Catalyzes the hydrolysis of 3-deoxy-D-manno-octulosonate 8-phosphate (KDO 8-P) to 3-deoxy-D-manno-octulosonate (KDO) and inorganic phosphate. 3-deoxy-alpha-D-manno-2-octulosonate-8-phosphate + H2O = 3-deoxy-alpha-D-manno-oct-2-ulosonate + phosphate Mg(2+). The phosphatase activity is also stimulated by cobalt ions, whereas baryum, zinc, and manganese ions are less effective stimulators. Inhibited by calcium, cadmium, mercury, and copper ions. kcat is 175 sec(-1). Optimum pH is 5.5. High catalytic activity is observed between pH 5.5 and 7.0. Carbohydrate biosynthesis; 3-deoxy-D-manno-octulosonate biosynthesis; 3-deoxy-D-manno-octulosonate from D-ribulose 5-phosphate: step 3/3. Bacterial outer membrane biogenesis; lipopolysaccharide biosynthesis. Homotetramer. Belongs to the KdsC family.