Effective recycling of Co and Sr from Co/Sr-bearing wastewater via an integrated Fe coagulation and hematite precipitation approach.

Flocculant overdose has been considered an inefficient technique for precipitating heavy metals from wastewater at low levels due to the high yield of hazardous waste sludge that should be treated properly before it can be disposed of safely in landfills. This problem was effectively solved in this study via a novel method that recycles sludge separately into high-purity hematite and heavy metal-bearing products. The wastewater, which contained 10.3 mg/L of Co and 4.8 mg/L of Sr, was coagulated by adding ferric salt to generate Co/Sr-bearing sludge. The sludge was dissolved in HNO, followed by hydrothermal treatment with the addition of organic matter (e.g. methanol or isopropanol). Without the addition of organic matter, only 56.5% of total Fe was removed as irregular hematite particles, whilst Co/Sr remained unchanged in the acid. Over 99.5% of total Fe was eliminated as hematite nanoparticles with 97.7% FeO content, but more than 98% Co/Sr remained in the acid when methanol with a molar ratio (M/M) of 5 was added. Nearly 100% Co was precipitated by adjusting the pH of the acid to 8 to generate Co hydroxide with 83.9% purity. Meanwhile, the residual Sr was further precipitated by adding NaCO to generate SrCO with 96.8% purity. Isopropanol achieved total Fe removal similar to that of methanol. The optimal molar ratio (M/M) was 1, which corresponded to the removal of 98.7% total Fe. Methanol and isopropanol can react with NO in acid to reduce NO concentration and improve acid pH, promoting hydrolysis followed by the crystallisation of ferric Fe with hematite as the final product. This paper is the first report on an environment-friendly method for enriching Co/Sr without generating any waste.