Research On Improving The Microenvironment Of Pb-contaminated Agricultural Wastes By Incubating With White-rot Fungus And Its Mechanism

Recently, agriculture waste is seriously polluted by heavy metals. For the current heavy-metal contamination of agricultural waste, the proposed work was studied for the degradation of Pb-polluted rice straw by white-rot fungi during fermentation. The aim was to examine the effect of different Pb concentrations on the degradation of rice straw during solid-state fermentation by white-rot fungi and the differences in Pb passivation ability of white-rot fungi under different Pb concentrations, as well as lead migration and conversions in solid state fermentation process. To better understand the interaction between heavy metals and white-rot fungi, passivation mechanism of heavy metals by white-rot fungi extracellularly was discussed. Finally, further research on the enhanced adsorption of Pb by white-rot fungi was performed.Firstly, the physiological processes of white-rot fungi under various Pb²⁺ concentrations during solid-state fermentation were examined. The results showed that high concentration of Pb²⁺ had an inhibitory effect on the biomass, enzyme activity and organic acids of extracellular secretions, on the contrary, low concentration of Pb²⁺ expressed a stimulative effect. Especially, low Pb²⁺concentration promoted the formation of manganese peroxidase (MnP), meanwhile, the content of oxalic acid also increased. Relatively to strong inhibition effect of enzyme activity under high concentration of Pb²⁺, the oxalic acid extracellular secretion of white-rot fungi was relatively much less affected by the concentration of Pb²⁺, and demonstrated a proportional correlation with the concentration of Pb²⁺ during the later stage of fermentation.Next, the energy flow, carbon cycle in the degradation of Pb-contaminated straw by white-rot fungi, as well as the effect of different Pb²⁺ concentrations on lignin degradation were studied. It's found that white-rot fungi had some ability to tolerate Pb²⁺ toxicity. Higher Pb²⁺ concentration showed more interference to the degradation of lignin and other organic substances by white-rot fungi in the early stage of fermentation. However, white-rot fungi had gradually adapted to Pb²⁺ toxicity and the energy cycle in the solid-state fermentation substrates slowly recovered in the late fermentation. It was proved that the degradation of lignin by white-rot fungi was accompanied with its adaption to low concertration of Pb²⁺.The transformation of lead (Pb) fractions and the alleviation of Pb-hazards in solid-state fermentation of Pb-contaminated solid waste by white-rot fungi were also studied. White-rot fungi effectively reduced the bioavailability of Pb²⁺ in all of the treatments after 45-days solid-state fermentation. It was shown that the concentration of soluble-exchangeable Pb²⁺ and carbonate-bound Pb²⁺ were reduced during the solid-state fermentation, while the concentrations of organic-bound Pb²⁺ and residual Pb²⁺ increased markedly after fermentation. It was proved that the degradation of lignin by white-rot fungi was accompanied with its adaption to low concertration of Pb²⁺.In addition, the biomass titration revealed that the cell surface of white-rot fungi contained three types of functional groups at least, ie. carboxyl, phosphoryl and phenolic groups. The Fourier transform infrared (FTIR) spectrometry showed that these three types of functional groups of white-rot fungi were mainly responsible for Pb²⁺ adsorption. The results explored the extracellular immobilization mechanisms for white-rot fungi to reduce the bioavailability of Pb²⁺ to some extent. At last, a novel magnetic biosorbent based on immobilized white-rot fungal was developed to improve the endurance of white-rot fungal to the toxicity of the heavy metals.