Application of Walnut Shell-Based Composite Activated Carbon Adsorbents for the Removal of Phosphate Ions from Secondary Wastewater

Phosphorus is essential for plant growth and freshwater ecosystems. However, excessive concentrations in aquatic environments accelerate eutrophication and threaten aquatic life and biodiversity. In this study, walnut shell-derived activated carbon (WAL-AC) was modified with magnesium (WAL-AC@Mg), calcium (WAL-AC@Ca), and tin (WAL-AC@Sn) in different mass ratios, as well as with chitosan/magnesium (WAL-AC@CS/Mg), and evaluated for the removal of PO43− from aqueous solutions and real wastewater. Among all modifications, magnesium-activated WAL-AC (WAL-AC@Mg4:1) performed best, achieving consistently high phosphate removal efficiency (approximately 95%; 95.3% at pH 7.0) across the tested pH range (3–9). The adsorption of WAL-AC@Mg4:1 followed pseudo-second-order kinetics and the Langmuir isotherm (Qm = 41.80–46.13 mg/g, 303–323 K), with thermodynamics confirming spontaneous, endothermic uptake. FTIR, and SEM-EDS analyses revealed surface Mg-O sites, a P-O band at 996 cm−1, and phosphorus accumulation (up to 8.61 at.%) after adsorption, consistent with ligand exchange and possible Mg-phosphate precipitation. WAL-AC@Mg4:1 remained effective in real secondary wastewater (73% removal) and maintained approximately 34% removal after five regeneration cycles. A preliminary agronomic test showed that WAL-AC@Mg4:1 potentially recovered from wastewater supported the growth of tomato seedlings, providing a preliminary proof-of-concept for phosphorus recovery and reuse rather than a demonstrated fertilizer benefit. These findings identify magnesium-functionalized WAL-AC as the most promising adsorbent among those tested, derived from a low-cost agricultural waste precursor, combining phosphate removal from secondary wastewater with resource recovery.