Impact of heavy metal laden algal biomass on hydrothermal liquefaction and biorefinery approach

Disposal of metal contaminated biomass after bioremediation poses challenges due to non-availability of suitable techniques. In the present study, an attached algal biofilm reactor (ABR) of 3 L capacity was used for remediating six heavy metals (Zn, Cu, Cr, Ni, Pb and Cd) from a metal mix and subsequently the biomass after bioremediation was hydrothermally liquefied to see the fate of the heavy metals. The algal biofilm was a consortium of Phormidium and Chlorella which was able to remove between 50-90 % of the heavy metals after 6 d. The metal removal trend followed the order Zn > Cu > Cr > Ni > Pb > Cd. Hydrothermal liquefaction of the metal contaminated biomass was done at 230 C of temperature, 27 bar of pressure, water to biomass ratio of 4, K2CO3 as catalyst and holding time of 20 min under N-2 environment in a high temperature pressure reactor. The biocrude obtained in presence of metals had a HHV of 20 MJ kg(-1) compared with 19.32 MJ kg(-1) in control (without metal). The heavy metal analysis of the solid and aqueous fraction showed that >70 % of the metals had partitioned into the solid fraction whereas <1 % were in the aqueous fraction. The aqueous fraction was also rich in nitrate and phosphate which could be reused for algal cultivation. Hence, hydrothermal liquefaction could be a very useful technique for valorisation of metal contaminated biomass for a sustainable biorefinery. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.