There are almost 46,000 abandoned mines all over the United States. These mines produce heavy-metal-rich sulfuric acid solution called acid mine drainage (AMD), which has brought long-term water and soil pollution impact, according to the US department of the interior [1]. Different technologies have been used to remediate the surface water, groundwater and soil contaminated by the AMD. These methods include physical, biological, and chemical methods which are generally chemical intensive or the treatment period is relatively long. A better approach for AMD treatment is using microbial fuel cell (MFC), a type of bioelectrochemical systems (BES). MFC uses microorganisms to convert the chemical energy in the biodegradable materials to electricity and at the same time removes the heavy metals in AMD by precipitation and enhances the pH of the drainage. Dry sludge from wastewater treatment plant was used as electron donors for the first time for AMD remediation in the MFC. This treatment technology offers a cheap and environmentally benign approach for integrated waste utilization, energy production and wastewater treatment. Using BES with different mechanisms, different heavy metals have been removed, such as nickel, copper, lead, cadmium and zinc. [2&3&4]. In this study, AMD was treated in the cathode chamber of the MFC with continuous air injection into the chamber, and dry sludge was placed in the anode chamber of the MFC. AMD samples were taken from an abandoned coal-mine in Illinois Coal Basin around Carbondale IL, and the dry sludge was from Carbondale Southeast Wastewater Treatment Plant in Carbondale, IL. The plant used activated sludge tank as secondary treatment process. The dry sludge was formed after activated sludge digestion and drying. Anion exchange membrane (Membrane International Inc) was used to separate the two chambers. Carbon fiber brush was used as the anode electrode, and platinum coated carbon sheet was used as the cathode electrode. With catalyst used, the removal speed of heavy metals can be enhanced. The volume of each chamber was 150 mL. The dry sludge in the anode chamber of the MFC was saturated with D.I. water and a mixture of bacteria from different sources was inoculated into the chamber. No external nutrients were added into the chamber as the dry sludge contains reach nutrition, including organic compounds and different salts. Air was injected into the cathode chamber filled with the AMD. A multimeter was used to record the whole cell potential with an external resistor of 1 kΩ resistance, and the potential of each half cell was also recorded by a data logger (DataLoggerInc) with an Ag/AgCl reference electrode placed in the cathode chamber. The results showed that the closed circuit voltage of the whole cell was kept increasing during the 5 days of operation with the maximum reached voltage of 0.1 V. With the density peak method, the internal resistance of the MFC was measured to be 650 Ω. The pH and the ORP of the AMD changed from 2.57 and 619.9 mV to 4.49 and 263.2 mV respectively after 5 days. White-gray colored precipitation was observed to be formed on the bottom of the cathode chamber. The color of the AMD was changed from yellow-brown before the test to transparent after 5 days. Scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) was used to test the composition of the precipitate and to observe its morphology. The EDS test showed the existence of Al, Cu, and Fe in the precipitation. Precipitates with different sizes were formed with the maximum less than 5 µm. The preliminary results from this study indicated the possible treatment pathway of AMD by MFC with electricity generation during the treatment process and by using dry sludge from the wastewater treatment plant; no additional chemicals are needed for the treatment process.

References

[1]. US department of the interior, bureau of land managment. 2014. Abandoned Mine Lands. http://www.blm.gov/wo/st/en/prog/more/Abandoned_Mine_Lands.html

[2]. Wang, H., Ren, Z.J., 2013. A comprehensive review of microbial electrochemical systems as a platform technology. Biotechnology advances. Volume 31, issue 8, pages 1796-1807.

[3]. Logan, B.E., Rabaey, K., 2012. Conversion of wastes into bioelectricity and chemicals by using microbial electrochemical technologies. Science 337 (6095), 686-690.

[4]. Wang, H., Ren, Z.H., 2014. Bioelectrochemical metal recovery from waste water: A review. Water research, (66 ) 219-232.