ENVIRONMENTAL REMEDIATION APPLICATIONS OF GEOTHERMAL CALCIUM SILICATES

Abstract

The deposition of amorphous silica from geothermal brines within pipework and reinjection wells is a significant problem, which limits the effectiveness of geothermal resource utilisation. We have developed a method to remove this silica from the geothermal brine as a nanostructured calcium silicate hydrate. In addition to preventing the formation of silica scale, the recovered material has several potential useful applications, including environmental remediation of various metal and other inorganic species.
The material, which may also be prepared from synthetic silica sources, has a moderately high surface area of up to 400 m2 g-1. The calcium component of the silicate is chemically active and reacts rapidly with anions such as phosphate to form insoluble calcium phosphate phases. Additionally, hydroxide is released as the silicate reacts with water. This, in turn will react with many heavy metal cations, forming insoluble hydroxide species. In both cases, these insoluble products are retained within the silicate material.
The high surface area and the reactivity of the calcium within the silicate make this a promising material for the remediation of environmental pollution. The uptake capacity of the silicate towards metal ions, such as copper or iron is 5-10 mmol metal per gram of silicate. The silicate is also capable of removing phosphate from eutrophic water sources, where the phosphate concentration becomes problematic at concentrations less than 10 ?mol dm-3. When removing phosphate from more concentrated sources, the silicate has an uptake capacity of 5 mmol phosphate per gram of silicate. The resulting phosphate-containing product has potential for application as a fertilizer.
The uptake characteristics and potential applications of both synthetic and geothermal-derived versions of this calcium silicate in environmental remediation will be discussed.