Compared to aluminum coagulants, iron hydroxides are insoluble over a wider pH range and less soluble than aluminum hydroxide. Iron coagulants form a heavier stronger floc, and form iron-metal complexes that co-precipitate a number of heavy metals. Treatment with alkaline materials such as lime or sodium hydroxide will raise the pH of a solution causing metallic hydroxide compounds to become insoluble and precipitate. The addition of ferric sulfate will enhance the precipitation of heavy metals beyond what is expected from the theoretical solubility curve.