Characterization of heavy metal changes during rice husk charcoal-enhanced vermicomposting of municipal sludge
The effects of rice husk charcoal-enhanced vermicomposting on sludge properties and heavy metals were investigated.Here,we took municipal sludge,added different proportions of rice husk charcoal to enhance vermicomposting and investigated the effects of different biochar dosages(2%,4%,6%,and 8%based on dry mass of sludge)on the bioavailability and speciation of heavy metals in the process of municipal sludge vermicomposting.During the composting process,pH first increased and then decreased,conductivity decreased and then increased,and organic matter decreased.After the addition of different ratios of rice husk charcoal reinforcement,the conductivity and organic matter decreased by 28.96%-47.46%and 2.84%-18.87%,respectively compared with the control group.The rice husk charcoal reinforcement caused a nutrient increase in the composted sludge,and the total nitrogen,total phosphorus,and total potassium increased by 9.89%,24.39%,and 2.51%at 8%addition,respectively.The addition of rice husk charcoal caused the compost heavy metals to continuously transform from easy to migrate to challenging to migrate.The proportion of Cd,Cu,and Ni residue states increased by 3.57,19.12,and 9.27 percentage points,respectively,and the bioavailability decreased by 56.25%,34.22%,and 28.20%,respectively,at 8%addition.In comparison,adding 4%rice husk charcoal could transform Pb from an exchangeable and carbonate-bound state to a more stable organic-bound state,and the bioavailability was reduced by 27.69%.Correlation analysis showed that rice husk charcoal-enriched vermicomposting reduced the bioavailability of most sludge heavy metals by increasing the compost pH.Therefore,rice husk charcoal-enriched vermicomposting can significantly enhance urban sludge nutrients,improve physicochemical properties,and increase passivation efficiency of sludge heavy metals.
earthwormrice husk charcoalmunicipal sludge compostingheavy metal speciation
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