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 PO43− adsorption capacity (Qe = 23.0 mg/g) and magnetic separability (90.1%) of SS@SCA compared to pristine carbonaceous adsorbents (SCA) prepared through simple pyrolysis of SCGs (Qe = 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 Qe: 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, PO43−-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.