Abstract
Biochars were prepared via the pyrolysis of sewage sludge at temperatures ranging from 350–550 °C. The properties and behaviors of heavy metals in the biochars were investigated. The results indicated that the pH values and ash contents of the biochars increased, while biochar yield and C, H, and N contents decreased with the increasing temperature. A high pyrolysis temperature contributed to a developed biochar pore structure. The specific surface area and pore volume of the biochars increased, while the average pore width decreased, with the increasing temperature. Heavy metals in the biochars were further enriched with the increasing temperature. TCLP tests demonstrated that the leaching potential of heavy metals from the biochars significantly decreased with the increasing temperature, indicating the decrease of potential ecological risks of heavy metals to the environment. Additionally, BCR tests confirmed the transformation of heavy metals from mobile fractions (F1 and F2) to stable fractions (F3 and F4). The evaluation results showed that a high pyrolysis temperature can effectively inhibit the ecological risks of heavy metals in the biochars. Thus, the conversion of sewage sludge into biochar via pyrolysis is a promising method for the safe disposal of sewage sludge.
Highlights
Sewage sludge is a by-product of municipal wastewater treatment plants
The biochar yield decreased from 73.33% to 61.28% with the increase of pyrolysis temperature from 350 to 550 °C
The contents of these elements gradually decreased with the increase of pyrolysis temperature, indicating the increased thermal decomposition of organic matter at higher temperatures
Summary
Sewage sludge is a by-product of municipal wastewater treatment plants. Many municipal sewage treatment plants have been constructed, and the amount of discharged sewage sludge is increasing dramatically. Sewage sludge contains a large amount of heavy metals and pathogenic microbes that can cause serious environmental pollution and even threaten human health once directly discharged into the environment [1]. The proper disposal of sewage sludge has attracted great interest, and the organic composition in sewage sludge is considered as an important biological resource. Pyrolysis may provide a feasible and attractive strategy for sludge recycling [2], since it can reduce sludge volume, kill pathogens and parasites [3], and convert the organic matter in sewage sludge into bio-oil, pyrolysis gas, and biochar [4]. It is a potential amendment to contaminated soil, and its application to
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