The heavy metals present in sludge are very harmful to the environment and the human body. It is necessary to remove them before sludge disposal. In this study, an ex situ method for removing heavy metals from sludge using electroosmosis was designed, and experiments were conducted to study the effects of electroosmosis voltages (30 V, 40 V and 50 V), citric acid concentrations (0.03 mol/kg, 0.06 mol/kg, 0.09 mol/kg and 0.12 mol/kg) and the power supply (continuous and interrupted) on the removal rate of heavy metals (Cu, Cr, Cd, Zn and Pb). The study found that the combination of the citric acid pretreatment and electroosmosis can effectively improve the removal rate of heavy metals. An appropriate increase in the voltage and citric acid concentration can improve the removal rate of heavy metals. The best combination was determined to be 40 V and 0.09 mol/kg. Under these conditions, the removal rates of Cu, Cr, Cd, Zn and Pb, were 14.39%-41.28% (continuous power supply) and 21.78%-42.36% (interrupted power supply). The interrupted power supply effectively improved the removal rates of Cd and Zn but reduced the removal rates of Cr and Pb. The power supply mode had no significant effect on the removal rate of Cu. Heavy metal speciation was analyzed by the BCR method, and the relationship between the ratio of easily removed metal speciation and the removal rate was studied. The increase in the ratio of easily removed speciation was accompanied by an increase in the removal rate, indicating that citric acid and electroosmosis increased the removal rate by increasing the ratio of easily removed speciation. However, the high pH value around the cathode caused by the electrochemical reaction was not conducive to the removal of heavy metals.

Introduction

Sewage sludge, a byproduct of sewage treatment processes, is rich in nitrogen (N), phosphorus (P), potassium (K) and organic matter. It is a popular practice to dispose of sewage sludge via land use, and many countries currently use this method. However, it can be difficult to convince the government and land users to agree on this method because of the presence of toxic metals and bacteria in the sludge. Therefore, it is necessary to separate the heavy metals from the sludge [1,2]. The methods for the removal of heavy metals from sludge are chemical extraction, electrokinetic remediation, filtration, ion exchange, adsorption on activated carbon, membrane technologies, microbiological leaching, phytoremediation, etc. [3-9]. The electrokinetic process aims to remove the heavy metals using an electric field, which promotes several complex mechanisms such as electrodialysis electric migration, electrophoresis, and electrolysis. This approach is conducive to the transportation of pollutants [10,11]. These effects cause the heavy metals to concentrate around the cathode, which allows for their removal. The advantages of electrokinetic remediation are that it is simple (i.e., simple equipment), flexible (i.e., can be used for in situ or ex situ treatments), relatively inexpensive, effective and environmentally friendly [12,13].