| Biochar is an insoluble stable carbon-rich and highly-aromatic material that isprepared during the pyrolysis of biomass in the present of little or no oxygen.Recently, biochar have received much attention as an effective adsorbent for heavymetal removal because of their several unique properities, such as widespreaddistribution of raw material, one-step process, low producing cost, aboundant surfacefunctional groups and higher specific area compared with raw biomass. However,biochars prepared under traditional nitrogen atmosphere present limited adsorptioncapacity. Therefore, it is urgent to develop a new method for achievement of betteradsorption performanceAs a typical activation agent, carbon dioxide can gasify pyrolytic carbon at hightemperature, significantly enhancing its specific area and multiporous structure andthen transforming pyrolytic carbon into activation carbon. Taking advantage of thisfunction, a method was proposed to improve biochars’ ability for heavy metalremoval by prolyzing raw biomass under CO2atmosphere.In this experiment, Spartina alterniflora, a salt-marsh hydrophyte and RiceStraw, a freshwater terrestrial plant were employed as raw biomasses. Biochars werepreprared under various pyrolytic atmospheres and temperatures (300-700oC) andtheir physical and chemical properities were characterized by yield calculation,elemental analysis, TG-MS, BET and scanning electron microscopy. Afterwards,Cu(II) adsorption performance of biochars was investigated by batch adsorptionexperiments and optimal Cu(II) sorption conditions were found out via exploring onthe effects of pyrolytic atmosphere, temperature and initial solution pH. The keneticsof heavy metal sorption and inorganic ions release were also studied. Finally,potential mechanisms of Cu(II) sorption were proposed based on the analysis offourier transform infrared specturms, supernant components, acid and water washedtests and XRD results. The role of CO2involved in Cu(II) removal was investigated as well. The results were listed as follows:(1) Thermal cracking was the main process at low temperature interval sinceCO2only served as inert atmosphere. After600oC, the mass loss rate of biocharswas determined by both CO2gasification and thermal cracking. Gasification greatlyenhanced specific area and multiporous structures (close to the level of activationcarbon) and lowered down the carbon contents of biochars at700oC. But elementaloxygen was introduced into biochars via oxygation reaction during the same process.(2) The initial gasification temperature of Spartina alterniflora-derivedbiochar (SABC) was lower than that of Rice straw-derived biochar (RSBC). Possibleexplanation may be due to the lower Si and higher Na contents in Spartinaalterniflora.(3) The optimal Cu(II) sorption conditions for SABC were being pyrolyzedunder700oC and CO2atmosphere with initial solution pH5.0. The maximum Cu(II)sorption capacity for SABC C700was89.12±2.77mg/g,50%higher than thatgenerated under N2atmosphere.(4) The kenetics of Cu(II) removal by SABC C700presented obvious twostages with highly correlation with that of Ca2+/Mg2+in shape and equilibrium time.(5) The surface founctional groups like carboxyl could complex with Cu(II).However, the strong comparative adsorption by H+inhibited the Cu(II) removal.Unsaturated bonds on the surface of biochars could be oxygated by CO2andtransformed into carbonxyl, thus increasing the adsorption capacity of biochars atlow pH.(6) K+/Na+were not involved in the electrostatic ions exchange as expectedbut simple dissolution process after analyzing the surpertant componts, whichimposed trivial contribution to Cu(II) removal. Soluble Ca2+/Mg2+increased solutionpH, prompting more hydrolyzed copper formation that reduced the electrostaticrepulsion between adsorbate and adsorbent and Cu(OH)2precipitation. This processwould account for about75%of maximum Cu(II) removal amount. At relatively high pH, insoluble Ca2+/Mg2+released CO32-slowly, where basic cupric carbonatewas formed. On the other hand, the Ca2+/Mg2+bond to fountional groups particaptedin the complexed ion exchange with Cu(II). The two prcess with Ca2+/Mg2+wouldoccpupy about24%of maximum Cu(II) removal amount. CO2inhibited the thermaldecomposition of CaCO2and MgCO3and improved the porous structure, allowingbetter reaction between inorganic cations and Cu(II). |
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