Adsorption Efficiency And Mechanism Of Iron-modified Rice Husk Biochar For SMT In Water

Biochar(BC)is a product of biomass conversion through pyrolysis or incomplete combustion.It not only solves the problem of recycling a large number of urban and agricultural wastes,but also reduces environmental contamination by adsorption and distribution with its special structures.However,the adsorption capacity of traditional biochar is limited,and the structure-function relationship and mechanism between biochar and pollutant are unclear.In this study,two types of rice husk BCs,i.e.,BC-organic composite materials(biochar mycelial pellets(BC-MP))and BC-inorganic particle composite materials(magnetite-coated biochar(MBC))were prepared.Through structural characterization and comparison of adsorption performance,the iron-modified biochar with the best adsorption capacity was selected to investigate the structure-function relationship and mechanism for sulfonamides(SAs)adsorption.Structural characteristics of BC-MP and MBC and their adsorption performance for sulfamethazine(SMT)were explored.The coupling mechanism of biochar(BC)and mycelium(MP)in BC-MP is clarified as surface adsorption,hydrogen bonding between extracellular polymer and BC,and electrostatic interaction.Compared to single MP,the stability of BC-MP is improved with increased structure density,and the number of active adsorption sites increases due to larger specific surface area.The existing form of iron in MBC is magnetite.With structural modification by magnetite,the specific surface area of MBC decreases,and the polarity and pore volume increase.Fe-O bond forms on the surface of magnetite modified biochar.The apparent equilibrium time for MBC(120 h)to adsorb SMT is lower than that of BC-MP(144 h),and the apparent adsorption capacity of MBC(0.083 mmol/L)for SMT is 6.15 times higher than that of BC-MP(0.013 mmol/L).MBC has higher application potential than BC-MP due to its high pore volume and complex changes of structure and composition.Structural evolution characteristics of MBC under different preparation conditions(pyrolysis temperature(T)and iron/carbon mass ratio(r))were analyzed.MBC prepared at specific T(350,500 or 700℃)and r(0.1-2)is referred as MBCr-T.When r is 0,the samples referred as BCT.The increased pyrolysis temperature(from 350℃to 700℃)stimulates the alteration of original BC(r=0)organic carbon from amorphous phase to aromatic phase.Meanwhile,the surface polarity decreases,but the specific surface area,pore volume and the degree of surface defects increase.For MBC with low r values(0.1-0.5),magnetite particles are dispersible on BC surface.When r further increases,iron-containing particles agglomerate and block the pores of BC.Compared to BC,without considering pyrolysis temperature,the polarity and pore volume of MBC increase.To be noticed,the appearance of Fe-O structure causes the Fe-O-C complexation confirmation(the formation of oxidized iron on the surface of biochar),resulting in the alternative appearance of inorganic and organic structures in BC,and further accelerating hydroxylation/hydration on the ferric oxide/aromatic layer.At medium and low pyrolysis temperature(350℃and 500℃),iron compositions benefit the condensation of the carbon structure.While at high pyrolysis temperature(700℃),the magnetite coatings promote the defects occurrence at the surface of BC.The structure-function relationship between MBC and SMT was analyzed.For BC without coating(pyrolysis temperature 500℃),it shows a relatively high adsorption capacity of 0.028 mmol/g due to its structure that promotes turbulence and SMT adsorption.Comparatively,MBC0.5-700 has a much higher adsorption capacity for SMT(0.083 mmol/g)due to its highest degree of structural defects under our experimental conditions.Two-compartment kinetic model appropriately describes both the adsorption of SMT on BC and MBC.It could distinguish the contributions of different components in adsorbents.For BC,fast adsorption tends to occur in amorphous carbon components,while slow adsorption tends to occur in condensed carbon components.For MBC,magnetite modification increases the components of fast-adsorption,further accelerated the fast adsorption kinetic rate.The adsorption mechanisms include the"species adsorption model"dominated by the species of SMT in aqueous solution and"deviated species adsorption model"dominated byπbond-assisted hydrogen bond between BC/MBC and SMT.Compared to BC,the magnetite coating not only introduces more hydrated oxides as hydrogen bonding sites for BC,but also improvesπ-bond polarity.Persistent free radicals(PFRs)on BC surface and/or active ferric oxides on MBC may cause partial degradation of SMT,but such small fraction of SMT degradation does not impact the estimation of the capacity of SMT adsorption by BC and MBC.Dubinin-Ashtakhov model appropriately describes the adsorption of SMT on BC and MBC.The affinity and capacity of MBC for SMT adsorption is higher than that of BC,because of the surface hydration of iron oxide and carbon matrix,and the increasing heterogeneity of MBC.Among all the MBC prepared in this study,MBC0.5-700 has the highest estimated adsorption capacity of 0.3207mmol/g.Site energy theory based on the adsorption isotherm model was used to further clarify the adsorption mechanisms at the BC-SAs interface.The energy distribution curves of BC and MBC sites show a unimodal distribution under the medium temperature condition(500℃),but show a bimodal distribution under the low temperature(350℃)or high temperature(700℃)condition.Moreover,there is a significant peak difference between BC and MBC in the high-energy region.MBC0.5-700 shows the highest peak energy,average site energy(E_m)and heterogeneity(σ_e~*),which further verifies its optimal efficacy for SMT adsorption.The adsorption capacity of BC and MBC is positively correlated to their E_m and pore volume,and E_m grows with increasing oxygen content,indicating thatπ-πinteraction and pore filling are important mechanisms for SMT adsorption on BC and MBC.The site energy distribution of MBC0.5-700 to four selected types of SAs is significantly different in both high-energy and low-energy areas due to their different molecular properties.E_m is negatively correlated to SA molecular size and solubility,indicating that pore filling,π-πinteractions and hydrogen bonds are the main mechanisms and leading to the adsorption differences of various types of SAs.