Extracting heavy metals from electroplating sludge by acid and bioelectrical leaching using Acidithiobacillus ferrooxidans

Electroplating sludge (EPS) is a potential secondary resource; the heavy metals it contains show microbial toxicity in the bioleaching process and can be recycled as well. The aim of this work was to utilize optimized acid leaching to reduce the toxicity of heavy metals in EPS. Bioelectrical reactors (BERs), which can directly supply electron donors and acceptors for Acidithiobacillus ferrooxidans (A. ferrooxidans), were combined with acid leaching to achieve the progressive attack by bacteria on heavy metals in EPS. The results showed solution pH in acid leaching and voltage in BERs bioleaching played important roles in heavy metals extraction. Under the optimized acid leaching condition of processing pH 2.2, 75.8% of the copper (Cu), 84.4% of the zinc (Zn), 80.9% of the chromium (Cr), and 65.8% of the nickel (Ni) were leached in 9 h. An additional extraction of 11.7% Cu, 4.6% Zn, 6.0% Cr, and 11.7% Ni from acid-leached EPS residuum was produced in 0.2 V BERs compared with shaken bioleaching in 60 h. Various characterization methods such as transmission electron micrographs and cyclic voltammograms confirmed the Fe²⁺ oxidation ability of A. ferrooxidans was improved in the 0.2 V BERs, and the response of rus operon was downregulated by 2.3–3.5 times as determined by quantitative real-time polymerase chain reaction. These findings have great significance in constructing a model of heavy metals extraction and Fe²⁺ oxidation by A. ferrooxidans in BERs. The results of the leaching process are also significant for understanding how to implement these processes on an industrial scale.