Interaction of Aluminium with Silica in Geo-fluids: Investigation of a Proposed Compositional Control Mechanism of Natural Waters and Aluminosilicate Precipitation Reactions in Aqueous Systems

Abstract

Metastable, amorphous aluminosilicates such as allophane have long been suspected to control the solubility of Al and SiO2 in groundwater and soil pore-fluids. However, few efforts to determine the trigger mechanisms and rate of these important and ubiquitous geochemical reactions have previously been attempted. Meanwhile, at much higher temperatures, amorphous aluminosilicate scales are precipitated from geothermal fluid within pipelines and geothermal reinjection wells, causing costly formation damage, and pipe and valve blockages. However, despite >40 years of research, a trigger mechanism for the formation of these aluminosilicate scales has yet to be conclusively identified. Allophane deposited from spring water at Silica Rapids, Mt Ruapehu, New Zealand, provides the lowtemperature example of aluminosilicate precipitation for this study, while geothermal aluminosilicate scales deposited in pipelines at the San Jacinto-Tizate power project, Nicaragua, supply the high temperature case. Solid samples from both locations are characterised in terms of composition via Xray fluorescence (XRF) and field emission scanning electron microscopy (FESEM), and structure via X-ray diffraction (XRD). These materials are then compared to those produced in solution experiments, conducted at both low and high-temperature, under pH and compositionally-controlled conditions. A hypothesis, wherein the supersaturation of gibbsite/boehmite may be the trigger for both low and high-temperature reactions was tested, and a qualitative understanding of the rates of both aluminosilicate precipitation reactions obtained. Geothermal aluminosilicate scales are X-ray-amorphous and enriched with K, Ca, and Al; with (K/Na)[scale]/(K/Na)[brine], (Ca/Na)[scale]/(Ca/Na)[brine], and (Al/Si)[scale]/(Al/Si)[brine] ratios of ~6, 40, and 200-800, respectively (mass basis), while [K+Na+2Ca]/Al and Si/Al mole ratios are ~1.0, and as low as 5.1, respectively. Synthetic aluminosilicates produced in K-spiked geothermal experiments attain identical (K/Na)[solid]/(K/Na)[brine] ratios to geothermal scales, while [K+Na+2Ca]/Al and Si/Al mole ratios are ~1.0, and as high as 4.1, respectively. Silica Rapids allophane contains ≤0.5 wt.% K2O+Na2O+CaO, with Si/Al ratios of ~0.65. Synthetically-produced allophane similarly contains <0.5 oxide wt.% alkali elements, and is Al-rich; with Si/Al mole ratios of 0.65-0.97. The onset of both synthetic Silica Rapids and geothermal aluminosilicate formation is found to coincide with gibbsite saturation, while the rate of both reactions is rapid, in accordance with field observations. Based on these results, some possible natural hydrothermal analogues to geothermal aluminosilicate scales are proposed, and potential geothermal scale mitigation strategies examined.