Equilibrium, kinetics and thermodynamics of cadmium ions (Cd^2 +) removal from aqueous solution using earthworm manure-derived carbon materials

Carbon materials (CMs) derived from earthworm manure via slow pyrolysis at 400–600 °C were employed as adsorbents for the removal of cadmium ions (Cd^2 +) from aqueous solution. The influences of adsorption conditions (including initial concentrations, contact time, ambient temperature, solution pH, ionic strength and adsorbent dosage) on the removal of Cd^2 + using the developed CMs were investigated in addition to the potential mechanism studies in the adsorption process. The increase of initial concentration promoted the adsorption capacity and confined the removal rate of Cd^2 +. Adsorption was vigorously commenced within the initial contact time of 1 h, accounting for 63.81–76.52% to the total adsorption amount. It is found that the adsorption performance was not significantly influenced by the solution pH between 3.5 and 5.5 and the ionic strength between 0 and 0.01 mol NaNO₃ /L. Overall, the optimum initial concentration, contact time, ambient temperature, solution pH, ionic strength and adsorbent dosage were 80 mg/L, 24 h, 25 °C, 5.5, 0 mol NaNO₃ /L and 2 g/L, respectively, in terms of single factor test. The maximum adsorption capacity of the CM prepared from 400, 500 and 600 °C were 32.468, 19.646 and 14.599 mg/g, respectively. The uptake of Cd^2 + by the CMs was subjected to the spontaneous and endothermic processes. Alkali and alkaline earth metallic, especially for Ca^2 + and Mg^2 +, can accelerate the adsorption through ion exchange interactions. Complexation and precipitation driven by surface functional groups involving oxygen-containing groups and aromatic C[dbnd]C, and salt minerals (phosphates, carbonates or silicates) may also make important contributions to the adsorption process.