Is Photocatalysis Ready for Scale Yet?

Abstract

Despite being frequently proposed as a low-carbon solution for wastewater treatment and solar fuel production, the feasibility of photocatalytic processes in large-scale deployments remains unclear. This review evaluates the scalability of photocatalytic technologies by synthesizing a decade (2015–2025) of techno-economic analysis (TEA) and life-cycle assessment (LCA) studies. Using a systematic search and programmatic screening, 77 assessment-focused publications were identified from an initial corpus of 854 studies. Across applications, TEA and LCA consistently highlight two dominant barriers to scale-up: high electricity demand in UV-driven systems and significant cradle-to-gate impacts associated with catalyst synthesis, particularly for nanostructured materials. When solar irradiation replaces artificial light, environmental and economic hotspots shift from energy use to material production, catalyst durability, and reuse assumptions. Wide variability in reported costs and impacts reflects heterogeneous methodologies, limited pilot-scale data, and a lack of standardized reporting. Overall, assessment-based evidence indicates that photocatalysis is not yet ready for widespread industrial deployment as a large industrial process. However, continuous advances in solar-driven reactor design, low-impact and circular catalyst synthesis, hybrid process integration, and harmonized TEA/LCA frameworks could substantially improve its prospects for scalable, climate-positive implementation, especially in the context of emerging green energy alternatives.

1. Introduction

Rapid industrialization and the global rise of emerging contaminants have intensified the demand for water-treatment and resource-recovery technologies. Conventional physicochemical and biological processes often struggle to meet increasingly stringent discharge standards for persistent pollutants [1], leading to growing interest in Advanced Oxidation Processes (AOPs). Among these, photocatalysis has gained prominence for its ability to mineralize recalcitrant compounds and drive solar-to-chemical energy conversion pathways such as hydrogen production or photoreforming [2,3,4,5].

Despite decades of laboratory success, the large-scale implementation of photocatalysis remains limited [6,7]. It is important to note that while photocatalysis is commercially mature in surface applications, such as self-cleaning glass and antimicrobial coatings [8], these represent passive material functionalities rather than the active volumetric processes required for water treatment and energy conversion. Consequently, most studies in this field consider idealized conditions, such as high UV irradiance, simplified aqueous matrices, and batch configurations, which in most cases do not reflect large-scale practical applications [9]. This “scaling gap” introduces large uncertainties in predicting industrial viability, as performance is strongly influenced by reactor hydrodynamics, natural solar variability, catalyst stability, and mass transfer limitations [6,7,10], which dominate both operational and cradle-to-gate impacts [11]. As a result, it remains unclear whether photocatalytic technologies can compete with established treatment and renewable energy processes.

Techno-economic analysis (TEA) and Life Cycle Assessment (LCA) have emerged as essential tools for assessing the scalability and sustainability of industrial processes. However, the existing TEA and LCA literature in the field of photocatalysis is highly heterogeneous: studies differ substantially in system boundaries, functional units, operational assumptions, catalyst lifetimes, and reactor configurations. These divergences, together with limited pilot-scale data and scarce reporting of life-cycle inventories and photonic metrics, make it difficult to determine the true readiness of photocatalysis for industrial deployment [12,13]. The urgency of integrating environmental and economic metrics is further amplified by increasingly stringent regulatory frameworks. Policies such as the EU Water Framework Directive, the revised Municipal Wastewater Treatment Directive, and global commitments to decarbonization and circular economy targets necessitate the adoption of technologies that are not only effective but also demonstrably sustainable [14,15]. Photocatalytic systems are frequently promoted as low-carbon alternatives; however, without rigorous evidence of LCA and TEA, their contribution to meeting these regulatory and policy objectives remains uncertain [12,13]. Establishing clear sustainability frameworks is therefore crucial to guiding research priorities and supporting future industrial deployment.

Addressing these gaps, this review synthesizes a decade (2015–2025) of techno-economic analysis and life-cycle assessment studies to evaluate whether photocatalytic technologies are ready for large-scale deployment. Rather than reviewing photocatalytic materials or reactor concepts in isolation, the manuscript focuses exclusively on assessment-based evidence across the full photocatalytic domain, encompassing both wastewater treatment and solar fuel production.

2. Methodology

This scoping review was conducted through a multi-stage process involving a systematic literature search followed by a semi-automated, programmatic analysis to screen, categorize, and thematically analyze the resulting dataset. The workflow was designed to ensure reproducibility and to efficiently manage a large volume of literature.

2.1. Search Strategy and Data Extraction

A systematic search was performed on the Scopus database on 28 October 2025, to identify relevant peer-reviwed publications, excluding grey literature and preprints. The search was designed to capture studies focusing on the intersection of photocatalysis for water treatment and its techno-economic or life-cycle assessments. The following search string was used, targeting the title, abstract, and keywords of the articles:

• TITLE-ABS-KEY((photocataly* AND (water OR wastewater OR effluent*))
• AND (("techno-economic" OR "cost analysis" OR "economic feasibility")
• OR ("life cycle assessment" OR "LCA" OR "environmental impact")))

The initial search, limiting the publications to a 10-year window (2015–2025), yielded 854 publications. The complete metadata for these records, including authors, title, year, source, affiliations, abstract, and author keywords, were exported in CSV format for subsequent analysis.

2.2. Screening and Prioritization Framework

To facilitate a structured and efficient screening of the extensive dataset, an approach was developed using Python (v3.9) with the Pandas library for data manipulation [16,17]. This framework was used for both the initial tiered prioritization and for a subsequent supplementary analysis focused on scalability.

2.2.1. Data Cleaning and Preprocessing

The raw dataset was first loaded and preprocessed to ensure data quality. Duplicate entries, identified based on identical titles, were removed, retaining the first occurrence. Missing values in abstract, title, and affiliation fields were replaced with empty strings to prevent errors during text processing. Additionally, the geographic origin of each publication was determined by programmatically parsing the “Affiliations” column to extract unique country names for each entry, accounting for publications with international co-authorship.

2.2.2. Keyword-Based Relevance Scoring and Tiered Prioritization

The core of the framework is a keyword-based scoring system designed to quantify the relevance of each publication. Keywords were grouped into eight distinct categories: economic assessment, environmental assessment, core process, application context, contaminant type, specific catalyst, energy source, and major product. The keywords are listed in

Table 1. Keyword Categories Used for Programmatic Relevance Scoring.

Category Name	Keywords (Case-Insensitive)
Economic Assessment	techno-economic, technoeconomic, cost analysis, economic feasibility, economic assessment, economic viability, cost-effectiveness, cost-effective, operating cost, production cost
Environmental Assessment	life cycle assessment, LCA, life-cycle assessment, environmental impact, environmental assessment, carbon footprint, global warming potential, GWP, ecotoxicity, sustainability assessment
Core Process	photocatalysis, photocatalytic, photodegradation, photo-Fenton, photoelectrochemical, photo-oxidation
Application Context	wastewater, water treatment, water purification, effluent, remediation, decontamination, water reuse
Contaminant Type	dye, textile, pharmaceutical, antibiotic, pollutant, contaminant, microplastic, pesticide, phenol
Specific Catalyst	TiO2, titanium dioxide, Zn0, zinc oxide, g-C3N4, carbon nitride, MOF, metal-organic framework, nanoparticle, nanocomposite, mxene
Energy Source	solar, sunlight, UV, visible light
Major Product	hydrogen, H2, H2O2, hydrogen peroxide

. A relevance score was calculated for each paper by scanning its title and abstract for these keywords. To reflect the greater importance of terms appearing in the title, a weighting system was applied: keywords found in the title or in the authors’ keywords contributed a weight of 3 to the score, while keywords in the abstract contributed a weight of 1. A Total Relevance Score (TRS) was computed by summing the scores across all categories. The weighting scheme was designed to reflect the informational hierarchy typically observed in bibliometric and text-mining analyses [18].

To manage the large volume of literature and focus the review on the most pertinent studies, a four-tiered screening strategy was implemented. The highest priority, Tier 0, was designed to capture papers explicitly focused on the review’s core questions; a publication was assigned to this tier if it contained one or more keywords from the economic assessment or environmental assessment categories within its title or author-provided keywords. The subsequent tier, Tier 1, consisted of the remaining papers that met a broader high-relevance criterion, containing at least one assessment-related keyword in their title, author keywords, or abstract. These papers were considered highly relevant but potentially discussing the assessments in less detail. Following this, Tier 2 was populated with articles that did not qualify for the first two tiers but exhibited a high overall relevance, defined as having a TRS within the top 30th percentile of the dataset; these articles were identified as likely sources of important technological and contextual background. Finally, all remaining publications were assigned to Tier 3, a low-priority group that was de-prioritized for manual screening.

The papers from Tier 0 were analyzed individually and in full by the authors, checking which papers included TEA and/or LCA investigations or discussions explicitly. This resulted in a collection of 77 documents that were analyzed in depth.

2.3. Thematic Analysis via Topic Modeling

To identify latent research themes within the literature corpus in an unsupervised, data-driven manner, we employed Latent Dirichlet Allocation (LDA), a generative probabilistic topic model [19]. The analysis was performed using the Python libraries Gensim and NLTK [20,21]. The combined text from titles and abstracts was preprocessed by removing common English stop words and domain-specific terms (e.g., “photocatalysis”, “wastewater”) that were too frequent to aid in topic differentiation. Words were then lemmatized to their root form. An LDA model was trained on the resulting corpus to identify 7 distinct topics. Each paper in the dataset was subsequently assigned a dominant topic based on the highest contribution score, providing a thematic map of the research landscape.

3. Results

3.1. Overview of the Analytical Dataset

A quantitative overview of the assessment-oriented photocatalysis literature identified in this review reveals that techno-economic and life-cycle assessments represent only a very small fraction of overall photocatalysis research, and that the final assessment-focused corpus is both geographically diverse and thematically distinct from the broader materials-dominated literature.

The initial search returned a total of 854 publications. A simple search for photocatalysis (TITLE-ABS-KEY((photocataly* AND (water OR wastewater OR effluent*))) returned 84,903 documents—which gives us a panorama of the relative minor role of TEA and LCA studies in this field, responding to almost 1% of its research output. However, this initial dataset still included irrelevant documents, so it went through a screening procedure to be progressively refined according to increasing methodological relevance.The process eliminated duplicates, off-topic papers, and those lacking explicit techno-economic or life-cycle analysis components. The screening funnel narrowed the dataset from 854 initial records (849 unique papers) to 200 (Tier 3), 49 (Tier 2), 439 (Tier 1), 161 (Tier 0), and ultimately 77 core publications selected for detailed analysis. These papers form the analytical core of this review, as they explicitly evaluate the scalability or sustainability of photocatalytic systems through quantitative assessment frameworks, representing 0.09% of the research on photocatalysis.

Figure 1 highlights the geographical distribution of the papers in the dataset used in this review. Figure 1a shows the distribution of the full dataset, and Figure 1b focuses on the papers selected for deeper discussion. These results show the prominence of China and India in these topics, particularly in the full dataset, where their contribution corresponds to almost 30% (417 papers) of the dataset. The finally selected dataset shows an increased diversity of the sources, with a significant increase in the contributions of the US, Canada, Japan, and the MENA (Middle-East & North Africa) region—particularly Egypt, whose contribution reaches 8.6% of the authorship.

The classification of the dataset is better visualized in Figure 2, which shows the progressive refinement of the research focus across the tiers to the final selection. Figure 2a shows how the most frequent keywords evolve from Tier 3 to the final selection of assessment-oriented papers. The lower tiers are dominated by broad terms such as wastewater treatment, adsorption, and photocatalyst, reflecting general applications of photocatalytic processes for environmental remediation. In contrast, the final tiers shows a clear rise in life cycle assessment and cost analysis, signaling a shift towards studies that explicitly address process scalability and sustainability.

This is further confirmed in Figure 2b, which provides a complementary perspective based on topic modelling using LDA. The topics identified in the lower tiers focus mainly on TiO₂, oxidation, and pollutant degradation, themes typical of laboratory-scale photocatalysis research. In upper tiers, topics related to economic assessment, energy production, and hydrogen generation become more prominent, indicating a transition from materials development to system-level evaluation. Quantitatively, nearly two-thirds (63%) of the final dataset still focus on materials development and mechanistic studies, while about one-third (37%) directly engage with economic or environmental assessment frameworks. This imbalance shows that, although sustainability assessment of photocatalytic systems is emerging as a recognized subfield, it remains secondary to materials-focused research.

Taken together, the two heatmaps illustrate the refinement of the dataset and the thematic convergence between keyword scoring and LDA. Figure 2a shows that sustainability-oriented terms only begin to dominate at the highest relevance tiers, while Figure 2b confirms a parallel shift from mechanistic topics to system-level themes. This alignment between independent text-mining approaches strengthens the evidence that the tiered screening captures a genuine structural transition in the literature—from pollutant degradation and materials-centered studies toward TEA, LCA, hydrogen-production routes, and broader sustainability considerations. The emergence of energy-production and economic-assessment topics in the upper tiers also suggests that photocatalysis is gradually expanding beyond traditional environmental remediation, with new research clusters connecting photocatalytic materials to energy transitions and circular-process intensification.

3.2. The Core Literature

The 77 core publications selected for this review are summarized in Table 2, and reflect a maturing research landscape focused on integrating technological innovation with quantitative sustainability metrics for photocatalytic systems. Regarding technological diversity, the literature confirms the ongoing prominence of well-established materials, especially titanium dioxide (TiO₂) and its composites, which are popular for their low cost, stability, and high reactivity [4,12]. However, substantial research effort is now focused on overcoming TiO₂’s UV dependence and separation issues by developing nanostructured and composite materials [22,23]. This includes modifications such as Ag-TiO₂, GO-TiO₂, and TiO₂-steel slag nanocomposites [24]. There is a marked trend toward emerging materials that utilize visible light and offer dual functionality, such as graphitic carbon nitride (g-C₃N₄), noted for its visible light response and low fabrication cost [3,23], and Metal-Organic Frameworks (MOFs), often used in composites for pollutant degradation and hydrogen production [25,26]. The cutting edge of material development is represented by novel green synthesis techniques using agro-industrial waste and advanced systems such as piezo-photocatalysts (e.g., MOF-based K@Z-2 heterojunctions), which integrate degradation efficiency with complex toxicity and life-cycle sustainability assessment [27].

In terms of the key applications of photocatalysis that include economic or environmental assessments, the corpus reveals a persistent and strong emphasis on Wastewater Treatment (WWT) [24,28]. Key target pollutants include industrial dyes (such as Methylene Blue and Remazol Brilliant Blue) [29,30,31], phenols [4,11,25], and various micropollutants such as pharmaceuticals (e.g., ciprofloxacin) [27,30,32] and pesticides (e.g., 2,4-DCP) [12]. Simultaneously, a significant and increasing portion of the literature is dedicated to energy-related applications, particularly hydrogen (H₂) production via photocatalytic water splitting or photoreforming [33,34,35]. These two applications are often linked through the synergistic approach of simultaneously degrading environmental waste while generating clean fuel [34,36].

Regarding the assessment types, the selected papers explicitly employ either TEA, or LCA, or often both methodologies to evaluate scalability, cost-effectiveness, and environmental impacts [4]. TEA studies calculate economic metrics such as Net Present Value (NPV) and Internal Rate of Return (IRR) to determine the commercial feasibility of scale-up [12,29,37]. Meanwhile, LCA provides the critical environmental perspective, frequently identifying energy consumption (especially from non-renewable sources or artificial UV light) and catalyst production or disposal as primary environmental hotspots [38,39,40,41]. Furthermore, a portion of the core literature consists of review articles (15 out of 77), including reviews focused on phenolic compound treatment [25], micropollutant abatement [28], or specific photocatalytic materials [3].

These review articles typically adopt a narrow scope, concentrating on specific applications, pollutant classes, or catalyst families. Other reviews address hydrogen production pathways, often detailing reactor configurations, scale-up considerations, and, in some cases, techno-economic or life-cycle aspects for photoelectrochemical systems or photocatalytic photoreforming. However, such reviews remain largely application- or material-specific and do not integrate assessment-based evidence across the full range of photocatalytic technologies. The increasing inclusion of sustainability and economic feasibility considerations in these reviews nonetheless indicates a maturing field increasingly concerned with deployment-relevant challenges. Figure 3 illustrates this evolution, highlighting the growing prevalence of studies performing both economic and environmental assessments.

In contrast, the present manuscript differs fundamentally in scope and methodology by synthesizing evidence exclusively from a decade of TEA and LCA studies across the entire photocatalytic domain, encompassing both wastewater treatment and solar fuel production. Rather than reviewing photocatalytic performance or reactor concepts in isolation, this work systematically diagnoses the economic and environmental barriers that constrain large-scale deployment, identifies consistent system-level trends, and reconciles divergences arising from heterogeneous assumptions and assessment boundaries.

Table 2. Summary of Photocatalytic and Related Process Assessments from the Literature Selection.

References	Technology	Application	Assessment Type
Aghel (2025) [4]	Fe3O4, SiO2, TiO2, Ag-TiO2 (Immobilized/Suspended)	WWT (Phenol removal)	Both (LCA & LCC)
Ahmaruzzaman (2024) [25]	Review of Phenolic Treatment Technologies	WWT (Phenolic WW)	Review (Cost/Sustainability)
Alhalafi (2025) [42]	ZnO Quantum Dots	WWT (Dye Degradation)	Both (Economic & Ecotoxicity)
Alsayegh (2019) [43]	Photocatalytic Water Splitting	H2 production	TEA (Economics/Energy Analysis)
Aydin (2021) [36]	Review (Photocatalysis, PEC, Fermentation)	H2 production/WWT	Review (LCA, TEA, Other)
Baaloudj (2022) [44]	Bi12TiO20 Catalytic System	WWT (Pharmaceutical WW)	TEA (Economic Evaluation)
Bahadur (2020) [45]	TiO2 in TADOX	WWT (COD reduction)	TEA (Techno-Economic Feasibility)
Balakrishnan (2021) [22]	UV/Chitosan-TiO2 Beads	WWT (Pesticide abatement)	Other (Energy & Cost Analysis)
Barman (2021) [46]	FA_NPC*O Catalyst	Other (HMF production from waste)	LCA (Environmental Performance)
Bhargava (2023) [47]	UV photo-Fenton, UV/TiO2 photocatalysis	WWT (Textile WW)	TEA (Techno-Economic Feasibility)
de Boer (2022) [28]	Review (Photocatalytic, Fenton-based, UV)	WWT (Tertiary, Micropollutants)	Review (LCA, TEA)
Carvalho (2023) [48]	g-C3N4 nanosheets, nano-TiO2 synthesis	H2 production (Synthesis context)	LCA (Synthesis impacts)
Chauhan (2025) [49]	SCBW-10 Composite Photocatalyst	WWT (OTC degradation)	LCA (Environmental Performance)
Chawla (2020) [29]	Anatase and P25 TiO2	WWT (Dye degradation)	TEA (Techno-economic evaluation)
Chen (2025) [26]	MOF-based Photocatalysis	WWT/Other (H2O2 gen., Cr(VI) photoreduction)	Both (TEA & LCA proxy)
Chen (2025b) [27]	K@Z-2 Piezo-photocatalysis	WWT (TC removal)	LCA (Sustainability Assessment)
Costamagna (2020) [50]	Rare Earth doped ZnO	WWT (Phenol degradation)	LCA (Environmental Performance)
Dihan (2025) [12]	Review (Photocatalytic/PEC, e.g., TiO2, Cd0.5Zn0.5S)	H2 production	Review (LCA, TEA)
Dominguez (2018) [51]	Photocatalysis (TiO2), PV-Photocatalysis	WWT (Greywater reuse)	LCA (Technology Comparison)
Dubsok (2022) [38]	Nano-TiO2/FeCl3 Photocatalysis	WWT (Cyanate degradation)	LCA (Environmental Performance)
Elgarahy (2022) [33]	Review (Gold-TiO2, Copper-TiO2 etc.)	H2 production	Review (TEA, Cost analysis)
Elhami (2023) [52]	Chl/TiO2 Photocatalyst	WWT (Rhodamine B removal)	LCA (Environmental Performance)
Feijoo (2020) [53]	Fe3O4 Nanoparticles (Photo-Fenton)	WWT (Antibiotic removal/WW)	LCA (Environmental Performance)
Feng (2024) [54]	Fe-C3N4 (Photo-Fenton-like)	WWT (Livestock WW, TC-HCl)	Both (LCA & Technical)
Foteinis (2018) [2]	Solar Photo-Fenton (Ferrioxalate-assisted)	WWT (Pharmaceutical WW)	LCA (Environmental Performance)
Foteinis (2018b) [55]	Light-driven AOPs (UV-A/TiO2, Photo-Fenton)	WWT (EE2 removal)	LCA (Comparative)
El Golli (2024) [30]	PTC-based process, TiO2	WWT (Water treatment)	TEA (Economic Assessment)
Gomes (2018) [56]	Solar Photo-Fenton (CPC systems)	WWT (Leachate treatment)	TEA (Cost analysis)
Gowland (2024) [39]	Immobilized vs. Suspended TiO2	WWT (NOM removal)	LCA (Environmental Performance)
Hargreaves (2020) [57]	Thermal, Electro-, Photo-catalysis	Other (NH3, H2O2 production)	Review (Sustainability)
Jia (2025) [58]	UiO-66/CN Photocatalyst	WWT (DMP removal)	LCA (Environmental Performance)
Luo (2024) [34]	CDs/CdS/CNU Photocatalyst	WWT & H2 production	LCA (Environmental Performance)
Magdy (2021) [11]	Solar Photo-Fenton, TiO2, TiO2/AC, Electro-Fenton	WWT (Phenolic WW)	Both (LCA & Economic)
Mao (2024) [59]	Solar-driven Photocatalytic Membrane Reactors (PMR)	WWT (PDWW treatment)	LCA (GHG Assessment)
Maurya (2023) [23]	TNRs, CNF:TNRs, g-C3N4-S, BiOI/g-C3N4-S	H2 production	TEA (LCOH Model)
Maurya (2023b) [60]	TNRs, CNF:TNRs, g-C3N4-S, BiOI/g-C3N4-S	H2 production	LCA (GHG/EPBT)
McKee (2022) [40]	UV-LED/TiO2 vs. UV-BL/TiO2	WWT (BPA removal)	LCA (Environmental Performance)
Mule (2021) [61]	TiO2/H-MOR (Visible-light-driven)	WWT (Pesticide removal)	Review (Technical)
Ngulube (2024) [31]	ZnO@Mg(OH)2 Nanocomposite	WWT (Dye-laden wastewater)	Both (LCA & TEA)
Notarnicola (2023) [62]	UV-C+TiO2 and related UV-C systems	WWT (CECs removal, tertiary treatment)	LCA (Comparative)
Oviedo (2025) [37]	nSOD@CuO-NPs Nanocatalyst	WWT (Organic pollutants degradation)	TEA (Cash Flow Analysis)
Patiño-Arévalo (2025) [63]	Hom./Het. Fenton/Fenton-like (CA-Fe-II/CA-Fe-III)	WWT (Phenol degradation)	Both (LCA & Cost)
Pelayo (2023) [64]	UV-A/TiO2, UV-C/TiO2 and related systems	WWT (CECs removal)	Both (Economic & LCA proxy)
Pesqueira (2020) [65]	Review (Photocatalysis, PF, OZ, etc.)	WWT (PSs and CECs removal)	Review (LCA)
Pesqueira (2021) [66]	Solar TiO2 Photocatalysis, Photo-Fenton	WWT (Micropollutant removal)	LCA (Comparative)
Pesqueira (2024) [32]	Solar/GO-TiO2 Photocatalysis vs. Solar/TiO2	WWT (PSs and CECs removal)	LCA (Comparative)
Pini (2017) [67]	Nano-TiO2 coated float glass	Other (Material Production/Coating)	LCA (Synthesis)
Pookmanee (2025) [68]	TiO2 and N-TiO2 Photocatalysts Synthesis	Other (Material Synthesis)	LCA (Comparative) (Synthesis impacts)
Qureshi (2024) [69]	Review (PEC, PC water splitting)	H2 production	Review (LCA, TEA)
Ran (2025) [70]	Heterogeneous Coupling Systems (e.g., Light-US-PMS)	WWT (Phenolic WW)	LCA (Comparative GWP)
Rezaie (2025) [5]	rGH,Fe3O4@SnO2/Ag-based	WWT (2,4-DCP removal)	Both (LCA & Cost)
Rodríguez (2018) [71]	Various WW treatments (incl. Photocatalysis context)	WWT (Chrome plating WW)	Both (LCA & TEA)
Rumayor (2022) [35]	Photocatalysis (Photoreforming, TiO2)	H2 production (from glycerol waste)	LCA (Prospective ex-ante)
Sahoo (2024) [3]	Review (g-C3N4-based photocatalyst)	WWT (Organic pollutant remediation)	Review (LCA)
Sendão (2025) [41]	CDs Nanocomposites (TiO2/Ag)	WWT (MB photodegradation)	LCA (Synthesis/FU-based)
Serik (2026) [72]	Solar H2 by Photocatalysis (Review context)	H2 production	Review (TEA, Sustainability)
Shah (2024) [73]	BCN Nanomaterials Synthesis	Other (Material Synthesis)	LCA (Synthesis impacts)
Sharmila (2019) [74]	TADOX treatment (implied context)	WWT (Sludge pre-treatment)	TEA (Economic Analysis)
Souza (2023) [75]	TiO2 Photocatalytic Surface (UVA-LED)	WWT (Greywater reuse)	LCA (Environmental Performance)
Srivastava (2023) [76]	Review (Photonic energy materials)	H2 production (Solar H2)	Review (LCA)
Supramaniam (2025) [77]	Photocatalysis (Review Context)	H2 production (Review)	Review (Technical)
Tsalidis (2022) [78]	P25-TiO2, Cu2O-coated/P25-TiO2	H2 production (Generation)	LCA (Prospective)
Wang (2021) [79]	Magnetic AgVO3/ZnFe2O4 Nanocomposites	WWT (Dye degradation)	Other (Technical)
Wang (2024) [80]	KPTI/PTI Photocatalysts	Other (Material Synthesis/OER)	LCA (Equivalent CO2 emissions)
Yang (2022) [81]	Suspended Photocatalytic System	H2 production	LCA (Environmental Performance)
Yaqub (2023) [82]	Membrane/Chemical processes (UV included in context)	WWT (Textile ZLD Scenarios)	TEA (Cost analysis)

Table 2. Summary of Photocatalytic and Related Process Assessments from the Literature Selection.

References	Technology	Application	Assessment Type
Aghel (2025) [4]	Fe3O4, SiO2, TiO2, Ag-TiO2 (Immobilized/Suspended)	WWT (Phenol removal)	Both (LCA & LCC)
Ahmaruzzaman (2024) [25]	Review of Phenolic Treatment Technologies	WWT (Phenolic WW)	Review (Cost/Sustainability)
Alhalafi (2025) [42]	ZnO Quantum Dots	WWT (Dye Degradation)	Both (Economic & Ecotoxicity)
Alsayegh (2019) [43]	Photocatalytic Water Splitting	H2 production	TEA (Economics/Energy Analysis)
Aydin (2021) [36]	Review (Photocatalysis, PEC, Fermentation)	H2 production/WWT	Review (LCA, TEA, Other)
Baaloudj (2022) [44]	Bi12TiO20 Catalytic System	WWT (Pharmaceutical WW)	TEA (Economic Evaluation)
Bahadur (2020) [45]	TiO2 in TADOX	WWT (COD reduction)	TEA (Techno-Economic Feasibility)
Balakrishnan (2021) [22]	UV/Chitosan-TiO2 Beads	WWT (Pesticide abatement)	Other (Energy & Cost Analysis)
Barman (2021) [46]	FA_NPC*O Catalyst	Other (HMF production from waste)	LCA (Environmental Performance)
Bhargava (2023) [47]	UV photo-Fenton, UV/TiO2 photocatalysis	WWT (Textile WW)	TEA (Techno-Economic Feasibility)
de Boer (2022) [28]	Review (Photocatalytic, Fenton-based, UV)	WWT (Tertiary, Micropollutants)	Review (LCA, TEA)
Carvalho (2023) [48]	g-C3N4 nanosheets, nano-TiO2 synthesis	H2 production (Synthesis context)	LCA (Synthesis impacts)
Chauhan (2025) [49]	SCBW-10 Composite Photocatalyst	WWT (OTC degradation)	LCA (Environmental Performance)
Chawla (2020) [29]	Anatase and P25 TiO2	WWT (Dye degradation)	TEA (Techno-economic evaluation)
Chen (2025) [26]	MOF-based Photocatalysis	WWT/Other (H2O2 gen., Cr(VI) photoreduction)	Both (TEA & LCA proxy)
Chen (2025b) [27]	K@Z-2 Piezo-photocatalysis	WWT (TC removal)	LCA (Sustainability Assessment)
Costamagna (2020) [50]	Rare Earth doped ZnO	WWT (Phenol degradation)	LCA (Environmental Performance)
Dihan (2025) [12]	Review (Photocatalytic/PEC, e.g., TiO2, Cd0.5Zn0.5S)	H2 production	Review (LCA, TEA)
Dominguez (2018) [51]	Photocatalysis (TiO2), PV-Photocatalysis	WWT (Greywater reuse)	LCA (Technology Comparison)
Dubsok (2022) [38]	Nano-TiO2/FeCl3 Photocatalysis	WWT (Cyanate degradation)	LCA (Environmental Performance)
Elgarahy (2022) [33]	Review (Gold-TiO2, Copper-TiO2 etc.)	H2 production	Review (TEA, Cost analysis)
Elhami (2023) [52]	Chl/TiO2 Photocatalyst	WWT (Rhodamine B removal)	LCA (Environmental Performance)
Feijoo (2020) [53]	Fe3O4 Nanoparticles (Photo-Fenton)	WWT (Antibiotic removal/WW)	LCA (Environmental Performance)
Feng (2024) [54]	Fe-C3N4 (Photo-Fenton-like)	WWT (Livestock WW, TC-HCl)	Both (LCA & Technical)
Foteinis (2018) [2]	Solar Photo-Fenton (Ferrioxalate-assisted)	WWT (Pharmaceutical WW)	LCA (Environmental Performance)
Foteinis (2018b) [55]	Light-driven AOPs (UV-A/TiO2, Photo-Fenton)	WWT (EE2 removal)	LCA (Comparative)
El Golli (2024) [30]	PTC-based process, TiO2	WWT (Water treatment)	TEA (Economic Assessment)
Gomes (2018) [56]	Solar Photo-Fenton (CPC systems)	WWT (Leachate treatment)	TEA (Cost analysis)
Gowland (2024) [39]	Immobilized vs. Suspended TiO2	WWT (NOM removal)	LCA (Environmental Performance)
Hargreaves (2020) [57]	Thermal, Electro-, Photo-catalysis	Other (NH3, H2O2 production)	Review (Sustainability)
Jia (2025) [58]	UiO-66/CN Photocatalyst	WWT (DMP removal)	LCA (Environmental Performance)
Luo (2024) [34]	CDs/CdS/CNU Photocatalyst	WWT & H2 production	LCA (Environmental Performance)
Magdy (2021) [11]	Solar Photo-Fenton, TiO2, TiO2/AC, Electro-Fenton	WWT (Phenolic WW)	Both (LCA & Economic)
Mao (2024) [59]	Solar-driven Photocatalytic Membrane Reactors (PMR)	WWT (PDWW treatment)	LCA (GHG Assessment)
Maurya (2023) [23]	TNRs, CNF:TNRs, g-C3N4-S, BiOI/g-C3N4-S	H2 production	TEA (LCOH Model)
Maurya (2023b) [60]	TNRs, CNF:TNRs, g-C3N4-S, BiOI/g-C3N4-S	H2 production	LCA (GHG/EPBT)
McKee (2022) [40]	UV-LED/TiO2 vs. UV-BL/TiO2	WWT (BPA removal)	LCA (Environmental Performance)
Mule (2021) [61]	TiO2/H-MOR (Visible-light-driven)	WWT (Pesticide removal)	Review (Technical)
Ngulube (2024) [31]	ZnO@Mg(OH)2 Nanocomposite	WWT (Dye-laden wastewater)	Both (LCA & TEA)
Notarnicola (2023) [62]	UV-C+TiO2 and related UV-C systems	WWT (CECs removal, tertiary treatment)	LCA (Comparative)
Oviedo (2025) [37]	nSOD@CuO-NPs Nanocatalyst	WWT (Organic pollutants degradation)	TEA (Cash Flow Analysis)
Patiño-Arévalo (2025) [63]	Hom./Het. Fenton/Fenton-like (CA-Fe-II/CA-Fe-III)	WWT (Phenol degradation)	Both (LCA & Cost)
Pelayo (2023) [64]	UV-A/TiO2, UV-C/TiO2 and related systems	WWT (CECs removal)	Both (Economic & LCA proxy)
Pesqueira (2020) [65]	Review (Photocatalysis, PF, OZ, etc.)	WWT (PSs and CECs removal)	Review (LCA)
Pesqueira (2021) [66]	Solar TiO2 Photocatalysis, Photo-Fenton	WWT (Micropollutant removal)	LCA (Comparative)
Pesqueira (2024) [32]	Solar/GO-TiO2 Photocatalysis vs. Solar/TiO2	WWT (PSs and CECs removal)	LCA (Comparative)
Pini (2017) [67]	Nano-TiO2 coated float glass	Other (Material Production/Coating)	LCA (Synthesis)
Pookmanee (2025) [68]	TiO2 and N-TiO2 Photocatalysts Synthesis	Other (Material Synthesis)	LCA (Comparative) (Synthesis impacts)
Qureshi (2024) [69]	Review (PEC, PC water splitting)	H2 production	Review (LCA, TEA)
Ran (2025) [70]	Heterogeneous Coupling Systems (e.g., Light-US-PMS)	WWT (Phenolic WW)	LCA (Comparative GWP)
Rezaie (2025) [5]	rGH,Fe3O4@SnO2/Ag-based	WWT (2,4-DCP removal)	Both (LCA & Cost)
Rodríguez (2018) [71]	Various WW treatments (incl. Photocatalysis context)	WWT (Chrome plating WW)	Both (LCA & TEA)
Rumayor (2022) [35]	Photocatalysis (Photoreforming, TiO2)	H2 production (from glycerol waste)	LCA (Prospective ex-ante)
Sahoo (2024) [3]	Review (g-C3N4-based photocatalyst)	WWT (Organic pollutant remediation)	Review (LCA)
Sendão (2025) [41]	CDs Nanocomposites (TiO2/Ag)	WWT (MB photodegradation)	LCA (Synthesis/FU-based)
Serik (2026) [72]	Solar H2 by Photocatalysis (Review context)	H2 production	Review (TEA, Sustainability)
Shah (2024) [73]	BCN Nanomaterials Synthesis	Other (Material Synthesis)	LCA (Synthesis impacts)
Sharmila (2019) [74]	TADOX treatment (implied context)	WWT (Sludge pre-treatment)	TEA (Economic Analysis)
Souza (2023) [75]	TiO2 Photocatalytic Surface (UVA-LED)	WWT (Greywater reuse)	LCA (Environmental Performance)
Srivastava (2023) [76]	Review (Photonic energy materials)	H2 production (Solar H2)	Review (LCA)
Supramaniam (2025) [77]	Photocatalysis (Review Context)	H2 production (Review)	Review (Technical)
Tsalidis (2022) [78]	P25-TiO2, Cu2O-coated/P25-TiO2	H2 production (Generation)	LCA (Prospective)
Wang (2021) [79]	Magnetic AgVO3/ZnFe2O4 Nanocomposites	WWT (Dye degradation)	Other (Technical)
Wang (2024) [80]	KPTI/PTI Photocatalysts	Other (Material Synthesis/OER)	LCA (Equivalent CO2 emissions)
Yang (2022) [81]	Suspended Photocatalytic System	H2 production	LCA (Environmental Performance)
Yaqub (2023) [82]	Membrane/Chemical processes (UV included in context)	WWT (Textile ZLD Scenarios)	TEA (Cost analysis)

3.3. An Overview of the Scaling Challenges of Photocatalytic Technologies

Across the reviewed literature, scaling challenges in photocatalysis emerge as systemic rather than focusing only on a single aspect of the process. Assessment studies consistently identify material sustainability, cost competitiveness, and health or toxicity concerns as the most frequently reported barriers, while engineering and standardization issues appear less often but remain qualitatively critical. Together, these findings indicate that scale-up limitations arise from interactions between materials, process design, and assessment assumptions.

Our screening reveals a hierarchy of barriers reported across the literature (Figure 4). The most frequently cited challenge is Toxicity/Health Risk (n = 42), followed closely by Material Synthesis Impacts (n = 39) and Cost Competitiveness (n = 38). Engineering-specific issues, such as Scalability and Standardization, appear less frequently in keyword counts but are qualitatively highlighted as critical bottlenecks. Notably, while `Energy Consumption’ is a dominant hotspot in LCA quantitative results (as detailed in Section 3.4), it appears as a qualitative keyword in only 34 of the reviewed papers, suggesting a gap between quantitative evidence and qualitative reporting.

Strategies to Accelerate Scale Up

Strategies to overcome the aforementioned limitations have largely focused on material engineering and process design aimed at improving light utilization, charge separation, and long-term stability. At the material level, band-gap engineering through metal and non-metal doping (e.g., Ag–TiO₂, N–TiO₂), heterojunction formation, and Z-scheme architectures has been widely explored to extend absorption into the visible range and suppress electron–hole recombination [23,26,70,83,84]. Additional enhancement routes include defect engineering, such as the controlled introduction of oxygen vacancies, plasmonic activation using noble or noble-metal-free nanostructures, and cocatalyst deposition to promote charge transfer and surface reaction kinetics [3,78,80]. Complex material engineering, specifically facet-controlled cocatalyst deposition (like Rh/Cr₂O₃ and CoOOH on SrTiO₃:Al), has been shown to achieve an External Quantum Efficiency (EQE) of up to 96% at 350–360 nanometres, equivalent to an Internal Quantum Efficiency (IQE) of almost unity [85], providing a definitive answer that overall water splitting free from charge recombination losses is technically feasible.

Beyond intrinsic activity, durability and resistance to deactivation are critical for scalability. Core-shell architectures, protective overcoats (e.g., amorphous oxide layers), and engineered interfaces have been proposed to mitigate photocorrosion, metal leaching, and fouling in aqueous environments [31,57]. Catalyst regeneration strategies, including thermal, chemical, or photonic treatments, have also been reported to partially restore activity after deactivation, thereby reducing the frequency of replacement and material demand [5,42].

In addition to challenges related to efficiency and durability, toxicity and secondary pollution risks associated with photocatalytic systems represent an important barrier to industrial deployment and increasingly influence scale-up strategies. These risks arise primarily from nanomaterial release and metal leaching, which can compromise both environmental safety and regulatory acceptance, while bulk TiO₂ is generally regarded as non-toxic, surface modifications intended to enhance catalytic performance can substantially alter its safety profile; for instance, coating TiO₂ nanoparticles with vanadium pentoxide has been shown to increase cytotoxicity by up to 400% [78]. Similarly, cadmium-based (CdS) photocatalysts remain highly restricted for large-scale applications due to their association with chronic diseases and their propensity for metal-ion leaching into treated effluents [3]. Quantitative life-cycle assessments further reveal that certain dopants introduce disproportionate environmental burdens, with silver nitrate contributing more than 92% of the terrestrial ecotoxicity potential during catalyst synthesis in silver-modified systems [5]. As a consequence, stringent containment and recovery requirements become unavoidable, often necessitating complex and energy-intensive separation stages such as ultrafiltration or high-speed centrifugation, which constitute critical engineering and economic bottlenecks at scale [39]. Moreover, the potential formation of toxic intermediate by-products sometimes more hazardous than the parent compounds, as reported for phenol and dye degradation pathways, further complicates scale-up decisions [54]. Collectively, these factors impose a fundamental trade-off between maximizing catalytic efficiency and ensuring environmental and operational safety, reinforcing the need for earth-abundant, chemically stable, and non-toxic materials, together with immobilized catalyst configurations and benign synthesis routes, as core enablers of industrially viable photocatalytic processes.

Process and reactor engineering approaches play an equally important role. The immobilization of photocatalysts on membranes, glass substrates, or structured supports can facilitate continuous-flow operation and catalyst recovery, although this often comes at the cost of reduced apparent reaction rates compared to slurry systems [4,39,86]. Reactor configurations prioritising efficient photon management, such as thin-film reactors, modular photoreactors, and solar collectors including compound parabolic concentrators (CPCs), have been shown to improve light utilisation under both artificial and solar irradiation [2,56,59]. Hybrid treatment schemes integrating photocatalysis with adsorption, membrane separation, or electrochemical polishing have also been proposed to enhance robustness and mitigate fouling and mass-transfer limitations [64,65].

More recently, data-driven optimisation and automated control strategies have emerged as tools to manage the inherent variability of photocatalytic systems, particularly under solar operation where irradiance fluctuates temporally and seasonally [72,75].

Collectively, these material and process-level strategies provide a broad toolbox for addressing known technical limitations; however, their relevance for large-scale deployment ultimately depends on whether they translate into measurable economic and environmental benefits at the system level.

3.4. TEA and LCA for Photocatalysis

Techno-economic and life-cycle assessments provide complementary perspectives on the feasibility of photocatalytic technologies, revealing consistent hotspots that are not apparent from performance metrics alone. Across the literature, electricity consumption and catalyst production dominate environmental impacts and operating costs, although their relative importance depends strongly on irradiation mode and system boundaries. This subsection synthesizes methodological trends and recurring hotspots identified through TEA and LCA studies.

LCA is a standardized, comprehensive methodology for assessing the environmental effects of materials and services across their entire life cycle, including resource consumption, energy use, and waste generation, with procedures generally defined by ISO 14040 and ISO 14044 standards [49,65]. TEA, on the other hand, focuses on market feasibility, revenue forecasts, and profitability metrics [4,12]. Integrating these holistic assessments alongside technical performance is crucial, especially for comparing different incoming technologies.

3.4.1. LCA Scope and Methodology Trends

The first stage of LCAs is defining the system boundaries and the functional units (FUs). A common system boundary in photocatalysis LCA studies is the “cradle-to-gate” approach, which encompasses raw material extraction up to the synthesis and operational phases but often excludes maintenance and end-of-life stages due to challenges in obtaining detailed industrial data [4,38,41,52,62]. The operational phase is frequently the most studied life cycle stage, as it is often assumed to hold the main environmental impact [34], though the inclusion of construction and decommissioning phases is slowly increasing [28]. These studies typically include inputs such as catalyst synthesis, chemical consumption, and electrical energy utilization [4,5,31,38]. The definition of a consistent functional unit (FU) is critical for robust comparative analysis. Common FUs in photocatalysis LCA include the degradation of a specific mass of pollutant (e.g., 1 kg of phenol) [4,5,49,52], the treatment of a unit volume of water (e.g., 1 m³ of wastewater) [11,66], or the production of a unit mass of material (e.g., 1 kg of photocatalyst) [41,54,80]. For hydrogen production, the FU is often defined as 1 kg of H₂ [35].

The impact assessment is carried out by using both midpoint (problem-oriented) and endpoint (damage-oriented) methods. In midpoint categories, the magnitude of the impact in the initial stages of the cause-effect chain are assessed, generating an intermediate result that is deterministic and often traceable [87]. Widely utilized Life Cycle Impact Assessment (LCIA) methodologies include ReCiPe (often ReCiPe 2016) [2,34,39,41,49,52,63], IPCC (e.g., IPCC 2013) for quantifying Global Warming Potential (GWP) [2], CML [11,35], IMPACT 2002+ [38], and TRACI (Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts) [48,54]. Key midpoint impact categories frequently assessed in these studies encompass GWP [35,50], various forms of toxicity (human, ecosystem, aquatic, terrestrial) [5,39,40,49,53], and resource depletion metrics such as Cumulative Energy Demand (CED) [35,39,49] and Abiotic Depletion Potential (ADP) [11,69].

As shown in Figure 5, the studies reviewed in this work employ a variety of LCIA methods, with ReCiPe being the most widely adopted (approximately 35%), followed by CED/LCC, IPCC-based GWP, TRACI, and USEtox. Less common methods, such as IMPACT 2002+, CML 2000, Eco-indicator 99, and EF 3.0, appear in fewer than 5% of the studies. The variability is due to the fact that the choice of framework is influenced by the availability of regional databases, or the major focus of the impact assessment. The most commonly reported impact categories include GWP, ecotoxicity, resource depletion, and acidification, highlighting the emphasis on energy- and climate-related impacts. Categories like eutrophication, ozone depletion, and cumulative energy demand are less frequently studied.

3.4.2. Environmental Hotspots

Electricity consumption is consistently identified as the primary environmental hotspot in photocatalytic processes that rely on artificial light sources [11,40,42,48,55]. This energy demand is a primary driver for impacts such as GWP, non-renewable energy consumption, and human health toxicity, with one study attributing significant damage to human health to the lack of renewable electricity in the energy mix [39]. In contrast, for specific processes such as photocatalytic H₂O₂ generation, a low carbon footprint of 0.088 kgCO₂ eq/kgH₂O₂ has been reported [26], demonstrating the potential for low-impact pathways.

Another major environmental hotspot is attributed to the production of catalyst materials and their chemical precursors [41]. For example, in the sol-gel synthesis of nano-TiO₂/FeCl₃, ethanol consumption accounted for most environmental impacts (82%) [38], while for g-C₃N₄ nanosheet synthesis, relying mostly on organic materials, electricity input showed overwhelming contributions across all midpoint categories [48]. In other cases, such as nanocomposite synthesis, precursors such as titanium tetraisopropoxide (TTIP) and isopropanol were responsible for over 60% of impacts [41]. Furthermore, the synthesis process for GO-TiO₂ was found to have higher potential environmental impacts than commercial TiO₂-P25, largely due to the use of hydrofluoric and boric acids [32]. Overall, material extraction and synthesis processes have been found to contribute as much as 83% to 89% of total GHG emissions in some pathways [60].

Part of the efforts to reduce environmental impact revolve around using natural light sources. Solar-driven photocatalysis is consistently identified as the most environmentally sustainable option when compared to systems using artificial UV lamps, with a reduction of about five times the environmental footprint [55]. Studies have demonstrated that solar-TiO₂ photocatalysis exhibits a substantially lower environmental load than systems using UV-LEDs (71% decrease) or conventional UV blacklight (UV-BL) lamps (89% decrease) [40]. This trend is further supported by findings that UV-LED systems themselves offer a 61% reduction in environmental footprint compared to UV-BL systems [40]. Similarly, solar-driven processes for cyanate degradation showed the lowest life cycle burden [38]. One analysis identified solar TiO₂-P25 treatment as the most suitable option among five solar-based treatments for micropollutant removal, assuming the catalyst is reused [66]. When artificial light is replaced by solar irradiation, the environmental impact burden shifts from energy consumption primarily to the contribution of the catalyst itself [2,40]. Figure 6 illustrates the environmental impact comparison between photo-Fenton and TiO₂, as identified by Pesqueira and colleagues [66]. Their study assessed five solar-based treatments, including solar photocatalysis using TiO₂ (P25) and solar circumneutral photo-Fenton, for the removal of micropollutants from secondary-treated urban wastewater using a pilot-scale Compound Parabolic Collector (CPC) photoreactor. They concluded that the solar TiO₂ treatment appeared to be the most suitable option, aiming for a balance between ecotoxicity reduction and incurred environmental impacts, particularly if the TiO₂ was reused at least five times. The environmental downfall of photo-Fenton was identified as the need for acidification and subsequent neutralization.

The environmental comparison between suspended (slurry) and immobilized catalyst configurations yields varied results depending on the specific application. For instance, in the context of phenol degradation, an immobilized Ag-TiO₂ pottery plate demonstrated significantly lower environmental footprints and costs, with the lowest reported GWP (672 kgCO₂ eq) and CED (3.17 kWh) [4]. In contrast, for the removal of natural organic matter (NOM), a slurry photocatalysis system showed an 87% reduction in environmental footprint compared to an immobilized system, an effect attributed primarily to the high-impact chemical components required for the immobilization process [39].

When benchmarked against other AOPs, photocatalysis often holds a favorable environmental position, particularly when solar-driven, as indicated in Figure 6. For phenol removal, a comparative ranking of Eco-indicator scores placed solar photo-Fenton as the most environmentally friendly technology (0.044 pt), followed by photocatalysis with bare TiO₂, adsorption by activated carbon (AC), and photocatalysis with TiO₂/AC, with electro-Fenton having the highest impact (1.48 pt) due to its intensive electricity consumption [11]. For hydrogen production, prospective LCAs have shown that photocatalysis and photoelectrochemical methods have remarkably lower acidification potential and GWP compared to conventional Steam-Methane Reforming (SMR), identifying direct solar conversion via photoreforming as a highly promising route [35,69].

3.4.3. Overview of Techno-Economic Assessment Reports

Techno-economic assessments of photocatalytic processes reveal both the long-term promise of solar-driven Advanced Oxidation Processes (AOPs) for wastewater remediation and the significant economic barriers that still limit the large-scale deployment of renewable hydrogen production technologies. Across the literature, reported costs are remarkably broad, reflecting the early technological maturity of these systems and the substantial methodological heterogeneity among different TEA studies. Variability arises from differences in reactor configuration, sunlight-to-hydrogen (STH) efficiency, catalyst stability and reuse, operating and environmental conditions, and the allocation of capital (CAPEX) and operating (OPEX) expenditures. Understanding these divergences is therefore essential for interpreting cost competitiveness and identifying realistic pathways for technological improvement.

Table 3. Summary of Techno-Economic Assessment Costs for Photocatalytic Processes.

Application	Technology	Cost Metric	Reported Cost	Ref.
Hydrogen Production	Photocatalytic Water Splitting Reforming (SMR)	LCOH	4.90–7.80 USD/kg H2	[23]
	Steam Methane Reforming (SMR)	LCOH	0.85–2.00 USD/kg H2	[23]
	Particulate Suspension Systems	LCOH	$18.32/kg H2	[12]
	Photoelectrochemical (PEC) Systems	LCOH	8.43–19.98 USD/kg H2	[12,69]
	Photoreforming of Plastic Waste	LCOH	5.50–6.00 USD/kg H2	[77]
Wastewater Treatment	Phenolic Wastewater (per m3)
	Adsorption by Activated Carbon	Treatment Cost	0.74 USD/m3	[11]
	Solar Photo-Fenton	Treatment Cost	1.55 USD/m3	[25]
	Solar Photocatalysis (bare TiO2)	Treatment Cost	1.66 USD/m3	[25]
	Photocatalysis/Adsorption (TiO2/AC)	Treatment Cost	2.19 USD/m3	[25]
	Electro-Fenton	Treatment Cost	6.12 USD/m3	[11]
	Textile Dye Wastewater (per m3)
	TiO2-steel slag nanocomposite	Operational Cost	0.84 USD/m3	[24]
	ZnO@Mg(OH)2 core-shell nanocomposite	Capital & Op. Cost	2.70–4.00 USD/m3	[31]
	Methylene Blue (ANN Optimized)	Total Cost	7.70 USD/m3	[88]

summarizes representative TEA results for hydrogen production and wastewater treatment systems.

In TEAs focused on hydrogen generation through photocatalytic water splitting, photoelectrochemical (PEC) water splitting, or photoreforming of organic wastes, economic projections vary primarily with assumptions concerning photoreactor design, STH efficiency, and catalyst lifetime. Photocatalytic water-splitting analyses generally model systems composed of panel-integrated photocatalytic cells [23]. PEC-based assessments evaluate semiconductor photoelectrodes with engineered charge-separation layers [69], whereas photoreforming studies examine slurry-based or panel-modified systems designed for organic feedstocks [77]. These reactor and device configurations differ substantially in light-harvesting efficiency, H₂/O₂ separation demands, and materials requirements, which contribute to the wide variation in projected hydrogen-production costs across the analyses. A consistent finding across them is that reactor panel manufacturing and labour dominate costs, accounting for roughly 30–37% and 32–46%, respectively, [23]. Material costs add a further 13–29%—driven largely by the use of precious metals, while panel acquisition represents the single largest capital item, highlighting that reducing catalyst price alone cannot offset the high CAPEX associated with panel-based photocatalytic reactor systems.

STH efficiency assumptions for H₂ production span a broad range, from conservative values below 1–2% for particulate and PEC systems [12,69], to values exceeding 5% in plastic-waste photoreforming scenarios [77], and up to 10–11% for thin-film photocatalytic panels and Photovoltaic–Photoelectrochemical (PV-PEC) hybrid configurations [12]. Since the Levelized Cost of Hydrogen (LCOH)—the lifetime-averaged cost of producing 1 kg of H₂—depends inversely on the STH efficiency, even small differences in efficiency assumptions can shift projected LCOH values by nearly an order of magnitude. Catalyst durability is another critical variable. Metal-oxide and carbon-nitride photocatalysts such as TiO₂ and g-C₃N₄ are widely regarded as more stable under prolonged irradiation, leading many TEAs to assume comparatively long operational lifetimes for these materials [12]. Conversely, combined TEA+LCA studies of photocatalysts synthesized through ligand-intensive or multi-step routes emphasize that ligand and precursor costs, rather than long-term durability, often dominate both economic and environmental burdens [26]. These distinct assumptions regarding efficiency, stability, and synthesis complexity substantially affect CAPEX amortization and OPEX, contributing to the large variability observed across hydrogen-production TEAs.

The LCOH in photocatalytic water-splitting pathways is estimated to range from 4.9 to 7.8 USD kg⁻¹ H₂ according to the detailed assessment of [23]. This range was determined for stable photocatalysts using mainly earth-abundant elements (TNR, CNF:TNR hybrids, sulfur-doped g-C₃N₄, and BiOI/g-C₃N₄ heterojunctions), designed for a 5 tonnes/day production capacity. The techno-economic framework integrates site-specific solar availability, light absorption and charge separation efficiencies, slurry recirculation energy demand, and H₂/O₂ separation requirements. Although promising relative to earlier photocatalytic TEAs, these values remain substantially above the 0.85–2 USD kg⁻¹ H₂ typical of Steam Methane Reforming (SMR). More conservative TEAs for photocatalysis indicate much higher hydrogen-production costs, with some particulate suspension and PEC systems reaching LCOH values of 18.3–19.98 USD kg⁻¹ H₂ [12], primarily driven by low STH efficiencies, shorter electrode lifetimes, and higher specific CAPEX associated with complex reactor designs.

Photoreforming of plastic waste appears relatively more advantageous, with costs of 5.5–6 USD kg⁻¹ H₂ [77], mainly due to the negative-cost feedstock and more favorable reaction energetics. However, these estimates depend on high conversion rates and stable long-term catalyst performance, which have not yet been confirmed at scale. Prospective TEA–LCA analyses [35] show that photoreforming only becomes competitive when STH efficiency exceeds several percent, and catalyst durability approaches multi-year operation.

In photocatalysis for wastewater treatment, TEAs focus on comparing reactor configurations, catalyst immobilization strategies, and chemical reagent consumption. Studies evaluating the treatment of phenolic compounds, dyes, and pharmaceutical contaminants employ a wide range of reactor types, including slurry and immobilized TiO₂ systems [39], solar photo-Fenton reactors [25], parabolic-trough collectors [30], and hybrid photocatalysis–adsorption units [11]. These configurations differ substantially in hydraulic retention time, mass transfer characteristics, solar capture efficiency, and catalyst recovery requirements. Typically, solar-driven processes, such as photo-Fenton and TiO₂ slurry reactors, tend to exhibit lower operating costs due to their minimal external energy demand, whereas energy-intensive systems, such as electro-Fenton, incur significantly higher OPEX.

Regarding catalyst immobilization and formulation, the reviewed studies consistently show that systems employing waste-derived or readily available support can substantially reduce material costs in addition to the already mentioned environmental burdens. For example, ref. [24] reports that a TiO₂–steel slag composite catalyst prepared from an abundant metallurgical by-product achieves a treatment cost of 0.8428 USD m⁻³, among the lowest values documented for heterogeneous photocatalysis in dye remediation. In contrast, catalysts synthesized through more complex, multi-step routes, such as the ZnO@Mg(OH)₂ core–shell nanocomposite, require high-purity precursors and energy-intensive processing. Their techno-economic evaluation indicates capital and operating costs of 2.7 USD m⁻³ and 4.0 USD m⁻³, respectively, [31], substantially higher than those of systems based on industrial-waste supports.

Techno-economic studies focusing on reagent and chemical consumption are carried out particularly in comparison with hybrid AOPs, such as photo-Fenton and UV/H₂O₂. These processes typically require substantial inputs of H₂O₂, iron salts, acids, or bases, and the assessments usually explore reagent dosage, replenishment rates, and possibilities for recovery or recycling. In addition to reagent consumption, acid/base usage for pH correction represents a significant operational requirement in many AOP configurations, particularly in optimized photo-Fenton systems that work in pH 3, requiring acidification and subsequent neutralization to comply with discharge regulations [25,28]. These steps can dominate both chemical demand and sludge production, especially when iron-based catalysts require precipitation and removal [63,66]. The use of slurry photocatalysis also introduces new downstream demands, as streams with suspended TiO₂ typically require filtration or membrane-based recovery stages, prior to reuse or disposal [74,75]. The need for final pH adjustment and solids removal to meet legal effluent-quality standards, therefore, adds to OPEX. For instance, for a simulated 10 m³/day facility treating dye-laden wastewater using a suspended magnetite-based photocatalyst, the electrical cost for running auxiliary equipment (which includes separation and compression equipment) was estimated at USD 962 per month. This figure represented roughly 65% of the total monthly running cost [88]. In some cases, chemical reagents represent a major fraction of OPEX, overshadowing catalyst or reactor costs, particularly in solar photo-Fenton and UV/H₂O₂ configurations treating pharmaceutical or industrial effluents [2,11,25]. This dependence on consumables is especially pronounced when operation demands continuous pH control or when pollutant concentrations fluctuate substantially, leading to higher and more uncertain reagent demand. In addition, LCA–TEA analyses for advanced photocatalysts show that the cost and production of auxiliary oxidants can become dominant hotspots in both economic and environmental terms [5,26].

Regarding irradiation mode and energy source, wastewater-treatment TEA studies—similarly to what was observed in LCA—show that the use of artificial UV lamps makes electricity demand the dominant contributor to operating costs. In UV-driven photocatalytic systems, electricity for lamps, pumps, and mixers can represent around 60–70% of the total running cost, as reported for semi-industrial (10 m³/day) dye-treatment units [1,88]. In artificially irradiated systems, TEA outcomes become highly sensitive to electricity prices, lamp or LED replacement frequencies, and their end-of-life destinations. Conversely, solar-driven photocatalysis eliminates this electricity burden, shifting economic relevance from OPEX to CAPEX associated with solar-collector design and manufacture. Solar parabolic-trough reactors, for example, achieve comparable removal efficiencies to artificial-UV systems while exhibiting substantially lower annual treatment costs and negligible grid-electricity consumption [30]. The comparison revealed that the total annual cost (TAC) for the solar PTC (€1626.20/year) was substantially lower than the TAC for the comparable artificial UV equipment (€5068.60/year), both working with TiO₂ P25. This represents the PTC system operating at approximately 32% of the annual cost of the UV system. In particular for electricity, the annual electricity cost for the solar PTC was only €21.60/year, calculated for the operation of the pump and solar tracker. In contrast, the comparable UV system incurred €723.60/year in electricity costs. In the solar PTC scenario, electricity costs account for only 1.3% of the total annual cost, whereas the artificial UV system allocates 14.3% of its total annual cost to electricity.

Finally, some assessments also explore different scale-up assumptions—particularly regarding treatment capacity, hydraulic residence time, and catalyst reuse. Many assume multiple reuse cycles for immobilized catalysts—such as five cycles for TiO₂–steel slag composites [88] or up to ten cycles for hybrid photocatalysis–adsorption systems [11]—even though these levels of durability are not yet demonstrated at pilot scale. Moreover, most TEAs apply fixed influent characteristics, despite the fact that variations in pollutant concentration, organic load, and water chemistry can substantially increase oxidant demand and irradiation requirements in processes such as solar photo-Fenton and UV/H₂O₂ [11,30]. These scale-up and influent uncertainties remain major sources of inconsistency across wastewater treatment TEAs.

4. Discussion

The preceding sections show that photocatalytic processes are evolving from primarily laboratory-scale concepts into technologies with credible prospects for deployment at scale. This transition is supported by advances in semiconductor engineering, reactor and process design, and, critically, by the progressive integration of quantitative sustainability frameworks such as techno-economic and life-cycle assessments. Overall, the core literature confirms that technical performance alone is no longer a sufficient criterion: economic feasibility and environmental compatibility increasingly determine which photocatalytic systems can realistically progress towards industrial implementation.

4.1. A Field in Transition

The literature analysis evidences a field in active transition: while a substantial fraction of publications focuses on mechanistic understanding, a growing subset is embedding quantitative sustainability metrics into photocatalytic design. This structural shift is largely a response to the consistent evidence from TEA and LCA studies that technical performance alone does not guarantee scalability.

The primary driver of this transition is the recognition that electricity consumption typically governs both operating costs and environmental burdens in UV-driven systems. Several studies attribute more than 90% of the total environmental burden to electricity use in artificial lighting setups [13,40,62,64]. Consequently, research efforts have increasingly pivoted toward solar-driven configurations to mitigate these energy penalties. However, assessment data reveal that this shift transfers the environmental burden from operation to the “cradle-to-gate” impacts of catalyst production, where TiO₂-based and nanostructured materials often dominate embodied GHG emissions [38,41,60,68].

In response to this newly identified bottleneck, the literature shows a marked trend toward “circular” material design. Strategies such as using industrial residues, agro-industrial biochar, or calcination-free synthesis are gaining traction not merely as novelties, but as necessary measures to reduce the high embodied impacts of nanostructured photocatalysts [4,50]. Furthermore, the move toward noble-metal-free plasmonic materials seeks to align intrinsic catalyst properties with long-term sustainability goals [89,90].

Similarly, the persistent gap in cost competitiveness—with hydrogen production costs (4.9–7.8 USD kg⁻¹ H₂) remaining well above conventional benchmarks like steam methane reforming [23,60,69]—is pushing the field toward integrated assessment frameworks. Rather than optimizing reaction rates in isolation, emerging studies are using ex-ante LCA and digitization tools to identify “efficiency-impact” trade-offs before pilot implementation [35,65]. For instance, the “Material Properties and Sustainability” (MAPS) approach integrates measurable properties (e.g., band gap, photoluminescence) with impact metrics to computationally screen for stable, environmentally benign candidates before laboratory synthesis [12,58,73].

Finally, this systems-level perspective is essential for resolving engineering trade-offs, such as the choice between immobilized and slurry systems. While slurry configurations often offer higher reaction rates, they incur significant separation costs; conversely, immobilization simplifies recovery but may require chemically intensive support synthesis [4,39]. This evolution indicates that photocatalysis is progressively being repositioned: no longer just a quest for higher quantum yields, but a challenge of integrating robust materials into economically viable, solar-efficient processes.

4.2. Key Limitations to Further Expansion

Despite encouraging progress, several factors continue to limit the widespread industrial implementation of photocatalytic processes.

A primary constraint is the low overall energy-conversion efficiency observed in solar fuel applications. For most presently demonstrated systems, solar-to-hydrogen (STH) efficiencies are often below 1% [12,72], and remain well below the 5–10% range considered necessary for economic competitiveness, even under optimistic assumptions [23,69]. For instance, even one of the most efficient photocatalytic composite (Rh/Cr₂O₃ and CoOOH on SrTiO₃:Al) despite its near-perfect quantum efficiency shows an STH of only 0.65% [85]—highlighting why low conversion yields remain the major practical bottleneck limiting industrial deployment for solar fuel applications. In addition to these low yields, intrinsic photonic constraints, such as limited quantum efficiency, shallow light penetration, and inefficient photon utilization in powdered systems, further restrict reactor productivity. Similarly, pollutant degradation studies conducted under realistic water matrices frequently report lower performance than in simplified laboratory conditions, due to light attenuation, competing species, complex mixtures, and non-ideal hydrodynamics.

Economic barriers remain significant. High capital expenditures (CAPEX) for photocatalytic reactors (including large-area solar collectors, containment, and auxiliary units), combined with non-negligible operating costs, lead to total annualized costs that are often dominated by infrastructure: for solar-driven systems, the CAPEX of the facility (investment costs) can constitute 75.6% to 89.7% of the Total Annual Costs (TAC) [31,43]. Even in wastewater treatment, the high cost of the process can be up to 10 times greater than conventional treatment methods [13]. In the absence of favorable policy instruments (e.g., carbon pricing, green premiums, or renewable-electricity incentives), photocatalytic routes for energy carriers and tertiary treatment may struggle to compete with incumbent technologies.

Environmental and health uncertainties associated with nanomaterials constitute another critical limitation. The potential release of metal ions (e.g., Cd²⁺ from CdS, Zn²⁺ from ZnO) and nanoparticulate residues raises ecotoxicological concerns that are only partially captured in current LCAs, given the paucity of robust fate, transport, and toxicity data [27,34,53]. Many assessments are forced to rely on generic inventory datasets or simplified assumptions, which introduce substantial uncertainty into comparative conclusions [4,67].

Furthermore, scale-up remains weakly documented. As highlighted in our screening, most quantitative assessments are based on bench-scale or pilot-scale data, with limited disclosure of process parameters, reactor dimensions, or long-term operating behaviour [51,65]. The scarcity of transparent scale-up information complicates the transferability of LCA and TEA results, hindering the development of standardized design and evaluation methodologies for both wastewater treatment and hydrogen production. Finally, catalyst deactivation, photocorrosion, and separation remain significant challenges, particularly for slurry systems, where solid–liquid separation can be energy-intensive and costly (e.g., ultrafiltration or complex recovery schemes). Moreover, the absence of long-term durability and regeneration data forces most LCA and TEA studies to assume catalyst lifetimes that are rarely validated experimentally, leading to a significant underestimation of environmental and economic impacts.

Finally, the scalability of direct photocatalysis must be contextualized against emerging adjacent technologies that decouple light harvesting from chemical conversion. Photovoltaic–electrocatalysis (PV-EC) coupling, for instance [91], offers a modular alternative where photon capture and catalytic turnover occur in separate, optimized units. This decoupling enables clearer techno-economic accounting, as it leverages the established cost curves of commercial PV modules and electrolysers, contrasting with the high uncertainty associated with novel slurry or panel-reactor designs. Similarly, photothermal catalysis [92] is gaining traction as a strategy to surpass the photonic efficiency limits of conventional semiconductors. By utilizing the full solar spectrum to drive thermal activation alongside charge generation, photothermal systems can significantly lower the energetic barrier for difficult transformations (e.g., CO₂ hydrogenation), offering a distinct deployment route that circumvents some of the photonic transport bottlenecks inherent to UV-driven suspension systems.

4.3. Strategic Roadmap for Scale-Up

From a larger-scale deployment perspective, the relevance of photocatalytic technologies is shaped not only by technical feasibility but also by alignment with market needs and regulatory drivers. Markets consistently prioritize solutions that minimize operating costs, reduce chemical consumption, and demonstrate long-term reliability. Consequently, the near-term deployment of photocatalysis is most plausible in application-specific niches rather than as a universal replacement for incumbent processes.

Building on the TEA and LCA evidence, seven research directions emerge as critical for bridging the gap between laboratory innovation and industrial implementation:

1.

Photon-efficient reactor design. TEA and LCA consistently identify electricity consumption as the dominant hotspot in UV-driven systems. Future research should prioritize reactor configurations that maximize photon utilization and minimize electrical input, such as thin-film geometries, light-guiding architectures, and solar-first designs, rather than focusing solely on intrinsic catalytic activity.

2.

Scalable, low-impact material synthesis. Assessment results show that complex, multi-step synthesis often trades performance gains at the expense of high cradle-to-gate impacts. Research should prioritize materials producible via low-temperature, solvent-efficient, and high-yield routes (e.g., aqueous sol-gel) using abundant precursors, even if this entails accepting moderate activity in exchange for manufacturability [4,50].

3.

Standardized assessment frameworks. The comparability of existing studies is severely limited by heterogeneous assumptions regarding functional units, energy inputs, and system boundaries. Adopting harmonized reporting protocols for photon fluxes, reactor geometries, and life-cycle inventories is essential to facilitate robust, evidence-based decision-making [12,60].

4.

Validated lifetime and regeneration benchmarks. A major source of uncertainty in economic and environmental models is the lack of long-term durability data. Future studies must systematically quantify degradation rates and regeneration efficiency under representative operating conditions, as catalyst replacement frequency is a primary driver of life-cycle burdens.

5.

Shift to pilot-scale integrated assessment. Many favorable projections rely on optimistic laboratory assumptions that fail under continuous-flow conditions. Research must pivot toward integrated pilot systems that combine reactors, hydraulics, and downstream separation, supported by ex-ante TEA/LCA to identify technological lock-in risks early in development [35].

6.

Holistic safety and risk integration. Beyond performance, the potential release of nanomaterials and toxic leachates remains a critical barrier. Experimental evaluation of spent catalyst toxicity and fate-and-transport modeling should be incorporated routinely into process development, particularly for treated effluents intended for agricultural reuse [13,42].

7.

Targeting high-value niches. Rather than pursuing direct competition with low-cost biological treatments, research should identify applications where photocatalysis provides unique system-level value. This includes decentralized treatment, removing recalcitrant contaminants (e.g., APIs) where chemical inputs must be minimized, or coupling with renewable energy for waste valorization.

These priorities are synthesized in Figure 7, which maps the trajectory from laboratory innovation to industrial implementation. By aligning research objectives with these assessment-driven priorities, future developments can effectively translate photocatalysis from a promising scientific concept into a scalable, climate-positive technology.

By aligning research objectives with these assessment-driven priorities, future developments can effectively translate photocatalysis from a promising scientific concept into a scalable, climate-positive technology.

5. Conclusions

This review assessed whether photocatalytic technologies for wastewater treatment and solar fuel production are ready for large-scale implementation. Based on ten years of TEA and LCA data, the answer is clear: photocatalysis is not yet suitable for industrial deployment. Across different applications and reactor types, two main constraints consistently limit sustainability.

First, UV-driven systems have very high operational demands due to electricity consumption for lamps, pumps, and auxiliary equipment. Even in optimized cases, electricity remains the main cost and environmental hotspot. Second, when solar irradiation is used to cut energy needs, focus shifts to the cradle-to-gate impacts of catalyst production, which can account for over half of total emissions for nanostructured or multi-step materials. These burdens, combined with limited catalyst durability, low photon-to-product efficiencies, and incomplete H₂/O₂ management strategies, currently hinder the large-scale production of photocatalytic hydrogen.

Regarding wastewater treatment, the literature indicates that solar photo-Fenton and solar-assisted heterogeneous systems show the most promise but still face issues related to catalyst recovery, reagent consumption, and operation with varying water types.

More broadly, inconsistent methodologies across TEAs and LCAs, limited pilot-scale validation, and the lack of standardized functional units and photonic metrics hamper meaningful comparison and reliable scale-up predictions. Nonetheless, the way forward is clear. Solar-integrated reactors, immobilized or waste-derived catalysts, calcination-free synthesis methods, hybrid treatment systems, and the integration of environmental and economic metrics during material design all offer promising paths to reduce impacts and improve feasibility.

Achieving industrial readiness will require coordinated progress in reactor engineering, sustainable material development, and harmonized reporting frameworks, along with demonstration-scale studies conducted under realistic solar and hydraulic conditions. With these advancements, photocatalysis could evolve from a promising laboratory technique into a credible, scalable, and climate-positive solution for water treatment and solar fuel production.

Across the analyzed literature, several conclusions emerge as robust, appearing consistently across applications, reactor concepts, and assessment frameworks. In particular, both TEA and LCA studies converge on the dominance of electricity consumption as the primary economic and environmental hotspot in UV-driven photocatalytic systems, as well as on the shift toward catalyst production impacts when artificial irradiation is replaced by solar light. Similarly, the finding that complex, energy- and material-intensive catalyst synthesis routes can negate gains achieved through improved photocatalytic activity is observed across multiple independent studies. In contrast, conclusions related to absolute cost competitiveness, scale-specific feasibility, or direct comparisons with incumbent technologies remain strongly dependent on modelling assumptions, system boundaries, and limited pilot-scale data. These dataset-dependent insights should therefore be interpreted with caution, underscoring the need for standardised reporting and validation at larger scales.