Phytostabilisation on post-flotation sediment waste: mobility of heavy metals and stimulation of biochemical processes by mineral-organic mixtures

Abstract

Abstract Purpose The study aimed to determine the effect of the addition of innovative combinations of organic-mineral mixtures obtained from biochar (BC), zeolite (Z), soil (S), poultry litter (PL), and slurry (SL) to post-flotation sediment (PFS) on (i) heavy metal mobility, (ii) heavy metal accumulation in willow, and (iii) PFS respiratory activity. Materials and methods The tests were carried out under laboratory conditions in containers with 500 g of PFS, to which 1% (w/w) of organic-mineral mixtures were added. Willow was grown for 90 days on substrates with the addition of organic and mineral mixtures. Results and discussion The addition of mixtures BC + Z + S + PL and BC + Z + S + SL to PFS significantly reduced the pH to, respectively, 7.12 and 7.02. This can be attributed to the release of the hydrogen load combined with organic anions deriving from the mineralisation of organic materials and the nitrification process. The addition of BC + Z and BC + Z + S mixtures to PFS reduced the content of Zn-H2O by 65%, Cd-H2O by 48%, and Ni-H2O by 30%. The addition of BC + Z + S + PL and BC + Z + S + PL mixtures to PFS increased the content of water-extracted Pb, respectively, 40 and over 60 times. The content of bioavailable heavy metals (extraction with 1 M NH4NO3) in PFS was comparable in all treatments to which mixtures were added. Altered mobility of heavy metal ions may be associated with a change of substrate properties, including redox potential, pH value, as well as the introduction into the soil of materials with significantly developed sorption surfaces. In the first 2 weeks of incubation of mixtures with PFS, respiratory activity was very low, except for that in BC + Z + S + PL and BC + Z + S + SL treatments. In these treatments, oxygen consumption was more than 50 times higher compared to the control treatment and more than 10 times higher in relation to BC + Z and BC + Z + S treatments. Conclusions The mixtures of BC + Z and BC + Z + S effectively reduced the content of water-extracted heavy metals in PFS. BC + Z + S + PL and BC + Z + S + PL mixtures were not effective in reducing water-extracted mobile heavy metals in PFS. The introduction into PFS of mixtures partially composed of biologically unstable materials (PL, SL) increased the biochemical activity measured by respiratory activity and reduced biomass increment of willow aerial parts. The adverse response of willow to the introduction of mixtures with poultry litter or slurry into PFS indicates the need to verify the amount of these materials in the mixtures or to stabilise them by biological or thermal processes.