Tandem catalytic strategy enables selective upcycling of polystyrene waste into pure toluene

Methylene blue is a widely used dye in the textile industry, but its non‑biodegradable and carcinogenic nature poses serious ecological and health risks. The United States Environmental Protection Agency has set a maximum concentration below 0.2 ppm. Conventional treatment methods such as adsorption merely transfer pollutants to a solid phase, while advanced oxidation processes require precise chemical dosing or are energy‑intensive. In a study published in ENG. Chem. Eng., researchers at Jiangnan University and collaborators report a dielectric barrier discharge microfluidic plasma system that synergizes microfluidics with plasma for continuous and efficient methylene blue degradation.

The reactor consists of a polytetrafluoroethylene capillary wrapped around a quartz tube. A copper rod inside the quartz tube serves as the high‑voltage electrode, while a copper mesh around the capillary acts as the ground electrode. Both the capillary and quartz tube function as dielectric barriers, enabling stable dielectric barrier discharge plasma inside the capillary. Argon carrier gas and methylene blue solution form a gas‑liquid biphasic flow through the plasma zone.

Systematic investigation of key parameters revealed that degradation efficiency increases with longer residence time and higher plasma power, and decreases with higher initial methylene blue concentration. At a plasma power of 5.6 W, a 20 mg·L⁻¹ methylene blue solution achieved 99.72 % degradation at a residence time of 15.95 seconds. Increasing the plasma power to 17.3 W enabled complete (100 %) degradation of a 100 mg·L⁻¹ methylene blue solution at the same residence time.

Kinetic analysis using pseudo‑zero‑order, pseudo‑first‑order, and pseudo‑second‑order models showed that the degradation process follows pseudo‑first‑order kinetics, with a correlation coefficient (R²) of 0.9969. The rate constant decreased from 0.369 to 0.151 s⁻¹ as the initial methylene blue concentration increased from 20 to 100 mg·L⁻¹, attributed to competition for reactive radicals.

Optical emission spectroscopy identified hydroxyl radicals (OH band at 309 nm), atomic hydrogen (Hα at 656 nm, Hβ at 486 nm), and atomic oxygen (777 nm) as the dominant reactive species. Radical quenching experiments with isopropanol—a well‑known hydroxyl radical scavenger—significantly suppressed degradation. At a residence time of 15.95 seconds, the degradation efficiency decreased from 99.72 % to 88.71 %, and the first‑order rate constant dropped from 0.369 to 0.134 s⁻¹, confirming that hydroxyl radicals are the pivotal oxidative species.

Mass spectrometry analysis revealed a degradation pathway initiated by hydroxyl radical‑driven demethylation and ring cleavage, producing intermediates such as azure B, azure A, and azure C, followed by further oxidation to low‑molecular‑weight organic acids and inorganic ions.

The energy yield for a 100 mg·L⁻¹ methylene blue solution at a residence time of 6.85 seconds was 73.10 g·kWh⁻¹. Unlike conventional batch plasma reactors, this microfluidic system operates continuously, with stable discharge confirmed by periodic voltage‑charge signals and consistent optical emission spectra.

While current throughput is limited by a single capillary, the system can be scaled up by numbering up multiple microfluidic channels. This work establishes microfluidic plasma processing as an intensified, efficient, and continuous strategy for organic pollutant treatment in water.