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biochar
发布日期:2018-12-21  作者:邵晨  浏览量:99

Synthesis andcharacterization of a novel MnOx-loaded biochar and its adsorption properties for Cu2+ inaqueous solution

http://dx.doi.org/10.1016/j.cej.2013.12.061
Introduce:Biochar (BC) is produced by oxygen-limited pyrolysis of carbonrich biomassessuch as crop straw, sludge and wood.BC has strong adsorption affinity toorganic contaminants and heavy metals due to the porous structure andheterogeneous surface chemistry.The intense heattreatment inevitably results in fewer ion exchange functional groups on BCsurfaces , which to a great extent reduces the binding sites of BC to metalions.Thus, some efforts have been made to enhance the adsorption capacity of BCthrough designing BC-based composites (i.e., engineered BCs), includingFe2O3-coated BC , MgO-BC nanocompositeand nano-zerovalent iron-loaded BC .
Treatment:BC was produced from corn straws through slow pyrolysis at 600 ℃ for 3 h in amuffle furnace under N
2 atmosphere. The obtained BC was ground topass through a 0.15 mm sieve, and 5 g of the BC sample was soaked with 40 mL ofKMnO4 solution. The weight ratio of KMnO4 to BC wasselected as 1:40 (2.5%), 1:10 (10%) and 3:5 (60%), and thus the final productswere labeled as 2.5%MBC, 10%MBC, and 60%MBC, respectively. The suspension wasmixed ultrasonically for 2 h and was then oven dried at 80 ℃. The mixture of BCand KMnO4 was heated at 600 ℃ for 0.5 h under N2 to produceMnOx-loaded BC. The obtained samples were rinsed thoroughly with deionizedwater to remove impurities and dried at 80 ℃. Untreated BC was also included asadsorbent for comparison.


Adsorptionexperiment: The adsorption experiments were performed in 40-mL glass vials at25 C. The Cu2+ solutions were prepared by dissolving its nitrate salt,purchased from Sigma–Aldrich, in 0.01 mol/L NaNO3 as background electrolyte.The initial concentration of Cu2+ in the solution was in the range of 6.35–127mg/L. 100 mg of the adsorbents and 20 mL of Cu2+ solutions were added to thevials. The initial pH of the solutions was adjusted to 6.0 ± 0.1 by 0.1 mol/LNaOH and HNO3 solution. All the vials were sealed and shaken at 150 rpm in arotary shaking incubator for 24 h to reach apparent equilibrium based on thepreliminary study. Then the solutions were sampled and centrifuged, theconcentrations of Cu2+ in the supernatant were determined using AAS. Alladsorption experiments were conducted in triplicate.
Desorptionexperiment: For desorption experiment, 100 mg of Cu-loaded adsorbents from theadsorption test was added into 20 mL 0.01 mol/L NaNO3 solution after washingand drying. The vials were shaken at 150 rpm for 24 h to reach apparentequilibrium, then the Cu2+ concentrations in the solution were analyzed.

Result:

After KMnO4 modification, therelative element content of C and H decreased sharply. In contrast, as the KMnO4ratio increased, the volume O content increased from 5.16% (BC) to 25.6% (60%MBC). The measured surface O content also increased from 15.3% to 38.0%. Theincreased surface Mn content on the treated BC indicates the deposition ofmanganese oxide.


To further examine the properties of themanganese oxide on the surface of the KMnO4 treated BCC, the samples weresubjected to SEM-EDS and XPS analysis. The SEM image shows that the surface of2.5% MBC consists of a porous structure that looks smoother than 10% MBC and60% MBC. It was found that a layered structure containing a large amount ofvacant manganese oxide was formed on the surface of 10% MBC, however, thecarbon surface was gradually covered by the agglomerates of the bulk particles,and the manganese oxide load was increased to 60%. EDS analysis confirmed thepresence of manganese oxide on BC. As the KMnO4 ratio increased, the atomicpercentages of surface Mn and O increased from 1.06% and 6% to 5.6% and 20.4%,respectively.


Comparedwith 2.5% MBC and 10% MBC, 60% MBC has a lower adsorption capacity for Cu2+,indicating that the proper surface loading of MnOx to BC is one of the keyfactors controlling adsorption. Previous studies have shown that thehigh-energy adsorption sites of metal ions are mainly located at the edges,defects and vacancies of the manganese oxide interlayer. However, excessiveloading of MnOx (e.g., 60% MBC) may form agglomerates of bulk particles on thecarbon surface. Therefore, 10% MBC has more Cu 2+ binding sites onthe surface than 60% MBC, and thus has a higher adsorption capacity for Cu 2+.


As thesurface MnOx content increases, the amount of Cu2+ desorption in allsamples is greatly reduced. Calculate the maximum desorption rate of Cu2 +(% = desorption of Cu2 + / corresponding Cu2 + adsorption)and follow BC (11.6%) > 2.5% MBC (7.9%) > 10% MBC (0.7%) > 60% MBC(0.66%) order. The results show that the micro/nano-MnOx coating can greatlyimprove the binding affinity of Cu2+ to the BC surface, thusreducing its desorption of BC.


Conclusion: A novel engineered adsorbent(i.e., MnOx-loaded BC) was successfullysynthesized via KMnO4 modification of corn straw BC in a nitrogen atmosphere at600 _C. Layered structures of micro/nano-MnOx were well dispersed on the BC surface heattreated with10% KMnO4. The unique nanostructure makes the MnOx-loaded BCs has much stronger adsorption capacity for Cu2+than original BC with the maximum adsorption capacity as high as 160 mg/g. Theincreased adsorption of Cu2+ on the MnOx-loaded BCs was mainly due to the formation of surfacecomplexes with MnOx and O-containinggroups. Besides, other mechanisms including cation-exchange and cation-p bondingmay also be involved in the Cu2+ adsorption. Meanwhile, the desorption dataevidenced that the adsorbed Cu2+ on MnOx-loaded BCs was more stable than on original BC,indicating that MnOx/BC composites, as alow-cost adsorbent, may be more effective in various environmentalapplications, such as wastewater treatment or immobilization of heavy metals(e.g., Cu2+) in contaminated soils.


Adsorption of Cu(II)and Cd(II) from aqueous solutions by ferromanganese binary oxide
biochar composites

https://doi.org/10.1016/j.scitotenv.2017.09.220

Sorbent preparation:
The synthesis of BC is described elsewhere (Song et al., 2014). Briefly,untreated BC, used as a reference adsorbent, was produced by slow pyrolysis ofcorn straw at 600 °C for 2 h in a mufflefurnace under an atmosphere of N2 (nitrogen flow rate=300 cm3/min). FMBC were prepared as follows. BC(5 g) was immersed in a mixture of KMnO4 (0.24 M, 40 mL) and Fe(NO3)3 (0.18 M,40 mL) solutions, ultrasonically dispersed for 2 h, dried in a water bath (95°C) for 22 h, and finallypyrolyzed at 600 °C under N2 for 0.5 h to obtain composites with an Fe: Mn: BCmass ratio of 1:3:32. The obtained samples were thoroughly rinsed, washed withdeionized water, and dried in an oven at 80 °C for 4 h.


Adsorption experiments:
For adsorption kinetic studies, 0.5g samples were added to Cd(II) or Cu(II)solutions (500mL, 50mg/L) stirred at 1000 rpm. Aliquots (0.5mL) were sampled atdifferent time intervals (1, 3, 10, 30, 60, 120, 180, 225, 280, 720, and 1440min), filtered through a 0.22-μm filter, and analyzed by AAS. All adsorption experimentswere performed in 40-mL brown glass vials, which were filled with BC or FMBC (0.02g). Subsequently, aqueous NaNO3 (0.01 M; 19.5,19.0, 18.5, 18.0, 17.5, 17.0, 15.5, and 14.0 mL) was added as a backgroundelectrolyte, and the mixture was diluted to 20mL with a Cu/Cd solution (0.01M),achieving initial Cu/Cd concentrations of 0190 mg/L.

The influence of pH onadsorption was investigated by varying the pH between 3 and 6 using 0.10 M NaOHand HNO3 solutions. Adsorption thermodynamics was examined at temperatures of15, 25, and 35 °C. The effect of solution ionic strength on adsorption wasinvestigated by varying the concentrations of NaNO3 (0.0010.1 M), and the influence of humic acid (HA) concentration was determined by varying itbetween 5 and 60mg/L. Suspensions were shaken using an end-over-end tumbler ina thermostatic oscillator at 25 ± 0.5 °C for 22 h, left to stand for 2 h toreach equilibrium, and filtered through aWhatman No. 42 filter, with the concentrations of Cu(II) and Cd(II) in thefiltrate subsequently determined by AAS. The adsorptioncapacity of FMBC was calculated as the difference between concentrationsdetermined before and after the adsorption equilibrium was established. Allexperiments were repeated fourfold. A standard sample was examined after everyten samples. The recovery percentages equaled 94.6103.4%, with the relative standard deviations of determination being <2.6%.<>

Result:


The Cu(II) and Cd(II) adsorption capacities of FMBC increase with time(Fig. 1), for example, a rapid increase in Cd(II) adsorption capacity duringthe first 200 minutes. Compared to BC, FMBC reaches the adsorption equilibriumfaster. The Cu(II) adsorption capacity of FMBC increased rapidly during thefirst 250 minutes and reached equilibrium after 750 minutes, which wasdifferent from the Cd(II) results. Therefore, during the first 3 hours, therate at which FMBC adsorbs Cu(II) and Cd(II) from the solution changes rapidlyand slows down over time.

The k1, k2 and Qe values of Cu and Cd and the correlation coefficient (R2= 0.969-0.972) are listed in Table 2, indicating that the adsorption processcan be described by quasi-first-order kinetics. However, the R2 values for0.999 (Cu) and 0.998 (Cd) obtained for the pseudo second kinetics are stillbetter, indicating that chemisorption is the rate determining step.


The Cu(II) and Cd(II) adsorption capacities were determined by plottingthe amount of Cu(II) and Cd(II) adsorbed after 24 hours of equilibrium with theequilibrium concentration in the solution (Figure 2). For FMBC It is muchhigher than BC (approximately 5 times higher at the same Cu(II) and Cd(II)equilibrium concentrations). To simulate the adsorption isotherms of Cu(II) andCd(II), experimental data were fitted using Langmuir and Freundlich isotherms.

The above model fits well the experimental adsorption data of BC and FMBC, and theparameters obtained are summarized in Table 3. Based on the Langmuir parameter,FMBC has a higher affinity for Cu and Cd than BC (Table 3), as they reflect.The maximum adsorption capacity (Qm) of Cu(II) (64.9 mg / g) and Cd (II) (101.0mg / g) is larger than BC (21.7 and 28.0 mg / g). These increased adsorptioncapacities can be explained by the high affinity of manganese oxide with copperand cadmium on the surface of FMBC to produce a stable internal sphericalcomplex or coordination with abundant surface functional groups (-OH, -COOH),Fe- of FMBC The MnOx phase promotes the adsorption of Cd(II) and Cu(II) bydirect electrostatic retention, ion exchange, surface complexation andprecipitation.


The temperature has little effect on the adsorption of Cu(II) and Cd(II)on FMBC (Fig. S2). ΔG<0,>0 and ΔH>0, that is, the adsorptionprocess is spontaneous and endothermic.


When the pH was increased from 3 to 6, the adsorption capacity of Cu(II)and Cd(II) of FMBC showed a significant increase (Fig. 3). In general, pH canaffect the adsorption capacity in two ways, by affecting the charge density ofthe surface to promote/block electrostatic interactions or affect theconcentration of metal ions in aqueous solutions by affecting ion exchange andmetal deposition reactions (Jefferson et al., 2015). . The surface of the FMBCis positively charged at a pH below its zero-load hot spot (pH = 9.2), whichmeans that metal ion adsorption is advantageous at high pH because highconcentrations of H3O + at low pH lead to Metal ions effectively compete foradsorption sites (Lisha et al., 2010).


A negatively charged adsorption site, and due to electrostatic repulsion,the increased Na+ concentration does not hinder the loading of Cu(II) andCd(II) at the surface location. Therefore, the studied Cu / Cd / FMBC systemwas not described by competitive adsorption, consistent with the results ofZhou et al. (2017). In addition, previous reports have shown that manganeseoxides have higher affinities for heavy metals such as Cu(II) and Cd(II) thanfor Na+.


At high HA concentrations, the adsorption capacity of Cu(II) and Cd(II) ofFMBC is significantly increased (Fig. S4), revealing that HA can enhance the affinityof FMBC for these metal ions by modifying functional groups (for example, OHand COOH). The negative charge on the surface of these composites increases thenegative charge on their surface, enhancing the adsorption of Cu(II) and Cd(II)through electrostatic attraction and complexation. For example, Cu(II)adsorption of multi-walled carbon nanotubes is enhanced by increasing theinitial natural organic concentration at low pH and high pH (Sun et al., 2012).


The FTIR spectra of the FMBC surface functional groups before and afteradsorption are shown in Figure 4, revealing new bands appearing at 524, 700,1103, 1450 and 3450 cm-1. The broadband at 3450 cm-1 is attributed to thestretching vibration of the hydroxyl group participating in extensive hydrogenbonding, and the broadband at 524 and 700 cm-1 is attributed to the -OHdeformation vibration of manganese oxide. Overall, the results obtaineddemonstrate a qualitative difference between the surface functional groups ofFMBC before and after adsorption, revealing that most of the hydroxyl groupsare consumed in the adsorption of Cu(II) and Cd(II) to form a strong single ormultidentate inner sphere complex ( For example, COO-M (M = Cu or Cd) andMn-OM) result in a shift in the characteristic OH band. In addition, thesespectra provide further evidence of strong interactions between Cu/Cd andmanganese oxide in FMBC. Similarly, it has been reported that the vibrationassociated with the hydroxyl groups of the nano-MnO2-supported BC compositessignificantly changes after adsorption of Cu(II) (Zhou et al., 2017).


The deconvolution of the peaks indicates that Cu(II) on the surface ofFMBC exists as CuO (48.1 wt%), CuCO3 (25.1 wt%) and Cu(OH)2 (26.8 wt%), whileCd(II) is Cd ( OH) 2 (59.4 wt%) and CdO (40.0 wt%) are present. These resultsclearly indicate that the deposition and chelation of Cd(II) and Cu(II) play animportant role in the adsorption process.

Although the complexation between Cd(II), Cu(II) and Fe MnOx and theO-containing group is strong on FMBC, adsorption with respect to the cation-πinteraction should also be considered. Harvey et al. (2011) reported thatplant-derived BC produced above 350 °C includes an electron-rich domain on thearomatic structure, and Cd(II) is adsorbed by a cation-π bond. In this study,BC was produced by pyrolysis of corn stover at 600 °C. Therefore, it is assumedthat the graphene-like domains can coexist with the Fe-Mn oxide on the surfaceof the FMBC. Our previous studies have found that Cu(II) is adsorbed on MnOx-loadedbiochar by cation-π interaction in an aqueous solution of pH 6.0 (Song et al.2014). Thus, a cation-π interaction can be included as another contributor tothe adsorption of Cd(II) and Cu(II) on FMBC.

Conclusions
A novel engineered adsorbent (i.e., MnOx-loaded BC) was successfullysynthesized via KMnO4 modification of corn straw BC in a nitrogen atmosphere at600 _C. Layered structuresof micro/nano-MnOx were well dispersedon the BC surface heat treated with 10% KMnO4. The unique nanostructure makesthe MnOx-loaded BCs has muchstronger adsorption capacity for Cu2+ than original BC with the maximumadsorption capacity as high as 160 mg/g. The increased adsorption of Cu2+ onthe MnOx-loaded BCs wasmainly due to the formation of surface complexes with MnOx and O-containinggroups. Besides, other mechanisms including cation-exchange and cation-p bonding may also beinvolved in the Cu2+ adsorption. Meanwhile, the desorption data evidenced thatthe adsorbed Cu2+ on MnOx-loaded BCs was more stable than on original BC,indicating that MnOx/BC composites, as alow-cost adsorbent, may be more effective in various environmentalapplications, such as wastewater treatment or immobilization of heavy metals(e.g., Cu2+) in contaminated soils.

from:陈韵如

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