Preparation of high adsorption capacity bio-chars from waste biomass
Highlights
► High-adsorption-capacity bio-chars were obtained from fast pyrolysis of biomass. ► Retention time is a key parameter influencing the capacities of bio-chars. ► Mechanism of the formation of bio-chars is consistent with experimental results.
Introduction
Because of the distinct advantages of high surface area, large pore volume and broad pore size distribution, carbonaceous materials, e.g., activated carbons (ACs), have been widely employed as adsorbents to remove pollutants (Michailof et al., 2008, Lee et al., 2010, Carvalho et al., 2007, Guo et al., 2008). The adsorption capacity of ACs depends mainly on their pore structure and surface chemical state (Haghseresht et al., 2002, Liu et al., 2010, Sze and McKay, 2010). Surface oxygen/nitrogen functional groups have been proven to be able to significantly enhance the adsorption capacity of ACs by chemical mechanism (Considine et al., 2001). Many efforts have been expended to increase these functional groups by surface chemical modification. Indeed, the modified ACs exhibit better adsorption performance than the original ones (Chingombe et al., 2005, Yin et al., 2007, Starck et al., 2006, Ania et al., 2007, Villacañas et al., 2006). However, the modification process is sometimes very complex and time-consuming, and the commercial ACs are rather expensive, e.g., average $2500 per ton in USA (Kirschner, 2006). An economic approach is utilization of agricultural waste biomass (e.g., rice-husk, corncob, straw, cotton stalks and so on) as precursors for the preparation of ACs. Slow pyrolysis of the biomass at high temperature is widely used to produce the low-cost ACs (Jia and Lua, 2008). High volatile contents in the biomass are more favorable for creating highly porous structures within the AC matrix in the slow pyrolysis process. Comparing to the slow pyrolysis process, fast pyrolysis of the biomass has more advantages because of its short operation time and high energy recovery. Fast pyrolysis can be described as an anaerobic thermal decomposition process of the biomass which occurs at a mediate temperature range of 623–873 K with a high heating rate to the biomass particles and a short hot vapor residence time (often less than 2 s) (Czernik and Bridgwater, 2004). Bio-oil, the main product of the biomass fast pyrolysis, has a great potential use as a high-quality fuel or to extract high valuable chemicals (Huber et al., 2006, Zeng et al., 2011). The bio-char, a by-product of biomass fast pyrolysis (Muradov et al., 2010), is often used as a solid fuel (Abdullah et al., 2010, Kim and Parker, 2008). Considering that the surface of bio-char is abundant in oxygen/nitrogen functional groups (Cheng et al., 2010), the bio-char can be used as a modification-free adsorbent for the removal of pollutants, and thus has good economical and environmental prospects.
In the fast pyrolysis process, the temperature is a key factor affecting the characters of bio-chars (Boateng et al., 2007, Mullen et al., 2010). While the pyrolytic retention time (RT), another key parameter, is difficult to be adjusted due to the limitation of conventional pyrolysis reactors (e.g., fixed-bed or fluidized-bed reactors). The characters of bio-chars (e.g., surface area and functional groups content) significantly influence their adsorption capacities. To the best of our knowledge, so far little efforts have been made to use the bio-chars from fast pyrolysis as adsorbents and investigate the effects of RT on the adsorption performance of bio-chars. Therefore, in this work bio-chars were obtained from the fast pyrolysis of two abundantly available agricultural waste biomass (i.e. rice-husk and corncob) in a screw pyrolysis device, whose RT is adjustable. Then, phenol, a ubiquitously spread hazardous organic pollutant, was selected as a model organic pollutant to evaluate the adsorption capacity of the bio-chars obtained at different RTs. Additionally, on basis of the adsorption experimental results and further chemical analysis, the mechanisms for the phenol adsorption by the bio-chars were proposed. Finally, an economic evaluation of utilization of the bio-chars as adsorbents was performed.
Section snippets
Materials
All reagents used in this work were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Shanghai, China. Rice-husk and corncob were gathered from a farm of Hefei, southeast of China. All the biomass materials were crushed by a high-speed rotary cutting mill, screened to limit the particle size in range of 40–60 mesh and dried at 383 K for 7 h to remove the moisture.
Biomass pyrolysis and pretreatment of the obtained bio-char
The biomass pyrolysis was conducted in a self-designed screw pyrolysis device with a RT adjustment (Fig. 1) at
Characterization of the feedstocks and bio-chars
The general characteristics and constituents of two feedstocks (rice-husk and corncob) were analyzed and are listed in Table 2. As the results of proximate analysis show, the moisture, volatile matter, and fixed carbon contents of the rice husk and corncob are different significantly, while the ash contents of both biomasses are only slightly different. As for their elemental compositions, all the C, H, N, and O contents of both biomasses are only slightly different.
The surface elemental
Conclusions
High-adsorption-capacity and modification-free carbonaceous materials (bio-chars) were obtained by fast pyrolysis of rice-husk and corncob at different RTs. The pyrolysis RT was a key factor affecting the surface areas and CO-FG of the bio-chars, and further influencing their adsorption capacities. RH-1.6 exhibited a higher phenol adsorption capacity (589 mg g−1) than other bio-chars and even surface-modified ACs. Hydrogen binding and complexation between phenol and functional groups on
Acknowledgements
This work was supported by the National Natural Science Foundation of China (50978242), National Water Project (2009ZX07528-006-01-02) and the Fundamental Research Funds for the Central Universities (WK2060190007).
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