Circular upcycling of waste toner into eco-safe superparamagnetic nano-Fe3O4@C core-shell pigment for sustainable magnetic leather coatings.

The widespread use of laser printing and photocopying technologies has led to the accumulation of waste toner powder (WTP), a chemically stable and potentially carcinogenic pollutant. Herein, a scalable pyrolysis approach is demonstrated for converting WTP into a functional magnetic pigment for magnetic leather finishing. Pyrolysis at 600 °C for 2 h, guided by thermogravimetric analysis, enabled controlled phase transformation monitored by hyphenated TGA-FTIR. The resulting material (WTP-600) was comprehensively characterised using VSM, FTIR, XPS, pXRD, FESEM, HRTEM, EDX, 57Fe Mössbauer analysis, ICP-OES and TOC analysis. These analyses confirm near-complete removal of the polymeric fraction during pyrolysis, leading to an ≈ 97% reduction in volume of pristine WTP and the formation of a carbonaceous shell coated on a superparamagnetic nanocrystalline Fe3O4 core (Fe3O4@C). The carbonaceous surface of WTP-600 is enriched with hydroxyl functionalities; it is eco-friendly, and its magnetic strength is ≈ 2.3-fold higher than that of pristine WTP. Both WTP and WTP-600 were formulated with a commercial leather-finishing dispersion and applied as surface finishing on leather. As anticipated, the magnetic strength of WTP-600-coated leather was approximately 2.3 times higher than that of WTP-RT-coated leather. Evaluation of coating performance parameters and organoleptic assessment demonstrated that WTP-600-coated leather exhibited superior surface finishing and tactile properties compared to WTP-RT-coated leather. Magnetic leather of various colours was successfully synthesised by incorporating different pigments into WTP-600, resulting in only a marginal reduction in magnetic properties. The magnetic pigment-coated leather was found to be microbiologically stable, environmentally safe, and cytocompatible with human epidermal HaCaT keratinocyte cell lines. The prepared magnetic leathers demonstrate significant potential for advanced applications, including adhesive-free wall tiling, educational tool fabrication, and energy harvesting from human motion.