Influence of magnesium content and phosphoric acid treatment on cadmium adsorption onto sugarcane leaf biochar

Biochar is an attractive low-cost sorbent for removal of heavy metals such as cadmium (Cd) from solution. This study presents modification of sugarcane leaf biochars with phosphoric acid (H₃PO₄) and magnesium oxide (MgO) and the influence of Mg:P atomic ratio (varied from 0 to 2) on the Cd²⁺ adsorption performance was evaluated. Compared to unmodified biochar (BC), pre-pyrolysis modification with H₃PO₄ alone (P-BC) greatly increased the surface area from 296 to 505 m²/g but its Cd²⁺ adsorptivity decreased by 5-folds due to high acidity. Incorporation of alkaline MgO increased the Cd²⁺ adsorption and pH, despite a reduction in surface area, confirming that Mg played a predominant role in Cd²⁺ removal. When Mg:P > 1 (1.5MgP-BC and 2MgP-BC), more than 99% of the Cd²⁺ was removed, but the solutions were alkaline with pH > 8. Adsorbent 1MgP-BC presented an overall good performance with a maximum Cd²⁺ adsorption capacity of 36.4 mg/g (compared to 12.8 mg/g for BC) and neutral pH. The adsorption kinetics of Cd²⁺ onto both BC and 1MgP-BC were best described by the Elovich and pseudo-second order models, suggesting chemisorption as primary adsorption mechanism. Adsorption isotherms followed the Langmuir (for 1MgP-BC) and Freundlich isotherm models (for BC). Characterization of the post-adsorption biochars with XRD, FTIR, XPS and SEM–EDX confirmed that Cd²⁺ adsorption onto BC was governed by precipitation of CdCO₃ and interaction with specific surface sites, whereas for 1MgP-BC the Cd²⁺ was primarily adsorbed onto Mg phosphates on the biochar surface and precipitated as Cd₃(PO₄)₂.