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.

Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2012.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 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

Taxonomic Identification and Characterization of African Nightshades (Solanum L. Section Solanum)

African nightshades play an important role in meeting the nutritional needs ofrural households, and are reported as being particularly rich in protein, vitamin A, iron and calcium. Nightshades are among three top priority African indigenousvegetables identified for improvement and promotion through research. A majorconstraint facing this objective is the scantiness of taxonomic and nomenclaturalknowledge on African nightshades resulting in extensive synonymy andconfusion. As a consequence, the toxic species are difficult to discriminate fromthose with high nutritional value. It is also difficult to identify species with goodagronomic traits for genetic enhancement. This study was conducted to identify,characterize, and delimit African nightshade species. Fifty accessions ofSolanum section Solanum from eastern, southern and western Africa were raisedin a greenhouse at the Botanical and Experimental Garden, Radboud University,Nijmegen, the Netherlands. A descriptor list with 48 vegetative and reproductivecharacters was developed and used to characterize flowering and fruiting plants.Counting of chromosome was done on root squash preparations from one weekold seedlings, aided by digital enhancement of microscopic images. Nine species were represented in the study material, including two diploids: Solanum americanum, and Solanum chenopodioides; five tetraploids: Solanum retroflexum, Solanum villosum, Solanum florulentum, Solanum grossidentatum and Solanum tarderemotum; and two hexaploids: Solanum nigrum and Solanum scabrum. Most of the section Solanum species were distinguishable and easily identified. The exception was S. florulentum and S. tarderemotum which were identified tentatively and assigned respective names, but are difficult to differentiate and require further studies. The S. florulentum/tarderemotum group has three distinguishable variants and further studies are needed to determine the taxonomic status of each as a separate species, subspecies or genotypic/phenotypic variants. Furthermore, S. retroflexum, S. villosum and S. scabrum each had a high degree of within-species variation, and further studies are recommended to determine whether the variations within each constitute subspecies.Key words: Solanum sp., Identification, Characterization 

Experimental and Computational Fluid Dynamics study of microcarrier suspension during the cultivation of Mesenchymal Stem Cells in an ambr250 bioreactor

The ambr250 unit is a fully automated disposable 100-250 ml bioreactor for R&D that has been developed by TAP Biosystems, now part of Sartorius-Stedim, widely used for scale down and scale up modelling studies. Recently, mesenchymal stem cells (MSCs) have become strong candidates for cell-based therapies based on in vitro growth on microcarriers in stirred bioreactors. However, to fully realize the MSCs potential, a number of key processing issues need to be addressed because of the huge number of cells that are required. Thus, the fluid dynamics characteristics of the stirred ambr250 bioreactor must be sufficiently well understood to enable scale-up to larger bioreactors to be efficiently accomplished particularly because of the special issues arising from the presence of the particulate solid phase. One of the most critical aspects for MSC cultivation on microcarriers is the minimum agitator speed required to achieve complete microcarriers suspension, NJS. Under these conditions, the surface area of all the attached cells is available for transfer of nutrients (including oxygen) to the cells and metabolites from them, whilst higher speeds hardly increase these transport processes and may lead to damaging fluid dynamic stresses being generated1. This suspension condition can be studied experimentally if equipment is specially modified to make easy visual observation of the two-phase flow in the bioreactor but during actual growth that is very difficult. Therefore, it is extremely beneficial to both measure NJS and compare the measured values with predictions based on computational fluid dynamics (CFD) for validation. Once validated, then CFD is a very useful tool for analyzing flow patterns, mixing time, mean and local specific energy dissipation rates and other parameters important for scale up. In this work we examined the fluid dynamics of the two-phase particle-liquid system with an experimental analysis and a CFD simulation using a lattice-Boltzmann base software and particle tracking of an ambr250 vessel at different stirring conditions. Cell culture was also performed in parallel to analyse the cell growth at and around NJS and the results were compared to the performance in a spinner flask bioreactor. The CFD and experimental results will be discussed in detail along with their scale-up implications. References 1) Nienow, A. W., Coopman, K., Heathman, T. R. J., Rafiq, Q. A. and C. J. Hewitt (2016). “Bioreactor Engineering Fundamentals for Stem Cell Manufacturing”. In: “Stem Cell Manufacturing”, (Eds. J.M.S. Cabral, C.L. de Silva, L. G. Chase and M. M. Diogo), Elsevier Science, Cambridge, USA; Chapter 3, pp 43 – 76

System characteristics and management practices for small ruminant production in “Climate Smart Villages” of Kenya

The CGIAR research programme on Climate Change Agriculture and Food Security, in collaboration with several partners is testing a portfolio of interventions to address the threat of changing climatic conditions for smallholder farming communities living beside river flood plains, grouped into “Climate Smart Villages” (CSVs). We present characteristics of farms in CSV in relation to small ruminant (SR) production and the scenario for a breeding and improvement programme. Information was collated using participatory systems research methods from 140 households in seven CSVs in Nyando basin, Kenya. Although most households were headed by men, there were a higher proportion of adult women within the communities, and literacy levels were moderate. A total of 58 percent of the population owned <1 ha of land for growing crops and rearing on average 6.96 ± 3.35 Tropical Livestock Units comprising different species of animals. Women headed households owned more sheep which were mainly crosses of unspecified local breeds, than Goats which were mainly the Small East African breed-type. Mating among the SR was random, with no control of inbreeding as flocks mixed in grazing fields and at water points. Farmers desired large and resilient animals for better market prices; however, growth rates were slow. The SR flocks were dynamic with 31 percent of the animals moving in and out of flocks in a year. A community breeding programme optimally using available resources and incorporating gender integrated innovative technologies could be implemented for the CSV, alongside strong capacity development on animal husbandry, health and marketing of products

A novel use of high density SNP assays to optimize choice of different crossbred dairy cattle genotypes in smallholder systems in East Africa

We present use of SNP technologies to obtain for the first time, rapid, large-scale, in situ estimates of performance of crossbred cows in smallholder herds. Compared to historical approaches, our approach allowed optimum crossbreed choices to be determined more rapidly and without question about the relevance of the environment. High-density SNP assays were used to estimate breed composition for a monitored population comprising 1292 cows from 610 smallholder farms in Kenya. The achieved milk yields are much lower than generally assumed and lactation curves are very flat out to 400 days. In poor production environments, lower grade crosses (containing <60% exotic dairy alleles) had the same yield as high grade crosses. Allowing for the larger size and maintenance requirements of high grade exotics, lower grade exotics will be the most economically productive animals in these environments

Use of high density SNP genotypes to determine the breed composition of cross bred dairy cattle in smallholder farms: Assessment of reproductive and health performance

Reproductive performance and disease data were recorded for 2 years on 1,824 dairy cows in smallholder farms using participatory approaches and onfarm recording. Most animals experienced one service to conception. Calving intervals were long, ranging between 261 and 761 days, with an average of 451±101days. Herd level of production (HeL) had significant effect on calving interval. However, there was no difference between crossbreds with different levels of exotic breed percentage or in different HeL classes in disease incidence. Most animals had less than 2 treatment events, despite the high disease burden in the study areas. Mortality rates were low, ranging from 2.13% to 2.65%. Even though the crossbred animals had higher performance compared to indigenous animals, the gains obtained were below what would be possible with better management. These results suggest that crosses with low exotic proportions would be the most optimal for the production systems studied