Performance And Mechanism Study On Engineered Hydrochar Modified By Metal Salts Or Industrial Red Mud For Heavy Metals Immobilization

With the development of industrial processes in modern generations,the contamination of heavy metals on surface water bodies as well as underground systems have gained extensively attention in recent years.Various technologies were applied to solve these problems,and among them,adsorption,especially biomass-derived adsorptive materials,are widely employed for wastewater purification with the advantages of cost-effectiveness,facile preparation,and easy operation.However,raw bio-adsorbents are lack of efficiency,and further modifications are normally indispensable for these kinds of functional materials.Using the common,abundant,and environmentally friendly mineral elements,such as iron,magnesium,and aluminum,for biomass fabrication is an effective method,which could obtain magnetic or surface optimized biochar with the relatively easier steps.The conventional method to obtain this engineered biochar is a two-step coprecipitation process,which is complex,and the obtained biochar is normally lack of efficiency and insufficient recyclability.Hence,more fabrication method is needed for bio-adsorbent preparation.Hydrothermal carbonization is a novel and emerging method for bioresource convention,which attracted a lot of attention in recent decades.Under the high temperature and pressure of hydrothermal process,biomass could be converted into hydrochar(HC),while external functional materials also could be loaded into autoclave which would be loaded onto the surface of adsorbent.Hence,in this work,iron slats were applied to magnetize HC and successfully prepared Fe fabricated hydrochar(Fe/HC).Besides,Mg and Al salts were employed for biomass surface modification,and Mg Al fabricated hydrochar(Mg Al/HC)was obtained with the three-dimensional layered double hydroxide(LDH)structure loaded on its surface.Furthermore,under the conception of waste control by waste,an abundant industrial byproduct,red mud(RM),was applied to modify HC,as the substitution of mineral salts,and novel RM/HC adsorbent was prepared.Last but not least,a fixed-bed column system was employed to evaluate the dynamic adsorption capability of different obtained adsorbents,while practical electroplating effluent was conducted as target wastewater,and was applied to analysis the industrial applicability of various HC adsorbents.The major conclusions of this word are as follows:(1)Magnetic Fe/HC adsorbent was prepared by adding Fe salts into hydrothermal autoclave,and Fe was existed as?-Fe₂O₃ on the surface of Fe/HC.The Hg(II)removal mechanism was analysis by changing heat treatment atmosphere of magnetic Fe/HC,and the major mechanisms were formation of complexation and coprecipitation effects.Fe ions were tightly adhered on the pores of Fe/HC under this one-pot hydrothermal process,leading to the good efficiency and better recyclability of Fe/HC,compared with magnetic biochar prepared by the conventional coprecipitation approach.(2)Mg Al/HC adsorbent was obtained by fabricating Mg Al-LDH nanosheets onto the surface of HC,which demonstrated excellent surface area(enhanced with 44.5%)and copious oxygen-contained functional groups,resulting in the good adsorption capacity for heavy metal immobilization.Different mechanisms were obtained for Pb(II)and Cr(VI)removal.Moreover,simultaneous removal of these two contaminants from the binary system,the Pb Cr O₄ complexation was formed and synergetic effect was observed.(3)RM was firstly conducted as the source of mineral elements for HC fabrication,and magnetic RM/HC was successfully synthesized.Interestingly,the high pyrolysis temperature of RM/HC for adsorbent magnetization also facilitated the convention of Fe(III)to Fe(II)and Fe(0),while the organic components in HC acted as reductants of iron species.The synthesized RM/HC adsorbents with reactive Fe(0)/Fe(II)species demonstrated excellent reductive and adsorptive abilities for Cr(VI)elimination.(4)A fixed-bed column system was established to evaluate the dynamic adsorption potential of Fe/HC,Mg Al/HC,and RM/HC.Simulated wastewater as well as practical electroplating effluent were used as target heavy metal polluted solution,which applied in this dynamic adsorption system.The influence of essential operation parameters,including initial pollutants concentration,p H,velocity of inlet solution,as well as length of fixed-bed,were comprehensively investigated,and various dynamic adsorption models(Adams-Bohart model,Thomas model,Yoon-Nelson model)were employed to mimic the surface interactions and predict the breakthrough curves of HC adsorbents.The economic analysis was applied to evaluate the applicability of these hydrochar based adsorbents.Based on the aforementioned conclusions,this study focused on the preparation of efficient heavy metal bio-adsorbents via hydrothermal carbonization with the modification of common mineral elements,e.g.Fe,Mg,Al,which obtained from mineral salts and industrial byproducts.The applicability and practicality on real industrial effluent purification of prepared magnetic or surface modified adsorbents were also investigated.