Adsorption of Cd(II), Cu(II), and Zn(II) by granules prepared using sludge from a drinking water purification plant

A granular sludge-clay (GSC) adsorbent was prepared based on the sludge obtained from drinking water purification plant and clay, with a mass ratio of 1:2. The adsorption of Cd(II), Cu(II), and Zn(II) onto GSC was also investigated. The adsorption process was highly pH-dependent, with the maximum adsorption of Cd(II), Cu(II), and Zn(II) occurring at pH 4.0, 5.0, and 5.0, respectively. Kinetics data correlated well with the pseudo-second order kinetics model, suggesting that the Cd(II), Cu(II), and Zn(II) adsorption processes were chemisorption. The equilibrium data were fitted well by the Langmuir isotherm model with the maximum Cd(II), Cu(II), and Zn(II) adsorption capacities of 1.53, 2.76, and 1.23 mg/g at 298 K and pH 5.0, respectively. Temperature effect showed same tendency on the Cd(II), Cu(II), and Zn(II) adsorption, exhibiting higher adsorption capacities with higher temperature. The results of competitive adsorption demonstrated that Cu(II) was preferentially adsorbed by GSC, resulting in a higher removal efficiency both in the bi-metal and tri-metal systems. The major adsorption mechanisms of Cd(II), Cu(II), and Zn(II) by GSC were the electrostatic attraction, hydroxyl substitution, and surface complexation. GSC synthesized using readily available raw materials (sludge and clay) is therefore a very promising adsorbent for removing Cd(II), Cu(II), and Zn(II), particularly Cu(II), from surface water and groundwater.