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Article

Environmental Assessment of Tannin Extraction from Bark Residues for Application in Water Treatment

by
Carla L. Simões
1,*,
Alice B. P. Santos Neto
2,
Ana C. Rodrigues
3,
Ricardo Ferreira
3 and
Ricardo Simoes
2,4
1
TecMinho—Associação Universidade-Empresa para o Desenvolvimento, 4800-058 Guimarães, Portugal
2
School of Design, Polytechnic Institute of Cavado and Ave (IPCA), 4750-810 Barcelos, Portugal
3
PROMETHEUS, Unidade de Investigação em Materiais, Energia e Ambiente para a Sustentabilidade, Escola Superior Agrária, Instituto Politécnico de Viana do Castelo, 4900-347 Viana do Castelo, Portugal
4
2AI-School of Technology, Polytechnic Institute of Cavado and Ave (IPCA), 4750-810 Barcelos, Portugal
*
Author to whom correspondence should be addressed.
Biomass 2025, 5(1), 15; https://doi.org/10.3390/biomass5010015
Submission received: 26 November 2024 / Revised: 28 February 2025 / Accepted: 4 March 2025 / Published: 6 March 2025
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Abstract

:
This study explores the extraction and utilization of tannins from Acacia sp. bark residues for water treatment applications. As a by-product of forest management, Acacia sp. bark is valorized through tannin-based coagulant production, contributing to the circular (bio)economy. A systematic review with bibliometric analysis was first conducted to assess the technical–scientific landscape, identifying methodologies and technologies applied to extract and produce natural tannin-based coagulants from Acacia sp. bark residues for water treatment. From the portfolio of analyzed publications, and which followed the thematic axis addressed and the inclusion criteria, only a single study focuses on performing a life cycle assessment (LCA). Due to the relevance of the topic and the clear lack of existing literature, an environmental assessment of the extraction and production of condensed tannins was performed using the LCA methodology from a gate-to-gate perspective. Among the six process stages, spray drying and adsorption (purification) were the primary sources of environmental impact due to their high energy consumption and makeup ethanol use, respectively. The most effective strategy to enhance environmental performance would be reducing water consumption in extraction, thereby lowering energy demand in spray drying. Since both extraction and spray drying require significant energy, decreasing water use and allowing higher moisture content in the condensed tannin extract would mitigate energy consumption. The LCA study thus proved essential in guiding process development toward a reduced environmental footprint.

1. Introduction

Clean water is essential for human life, sustainable development, and the preservation of ecosystem health. Major human activities—including agriculture, livestock production, manufacturing, power generation, and domestic uses—depend on the availability of water in adequate quantity and of quality at the point of demand [1,2,3,4]. It is increasingly acknowledged that conventional water resources, such as rainfall, snowmelt, and runoff stored in lakes, rivers, and aquifers, are insufficient to meet human needs in water-scarce regions [2,5,6,7].
While improvements in water consumption efficiency can help to reduce the gap between water demand and supply, these measures must be integrated with strategies that enhance both supply and quality [8]. Traditional supply improvement methods, such as reservoir construction, surface water diversion, and pipeline development, are often limited by geographic and climatic conditions, face significant public opposition, and frequently overlook water quality [2].
Unconventional strategies present considerable potential for bridging the water demand–supply gap and ensuring a secure water future. These include localized methods such as fog-water and rainwater harvesting, desalination, and wastewater treatment and reuse [5,9,10,11]. Wastewater treatment focuses on enhancing the quality of used water by reducing contaminant levels below sectoral thresholds for intentional reuse or mitigating the environmental impacts of discharge [12].
Globally, it is estimated that domestic and municipal wastewater production totals 360 km3 per year, of which approximately 41 km3 per year (11.4%) is treated and reused, 149 km3 per year (41.4%) is treated and discharged, and 170 km3 per year (47.2%) remains untreated and is released directly into the environment [2,13].
Conventional water treatment relies on a combination of physical, chemical, and biological processes to remove solids, organic matter, and, in some cases, nutrients from wastewater [14,15]. Among these, coagulation–flocculation is one of the oldest and most widely employed methods in water and wastewater treatment plants. This process removes impurities, particularly suspended particles and colloids, by destabilizing and aggregating them into larger particles that can rapidly settle and be easily separated. The consistent use of coagulants in water treatment processes persists due to its simplicity and cost-effectiveness in accelerating the removal of suspended impurities [16,17]. This underscores the critical role of coagulation in providing clean water for consumption and ensuring treated wastewater can be safely reclaimed or discharged.
The use of chemical coagulants and flocculants, however, is associated with certain environmental impacts. Typically, the addition of inorganic salts, polymers, or their combinations results in the aggregation of fine particles, which can then be removed through sedimentation. Inorganic coagulants present drawbacks such as the requirement for large dosages, limited pH operational ranges, and potential corrosion issues [18]. In contrast, nature-based polymers offer a potentially more sustainable solution, producing smaller sludge volumes, reducing ionic loads, and lowering aluminum content in treated water compared to traditional alum salts [19]. Cationic acrylamide-based polyelectrolytes are commonly used due to their range of molecular weights and charge densities, but they raise significant environmental concerns due to their low biodegradability, which leads to accumulation in ecosystems [20]. Regulations such as the German Fertilizer Ordinance (DüMV) restrict the use of synthetic polymers in sewage sludge for agricultural purposes unless the polymers demonstrate at least 20% biodegradation within two years [21,22,23]. Additionally, both synthetic polymers and inorganic salts are derived from non-renewable resources. The global market for coagulants and flocculants was valued at USD 10.4 billion in 2023 and is projected to grow to USD 12.6 billion by 2028, with an annual growth rate of 3.8% [24]. This demand has driven the development of next-generation coagulants, including nature-based alternatives that align with sustainability goals [16].
Nature-based coagulants, which may or may not be plant-based, enhance contaminant removal processes while offering environmental, economic, and social benefits. Tannins, organic compounds derived from plant parts such as the bark of Acacia sp., are among the most common natural substances used for coagulant production. These compounds act by neutralizing charges and forming bridges between particles, facilitating floc formation and sedimentation [25,26,27]. Tannins, which are classified as hydrolysable or condensed based on their structure, exhibit excellent metal-removal properties and offer advantages such as biodegradability, low toxicity, cost-effectiveness, and ease of application [28]. Commercially available tannin-based coagulants demonstrate the feasibility of using tannins for water treatment [20].
Traditional coagulants/flocculants of metallic origin applied in the treatment of drinking water and wastewater have generated new concerns. These concerns relate on the one hand to the health of consumers who drink water treated with these metallic salts, and on the other to the environmental pollution potentially associated with the sludge obtained at the end of the treatment process [29].
Currently, biocoagulants/bioflocculants are emerging as potential substitutes for inorganic salts. These organic coagulants can be obtained from animals (chitosan and crustaceans), microorganisms (fungi, algae, and bacteria) and plants (bark, leaves, and seeds) [30,31]. Tannins have been studied as a possible environmentally friendly coagulant and flocculant [32].
The search for innovative solutions in water treatment has led to the need to develop safer and more sustainable coagulants that can replace traditional inorganic coagulants. It is imperative to find alternative processes and products that are less toxic and do not pose risks to public health. Although the road to developing efficient and harmless coagulants is challenging, it is not impossible. This research highlights the importance of exploring the circular (bio)economy as a promising opportunity in the sustainable synthesis of biocoagulants for water treatment. In that context, the present study is focused on a new method for producing a nature-based coagulant from condensed tannins extracted from Acacia sp. bark residues for water treatment applications, and addresses specifically the process of extraction, purification, and concentration of condensed tannins from Acacia sp. bark, a forest by-product resulting from debarking the trunk, bark removal being one of the most effective strategies for controlling this woody invasive species.
During the conceptual development of the production process, it is critical to explore alternative production pathways and assess their environmental impacts to support sustainable decision-making processes. Life cycle assessment (LCA) is a well-established method for evaluating the environmental performance of systems [33,34,35,36]. LCA has been used to assess the environmental impacts of tannin extraction and production from biomass [20,37,38,39,40,41], but most studies focus on other applications rather than water treatment.
Notably, only one LCA study has examined the extraction of tannins from Norway spruce bark, identifying the extract drying process as a major contributor to environmental impact [40]. Another study assessed the production of cationized tannin-based flocculants from Norway spruce bark, highlighting the environmental impact of reagents in the cationization step and potential issues in the purification step related to ethanol reuse and resin longevity [20]. Another LCA study compared three large-scale extraction technologies—hot water extraction (HWE), ultrasound-assisted extraction (UAE), and supercritical fluid extraction (SFE)—for the extraction of phenolic compounds from spruce bark. The results indicated that HWE had the lowest environmental impact per unit of extracted compound unless the yield from UAE and SFE was significantly higher due to the environmental burden of ethanol consumption [38]. Other LCA studies have explored tannin extraction from softwood bark for producing leather dyes, adhesives, and resins [37,39,41]. This underscores the lack of LCA studies focusing on tannin-based natural coagulants for water treatment.
This paper aims to highlight the process of condensed tannin production from Acacia sp. bark as a circular (bio)economy and sustainable process, considering (i) on the one hand that Acacia bark is a sustainable forest management by-product resulting from debarking the trunk, bark removal being one of the most effective strategies for controlling this woody invasive species; (ii) on the other hand, that tannin-based coagulants are a promising alternative to metallic coagulants traditionally used in water treatment plants. No similar approach was found in the literature and here lies the novelty of our study.
To advance technical and scientific understanding in this field, this study is structured around two research lines. First, a comprehensive analysis of tannin extraction and application for water treatment is conducted using a systematic bibliometric approach to review the state of the art. Second, the environmental impact of extracting condensed tannins from Acacia sp. bark residues for producing a nature-based coagulant is assessed. Application of the LCA methodology for the environmental evaluation of the condensed tannin production process provides a better understanding of the process of extraction and concentration of condensed tannins, as a raw material for the production of tannin-based coagulants.

2. Materials and Methods

2.1. Bibliometric Analysis

The primary objective of the systematic review with bibliometric analysis is to gain insight into the current technical and scientific landscape surrounding the extraction of tannins towards the production of nature-based coagulants for water treatment. Specifically, this review aims to identify and analyze the methodologies and technologies employed in these processes.
A comprehensive survey of studies available in leading reference databases was conducted to examine the international body of work related to tannin extraction from bark and the production of tannin-based natural coagulants for water treatment. This approach follows a hybrid model characterized by a structured systematization of stages. Bibliometric analysis was chosen as the key approach due to its capability to be applied across various disciplines, particularly in generating scientific production indicators [42].
The research followed a four-stage systematic methodology: (1) database search, (2) meta-analysis, (3) bibliometric analysis, and (4) systematic analysis (Figure 1).

2.1.1. Database Search

This study utilized the Web of Science and Scopus databases. The identification and selection of scientific publications followed the systematic methodology outlined in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) framework, encompassing the phases of identification, selection, eligibility, and inclusion [43].
This research was conducted in September 2023. During the initial identification stage, the keywords “tannin” AND “bark OR biomass” AND “extraction OR production” AND “water” were used to search for relevant publications within the databases. During the selection stage, exclusion criteria were applied to filter out publications that lacked a DOI, did not provide full-text access, were not open-access, or were published more than 10 years ago (prior to 2013). In the eligibility stage, the titles and abstracts of the remaining publications were reviewed. The final inclusion stage involved a full-text assessment to exclude studies misaligned with the thematic focus or whose content deviated from the abstract and study scope. Bibliometric data from the review process were exported in .bib and .xls formats for analysis.

2.1.2. Meta-Analysis

The bibliometric data were organized based on key publication attributes, including year, authorship, journal, citations, and keywords. Additionally, the data were categorized by research objectives, methodologies, and applied technologies.

2.1.3. Bibliometric Analysis

Bibliometric analysis employed both qualitative and quantitative indicators. Microsoft Excel and R Studio Open Source Edition (AGPL v3) (Bibliometrix) were used to generate graphs, tables, and figures that provided a comprehensive representation of the data.

2.1.4. Systematic Analysis

To assess the state of the art on tannin extraction from bark and its application in water treatment, the collected publications were systematically analyzed. Summary tables were developed to effectively synthesize the extracted information.

2.2. Life Cycle Assessment

The primary objective of the LCA study was to evaluate the potential environmental impacts associated with the extraction of condensed tannins from Acacia sp. bark residues towards the production of a tannin-based coagulant. Additionally, a secondary aim was defined: to identify the life cycle phase that contributes most significantly to the overall environmental impact. The LCA methodology, which encompasses all stages of a product’s life cycle [33,34,35,36,44], was employed in compliance with ISO 14040 and ISO 14044 standards [45,46]. This methodology comprises four main stages: goal and scope definition, life cycle inventory analysis (LCI), life cycle impact assessment (LCIA), and interpretation of results.

2.2.1. Definition of Goal and Scope

The LCA study aimed to quantify the environmental impacts associated with the extraction of condensed tannins from Acacia sp. bark residues collected in northern Portugal and the subsequent production of a nature-based coagulant. The Acacia sp. bark, a by-product of forest management, is valorized through its use in tannin-based coagulant production, contributing significantly to the circular (bio)economy. The environmental assessment’s objective was to identify and optimize the most sustainable pathways for product development. The defined functional unit (FU) for this LCA is 1 kg of condensed tannins yield following post-extraction treatment from Acacia sp. bark residues.
A gate-to-gate approach was adopted for this study. The system boundary begins with the drying and shredding of Acacia sp. bark residues, which are then loaded into a solid–liquid extraction unit using water as the solvent. The aqueous extract undergoes purification via adsorption, followed by solvent removal through evaporation. Finally, the extract is spray-dried to eliminate any remaining water content (Figure 2). The production and collection stages of Acacia sp. bark were excluded from the system boundary, as they are considered part of the sustainable forest management system. This system inherently involves strategies for controlling the growth of invasive species, such as Acacia sp., and promoting biodiversity.

2.2.2. Life Cycle Inventory

The life cycle inventory was developed based on data gathered during process simulation with SuperPro Designer® software 10 [47], a process simulator software that facilitates the modeling, evaluation, and optimization of integrated processes, specifically developed for the simulation of bioprocess unit operations [48,49]. For the simulation process, data related to mass and energy flow were obtained from lab-scale experiments mainly focused on the optimization of the process of extraction of condensed tannins from the bark of Acacia dealbata, as described by Ferreira et al. (2023) [50]. Under the conditions tested and considering the lower energy requirements, the results suggest that the most sustainable operating conditions corresponded to the ethanolic extraction with a yield of 1.9%. The highest concentration of condensed tannins was detected in samples of Acacia sp. with greater vegetative development (trunk perimeter of 20 cm), corresponding to 246.2 ± 9.4 milligrams per gram of dry extract. Based on these experimental results, a first approach to designing the industrial process was devised using simulation with SuperPro Designer® reflecting the mass balance of the process of extraction and production of 1 kg of condensed tannins from Acacia sp. bark (FU).
The results of this LCA will be fundamental for the design of a pilot-scale unit, which should always precede the implementation of the process on an industrial (real) scale. Such experiments allowed identifying all process flows and collecting all process data, both input and output streams, including materials, energy requirements, products, on-site emissions, and waste streams. Moreover, taking as an initial premise the LCA methodology to determine the environmental impacts attributed to condensed tannin production, the scope of the analysis, as well as the FU, the system boundaries, and the life cycle assessment method, has been defined. Accordingly, the FU of 1 kg of product (condensed tannins) was considered during process simulation with SuperPro Designer®, and the process data were recalculated.
All flows associated with equipment and capital goods employed during the extraction and production of condensed tannins from Acacia sp. bark residues were excluded from this study, since it is not usual to consider the infrastructure in LCA studies [44], and also, there is a lack of available LCA data on infrastructure related to the production of tannin-based coagulants. The Ecoinvent database [51] was used as a main secondary data source, and missing data were collected from literature and laboratory data. The Ecoinvent database was selected since it is a globally leading database and has a long history of gathering data [52]. Ultimately, the LCI was performed in SimaPro 8.5 software [53]. An overview of mass and energy flows to produce 1 kg of condensed tannins yield after post-extraction treatment from bark residues (FU) is presented in Figure 2. In this context, it is important to highlight that the entire process has been designed and planned to meet sustainability criteria wherever possible, minimizing the consumption of resources (mainly energy) and the production of waste and wastewater. To that regard, to dry the initial biomass, a conventional drier was used after a preliminary solar-assisted drying stage, aiming at reducing energy consumption to 2 kWh for process sustainability.

3. Results

3.1. Bibliometric Analysis

This research was conducted during September 2023, and included publications between the years of 2013 and 2023. Table 1 lists the number of publications found in the databases considered in this study during the database search.
Based on a combined search of the keywords, 345 publications were found; after the exclusion criteria, 133 publications were available. The following criteria were defined for the inclusion of these publications for subsequent systematic analysis: the publication must contain analyses of the use of tannins, whether from bark or another part of plant biomass, and must be used in water treatment. A portfolio of 12 publications was selected after reading the titles and abstracts of all 133 publications. Publications that did not belong to the thematic axis addressed were excluded, culminating in a sample of 12 publications for detailed analysis, one of which is a literature review article [54]. Many of the publications found focus on other applications, such as tannin characterization [55,56], antimicrobial properties [57,58], leather tanning [59], and the adhesive and chemical industry [60,61,62]. Thus, Table 2 presents the list of the 12 publications that were selected for a full analysis.
The difference between the number of publications located and the number of publications fulfilling the inclusion criteria demonstrates the importance of the methodology applied for the selection of a representative sample of publications on the theme addressed, allowing the minimization of biases.
Figure 3 presents the chronological distribution of relevant publications over the years. The increasing interest in this topic in recent years is evident, highlighting its relevance, as the number of publications from the past six years accounts for 100% of the identified studies. It is important to note that the number of publications for 2023 represents only a partial count for the year, as the search was conducted in September.
Publications on this topic were identified in 12 different journals, each represented by a single publication, as shown in Table 2. Among the 67 authors contributing to these 12 publications, the European continent had the highest representation, with 51 authors (approximately 76%), followed by the Asian continent with 13 authors (approximately 19%). The American continent had three authors, accounting for approximately 5%. These 67 authors are affiliated with various institutions. Collectively, the 12 publications indexed in the Web of Science database have accumulated a total of 120 citations. Table 3 provides a summary of each study’s objectives and the methodologies employed to achieve them.
The results were distributed and analyzed in relation to the origin of the tannin, the process by which it was obtained, and the material to be eliminated or reduced in the water treatment process. Each category considered different parameters, as shown in Table 4; the literature review article was not considered for analysis. Only Carlqvist et al. [20] and Tomasi et al. [72] did not carry out experiments on using tannin in water treatment, focusing their studies on obtaining/extracting tannin.
Of the publications analyzed, 81.82% used tannin from tree bark, which was the main source of tannin in the studies analyzed; only Jing et al. [68] used pulp, which was the source of only a few studies. With regard to water treatment, 45.5% of the studies analyzed the removal of color and turbidity in water with the use of tannins, and these are the materials that have been studied the most. The removal of organic material or metals was analyzed in only three studies.
From the portfolio of the 12 publications that were analyzed, and that followed the thematic axis addressed and the inclusion criteria, only one study focuses on performing a LCA [20], which assessed the production of cationized tannin-based flocculants from Norway spruce bark. This underscores the lack of LCA studies focusing on tannin-based natural coagulants for water treatment.

3.2. Life Cycle Assessment

The ReCiPe midpoint Hierarchist V1.01 method was employed to evaluate the environmental midpoint impacts [73]. This model is well suited to European conditions, aligning with the geographical context of this LCA study. Of particular interest is the Global Warming Potential (GWP), given the current emphasis on environmental and sustainability targets. Consequently, this category will be analyzed in detail first (Figure 4), followed by an assessment of 18 environmental categories (Figure 5).
Figure 4 provides an overview of the relative contributions to GWP for each process step. The spray drying phase exhibits the highest potential impact on GWP in this analysis and serves as the primary hotspot for many ReCiPe impact categories, as will be further demonstrated in the summary of all 18 impact categories (Figure 5). This high impact is attributed to substantial energy consumption, based on the Portuguese electricity mix. The Portuguese electricity mix used represents 0.115 kg CO2 eq/kWh, and its production is from 40% renewable sources. The spray drying process involves the atomization of the extract solution containing tannins, into droplets by spraying followed by the rapid evaporation of the sprayed droplets into solid powder by hot air. This process requires drying materials with high liquid contents to form droplets, which leads to high energy consumption to evaporate the liquid [74]. The adsorption step holds the second-largest potential GWP impact, primarily due to the use of makeup ethanol and adsorption resin, despite the assumption of reuse of condensed ethanol vapor. Other process steps, including drying, shredding, extraction, and evaporation, are also notable energy consumers. The total power consumption for each stage of the process was determined on the basis of the equipment’s power rating and the duration of operation. For the simulation with SuperPro Designer, the most suitable equipment available on the market was identified that simultaneously meets the process requirements for a given stage and whose operation requires the minimum amount of resources, including energy.
Based on collected data, the total GWP for the life cycle system is estimated at 64.31 kg CO2 eq per functional unit (FU), with the GWP for spray drying and adsorption steps estimated at 30.48 kg CO2 eq/FU and 19.52 kg CO2 eq/FU, respectively.
On a broader scale, Figure 5 shows the relative contributions to all ReCiPe impact categories for the production of condensed tannins. The burden distribution is generally similar across most impact categories, dominated by the spray drying and adsorption processes. Exceptions include stratospheric ozone depletion, marine eutrophication, terrestrial ecotoxicity, human non-carcinogenic toxicity, and land use, where the adsorption process is the main contributor. In these environmental impact categories, the main environmental burden is the adsorption step largely due to the use of makeup ethanol and adsorption resin, even with the assumption of reuse of condensed ethanol vapor. These results underline the environmental impact of reagents/chemicals in the extraction and production of condensed tannins from Acacia sp. bark residues.
These findings align with previously published studies [20,40] when considering the functional unit and system boundaries. A potential strategy to enhance the environmental performance of the system involves reducing water usage during the extraction step. Since both extraction and spray drying are energy-intensive, lowering water content could reduce energy demand. The data for the adsorption step further emphasize the benefits of ethanol reuse for elution and the use of a longer-lasting resin.
It is noteworthy that this method for producing condensed tannin extract from Acacia sp. bark residues for water treatment applications will be further refined to deepen the understanding of its environmental impacts. Future work may explore eliminating the spray drying step to reduce energy consumption, as the primary aim is to produce condensed tannins for water treatment, where they are rehydrated in subsequent steps. Figure 6 presents a comparison of the environmental impact between the dry condensed tannin extract and an alternative scenario without the spray drying step. The alternative scenario is analogous to the dry condensed tannin extract but devoid of the spray drying step. Thus, for the LCA of the alternative scenario, the parameters used in all previous steps are equal to those of the dry condensed tannins scenario, and only the final step of the process is omitted.
Results indicate that omitting the spray drying phase could reduce the environmental impact by approximately 50% in several categories, including GWP, ionizing radiation, fine particulate matter formation, terrestrial acidification, freshwater eutrophication, freshwater ecotoxicity, marine ecotoxicity, human carcinogenic toxicity, and fossil resource scarcity. The total GWP of the condensed tannins without spray drying is estimated at 33.84 kg CO2 eq/FU, representing a potential reduction of 47.4% in environmental impact. However, while the ethanol solution of tannins may not be directly usable for water treatment, alternative processing methods could be considered to remove ethanol without relying on energy-intensive spray drying. For example, vacuum evaporation or low-temperature distillation could be employed to selectively remove most of the remaining ethanol, leaving a more concentrated aqueous tannin solution. This approach could significantly reduce energy consumption compared to spray drying while still ensuring that the final product is free of ethanol and suitable for water treatment applications. Another alternative scenario may consider that the liquid tannin concentrate could be further processed by dilution with water, followed by a simple drying method such as air drying or drum drying, which might be more energy-efficient than spray drying.
The production of Acacia and Acacia sp. bark collection stages were excluded from the system studied because it is assumed that it should be considered as part of the sustainable forest management system which requires the implementation of strategies to control the growth of invasive species (such as Acacia sp.) and promote biodiversity. However, an integrated approach to environmental impact assessment may have to be considered to enable large-scale production of Acacia sp. bark tannin-based coagulants, including, for instance, other sources of plant-based tannins.

4. Conclusions

The present study addressed the extraction process and use of tannins from bark residues for application in water treatment.
It was found that there is a scarce number of publications on the subject of “extraction process and use of tannins from bark for water treatment” published in the most recognized databases worldwide, which became evident as more specific keywords were included. For each added specific keyword, the number of relevant publications significantly decreased, until only 133 publications of interest remained. After an exhaustive analysis of those 133 publications, 12 were considered to have content actually focused on the area of interest, one of which is a literature review article.
These 12 publications were analyzed with respect to three factors: (1) the origin of the tannin, (2) the process by which it was obtained, and (3) the material to be eliminated or reduced in the water treatment process. Results show that only two publications did not carry out experiments on using tannins in water treatment, focusing their studies on obtaining/extracting tannins. The main source of tannins in the selected publications was tree bark, followed by wood, and the least used was pulp. In water treatment processes, removal of color and turbidity are the main objectives of tannin application. Only one of the selected studies focuses on LCA, highlighting the scarcity of LCA research on tannin-based natural coagulants for water treatment.
The second part of this study examined the environmental impact of condensed tannin production from Acacia sp. bark residues using a gate-to-gate LCA. Among the six process steps analyzed, spray drying and adsorption (purification) emerged as the primary contributors to environmental impact, driven by high energy consumption and the resin used in the adsorption column, respectively.
Enhancing the environmental performance of the system requires improving the sustainability of the extraction and spray drying steps. The LCA highlights water consumption reduction during extraction as a key strategy to minimize environmental impact. Since both extraction and spray drying are energy-intensive, lowering water use and allowing a higher moisture content in the condensed tannin extract would reduce overall energy demand. For the adsorption step, the analysis underscores the importance of ethanol reuse during elution and selecting a resin with a longer lifespan to mitigate environmental effects. By implementing these improvements, resource efficiency can be significantly enhanced, reducing the overall footprint of the process. This LCA study successfully identified critical areas for optimization, providing valuable insights for process development aimed at minimizing environmental impact.
The LCA methodology application for the environmental evaluation of the condensed tannins production process provides a better understanding of the process of extraction and concentration of condensed tannins from Acacia sp. bark residues., as a raw material for the production of tannin-based coagulants.

Author Contributions

Conceptualization: C.L.S. and R.S.; methodology: C.L.S. and A.B.P.S.N.; formal analysis: C.L.S. and A.B.P.S.N.; investigation: C.L.S., A.B.P.S.N. and R.F.; writing—original draft preparation: C.L.S. and A.B.P.S.N.; writing—review and editing: C.L.S., A.B.P.S.N., R.F., A.C.R. and R.S.; supervision: C.L.S. and R.S.; funding acquisition: R.S. and A.C.R.; project administration: R.S. and A.C.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Project TECH (reference NORTE-01-0145-FEDER-000043), financed by the European Fund for Regional Development (FEDER) through Programa Operacional Regional do Norte (NORTE2020), and Portuguese Foundation for Science and Technology (FCT) through projects UIDB/05549/2020 and UIDP/05549/2020.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data will be made available by the authors on reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Systematic flowchart of the bibliographical research.
Figure 1. Systematic flowchart of the bibliographical research.
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Figure 2. System boundary, showing the production system with mass and energy flows to produce 1 kg of condensed tannins (FU) from Acacia sp. bark residues.
Figure 2. System boundary, showing the production system with mass and energy flows to produce 1 kg of condensed tannins (FU) from Acacia sp. bark residues.
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Figure 3. Chronological distribution of relevant publications over the years.
Figure 3. Chronological distribution of relevant publications over the years.
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Figure 4. Relative contributions (in %) to the GWP for each process stage for the production of 1 kg of condensed tannins yield after post-extraction treatment from Acacia sp. bark residues (FU).
Figure 4. Relative contributions (in %) to the GWP for each process stage for the production of 1 kg of condensed tannins yield after post-extraction treatment from Acacia sp. bark residues (FU).
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Figure 5. Relative contributions (in %) to the impact categories of ReCiPe per subsystem in the overall production of 1 kg of condensed tannins yield after post-extraction treatment from Acacia sp. bark residues (FU).
Figure 5. Relative contributions (in %) to the impact categories of ReCiPe per subsystem in the overall production of 1 kg of condensed tannins yield after post-extraction treatment from Acacia sp. bark residues (FU).
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Figure 6. Comparison of relative contributions (in %) for all midpoint impact categories of the dry condensed tannins and dry condensed tannins without spray drying.
Figure 6. Comparison of relative contributions (in %) for all midpoint impact categories of the dry condensed tannins and dry condensed tannins without spray drying.
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Table 1. Number of studies found according to the keywords defined in the databases.
Table 1. Number of studies found according to the keywords defined in the databases.
KeywordsWeb of ScienceScopus
“tannin” 33,41651,848
“tannin” + “bark OR biomass”36281189
“tannin” + “bark OR biomass” + “extraction OR production”99931
“tannin” + “bark OR biomass” + “extraction OR production” + “Water”3414
Table 2. List of studies selected for systematic analysis of the research topic, in ascending order of year of publication.
Table 2. List of studies selected for systematic analysis of the research topic, in ascending order of year of publication.
AuthorTitleJournal
Bacelo et al., 2018 [63]Recovery and valorisation of tannins from a forest waste as an adsorbent for antimony uptakeJournal of cleaner production
Grenda et al., 2018 [64]Tannin-based coagulants from laboratory to pilot plant scales for coloured wastewater treatmentBioresources
João and Júnior, 2019 [65]Utilization of the Pinus bark for tannin extraction to apply with as coagulant in the treatment of industrial effluentsRevista Virtual de Química
Panzella et al., 2019 [66]Exhausted woods from tannin extraction as an unexplored waste biomass: evaluation of the antioxidant and pollutant adsorption properties and activating effects of hydrolytic treatmentsAntioxidants
Grenda et al., 2020 [67]Up-scaling of tannin-based coagulants for wastewater treatment: performance in a water treatment plantEnvironmental science and pollution research
Carlqvist et al., 2020 [20]Life cycle assessment of the production of cationized tannins from Norway spruce bark as flocculants in wastewater treatmentBiofuels bioproducts and biorefining (Biofpr)
Kavitha and Kandasubramanian, 2020 [54]Tannins for wastewater treatmentSN applied sciences
Jing et al., 2022 [68]Simultaneous adsorption of Cu2+ and Cd2+ by a simple synthesis of environmentally friendly bamboo pulp aerogels: adsorption properties and mechanismsPolymers
Bello et al., 2022 [69]Effects of tree harvesting time and tannin cold/hot-water extraction procedures on the performance of spruce tannin bio coagulant for water treatmentChemical engineering journal
Nicomel et al., 2022 [70]Selective copper recovery from ammoniacal waste streams using a systematic biosorption processChemosphere
Jinze et al., 2023 [71]Willow bark proanthocyanidins with potential for water treatment: chemical characterization and zinc/bisphenol A removalSeparation and purification technology
Tomasi et al., 2023 [72]Microwave-assisted extraction of polyphenols from Eucalyptus Bark—A first step for a green production of tannin-based coagulantsWater
Table 3. Summary of the objective and methodology of each publication, in ascending order of year of publication.
Table 3. Summary of the objective and methodology of each publication, in ascending order of year of publication.
AuthorObjectivesMethodologies and Technologies
Bacelo et al., 2018 [63]Removal of antimony that may be present in mine drainage and mine flotation wastewater and/or from much more dilute solutions, such as contaminated surface water or groundwaterTreatment of raw bark chemically to decontaminate synthetic solutions and industrial effluents on a laboratory and pilot scale and by regeneration, pyrolysis, or incineration of saturated bark
Grenda et al., 2018 [64]Producing nature-based treatment agents to remove color from waterModified tannin extract on a laboratory and pilot plant scale using the Mannich condensation reaction
João and Júnior, 2019 [65]Reuse Pine bark to extract tannins and use it as a coagulant in the treatment of effluent generated in the household cleaning products industryExtraction of tannins using different solvents such as 5% sodium bisulphite, ethanol, and water
Panzella et al., 2019 [66]To characterize the antioxidant properties and other properties of practical interest of chestnut wood and quebracho wood, together with those of a chestnut wood fiber produced from steamed chestnut wood for the adsorption and removal of pollutantsTannins extracted by immersing the wood chips in an autoclave with water at 120 °C under pressure; the extracts were concentrated in a multiple-effect evaporator under vacuum until the water was removed, and the tannin powder was obtained by spray drying
Grenda et al., 2020 [67]Production of nature-based cationic coagulants for wastewater treatment using different origins and sources of tannin to analyze the performance of the eco coagulants obtained and their biodegradability in an effluent treatment plant for industrial effluent treatmentModification of tannins through Mannich aminomethylation with formaldehyde and dimethylamine hydrochloride for water treatment on a pilot plant scale
Carlqvist et al., 2020 [20]Elaborate the LCA of the production of a flocculation agent based on cationic tannins from Norway spruce bark, from the production of spruce trees to the flocculation product, identifying the potential for optimization and comparing it with the three conventional flocculants normally used: polyacrylamide, aluminum sulfate, and iron (III) chlorideThe Ecoinvent 3.6 database was used to assess the life cycle impact (LCA) of the extraction, purification and drying, and cationization phases of tannins for the production of flocculants in wastewater treatment
Kavitha and Kandasubramanian, 2020 [54]Analyze the literature and describe polyphenols and their efficiency in removing cationic heavy metal ions and dyes from polluted industrial wastewaterLiterature review
Jing et al., 2022 [68]Synthesis of a tannin/chitosan/bamboo pulp aerogel (TCPA) as an ecological, renewable, and low-density adsorbent for the treatment of wastewater containing mixtures of heavy metalsAerogel synthesized using a simple freeze-drying method and analyzed using FTIR, XPS, SEM, TEM, TGA, and surface area and porosity methods
Bello et al., 2022 [69]Investigating the effect of seasonal variation (winter and summer) on spruce bark tannin extracts as biocoagulants for water treatment applicationsTannins obtained directly after dry peeling in the factories, extracted using three different water extraction procedures—cold water extraction (21 °C), cold water extraction plus hot water extraction (85 °C), and direct hot water extraction (85 °C)—and synthesized using the Mannich reaction
Nicomel et al., 2022 [70]Analyzing 23 biosorbents for Cu adsorption from a synthetic Cu-NH 3 leachateBatch experiments, discontinuous experiments, and adsorption analysis at different pHs
Jinze et al., 2023 [71]Study the chemical structure of crude extracts rich in proanthocyanidins from the bark and carry out tests as adsorbents for artificial wastewater treatmentSoft water extraction and chromatographic fractionation, purification by nuclear magnetic resonance (NMR) and ultra-performance liquid chromatography mass spectrometry and adsorption tests
Tomasi et al., 2023 [72]Optimizing the extraction of tannin from eucalyptus bark to produce a coagulant for water and effluent treatmentDifferent extraction techniques, such as SLE, PLE, UAE, and MAE, using distilled water as a solvent, vacuum filtration through a glass microfiber membrane, calculation of extraction yield (EY), analysis of polyphenol (TPC) and condensed tannin (CT) content, microwave-assisted extraction, and JMP statistical software (trial version 16)
Table 4. Summary of the main parameters analyzed in each study.
Table 4. Summary of the main parameters analyzed in each study.
AuthorsTannin Extraction for Use in Water Treatment
Tannin OriginExtraction
Methodology		Water Treatment
BWPPYCMTCLMTOMCC
Bacelo et al., 2018 [63]+ + + +
Grenda et al., 2018 [64]++ +++++
João and Júnior, 2019 [65]+ +++++ ++
Panzella et al., 2019 [66] + + ++++++
Grenda et al., 2020 [67]++ +++++
Carlqvist et al., 2020 [20]+ + +
Jing et al., 2022 [68] ++++ +
Bello et al., 2022 [69]+ + +++ +
Nicomel et al., 2022 [70]+ +++ +
Jinze et al., 2023 [71]+ +++ +
Tomasi et al., 2023 [72]+ +++
Legend: B = bark; W = wood; P = pulp; PY = physics; M = mechanics; C = chemistry; T = turbidity; CL = color; MT = metals; OM = organic matter; CC = chemical compounds.
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MDPI and ACS Style

Simões, C.L.; Neto, A.B.P.S.; Rodrigues, A.C.; Ferreira, R.; Simoes, R. Environmental Assessment of Tannin Extraction from Bark Residues for Application in Water Treatment. Biomass 2025, 5, 15. https://doi.org/10.3390/biomass5010015

AMA Style

Simões CL, Neto ABPS, Rodrigues AC, Ferreira R, Simoes R. Environmental Assessment of Tannin Extraction from Bark Residues for Application in Water Treatment. Biomass. 2025; 5(1):15. https://doi.org/10.3390/biomass5010015

Chicago/Turabian Style

Simões, Carla L., Alice B. P. Santos Neto, Ana C. Rodrigues, Ricardo Ferreira, and Ricardo Simoes. 2025. "Environmental Assessment of Tannin Extraction from Bark Residues for Application in Water Treatment" Biomass 5, no. 1: 15. https://doi.org/10.3390/biomass5010015

APA Style

Simões, C. L., Neto, A. B. P. S., Rodrigues, A. C., Ferreira, R., & Simoes, R. (2025). Environmental Assessment of Tannin Extraction from Bark Residues for Application in Water Treatment. Biomass, 5(1), 15. https://doi.org/10.3390/biomass5010015

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