Abstract [Background] In order to remediate heavy metal-contaminated calcareous soil, a comparative research on the effect of lignite-based materials on heavy metals speciation of different organo-mineral complexes were conducted for screening out highly effective amendments andunderstanding the restoration mechanism of pollution degradation. [Methods] All these materials weremixed with lead-contaminated soiland incubated for 120 days at 25 and 65% relative humidity,thereafter organo-mineral complexes were extracted, and mass fraction of different Pb chemical speciationof soil complexes were assessed. [Results] The results indicated that water-stable complexes were increased with the application of lignite organic materials,the treatments with humic acid,lignite,nitrified lignite and sulphonated lignite transformed more of G1 and G2 complexes from G0, and other results were as follows:1) The distributions of 6 Pb chemical speciation (ion-exchangeable Pb, Fe-Mn-oxide-bound Pb, carbonate-bound Pb, organics-weakly-bound Pb, organics-strongly-bound Pb, and residual Pb)varied in the different complexes G0(water-dispersing complex), G1 (NaCl-dispersing complex),and G2 (NaCl-grinding-dispersing complex ). For all amendments, the order of the abundance for ion-exchangeable Pb, Fe-Mn-oxide-bound Pb, and carbonate-bound Pbwas G0 > G1 > G2; the ion-exchangeable Pb decreased 8.74% - 32.22% from G0 to G1, and 2.73% - 26.74% from G1 to G2; organics-weakly-bound Pb, organics-strongly-bound Pb, and residual Pb in G1 and G2 complexes were more than that in G0. 2) Applying organic materials decreased the content of ion-exchangeable Pb averagely 2.73% - 32.22% in 3 complexes, while increased the organics-weakly-bound Pb at a maximum of 51.23% and organics-strongly-bound Pb at a maximum of 67.65%, no significant effect on residual Pb. Organics-weakly-bound Pb in G0 was less significantly affected than that in G1 and G2 by these organic materials, and the content of organics-weakly-bound Pb in G1 complexes treated with lignite, humic acid, nitrified lignite and sulphonated lignite were higher than original soil in the range of 35.06% -51.23%, and in G2 higher in the range of 27.11% - 41.35%. All lignite-based materials increased the content of organics-strongly-bound Pb in the range of 5.44% - 38.29%, but had no significant effect on residual Pb of G0, G1 and G2. 3) Compared with raw lignite, the treatments with humic acid, active charcoal, alkalization and calcium-loaded lignite decreased markedly the content of ion-exchangeable Pb in G0 group, but all modified lignite increased ion-exchangeable Pb content in G2 group, as well as sulphonation and demineralization increased ion-exchangeable Pb in G1 remarkably. Except humic acid,many modified lignite-based materials resulted in lower content of organics-strongly-bound Pb in all three groups of complex. There were no significant differences on content of carbonate-bound Pb in all complexes between original and modified lignite. Alkalization and calcium-loaded lignite markedly decreased residual Pb in G0, and the same changes in G1 and G2. Simultaneously, alkalization and calcium-loaded lignite increased significantly Fe-Mn-oxide-bound Pb in G1. [Conclusions] In conclusion, applying lignite-based amendments is effective in increasing water-stable complexes and prompting more organically-bound Pb speciation in contaminated soil, decreasing the fraction of ion-exchangeable Pb, thus it helps establish stable soil structure, enhance the capacity of resisting soil erosion, and decrease the environmental exposure risk by surface runoff and deep percolation. Organic lignite-based amendments improve remarkably content of organically-bound Pb, nevertheless, organic matter has little effect on Fe-Mn-oxide-bound, residual and carbonate-bound Pb. Taking immobilization and economy efficiency into account, raw lignite, humic acid, active charaoal, nitrified lignite and sulphonated lignite are recommended for remediating Pb-contaminated soil.
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