i AbstractSilica precipitation and silica scale formation on membrane surfaces in RO desalination present a significant operational challenge in water purification. Silica scale formation on the membrane surface leads to flux decline, RO system productivity lost, membrane degradation, and increased cost of chemicals. In spite of the apparent simplicity of silica's composition and scale formation on the membrane surface, fundamental questions and empirical knowledge persist about the formation, solubility and behaviour of silica species during the silica polymerisation, precipitation and scale formation.The broad objective of the present study was to define conditions and factors affecting silica polymerisation and precipitation in coal seam gas (CSG) waters in Australia. The scope of the problem was narrowed to focus on dissolved silica species studied by 29 Si NMR spectroscopy, removal of silica by coagulation as pre-treatment step for RO desalination and silica fouling patterns in RO desalination for a range of salinities in both synthetic and CSG waters to develop a conceptual model of silica precipitation and deposition on the membrane surface.The CSG industry in Australia generates significant quantities of CSG water, especially during the first 3-5 years of reservoir development when the hydraulic pressure needs to be released to extract the gas from coal seam. To avoid high cost of brine treatment and residual disposal frequently requires high recovery RO desalination to treat CSG water to level acceptable for further re-use. Furthermore, CSG waters in Australia have all four critical parameters, which potentially lead to silica precipitation prior it reached the solubility limit. These parameters include medium to high salinity, medium silica concentrations, high alkalinity at pH9 and slightly elevated aluminium concentrations.Practical field, theoretical and laboratory research works have been undertaken to study industry's concerns and bring fundamental and practical solutions to the problem. In Sodium ions depress silica solubility at the same time preventing silica from deposition on the membrane surface. The majority of dissolved silica polymerised in the bulk solution and was discharged in the reject stream of the RO system. To conclude practical silica solubility or the solubility defined empirically is a key for prevention of silica polymerisation in RO desalination systems.iii

Highlights

Silica fouling patterns in a sodium–silica system and the effect of pH on residual dissolved silica concentrations are reported.

Dissolved silica (SiO 2) is supplied to the environment by chemical and biochemical weathering processes despite the fact that dissolved silica has many stable and unstable dissolved forms (silica species). The processes involve ion substitution and chelate forming reactions which remove mineral lattice cations. The concentration of dissolved silica in natural waters is determined by a buffering mechanism which is thought to require the sorption and desorption of dissolved silica by soil particles. For instance average concentration of silica in some groundwater like coal seam gas water ranges between 0.1 and 80.0 ml/L. The dissolution process of silica and silicates from rocks into water is mainly due to hydrolysis of silica-oxygen-silica bonds, resulting in the liberation of silicic acid (Si(OH) 4) and silicates into aqueous phase. It is difficult to define precisely the term 'aqueous silica' as there is an array of silica species possible. Temperature, pH and ionic strength have a substantial influence on the solubility of amorphous silica and forms of silica present in a solution. This phenomenon of silica chemistry can be explained by presence of various silica species, which frequently define silica solubility and physicochemical reactions. It appears that some silica species behave as organics. For seawater the composition is relatively balanced; though, this might not explain low silica precipitation in seawater desalination.