Making light work of heavy metal contamination: The potential for coupling bioremediation with bioenergy production

Intense anthropogenic activity continues to expose the natural environment to heavy metal contamination. Whilst a number of physical and chemical solutions for remediation exist, the use of higher plants and algae for clean‐up of contaminated landscapes, termed “phytoremediation” and “phycoremediation”, respectively, offer an attractive and sustainable alternative. However, these remediation processes will always lead to a high‐moisture, heavy metal‐contaminated biomass, which must be further processed to partition, or render inert, the metal contaminants. Conversion of this metal‐rich biomass into second‐generation biofuels offers a useful route to subsidise the economics of remediation activities. Here we briefly review the various methods for bioremediation of heavy metals, and discuss the potential to produce bioenergy from these biomass sources. Ultimately, coupling the bioremediation activity to bioenergy production gives far‐reaching social and economic benefits; however, established processes such as direct combustion and anaerobic digestion risk releasing heavy metals back into the environment. Alternatively, thermochemical conversions such as pyrolysis or hydrothermal liquefaction (HTL) offer significant advantages in terms of the segregation of metals into a relatively inert and compact solid phase while producing a biocrude oil for bioenergy production. In addition, preliminary work suggests that the HTL process can also be used to partition essential macronutrients, such as N, P and K, into an aqueous medium, allowing additional nutrient recycling.