Preparation Of Corn Straw-based Biochars And Biochar-supported NZVI Composites For Removal Of Atrazine In Water

The redundant solid biomass wastes produced in agricultural production,such as crop straw,accumulation and incineration,and the large-scale use of pesticides,such as atrazine herbicide,have led to serious environmental pollution and threatened human health.Reasonable disposal of agricultural straw is of great significance for the utilization of waste biomass resources and environmental improvement;efficient and low-cost solution to the problem of organic pollution caused by large-scale application of pesticides is of great significance for environmental pollution remediation and drinking water safety.Conversing waste biomass into biochar can not only scientifically utilize waste biomass,but also use the produced biochar to remove contaminants in the environment.At present,technologies of using biochar and its functional materials to adsorb or activate persulfate advanced oxidation for organic pollutants removal have attracted much attention in the field of environmental pollution remediation due to their advantages of high efficiency,rapidity and environmental friendliness.In this study,corn straw is used as biomass raw material to design high-performance biochars and explore their formation mechanism.Atrazine is used as the target pollutant to study the adsorption performance of the representative biochar.Simultaneously,biochar-supported nZVI composites were synthesized to activate persulfate(PS,Na₂S₂O₈,₂₈^2-)for oxidation degradation of atrazine.The main results and conclusions are as follows:?1?The hydrothermal carbonization process of corn straw was studied.Corn stover is hydrothermally carbonized at different temperatures?180?,200?,220?,240?and 260??for 4 h,for different residence times?1 h,2 h,4 h,12 h and 24 h?at 220?,or by adding acid?1%and 2%?at 220?for 4 h.The yields of hydrochars and changes of fixed carbon ratio,element composition,surface functional group,micro structure and molecular composition are analyzed,and the nitrogen recovery in the by-products is analyzed to further understand the elemental transformation in the progress of hydrothermal carbonization.The results indicate that a higher temperature,longer residence time or acid addition can cause lower yields and simultaneously reduce the H/C?0.76-1.33?and O/C?0.19-0.68?ratios of the hydrochars by 13.1%-50.5%and 9.3-73.4%compared with feedstock,respectively,which is consistent with the results of functional group analysis.With the increase of reaction degree,the number of functional groups on the hydrochar surfaces decreased,which indicated that dehydration or decarboxylation occurred in the hydrothermal carbonization process of corn straw.The SEM images of hydrochars show that the formation of carbon spheres is the cause of changes in the surface morphology of hydrochar.Py-GC/MS analyses showed that furan compounds decreasing or disappearing and phenolic compounds increasing significantly promotes the aromaticity of hydrochar during the hydrothermal carbonization,especially under severe conditions.The nitrogen recovery efficiency increased with the reaction severity,so that the liquid by-product from corn stovers by hydrothermal carbonization may be used as a fertilizer due to its abundant nutrients.?2?Porous biochars with high performance are further prepared and characterized.Two types of porous biochars were prepared by using hydorchar or corn straw as carbon precursors and potassium oxalate as activator,which were mixed uniformly at a mass ratio of 1:1 and annealing at800?for 1 h in inert gas?high purity argon?.The characterization results show that the hydrochar-derived porous biochar retains the carbon microsphere structure of the original hydrochar,and there is a partial carbon sheet structure.While the microstructure of the corn straw-derived porous biochar presents a graphene-like nano-carbon sheet structure.The specific surface area and the total pore volume of hydrochar-derived porous biochar are 1737 m²/g and 0.8555 cm³/g,respectively,which are larger than those of corn straw-derived porous biochar?1253 m²/g and 0.5604 cm³/g?,which may be due to the fact that the original closed plug holes and channels of hydrochar are opened and expanded during the high-temperature activation.The result of thermogravimetric analysis?TGA?shows that a large amount of gas is produced by potassium oxalate at high temperature to form pore structure,and K₂CO₃ can be used as in-situ template to form nano carbon sheet structures.In addition,based on the characterization results,it is found that the hydrochar-derived porous biochar is a kind of super-small mesoporous/microporous carbon?MPC?,and the corn straw-derived porous biochar is a kind of graphene-like carbon sheet?CS?.?3?The influence of the ratio of hydrochar and potassium oxalate on the performance of MPC was studied,and the representative MPC was selected to study the adsorption performance and mechanism of atrazine in aqueous solution.The results show that when the ratio of hydrochar to potassium oxalate is 1:3,the specific surface area of porous biochar is the largest?MPC-1:3?,reaching 2523 m²/g.The as-fabricated carbon material rapidly removes atrazine in the first 3 h at the initial concentration of 20 mg L^-1 with an adsorption efficiency of 93.6%.The adsorption of atrazine onto MPC-1:3 was in accordance with pseudo second-order kinetics and Langmuir isotherm adsorption model.The theoretical maximum adsorption capacity was 501.3 mg/g,which was better than that of the representative carbon adsorbents reported in other literatures.The small mesoporous/microporous structure of MPC matches the molecular movement diameter of atrazine?0.72 nm?,which is conducive to pore packing.Meanwhile,?-?interaction,H-bond interaction and hydrophobic interaction are the main mechanisms of efficient adsorption of atrazine by MPC.In addition,95%of the adsorption capacity of MPC-1:3 can be recovered by simple annealing treatment.?5?CS-supported nanoscale zero-valent iron?nZVI?composites?nZVI@CS?were designed to enhance atrazine removal by persulfate?PS?activation.The removal efficiencies of nZVI@CS-800were higher than that of nZVI-PS,implying that the synergy between CS and nZVI was achieved and promoted the removal efficiency of atrazine.CS facilitates the dispersion of nZVI due to its vast SSA,which could prevent nZVI particles from self-aggregation.Simultaneously,the adsorption capacity of composite promotes the degradation rate of atrazine.Radical quenching and electron spin resonance tests indicate that both SO₄?^-and?OH are produced in the nZVI@CS-PS systems,and SO₄?^-plays a more important role in the oxidative degradation of atrazine.CS can directly activate PS to generate SO₄?^-,and as an electron shuttle,it can promote the conversion of Fe³⁺to Fe²⁺and improve the activity of the catalyst.Therefore,the nZVI@CS can enhance SO₄?^-generation,accelerating atrazine degradation.The oxidative degradation of atrazine is favored at high temperature or low p H,while it is inhibited when the amount of PS or catalyst becomes excessive.Furthermore,it may be impacted by organic compounds or anions in the environment.The composite material shows good reusability,which indicates that the carbon material improves the stability of the material and delays the oxidation of the material surface.