Characterization of Bio-Adsorbents Produced by Hydrothermal Carbonization of Corn Stover: Application on the Adsorption of Acetic Acid from Aqueous Solutions

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.

Effect of Process Conditions on Hydro-Char Characteristics and Chemical Composition of Aqueous and Gaseous Products by Hydrothermal Processing of Corn Stover with Hot Compressed H₂O: Structural Evolution of Hydro-Char and Kinetics of Corn Stover Decomposition

“…al. [7], between 25 and 800 °C, 10 °C/min, under N2 atmosphere, as well as by Mohammed et al [21], between 25 and 600 °C, 10 °C/min, under N2 atmosphere. Figure 2 illustrates the TG/DTG of hydro-chars obtained by hydrothermal processing of corn Stover at 250°C, biomass/H2O ratio of 1:10, 60, 120, and 240 minutes, using a reactor of 18.927 L. One observes the presence of 03 (three) reaction steps, being according to the similar results reported in the literature [7,21].…”

“…[7], between 25 and 800 °C, 10 °C/min, under N2 atmosphere, as well as by Mohammed et al [21], between 25 and 600 °C, 10 °C/min, under N2 atmosphere. Figure 2 illustrates the TG/DTG of hydro-chars obtained by hydrothermal processing of corn Stover at 250°C, biomass/H2O ratio of 1:10, 60, 120, and 240 minutes, using a reactor of 18.927 L. One observes the presence of 03 (three) reaction steps, being according to the similar results reported in the literature [7,21]. The TG/DTG curves for the hydro-chars obtained at 240 and 120 minutes by hydrothermal processing of corn Stover at 250 °C, 1:10 biomass-to-H2O ratio, using a reactor of 18.927 L, are identical, showing mass losses between 50.63 and 49.15% (wt.)…”

“…The morphological, crystalline, and thermogravimetric characterization was performed by thermo-gravimetric analysis (TG/DTG), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD), described in details elsewhere [5,7].…”

“…(347.05 million) [3]. In this context, just as tons of corn are produced, tons of agricultural waste are discarded [4], being a potential lignocellulosic-base residue for use, after submitted to hydrothermal processing [5][6], into a carbonaceous material with bio-adsorbent properties [5,7].…”

“…The hydrothermal carbonization of lignin-cellulose-base materials including agricultural waste residues, such as corn Stover, with hot compressed H2O in the sub-critical is a thermo-chemical process of great potential [6][7][8][9][10][11][12][13][14][15][16][17], and the literature reports some studies on the hydrothermal carbonization of corn residues [5-6, 11, 14, 16-27], such as Stover [5-6, 14, 17-19, 21-25], corn Stalk [20], corn Silage [11], and until corn Stover activated with surfactant (Tween80, Span80, sodium dodecyl-benzenesulfonate, sodium lignosulfonate, PEG 400) [26], and corn Stover activated with KOH [27].…”