Phosphorus-rich effluents generated during fertilizer production require efficient treatment technologies to minimize their environmental impact. This study evaluates the adsorption of phosphate ions using a carbonaceous material before and after surface modification with iron under both batch and continuous-flow operating conditions. The modified adsorbent was prepared by a simple wet impregnation method to introduce iron active sites capable of enhancing phosphate uptake. The influence of operating mode and solid–liquid contact time on adsorption performance was investigated. Surface modification improved phosphate uptake through electrostatic attraction, ligand exchange, and surface complexation mechanisms. Under continuous-flow conditions, extending the contact time by solution recirculation increased adsorption efficiency, demonstrating the importance of residence time in fixed-bed systems. The results indicate that the proposed material is suitable for the treatment of highly concentrated phosphorus-containing wastewaters and that continuous adsorption can provide performance comparable to, or slightly better than, batch operation when sufficient contact time is ensured.