| Chromium(Cr)is a common toxic metal in wastewater,which comprises mainly of two stable oxidation states such as hexavalent Cr(Cr(Ⅵ))and less toxic trivalent Cr(Cr(III)).Zero-valent iron(ZVI)has a remarkable ability to remove Cr(Ⅵ).However,less stability and dispersion of ZVI could enhance surface passivation and steric attraction between particles and thus hinder electron transfer in Cr(Ⅵ)reduction.Biochars(BCs)are biomass-derived carbonaceous materials with abundant porous structure and electroactive properties,thus regarding as a supporter for dispersing ZVI.At specific temperatures,BC properties e.g.,redox and electron transfer capacity are largely inherited from its precursor biomasses compositions(i.e.,cellulose,hemicellulose,lignin,and ash).The variation in electron donating capacities(EDCs)of BCs prepared by different biomass can thus give rise to different electron transfer capacity of ZVI,resulting in significant difference in terms of Cr(Ⅵ)reduction.However,there is no systematic investigation on the contribution of each major component in biomass to overall BC’s redox properties,which is of vital significance to improve performance of ZVI and Cr(Ⅵ)reduction.In this study,10 kinds of biomass samples with different compositions in 2 types(stalk-type and wood-type)were applied for investigating the influence of lignocellulosic constituents(cellulose,hemicellulose,lignin,and ash)on electron transfer and Cr(Ⅵ)reduction capacity of prepared biochars-supported zerovalent iron(ZVI/BCs).Also,either single or multiple biopolymers(cellulose,hemicellulose,and lignin)was pyrolyzed at 600℃ to prepare BCs to support ZVI(ZVI/BC).The ash,to mimick minerals in natural pinewood,was co-pyrolyzed with triple components to understand its contribution to BC’s properties.Through characterization,adsorption experiment,and electrochemical test,the decomposition behavor of precursor biopolymers was clarified,and the contribution of each component to the electron transfer capacity in Cr(Ⅵ)reduction by ZVI/BC composites was discriminated.The main results are as follows:(1)Cellulose,hemicellulose,and lignin were the main components of all the samples,and wood-type biomass showed a high lignin content with the average value more than 35.58 wt.%,while stalk-type biomass mainly contained cellulose and hemicellulose.The content of ash in wood-type biomass was quite limited(～4.61 wt.%),while stalk-type biomass had higher ash content,among which rice biomass even reached up to 17.80 wt.%.As to the element composition of different samples,it can be found that the N contents of the pyrolyzed biochars were similar,while the C and H contents followed the order of wood-type>stalk-type.(2)The EDCs of biochars derived from ten biomass pyrolyzed at 600℃ was from 0.069 mmol e-g-1to 1.094 mmol e-g-1 and the electron accepting capacity(EAC)was from 0.040 mmol e-g-1 to 0.604 mmol e-g-1.The redox properties of both stalk and wood chars produced at 600℃ were dominated by the electron donating moieties.The EDC value of wood-type biochars was 0.633 mmol e-g-1 on average,while that of stalk-type biochars was 0.134 mmol e-g-1,suggesting that biomass feedstock had significant influence on the redox properties of biochars.(3)The partial least-squares(PLS)modeling was implemented to evaluate the quantitative contributions of redox properties and structure characterizion to electron transfer capacity in Cr(Ⅵ)reduction by ZVI/BC composites.Cr(Ⅵ)reduction capacities were positively correlated with EDC values,whereas the peak intensity ratio of the D to G band(ID/IG)and Tafel corrosion potential were negatively correlated.The higher graphitization and EDC values of biochars could be beneficial to the electrochemical performance of ZVI/BC,and further promote Cr(Ⅵ)reduction.Principal component analysis(PCA)was used to evaluate the contribution of different biomass components(cellulose,hemicellulose,lignin,and ash)to Cr(Ⅵ)reduction.The results showed that cellulose or hemicellulose was beneficial for ZVI to transfer electrons to Cr(Ⅵ).(4)The pyrolysis behavior of single or mixed biomass components at 20-600℃ under nitrogen was compared by Thermogravimetric analyses(TGA)analysis.The weight loss of cellulose,hemicellulose,and lignin reached peak values of 0.37,1.35,and 0.11 wt.%/℃.Lignin was more thermally stable and decomposed slowly.Cellulose(BCC)and hemicellulose(BCH)-derived BCs has greater C content,H/C ratio,surface area and mass loss than BCs derived from lignin or lignin-containing biopolymer blends(BCLX).(5)As per sorption and X-ray photoelectron spectroscopy(XPS)analysis,Cr(Ⅵ)reduction by ZⅥ harbored by BCC and BCH was 19.69-17.38 g kg-1,compared to 7.01-5.47 g kg-1 for BCLX.ZVI/BC prepared by three-biopolymer blends with(14.17 g kg-1)or without(13.83 g kg-1)mineral approximated pinewood-BC(BCp)(14.32 g kg-1)for Cr(Ⅵ)reduction,suggesting minerals are not important constituent.Tafel analysis showed BCC and BCH,with lower ID/IG ratio owing to greater graphitization,were more conducible to transfer electron of ZVI in Cr(Ⅵ)reduction than BCLX.Thus,cellulose,hemicellulose,and lignin can offer a good prediction of property of natural biomass,in which BCC and BCH favor electron transfer of ZVI but BCL is not electroactive.(6)Pine and bamboo biomass with different lignin structures were used as matrix to prepare biochar-supported ZVI composites.Langmuir isotherm showed removal capacities of Cr(Ⅵ)followed the order of PBC/ZVI(5.93 g kg-1)>BBC/ZVI(3.61 g kg-1)>BBC(3.55 g kg-1)>PBC(2.59 g kg-1).Desorption and XPS of four Cr-spent sorbents suggested reduction accounted for 79-88%of Cr(Ⅵ)detoxification.PBC/ZVI can reduce Cr(Ⅵ)to a larger extent,corresponding to greater quantity of transferred electrons than BBC/ZVI.Lignin-BC had lower electron transfer rates than cellulose-BC and hemicellulose-BC.BC prepared by lignin extracted from pinewood exhibited faster electron transfer rate than that from bamboo.Thus,unfavorable lignin type in bamboo likely resulted in compromised electron transfer of BBC and low Cr(Ⅵ)reduction by BBC/ZVI. |
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