The Phytotoxicity Alleviation And Adsorption Mechanism Of Toxic Metals Of Acidic Soil By Biochar Amendment

Biochar is a carbon-rich material that is produced during the pyrolysis of biomass, and it has received increasing attention as an effective approach to acidic soil amelioration and contaminated soil remediation. This paper introduced the structure and composition of biochar, and focused on the progress of acidic soil improvement and heavy metal contaminated soil remediation. Until now, the mechanism of acidic soil amelioration by biochar amendment was not well understood, and the adsorption and phytotoxicity alleviation mechanism of aluminum and heavy metal by biochars has not been systematically elucidated. In this dissertation, biochars derived from rice straw and cattle manure were prepared under different pyrolytic temperatures. Through the structural characterization of biochar by element analysis, FTIR, XRD, SEM and Zeta potential, the Al binding mechanisms by biochars were discovered, and then the Al phytotoxicity alleviation mechanisms by biochar in hydroponic system were investigated. Furthermore, the mechanisms of Al phytotoxicity alleviation in acidic soil were revealed. Simulating aging process of biochar by oxidation, the Al and Cd binding mechanisms by aging biochar were illustrated, and the Cd phytotoxicity mitigation mechanisms by biochar were clarified when response to the coupling of greenhouse effect and soil acidification. These results provide a theoretical basis for acid soil improvement and contaminated soil remediation. The main original conclusions of this work are drawn as follows:(1) Revealed the mechanism and structure-effect relationship of biochar and Al interaction, and found that the oxygen-containing organic components and the scattering silicon particles of biochar are two main regions of Al adsorption. The complexation of Al with organic hydroxyl and carboxyl groups and the surface adsorption and coprecipitation of Al with silicate particles both contributed to the Al adsorption of the biochars. According to the FTIR of biochar, the difference peak of carboxyl asymmetric and symmetric showed a significant increase after Al loaded, indicating the surface complexation of carboxyl group and Al. After the biochars were loaded with Al, the zeta potentials of the biochars and ashes increased as a function of pH. The charge reversal was caused by the Stern-layer adsorption of hydrolyzed aluminum species on the silicate surfaces via hydrogen bonds.(2) Effective alleviation of Al phytotoxicity by biochar was found, and its mechanism included the liming effect of biochar (alkaline) to change form of Al, the adsorption by biochar to reduced the available Al and the alleviation was the formation of aluminosilicate by dissolution silicon and Al. Hydroponic experiments showed that upon addition of0.02%biochar to the exposure solution, the inhibition of plant growth by Al was significantly reduced while the toxic threshold was extended from3to95?mol/L Al3+. Due to the biochar liming effect, the aluminum species were converted to Al(OH)2+and Al(OH)2+monomers, which were strongly adsorbed by biochar; furthermore, the highly toxic Al3+evolved to less toxic Al(OH)3and Al(OH)4-species. Further used biochar in acidic soil, it has proven a dual function in Al phytotoxicity alleviation. Soil exchangeable Al, Al concentration and hematoxylin stai ning showed that the biochar reduced soil exchangeable Al, thus reduce the migration to plants. Meanwhile, Morin staining and SEM-EDS showed silicon dissolution from biochar can enter in plants, then form Al-Si compounds with Al in the epidermis of wheat roots, alleviating the Al phytotoxicity in plants.(3) Elucidated the biochar structural evolution by aging process as well as the adsorption and resistance mechanisms controlling of soil toxic metals Al and Cd during soil acidification. Simulated biochar aging process in acidic soil and found that biochar aging process lead to the leaching of base cations and the increase of surface carboxyl and hydroxyl functional groups. Additional carboxyl and hydroxyl groups provided new adsorption sites of Al and Cd, thus promoting the adsorption of Al and Cd biochar, indicating that biochar had long-term effect on harmful metals immobilization in acidic soil. Further studies showed that soil acidification and warming effect have a synergistic effect on the phytotoxicity of Cd. Under the coupling of the soil acidification and warming effects, biochar showed effective alleviation of Cd phytotoxicity.