sustainable Development of the Subsurface [Compernolle et al., 2023] aims for holistic planning of the subsurface with researchers looking at sustainability from (hydro)geological, engineering, economics, sociological and environmental perspectives. A Principles, Criteria & Indicators (PC&I) framework is developed as a decision support system to incorporate these different views, different tools to deal with uncertainty and the different values at play regarding the sustainable use of GS.A PC&I is a hierarchical framework consisting of three levels. The first level, the principles, encompasses the universal values that determine sustainability. These are established via a two-round Delphi survey, consulting experts until consensus is reached. The second level consists of criteria which are measurable conditions for the level of applicability of the principle. In this project, the criteria are derived through expert interviews and a focused literature study. Afterwards they are validated and given weight to with an expert survey. To describe the characteristics of the real situation and benchmark against the criteria, indicators are defined at the third level. The information from the involved disciplines is used to create the integrated framework and the framework feeds back into the research by setting some boundaries and specific subjects to measure, model and analyse. The information flows back and forth between the disciplines in the shape of stakeholder workshops, (hydro)geological models, techno-economic assessments, life cycle assessments, real options games and causal loop diagrams.With this comprehensive decision support system, we hope to guide decision makers towards a sustainable development of the subsurface, helping the energy and climate transition without mortgaging the possibilities for future generations to make use of GS.Compernolle, T.; Eswaran, A.; Welkenhuysen, K.; Hermans, T,; Walraevens, K.; Camp, M.; Buyle, M.; Audenaert, A.; Bleys, B.; Schoubroeck, S.; Bergmans, A.; Goderniaux, P.; Baele, J.; Kaufmann, O.; Vardon, P.; Daniilidis, A.; Orban, P.; Dassargues, A.; Serge, B.; Piessens, K. Geological Society Special Publication (2023) 528, 101-121, DOI: 10.1144/SP528-2022-75