Removal of trivalent chromium by electrocoagulation

The present work deals with removal of trivalent chromium (Cr3+) from aqueous solutions in a batch stirred electrocoagulator with mild steel electrode pair. Effects of operating time, stirrer rpm, current density, initial pH, initial concentration of Cr3+ and supportive electrolyte (NaCl) concentration have been investigated. Removal of Cr3+ during electrocoagulation (EC) is due to combined effect of chemical precipitation, co-precipitation, sweep coagulation and adsorption. At higher current density and solution pH, remarkable removal of Cr3+ was observed. Single stage of electrocoagulation could reduce Cr3+ concentration from 1000 mg/l to the discharge limit of 2 mg/l. Three different regimes of Cr3+ removal rate (g/m3 min) viz. (i) sluggish, (ii) high and (iii) decline rate regimes are observed with progress of EC. Current efficiency of about 100% with respect to Fe dissolution from electrodes was observed. NaCl not only increases the conductivity of the solution but also has impact on electrode passivation and removal of Cr3+. High resolution X-ray diffraction study of sludge revealed its amorphous nature. Electrocoagulated sludge is highly rich in Cr3+ and the maximum value of the mass ratio, Cr3+/Fe in sludge was found to vary from 3.5 to 4.0. Around 4% additional removal was noticed after 60 min of EC with same duration of post-EC contact. This is attributed to chemical precipitation and co-precipitation effects.