In order to test the buffering effect of iron, a series of experimental runs were conducted with varying initial iron concentrations. The results, shown in Fig. 3, clearly demonstrate how the pH of the initially iron-free system continues to increase with time, whereas the three other isotherms, representing initial iron concentrations of 100, 250 and 500 mg/l, decline slowly after having reached their respective peak values. Note the break on the horizontal axis. It is also worth noting that the height of the peak is negatively correlated with the initial iron concentration, i.e. the lowest peak is obtained for the system with the highest initial iron concentration.