Model validation of cu(Ⅱ)- kaolinite systems by in-situ measurement of free metal ion activities

Abstract

Surface complexation modelling methods are extensively used to quantify the chemical speciation at the mineral-water interface. However, the input parameters are often not uniquely defined. In 1-pK modelling of metal ions adsorption, the basic Stern layer model (1-pK SLM) calculations require six parameters, viz., specific surface area, site density, inner and outer layer capacitance, and intrinsic acidity/binding constants. The number of input parameters by introducing free Cu²⁺ ions as an observation to determine its binding constants was reduced by 1-pK SLM. The copper selective electrode (Cu-ISE) was calibrated using ethylenediamine/Cu²⁺ metal ions buffer to extend the Nernstian Cu²⁺ detection limit to 10-15 M. Spectral data also suggests bidentate complexation of Cu²⁺ with =AlOH⁻⁰‧⁵ and ion exchange complexes between Cu²⁺ and ≡Si-OH sites as follows, ={AlOH}₂Cu⁺ pK = -2.8, (≡SiO₂)Cu⁰ pK= - 8.4 and (--X)₂Cu pK = -2.02. The ISE measurements of free Cu²⁺ seem to interfere with the presence of CuOH⁺, particularly when the solution is pH > 7.0. When the analytical data are corrected for CuOH⁺ interference, the measured {Cu²⁺} of the Cu-kaolinite system is predicted well with 1-pK SLM modelled data. Using this method, the number of input parameters reduced to five while simulating Cu²⁺ - kaolinite interface processes reasonably well is considered a novel method concerning the experiment results. Quantifying chemical speciation at mineral-water interfaces could be done using consistent parameters with reference to the above experiment.