PhD Thesis Defence Presentations - Panagiota Natsi

In numerous applications in both medicine and industry, the formation of deposits on process equipment and parts consisting of both organic and (mainly) inorganic salts, including calcium carbonate, is the cause of operational problems. Mineral deposits associated with the development of microorganisms (biological deposits) are complicated forms which need special attention. The formation calcium carbonate, associated with the growth of biological substrates, addressed to in the present work, is a challenging research topic. In the first part of this work, the mechanism of spontaneous precipitation of calcium carbonate in supersaturated solutions and the effect of the presence of heavy metals was investigated. The free drift spontaneous precipitation and the corresponding repetitive study at constant supersaturation, were used to establish the stability diagram of the calcium carbonate supersaturated solutions. Precipitation in supersaturated solutions, corresponding to the unstable solutions domain, yielded measurements of the induction time, lapsed from the preparation of the supersaturated solutions to the onset of the precipitation of the calcium carbonate, and of the rate of precipitation. From the dependence of the rate of precipitation on the corresponding saturation ratio with respect to calcite (5.89<SR_vaterite<9.12, 0.10 M NaCl), it was possible to identify that the rate determining step was surface diffusion of the growth units to the growing calcium carbonate supercritical nuclei. The rate–supersaturation ratio data, for supersaturated solutions with ionic strength 0.1 and 0.15M, gave satisfactory fit to the polynuclear crystal growth model. Fit of the data to the Avrami model gave apparent order value for the precipitation, n, gave different values, for low (0.47<SR_vaterite<0.58) and for higher relative supersaturation values (1.42<SR_vaterite<2.01). However, the Avrami model, is concerned only with the growth in size, of a fixed number of nuclei, as a function of time. The surface energy, calculated from the plots according to this model, was 114mJ^-1·m^-2 and the number of modules, which constitute the critical nuclei, equal to 4.

The influence of the presence of Acutodesmus Obliquus microalgae growth medium in the supersaturated solutions, on the spontaneous precipitation of calcium carbonate (0.15M NaCl, 25^oC, pH=8.50) was studied next using stable supersaturated solutions according to the stability diagram. Supersaturated solutions (11.48<SR_calcite<53.70), in which the culture medium was used as the matrix, suppressed completely the precipitation of CaCO₃ and the solutions remained stable. However, modification of the supersaturated solutions to contain 1% w/w of medium (11.48<SR_calcite<79.43), resulted in an acceleration of the precipitation, as much as by 85% of the rate value in the absence of microalgae nutrients medium. The presence of the nutrient medium in the supersaturated solutions resulted in the stabilization of vaterite.

The effect of the presence of heavy metals (Zn, Ni, Cd, Cu) in the supersaturated solutions during spontaneous precipitation of CaCO₃ was studied next. In the presence of metals, at a concentration of 20µM in the supersaturation ratio range 26.91<SR_calcite<79.43, the rate of precipitation of calcium carbonate was drastically reduced (inhibition of 60% in the presence of Ni and Zn), and the corresponding induction times were significantly longer. In the presence of metals in the supersaturated solutions, higher surface energy values for the precipitated solid, were calculated. The presence of the metals investigated at the corresponding concentration range, did not have any effect on the precipitation mechanism. The presence of zinc in the supersaturated solutions (ionic strength 0.1M NaCl & 47.86<SR_calcite<112.2), shifted their stability domain to higher relative supersaturation values. The rate of calcium carbonate precipitation in the presence of zinc was reduced by 98%, with respect to the corresponding in its absence. In the presence of zinc, the aragonite was stabilized.

The second part of the present work was concerned with the study of heterogeneous precipitation of CaCO₃, following the inoculation of the supersaturated solutions with calcite and biological substrates. In stable supersaturated solutions, (SR_calcite=1.66-9.55), for ionic strength 0.15 M NaCl, the rate of crystal growth increased with supersaturation following a quadratic mode. Keeping the supersaturation value constant in the solutions, the precipitation rate was proportional to the total surface of the substrate, confirming that the number of active crystallization centers is constant. The presence of zinc on the supersaturated solutions at concentrations up to 3·10^-5M for ionic strength 0.1M (SR_calcite=7.41) and 0.15M NaCl (SR_calcite=3.55) reduced the rate of crystal growth of calcium carbonate. The rate reduction was higher with, increasing the zinc concentration, reaching 90% of the corresponding rate in its absence. The crystal growth rates of calcium carbonate, measured as a function of zinc concentration, were fitted to a model based on the assumption that the rate reduction is due to adsorption and poisoning of active sites for crystal growth. The adsorption was assumed to take place according to the Langmuir model. The presence of zinc in the supersaturated solutions resulted in a complete inhibition of the crystallisation of calcium carbonate, even at concentrations below the concentration, needed for complete monolayer coverage of the substrate from the zinc chemicals species. The identification of the crystalline solid formed on the substrate confirmed the absence of metal oxide and/or hydroxide. The study of zinc absorption in calcite showed that zinc’s absorption in the solid substrate is initially (the first 10-20minutes) rapid and slower thereafter, approaching equilibrium, corresponding to the maximum amount of zinc that may be absorbed. The concentration measurements of zinc in solutions as a function of time, during adsorption, gave a good fit to the pseudo-first order model. Equilibrium adsorption data gave satisfactory fir to the Langmuir isotherm presence in the supersaturated solutions, resulted in a reduction in the rate of precipitation of calcium carbonate (by 60%). Measurements of nickel adsorption on calcite confirmed the high affinity of the calcium substrate for the metal. Finally, the absorption of cadmium on calcite was slower, in comparison with zinc and nickel. It has also been shown that calcite exhibited selectivity for the elimination of cadmium from solutions reaching 85% of the initial concentration.

The effect of biological substrates, on the nucleation and growth of calcium carbonate, was studied by the inoculation of stable supersaturated solutions with substrates prepared from the initial culture of the microalgae Acutodesmus obliquus. The microalgae was cultured in Basal Bold’s Medium, in order to measure its rate of growth by measurements of the concentration of chlorophyll produced and the cell density as a function of time. The identification of the chemical composition of the microalgae confirmed the presence of carboxyl and amino groups. The formation of calcium carbonate from supersaturated solutions (7.94<SR_calcite<104.71) in culture has been shown to depend on cell density. Higher precipitation rates of calcium carbonate were measured by increasing the cell density of cultures of microalgae. The precipitation rate of CaCO₃ depends on the illumination of the solutions and the presence of medium for microalgae growth. These parameters resulted in an acceleration of precipitation rate of calcium carbonate. The measurement of carbonate concentration confirmed the growth of microalgae by photosynthesis during formation of calcium carbonate. The importance of photosynthesis in the precipitation of calcium carbonate was also confirmed by the study of substrates of microalgae, which were dried in the air at 25°C. The presence of these substrates in supersaturated solution with respect to calcium carbonate, has resulted in the increase of the precipitation rate, in comparison with the respective rates in the presence of active microalgae culture. Inoculation of stable supersaturated solutions with microalgae cultures air dried at 70°C, did not induce precipitation. Composite substrates of microalgae with calcite, precipitated in air-dried microalgae, were more effective in inducing doubling of the rate of precipitation, in comparison with the corresponding rate obtained with calcite.

In the last part of the study on the effect of biological substrates on the crystal growth of calcium carbonate, the effect of heavy metals was investigated. The active microalgae culture in the presence of calcite, increased zinc absorption. Similar results were obtained for calcite precipitated on air-dried microalgae. The precipitation of calcium carbonate in supersaturated solutions in the presence of zinc and active microalgae was reduced by 80% (of the respective rate in their absence), while the growth rate of microalgae culture was reduced. Nickel was not absorbed into substrates of air-dried cultures of microalgae. However, in solutions, in the presence of nickel and active microalgae culture, the rate of precipitation of calcium carbonate was reduced by 88%. The presence of cadmium in the supersaturated solutions with respect to calcium carbonate, upon inoculation with air dried microalgae culture, cancelled CaCO₃ precipitation. The presence of Ni, Zn, Cd and Cu (C_Me=10^-5M), in the supersaturated solutions resulted to significant reduction of their concentration in the respective supersaturated solutions. The effect of the presence of cadmium in the supersaturated solutions was significant. The binding of metals on the test substrates may be attributed to surface complexation. Calculations of the capacitance of the substrate electrolyte double layer in the presence of metals from potentiometric titrations data on dry microalgae substrates corroborated this suggestion.