Hydrogeochemical modeling of the speciation and leaching of fly ash co-disposed with water, brines and organics : a case study of Sasol-Eskom coal ash disposal, South Africa.
Mbugua, John Mwai.
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Two coal utility plants in South Africa selected (one from Sasol and another from Eskom) for this study produce large volumes of fly ash (over 40 Mt from Eskom at Tutuka, and 3 Mt from Sasol Synfuels at Secunda annually), and brines as by-products during coal processing. Co-disposal of the brines and fly ashes has been a normal practice in these coal-utility plants for decades. Long-term management of fly ash is necessary and requires an understanding and knowledge of how the different waste materials interact with water and brines in different chemical situations. However the geochemistry of their interactions, the leaching and mobility of elements in these disposal systems has not been fully understood. This work gives insights into the chemical processes taking place in the brine-water/brines systems that govern the concentrations of major and minor elements in ash leachates under different environmental conditions. The possible presence of organic compounds (subsequently referred to as 'organics') in brines and their effects on the leaching chemistry of fly ash was also studied. Sustainability and long term impact of the co-disposal of fly ash and brines on the environment was studied through static (batch tests) modeling of the pH-dependent acid neutralization capacity (ANC) tests and columns modeling for dynamic leach tests. The modeling was based on experimental results from other Sasol-Eskom ashbrine project collaborators. Modeling results of the ANC tests were in good agreement with the reported experimental results, which revealed that the release trends of various elements (including trace, heavy elements and contaminants) contained in fly ash into solution is highly pH dependent. However Na, K, Mo and Li exhibited constant solubilisation which was independent of pH changes from all the scenarios. The presence of different constituents of brines subjected to ANC resulted to different ANC capacities ranging from 0.98 moles H⁺/Kg dry ash (of ash-organics mixed with Mg-brines) to 3.87 H⁺/Kg dry ash for those with the C(4) brines. As expected, those constituents from the cationic brines were found on the lower region of acid addition (in the order Mg-brines < Ca-brines < Na-brines) while the anionic brines were found at the upper region of acid addition (in the order S(6)-brines < Cl-brines < C(4)-brines). In the middle region of acid addition were three important scenarios: that of ash with brine, ash without brines (i.e. ash with DMW) and ash with both ASW organics and combined brines. It was from these three scenarios that a generalization of the effect of brines and organics on the ANC was inferred. The ANC of ash with demineralised water (DMW) was 2.33 mol H⁺/Kg dry ash and that of ash with ASW organics lower at 2.12 mol H⁺/Kg dry ash which was the same value as that of ash with combined brines. This indicated that brines decreased the ANC of ash by about 9.01 % and which could be attributed to the acid-base neutralization process and the dynamics of solid phase dissolutions in response to the acid addition. Both fly ashes exhibited a typical pH > 12 (suspension in demineralised water) and the predominant cation even at this high pH is Ca²⁺ (at concentration > 0.002 mmol/L). This indicates that dissolution of CaO and formation of OH⁻ species at pH > 10 contributes to acid neutralisation capacity of both fly ashes and is the greatest contributor to the acid neutralizing capacity of both fly ashes. Two broad leaching behaviours as a function of pH were observed from the three fly ash-ASW organics-brines scenarios (i) leaching of Ca, Mg, Ni and Sr follows a cationic pattern where the concentration decreases monotonically as pH increases; (ii) leaching of Al, Fe, Ti and Zn follow an amphoteric pattern where the concentration increases at acidic and alkaline pH, although Al showed some anomaly from pH 11 where the concentration decreased with the increase in pH. Al showed an amphoteric pattern in which its release increased between pH 12.8 and 11 for all the scenarios and then decreased with decrease in pH down to neutral pH of 7. The batch leaching simulation results from hydrogeochemical modeling also showed that mineral dissolution, precipitation and new phase formation during ash-organics-brines interactions was controlled by pH. The newly formed phases however remain in equilibrium with the ash-brines-organics mixture. Each individual mineral phase dissolution/precipitation/formation system controls the concentration and speciation of the respective constituent elements as evidenced by the log C-pH diagrams obtained from the modeled scenarios. The ash-brines-organics interactions do exhibit and affect the mineralogical chemistry of fly ash. However the extent to which these interactions occur and their effect, varies from one scenario to another, and are dependent on the amounts and type of the constituent brine components. Organics do have a significant effect on dissolution characteristics of few minerals such as calcite, mullite, kaolinite, Ni₂SiO₄, and SrSiO₃ due to complexation effect. The effect is quantitatively conspicuous for calcite mineral phase and for the formation of some new phases such as Fe(OH)₃(am)-CF and portlandite. The composition of the liquid phase from acid neutralisation capacity experiments was successful.Hydrogeochemical modeling was used as a means to provide insights and understanding of the complex reactions taking place, speciation and mineralogical changes occurring. These changes would serve to predict future environmental scenarios when pH conditions change. In this study, an extension of the application field of PHREEQC hydrogeochemical code for modeling and simulation of equilibrium; kinetic and transport mechanisms associated with the interaction of water; and organics and brines with fly ash during their co-disposal is successfully demonstrated. The parameters associated with these mechanisms were used as inputs into the PHREEQC program using modified Lawrence Livermore National Laboratory (LLNL) database for inorganic brines and MINTEQ.V4 database for organics, and used to model the results of ANC test data for the fly ashes. A special reference is made to two separate modeled mineralogical ash recipes from two of the South African power utility plants' fly ash systems, namely, Tutuka and Secunda. The effects of brines in the leaching of major, minor and trace elements at various pH values and the mineralogical changes associated with the intermediate and final products from the interactions of ash-brines systems under different scenarios are qualitatively and quantatively discussed. Multiphase saturation characteristics have been determined for mineral species in contact with water and brines. The modeling results indicated that several mineral phases could be controlling the species concentration in the leachates, and the ANC and column modeling results corroborated well in many aspects with the experimental results obtained from collaborating institutions (South Africa Universities and Research institutions). In addition, application of the PHREEQC model to the ash heap under different disposal systems was carried out to predict the heap leachate composition and geochemical transformations taking place in a period of time. Pore water chemical analysis, and moisture content analysis revealed that contact of the ash with water is a crucial factor in the mobilization of the contaminants with time. Maximum weathering/dissolution of the ash is observed in the top layer (1-3) m and at the point of contact with the subsurface water level which was in good agreement with the model results. The surface layer and the very lowest layers of the dump in contact with lateral flows experience the highest degree of weathering leading to depletion of species. The geophysical transformation of fly ash was also captured through the porosity change calculations and the results revealed that geochemical reactions do affect the porosity of fly ash during the weathering processes. These modelling results were in agreement with the hydraulic tests and salt leaching tests conducted during Sasol-Eskom ashbrine project in Phase I which suggested that salts captured in the ash will become mobile and leach from the fly ash over time. The data therefore indicates that ash dumps may not act as sustainable salt sinks. These findings may have some bearing on engineering decisions on fly ash reuse. From the above observations, it is apparent that release of large quantities of the salts in the ash depends on the extent of its interaction with brines being used for irrigation or with water, either through plug-in flow after a rainfall event or contact with groundwater. The results revealed effects of brine-water contact time with fly ash, the flow volume and velocity, the pH, the degree of saturation, hydrogeology and ash heap geometry as important factors that affect fly ash transformation and weathering. Overall, the ash heap modeling enhanced the understanding of the ash-brines interactions and demonstrated that leachate composition is determined by the following factors; (i) the mass flows from the pores of fly ash, (ii) the surface dissolution of the mineral phases, (iii) the various chemical reactions involved during the ash-brine and ash-water interactions, (iv) the interactions with a gas phase (atmospheric CO₂), (v) the composition of the initial fly ash, and (vi) by the leachate flow and hydrodynamics as captured in the conceptual model. Any ash handling system should therefore be designed to take these criteria into consideration to prevent environmental contamination. The modeling results also gave indications that the ash-brine co-disposal in dry ash systems would be an unsustainable way of locking up brine salts in the long run. In this Thesis, modeling results were used to support experimental data which further reaffirmed the important role hydrogeochemical modeling plays in liquid and solid waste management. Furthermore, hydrogeochemical modeling complements the work of analytical/environmental scientists as well as guiding the future solid waste management and engineering decisions.