LAUSR

The solid waste generated from tannery wastewater treatment is hazardous as it contains high level of chromium (Cr) which is carcinogenic. To reduce its environmental risk and health concern, recovery and recycling is attempted. The recovery of Cr was performed in three steps, viz., extraction of Cr from the tannery waste sludge, oxidation of Cr(III) to hexavalent Cr(VI), and finally reduction to Cr(III). Total chromium in the sludge was determined using EPA3050(B) method and found to be about 8514 to 9130 mg/kg. The first step, i.e. Cr extraction from tannery sludge, was carried out under three different conditions, viz., neutral, acidic, and oxidizing conditions. The acidic extraction using 4% H2SO4 (v/v) on raw sludge extracts 90% of total Cr in the sludge. Though extraction under oxidizing condition using piranha solution (H2SO4 + H2O2 in ratios 3:1) resulted in complete dissolution of sludge, due to its highly corrosivity, strong oxidizer, economy and experimental safety concerns thus acidic extraction was chosen for further experiment. The second step using advanced oxidation process (H2O2 + UV) at pH 10 achieved maximum oxidation (83%) to Cr(VI) at 3 h. Finally, the reduction of Cr(VI) to Cr(III) as chromium sulphate [Cr2(SO4)3] was performed using sodium sulphite at acidic pH 2 with overall recovery of about 83%. The study showed promising results for recovery of chromium, but the main problem associated with this process is the high dilution of chromium sulphate solution obtained.