Green synthesis of silver nanoparticles using Chromolaena odorata leaf extract for adsorptive removal of heavy metals and textile dyes from aqueous systems

Background

Water pollution by heavy metals and synthetic dyes is a critical global environmental challenge with severe consequences for ecosystems and human health. Pollutants such as lead (Pb²⁺), copper (Cu²⁺), cobalt (Co²⁺), iron (Fe²⁺), and textile dyes including methyl orange, methyl red, methyl blue, and Congo red are commonly released into water bodies from industrial effluents, textile manufacturing, mining, and agricultural runoff. These contaminants are persistent, bioaccumulative, and toxic, with risks including oxidative stress, neurodevelopmental damage, and ecosystem disruption. Conventional remediation methods such as chemical precipitation, ion exchange, membrane filtration, and activated carbon adsorption have been widely applied but remain constrained by cost, secondary waste generation, and operational complexity.

Green nanotechnology has emerged as a sustainable alternative, particularly the biosynthesis of silver nanoparticles (AgNPs) using plant extracts as reducing and stabilizing agents. Chromolaena odorata (locally known as Hagonoy) contains phytochemicals such as flavonoids, phenolic acids, terpenoids, and saponins that enable the ecofriendly synthesis of stable nanoparticles. While prior studies have reported antimicrobial and biomedical applications of C. odorata-mediated AgNPs, their environmental application in the removal of both heavy metals and dyes has received limited exploration. This study addresses this gap by synthesizing AgNPs via C. odorata leaf extracts and systematically assessing their adsorption performance under controlled laboratory conditions.

Methods

Fresh C. odorata leaves were shade-dried, powdered, and subjected to aqueous extraction. The extract was mixed with AgNO₃ solution to induce nanoparticle synthesis. Nanoparticle formation was confirmed by ultraviolet–visible (UV–Vis) spectroscopy, which exhibited a sharp surface plasmon resonance (SPR) peak at 428 nm, indicating stable AgNP synthesis.

Batch adsorption experiments were carried out using 50 mL of contaminant solution with concentrations ranging from 10–50 mg/L, treated with 0.1 g of AgNPs under agitation at 150 rpm. Removal efficiency was quantified using UV–Vis spectrophotometry, while adsorption capacity was calculated by mass balance. Kinetic modeling was conducted using pseudo-first-order (PFO) and pseudo-second-order (PSO) equations. Adsorption isotherms were fitted to Langmuir, Freundlich, and Temkin models, and thermodynamic parameters (ΔG°, ΔH°, ΔS°) were derived using the van ’t Hoff equation at temperatures of 25°C, 35°C, and 45°C.

Results

Nanoparticle characterization. The UV–Vis spectrum displayed an SPR peak at 428 nm, characteristic of spherical, monodispersed AgNPs stabilized by phytochemicals in the C. odorata extract.

Adsorption efficiency. AgNPs exhibited high removal efficiencies across contaminants. For heavy metals, removal efficiency followed the order Pb²⁺ (92.3%) > Fe²⁺ (88.5%) > Cu²⁺ (85.1%) > Co²⁺ (81.6%). For dyes, removal efficiency ranked as methyl orange (89.7%) > Congo red (86.4%) > methyl red (84.2%) > methyl blue (77.5%). Optimal performance was observed at near-neutral pH (6–7) for metals and slightly acidic pH (5–6) for dyes. Equilibrium was reached within 90–120 minutes.

Kinetics. Adsorption data aligned closely with the PSO model (R² > 0.99), confirming chemisorption as the primary mechanism. Metals generally achieved equilibrium faster (90 minutes) than dyes (100 minutes).

Isotherms. Langmuir modeling best described the adsorption process, suggesting monolayer adsorption. Maximum adsorption capacities (qmax) were 50 mg/g (Pb²⁺), 46 mg/g (Fe²⁺), 43 mg/g (Cu²⁺), and 44.5 mg/g (Co²⁺). Dyes exhibited slightly lower qmax values (40–42 mg/g). RL values of 0.36–0.50 indicated favorable adsorption. Freundlich constants (KF = 9.8–12.5) reflected moderate surface heterogeneity, while Temkin parameters confirmed significant adsorbate–adsorbent interactions.

Thermodynamics. Negative ΔG° values confirmed spontaneous adsorption. Positive ΔH° values (15–18 kJ/mol) indicated endothermic processes, and positive ΔS° values (118–140 J/mol·K) reflected increased randomness at the solid–solution interface. These thermodynamic properties are consistent with entropy-driven adsorption involving release of bound water molecules during adsorption.

Discussion

The superior removal of Pb²⁺ compared with other metals can be explained by its larger ionic radius and higher affinity for surface functional groups, which promote strong binding. In contrast, the relatively lower adsorption of methyl blue may result from steric hindrance and electrostatic repulsion, limiting access to active sites. The PSO kinetic model’s strong fit suggests chemisorption mechanisms, while Langmuir isotherm modeling confirms uniform monolayer adsorption. The thermodynamic data affirm that adsorption is spontaneous, feasible at higher temperatures, and enhanced by entropy-driven mechanisms.

These findings highlight the potential of C. odorata-mediated AgNPs as sustainable adsorbents, not only because of their efficiency but also because of their ecological implications. C. odorata is an invasive species in the Philippines and other tropical regions; using its leaves for nanoparticle synthesis provides a dual benefit by controlling an invasive weed while creating value-added applications in environmental remediation.

Conclusion

This study demonstrates the potential of Chromolaena odorata-mediated AgNPs as effective, ecofriendly adsorbents for removing heavy metals and synthetic dyes from aqueous solutions. High removal efficiencies, favorable adsorption kinetics, and thermodynamically feasible processes underscore their viability for wastewater treatment. Beyond laboratory findings, practical application requires addressing scalability, nanoparticle aggregation, and performance in complex wastewater matrices. Economic feasibility also depends on estimating how many kilograms of C. odorata leaves are required to synthesize sufficient AgNPs for treating several liters of wastewater in real-world settings.

Overall, this research advances green nanotechnology for environmental applications, aligning with global sustainability goals, particularly Sustainable Development Goal 6 (Clean Water and Sanitation). Future work should include pilot-scale validation, regeneration studies, and integration into hybrid treatment systems to ensure cost-effectiveness and long-term applicability.