Remediation Of Chromium-contaminated Soil By Modified Cotton Straw-based Biochar

Due to the rapid development of the current social industry,a large amount of chromium-containing wastewater and waste residues are discharged into the soil environment,and chromium pollution has become a serious threat to the ecological environment.A large amount of cotton straw remaining after the cotton harvest in Xinjiang has become an agricultural and forestry waste,and the preparation of biochar from straw is a new material for soil remediation of heavy metal pollution that has been widely paid attention to in recent years.By giving biochar a richer surface functional group and pore structure through modification,the effect of its modification pathway on the remediation of heavy metal contaminated soil has become a research hotspot.In this study,cotton straw-based biochar was prepared from waste cotton straw by two different modification pathways,modified first and pyrolysis first,using hydrochloric acid and sodium hydroxide as modifiers,controlling the pyrolysis temperature,pyrolysis time,modifier solid-liquid ratio and modifier concentration of cotton straw char,and designing a four-factor,four-level orthogonal experimental table for indoor soil incubation experiments.The modified cotton straw-based biochar was prepared according to the orthogonal table,which were hydrochloric acid modified and then pyrolyzed cotton straw char(H-BC),sodium hydroxide modified and then pyrolyzed cotton straw char(OH-BC),pyrolyzed and then hydrochloric acid modified cotton straw char(BC-H),pyrolyzed and then sodium hydroxide modified cotton straw char(BC-OH)and unmodified cotton straw char(BC),and the chromium effective state remediation efficiency was orthogonalized by indoor soil incubation experiments.The optimum cotton straw charcoal was selected by ANOVA with respect to the effective state restoration efficiency of chromium in the indoor soil culture experiment.Pot experiments were conducted using the preferred cotton straw charcoal to explore its remediation effect on chromium-contaminated soil and its effect on the growth and uptake of chromium by Brassica juncea.The main findings of the study were as follows:(1)By examining the content of the effective state of chromium in the indoor soil incubation experiment,it was found that the restoration efficiency of H-BC for chromium ranged from 46.62%-72.51%,OH-BC from 36.91%-67.38%,BC-H from 38.08%-69.60%,BC-OH from 35.41%-59.18%,and BC were39.96%-57.24%.The optimal solutions for obtaining H-BC from cotton straw charcoal prepared by each route based on the orthogonal test with extreme difference and ANOVA were: pyrolysis temperature of350℃,pyrolysis time of 4h,solid-liquid ratio of cotton straw to modifier of 1:2,and modifier concentration of 1 mol/L;OH-BC of 300℃,1.5h,1:8,2 mol/L;BC-H of 300℃,4h,1:4,2 mol/L;BC-OH was 400℃,1h,1:4,1 mol/L.BC was chosen to have the best chromium restoration effect of 350℃,2h.(2)Cotton straw charcoal prepared by different modification methods significantly reduced the effective state of chromium in the soil when added at 6%,the five treatments reduced 51.34%,44.65%,52.34%,44.76%,and 25.86%,respectively,compared with CK,and the priority order of their effects was H-BC>BC-H>OH-BC>BC-OH>BC.All treatments induced the conversion of chromium from the weak acid extracted state and the reducible state to the residue state,and the order of the conversion effect was the same as that of the effective state.(3)The growth of Brassica juncea was promoted by cotton straw charcoal prepared in different ways of modification,and compared with CK,the maximum increase in weight of aboveground part of Brassica juncea was 12.22 g for OH-BC and 1.78 g for BC-H.The chromium content in Brassica juncea decreased after the addition of cotton straw charcoal,and its reduction effect was large H-BC(68.70%)> BC-H(66.89%)> OH-BC(63.90%)> BC-H(62.16%)> BC(55.25%))> OH-BC(63.90%)> BC-H(62.16%)>BC(55.25%).The overall enrichment and translocation coefficients of soil chromium by Brassica juncea after the application of cotton straw charcoal treatment were reduced compared to the control.It can be concluded that the cotton straw modification can effectively reduce the enrichment capacity of Brassica juncea for heavy metals and the translocation capacity from the lower part of the ground to the above ground.In conclusion,H-BC is more effective in reducing the biological effectiveness of chromium in soil and has great value in remediation of heavy metal chromium pollution in soil applications.