BACKGROUND Brewer's Spent Grain (BSG), a form of lignocellulosic biomass more commonly known as barley waste was used to synthesize activated carbon (AC) and carbon nanotubes (CNTs). The produced materials were used in water remediation application. RESULTS A novel approach involving two activation steps; first, with phosphoric acid (designated BAC-P) and then using potassium hydroxide (designated BAC-K) was proposed for the production of AC and CNTs from BSG. The AC produced showed a surface area as high as 692.3 m(2) g(-1) with a pore volume of 0.44 cm(3) g(-1). This can help aid and facilitate the circular economy by effectively upcycling and valorizing waste lignocellulosic biomass to high surface area AC and subsequently, multi-walled carbon nanotubes (MWCNTs). Consequently, MWCNTs were prepared from the produced AC by mixing it with the nitrogen (N)-based material melamine and iron precursor, iron (III) oxalate hexahydrate, where it produced hydrophilic MWCNTs. Both AC and CNT materials were used in heavy metal removal (HMR), where the maximum lead absorption was observed for sample BAC-K with 77% removal capacity after the first hour of testing. CONCLUSION This result signifies that the synthesis of these upcycled materials can have an application in the areas of wastewater treatment or other AC/CNT end uses with a rapid cycle time. (c) 2019 Society of Chemical Industry