| Chromium(Cr)is a class I heavy metal,which cannot be degraded spontaneously in nature and is harmful to water bodies and soil.Existing treatment methods for Cr have different degrees of limitations.In this paper,a functional strain of chromium-tolerant bacteria was screened from soil contaminated with added exogenous Cr.Biochar made from different raw materials was used as a carrier for immobilizing the functional strain,and the effects of different types of biochar,adsorption time,rotational speed and biochar The effects of different types of biochar,adsorption time,rotational speed and biochar dosage on the immobilized microorganisms were investigated to determine the optimal conditions for biochar immobilization of microorganisms.The adsorption efficiency of biochar immobilized microorganisms on heavy metal ions was also predicted from the physicochemical properties of biochar and adsorption environment based on GBDT,random forest(RF)and support vector machine model(SVM)of machine learning theory.The main results of this study are as follows:(1)A comparison of the physicochemical properties of biochar made from different raw materials showed that straw biochar(YMB300)at 300℃and tobacco stalk biochar(YGB500)at 500℃had higher acidic functional group content and contained more-COOH and-OH,which were beneficial to the carrier to maintain the activity of microorganisms,and also had larger specific surface area and pore size,relatively complete skeleton structure and clearer pore channels.It is conducive to the adsorption and immobilization of microorganisms.(2)Ten strains were isolated from soil contaminated with added exogenous chromium,and the strain with the highest Cr removal rate was determined and identified by 16S r DNA as Aspergillus niger.The target strains were incubated for 60h at Cr(VI)concentrations of 50 and 500 mg-L-1 and were found to show high tolerance to both low and high concentrations of Cr(VI).(3)An orthogonal experiment with different levels of four factors was designed with different types of biochar raw materials,initial amount of biochar,difference in adsorption time and adsorption speed,etc.By measuring the fixation rate,it was found that the highest fixation rate of Aspergillus niger was achieved with straw biochar(YMB300)at 300℃,the volume ratio of Aspergillus niger suspension to biochar was4,the shaking speed was 180rpm and the adsorption time was 24 hours at 80.69%.At this level,the activity of Aspergillus niger was stronger.(4)When pH=3,the adsorption of Cr(VI)by the charcoal bacterium complex(BY)and the straw biochar at 300°C(YMB300)was the largest,with 94.12%and 64.92%,respectively.the adsorption process of Cr(VI)by BY and YMB300 could be better fitted to the quasi-secondary kinetic equation,and the adsorption process was more consistent with chemisorption;the adsorption of Cr(VI)by both BY and YMB300materials was better fitted to the Freundlich model.The adsorption of Cr(VI)by both BY and YMB300 was better fitted to the Freundlich model for molecular multilayer adsorption.(5)Three machine learning models were selected to train the data,and the GBDT model had an RMSE of 0.91 and R~2 of 0.98,with the lowest root mean square error and the best fitting output.analysis of the influence weights of the GBDT model showed that the pH value of the solution,the initial dosing amount,the pore size and specific surface area of the biochar were more significant in predicting the heavy metal adsorption efficiency.(6)When the charcoal bacterium complex(BY)was applied to chromium-contaminated soil,it was found that BY significantly improved the removal of Cr(VI)compared to free Aspergillus niger and single straw biochar,and also significantly promoted the conversion of chromium from the water-soluble and exchange state to the precipitated and residual state. |
