Remediation of a grey forest soil contaminated with heavy metals by means of leaching at acidic pH

PURPOSE: The purpose was to study the heavy-metal leaching from topsoil when acidolysis or combination of acidolysis/complexolysis was applied as an approach for in situ soil remediation. The question was how the acidolysis of ferric iron hydroxides from topsoil would effect on the heavy-metal leaching when soil pH was lowered with sulfuric acid in or without the presence of cut straw into the soil. MATERIALS AND METHODS: Four zero-suction lysimeters were used in this study. The heavy-metal acidolysis was studied by the soil irrigation with diluted solutions of sulfuric acid or its in situ generation as a result of the bacterial oxidation of S° by Acidithiobacilus thiooxidans. The sulfur requirement was calculated having in mind the higher amount of acid soluble iron in the topsoil due to its contamination by AMD. The combination of acidolysis/complexolysis was studied by cut straw addition to S° at ratio 1:1 and 3:1, respectively. Regular sampling and analyses of the pregnant soil solutions was applied. The pollutant content, their distribution among the geochemical fractions, and the content of amorphous ferric iron hydroxides were determined before and after soil bioremediation. RESULTS AND DISCUSSION: The reducible mobile fraction was the main fraction in which almost 50 % of each heavy metal was entrapped as a result of the soil pollution by AMD. The combination of acidolysis/complexolysis, realized by elemental sulfur and cut straw addition to horizon A of the AMD-affected soil at ratio of 1:1, allowed the concentration of lead, uranium, and arsenic after 9 months of remediation to be decreased near the relevant maximum admissible concentration (MAC), as their content in the exchangeable and specific adsorbed mobile fraction were drastically reduced. It determined the lower bioavailability of the pollutants at the end of the leaching stage. CONCLUSIONS: A mixture of elemental sulfur and cut straw at a ratio of 1:1 was an efficient method for in situ bioremediation of AMD-affected soil. The heavy-metal leaching from easily leachable fractions was a result of the joint action of acidolysis/complexolysis, and as a result, the contaminant content in horizon A was significantly reduced.