Abstract

Iron-based sorbents (IBS) are a promising tool for the removal of toxic metalloids, in particular, selenium (Se), from mining waste water. However, a barrier to the application of IBS is the absence of a sustainable and cost-effective substrate for their production. We demonstrate that IBS can be produced from the waste biomass that remains after the commercial extraction of agar from farmed seaweed (Gracilaria; Rhodophyta). The biosorbent is most effective when the waste Gracilaria biomass is treated with a ferric solution, then converted to biochar through slow pyrolysis. The resulting IBS is capable of binding both selenite (Se-IV) and selenate (Se-VI) from waste water. The rate of selenate (Se-VI) biosorption, the predominant and most intractable form of Se in industrial waste water, is minimally affected by temperature. Similarly, the capacity of the biosorbent for Se (q (max)) is unaffected by pH. The q (max) values for the optimised biosorbent range from 2.60 to 2.72 mg Se-VI g(-1) biochar between pH 2.5 and 8.0. Gracilaria waste is a sustainable substrate for IBS production and can be used to treat a costly waste problem. The use of Gracilaria waste as a substrate for waste water treatment could simultaneously improve the sustainability and profitability of seaweed farming by valorizing a low-value waste stream.