What ORP reading is desired in the first reaction tank when removing hexavalent chromium by hydroxide precipitation?

The desired ORP (Oxidation-Reduction Potential) reading of +250 mV in the first reaction tank for removing hexavalent chromium by hydroxide precipitation aligns with optimal conditions for facilitating the precipitation process. An ORP in this range indicates that the environment is sufficiently oxidative, which can aid in converting hexavalent chromium (Cr(VI)) into trivalent chromium (Cr(III)).

Trivalent chromium is less soluble and can be effectively removed from wastewater through precipitation. By maintaining an ORP around +250 mV, conditions are favorable for this conversion, ensuring that hexavalent chromium is efficiently reduced and precipitated as a solid, thereby allowing for effective removal during subsequent treatment processes.

An ORP reading that is too low would not support the necessary oxidation reaction to reduce Cr(VI), while excessively high ORP levels could lead to unwanted side reactions or the solubilization of precipitated metals. Hence, the +250 mV value strikes a balance necessary for optimal hexavalent chromium removal through hydroxide precipitation.