Sewage sludge carbonization for biochar applications: the fate of heavy metals

Biochars produced from sewage sludge show promise for use as soil amendments that could both enhance plant growth and sequester carbon. In a previous study, we showed that moist sewage sludge obtained from a rural, residential region of Oahu, Hawaii, could be carbonized, and the heavy-metal content of its biochar did not exceed United States EPA regulations limiting its use as a soil amendment. In this paper, we show that biochars produced from the nearby residential community of Hawaii Kai cannot be used as a soil amendment in Hawaii because their contents of Zn, Mo, and Cr exceed state regulations. Likewise, their contents of Cd, Cu, Ni, and Zn exceed Belgian regulations. These heavy metals are retained in the biochar during carbonization, whereas Hg and (to a lesser extent) As, Cd, and Se are released and thereby depleted in the biochar. Biochar within the carbonizer does not adsorb the heavy metals which are released during pyrolysis; consequently, these leave the carbonizer in its exhaust stream