1.  Introduction

The concept of “waste to wealth” has emerged as a cornerstone in sustainable development discourse, emphasizing the transformation of unwanted or discarded materials into valuable resources. Rooted in the philosophy of the circular economy, this approach seeks to minimize waste, extend the lifecycle of materials, and optimize resource recovery through closed-loop systems. Unlike the traditional linear model of ―take–make–dispose,‖ a circular framework envisions waste streams as raw materials for new processes, ensuring ecological balance and economic resilience. Globally, this paradigm has gained momentum as societies grapple with mounting waste accumulation, dwindling natural resources, and pressing climate challenges [1].

Within this context, biomass waste from invasive species has become an underexplored yet abundant feedstock. In India and across many tropical regions, Eichhornia crassipes (water hyacinth) and Lantana camara are notorious for their aggressive spread. Water hyacinth, often termed the ―world’s worst aquatic weed,‖ clogs waterways, depletes dissolved oxygen, and disrupts aquatic biodiversity [2]. Similarly, Lantana camara dominates vast stretches of forests and grasslands, displacing native flora and altering ecosystem services [3]. Parallel to these terrestrial and aquatic invasions, algal blooms—driven by eutrophication and nutrient runoff—pose severe ecological threats. Harmful cyanobacterial blooms reduce water quality, produce toxins dangerous to both humans and animals, and accelerate the degradation of freshwater ecosystems [4].

Traditionally, management strategies for such biomass have been reactive and unsustainable. Weeds are often removed manually or mechanically and subsequently burned, dumped, or left to decay, contributing to air pollution, greenhouse gas emissions, and secondary ecological damage [5]. Algal blooms, similarly, are frequently treated with chemical algaecides or physical removal, approaches that provide short-term relief but fail to address resource recovery or long-term sustainability [6]. These conventional practices not only squander opportunities for valorization but also perpetuate the waste problem.

In recent decades, researchers have explored pathways for converting weeds into biofuels, compost, animal feed, and handicrafts, while algae have been investigated for pigments, biofertilizers, nutraceuticals, and bioenergy production [7][8]. However, these efforts have largely remained fragmented, focusing on one biomass type at a time. This siloed approach overlooks the potential synergies of integrating diverse biomass sources into a single valorization system. A biorefinery framework that combines weeds and algae could enhance efficiency, diversify product streams, and reduce environmental burdens, creating a truly circular model.

Thus, the objective of this paper is to propose and analyze an integrated weed–algae valorization system within the circular economy framework. By examining the complementary properties of invasive weeds and algal biomass, and aligning them with sustainable technologies, this study envisions a pathway where ecological problems are converted into economic opportunities. The

work aims to highlight the feasibility, benefits, and challenges of such an integrated approach, offering insights into how waste streams can be redesigned into value chains that serve both environmental and societal goals.

2.  Review of Literature

• Weeds as biomass: composition and prior valorization

Invasive macrophytes such as Eichhornia crassipes (water hyacinth) and shrubs like Lantana camara are abundant, fast-growing sources of lignocellulosic biomass. Compositional analyses report that water hyacinth typically contains substantial cellulose and hemicellulose with relatively low lignin content (reported ranges: cellulose ≈16–38%, hemicellulose ≈19–33%, lignin ≈3–10% depending on tissue and study), making it amenable to biochemical and thermochemical conversions. These compositional properties have prompted investigation into hyacinth as a feedstock for cellulose nanofibers, bioethanol, biogas, and biochar production. Studies have demonstrated successful extraction routes (acid/alkaline pretreatment, enzymatic hydrolysis) and promising yields for both fermentable sugars and biochars suitable as soil amendments. [9][10][11]

Lantana camara presents a different biochemical profile — higher in phenolics and secondary metabolites and with tougher woody fractions — which has limited some biochemical routes but opened others. Lantana has been trialed as feedstock for anaerobic digestion (with pretreatment or co-digestion), composting (producing biomanure with pest-repellent properties), and for low-tech applications such as craft materials and fuelwood. Its allelopathic compounds complicate direct use as feed, but composting or thermochemical conversion can detoxify residues and yield useful soil amendments or energy products.[12][13][14]

2.2.  Algae as biomass: pigments, fertilizers, biofuels, and feed

Microalgae and cyanobacteria are chemically distinct from terrestrial weeds: they are rich in proteins, lipids, polysaccharides and pigments (chlorophylls, carotenoids, phycobiliproteins such as phycocyanin). This composition underpins multiple valorization pathways. Algal pigments are increasingly studied for food, cosmetic, pharmaceutical, and dye-sensitized solar cell applications because they combine strong light absorption with bioactivity. Algal biomass is also used as a biofertilizer and soil biostimulant (improving soil microbiome, nutrient availability and plant growth), and lipids/carbohydrates are convertible to biodiesel and bioethanol respectively. Several recent reviews document progress in algal biofertilizers and pigment extraction methods as well as techno-economic bottlenecks for scale-up.[15][16][17][22]

2.3.  Waste-valorization approaches: bioenergy, biochar, phytochemical extraction

The principal valorization modalities explored in the literature fall into biochemical (fermentation/anaerobic  digestion),  thermochemical  (pyrolysis/gasification  to biochar/bio-oil/charcoal), and extraction routes (solvent, enzymatic, supercritical extraction of pigments, polyphenols, oils). For water hyacinth, anaerobic digestion and bioethanol production have shown useful yields when appropriate pretreatments or co-digestion (e.g., with manure) are applied, while pyrolysis produces biochar with beneficial soil amendment properties and activated carbon useful for water purification. Multiple studies report that water-hyacinth-derived biochar has favourable adsorption properties for dyes and can improve soil biological activity; pyrolysis conditions markedly affect yield and surface chemistry. Likewise, Lantana has been tested for biogas production (often enhanced by cellulolytic microbial consortia) and composting to produce biomanure; composted Lantana can reduce pest incidence in treated soils. On the algal side, pigment extraction (phycocyanin, carotenoids), lipid extraction for biodiesel, and whole-biomass use as biofertilizer are well documented—although economic viability remains an obstacle for many single-product value chains.[10][13][18][19]

2.4.  Circular economy and biorefinery concept

The literature increasingly frames biomass valorization within a biorefinery or circular-economy perspective: instead of single-product valorization, an integrated cascade can produce multiple co-products (energy, soil amendment, chemicals, pigments), and residual streams from one process can feed another. Several recent reviews argue that such integrated, multi-stream biorefineries increase resource efficiency, improve overall economics by product diversification and reduce environmental externalities (e.g., avoiding landfill or burning). Yet most practical projects remain modular and siloed (weed-only or algae-only); few demonstrate a fully integrated weed–algae system that couples ecological removal, co-processing, and local reuse—particularly at the decentralized, community or watershed scale. This gap is the focus for proposed integrated systems that combine mechanical/ecological removal, sequential extraction (e.g., pigments → lipids → digestate → biochar), and circular reuse (digestate/biochar → agriculture; activated carbon → water remediation).[17][20][21]

3.  Proposed Integrated Weed–Algae Valorization System

• Conceptual framework

The integrated valorization model proposed here builds on the principles of the circular economy and biorefinery design, in which diverse biomass feedstocks are processed through complementary pathways to maximize product recovery and minimize waste. The system envisions Eichhornia crassipes (water hyacinth), Lantana camara, and algal biomass as primary inputs. Their combined utilization addresses two pressing ecological issues—weed proliferation and harmful algal blooms—while generating a portfolio of sustainable outputs.

The guiding principle is that the residue of one process becomes the feedstock of another, ensuring closed-loop cycling and minimal ecological burden [22,23].

3.2  Step 1: Biomass harvesting and pretreatment

Efficient harvesting is the first stage. For aquatic weeds and algae, mechanical removal via skimmers, booms, or dredgers is required, while terrestrial weeds like Lantana demand manual cutting or mechanized chippers. Pretreatment methods aim to improve processability:

• Drying and shredding reduce bulk and facilitate storage.

• Mechanical or chemical hydrolysis (acid/alkali pretreatments, enzymatic saccharification) breaks down lignocellulosic structures, particularly in Lantana and hyacinth, to release fermentable sugars [24].

• For algae, mild drying and bead milling can enhance lipid or pigment extraction without significant nutrient loss [25].

These pretreatments not only enhance the efficiency of downstream bioconversion but also help homogenize biomass quality, enabling co-processing of otherwise heterogeneous inputs.

3.3  Step 2: Conversion streams

• Bioenergy

One primary valorization stream is bioenergy generation through anaerobic digestion or fermentation. Hydrolyzed weed biomass yields fermentable sugars for bioethanol production, while co-digestion of weed and algal slurry enhances biogas yield due to complementary carbon-to-nitrogen ratios [26]. Algae contribute proteins and lipids, improving methane content, whereas weed biomass supplies structural carbohydrates. Digestate residues from anaerobic digestion can subsequently be valorized as organic fertilizer, closing the nutrient cycle [27].

(b)  Biochar

Thermochemical conversion through pyrolysis transforms dried weed biomass into biochar. Biochar serves dual purposes: as a soil conditioner that improves water retention and nutrient availability, and as a precursor for activated carbon. Studies show water-hyacinth-derived biochar exhibits high cation exchange capacity and can adsorb heavy metals and dyes from wastewater [28]. When co-pyrolyzed with algal residues, synergistic effects improve both carbon stability and surface area, expanding its utility in environmental remediation [29].

(c)  Biofertilizers

Residual slurries from anaerobic digestion, along with composted Lantana biomass, form nutrient-rich biofertilizers. These biofertilizers provide macro- and micronutrients, while also introducing beneficial microbial consortia that improve soil health [30]. Algal biomass in particular contributes plant growth-promoting substances such as auxins, cytokinins, and polysaccharides, enhancing the value of the final fertilizer product [31].

(d)  Pigments and phytochemicals

Microalgae are a well-established source of high-value pigments (e.g., carotenoids, phycocyanin) and bioactive compounds. Solvent extraction and chromatography techniques can be used to isolate these products before energy conversion, thereby creating a ―cascade‖ biorefinery model where high-value nutraceuticals are extracted first, followed by energy recovery from the residue [32]. Similarly, phytochemicals present in Lantana (notably phenolics and flavonoids) could be harnessed for natural pesticide or medicinal applications after detoxification [33].

(e)  Activated carbon

Finally, carbonization and chemical/steam activation of biochar yield activated carbon with high porosity and surface area. Activated carbon derived from weeds has demonstrated efficiency in adsorbing dyes, heavy metals, and organic pollutants from water streams [34]. In an integrated framework, algal residues could supply nitrogen dopants during activation, further enhancing adsorption capacity and catalytic properties [35].

3.4  Integration principle: waste-to-input cascading

The true innovation of the proposed system lies not in any single pathway but in its integration. Instead of discrete, siloed valorization efforts, the system connects streams:

• Extracted algal pigments leave behind carbohydrate-rich residue ideal for fermentation.

• Digestate from anaerobic digestion is applied to soil or reintroduced into algal cultivation ponds as a nutrient medium.

• Biochar not only acts as a soil amendment but also improves the performance of composting weeds by stabilizing nitrogen losses.

• Activated carbon derived from weed biochar can treat effluents from algal harvesting ponds, creating a feedback loop for cleaner water inputs.

In this cascading model, nothing is wasted: every by-product is redirected into another productive stream, maximizing resource recovery while minimizing ecological impact.

An integrated system could be deployed at the community or watershed scale, transforming invasive biomass from a nuisance into a cornerstone of local circular economies [36].

3.5  Anticipated benefits and challenges

Adopting this integrated weed–algae valorization framework offers several benefits: ecological restoration through weed/algae removal, diversified product streams that enhance economic feasibility, and a reduction in reliance on fossil-derived inputs. However, challenges remain in terms of logistics (continuous harvesting, seasonal variability), pretreatment costs, and the need for decentralized processing infrastructure. Research into scalable technologies, community participation, and supportive policies will be essential for practical implementation [37].

4.  Outputs and Applications

The integrated weed–algae valorization system yields a diverse portfolio of outputs that span multiple sectors, supporting both economic viability and ecological restoration.

4.1  Energy Applications

One of the primary outputs is bioenergy. Anaerobic digestion of weed–algae mixtures produces biogas rich in methane (CH₄). Co-digestion studies have reported methane yields of 220–280 mL CH₄/g volatile solids when water hyacinth is combined with microalgae, significantly higher than mono-digestion of weeds alone (typically ~150 mL CH₄/g VS) [38].

Similarly, Lantana camara biomass has demonstrated potential for bioethanol production, with yields up to 0.18 g ethanol/g dry biomass after enzymatic hydrolysis [39]. Electricity can also be generated by coupling biogas to microturbines or fuel cells, while algal lipids provide an additional biodiesel pathway, with conversion efficiencies of 20–30% lipid-to-biodiesel yield in optimized systems [40].

4.2  Agricultural Applications

Biofertilizers and soil enhancers form a critical agricultural output. Digestate from weed–algae digestion is nutrient-rich, containing 2–3% nitrogen, 1–1.5% phosphorus, and 1–2% potassium—comparable to conventional compost [41]. Water hyacinth biochar applied at 2–4 tons/ha has been shown to increase crop yields by 15–20% due to improved soil aeration and water retention [42]. Algal biofertilizers provide not only macronutrients but also phytohormones like auxins and cytokinins, which stimulate root growth and improve stress resistance in crops [43]. Together, these outputs reduce dependence on chemical fertilizers and promote regenerative farming practices.

4.3  Industrial Applications

The integrated model also generates high-value industrial products. Algal biomass is an established source of pigments: phycocyanin from cyanobacteria can reach extraction yields of 60–65 mg/g dry weight, while β-carotene and astaxanthin extractions yield 2–3% of dry weight under optimized solvent systems [44]. These pigments serve the nutraceutical, pharmaceutical, and food industries, with a global phycocyanin market value exceeding USD 200 million annually [45]. Additionally, weeds like Lantana camara contain phenolic and flavonoid compounds, which exhibit antioxidant and antimicrobial activity, offering potential as biopesticides or in medicinal formulations [46]. Algal residues are also explored for biopolymer production, particularly polyhydroxyalkanoates (PHAs), with reported conversion efficiencies of 0.2–0.4 g PHA/g substrate [47].

4.4  Environmental Applications

Finally, the system contributes directly to environmental sustainability. Biochar and activated carbon derived from weeds exhibit adsorption capacities of 50–120 mg/g for heavy metals such as Pb²⁺ and Cd²⁺, making them effective for wastewater purification [48]. Removal of water hyacinth from lakes has been shown to restore dissolved oxygen levels by up to 40% within months, improving fish productivity and biodiversity [49]. Additionally, biochar contributes to carbon sequestration, with stable carbon fractions persisting for hundreds of years and a sequestration potential of 2–3 tons CO₂ equivalent per ton of biochar applied [50]. Thus, valorization simultaneously addresses invasive biomass management and climate mitigation.

5.  Future Directions

The transition from fragmented valorization practices to integrated weed–algae biorefineries requires strategic planning and scaling. One promising avenue is the establishment of community-scale rural biorefineries near lakes, rivers, and wetlands, where invasive weeds and algal blooms are abundant. Locating processing hubs at the biomass source minimizes transport costs and generates localized employment [1].

Another direction lies in coupling valorization with wastewater treatment plants (WWTPs). Algal cultivation in WWTPs has already demonstrated potential for nutrient recovery and pollutant removal, while weeds like Eichhornia can act as phytoremediators. Integrating valorization here would enable simultaneous water purification and biomass conversion, creating dual environmental and economic benefits [2][3].

Biotechnological advances can further enhance yields. Genetic engineering of algae and microbes could optimize biofuel productivity, improve enzyme cocktails for lignocellulosic weed hydrolysis, and enable high-value metabolite extraction [4][5]. Finally, realizing this vision requires policy support and public–private partnerships. Government incentives, waste management policies, and corporate engagement can accelerate adoption, making integrated valorization systems feasible within the broader circular economy framework [6].

6.  Conclusion

The ―waste to wealth‖ paradigm offers a powerful lens for reimagining ecological problems as opportunities. Invasive weeds such as Eichhornia crassipes and Lantana camara, together with algal blooms, represent abundant yet underutilized biomass. Conventional management approaches—burning, dumping, or chemical suppression—worsen environmental degradation while missing valorization opportunities.

This paper proposed a novel integrated weed–algae valorization system that leverages bioenergy, biochar, biofertilizers, pigments, phytochemicals, and activated carbon within a circular biorefinery model. By designing complementary conversion pathways, the system ensures that the waste of one process becomes the input of another, maximizing efficiency and sustainability.

The approach not only reduces ecological burdens but also offers multidimensional benefits: renewable energy, agricultural inputs, industrial products, water purification, and carbon sequestration. While challenges remain in technology, logistics, and policy, the synergy between invasive weeds and algal biomass represents an untapped frontier in circular bioeconomy. With integrated strategies and supportive governance, these waste streams can be transformed into resilient value chains that drive both environmental restoration and economic development.

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