Waste bulrush transformed into copper-enhanced material for dye removal from wastewater

Industrial activities such as textile manufacturing, paper production, leather processing and food processing can generate wastewater containing synthetic dyes. Because many synthetic dyes are chemically stable, difficult to biodegrade and environmentally persistent, their release into waterways can pose risks to aquatic ecosystems and human health.

To address this challenge, researchers at Doshisha University in Japan devised a single-step co-pyrolysis strategy to turn agricultural waste into an amphoteric adsorbent that can remove synthetic dye contaminants. The approach was developed by Asif Ali, a Ph.D. student and MEXT scholar, and professors Michiaki Matsumoto and Yoshiro Tahara of the university’s Department of Applied Chemistry, Graduate School of Science and Engineering. The study appears in the Journal of Environmental Chemical Engineering.

Sharing the motivation for the study, Ali says, “An alarming 20% of toxic, recalcitrant textile wastewater is discharged untreated, causing severe damage to aquatic life and human health. So far, no single wastewater treatment method is universally suitable.” Conventional treatment methods, including membrane separation and advanced oxidation, can be constrained by high energy requirements, secondary products, fouling, maintenance and operating costs. Activated carbon provides an effective alternative for dye removal, but commercial activated carbon can be costly to produce and regenerate, while some metal-modification approaches require additional chemical reducing agents.

To overcome these hurdles, the research team used bullrush agricultural waste as a sustainable carbon precursor. Copper(II) nitrate trihydrate and potassium hydroxide (KOH) facilitated the in situ growth of zero-valent copper nanoparticles (Cu⁰) within a mesoporous framework. Through single-step co-pyrolysis, the team used the biomass’s own in-situ volatile reducing gases (CO and H₂) to reduce copper precursors. The process produced a zero-valent copper nanoparticle-enhanced bullrush activated carbon composite, ZVCu@BAC, without secondary chemical reductants.

To characterize the composite, the team used SEM, TEM, EDX mapping, FTIR, BET, XRD and TGA/DTA. Unlike unmodified BAC, which formed an amorphous, highly porous structure, the ZVCu@BAC composite exhibited a mesoporous architecture, with a high surface area of 984.5 m²/g and a total pore volume of 0.615 cm³/g. Needle-like Cu⁰ structures were uniformly distributed across and securely anchored to the carbon matrix without clumping.

The researchers then conducted comparative batch adsorption experiments to evaluate removal of the cationic dye methylene blue (MB) and the anionic dyes methyl orange (MO) and sunset yellow (SY). With an amphoteric interface (pH_pzc ≈9.0), the ZVCu@BAC composite achieved broad-spectrum removal of both cationic (MB, q_m = 62.31 mg/g) and anionic dyes (MO, q_m = 56.37 mg/g; SY, q_m = 35.76 mg/g).

The Langmuir isotherm best described the equilibrium data, while pseudo-second-order kinetics indicated that chemisorption played a major role in the rate-controlling process. Thermodynamic evaluations showed that the adsorption processes were highly spontaneous (ΔG^∘ < 0) and exothermic (ΔH^∘ < 0). Adsorption was supported by multiple mechanisms: electrostatic attraction, π─π stacking, pore filling and coordinate bonding with metallic Cu⁰ sites.

Further characterization showed that the composite retained 90.6% MB removal efficiency and 88.0% MO removal efficiency after six regeneration cycles using a 0.1 M KOH/acetone eluent. Together with an estimated production cost of about ¥1,800 per kilogram, the results suggest that the bullrush-derived ZVCu@BAC composite could be a cost-effective, sustainable and scalable option for industrial wastewater treatment.

The composite could be explored for simultaneous removal of mixed dyes during industrial wastewater treatment, with its wide operational pH tolerance expanding its potential applications. Its ability to undergo repeated solvent-based regeneration may also reduce the need for replacement.

Emphasizing the significance of the findings, Matsumoto says, “Our study findings present the opportunity to advance a circular economy: valorizing invasive, zero-cost bullrush weed biomass into a functional carbon framework through an eco-friendly, single-step co-pyrolysis route.”

The study brings us one step closer to keeping rivers and streams clean and protecting water resources.

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