Hexavalent chromium removal from tunneling wastewater using chemical and electrochemical techniques

Hexavalent chromium (Cr6+) is of particular environmental concern due to its toxicity and mobility and removing it from industrial wastewater is a challenging task. The present investigation deals with the removal of Cr6+ (similar to 150 ppb) of tunneling wastewater, which has a very basic pH (11-12). For this purpose, batch experiments with wastewater samples were conducted to determine whether sub-ppb concentrations of dissolved Cr6+ could be achieved by chemical (using ferrous sulfate) or electrochemical reduction (iron electrodes). Cr6+ is chemically reduced to less soluble Cr3+ species by Fe2+. The influence of pH, temperature, suspended solids, concentration of ferrous sulfate, current density and reaction time were evaluated in the removal of Cr6+. Further, predictive equations were developed within the studied ranges. The results showed that ferrous sulfate is a good reducing agent of Cr6+ at very basic conditions and at short reaction times. Optimal conditions were at pH = 12 degrees C and 22 degrees C, with the presence of suspended solids and at molar ratios 15: 1 (Fe2+/Cr6+); 97.8% of Cr6+ was removed under these conditions. On the other hand, electrocoagulation was better at reducing Cr6+ at lower pH; 99.3% of Cr6+ was removed at pH = 8, 0.5 mA cm(-2), and 8 min of reaction time. At very basic conditions, the removal of Cr6+ could be achieved by means of higher current density values. Overall, the removal of Cr6+ with ferrous sulfate was more efficient at higher pH and at reaction times lower than 5 min, while electrocoagulation showed enhanced removal at neutral pH and at longer reaction times.