Competitive adsorption of pnitrophenol and heavy metals on water-quenched blast furnace slag

Single component and competitive adsorption of p-nitrophenol (p-NP) and heavy metals on water-quenched blast furnace slag (WBFS) were investigated in this study. The adsorption isotherms, kinetics and thermodynamics were studied to explore adsorption characteristics and mechanisms. The results showed that heavy metals played a dominant role in controlling the competitive adsorption with p-NP in a sequence of Cu²⁺ > Cd²⁺ > Zn²⁺. The equilibrium data for adsorption of p-NP by WBFS could be well described by Freundlich isotherm model, while Langmuir isotherm could better fit the data for the adsorption of heavy metal ions on WBFS. The adsorption energy of p-NP calculated by D-R model in each system was -7.53, -7.07, -7.96 and -7.86kJ/mol, respectively. The results showed that WBFS adsorbed p-NP via physical adsorption, while adsorbed metal ions via chemical adsorption and ion exchange. The kinetic results demonstrated that the adsorption rate of p-NP decreased in the presence of heavy metal ions. And the corresponding time to reach adsorption equilibrium was prolonged. However, the patterns for the adsorption of heavy metal ions were not significantly influenced in the presence of p-NP. In the binary system, WBFS exhibited relative higher affinity and selectivity for Cu²⁺, Cd²⁺ and Zn²⁺ than p-NP. The adsorption data of p-NP and heavy metal ions were fitted well by the pseudo-second-order model in both single and binary systems. The values of ΔG decreased with increasing temperature, indicating the spontaneous nature for the adsorption of p-NP and Cu²⁺, Cd²⁺, Zn²⁺ was proportional to the temperature. The thermodynamics results also indicated the adsorption were an endothermic and entropy-increasing process.