Browsing by Author "Mbugua, John"

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Application of hydrogeochemical modelling in simulating the transportation of elements in fly ash heap under different disposal systems in South Africa
(2015-07-20) Mbugua, John
Ash heap modelling of South African fly ash from Tutuka was carried out and the duration of transportation projected for 20 years based on two disposal scenarios, namely; irrigation of ash with rainwater, and irrigation with brines. The hydrogeochemical modeling code, PHREEQC, was applied in the study which gave insights into the speciation, release and transport of elements from the water and brines–fly ash long term interactions. Tutuka ash–water heap model showed a general sharp decrease of total elemental concentrations released during the first 2.5 years simulation as the pH value dropped from 12.6 to 8.7, after which it remained constant and their concentration remained constant up to 20 years. The elements showing this trend included Ca, Mg, Al, Fe, Sr, Zn, Na, K, Li and C(4). Generally, brines caused sharp increase in released concentration of the elements Ca, Mg, S(6) and C(4) for the first 3 years of heap irrigation whereas with water irrigation an opposite trend was observed in which the elemental concentrations decreased. Much of the release chemistry of the elements was closely related to the phase dissolution/precipitation and formation as the major controlling factors. Generally therefore, the modelled leachate quality results revealed that many elements are mobile and move through the ash heap in a progressive leaching pathway. The model could therefore be used to provide reasonable leachate quality from the modelled Tutuka ash heap which may be reaching the ground water. Overall, the ash heap modelling enhanced the understanding of the environmental impacts of ash–water–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) the leachate flow and hydrodynamics as captured in the conceptual model. Further model validation is recommended with lysimeters to quantatively compare the simulated results against the experimental data and improve on the model.
Reactive-transport modeling of fly ash–water–brines interactions from laboratory-scale column studies
(2015-07-20) Mbugua, John
Dynamic leaching tests are important studies that provide more insights into time-dependent leaching mechanisms of any given solid waste. Hydrogeochemical modeling using PHREEQC was applied for column modeling of two ash recipes and brines generated from South African coal utility plants, Sasol and Eskom. The modeling results were part of a larger ash–brine study aimed at acquiring knowledge on (i) quantification and characterization of the products formed when ash is in contact with water–brines in different scenarios, (ii) the mineralogical changes associated with water–brine–ash interactions over time, (iii) species concentration, and (iv) leaching and transport controlling factors. The column modeling was successfully identified and quantified as important reactive mineralogical phases controlling major, minor and trace elements’ release. The pH of the solution was found to be a very important controlling factor in leaching chemistry. The highest mineralogical transformation took place in the first 10 days of ash contact with either water or brines, and within 0.1 m from the column inflow. Many of the major and trace elements Ca, Mg, Na, K, Sr, S(VI), Fe, are leached easily into water systems and their concentration fronts were high at the beginning (within 0.1 m from the column inflow and within the first 10 days) upon contact with the liquid phase. However, their concentration decreased with time until a steady state was reached. Modeling results also revealed that geochemical reactions taking place during ash–water–brine interactions does affect the porosity of the ash, whereas the leaching processes lead to increased porosity. Besides supporting experimental data, modeling results gave predictive insights on leaching of elements which may directly impact on the environment, particularly ground water. These predictions will help develop scenarios and offer potential guide for future sustainable waste management practices as a way of addressing the co-disposal of brines within inland ash dams and heaps.
SIMULATED MODELING: CONTRASTING Ca/Mg AND Mn+-NATURAL ORGANIC MATTER COMPETITIVE COMPLEXATION IN WATER
(2015-07-20) Mbugua, John
Complexing capacities of calcium and magnesium with natural organic matter (NOM)/selected organic chelating agents, in the presence and absence of other major and trace cations in water, were studied by anodic and cathodic square wave stripping voltammetry techniques (ASWSV and CSWSV). Under predetermined and optimized physico-chemical parameters (pH, temperature and ionic strength), calcium-NOM complexation and competitive complexation by other elements of known concentration (Pb, Zn, and Cu) were determined. Voltammetric measurements showed competition between calcium and the other cations for the binding sites of humic substances at depositing, measuring and equilibrating potential of -2V and +1V and scan time of 150seconds. Two systems were complexed by known concentrations of NOM fractions (Humic acid (HA), 1, 2-dihydroxybenzene (CAT/catechol) and tannic acid (TA)): Mn+-NOM; Mn+ Calcium-NOM complexes at different pH conditions. Results show significant differences in peak currents in the order Mn+> Ca/Mn+-TA/HA/CAT > Mn2+ TA/HA/CAT but differed depending on the metal species, type of humic chelant and pH. Visual Minteq and PHREEQC simulations show differences in distribution of scale causing species such as Ca2+, CaOH+, Mg2+ and MgOH+ that are highly dependent on the pH and concentration. This paper presents results of part of the work aimed at providing simulated modeling alternatives in combating scaling as a consequence of cooling water used in industry. It equally provides an assurance on the re-use of the scarce water resources by simulated water experiments and models
SIMULATED MODELING: CONTRASTING Ca/Mg AND Mn+-NATURAL ORGANIC MATTER COMPETITIVE COMPLEXATION IN WATER
(2015-07-20) Mbugua, John
Complexing capacities of calcium and magnesium with natural organic matter (NOM)/selected organic chelating agents, in the presence and absence of other major and trace cations in water,were studied by anodic and cathodic square wave stripping voltammetry techniques (ASWSV and CSWSV). Under predetermined and optimized physico-chemical parameters (pH, temperature and ionic strength), calcium-NOM complexation and competitive complexation by other elements of known concentration (Pb, Zn, and Cu) were determined. Voltammetric measurements showed competition between calcium and the other cations for the binding sites of humic substances at depositing, measuring and equilibrating potential of -2V and +1V and scan time of 150 seconds.Two systems were complexed by known concentrations of NOM fractions (Humic acid (HA), 1, 2-dihydroxybenzene (CAT/catechol) and tannic acid (TA)): Mn+-NOM; Mn+Calcium-NOM complexes at different pH conditions. Results show significant differences in peak currents in the order Mn+> Ca/Mn+-TA/HA/CAT > Mn2+-TA/HA/CAT but differed depending on the metal species, type of humic chelant and pH. Visual Minteq and PHREEQC simulations show differences in distribution of scale causing species such as Ca2+, CaOH+, Mg2+ and MgOH+ that are highly dependent on the pH and concentration. This paper presents results of partof the work aimed at providing simulated modeling alternatives in combating scaling as a consequence of cooling water used in industry. It equally provides an assurance on the re-use of the scarce water resources by simulated water experiments and models