The influence of biopolymers, alginate (Alg) and polyaspartate (pAsp), on the kinetics of calcite precipitation was studied using the constant composition method for supersaturation states ranging from 2.4 to 4.5, at pH = 8.5. Biopolymer presence changes the mechanism of calcite precipitation and its inhibition depending on the system history. In a system without polymers, calcite precipitates by spiral growth. Introducing polymers during growth inhibits step movement by pinning at a number of kink sites along step edges. Polymer adsorption induces growth by two-dimensional nucleation. Both polymers inhibit calcite growth. Inhibition is stronger at higher concentration and lower solution supersaturation. The interfacial free energy, a key parameter in the control of nucleation and growth, was estimated from the analysis of the precipitation rates, as well as data obtained from vapor adsorption, which are quite identical for calcite with either alginate or polyaspartate adsorbed. This is confirmed by electrokinetic measurements, which show similar ζ- potential values for calcite with each of the polymers. Increasing polymer concentration and adsorption time led to a progressive decrease of the effective interface free energy, which could explain the much lower supersaturation needed for the onset of surface nucleation.
Recrystallization requires dissolution in pore fluids and precipitation on existing particle surfaces in a process known as Ostwald ripening, where the smallest particles feed growth on the larger ones. Recrystallization conditions are optimized in industry when commercial products require larger crystals, and it is the dominant process in burial diagenesis, which turns sediments into rock. In chalk, the original coccolith elements are often still clearly distinguishable, which is a surprise considering the rapid rates of pure calcite recrystallization. We studied the rates of calcite precipitation on the surface of the particles in various chalk samples, using the constant composition method over a wide range of supersaturations. Our results show that dependence of calcite precipitation rate on supersaturation does not obey the parabolic law, which is characteristic for calcite growth. Instead, the dependence was exponential, which indicates surface nucleation-mediated growth. The rates of calcite precipitation on the chalk surfaces at the lowest supersaturation examined in the present work are 3 orders of magnitude lower than those on pure calcite surfaces. This means that, during chalk recrystallization, when supersaturation is extremely low, recrystallization can be significantly suppressed. The presence of organic compounds has an important influence on recrystallization kinetics, so along with other possible causes of inhibition, the mechanism of calcite precipitation is a factor in the extremely low rates of the chalk recrystallization.
Strontium adsorption has been studied by the method of acid-base potentiometric titrations at three different temperatures: 25, 50, 75C. The effect of pH, ionic strength, sorbate/sorbent ratio, and temperature on adsorption was investigated. Experimental data were simulated using two various surface complexation models, with two different electrostatic descriptions of the interface: the constant capacitance model (CCM) and the triple-layer model (TLM). Although the both models used are able to account for the acid-base reactions and surface complexation of strontium on birnessite, we consider that the TLM is more applicable for a description of heterophaseous system H+ MnOH Sr2+. Under conditions of low ionic strength and negatively charged surface, Sr2+ ions compete with the electrolyte ions and form outer-sphere complexes along with inner-sphere complexes. Consequently, using the CCM for description of strontium adsorption data could be mathematically satisfactory, but physically senseless. The equilibrium model proposed here consists of the complexes of inner (MnOHSr2+, MnOSr+, MnOSrOH0) and outer types ([MnO Sr2+]+). The corresponding intrinsic equilibrium constants of the formation of these surface complexes were calculated for 25,50, and 75C.
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