Dual-gated interfacial oxidation enables dry purification of dye-wastewater recovery salt in a plasma fluidized bed

Dye waste salts extracted from dye-wastewater is a recoverable NaCl/Na₂SO₄-rich solid whose purification requires removal of interfacial organic residues while preserving the inorganic salt matrix. Here, we identify dual-gated interfacial oxidation as a site-pathway mechanism in a dielectric barrier discharge plasma fluidized bed. The first gate controls site formation: bubbling-like transport of 179 μm particles at 0.151 m·s^-1 repeatedly delivers organic-rich salt interfaces into discharge-accessible regions, while dynamic dielectric contacts redistribute current release from sparse bursts to more frequent microdischarge events, increasing the pulse counts from 8 ± 2 to 15 ± 5 per half-cycle while decreasing the inter-event-interval coefficient of variation (IEI-CV, dimensionless) from 0.45 to 0.25. The second gate controls pathway selection: oxygen-related excitation must be high enough to activate interfacial oxidation but not overdriven into ionization-weighted transport disruption. At 15.0 kV, the power-normalized O(I) emission index at 777.2 nm (η_O) reached 0.516 and coincided with the effective particle-transport/oxygen-excitation window defined by the coupled particle Reynolds number (Re_P) and oxygen-related excitation index (Re_P-η_O window); further voltage increases raised the N₂⁺/N₂ emission ratio but suppressed particle transport, separating stronger discharge from useful oxidation. Independent O₂ perturbation further showed that the O(I)-based oxygen-related excitation response tracked the increase in TOC removal. Consequently, total organic carbon (TOC) removal peaked only when interfacial access and oxidation-relevant excitation converged, reaching 95.7%, with a TOC removal energy yield of 28.17 g_TOC·kWh^-1. Surface-organic depletion was accompanied by substantial carbon conversion, with CO₂-C representing the largest quantified fate of removed carbon (57.1%), while FTIR and XPS confirmed depletion of organic-rich surface coverage while XRD and ion chromatography confirmed retention of the dominant NaCl/Na₂SO₄ matrix. Together these findings highlight a resource-oriented strategy for closing material loops in zero-liquid-discharge wastewater treatment systems.