Removal and mechanism of Cu (II) and Cd (II) from aqueous single-metal solutions by a novel biosorbent from waste-activated sludge

The removal and mechanism of Cu2+ and Cd2+ from aqueous single-metal solutions were investigated by using a novel biosorbent from waste-activated sludge. A series of adsorption experiments was designed to disclose the effects of the key factors on the adsorption capacity of the biosorbent for the metal ions. The mass ratio of the biosorbent to metal ion was optimized as 2 to balance the adsorption capacity and the removal efficiency. A right shaking speed (150 r/min) not only ensured enough contact frequency between the sorbent and the adsorbate but also reduced the mass transfer resistance. The natural pH value (about 5.5) of the metal solutions benefited a high adsorption capacity of the biosorbent and avoided the consumption of acid or base for pH adjustment. The adsorption reactions belonged to the endothermic process between 15 and 45 °C. As the scanning electron microscopy (SEM) images showed, the meshy structure with long chains and many branches was ideal for the biosorbent to quickly capture the metal ions. The energy-dispersive X-ray (EDX) spectra confirmed that the adsorbed metal ions lay in the precipitates of the adsorption reactions. According to the FTIR analyses, the functional groups responsible for Cu2+ adsorption majorly consisted of O–H, N–H, COOH, CONH2, and the groups containing sulfur and phosphorus, while those for Cd2+ adsorption contained O–H, N–H, COOH, and CONH2. The differences in the responsible functional groups explained the phenomenon that the adsorption capacity of the biosorbent for Cu2+ was higher than that for Cd2+.