Chemical speciation and bioavailability of five heavy metals in soil of Beijing plain area in two years

To effectively monitor the overall variation trend of the chemical forms for 5 heavy metals (As, Cd, Hg, Pb and Zn) in the soil in Beijing plain area during the year 2015 and 2016, the authors employed the Tessier sequential extraction method to ana-lyze the morphological characteristics of these soil heavy minerals. The authors studied the factors controlling the bio-availability of these heavy minerals. The results show that there is only minor change in the morphology of soil heavy metals in these 2 years. The element with highest effective concentration is Cd (45.67%), which thus poses greatest potential ecological risk. The following ele-ment is Zn with concentration of 12.16%. Though Zn is largely incorporated in carbonates, the mobility of Zn is reduced by the al-kaline soil, thus having less ecologically hazardous effect. The elements As, Hg and Pb are immobile with little potential hazards on ecology. The orders of activity coefficients and migration coefficients of the heavy metal elements in topsoil in the study area are Cd > Zn > Pb > As > Hg and Cd > As > Zn > Pb=Hg respectively. This suggests that Cd has the highest values in both the 2-year biological activity coefficient and transfer coefficient, implying a strong biological activity and migration capability, while other soil heavy metals have smaller activity coefficients and migration coefficients, indicating relatively low potential ecological risks. In the study area, the factors that affect the bio-availability of heavy metals are complex, and the dominant one is the total amount of heavy metal elements, followed by pH, organic matter, CEC, and other physical and chemical properties, and the connections of these factors are complicated.