Wood ash amended biochar for the removal of lead, copper, zinc and cadmium from aqueous solution

Abstract

Heavy metals in motorway adjoined aqueous environments have increased at an alarming rate over recent years. This increase has been primarily attributed to anthropogenic activities such as the increase of motor vehicle use. Current remediation techniques, such as balancing ponds have the potential to leave toxic residue with the associated removal costs often proving prohibitive. In this study biochar and wood ash amended biochar were evaluated as remediators of inorganic vehicular pollutants found in motorway runoff, specifically Pb, Cu, Zn and Cd. Biochar from European larch ( Larix decidua (L.) Karst.) was produced via fast pyrolysis-gasification (485–530 °C for 90 s) and amended with wood ash post pyrolysis. Pristine larch biochar (BC), larch biochar cold mixed with wood ash (WA) and larch biochar sintered with wood ash (WAS) were studied to evaluate metal immobilisation mechanisms and maximum removal capacities. This study demonstrates that the amendment of biochar with wood ash increases Pb, Cu, Zn, and Cd immobilisation by an order of magnitude compared to BC. The addition of wood ash increases pH whilst adding minerals causing precipitation. Precipitation and ion exchange dominate metal immobilisation and were not correlated to surface area. Sustainability of feedstock, low feedstock/production costs and maximum measured contaminant removal (61.5 mg/g, 38.9 mg/g, 12.1 mg/g and 10.2 mg/g for Pb, Cu, Zn and Cd respectively) indicate that wood ash amended biochar is a viable option to immobilise Pb, Cu, Zn and Cd from motorway runoff. • The addition of wood ash to biochar increased metal removal by an order of magnitude. • Immobilisation is correlated to pH. • Increased pH and the presence of minerals drive precipitation. • Precipitation and ion exchange dominate immobilisation. • Wood ash amended biochar could remove heavy metals from motorway runoff.