Ginger Straw Waste-Derived Porous Carbons as Effective Adsorbents toward Methylene Blue

In this work, ginger straw waste-derived porous carbons, with high adsorption capacity, high adsorption rate, and good reusability for removing the toxic dye of methylene blue from wastewater, were prepared by a facile method under oxygen-limiting conditions. This study opens a new approach for the utilization of ginger straw waste, and the porous materials can be employed as great potential adsorbents for treating dye wastewater.

. Specific surface area and pore characteristics of GSPC at various temperatures.

Characterizations
The morphology of prepared samples was obtained on a Hitachi SU8220 field-emission scanning electron microscope (SEM, Japan) operated at 20 kV. X-ray diffraction (XRD) patterns were performed by a Bruker D8 diffractometer (Bruker, Germany) using the CuKa radiation. Fourier transform-infrared (FTIR) spectra were recorded using a Nicolet 6700 spectrophotometer (Thermo Fisher, USA). Nitrogen adsorption-desorption isotherm was measured using a Quadrasorb instrument (Quantachrome, USA) at 150 o C, and data analysis was performed with Quantachrome software. The average pore sizes were calculated from the nitrogen adsorption isotherms according to the nonlocal density functional theory (NLDFT) model. UV-vis absorption spectra were performed on a Perkin Elmer Lambda 25 spectrophotometer.

Synthesis of ginger straw derived porous carbons (GSPC)
In a typical process, 5.0 g ginger straw powder was transferred in a tube furnace under nitrogen flow with a heating rate of 5ºC min -1 and then held at 300, 400, 500, 600, 700 ºC for 1 h, respectively. Then, the as-prepared samples were washed with ultrapure water. Finally, the product was dried at 60 ºC in an oven for 24 h.

Batch adsorption experiments
Batch adsorption experiments were performed in glass bottles containing 10 mg of GSPC and 10 mL of MB aqueous solutions with the initial concentrations ranging from 100 and 300 mg L -1 . Subsequently, the mixtures were shaken in a shaking incubator at different times with a shaking speed of 200 rpm. Then, the mixtures were centrifuged and the supernatant concentrations of MB were determined by a UV-Vis spectrophotometer at 664 nm. The effect of solution pH on MB adsorption on GSPC was investigated by varying pH value from 2 to 12.
The effect of temperature on MB removal was also studied by keeping the temperature at 25-55ºC. The adsorption capacity of MB by GSPC was calculated via the following equations: qe (mg g -1 ) represents the equilibrium adsorption capacity of MB on GSPC, c0 (mg L -1 ) and ce (mg L -1 ) are the initial and equilibrium concentrations of MB, respectively, V (L) is the volume of solution, and m (g) is the mass of GSPC.

Adsorption kinetic and isotherm models
To understand the adsorption dynamics of MB-GSPC system in relation to time and depict the nature of solute-surface interaction between adsorbent and MB as well as to investigate the performance of adsorbent, three kinetic models (pseudo-first-order and pseudo-second-order) and two isotherm models (Langmuir and Freundlich) were studied.
The equations can be listed as follows: log( e − t ) = log e − qt (mg g -1 ) represents adsorption capacity of MB on GSPC at any time t (min), k1(min -1 ) and k2 (g mg -1 min -1 ) are pseudo-first-order and pseudo-second-order adsorption rate constants, respectively, t is contact time (min). ki (mg g -1 h -1/2 ) is intraparticle diffusion rate constant and ci (mg g -1 ) is a constant. qm (mg g -1 ) is maximum adsorption capacity of MB on GSPC, b (L mg -1 ) is Langmuir adsorption constant, k is indicator of adsorption capacity, and 1/n represents heterogeneity factor.

Regeneration experiments
For the regeneration study, 10 mg GSPC were added to 10 mL MB solution (100 mg L -1 ) at pH 12 and the mixture was shaken at 200 rpm for 60 min at 25 ºC. After adsorption and centrifugation, the supernatant MB solution was discarded leaving GSPC. Then, the MB-adsorbed GSPC were added to 10 mL of absolute ethanol and shaken at 200 rpm for 10 min. Subsequently, the GSPC were isolated from the solution by centrifugation and used for next cycle. The final concentrations of MB were determined by UV-vis spectra. The adsorption-desorption processes as described were carried out successively for five times.