Jelena Perić

Interaction of the zeolitic tuff with Zn-containing simulated pollutant solutions

The possibility of removing Zn2+ cations from wastewater by ion exchange using natural zeolites as exchangers has been investigated. The process of binding of zinc ions into zeolite structure has been established by several reaction mechanisms as a fast chemical reaction of ion exchange, accompanied by slower adsorption of different ionic species and possible precipitation or coprecipitation with the zeolite structure. The physicochemical phenomena such as hydrolysis and dissolution of surface layers are the result of interaction of zeolite with hydrogen or hydroxyl ions from the solution. Complexation of OH- with Zn2+ to form the zinc-hydroxy species strongly depends on pH value and affect the uptake mechanism as to lower dissolution of surface aluminosilicate layers. Structure imperfections as a surface property of mineralogical nonhomogeneous zeolitic grains can lead to formation of sorption surface sites with different energy, which affects the nonuniform distribution of different zinc species adsorbed. It is particularly possible in zeolitic tuff samples with relatively high content of aluminosilicates as minor mineralogical components, which is characteristic of Croatian deposits.

Phase transformation of calcium carbonate polymorphs

Abstract The phase composition of mineral aragonite and synthetic vaterite samples was determined qualititavely by using IR-spectrophotometric and X-ray diffraction analyses. The transformation of aragonite, A, and vaterite, V, into the stable modification calcite, C, was followed using differential scanning calorimetry analysis. In order to determine the kinetics and mechanisms of these phase transformations, a number of experimental DSC curves were elaborated mathematically and the stationary point theory was applied. The activation energy, Ea, and the enthalpy, ΔH , were found to be, respectively, 234.5 ± 5.6 kJ mol −1 and 122 Jg −1 for the phase transformation A → C, and 252.8 ± 48.7 kJ mol −1 and −21.2 Jg −1 for the V → C transformation.

Effect of precipitation conditions on the morphology of calcium carbonate : quantification of crystal shapes using image analysis

Extra-pure calcium carbonate was obtained in the laboratory by carbonation of a solution of calcium nitrate and monoethanolamine with carbon(IV) oxide under various operating conditions. The particle size and shape of the samples obtained were characterized by image analysis. The results show that the method of quantitative morphological characterization using defined shape descriptors enables us to get a better insight into the relationship of the mineralogical composition and particle size/shape to precipitation conditions.

Precipitation of calcium carbonate in a calcium nitrate and monoethanolamine solution

Abstract High-purity precipitated calcium carbonate has been produced in the laboratory by carbonation of a solution of calcium nitrate and monoethanolamine. The process was traced by measurements of the change in Ca concentration and in electrical conductivity of the solution. The κ−t and cCu−t curves were analyzed to determine the effect of temperature and concentration of carbon(IV) oxide on the process of carbonation. The samples obtained were mainly of aragonite and vaterite crystal modifications with traces of calcite. The relationship between the mineralogical composition and particle size (morphology) and operating conditions was examined as well.

Morphological development in calcium carbonate precipitation by the ethanolamine process

Morphological development in calcium carbonate precipitation by the ethanolamine process at 30 and 60°C has been examined using different techniques (quantitative image analysis, laser diffraction, XRD, and FT-IR). The initially grown phase is of vaterite modification that at higher temperatures (60°C) transforms to a more stable aragonite phase within the reactor itself. A comparison of the form and structure of calcium carbonate particles obtained during the process leads to a conclusion that crystal aggregation is the mechanism that determines the overall particle size at lower temperatures (30°C), while crystal growth dominates at higher temperatures. The results show that crystal characterization by quantitative image analysis permits a better understanding of the phenomena taking place in the reactor.

Investigation of dehydroxylation of gibbsite into boehmite by DSC analysis

The dehydroxylation of gibbsite into boehmite was investigated by means of DSC analysis under non-isothermal conditions in the temperature range 453–673 K at heating rates from 2.5 to 20.0 K min−1. Mathematical analysis of the experimental DSC curves revealed the mechanism and kinetics of the gibbsite dehydroxylation process. The kinetic curvesα=f(t) andα=f(T) are sigmoidal in shape; their inflection points and the νm point of the curvesν=f(T) andν=f(T) are interrelated and are defined by the concept of a stationary point. The activation energy for the first stage of gibbsite dehydroxylation in the temperature range 453–673 K is 132.92±8.33–142.26±8.33 kJ mol−1.

The examination of the phase transformation of aragonite into calcite by means of DSC analysis

Abstract The X-ray diffraction method was used to determine the content of aragonite and calcite in samples of precipitated calcium carbonate synthesized in the laboratory. The transformation of the aragonite modification present into the stable calcite structure was traced by means of DSC analysis. All the samples were analysed by DSC under non-isothermal conditions in the temperature range 624–772 K at heating rates of 5, 10, 15, 20 and 25 K min −1 . A mathematical analysis of experimental DSC curves defined the mechanism and the kinetics of the phase transformation of aragonite into calcite. The kinetic curves α = f(t) and α = f(T) are sigmoidal in shape, and their inflection points and the v m point of the curves v = f(t) and v = f(T) are interrelated and are defined by the concept of a stationary point. The kinetic curves indicate that v m corresponds to (d H /d t ) m of the endothermal peak of transformation of aragonite into calcite, which is characteristic of a topochemical reaction.

Testing of Breakthrough Curves for Removal of Lead Ions from Aqueous Solutions by Natural Zeolite‐Clinoptilolite According to the Clark Kinetic Equation

Abstract The kinetics of removal of lead ions from aqueous solutions using the column method with a fixed bed of natural zeolite clinoptilolite has been examined. Experimental results are presented as breakthrough curves, and tested according to the kinetic equation developed by Clark by means of the linear and non‐linear regression analysis. Both mathematical methods attain the same values for parameters A and r of the Clark equation. These parameters are used for the calculation of the sorption rate coefficient k and the removal capacity q. The removal capacity obtained from the experimental results and the capacity calculated using the Clark kinetic equation have shown excellent agreement. The Clark kinetic equation has been found suitable for description of removal of lead on zeolite for different experimental conditions. Based on the calculated parameters, the theoretical breakthrough curves have been plotted and compared with the experimental ones. Applicability of the kinetic equation used has been additionally confirmed by the good fit of experimental and modelled breakthrough curves. Based on the parameters calculated, the behavior of modelled breakthrough curves was predicted for different depths of fixed bed of zeolite.

The Effect of Zeolite Fixed Bed Depth on Lead Removal from Aqueous Solutions

Abstract The effect of zeolite bed depth on lead removal from aqueous solutions by the column method has been examined. The results indicate that the increase of bed depth delays the breakthrough point and exhaustion point, and increases the contact time of the zeolite – lead solution, and the height of the mass transfer zone, hz. The increase of the bed depth lowers the effect of axial dispersion on the mass transfer process. In order to predict the time necessary for exceeding the defined effluent concentration for a constant bed depth, the bed depth service time (BDST) approach has been used. Experimentally obtained breakthrough curves for the flow rate of 1 ml/min were used to derive the BDST approach equations. These equations were successfully used for modelling of the system for flow rates of 2 and 3 ml/min. The BDST equations have yielded modelled linear equations used for calculation of hz. The increase of the flow rate increases hz, which indicates that the zeolite–solution contact time is not sufficient. This may be attributed to the affect of axial dispersion on mass transfer on the solid-liquid interface.

The Influence of Various Admixtures on the Calcium Carbonate Precipitation from a Calcium Nitrate and Monoethanolamine Solution

Calcium carbonate was precipitated by carbonation of calcium nitrate and monoethanolamine solution. The influence of various organic admixtures on the crystallization, crystal morphology, and polymorphism of calcium carbonate has been studied using different techniques. The results show that sucrose and fructose present even at very high concentrations do not affect the duration or mechanism of the carbonation process at a higher degree, what is attributed to the specific quality of the ethanolamine process. At high concentration citric acid and ethylenediamine-tetrakis-N,N,N,N-(methylenephosphonic acid) had some effect on the process mechanism, phase composition, morphology and crystal aggregation of the calcium carbonate precipitated.