Degradation of inorganic polyphosphates (polyP). Releases orthophosphate processively from the ends of the polyP chain. Has a strong preference for long-chain polyphosphates and has only weak affinity for smaller size polyP of about 15 residues. [phosphate](n) + H2O = [phosphate](n-1) + H(+) + phosphate Activity is redox-regulated. Highly sensitive to inactivation by oxidation (PubMed:24560923). Strongly stimulated by potassium salts (PubMed:8380170). Optimum pH is 8. Homodimer. Deletion mutant accumulates polyP. Belongs to the GppA/Ppx family.