| With the rapid development of industry,the cadmium pollution in water is becoming more and more serious.Biochar and microbial adsorption have been widely studied in the treatment of cadmium-containing wastewater,while there are still defects in their application.The research shows that the use of biochar-microorganism synergistic system for the treatment of cadmium wastewater can make up for the shortage of single system.However,there are relatively few researches on synergistic systems,and there is a lack of in-depth research on the mechanism of synergistic systems.Therefore,this paper examined the adsorption characteristics of three kinds of biochar,and on this basis,compared the synergistic system formed by biochar and Bacillus subtilis,and the difference in cadmium removal ability for wastewater containing cadmium,and further explored the process and mechanism of cadmium removal in the synergistic system.The main results are as follows:(1)Using corn straw,peanut shell and pine as raw materials,the cadmium adsorption characteristics of the three kinds of biochar were investigated.The results showed that the three kinds of biochar all had the best adsorption effect on cadmium under the condition of pH 7,and the adsorption process was dominated by chemical action and monolayer adsorption.The adsorption effects of corn straw and peanut shell biochars were 37.4 mg·g-1and 35.4 mg·g-1,respectively,while that of pine wood was the worst,which was only 4.9 mg·g-1.Scanning electron microscopy(SEM)-energy spectrum(EDS)analysis and BET nitrogen adsorption analysis indicated that corn stalk biochar contains mineral elements,rough surface pores,large specific surface area and pore capacity,which may have a greater potential in the treatment of cadmium-containing wastewater.(2)The three kinds of biochar were combined with Bacillus subtilis to treat the wastewater containing cadmium,respectively.The results showed that the corn straw biochar-Bacillus subtilis synergistic system had the best cadmium removal effect(62%).Compared with single Bacillus subtilis system,the cadmium removal rate increased by 139.4%;and compared with the biochar system,the cadmium removal rate increased by 86.9%,which was greater than the sum of the removal effect of corn stalk biochar and single Bacillus subtilis system.The monitoring of OD value showed that corn stalk biochar could increase the maximum biomass of Bacillus subtilis.Based on the comparison of the removal process and characterization analysis between the synergistic system and the single system,the cadmium removal process by the synergistic system of corn straw biochar and Bacillus subtilis can be roughly divided into three stages:biochar dominated restoration stage,Bacillus subtilis dominated restoration stage and biofilm formation stage.The release of mineral elements on the surface of biochar and the biofilm formed by the attachment of Bacillus subtilis in the synergistic system are important factors for the long-term and stable removal of cadmium from water.(3)Further transcriptome analysis showed that Bacillus subtilis removed cadmium from wastewater by means of cellular functional transport,but did not bind it stably to the cells,and was directly harmed by cadmium toxicity,thus greatly inhibiting its growth and metabolic activity.The addition of corn stalk biochar not only activated the phosphotransferase system in Bacillus subtilis,enhanced the expression of cadmium binding gene LuxS and cadmium resistance gene czcD,but also provided energy material for Bacillus subtilis and enhanced the metabolic pathways of carbon,nitrogen and riboflavin related to Bacillus subtilis.In addition,the expressions of pyrG and pcrA related to phosphorus biosynthesis/fixation of Bacillus subtilis were significantly up-regulated,and there were obvious characteristics of early biofilm development.These results further verified that the construction of a synergistic system can achieve long-term and stable removal of cadmium from wastewater. |
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