Highly selective recovery of phosphate ions using a novel carbonaceous adsorbent synthesized via co-pyrolysis of spent coffee grounds and steel slags: A potential phosphatic fertilizer

•SSs were used as alternatives to pure chemicals for surface modification of SCA.•SS@SCA had superior adsorption capacity of PO43− ions and magnetic separability.•Adsorption of PO43− ions by SCA was predominantly controlled by ligand exchange.•Electrostatic surface complexation governed adsorption of PO43− ions by SS@SCA.•PO43−-loaded SS@SCA considerably facilitated seed germination rate and root growth.