Assessment of vegetable safety and heavy metal contamination risk in decomposed rape straw and coal gangue soilless culture substrate

Four species of vegetables, Chinese cabbage (Brassica chinensis), lettuce (Lactuca sativa), amaranth (Amarantus mangostanus), and spinach (Spinacia oleracea), were soillessly cultivated in biotron pots. The culture media were mixtures of decomposed rape straw and coal gangue in five volume ratios, which were 8∶2 (T₁), 7∶3 (T₂), 6∶4 (T₃), 5∶5 (T₄) and 4∶6 (T₅), respectively. The state of heavy metal contamination and potential ecological risk in the cultivation systems of the four species of vegetables were assessed by using Nemerow Pollution Indices and Hakanson Indices, respectively. The results showed that three cultivated media (T₁, T₂ and T₃) containing less coal gangue exhibited low levels of heavy metal contamination and less potential ecological risk. According to the green vegetable standards, vegetables cultivated in the three soilless media were valuated as good quality or safe in relation to heavy metal contamination. Cd and Hg were the main heavy metal contaminant and potential ecological risk factors in the compound media. The other heavy metals (Cr, Cu, Pb, Zn, As) had relatively low pollution capability. Furthermore, bio-availability of heavy metals (except for Cd and As) were strongly negatively correlated with organic matter, though slightly influenced by pH of matrix.