METALS SOLUBILITY IN BIOCHAR FROM DIFFERENT FEEDSTOCK AND PYROLYSIS PROCESSES

Biochar is a co-product of the pyrolysis process of biomass-to-energy conversion. About 15-40% of the feedstock is recovered as biochar in the process. Further use of biochar in soil is suggested as a mean to increase soil productivity, and to store and sequester much of the biochar-recalcitrant carbon. Yet, biochar application may result in elevated levels of soluble and bio-available metals, adversely affecting soil biota and downstream water quality and ecosystem. In this study we evaluated metals (Al, Ca, Cu, Cr, Fe, K, Mn, Mo, Mg, Na, Ni, and Zn) content and solubility in biochar from different pyrolysis processes and feedstock, including plant residue and chicken litter. Chicken litter was processed at 350 ºC or 700 ºC in a slow pyrolysis; additional chicken litter, alfalfa stem, bamboo, miscanthus, and sorghum feedstock were processed in fast pyrolysis at 450-500 ºC. Subsamples of all biochars were further steam-activated at 800 ºC. Total, water, AB-DTPA (ammonium bicarbonate diethylene triamine pentaacetic acid), Mehlich-3 (a soil nutrient extraction), and SPLP (synthetic precipitation leaching procedure; EPA Method 1312) extractable metals were determined. Mean metal enrichment factor by pyrolysis (i.e. metal concentration in biochar/metal concentration in feedstock) was 2.5±0.7. Metal enrichment by subsequent biochar activation (i.e. metal concentration in activated-biochar/metal concentration in biochar) was temperature dependent, increasing for metals in biochar produced at 350 ºC (ranging from 1.3 to 1.7) while having minor changes for the metals in biochar produced at 700 ºC (ranged from 0.92 – 1.03). Though metals concentration in water was feedstock dependent, their solubility decreases in the order alkali metals > alkaline earth metals > transition metals. While feedstock’s alkaline earth and transition metals solubility in SPLP and Mehlich-3 decreases upon pyrolysis and biochar activation, K solubility increases. Depending on feedstock pyrolysis processing and activation, biochar may contribute soluble and bio-available metals. Hence, understanding the impact of feedstock, pyrolysis process, and biochar activation on metal solubility are important factors in controlling and alleviating biochar input.