Influence of Pyrolysis Temperature on Biochar Property and Function as a Heavy Metal Sorbent in Soil

TLDR

Large‑scale biochar soil amendments are popular for yield improvement and contamination remediation, yet systematic studies linking biochar properties to functions such as heavy‑metal sequestration are lacking. The study aimed to determine how biochar properties affect heavy‑metal sorption by conducting a soil amendment experiment on acidic Norfolk soil with low metal retention. Cottonseed hulls were pyrolyzed at 200–800 °C and characterized for yield, moisture, ash, volatile matter, fixed carbon, elemental composition, BET surface area, pH, pHpzc, and ATR‑FTIR, and these results were compared with literature values for other biochars (grass, wood, pine needle, broiler litter) with and without post‑treatments. Cottonseed hull chars had intermediate ash content and lower BET surface area than other plant sources; the N:C ratio peaked at 300–400 °C and varied among biomass types, and the study found that volatile matter, oxygen content, and pHpzc of biochars governed heavy‑metal sequestration in Norfolk soil, indicating that biochar selection must be tailored to soil and target function.

Abstract

While a large-scale soil amendment of biochars continues to receive interest for enhancing crop yields and to remediate contaminated sites, systematic study is lacking in how biochar properties translate into purported functions such as heavy metal sequestration. In this study, cottonseed hulls were pyrolyzed at five temperatures (200, 350, 500, 650, and 800 °C) and characterized for the yield, moisture, ash, volatile matter, and fixed carbon contents, elemental composition (CHNSO), BET surface area, pH, pHpzc, and by ATR-FTIR. The characterization results were compared with the literature values for additional source materials: grass, wood, pine needle, and broiler litter-derived biochars with and without post-treatments. At respective pyrolysis temperatures, cottonseed hull chars had ash content in between grass and wood chars, and significantly lower BET surface area in comparison to other plant source materials considered. The N:C ratio reached a maximum between 300 and 400 °C for all biomass sources considered, while the following trend in N:C ratio was maintained at each pyrolysis temperature: wood ≪ cottonseed hull ≈ grass ≈ pine needle ≪ broiler litter. To examine how biochar properties translate into its function as a heavy metal (NiII, CuII, PbII, and CdII) sorbent, a soil amendment study was conducted for acidic sandy loam Norfolk soil previously shown to have low heavy metal retention capacity. The results suggest that the properties attributable to the surface functional groups of biochars (volatile matter and oxygen contents and pHpzc) control the heavy metal sequestration ability in Norfolk soil, and biochar selection for soil amendment must be made case-by-case based on the biochar characteristics, soil property, and the target function.