Treating Coal Ash with Microbial-Induced Calcium Carbonate Precipitation

Coal ash stored in impoundments is susceptible to structural stability concerns and to migration of its trace elements to surface and ground water sources. A novel approach to mitigate these concerns is the use of microbial induced calcium carbonate precipitation (MICP). MICP improves the strength and stiffness of granular material (DeJong et al. 2013) and immobilizes heavy metals (Fujita et al. 2010). An MICP treatment process for stabilizing coal ash using ureolytic bacteria Sporosarcina pasteurii is explored herein.

Three different coal ash materials (CA1, CA2, and CA3) collected from the southeastern region of the US were used in this study. MICP treatments of the coal ash materials were developed through column testing. The testing caps were equipped with bender elements to assess the shear wave velocity (SWV) as an indicator of the cementation progression due to the MICP process. The coal ash material was deposited within the columns as a slurry, and the treatments were conducted under saturated conditions. The columns were treated while under an overburden pressure of 100 kPa.

CA1 and CA3 experienced significant improvement (by a factor of about 8 and 4, respectively) in SWV using an MICP recipe consisting of 0.4 M urea, $0.2\mathrm{M}{\mathrm{CaCl}}_2$, and $0.1\mathrm{M}{\mathrm{NH}}_4\mathrm{Cl}$ (Fig. 1). However, CA2 experienced essentially no change in SWV when identical treatment processes and recipes were used.

Potential reasons for the inability to treat the CA2 with MICP were identified as: treatment inhibitors, bacteria filtration, particle surface texture, and mineral nucleation sites. Treatment inhibitors were categorized as either inhibiting the urease enzyme or inhibiting the precipitation of calcium carbonate. The activity of the urease enzyme was assessed by monitoring the rate of hydrolyzed urea (evaluated by measuring the pore fluid conductivity) within coal ash leachate compared to the rate of hydrolyzed urea in deionized water. The rates of hydrolyzed urea were not affected by the coal ash leachate. Iron was identified as a potential inhibitor of calcium carbonate precipitation; however, the iron concentrations of CA2 and CA3 were similar when evaluated with energy dispersive X-ray spectroscopy mapping.

CA2 had a finer particle size distribution compared to the other materials evaluated based on the hydrometer test; however, when the particle size distribution was modified to be coarser (similar to CA1 and CA3), MICP treatments were still found to be ineffective. Particle surface texture was also evaluated, with scanning electron images taken for the coal ash materials [Fig. 2(a) shows CA2]. Similar textures were observed in all the images.

Nucleation sites for calcium carbonate precipitation were also evaluated by assessing the carbon concentrations within the ash materials. The carbon content in CA2 was remarkably lower than the other two ashes (approximately 0.5% for CA2 compared to 3.42 and 8.46% for CA1 and CA3, respectively). The CA3 ash material was heated to 700°C for 24 h to remove as much of the carbon in the ash as possible. The heat-treated CA3 material was then treated with the MICP recipe and compared to the original CA2 and CA3 materials [Fig. 2(b)]. The results indicate that the removal of carbon did lead to an inability to treat the ash material using MICP such that original increase in SWV of CA3 was not obtained.