Texture-regulated kraft lignin-derived Ni/carbon catalysts for adsorption-assisted H₂O₂ oxidation of dyes in synthetic textile wastewater

Kraft lignin-derived carbons are promising catalytic supports for wastewater treatment, but increasing surface area does not necessarily improve metal retention or substrate-dependent oxidation performance. Here, raw and KOH-activated lignin-derived supports with distinct pore structures were loaded with the same nominal Ni content and evaluated for adsorption-assisted H₂O₂ oxidation of dye-containing water. Carbonization and activation increased the BET surface area from 1.2 m² g⁻¹ for raw lignin to 192.0, 1140.8, and 1504.5 m² g⁻¹ for CLow, CMid, and CHigh, respectively. However, CMid-Ni showed the highest actual Ni loading (4.83 wt%; 96.6% retention), whereas CHigh-Ni retained 4.58 wt% Ni (91.6%). Methylene blue (MB) and Congo red (CR) represented dyes with contrasting charge and molecular structure. At pH 4, CHigh-Ni showed the highest MB oxidation rate ( k app = 0.2154 min⁻¹), whereas CMid-Ni showed the highest CR oxidation rate ( k app = 0.1237 min⁻¹). Carbon-only controls showed measurable apparent dye disappearance after H₂O₂ addition, whereas Ni loading enhanced the apparent rates by 3.16–6.73-fold. In a synthetic textile wastewater matrix containing both dyes and salts, the same numerical ranking was retained, although the CR rate difference between CMid-Ni and CHigh-Ni decreased. H₂O₂ consumption, dissolved-phase TOC reduction, scavenger inhibition, and filtrate tests were consistent with an adsorption-assisted, predominantly surface-associated Ni-mediated oxidation process. Single-run Ni leaching remained below 1%, while cumulative Ni loss after five reuse cycles was 1.51%. These results show that lignin-derived Ni/carbon catalysts should be optimized by balancing pore accessibility, metal retention, and oxidant activation rather than by maximizing BET surface area alone.