| Soil pollution with heavy metals has increasingly become a severe global environmental concern in recent years. In China, more arable lands going to heavy metals pollution which are close to 16.1% of the entire agricultural land. Particularly, lead(Pb) and copper(Cu) contamination in croplands pose a severe health hazardous and environmental concern throughout soil-food chain transfer. Substantial efforts have been made to remediate polluted soils with organic and inorganic materials. In the present studies, the immobilizing effects of rice straw and its derived biochar, single superphosphate, multiwall carbon nanotubes and pyrolytic and non-pyrolytic castor plant residues were investigated in a combination of incubation pots and green house experiments in which plant growth, metals uptake by plants, concentrations of metals in the soil solution, their distribution among different fractionations, leaching characteristics, bioaccessibility and as well as mobility were determined in Pb/Cu co-contaminated soil. These results could provide scientific basis for exploring fundamental scientific and technological issues relevant to Pb/Cu polluted acidic soil by immobilized amendments application.Firstly, various techniques such as European Community Bureau of Reference(BCR) sequential extraction, Total Characteristics Leaching Procedure(TCLP), single extraction(Ca Cl2) and Simple Bioaccessibility Extraction Test(SBET) were employed to evaluate the simultaneous effectiveness of rice straw(RS) and its derived biochar(BC), multiwall carbon nanotube(MWCNT) and single super phosphate(SSP) to immobilize the Pb and Cu in co-contaminated soil. The BCR sequential extraction results suggested that with increasing BC and SSP amount, the acid-soluble fractions decreased while oxidizable and residuals proportions of Pb and Cu increased significantly. Compared to SSP, the application of BC amendment substantially modified partitioning of Cu from easily exchangeable phase to less bioavailable residual bound fraction. The TCLP and Ca Cl2-extracted Pb and Cu were reduced significantly by the addition of BC compared to RS and MWCNT, whereas the bioaccessibility of Pb significantly reduced with RS addition. SSP showed better results for Pb immobilization while marginal for Cu in cocontaminated soil. Overall, the addition of BC offered the best results and could be effective in both Pb and Cu immobilization thereby reducing their mobility and bioavailability in co-contaminated soil.In the adsorption experiment, rice straw derived biochar(BC) and soil emended with biochar 3%(BC 3%) and 6%(BC 6%) were used as adsorbent for simultaneously removal of Pb and Cu from aqueous solution, so as to evaluate the adsorption capacities of biochar(BC) and soil+biochar(BC 3% and BC 6%) for Pb and Cu. The adsorption data was fitted well by a Freundlich isotherm, with utmost Pb and Cu capacities 31 and 32.4 mg g-1 for BC, 3.9 and 4.03 mg g-1 for BC 3%, and 4.3 and 4.1 mg g-1 for BC 6%, respectively. The addition of biochar considerably enhanced the adsorption of Pb and Cu by this soil. Pb adsorbed more than Cu, and the enhancement of adsorption of Pb and Cu increased with increasing contact time and p H. The removal of metals by biochar was due to complexation of metals with ionized phenolic-O-, and also with the precipitation of metals with CO32- and/or PO43- present in biochar. These indicated that rice straw biochar could act as efficient surface sorbent, while its capability to treat diverse waste streams requires to be carefully assessed on separable basis.Plant growth trial was conducted to simultaneously investigate the effects of RS, BC, MWCNT and SSP on Pb/Cu immobilization and leaching by means of BCR and TCLP, and phytoavailability was assessed with rapeseed(Brassica napus L.) in cocontaminated soil. Moreover, residual influences of applied amendments on metals pyhtoavailability in promoting tomato(Lycopersicon esculentum) were also studied. Results demonstrated that BC was most effective among all amendments and prominently modified the distribution of Pb and Cu from acid soluble to less bioavailable residuals phase and increased the geochemical stability in acidic soil. The application of BC(6 %) reduced the uptake 46 % and 36 % of Pb and 77 % and 58 % of Cu in shoots and roots in rapeseed, while in tomato decreased 36 % and 66 % for Pb and 29 % and 61 % for Cu in shoots and roots, respectively. Metals concentrations in second grown tomato shoots treated with BC 6 % were below the permissible limit set by World Health Organization(WHO). SSP significantly reduced TCLP-Pb to 0.2 mg L-1 which was below critical limit(5 mg L-1). The low effectiveness of MWCNT on the immobilization of Pb and Cu in soil might be due to lesser amount of MWCNT applied and low p H of soil. Biochar can increase the immobilization of Pb and Cu and has the potential to reduce their hazard and bioavailability in two cropping patterns in co-contaminated soil. SSP was more effective for Pb immobilization, while bioavailability of Cu increased with increasing SSP addition. It should be carefully to assess the influence of SSP on Cu remediation in co-contaminated soil.Additional experiment was done to immobilize Pb and Cu by possible combinations of RS, BC, MWCNT and SSP amendments by means of BCR sequential extraction, TCLP and by growing rapeseed(Brassica napus L.) in contaminated soil. Results demonstrated that BC+MWCNT was more effective among all applied amendments and prominently modified the Pb and Cu distribution from acid-soluble to non-toxic residuals phase and increased the geochemical stability in acidic soil. Application of BC+MWCNT reduced the uptake of 86.3 % and 44.9 % of Pb, and 61.2 % and 57.6 % of Cu in shoots and roots, respectively, and thereby prominently increased the chlorophyll and soluble protein content in rapeseed. These metals concentrations in edible part of plant were within the normal range. The application of BC+SSP was effective for Pb immobilization than Cu. It should be careful to evaluate the impact of SSP on Cu remediation in co-contaminated soil. Hence, combine application of BC and MWCNT could reduce the hazards of Pb and Cu in contaminated soil by reducing uptake in plants.To evaluate the Pb and Cu subsequent transformation, immobilizing impact of pyrolytic and non-pyrolytic rice and castor straws and their efficiency to reduce the metals mobility and leachability in polluted soil was studied. Results highlight the potential of biochar over non-pyrolytic residues to enhance the immobilization of Pb and Cu in soil. Castor leaf derived biochar(CLB), castor stem derived biochar(CSB) and rice straw derived biochar(RSB) prominently decreased the mobility(acid-soluble fraction) of Pb 49.8 %, 31.1 % and 31.9 %, respectively, while Cu decreased 15.8 %, 11.5 % and 12 %, respectively, from their respective control. Sequential extraction showed that biochar treatments prominently modified the proportioning of Pb and Cu from acid soluble to less bioavailable fraction and increase its stability in contaminated soil, as compared to relative feed stocks and as well as controlled soil. Additionally, the soil p H increased markedly after the addition of biochar. Compared with control, the TCLPextractable Pb and Cu were reduced from soil 29.22 %-41.41 % and 5.7 %- 22.88 %, respectively, by the application of CLB. The immobilization and leachability of Pb and Cu was correlated with soil p H. The biochar effect on the Pb immobilization was much better as compared to Cu in co-contaminated soil. Over all amended biochar’s, addition of CLB offered best results and could be effective in both Pb and Cu immobilization thereby reducing their mobility and bioavailability in co-contaminated soil. |
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