Cascade utilization of phosphate-loaded low-temperature pyrolysis gypsum board for Cd(Ⅱ) and Pb(Ⅱ) immobilization from aqueous and soil systems

Phosphate-loaded pyrolyzed gypsum board was innovatively reused for Cd(Ⅱ)/Pb(Ⅱ) remediation. • Single and coexisting heavy metal systems were systematically evaluated in both aqueous and soil environments. • The recycled material achieved enhanced Cd(Ⅱ) adsorption while retaining high Pb(Ⅱ) removal capacity. • Phosphate-driven mineralization and ion exchange between hydroxyapatite and heavy metals enhance the immobilization. This study proposed a cascade utilization strategy for waste gypsum board by reusing phosphate-loaded pyrolyzed gypsum board (PGB150P) for Cd(Ⅱ) and Pb(Ⅱ) remediation in aqueous and soil environments. The results showed that PGB150 exhibited excellent Pb(Ⅱ) affinity (q max  = 1396.14 mg/g) but limited Cd(Ⅱ) adsorption capacity (161.21 mg/g). PGB150P significantly enhanced Cd(Ⅱ) adsorption (403.84 mg/g) while retaining high Pb(Ⅱ) adsorption performance (1271.13 mg/g). In the Cd(Ⅱ)–Pb(Ⅱ) system, the adsorption capacities of Cd(Ⅱ) and Pb(Ⅱ) decreased to 170.17 and 980.05 mg/g, respectively, with Pb(Ⅱ) showing a stronger competitive advantage. Adsorption followed the Langmuir model and was spontaneous endothermic chemisorption controlled by film and intraparticle diffusion. PGB150P exhibited a broad pH adaptability and strong resistance to common coexisting ions. Mechanistic quantification revealed that Cd(Ⅱ) removal by PGB150P was dominated by precipitation (58.56%) and ion exchange (34.7%), while Pb(Ⅱ) removal was also dominated by precipitation (62.41%) and ion exchange (31.7%). Soil immobilization experiments showed that after 28 days treatment, PGB150P increased the residual fractions of Cd and Pb to 29% and 50%, respectively. In the Cd-Pb system, the residual fractions were increased to 15% and 49%, respectively. After five adsorption–desorption cycles, PGB150P retained about 71% of its Cd(Ⅱ) capacity and 93% of its Pb(Ⅱ) capacity. Cost analysis indicated a total cost of only 39–54 USD/t, substantially lower than that of adsorbents prepared with exogenous phosphate or commercial hydroxyapatite. This study provides theoretical support for the high-value utilization of construction solid waste and the remediation of multi-media heavy metal pollution. Download: Download high-res image (253KB) Download: Download full-size image