Heavy metal and Pb isotopic compositions of soil and maize from a major agricultural area in Northeast China: Contamination assessment and source apportionment

Abstract

Environmental contamination by heavy metals is a public health concern. However, understandings of sources and pathways of heavy metal accumulation in crops, especially in major agricultural areas, is insufficient. In this study, atmospheric dust, coal, vehicle exhaust particles, maize and corresponding soils were collected from a major agricultural area in Northeast China and analyzed for heavy metal (Cd, Hg, Pb, As, Cu, Zn, Ni, Cr) contents and Pb isotopic compositions. Compared to the risk screening values for soil contamination of agricultural land in China, the soils in the study area have relatively low heavy metal concentrations. However, the contamination factor (CF), pollution load index (PLI) and geo-accumulation index (Igeo) indicated a slight degree of heavy metal (particularly Cd) accumulation during the past few decades. The concentrations of Cd, Cu, Hg and Pb decreased from leaves, roots, stalks to grains, whereas it decreased from roots, grains, leaves to stalks for Zn and from roots, leaves, stalks to grains for As, Cr and Ni. The calculated bioconcentration factor (BCF) values indicated that Cd had a high possibility of entering maize roots from soil, while it was relatively difficult for Cr and Pb to accumulate in maize roots. The order of the mean translocation factor (TF) values was Zn (1.41) > Cu (0.31) > Hg (0.27) > Cd (0.15) > Pb (0.11) > Cr (0.11) > As (0.11) > Ni (0.08), suggesting that Zn had the highest mobility from root to grain, while >90% of absorbed Ni was sequestered in roots. Based on a simple binary model, the approximate contributions of Pb in vehicle emissions and coal to atmospheric dust were 78.1% and 21.9%, respectively, suggesting that Pb in atmospheric dust was mainly from vehicle emissions. For maize organs, airborne Pb was the dominant source contributor to maize stalks and grains, with approximate contributions of 78.3% and 15.1% to leaves and roots, respectively. Our study suggested that Pb accumulation in maize plants might be mainly derived from atmospheric deposition related to vehicle emissions. Hence, more efficient managerial strategies should be adopted to protect food safety and human health.