Biochar for the removal of metals from solution: A review of lignocellulosic and novel marine feedstocks

Biochar, derived from the pyrolysis of biomass, has received great attention in literature for many applications. Owing to its high specific surface area and surface chemistry, the use of biochar as an adsorbent for heavy metals from solution has become a promising environmental application of this material. While research has primarily focused on generating biochar from lignocellulosic feedstocks, emerging feedstocks, such as marine-shell derived biochars have also shown promise as metal adsorbents with increased adsorptive capacity. In this paper, the mechanisms of metal adsorption from water are first analyzed from the perspective of the more commonly researched lignocellulosic biochar. Here, specific surface area, presence of organic functional groups, mineral content, and surface pH and charge are found to be controlling mechanisms for adsorption of metals, with the role of feedstock playing a primary role in determining these properties, along with pyrolysis conditions. This discussion leads into an analysis of the properties of marine-shell based biochar as well as its adsorption capacity for a range of metals as found in recent literature. It is found that for many divalent cations, such as lead (Pb2+), marine shell biochars have adsorption capacity that far exceeds that of traditional lignocellulosic biochar owing to the high cation exchange capacity and moderate surface area of this material. As a result, pyrolysis of this material may present a simple method of valorizing a common waste product from the fisheries and transforming it into a potent adsorbent for wastewater treatment.