افزایش حذف فلزات سنگین از لجن در تصفیه الکتروکینتیک با بیوسورفکتانت ها Biosurfactants enhanced heavy metals removal from sludge in the electrokinetic treatment

1. Introduction The biological treatment methods were commonly adopted by wastewater treatment plants (WWTPs) in the worldwide, which led to a large amount of sludge production [1, 2]. The latest statistics indicated that 6.25 million tons dry solids (DS) were generated in 2013 [3], and the amount of dewatered sludge (at a water content of 80%) has been increased from 30 million tons in 2012 to 34 million tons in 2015 in China [1, 3]. Nowadays, sludge disposal would be confronted with great challenges in China. Many sludge disposal methods have been used, such as utilization in building, land application, landfill and incineration [1, 3, 4]. As a matter of fact, land application is considered as the best way, because the sludge contains nutrient substances, such as nitrogen, phosphorus, potassium and organic matter [3]. However, wastewater treatment plants (WWTPs) treated the combining municipal and industrial wastewater, which lead to heavy metals concentrations increased in the sludge [5]. It is well known that several types of heavy metals, such as copper, zinc, chromium, lead, nickle, mercury and manganese, these heavy metals can be harmful to ecological environments and human beings, due to it can retain in soil limitlessly and accumulate in the environment, causing various disorders and diseases [5]. Therefore, heavy metal is restricted significantly in the utilization of sludge [1, 5]. Many techniques have been used to remove heavy metals from sludge, such as bioleaching, chemical extraction, electrokinetic treatment, ultrasound-assisted citric acid and super critical fluid extraction [2, 6, 7]. In these methods, the chemical 4 extraction has been used more than other methods, because it obtained a high removal efficiency in the short time. However, it can produce high cost and low pH in the treatment process. Also, the chemical reagents may pose secondary pollution through the soil and groundwater. Therefore, it is necessary to find a proper method for removing heavy metal from sludge. In particular, electrokinetic (EK) technology can remove inorganic and organic compounds from soils [8], and demonstrate low cost and high effective method for removing heavy metal from sludge, contaminated soil and sediment [8, 9]. EK technology involves the application a low direct current (0-1 A) or direct electric voltage gradient (0-2 V cm-1) across electrodes inserted into the contaminated soil, sludge, sediment, which can produce direct current electric field and cause heavy metal ions migration through the contaminated sites towards the anode or cathode [1, 8, 10], because three main migration mechanisms in the EK process: (1) electroosmosis, (2) electromigration, and (3) electrophoresis. For instance, toxic heavy metal ions are mainly transported by electromigration, which is the migration of ions towards an oppositely charged electrode [1, 8, 11]. In the EK process, water reduction occurs at the cathode, hydroxyl ions (OH- ) generated, which indicated high pH at the cathode.