Reusing sewage sludge ash as adsorbent for copper removal from wastewater

Abstract

The sewage sludge ash (SSA) can potentially be used for removal of heavy metals from wastewater due to its similar chemical composition to that of fly ash and blast-furnace slag. This study investigated the adsorptive characteristics of SSA including specific surface area, cation-exchange capacity (CEC), and pH of zero point of charge (pH ZPC). In addition, the adsorption isotherms of SSA with synthetic wastewater containing copper ions were tested. Experimental results indicated that the CEC and pH ZPC of SSA were 24.1–25.7 meq/100 g and 3.1–3.4, respectively, which exhibited the capability of removing heavy metals from wastewater. The adsorption test of applying SSA into synthetic wastewater revealed that the adsorption isotherm of SSA for copper ions generally followed the Langmuir model. The estimated maximum capacity of copper adsorbed by SSA was 3.2–4.1 mg/g and close to that of fly ash. The primary mechanisms of copper removal by SSA included electrostatic attraction, surface complex formation, and cation exchange. The precipitation of copper hydroxide occurred only when the dosage of SSA and the equilibrium pH of wastewater were at a high level (30–40 g/l and greater than 6.2, respectively). In general, similar to fly ash and blast-furnace slag, the SSA can be reused as an adsorbent for copper removal from wastewater and the removal efficiency was greater than 98%.