Treatment of mine water for the fast removal of zinc and lead by wood ash amended biochar

Lead and zinc mines are a primary source of environmental (post)-transition metal contamination resulting in major water pollution. In this study, the use of biochar amended with wood ash (WAS) was evaluated as a method to remediate zinc and lead contaminated mine water. Water from Nantymwyn lead mine, with zinc and lead concentrations as high as 12.1 mg L−1 and 1.7 mg L−1 respectively was used. The contact time for WAS to immobilise zinc and lead (1 min to 24 h), immobilisation mechanisms and maximum measured removal of lead and zinc were studied. FTIR spectroscopy and XPS was used to characterise WAS and the aqueous modelling program PHREEQC (pH redox equilibrium) was used to analyse mine water speciation. The fast removal performance of a biochar is a key indicator of its viability to be used as a green remediator. If the required contact time to remediate contaminated water is too long the sorbent becomes impractical. This study demonstrated that WAS removed 97% of zinc and 86% of lead within the first minute of contact with the mine water (0.5 g of WAS per 20 mL of mine water), with a maximum measured removal of 14.8 mg g−1 for zinc and 23.7 mg g−1 for lead (using 0.1–0.002 g of WAS per 40 mL of mine water). Fast removal was primarily a result of precipitation, and subsequent capture by WAS, and ion exchange. These findings show that WAS has the potential to be scaled up and deployed at mine sites to remediate contaminated water.