Removal of copper and nickel from municipal sludge using an improved electrokinetic process

Abstract

An improved electrokinetic (EK) process for the removal of copper and nickel from municipal sludge was investigated using a custom-made experimental setup. Optimal parameters were identified as an HNO3 electrolyte concentration of 0.200mol/L in the anode chamber; NaNO3 electrolyte concentrations of 0.800mol/L in the heavy-metals and cathode chambers; and a current density of 2.0mA/cm2. Adjustment of the sludge pH to 3.8 before treatment clearly increased the current efficiency (CE) and decreased the specific energy consumption (SEC) of the process. Under the optimal conditions in EK systems equipped with two and three sludge chambers, the removal efficiencies for copper and nickel were 51.5–82.9% and 46.6–75.1%, respectively, after 132h. The results of a sequential extraction procedure revealed that the binding states of the metals in the sludge were changed from difficult-to-extract to more easily extractable states after the EK process. Energy consumption calculations showed that the CE increased to 137.8% and the SEC decreased to 2.39kWh/kg of metals removed when three sludge chambers were equipped in the EK system. A mechanistic analysis of the current method demonstrated that it can be regarded as an effective technique for removing metals from municipal sludge.