Clean hydrogen production via methane pyrolysis (CH₄ → 2H₂ + C) generates large quantities of solid carbon. Realizing their applications is a critical gap to enable its broader adoption. Here, we show that carbon nano onions encapsulated with iron cores (Fe@CNO) produced in CH₄ pyrolysis using low-cost Fe ore catalysts can serve as a tri-functional adsorbent and catalyst for organic contaminant degradation. Fe@CNO has a high adsorption capacity and fast pseudo-second-order adsorption kinetics for antibiotics in wastewater. Surface oxidized Fe@CNO presents a high catalytic activity for in situ hydrogen peroxide production. Fe@CNO is also an efficient heterogeneous Fenton catalyst to degrade adsorbed antibiotics. Further, the Fe core is magnetic, enabling easy separation from solvents as reusable adsorbents and catalysts. We demonstrate that the adsorption and electro-Fenton process applies to various organic pollutants in different water matrices. This opens a new strategy for utilizing co-products from H₂ production for sustainable environmental applications.