Effects of Biochars and Compost Mixtures and Inorganic Additives on Immobilisation of Heavy Metals in Contaminated Soils

Abstract

Biochar (BC) as soil amendment has the potential to support the remediation of polluted soils. The aim of our study was to test metal extractability in a contaminated soil and metal accumulation in plants in response to the application of BC and compost-based and inorganic soil amendments. The soil used in our study was taken from a long-term polluted site in Austria (Arnoldstein). The treatments included three BC/compost mixtures (50/50 w/w; using three different BC types), two inorganic amendments (gravel sludge (GS) with siderite-bearing (Fe) material (GSFe, 50/50 w/w) and lime (L)) and an unamended control; each treatment consisted of 1.5 % in 1.2 kg soil. Poplar biochar (P-BC), GSFe and L significantly decreased the NH4NO3-extractable fraction of Pb, Zn and Cd, but not of Cu. The highest immobilisation effect within the organic amendments was found after P-BC, reducing NH4NO3-extractable Zn by 63 %. In contrast to the NH4NO3 extractions, heavy metal concentrations in the aboveground plant material (shoots of Lolium multiflorum) did not generally decrease. Still, Cd, Ni and Zn concentrations in L. multiflorum grown on the amended soil were lower after the P-BC treatment. Cu and Pb showed slightly higher plant concentrations upon amendment addition, indicating that NH4NO3 extractions did not entirely reflect plant availability. Our data show that the type of BC and the addition of compost are crucial when using sorbent additives as a remediation strategy for metal-polluted soils and that the desired immobilisation effects cannot be achieved for all heavy metals. Depending on the type of metal, specific biochars and compost mixtures can either cause immobilisation or mobilisation.