Date of Award
Master of Science in Chemical Engineering (MSChE)
Dr. Mercedes Rivero-Hudec
As many as 500 million computers became obsolete between the years of 2000 and 2007 (U.S. EPA, 2007), due in large part to the increasing rate of technological advances. Printed circuit boards (PCB) are the essential components of most electronic waste. Metals that are contained within PCB are potentially hazardous to the environment. These metals include copper, aluminum, chromium, mercury, zinc, lead, and nickel (Li et. al., 2004). If not contained properly, metals can leach into soils and possibly damage the ecosystem. This problem of growing electronic waste not only exists in the USA, but it also extends overseas in countries like China, India, and Pakistan .. An eco-friendly recycling technique needs to be investigated in order to deal with the exponential amount of electronic waste domestically. Bio leaching is using microbes to extract metals from material by mobilizing metal ions in solution. Bioleaching may hold the key to creating a process that is environmentally friendly and economically sound for extracting metals from PCB.
In this study, Acidiphilium acidophilum ATCC 27807 were grown in 9-K glucose liquid medium in a sterile environment at 26°C and plated onto 9-K glucose solid medium to insure purity. Printed circuit boards (PCB) that were involved in the study were shredded and sifted for particles greater than 0.841 mm and less than 1.680 mm. These particles were then briefly air cleaned to remove dust and autoclaved for sterility before use in experiments.
Experimental units ran under sterile conditions with 100 ml of 9-K glucose liquid solution in 250 ml baffled flasks at 26°C and 150 RPM. Experimental units that required cells were inoculated with 5ml of 4.26 x 108 cells/ml ± 1.1 x 10 7 cells/ml culture into 95ml of 9-K glucose liquid solution. Printed circuit board material was added to experimental flasks once Acidiphilium acidophilum ATCC 27807 reached the stationary phase. Experiments ran for 8 days in total. Cell densities and pH were recorded a least daily during experimental runs. Cell densities for live cells in the lag and exponential phases were obtained using a spectrophotometer (Milton Roy Spectronic 10001 plus, Item #335005) with a wavelength of 500 nm. Cells in the stationary and death phases, where live and dead cells coexist, were measured using the drop plating and dilution techniques. pH readings were taken using a pH probe AccTupH Catalog number 13-620-183 in conjunction with an electronic meter Accumet Basic AB 15 pH meter by Fisher Scientific. Calibrations of both the probe and meter were taken every 30 minutes, before and between experimental readings, to insure proper pH readings. Metal samples were processed after experiments ran for 8 days. Samples from experimental units were filtered and acidified with 2 N Optima Nitric Acid and stored at 4°C until they were ready to be analyzed. Samples were analyzed for Cu, Ni, Zn and Al using an Electron Inductively Coupled Plasma Mass Spectrometer (ICP-MS) at the RI- INBRE Centralized Research Core Facility at the University of Rhode Island.
Levels of 0, 8, 16, 24 and 32 g/L of PCB were tested with A. acidophilum. Two main controls were included in experiments: flasks with just cells and flasks with just PCB. This allowed for a comparison to the normal growth curve and for a basis for just chemical/medium leaching. The maximum amount of copper that was bioleached by cells was 77.7 mg/Lat 8 g/L of PCB. Total copper leached into solution yielded a 9.6% total copper recovery with 8.6% of the total copper leached specifically by A. acidophilum. Zinc was also bioleached by cells with a maximum amount of 3.9 mg/Lat 16 g/L of PCB. This yielded a 42.8% total recovery of zinc in solution; 36.3% of the total recovery was contributed by A. acidophilum. Nickel and aluminum were not bioleached. Experimental flasks with 8 g/L of PCB showed higher cell densities and lower pHs than the control curves of A. acidophilum.
Flasks specifically with 8 g/L of PCB and cells shows that Acidiphilium acidophilum ATCC 27807 found an energy source from the PCB that allowed for continued growth and production of an unidentified acid. This is also shown at levels of 16 g/L of PCB. The reason for metal specific bioleaching could be explained due to biofilm formation; this should be further investigated. And the reason for the death of cells in some of the PCB levels could be explained due to the high pH that these cells are not accustomed to living at.
Arora, Diana Goglia, "Using Acidiphilium Acidiphilium to Bioleach Metals from Printed Circuit Boards" (2008). Open Access Master's Theses. Paper 1419.