A Study of the Sorption Characteristics of Nanostructured Calcium Silicate
Authors:   , Cairns Mathew James 
Published: November 2021

<p>Nanostructured calcium silicate (NCS) is an X-ray amorphous silicate material consisting of randomly arranged platelets several tens of nanometres in size, forming agglomerates a few micrometres in size. This affords the material a high, readily accessible surface area of up to 600 m2 g -1 with chemically active surface-bound calcium and silanol groups being integral parts of the silicate structure. As such, it makes an ideal material for the sorption of many potential pollutant materials. However, NCS is highly thixotropic. This reduces its applicability for use as a sorbent material on a large scale, the thixotropic nature of NCS precluding its efficient separation from suspension. NCS, in contact with water, will ion-exchange surface-bound calcium with hydrogen ions, releasing calcium into solution, and leading to an increase in the pH value of the solution. The process may be exploited by using the material as a sorbent for cationic metal species forming insoluble hydroxides. This thesis demonstrates the use of NCS as a sorbent material for Cu2+, with the material exhibiting a sorption capacity for this ion of up to 10 mmol g -1. When the sorption capacity of the material is reached, all the calcium initially present in the NCS material (31-38 wt % CaO) is leached into solution. The copper is initially sorbed as an X-ray amorphous phase (most likely Cu(OH)2) but in the presence of excess copper, the more thermodynamically stable crystalline phase Cu2X(OH)3, X being chloride or nitrate, is formed. It was shown that the presence of calcium is necessary for this sorption to occur. When calcium was leached from the material prior to sorption studies, the sorption capacity of the material was significantly decreased. To aid the separation process of NCS from solution, bulk magnetite powder (Fe3O4), or superparamagnetic magnetite or maghemite (g-Fe2O3) were incorporated into the NCS structure during its synthesis. The addition of these additives to the NCS material reduced characteristics such as specific surface area or sorption capacity insofar as extra mass had been added to the system. The structure of the NCS was not degraded. The NCS material containing bulk magnetite powder was shown to be applicable to the sorption of phosphate in a continuous fluidised bed system, utilising the magnetic properties of the material to aid separation. Phosphate was chosen, as the sorption characteristics of NCS with respect to this ion were previously known. Attempts to use magnetic techniques to separate the superparamagnetic composites subsequent to copper sorption were unsuccessful. Although the composite materials exhibited similar sorption capacity for copper to the unmodified one, the acidic conditions of the copper solution degraded the composite, precluding the use of magnetic separation. Finally, composite materials of NCS and a conducting polymer, polyaniline, were prepared which provided potential redox-activity to a high surface area substrate. The sorption characteristics of this material were demonstrated with its use as a sorbent for the perrhenate ion. This rhenium ion was chosen due to its chemical similarity to pertechnetate, a component of many radioactive wastewaters. It was demonstrated that the sorption process proceeded via an electrochemical mechanism in which the polyaniline caused the perrhenate ion to be reduced to a rhenium oxide species.</p>