Coupling reactive transport and speciation models to study nanofiltration of metallic salts from an acid leachate

• Nanofiltration of a high-salinity, low-pH plant ash leachate is investigated. • The optimal ionic strength for maximising the Ni 2+ rejection rate was 0.37 mol kg −1 . • The processes governing the rejection rate are discussed. • A model coupling PHREEQC and Matlab was used to simulate these rejection rates. • Including speciation improved the fit between exp. and calc. rejection rates by 56%. This work proposes the application of membrane processes to filter the metals contained in a leachate of hyperaccumulating plant ashes. To better understand the phenomena involved in metal separation, a transport model based on the Nernst-Planck equation was used (SEDE model). The leachate was acidic and concentrated in various salts, including nickel, magnesium, and iron sulfate. It therefore exhibited a complex speciation. The objective was to study speciation influence on the modelling of the rejection rate. The leachate was diluted 2, 4, 8, 10 and 20 times to assess the quality of the nanofiltration and the influence of sulfuric acid and of salt concentration on the rejection rate of three metals of interest: nickel, iron, and magnesium. An optimum rejection rate was obtained for the leachate diluted 8-fold, at an ionic strength of 0.37 mol kg −1 , with a rejection rate of 85 % for the three metals. Pore swelling was observed for pure leachate nanofiltration, considerably reducing the rejection of all divalent ions. Iron was most affected by changes in speciation. By considering the speciation upstream of nanofiltration, the prediction of the retention rate of some divalent cations can be improved by up to 50 %, compared to experimental data. This work also constitutes a first step towards the evaluation of the quantity of recyclable protons after an acid leaching stage in hydrometallurgy.