Fig. 4 (a) Orthophosphate concentration (Cp) and (b) pH in solution of Ca–kaolinite (CaKL), Ca–montmorillonite (CaMT) and K–montmorillonite (KMT) suspensions (1:40) as a function of equilibration time. Rock phosphate was added to all suspensions at t = 0. Inoculation with E37 took place at t = 24 h (CaKL) or 6 h (CaMT, KMT)