| Biomass can be pyrolysed into biochar,which has been widely used as an environmental material in contamination remediation,achieving the win-win effect of "treatment by waste".In recent years,modified biochar has attracted much attention due to its enhancement in function and expansion in application.Fe is one of the most widely distributed and most abundant metals on the earth,and its oxides/iron ions/metal salts are often selected as modifiers to prepare iron-modified biochar.Iron-modified biochar has proven to be an effective adsorbent for removing organic pollutants and heavy metals from water or soil.However,this modified biochar is usually modified on the basis of the prepared biochar,facing problems such as complicated process and high cost.In this paper,a one-pot method was used to prepare iron-biochar composite by pyrolyzing the mixed solid of Fe2O3 and peanut shell biomass.The removal effect and mechanisms of iron-biochar composite on organic contaminants,methylene blue and acid orange 7 with various properties,and heavy metals Cr(Ⅵ)and Zn(Ⅱ)in water.The main conclusions are as follows:(1)In the present study,a low-oxygen(<0.5%)slow pyrolysis method was used to prepare peanut shell-derived pristine biochar(described as BC400 and BC700 according to the pyrolysis temperature)and its iron-biochar composites(described as BC400Fe2,BC700Fe2,etc.according to the pyrolysis temperature and the Fe mass percent)at 400℃and 700℃,using peanut shell fragments and pre-mixed Fe2O3(2%,5%,and 11%,w/w)with peanut shell fragments.In the iron-biochar composites pyrolyzed at 400℃,Fe existed mainly as Fe2O3,followed by Fe3O4,FeO,and FeO;while formed in the iron-biochar composites pyrolyzed at 700℃were Fe3O4,FeO,and FeO.The electron donating and accepting capacity(EDC/EAC)of the biochar pyrolyzed at 400℃ was stronger than that of the biochar pyrolyzed at 700℃.At the same pyrolysis temperature,the EDC/EAC value increased with the increase of Fe content.(2)The increase of Fe content enhanced the removal capacity of iron-biochar composites to methylene blue.For 400℃ biochar,with the increase of Fe content,the maximum sorption amounts of methylene blue were 52.9 mg/g,59.9 mg/g,62.2 mg/g and 75.8 mg/g,respectively.For 700℃ biochar,the maximum sorption amounts were 22.9 mg/g,27.0 mg/g,35.3 mg/g and 46.1 mg/g,respectively.The degradation of methylene blue dominated its removal by biochar,accounting for 58.9-94.2%of the removal rate.The abundant functional groups could promote the interactions between the dye and biochar.Therefore,the biochar pyrolyzed at lower temperature had a better removal efficiency of methylene blue,with an increase of 6.48-27.8 mg/g.The degradation of methylene blue by biochar was a non-radical oxidation-reduction process.For the iron-biochar composites pyrolyzed at 400℃,dissolved Fe3+,solid Fe0 and Fe2O3 in biochar played a major role in the removal of methylene blue;while it was the dissolved Fe3+ and solid Fe0 in the iron-biochar composites pyrolyzed at 700℃ that played an important part.LC-MS was used to determine the intermediate products in the system of methylene blue removal.It was found that compared to the pristine biochar,the iron-biochar composites could further destroy the central ring structure and aromatic structure of phenothiazine,thus degrading methylene blue more completely.(3)The increase of Fe content promoted the removal capacity of the iron-biochar composites to acid orange 7.Because acid orange 7 had higher polarity than methylene blue and its chemical structure(azo group)was more stable,the maximum adsorption capacity of biocharto acid orange 7 was higher.For 400℃ biochar,with the increase of Fe content,the maximum sorption amounts of acid orange 7 were 74.5 mg/g,84.7 mg/g,96.7 mg/g and 99.9 mg/g,respectively.For 700℃ biochar,the maximum sorption amounts were 63.8 mg/g,64.2 mg/g,65.5 mg/g and 73.8 mg/g,respectively.Sorption dominated the removal mechanisms of biochar to acid orange 7,accounting for 40.6-68.9%of the removal rate.The degradation of acid orange 7 by biochar was a non-radical process.Dissolved Fe3+,and solid FeO and FeO in the iron-biochar composites pyrolyzed at 400℃ and 700℃ played a major role in the process of remoⅥng the acid orange 7.The intermediates determined by LC-MS showed that the degradation of acid orange 7 was more complete by the iron-biochar composites.(4)The Cr(Ⅵ)was easier to be reduced under acidic conditions.With higher electron donating capacity,the iron-biochar composites were excellent electron donors,and hence Fe-containing biochar had a higher reduction efficiency of Cr(Ⅵ)than the pristine biochar.The biochar pyrolyzed at lower temperature was more prone to donate electrons,which enhanced its reduction efficiency of Cr(Ⅵ).The reduction product was determined to be Cr(Ⅲ)through XPS analysis.The positively charged Cr(Ⅲ)was removed through electrostatic adsorption to biochar,complexation with surface functional groups,and ion exchange with iron ions,thus remoⅥng the total Cr.Zn(Ⅱ)was mainly removed by biochar through electrostatic adsorption,complexation with surface functional groups and precipitation.Therefore,the removal efficiency of cationic heavy metal Zn(Ⅱ)was promoted by higher solution pH and lower pyrolysis temperature of biochar.The negative charge of the iron-biochar composites surface was weakened by the loading of Fe.Therefore,BC400 had had higher removal efficiency than BC400Fe11,with the removal rate of 52.1%and 39.5%,respectively.However,because BC700Fe11 had a higher solution pH than BC700,the removal efficiency of BC700Fe11(35.4%)for Zn(Ⅱ)was higher than that of BC700(27.5%).(5)In the combined system of Cr(Ⅵ)and Zn(Ⅱ),the precipitation of Zn(Ⅱ)with OH-could raise proton concentration in the solution,thus promoting the reduction of Cr(Ⅵ)to Cr(Ⅲ);while the sorption of Zn(Ⅱ)on the biochar could proⅥde sorption sites for Cr(Ⅵ),and Zn(Ⅱ)could compete with more polarized Cr(Ⅲ).Therefore,Zn(Ⅱ)could promote the removal of total Cr,and the addition of Cr(Ⅵ)could inhibit the removal of Zn(Ⅱ).The coexisting ions Ca2+ and SO42-had no significant effect on the removal of total Cr and Zn(Ⅱ)by biochar.The higher the concentration of acetic acid,the stronger the buffering effect on the system,which could buffer the pH increase caused by the biochar.Therefore,the addition of acetic acid promoted the removal of total Cr with an increase of 13.8-34.0%,and meanwhile decreased the removal efficiency of Zn(Ⅱ)by 13.6-46.6%.The present study elaborated the removal efficiency of iron-biochar composites on aqueous organic matters with different properties and combined heavy metals,and examined the effect of different iron compounds in iron-biochar composites on the removal of organic matters and the contribution of adsorption and degradation in the removal of organic matter by biochar.It also explored the removal mechanisms of biochar in the combined heavy metal system consisting of variable anion heavy metal Cr(Ⅵ)and cationic heavy metal Zn(Ⅱ),which aimed to provide new alternatives and theoretical foundations for industrial wastewater treatment and environmental remediation of groundwater and surface water. |
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