Gallium Isotope Effect of Ga-Si Complex Solutions in Water: Theoretical Study Based on Density Functional Theory

Abstract

A Ga isotope is a new proxy for different geochemical processes such as a weathering process, solution process, etc. Si (Si(OH)4) is ubiquitous in natural water bodies. However, studies on the Ga isotope effect about a Ga3+ aqueous solution reacting with Si (Si(OH)4) are lacking. In this study, the Ga isotope effect of this process will be studied using a theoretical calculation method based on first principles. The results show that the heavy Ga (71Ga) isotope enrichment ability of different Ga-Si complex solutions is different. The 1000lnβ (‰) sequence of different Ga-Si complex solutions is (OH)3GaOSi(OH)3.(H2O)30 ≈ (OH)3(H2O)2GaOSi(OH)3.(H2O)30 > (OH)2(H2O)3GaOSi(OH)3.(H2O)30 > (H2O)5GaOSi(OH)3.(H2O)30 > (OH)(H2O)4GaOSi(OH)3.(H2O)30. The results show that there are two different reaction mechanisms when a Ga3+ aqueous solution reacts with Si-bearing (Si(OH)4) water; that is, six-coordination Ga-Si complexes and four-coordination Ga-Si complexes are formed at low pH (acidic) and high pH (alkaline), respectively. Compared with a Ga-Si complex aqueous solution under acidic conditions, Ga-Si aqueous solutions under alkaline conditions preferentially enriched the heavy Ga isotope (71Ga). The Ga isotope fractionation factors (α) between Ga-Si complex solutions and Ga3+-bearing aqueous solutions are all negative, which indicates that light Ga (69Ga) isotopes preferentially enter the structure of Ga-Si complexes during the formation of Ga-Si complex solutions. At 50 °C, the Ga isotope fractionation factors (1000lnα) of five systems ((H2O)5GaOSi(OH)3.(H2O)30 vs. [Ga(H2O)6]3+(aq), (OH)(H2O)4GaOSi(OH)3.(H2O)30 vs. [Ga(H2O)6]3+(aq), (OH)3GaOSi(OH)3.(H2O)30 vs. [Ga(OH)3](aq), (OH)3(H2O)2GaOSi(OH)3.(H2O)30 vs. [Ga(OH)3](aq), and (OH)2(H2O)3GaOSi(OH)3.(H2O)30 vs. [Ga(OH)3](aq)) involved in this study are −0.12, −0.22, −0.07, −0.09, and −0.16 (‰), respectively. Excitedly, Si can affect the enrichment ability of the heavy Ga isotope (71Ga) in Ga-bearing complex aqueous solutions. This means that when Si is present in aqueous solutions, the enrichment capacity of the heavy Ga isotope (71Ga) of aqueous solutions will be effectively reduced. Ga in sediments is mainly derived from soluble Ga in the form of adsorbed (Fe, Mn) oxides/hydroxides, and the Ga isotope composition in sediments is heavier than that in basalt. The formation process of Ga-Si complex aqueous solutions influences the Ga isotope fractionation effect and also contributes to the composition of Ga isotopes in sediments. These key Ga isotope fractionation parameters obtained in this study will provide theoretical support for better explaining the reaction mechanism of Ga3+ complexes and Si-bearing (Si(OH)4) water bodies in solution processes and Ga isotope geochemical cycles.