Adsorption performance and mechanism of modified rice hull biochar for lead and cadmium removal in single and competitive systems

While adsorption in multi-metal systems is crucial for practical wastewater treatment, its efficacy and mechanisms remain insufficiently explored. This study investigates the adsorption performance and underlying mechanisms of Pb(II) and Cd(II) using modified rice hull biochar in single and binary solute systems. Biochar was functionalized through β-cyclodextrin and NaOH modifications, and characterization revealing that NaOH more effectively improved porosity, endowing NaOH-modified carbon (NBC) with a larger specific surface area and pore volume. In single-metal systems, β-cyclodextrin-modified carbon (BBC) demonstrated superior adsorption capacities. In binary systems, Pb(II) exhibited a strong competitive advantage over Cd(II). Sequential adsorption experiments confirmed that pre-sorbed Cd(II) could be displaced by subsequently added Pb(II), indicating higher affinity for the latter. Kinetic studies revealed that Pb(II) adsorption was better fitted by the pseudo-first-order model, while Cd(II) adsorption was better described by the pseudo-second-order model. Isotherm analysis demonstrated Pb(II) adsorption on NBC was better described by the Freundlich model, whereas on BBC it followed the Sips model. Cd(II) adsorption on NBC was better fitted by the Sips model, while on BBC it exhibited perfect agreement with the Langmuir model. The superior affinity for Pb(II) is attributed to its smaller hydrated radius, lower hydration energy, and stronger complexation capability. This work establishes NBC as an efficient and selective adsorbent for Pb(II) removal from complex wastewater systems.