Citric Acid Extraction Impact on Chemical and Bioavailable Forms of Metals in Soil

The presence of heavy metals in soils poses a serious threat due to these harmful elements being transported into the food chain. The aim of the presented research was to evaluate the effect of the extraction of selected heavy metals from soil with a 1 M aqueous solution of citric acid (CA) on the chemical (including mobile) forms of these elements and their bioavailability. A soil sample taken in an industrial area was extracted. Then the total content of selected heavy metals (Pb, Zn, Cu, Cd, Ni), their chemical forms (determined by sequential extraction according to the BCR procedure) and bioavailable forms (determined by one-step extraction with a 1 M HCl) were determined in both types of soil, before and after extraction. The tested soil contained significant amounts of the tested metals, and the amounts can be compared as follows: Pb ≥ Zn > Cu ≥ Cd > Ni. The greatest threat to the environment is associated with the presence of cadmium and lead in the tested soil, the content of which exceeds the limits set for soils in industrial areas. In addition, the level of presence of heavy metals in bioavailable and mobile chemical forms, was considered significant. No clear correlation was observed between the content of the analyzed metals in mobile and bioavailable forms. The tested soil contained significant concentrations of Pb (2141 mg/kg), Zn (2030 mg/kg), Cu (68 mg/kg), Cd (63 mg/kg), and Ni (23 mg/kg), which were reduced to 857, 589, 42, 28, and 14 mg/kg, respectively, after extraction with 1 M CA. The extraction process with a CA solution reduced the content of all metals, and the efficiency of the process can be compared as follows: Zn > Pb > Cd > Ni ≥ Cu, with efficiencies of 71%, 60%, 55%, 41% and 39%, respectively. The extraction process reduced the metal content of all the bioavailable and chemical fractions. The shares of metals in the mobile fractions decreased in favor of the immobilized fractions and ones more stable in the environment. After the process of leaching metals from the soil, a clear tendency towards equalization of the heavy metal content in the mobile and bioavailable fractions was observed.