Investigation of pH evolution with Cr(VI) removal in electrocoagulation process: Proposing a real-time control strategy

• A theory of the relationships between initial pH and chromic alkalinity for final pH was built.
• Final pH was conditioned by initial pH and chromic alkalinity rather than current density.
• The final pH can be detected by the features of pH evolution when p[Cr(VI)] = pHi and p[Cr(VI)] < pHi.
• A real-time control strategy for fluctuant (or stable) Cr(VI) wastewater treatment was proposed.

The pH value is easily monitored and widely used in real-time control processes. It is also a key parameter in removing Cr(VI) during electrocoagulation. We developed a theory of relations between initial pH (pHi) and chromic alkalinity (p[Cr(VI)]), and the final pH of a solution. That was, when p[Cr(VI)] = pHi, the final pH was neutral; when p[Cr(VI)] < pHi, the final pH was alkaline; and when p[Cr(VI)] > pHi, the final pH was acidic. Response surface methodology confirmed that final pH was influenced by initial pH and initial Cr(VI) concentration rather than by current density. Subsequently, the relationship between pH evolution and Cr(VI) removal was investigated for the aforementioned final pH conditions. Our results suggested that the point of final pH can be detected by the features of pH evolution in case of p[Cr(VI)] = pHi and p[Cr(VI)] < pHi. Rapid Cr (VI) removal rate was achieved when p[Cr(VI)] > pHi. However, the major fraction of dissolved Cr (III) and the uncertain point of final pH were observed in this condition. Finally, we proposed a real-time control strategy for treating fluctuant (or stable) Cr(VI)-contaminated wastewater based on the features of pH evolution.