Fig. 5 Gluconic acid (GA) + 2-ketogluconic acid (KGA) concentration in solution of Ca–kaolinite (CaKL), Ca–montmorillonite (CaMT), and K–montmorillonite (KMT) suspensions (1:40) as a function of equilibration time. Phosphorus was added as rock phosphate at t = 0. Inoculation with E37 took place at t = 24 h (CaKL) or 6 h (CaMT, KMT)