Chromium recovery of Tanning wastewater by precipitation with saline groundwater and acidification

Abstract

This research focused on the recovery of chromium from the tanning wastewater by treating the wastewater with the hydroxde and carbonate of metal prepared from saline groundwater. Magnsium, calcium and sodium are the main components of saline groundwater that can be precipitated as metal hydroxide and carbonate when react with alkaline reagent. Laboratory experiments were carried out to determine the optimum pH to produce the white slurry of magnesium hydroxide and calcium carbonate, the optimum dosage of the white slurry for removal of chromium, and recovery of chromium from the chromium precipitate. These experiments were performed using jar test apparatus. Saline groundwater used in this study contained calcium of 3,900-7,500 mg/l, magnesium of 2,100-2500 mg/l, pH of 6.8-7.4. When the pH of this saline groundwater was raised to 13.2 to 13.5 with sodium hydroxide, the white slurry of calcium carbonate and magnesium hydroxide concentration of 28,000-87,000 mg Ca/l and 13,000-38,500 mg mg/l was obtained. Under the condition of adding 16-35 ml of such white slurry to a one liter of tanning wastewater of which the pH was raised from 3.7-4.2 to 8.8-9.5, chromium concentration of 236-498 mg/l was reduced to 2.6 mg/l indicating 99% of chromium removal efficiency. Acidification of the chromium hydroxide precipitate with the concentration of sulfuric acid at the pH range of 1.6-2.8, recovery of chromium of about 63% at the optimum pH 2 was the result. For the model scale test with the reactor of about 6 liters with the optimum dosage of the white slurry at the pH of about 9, similar result to the jar test experiment for the chromium removal efficiency of about 96%. The findings indicated that using unwanted saline groundwater that is available on site is one effective treatment technology to treat tanning wastewater.