Effect of Bamboo Charcoal with Earthworms on the Form and Bioavailability of Heavy Metals in Steel Sludge

It is difficult to utilize steel mill sludge because of the high concentration of heavy metals included therein. To passivate the activity of heavy metals, reduce their toxicity, and create a new valuable product, in this study, 4 kg sludge was composted with a combination of earthworms and different ratios (2%, 4%, and 8%) of bamboo charcoal. The effects of bamboo charcoal and the combination of bamboo charcoal and earthworms on the changes of heavy metals in industrial sludge were examined. The results showed that under the 8% bamboo charcoal ratio, addition of bamboo charcoal alone increased the concentrations of heavy metals, while the addition of bamboo charcoal combined with earthworms reduced it in sludge. The application of bamboo charcoal transformed the available state to iron-manganese bound and residue fraction in the sludge, thus reducing the bioavailability of heavy metals. Addition of bamboo charcoal and earthworms promoted the transformation percentage of heavy metals in exchangeable, carbonate, organic matter, and iron-manganese fractions to the residue fraction in sludge, which further decreased the bioavailability of heavy metals. Vermicomposting with 8% bamboo charcoal achieved maximal heavy metal passivation. Correlation and redundancy analyses showed that composting with a combination of bamboo charcoal and earthworms mainly altered the total phosphorus content, which affected the bioavailability of heavy metals. Addition of earthworms and bamboo charcoal to sludge significantly reduced the total contents of heavy metals and their bioavailability, promoted the conversion of heavy metals to the residue fraction, and improved sludge availability.