Biochars from chlorine-rich feedstock are low in polychlorinated dioxins, furans and biphenyls

Chlorinated aromatic hydrocarbons likepolychlorinated dibenzo-p-dioxins and-furans (PCDD/F) andpolychlorinated biphenyls (PCB) areomnipresent intheenvironment duetohistoric production, use,and(unintended) release. Nowadays, theiremission andmaximum concentration inenvironmental compartments is strictly regulated. During biochar production, PCDD/F andPCBmaybeformed andretained onthesolidpyrolysis product. Industrial biochars certified, e.g.,under theEuropean Biochar Certificate (EBC), exhibit concentrations thatwerealways wellbelow threshold values forsoilapplication andevenanimal feed.However, thishasnotbeensufficiently testedforchlorine (Cl)richorganic material suchasmarine biomass orpolyvinyl chloride (PVC) contaminated feedstock. Here,weanalyzed PCDD/F andPCBcontamination inbiochars producedatdifferent temperatures fromdifferent biomasses withcomparatively highClcontents intherangefrom 0.2%to3.8%(w/w, seagrass, twotypesofsaltwater macroalgae, tobacco stalks, andPVCcontaminated wood). Allofthebiochars produced showed PCDD/F andPCBcontents wellbelowtheapplicable threshold values given bytheEBC( < 20ngTEQkg 1 forPCDD/F and < 2×105 ngkg 1 forPCB).TheEBCthresholds wereundershot by aminimum offactor1.5forPCDD/F (mostly factor20)andbyaminimum offactor90forPCB.Between 1and 27ppboffeedstock Clweretransformed toClbound inPCDD/F andPCBinthebiochars. Noconsistent correlation between biomass Clcontents andcontents ofPCDD/F andPCBwerefoundbuthigher Clcontents inthe feedstock ledtoamorediverse PCDD/F congener pattern inthebiochars. Pyrolysis ofPVC-amended wood resulted inconsistently higher contamination ofPCDD/F andPCBinthebiochars compared topyrolysis ofthe otherbiomasses, potentially duetodifferences inClspeciation inthefeedstocks i.e.,ClinPVCisalready covalently bound toanorganic carbon backbone. Ahighcontamination inPCDD/F andPCBinbiochar was intentionally triggered byseparation ofpyrogas andbiochar inthereactor at < 300 ◦Ctopromote condensation ofcontaminants onthesolidproduct. Between 20%and80%offeedstock Clwasreleased viathepyrogas, i.e., neutralization ofHClinburntpyrogas mightbenecessary whenpyrolyzing Cl-rich feedstock inindustrial biochar production. Ourresults indicate thatbiochars produced fromCl-rich feedstocks withproper biochar production process control areconform withEuropean certification guidelines forPCDD/F andPCBcontamination. The results opentheopportunity toexploit andvalorize sofarnon-used marine orotherwise Clenriched biomasses fortheproduction ofbiochar andcarbon sinks.