Effect of pyrolysis temperature on characteristics of heavy metals in different macroalgal biochars

Macroalgal biochars were produced from Sargassum fusiforme, Sargassum thunbergii, Sargassum vachellianum, Grateloupia turuturu, Chondria crassicaulis and Ulva pertusa at different pyrolysis temperatures (200, 300, 400, 500 and 600 ℃) under limited-oxygen conditions. The effects of pyrolysis temperature on the yield, heavy metal (Fe, Mn, Zn, Cu, As, Cr, Cd, Pb and Hg) contents and their relative enrichment factors of different macroalgal biochars were investigated. The results indicated that the yield of macroalgal biochar decreased with the increase of pyrolysis temperature. The contents of Fe, Zn, Mn, Cu, Cr and Pb in macroalgal biochars were significantly higher than those in their respective biomass. The As content in macroalgal biochars decreased first and subsequently increased with the increase of pyrolysis temperature, and the As content was not higher than that of raw biomass. The change trend of Cd content with the increase of pyrolysis temperature was related to macroalgae biomass. Hg wasn’t detected in the biochars. The contents of Zn, Cu, Cr, Pb and Hg in macroalgal biochars were all better than the basic quality grade stipulated by the European Biochar Foundation, but the contents of Cd and As in the partial biochars exceeded the threshold. In general, pyrolysis may promote the volatile migration of heavy metals in the macroalgal biochar. Furthermore, the correlation analysis showed that the heavy metal contents in the macroalgal biochars were critically dependent on the biomass and pyrolysis temperature. Cluster analysis demonstrated that the prepared biochars from the special macroalgae biomass could be approximately classified into the same cluster, which effectively reflected the differences in the characteristics of heavy metals in different macroalgal biochars. The above results can provide valuable information for guiding the production and application of macroalgal biochar, which improving the utilization of macroalgae.