Investigation into the Sources of Biochar Water-Soluble Organic Compounds and Their Potential Toxicity on Aquatic Microorganisms

During biochar production, a number of pyrolysis intermediates recondense on the surface of biochar where potentially they may play an interesting role in soil and freshwater environments. This study investigated the sources of biochar water-soluble organic compounds (WSOC) as a function of biomass materials induding cellulose and lignin and also as a function of pyrolysis temperature from 300 to 500 degrees C in relation to their potential toxicity to freshwater blue-green algae, Synechococcus. The amount of WSOC extracted from biochar, irrespective of biomass starting material, decreased significantly as a function of pyrolysis temperature. For biochars produced at pyrolysis temperatures below 400 degrees C, algal bioassays and advanced mass spectrometry analyses revealed that toxic WSOC extracted from pinewood-derived biochar was most likely due to mono-, di-, and trisubstituted phenolic compounds derived from lignin, while toxic WSOC extracted from cellulose-derived biochar was acidic and bio-oil like in nature. For pinewood biochar produced at temperatures above 400 degrees C, WSOC contained small to nondetectable (500 degrees C) amounts of phenolic compounds and decreased from 664.5 +/- 6.2 mg DOC/kg biochar (300 degrees C) to 75.39 +/- 0.42 mg DOC/kg biochar (500 degrees C) with no observable toxicity on blue-green algae growth. Additionally, this study investigated toxicity caused by an imperfect reactor condition, such as the presence of cold spots and insufficient evacuation of pyrolysis intermediates, leading to a significant amount of toxic WSOC.