Effect of phosphate pretreatments on properties of pyrolytic products from heavy-metal-contaminated biomass

Vegetation has successfully been used for remediation of heavy-metal-contaminated soils (phytoremediation). The heavy-metal-rich biomass after phytoremediation can be processed by pyrolysis, achieving bioenergy production and pollutant control. The pretreatment of the heavy-metal-rich biomass prior to pyrolysis, as an emerging factor, needs to be investigated. For this purpose, heavy-metal-rich biomass obtained from phytoremediation was engineered by impregnating with four phosphates (KH₂PO₄, K₂HPO₄, NaH₂PO₄, and Na₂HPO₄) prior to pyrolysis in the temperature range of 300–700 °C. The untreated (control) and water-wash-treated biomass were employed as references. Results showed that impregnated K₂HPO₄ and Na₂HPO₄ catalyzed the pyrolysis process, reducing the temperature for the highest mass loss rate from 343 °C in the control to ∼306 °C. The heavy metals enriched in the control also catalyzed the pyrolysis process. The bio-oils produced from the phosphate pretreatments contained a higher proportion of phenolic compounds while lower ketones compared to the control. Notably, upon the phosphate pretreatment, the heavy metal recoveries in the biochars at 500 °C were significantly increased, with more than 80% of As and more than 90% of Pb and Zn sequestrated in the solid products. This study showed that the application of phosphates as the pretreatment method catalyzed the pyrolysis of the heavy-metal-rich biomass, enhanced the heavy metal recovery in biochars, and thus provided a safe and value-added way to the phytoremediation–pyrolysis scheme.