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

This study rigorously investigated the viability of carbonaceous adsorbents (SS@SCA) prepared through one-step co-pyrolysis of steel slags (SSs) and spent coffee grounds (SCGs) for selectively capturing phosphate (PO43-) ions in aqueous solutions and their subsequent utilizing a phosphatic fertilizer to promote the plant growth. The higher PO4 3 adsorption capacity (Q(e) = 23.0 mg/g) and magnetic separability (90.1%) of SS@SCA compared to pristine carbonaceous adsorbents (SCA) prepared through simple pyrolysis of SCGs (Q(e) = 2.6 mg/g) were mainly attributed to the enriched metal oxides on its surfaces (i.e., CaO, MgO, Fe3O4, Fe2O3, Al2O3). Moreover, the main adsorption mechanism of PO43 ions toward SCA was changed from physisorption (ligand exchange by alcohol functional groups) to chemisorption (electrostatic surface complexation by CaO and MgO) via one-step co-pyrolysis of SCGs and SSs. This phenomenon enabled SS@SCA to selectively capture PO43 ions under the co-existence of Cl , NO3, SO42-, and CO32- ions (decreasing rates of Q(e): SCA = 38.5-65.4%; SS@SCA = 0.8-8.8%). PO43- - loaded SS@SCA more effectively promoted the seed germination (germination index (GI): 434-467%) and root growth (root length: 4.0-5.1 cm) of garden cress seeds (Lepidium sativum L.) than other liquid and solid matrices, including 20 mg/L PO43-, 100 mg/L PO43- , and SS@SCA (GI: 77-258%; root length: 0.9-3.0 cm). Hence, PO(4)(3- )loaded SS@SCA seemed to be potentially applicable to the agricultural field as a phosphatic fertilizer for accelerating the plant growth. Therefore, PO43-- loaded SS@SCA could be practically used as a phosphatic fertilizer to reinforce the plant growth.

Automated summary

This study investigated the effectiveness of carbonaceous adsorbents (SS@SCA) prepared from steel slags (SSs) and spent coffee grounds (SCGs) for capturing and utilizing phosphate ions. The SS@SCA showed higher adsorption capacity and magnetic separability compared to pristine carbonaceous adsorbents (SCA). The adsorption mechanism of PO43- ions changed from physisorption to chemisorption with the co-pyrolysis process, allowing selective capture of PO43- ions.