Concentrations of heavy metals in Eucalyptus and Pinus wood sawdust and smoke, Copperbelt province, Zambia

Abstract

The bulk of exotic forest plantations and sawmills in Zambia are within the reach of air pollutants from mines on the Copperbelt province. Up to 60% of every cubic meter of the timber processed is waste, of which 0,12 m3 is sawdust. Sawdust is largely used for various surface amendments and as a source of energy, but the dangers it poses to users are not known. Heavy metals assimilated by trees or adsorbed by sawdust from the environment may be a health hazard at certain levels. The amount of heavy metals in Eucalyptus and Pinus sawdust was evaluated on samples from Kitwe and Ndola to establish if the use of sawdust for energy and surface amendments was safe. Composite samples for each wood type were collected from each site and digested. The ensuing solution was filtered and analyzed by flame emission Atomic Absorption Spectroscopy
for heavy metals. Smoke from a burning test was trapped on Whateman 41 filter paper and the particulate matter that was trapped was extracted. The extract mixture was centrifuged to obtain a clear solution which was then analyzed for heavy metals by AAS. The heavy metal concentrations in Eucalyptus sawdust were 11,5-61,1 mg Pb/kg; 3,3-7,9 mg Cd/kg; 4,9-56,9 mg Cr/kg and 20,2-43,4 mg Ni/kg while that in Pinus sawdust were
17,1-32,8 mg Pb/kg; 5,1-8,6 mg Cd/kg; 9,9-28,2 mg Cr/kg and 18,7-67,4 mg Ni/kg. The concentrations of chromium and cadmium in both wood types from Kitwe exceeded the limit, and so the sawdust was deemed unsuitable for surface applications. This was the same for nickel in Pinus sawdust. The study showed that sawdust from both wood types was not safe for mulching, composting and animal bedding. The annual
exposure limits of 0,2 μg Cr/m3, 180 μg Ni/m3 and 0,2 μg Cd/m3 set by the World Health Organization were not exceeded by the smoke from both wood types. This suggested that heavy metals embedded on particulate
matter from Eucalyptus and Pinus sawdust which has been in storage for about two years in conditions found in Kitwe is unlikely to have adverse short-term health effects associated with heavy metals.