High-performance orthophosphate-modified biochar for removal of Cd (II) from aqueous solutions: Mechanisms and efficiency

The removal of cadmium (Cd) from aqueous solutions has become a global research hotspot due to its adverse effects on human health and the ecosystem. In this study, three phosphorus-enriched biochars (PBC-1, PBC-2, and PBC-3) were produced from Tamarix chinensis using KH₂PO₄, K₂HPO₄·3H₂O, and K₃PO₄·3H₂O as phosphate precursors, respectively. Among the synthesized materials, PBC-3 exhibited the highest Cd²⁺ removal efficiency. The adsorption of Cd²⁺ onto PBC-3 was governed by chemisorption and was well described by the pseudo-second-order kinetic model (R² > 0.99 and the lowest χ²). At ambient temperature, PBC-3 achieved a theoretical maximum adsorption capacity of 206.94 mg·g⁻¹ based on the Langmuir isotherm model (R² > 0.98 and the lowest χ²). Additionally, the phosphorus concentration in the solution at equilibrium was found to be close to zero. PBC-3 exhibited excellent adsorption performance over a relatively wide pH range (2–8). Mechanistic investigations revealed that phosphate functional groups played a dominant role in Cd immobilization, with mineral precipitation and ion exchange accounting for 52.92 % and 42.80 % of the adsorption, respectively. Correlation analysis revealed that higher H/C, O/C, and (N+O)/C ratios, as well as increased specific surface area, were positively associated with Cd²⁺ uptake. This study demonstrated the advantages of K₃PO₄·3H₂O-modified biochar in enhancing metal-binding capacity, highlighting its potential for the efficient treatment of Cd-contaminated aqueous solutions.