Acute Phase: It is hypothesized that ZS (zirconium silicate) is more effective than placebo control (alternative hypothesis) in lowering S-K levels in subjects with S-K between 5.0 - 6.5 mmol/l versus no difference between ZS and placebo control (null hypothesis).

Subacute Phase (randomized withdrawal): It is hypothesized that ZS once daily is more effective than placebo control (alternative hypotheses) in maintaining normokalemic levels (3.5 - 4.9 mmol/l) among subjects completing the Acute Phase versus no difference between each ZS dose and respective placebo controls (null hypotheses).

A total of 750 subjects with mild to moderate hyperkalemia (i- STAT potassium levels between 5.0-6.5 mmol/l, inclusive) will be enrolled in the study where they, in a double-blind fashion, will be randomized 1:1:1:1:1 to receive one of four (4) doses of ZS (1.25g, 2.5g, 5g, and 10g) or placebo control, administered 3 times daily (tid) with meals for the initial 48 hours (Acute Phase), followed by a Subacute Phase (randomized withdrawal) during which patients treated with active doses in the Acute Phase, who achieve normokalemia (i-STAT potassium values 3.5 to 4.9 mmol/l, inclusive) will be randomized to 12 days of subacute, once a day (qd) dosing. There will be a one-time randomization to assign the Acute Phase treatment and the Subacute Phase treatment. The Subacute Phase will include subjects who became normokalemic on active drug and those who became normokalemic on placebo. The former will be randomized in a 1:1 ratio between the same dose of ZS they received during the acute phase but only administered once a day (qd) or placebo, qd. Subjects on placebo during the Acute Phase who are normokalemic in the morning of Study Day 3, will be randomized to receive either 1.25 or 2.5 g ZS, qd as Subacute Phase treatment.

Safety and tolerability will be assessed on an ongoing basis by an Independent Data Safety Monitoring Board (DSMB). Each active dose group will consist of 150 patients per treatment group including the placebo control group for a total of 750 patients; the 1:1:1:1:1 allocation helps to optimize the multiple active dose comparisons to the respective placebo controls for the Subacute Phase.

Inclusion Criteria:

• Provision of written informed consent.
• Over 18 years of age.
• Mean i-STAT potassium values between 5.0 - 6.5 mmol/l inclusive, at screening (Study Day 0).
• Ability to have repeated blood draws or effective venous catheterization.
• Women of childbearing potential must be practicing a highly effective method of birth control.

Exclusion Criteria:

• Pseudohyperkalemia signs and symptoms, such as excessive fist clinching hemolyzed blood specimen, severe leukocytosis or thrombocytosis.
• Subjects treated with lactulose, xifaxan or other nonabsorbed antibiotics for hyperammonemia within the last 7 days.
• Subjects treated with resins (such as Sevelamer acetate or Sodium polystyrene sulfonate [SPS; e.g. Kayexalate®]), calcium acetate, calcium carbonate, or lanthanum carbonate, within the last 7 days.
• Subjects with a life expectancy of less than 3 months.
• Subjects who are HIV positive.
• Subjects who are severely physically or mentally incapacitated and who in the opinion of investigator are unable to perform the subjects' tasks associated with the protocol.
• Women who are pregnant, lactating, or planning to become pregnant.
• Subjects with Ketoacidosis/Acidemia.
• Presence of any condition which, in the opinion of the investigator, places the subject at undue risk or potentially jeopardizes the quality of the data to be generated.
• Known hypersensitivity or previous anaphylaxis to ZS or to components thereof.
• Previous treatment with ZS
• Treatment with a drug or device within the last 30 days that has not received regulatory approval at the time of study entry.
• Subjects with cardiac arrhythmias that require immediate treatment.
• Insulin-dependent diabetes mellitus
• Subjects on dialysis