## 6 Discussion
The model mainly overestimates the CaCO\({}_{3}\) precipitation in the inlet half of the column; see Figure 4. One reason might be that the amount of adsorbed urease and, thus, ureolytic activity in the inlet half is overestimated by the first-order adsorption kinetics used (Equation (18)), although the concentration measurements can still be reproduced with the used adsorption kinetics. This might be improved by using other, more complex adsorption kinetics as discussed in [37]. Even though less precipitates were observed in the experiment, the Ca\({}^{2+}\) concentrations were observed to decrease down to zero or close to zero during the no-flow periods. The calibrated model reproduces the experimentally observed complete consumption of calcium by precipitation when either the injected calcium concentration is reduced or the calculated apparent dissociation constant of ammonia-ammonium is increased. In the experiment, Ca\({}^{2+}\) might have formed complexes with the biomass in the system or been trapped in clogged dead-end pores as observed in [60; 61], making it unavailable for both measurements and precipitation, which could explain the lower than expected concentration measurements and the low amount of precipitated CaCO\({}_{3}\) in the experiment. Some of the experimentally observed reduction in calcium might have been due to precipitation of calcium with other anions, which were not considered in the model geochemistry, such as phosphates, which were present in the cell-growth medium. The calibration would suggest that up to one sixth of the injected calcium would have precipitated as non-carbonate minerals, which seems an unlikely high amount. As discussed above, another explanation for lower calcium carbonate measurements in the columns might be the potential transport of suspended CaCO\({}_{3}\) precipitates and calcium-biomass complexes out of the column. These precipitates and calcium-biomass complexes would not have been captured as calcium in the liquid phase since samples were filtered after extraction from the column. It is challenging to compare the experimental measurement to the model predictions for column #2, as this experiment featured a changed injection strategy, only injecting cell suspension (crude urease) before every odd-numbered mineralization-medium injection and doubling the injected concentrations of urea and calcium. Nevertheless, the model can to some extent reproduce the observed lower ureolytic activity during the mineralization periods not preceded by a urease injection. This is also reflected in the predicted volume fractions for the resulting CaCO\({}_{3}\) precipitation. Although the model overestimates the amount of precipitated CaCO\({}_{3}\) for column #2 with the 0.66 M mineralization medium concentration, the model does not predict a significantly increased volume fraction of calcite compared to column #1 with half the calcium injected. This matches qualitatively the experimental observations. The on average lower ureolytic activity and, in comparison to the injected calcium, low amount of precipitated calcite are all due to the reduced volume fraction of adsorbed urease, which is, for column #2, significantly smaller for the even-numbered injections not preceded by a urease injection; see Figure 7. The doubled injected Ca\({}^{2+}\) concentration of column #2 leads to more rapid inactivation of adsorbed urease due to the initially higher precipitation rate and the precipitation-rate-dependent term in the inactivation rate equation (Equation (21)). Accounting for some variability in the experimental setup, leading to variable inactivation of urease over the column length and during the course of the experiment, the model is able to reproduce the qualitative behavior of column #2.