Effects of acidity on acute toxicity of aluminium-waste and aluminium-contaminated soil

The total heavy metal concentrations of Al-waste and Al-contaminated soil were many times higher than that found in the control soil, which might pose toxic effects on nearby ecosystems under acidic condition. The present study aimed to detect the amount of Al, Cu, Zn, Mn, Pb, Ni and Cd extracted by distilled water and ammonium acetate at pH 3.8, 4.8, 5.8, 6.8 and 7.8. The acute toxicities of water extracts were assessed by two bioassays. Results showed that concentrations of heavy metals, especially Al, were the highest in extracts from Al-contaminated soil extracted with NH₄OAc, followed by Al-waste. The control displayed relatively low levels of metals. More heavy metals were extracted at acidic pH than at neutral pH. Distilled water extracts exhibited lower levels of metals than those extracted with NH₄OAc. The first bioassay, in terms of seed germination and root elongation of B. parachinensis, indicated that the germination rates were seriously retarded by Al-waste even at neutral pH. Less than 25% seeds were germinated in Al-waste and the toxic effect was more obvious at pH 3.8. Root growth in Al-waste and Al-soil was slower than in the control and no seedling in Al-waste had roots longer than 2 cm at the end of this study. In the second bioassay, the photosynthetic rate of Chlorella pyrenoidosa was significantly inhibited by Al-waste when compared with the control, although the pH effect was not clear. This study revealed that the metal availability was pH dependent and their toxicity could be rapidly assessed by two simple bioassays.