removal of hexavalent chromium from water.

First Stage

Sulfur dioxide (SO2), sodium bisulfite (NaHSO3), or sodium meta-bisulfite (Na2S2O5) are examples of chemical reducing agents that are frequently used in treatment to convert hexavalent chromium to trivalent chromium.^[1]This reaction will proceed quickly in the pH range of 2 to 3. Maintaining the wastewater between this pH range will reduce the amount of time it is retained. This is achieved by adding an acid, such as sulfuric acid (H2SO4), using a pH controller.

Second stage

To raise and maintain a pH of 8 or higher in the wastewater, calcium hydroxide, also referred to as lime, or Ca (OH)₂, must be added using an additional pH controller after the first-stage process is finished. This is required in order for chromium hydroxide to precipitate. It is simple to separate and redirect the precipitate for disposal.^[1] In order to achieve compliance, hexavalent chromium is typically reduced to trivalent chromium through precipitation, yielding chromium hydroxide, a safe and non-toxic material. The primary drawback of this treatment approach is that, in order to ensure a quick reduction rate, the wastewater must be reduced to a pH of two to three. Next, before being released, the wastewater needs to be neutralized. These processes use a lot of chemicals and often result in a lot more sludge from unreacted precipitants. ^[1]