Abstract
In this work, the influence of temperature on textural, morphological, and crystalline characterization of bio-adsorbents produced by hydrothermal carbonization (HTC) of corn stover was systematically investigated. HTC was conducted at 175, 200, 225, and 250 °C, 240 min, heating rate of 2.0 °C/min, and biomass-to-H2O proportion of 1:10, using a reactor of 18.927 L. The textural, morphological, crystalline, and elemental characterization of hydro-chars was analyzed by TG/DTG/DTA, SEM, EDX, XRD, BET, and elemental analysis. With increasing process temperature, the carbon content increased and that of oxygen and hydrogen diminished, as indicated by elemental analysis (C, N, H, and S). TG/DTG analysis showed that higher temperatures favor the thermal stability of hydro-chars. The hydro-char obtained at 250 °C presented the highest thermal stability. SEM images of hydro-chars obtained at 175 and 200 °C indicated a rigid and well-organized fiber structure, demonstrating that temperature had almost no effect on the biomass structure. On the other hand, SEM images of hydro-chars obtained at 225 and 250 °C indicated that hydro-char structure consists of agglomerated micro-spheres and heterogeneous structures with nonuniform geometry (fragmentation), indicating that cellulose and hemi-cellulose were decomposed. EDX analysis showed that carbon content of hydro-chars increases and that of oxygen diminish, as process temperature increases. The diffractograms (XRD) identified the occurrence of peaks of higher intensity of graphite (C) as the temperature increased, as well as a decrease of peaks intensity for crystalline cellulose, demonstrating that higher temperatures favor the formation of crystalline-phase graphite (C). The BET analysis showed 4.35 m2/g surface area, pore volume of 0.0186 cm3/g, and average pore width of 17.08 μm. The solid phase product (bio-adsorbent) obtained by hydrothermal processing of corn stover at 250 °C, 240 min, and biomass/H2O proportion of 1:10, was activated chemically with 2.0 M NaOH and 2.0 M HCl solutions to investigate the adsorption of CH3COOH. The influence of initial acetic acid concentrations (1.0, 2.0, 3.0, and 4.0 mg/mL) was investigated. The kinetics of adsorption were investigated at different times (30, 60, 120, 240, 480, and 960 s). The adsorption isotherms showed that chemically activated hydro-chars were able to recover acetic acid from aqueous solutions. In addition, activation of hydro-char with NaOH was more effective than that with HCl.
Highlights
Hydro-char is porous carbonaceous material with reactive, functionalized/aromatic surfaces [1]
The agglomerated micro-spheres and fragmentation indicates that cellulose and hemi-cellulose were decomposed, as reported by Xing et al [64], demonstrating that temperature had generated significant alterations on the morphological structure of corn stover by destructing the plant cell walls [6]
The adsorption kinetic data of acetic acid on hydro-char was correlated with a first order model, exhibiting root-mean-square error (r2 ) between 0.969 and 0.999, as shown in Table 2, which is in agreement with the results described by Machado et al [6]
Summary
Hydro-char is porous carbonaceous material with reactive, functionalized/aromatic surfaces [1] These morphological and textural properties make hydrochar a potential adsorbent to remove/recover chemical contaminants from process water [1,2,3,4,5], until the process aqueous phase produced by hydrothermal carbonization of biomass [6]. Hydro-chars have been applied as adsorbents due to their capacity to remove and or selectively absorb polar and non-polar organic compounds, including acetic acid [6], bisphenol A, 17α-ethinyl estradiol and phenanthrene [22], 2-naphthol [11], pyrene [22]; phenolic compounds (phenol, guaiacol, vanillyl alcohol, and resorcinol) [31]; dyes, such as methylene blue [8,9,23,24,26,27,29,36,38,39,40,41,42,43,44,45,46,47], methyl orange [36,49], methylene green [48], and Congo red [11]; herbicides, such as fluridone, norflurazon [18], and isoproturon [19]; pharmaceuticals, such as triclosan, estrone, carbamazepine, acetaminophen [12], sulfamethoxazole, diclofenac, bezafibrate, carbamazepine, atrazine [16,51], tetracycline [20], diclofenac sodium, salicylic acid, and flurbiprofen [21]; alkaline (K+ , Na+ ) and alkaline earth metals (Mg2+ , Ca2+ ) [31]; heavy metals, such as lead [2,3,4,10,15,23,24,25,28,30,32,42], cadmium (II) [2,4,5,10,13,24,32,38], uranium(VI) [1,35], antimony [13], copper(II) [4,5,10,14,17,24,39,50], nickel [4], zinc [10], and chromium(II) [33]; and fertilizers such as phosphate [23,34], orthophosphate [50], and ammonium [34].
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