Textural properties of synthetic nano-calcite produced by hydrothermal carbonation of calcium hydroxide

The hydrothermal carbonation of calcium hydroxide (Ca(OH)2) at high pressure of CO2 (initial PCO2 1/4 55 bar) and moderate to high temperature (30 and 90 1C) was used to synthesize fine particles of calcite. This method allows a high carbonation efficiency (about 95% of Ca(OH)2-CaCO3 conversion), a significant production rate (48 kg/m3 h) and high purity of product (about 96%). However, the various initial physicochemical conditions have a strong influence on the crystal size and surface area of the synthesized calcite crystals. The present study is focused on the estimation of the textural properties of synthesized calcite (morphology, specific surface area, average particle size, particle size distribution and particle size evolution with reaction time), using Rietveld refinements of X-ray diffraction (XRD) spectra, Brunauer-Emmett-Teller (BET) measurements, and scanning electron microscope (SEM) and transmission electron microscope (TEM) observations. This study demonstrate that the pressure, the temperature and the dissolved quantity of CO2 have a significant effect on the average particle size, specific surface area, initial rate of precipitation, and on the morphology of calcium carbonate crystals. In contrast, these PTx conditions used herein have an insignificant effect on the carbonation efficiency of Ca(OH)2. Finally, the results presented here demonstrate that nano-calcite crystals with high specific surface area (SBET 1/4 6-10m2/g) can be produced, with a high potential for industrial applications such as adsorbents and/or filler in papermaking industry

Experimental and theoretical consideration of the factors influencing cationic pollutants retention by seashell waste

BACKGROUNDSeashell waste (SW) is rich in biogenic calcium carbonate and potentially can substitute geological sources in various applications, such as the separation of heavy metals and radionuclides from contaminated solutions. This study aims to compare SW sorption efficiency towards different chemical species (Cu2+, Zn2+, Pb2+ and Sr2+) and to evaluate the effects of various factors based on the experimental data and modeling approach. RESULTSThe reaction of SW with aqueous metal solutions is a combination of several processes that result in metal retention, Ca2+ release, and changes in pH. SW demonstrates variable selectivity for investigated cations, depending on their concentrations and reaction times. Maximum sorption capacities declined in the order Zn2+ gt Pb2+ approximate to Sr2+ gt Cu2+. The model based on general regression neural network (GRNN) architecture was developed, which enabled prediction of removal efficiency taking into account the process specific, metal specific parameters and their non-linear interactions. Initial concentration and covalent radius of a cation exhibit the highest, while the initial pH the lowest significance. CONCLUSIONEcological problems caused by SW accumulation in coastal areas could be mitigated by mastering technologies for their practical utilization. The results obtained facilitate the understanding of cationic pollutants removal by SW in the range of experimental conditions, while the GRNN approach demonstrates advantages in modeling complex sorption processes