Preparation And Characterization Of Biochars Derived From Oil Crops And Its Adsorptive Properties For Cr(Ⅵ) And Cu(Ⅱ)

Biochar is a kind of carbonaceous solid material, which is produced from biomass duringpyrolysis under limited or no oxygen condition. Recently, biochar has received muchattention because of its large effects on mitigating climate change, improving soil propertiesand remediating contaminated soil. Due to its large surface area, sufficient surface charge andabundant functional groups, biochar has good adsorption properties, which can absorb heavymetals in soil and reduce the bioavailability of them, ultimately be expected to realize theremendiation of soil contaminated by heavy metals. However, because of heterogeneity ofbiochar and various properties among different heavy metals, the adsorption performance andmechanisms will be various when the biochar and heavy metals vary.In order to explore mechanism of immobilization of heavy metals by different biochar,various biochars were prepared at300~600oC, which were converted from straws andgreaves of typical oil crops in Gansu Province.(Biochars derived from flax (Linumusitatissimum L.) straws are hereafter referred to as LS300~600, cole (Brassica campestris L.)straws as BS300~600, flax(Linum usitatissimum L.) greaves as LO300~600and cole(Brassica campestris L.) greaves as BO300~600); The biochars were characterized byelemental analyzing, surface and porosimetry analyzing, scanning electron micrographs andFTIR spectra; The batch equilibrium adsorption experiment was done in solution system toexamine the adsorption performance of Cr(VI) or Cu(Ⅱ) onto biochars, to analyze therelationship between structures and adsorption properties of biochars together with theprevious characterization results, and to explore the adsorptive mechanism; The interaction ofCr(VI) and Cu(Ⅱ) in multi-metal system was also investigated. The result can provide atheoretical reference for immobilization of heavy metals in soil using biochars. The mainresults of the paper are as following:Various biochars were prepared under limited oxygen condition at300~600℃, labeledas LS300~LS600, BS300~BS600, LO300~LO600and BO300~BO600, respectively.Compared the adsorption capacity of Cr(VI) by homologous biochars under differentpyrolysis temperatures, the optimal biochars which have largest adsorption capacity, i.e.LS600, BS600, LO400and BO400were used for characterization and adsorption experiment.Compositions and structures of LS600, BS600, LO400and BO400were characterized.The elemental analysis shows that the content of C in biochars is the highest, following byoxygen, whereas H, N and S are little. The calculated ratios of H/C or (O+N)/C can providesome predictions about polarity and aromatic. LS600and BS600have higher degree ofaromatization, whereas LO400and BO400have stronger polarity and hydrophilicity. Theresults of surface area and pore distribution show that BET surface area of four biochars arebetween1.71~157.52m2/g, total pore volume of a single point are within3.18×10-3~1.03×10-1cm3/g and the average pore diameter are between24.52~82.61. The BET surface area and pore volume of LS600and BS600are much larger than those of LO400andBO400, which suggest that LS600and BS600have a relatively well-developed pore structure,and LO400and BO400have little porous structure. SEM results also strongly proved the twostraws biochars have more decentralized structure with large numbers of pores, and twogreaves biochars have smooth surface and few pores. It can be concluded that LS600andBS600have the structural advantages to adsorb heavy metals. Surface functional groups wereanalyzed by FTIR spectra, and the results indicate that biochars often contain hydroxyl,carboxyl and lactone functional groups.The effects of pH values, time, initial Cr(VI) concentration, kinetic and thermodynamicson adsorption were studied. It shows that the pH plays a significant role on the adsorption ofCr(VI) on biochars. The adsorption of Cr(VI) include rapid adsorption and slow balance withtime. With an increase of the initial concentration of Cr(VI) from10mg/L to300mg/L, theadsorption capacity of Cr(VI) on biochars gradually increase. The adsorption kinetics ofCr(VI) are well described by the pseudo-second-order-kinetic model and the correspondingcoefficients R2for LS600, BS600, LO400and BO400are0.9970,0.9902,0.9904and0.9698,respectively. The adsorption isotherm follows the Langmuir model, whose coefficients R2arebetween0.9027and0.9687. Compared with the characterization, it can be concluded that themechanism of Cr(VI) adsorption onto biochars mainly include chemical bonds andelectrostatic attraction.The effects of pH values, time and initial concentration on the adsorption of Cu(Ⅱ) ontobiochars were also studied. The results show that slightly acidic condition is more conduciveto the adsorption; adsorption capacity of Cu(Ⅱ) gradually increase over time; in the range of10~300mg/L of initial Cu(Ⅱ) concentration, adsorption capacity is increases. Adsorptionkinetics of Cu(Ⅱ) onto biochars follow the pseudo-second order model, and the isotherm fitsthe Langmuir model. The main mechanisms of adsorption are chemical bonds and ionexchange reaction.Compared with the single solution system, the interaction of Cr(VI) and Cu(Ⅱ) inmulti-metal system was also investigated, the adsorption capacity of Cr(VI) is slightlyenhanced and of Cu(Ⅱ) is significantly higher than those of in single system, which indicatesthat two metals are favorable synergistic effects. The main mechanisms including chemicaladsorption of Cr(VI) and Cu(Ⅱ), as well as electrostatic attraction between two metals.