Discussion · Figure 8

The same chemistry, in reverse.

Underground, acid lets the nickel loose. In the plant, half-burned dolomite catches it again. One equilibrium, run forwards and backwards.

UNDERGROUND · MOBILISATION IN THE PLANT · IMMOBILISATION STEP 1 Acid input from the surface atmospheric CO₂ + organic N → H⁺ + NO₃⁻ STEP 2 pH falls below 6.5 the groundwater turns calcite-aggressive STEP 3 Calcite of the matrix dissolves CaCO₃ + H⁺ → Ca²⁺ + HCO₃⁻ STEP 4 Trace Ni²⁺ is released into solution up to 61 µg/L · three times the legal limit STEP 1 Half-burned dolomite contacts the water Magnodol (CaCO₃·MgO) in the spray-aerated open filter STEP 2 pH rises back to neutral calcite equilibrium re-established · alkalinity restored STEP 3 Fresh calcite and MnO₂ precipitate Ca²⁺ + HCO₃⁻ → CaCO₃ · Mn²⁺ + O₂ → MnO₂ STEP 4 Ni²⁺ is captured by both phases r² = 0.66 onto MnO₂ · r² = 0.86 into new CaCO₃ dissolved Ni²⁺ travels from aquifer to plant and is removed below the 20 µg/L limit before distribution

Schematic of the mobilise–immobilise mirror that emerges from the data presented in Figures 4 through 7. In the aquifer (left column), acid input from the surface acidifies the groundwater, the calcite cement of the older main terrace dissolves, and trace nickel is released into solution at concentrations up to 61 µg/L. In the spray-aerated open filter (right column), contact with half-burned dolomite (Magnodol, CaCO₃·MgO) raises the pH, fresh calcite reprecipitates, and manganese is oxidised to MnO₂. The same dissolved Ni²⁺ released underground is captured by both newly formed phases, with the published correlations r² = 0.66 for the manganese-oxide fraction and r² = 0.86 for the carbonate-bound fraction; the treated water consistently meets the 20 µg/L TrinkwV limit. *