Heavy metals are of interest due to their deleterious impacts on both human and ecosystem health. This study investigated the effectiveness of wood ash in immobilizing the heavy metals Pb, Cd, Cu and Zn from aqueous solutions. The effects of initial metal concentrations, solution pH, ash dose and reaction time on metal sorption, as well as the metal sorption mechanisms were studied. To investigate the effect of initial metal concentrations, solutions containing Cd, Zn (25, 50, 75, 100 or 125 mg L −1 ), Cu (25, 50, 75, 100, 125, 150 or 175 mg L −1 ) or Pb (250, 500, 750, 1000, 1250, or 1500 mg L −1 ) were reacted with 10 g L −1 ash for two hours. For the effect of pH, solutions containing 100 mg L −1 of Cd, Cu or Zn or 1500 mg L −1 of Pb were reacted with 15 g L −1 ash over a pH range of 4 to 7. The wood ash was effective in immobilizing the four metals with a sorption range of 41-100 %. The amounts of metals retained by the ash followed the order of Pb > Cu > Cd > Zn. As expected, absolute metal retention increased with increasing initial metal concentrations, solution pH and ash dose. Metal retention by the ash exhibited a two-phase step: an initial rapid uptake of the metal followed by a period of relatively slow removal of metal from solution. Metal retention by the ash could be described by the Langmuir and Freundlich isotherms, with the latter providing a better fit for the data. Dissolution of calcite /gypsum minerals and precipitation of metal carbonate/sulfate like minerals were probably responsible for metal immobilization by the ash in addition to adsorption.
