Date of Award
Master of Science (MS)
Environmental Engineering and Science
KHAN , ABDUL A.
SCHLAUTMAN , MARK
An environmentally friendly method for the disposal of scrap tires is currently unavailable; as a result, ultimate disposal of used vehicle tires continues to be a major challenge around the world. In the United States (US), during the past two decades, scrap tires have been generated at the rate of approximately one tire per person per year (i.e., approximately 290 million new scrap tires every year). It is estimated that there are currently 2 billion tires stockpiled in the US. Due to various problems involved in the disposal of scrap tires, different alternatives for recycle and reuse have been examined; however, one concern is the leaching of different tire constituents (organic and inorganic) with time, and their subsequent potential harmful impacts on the environment.
The main objective of this study was to perform a systematic investigation to examine the leaching of dissolved organic carbon and selected inorganic constituents from crumb rubber and tire chips under different water chemistry conditions: at three different pH values (4.0, 7.0 and 10.0) in distilled and deionized water, and in the stimulants of acidic rain water (pH 3.0), hard groundwater (pH 8.3) and soft groundwater (pH 6.3) These are the water chemistry conditions that are likely to be encountered during scrap tire (crumb rubber or tire chips) reuse applications. One hundred grams of crumb rubber (8x14 mesh size) or tire chips (1'_1', 2'_2', 4'_2' and 6'_2') were soaked and mixed in each leaching solution with a solid to solution ratio of 1:20 at room temperature for one month period. Samples were periodically collected and analyzed for selected inorganic constituents (sulfur (S), zinc (Zn), cadmium (Cd), chromium (Cr), arsenic (As), potassium (K), phosphorus (P), sodium (Na), manganese (Mn), iron (Fe), calcium (Ca), magnesium (Mg), aluminum (Al), copper (Cu), lead(Pb), selenium (Se), molybdenum (Mo) and nickel (Ni)), dissolved organic carbon (DOC), ultraviolet absorbance at 254 nm (UV254) and dissolved nitrogen (DN). Toxicity Characterization Leaching Procedure test was also performed to assess the toxicity of the leachates.
Results showed that the best condition for using scrap tire chips in environmental reuse applications was around the neutral pH conditions. Leaching of dissolved organic carbon and selected elements was minimal around the neutral pH values. pH was a more important parameter than conductivity of the solution in controlling the leaching of DOC and selected elements from scrap tires. The changes in conductivity did not have a significant impact on the leaching of organics or inorganics. When tire chips were exposed to acidic conditions, Fe by far was the most significant metal leaching from tires at very large quantities (up to ~800 mg/ 100 g tire). The presence of organics significantly increased the Fe concentrations in water (e.g., ~ 20 mg/L at pH 4) above its solubility. Mn was the second metal observed leaching at acidic conditions; however at amounts (2-5 mg/ 100 g tire) significantly lower than Fe. When the tire chips were exposed to basic conditions, the leaching of DOC significantly increased (reaching 27 mg/ 100 g tire). For crumb rubber, leaching of DOC reached up to ~120 mg/ 100 g tire, indicating that organic components in tires are more prone to leaching under basic conditions. Under the basic conditions, the leaching of inorganics, including Fe (<1 mg/ 200 g tire) was significantly lower. As, Cd, Cr, Cu, Mo, Se, Ni and Pb were always below the detection limits for all conditions tested during one month of leaching experiments.
The SUVA254 values of the leaching solutions remained in the range of 1.5 to 3.0 L/mg-m during the experiments. A gradual increase in the SUVA254 values after the first week was observed, indicating an increase in the fraction of aromatic carbons leaching from the tires over time. The presence of some aromatic compounds in the leachate solutions was confirmed with gas chromatograph coupled with tandem mass spectrometer scans. Some of these compounds (e.g., aniline, benzothiazole, benzothiazolone) have been also reported in previous studies.
Analysis of DOC leaching data showed that the mass of DOC leached during the first 12 hr consisted of 40-50% of the leaching during the first week and 20-25% of the leaching during the four weeks of experiments. Although the cumulative DOC mass leached from tires depended on tire size and leaching solution, the leaching rate remained constant regardless of tire type and leaching solution composition. Analysis of leaching rate of four metals (Zn, Fe, Al, Mn) at acidic conditions showed a rapid initial leaching rate for Zn, followed by a slower but constant rate, while there was a constant rate of leaching for Fe, Mn and Al from the beginning of the experiments without showing any sign of slowing down. This observation was attributed to the release of the Zn from the rubbery portion of the tires due to the relatively similar leaching patterns observed for DOC and Zn. On the other hand, Fe, Al and Mn are probably coming from the wires in tire chips and showed a continuous and constant rate of dissolution.
Crumb rubber showed significantly higher degree of leaching than tire chips for all detected constituents except Fe, since the main source of the iron in tire chip was the wires that were removed prior to preparing the crumb rubber. Among the tire chips that were within the particle size range of practical applications, leaching from 1'_1' size tire chips, in general, was higher than the other particle sizes. The difference in leaching among other particle sizes (2'_2', 4'_2' and 6'_2') was relatively small or negligible for most of the parameters monitored.
Selbes, Meric, "LEACHING OF DISSOLVED ORGANIC CARBON AND SELECTED INORGANIC CONSTITUENTS FROM SCRAP TIRES" (2009). All Theses. 664.