Experimental and computational studies into the effect of salinisation on Zn in solid-solution interactions relevant to tropical soils

Abstract

Salinization of soils and aquifers in coastal areas worldwide increases due to seawater level rise. Increasing concentrations of Na+ and Cl- associated with the salinity increase change the geochemical cycles of heavy metals like Zn and hence have a fundamental impact on the adsorption of Zn in soils. Zinc is a critical pollutant metal in soils due to industrial activities and solid-solution interface processes arguably exercise the most important control of the Zn cycle in soils and sediments. To this end, surface complexation models (SCMs) are a promising technique to define the adsorption of Zn at solid-solution interfaces in different environmental conditions. These models are based on experimental data, and whilst SCM has been successfully used and parametrised for Zn and other metals in saline solutions < 0.1 mol L-1 NaCl, this has not been achieved for solutions with higher salinity. This thesis aim was to better understand Zn adsorption on soils and goethite in NaCl saline solutions and evaluate surface complexation models (SCMs) performance to predict the effect of salinisation on zinc (Zn) interface processes in tropical soils. This work found that the Davies equation successfully corrects for changes in activities up to 0.7 mol L-1 ion strength. Zn species in the aqueous solution are largely controlled by complexation with inorganic ions (Cl, HCO3) and dissolved organic carbon (DOC), especially at pH between 4 and 8. Zn complexation with Cl ions increases with increasing NaCl concentration from 0.1 to 0.3 and 0.7 mol L-1, therefore, decreasing Zn adsorption on soils at pH 3 to 6. The addition of carbonate ion (HCO3) in NaCl solutions does not change Zn adsorption in soils at [Zn]init = 0.08 mmol L-1, however, an increased (30 - 35%) of Zn adsorption is observed at pH 6 to 7.5 at [Zn]init = 0.8 mmol L-1. Multi-surface models (MSMs) with experimentally determined model parameters and generic surface complexation databases predict well Zn adsorption on soils in NaCl concentrations in the presence of carbonate. Model underprediction is generally observed at a low pH range (3 – 5) in 0.3 and 0.7 mol L-1 NaCl solution which might be primarily due to the absence of ternary surface complexation constants (≡FeO-ZnCl and ≡FeO-ZnHCO3) in the database of iron (hydr)oxides. To explore ternary surface complexes of Zn on goethite in 0.3 and 0.7 mol L-1 NaCl solution at low pH, a constant capacitance model protocol is established and successfully determines the stability constants of ternary surface complexes at [Zn]init = 0.08 mmol L-1. At [Zn]init = 0.8 mmol L-1, ternary surface complexes are not reported by model estimation. The results of this thesis highlighted that existing surface complexation databases well predict Zn adsorption in low concentrations of NaCl, but at higher concentrations (> 0.1 mol L-1 NaCl), the model databases need to incorporate ternary surface complexes.