Bioremediation And Its Mechanisms Of Polyvinyl Alcohol-immobilized White-rot Fungi In The Composite-polluted Wastewater

Immobilized microorganism technology is utilized to make freely suspended target cells immobilized inside or on the surface of carrier materials by physical or chemical means, and the immobilized microorganisms are efficiently employed as biocatalysts. The technology has a lot of advantages, such as high microbial activity, high cell concentration, favorable biological tolerance to stress, strong resistance to shock loads, superior removal efficiency, and good reusa bility. It has been widely used to treat wastewater. Wastewater pollution is becoming more and more serious and its contaminated components are spread from single contaminant to multipollutant in recent years. Contamination caused by a combination of heavy metal and organic pollutants has attracted researchers' attention because both of the two pollutants are toxic and more difficult to remove, have aroused serious environmental pollution, and have damaged human health strongly.Some of the advantages of polyvinyl alcohol(PVA) include low cost, good hydrophilicity, high mechanical strength, excellent chemical stability, strong resistance to biodegaradation, and nontoxic to microorganisms. Therefore, PVA is widely used as the immobilized carrier material. However, PVA can form agglomeration due to its large viscosity and water swelling during the process of immobilized preparation. In the present research, sodium alginate, powdered activated carbon, silicon dioxide, and zeolite powder w ere added into the PVA immobilized carrier as additives to improve the mechanical properties and adhesion phenomenon of PVA material. The PVA composite immobilized carriers prepared was utilized to immobilized Phanerochaete chrysosporium(P. chrysosporium), the model strain of white rot fungi, which is well known for its admirable biosorption capacity for heavy metal and unique biodegradation for xenobiotics. PVA-immobilized P. chrysosporium was employed to the bioremediation of composite-polluted wastewater containing both cadmium(II) and 2,4-dichlorophenol(2,4-DCP).In the present study, the effects of biomass dosage, contact time, and initial concentrations of cadmium and 2,4-DCP on cadmium removal and 2,4-DCP degradation capacities in a batch system wer e systematically researched. The p H of solutions and the concentration of extracellular proteins secreted by PVA-immobilized P. chrysosporium during the reaction process were investigated. The mechanisms of PVA-immobilized P. chrysosporium for cadmium removal and 2,4-DCP degradation were also explored. The optimum removal efficiency obtained were 78% for cadmium and 95.4% for 2,4-DCP when the initial concentrations of cadmium and 2,4-DCP were 20 and 40 mg/L, respectively. The resistance of P. chrysosporium to cadmium and 2,4-DCP had been significantly enhanced owing to the protection of PVA-immobilized carrier, leading to the removal rates of cadmium beyond 60% even at higher initial cadmium concentrations, and the 2,4-DCP degradation rates more than 90% with an increase in the initial concentrations of 2,4-DCP. The secretion of extracellular proteins was boosted with an increase in the concentrations of cadmium and 2,4-DCP within a certain range. And the changes in p H values of solutions might be closely associated with the initial concentrations of cadmium and 2,4-DCP. This article demonstrated that low concentrations of 2,4-DCP could be used as carbon and energe sources and that extracellular proteins secreted might be utilized as nitrogen source by PVA-immobilized P. chrysosporium for the further removal of cadmium and 2,4-DCP. Furthermore, the experiment of adsorption-desorption indicated that PVA-immobilized P. chrysosporium had good reusability. The desorption efficiency of cadmium adsorbed on the tested biosorbent was u p to 98.9%, remaining fairly constant in cadmium uptake capacity after three consecutive cycles. The high efficacy of 2,4-DCP degradation was still achieved during this process. And the shape of PVA-immobilized P. chrysosporium maintained intact, without visual deterioration.According to the analyses of scanning electron microscopy(SEM) and energy-dispersive X-ray analysis(EDAX), cadmium removal was correlated with the immobilized carriers and P. chrysosporium hyphae with a network structure surrounding on the surface of PVA-immobilized P. chrysosporium. After adsorption, the hyphae of PVA-immobilized P. chrysosporium became rough, tight, and were coated with a layer of gunk containing cadmium. Fourier transform infrared spectroscopy(FTIR) analysis showed that hydroxyl, carboxyl, and amino functional groups in PVA-immobilized P. chrysosporium played an important role in cadmium removal. The mechanism underlying the degradation of 2,4-DCP by PVA-immobilized P. chrysosporium into fumaric and 1-hexanol was analyzed by using gas chromatography-mass spectrometry(GC-MS). Coupled with above results obtained, PVA-immobilized P. chrysosporium exhibited excellent performance for simultaneous removal of heavy metals and organic pollutants, which was suggested to provide theoretical instruction for the application of PVA-immobilized microorganism technology in the field of wastewater treatment.