| Biochar is extensively used as an effective soil amendment for environmental remediation.In addition to its strong contaminant sorption capability,biochar also plays an important role in chemical transformation of contaminant due to its inherent redox-active moieties.However,the transformation efficiency of inorganic contaminants is generally very limited when the direct adsorption of contaminants on biochar is inefficient.The present study demonstrates the role of Fe ion as an electron shuttle to enhance Cr???reduction by biochars.Batch experiments were conducted to examine the effects of Fe???levels,pyrolysis temperature of biochar,initial solution pH,and biochar dosage on the efficiency of Cr???removal.The results are following:1)The surface of walnut shell biochar is rough and plicated with small round holes of varied sizes.The specific surface area and pore volume were big,and increased with increasing pyrolysis temperature.And the surface area of BC500?BC600 and BC700 was over 400 m2g-1?The walnut shell biochars had rich functional groups on its surface,including-OH,-CHO,C=O,-COO-and aromatic ring,etc.There was a difference in the types and abundance of functional groups on the surface of walnut shell biochar prepared by different pyrolysis temperature,and the functional groups of biochars prepared at higher temperatures were more abundant.The isoelectric point of biochar in aqueous solution was 2.9,and when the pH was over 2.9,Zeta potential of biochars was negative.Walnut shell biochar was found that it had good physical and chemical properties through to the characterization of biochar,and it was suitable for removal of heavy metal pollutants;2)Fe???can effectively enhance the removal of Cr???from biochar.In the case of Fe???,the removal efficiency of biochar was at least 20%higher than when Fe???did not exist.In addition,the Fe???can accelerate the biological carbon removal of Cr???;3)Biochar produced at higher pyrolysis temperatures?e.g.,700°C?favored Cr???removal,especially in the presence of Fe???.In the presence of Fe???,the removal efficiency of Cr???by BC300 and BC400 increased by 5%,while the removal efficiency of Cr???by BC500?BC600?BC700 increased at least by20%.In a certain range,the removal efficiency of Cr???by"Biochar-Fe???-Cr???"ternary system increases with the increase of biochar concentration,while a higher biochar dosage proved unfavorable likely due to the agglomeration or precipitation of biochar.4)The efficiency of Cr???removal by biocahr was shown to increase with increasing Fe???concentration.In the case of Fe???concentration of 1000?g L-1,the removal efficiency of Cr???by the biochar can reach 60%in 2h,and the removal efficiency is 80%at 12h,indicating that high concentration Fe???can effectively reduce the concentration of Cr???in water in a short period of time.5)"Biochar-Fe???-Cr???"was more effective in removing chromium under acidic conditions?pH?5.0?,and the removal efficiency of Cr???decreased with the increase of pH.There was not obvious change on the removal efficiency of Cr???by"Biochar-Fe???-Cr???"under the condition of neutral or alkaline?pH?6.0?.Fe???did not work probably because of the formation of the Fe?OH?3precipitation.6)Fe???played a role as the electron shuttles between biochar and Cr???.The mechanism of Cr???removal may take place through?1?the reduction of adsorbed Fe???(>Fe3+)to Fe?II?by biochar;?2?the dissolution of adsorbed Fe?II?(>Fe2+)into the aqueous phase;?3?the reduction of Cr???to Cr???by Fe?II?in the solution;and?4?the adsorption of Cr???onto BC500.The resulting Fe???can then re-adsorb onto biochar and serve as an electron shuttle again. |
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