Separation of waste starch in OCC pulping wastewater
The wastewater from the multi-disc thickener in the OCC re-pulping was ¦ ltrated by Busher funnel with 325 mesh ¦ rstly. Then, the ¦ ltration was centrifuged at 4500 rpm for 20 min using a high-speed centrifuge. The supernatants were subsequently placed in a dialysis bag with MW-CO 8000– 12000(D).Then, a water-soluble organic solution (waste starch and water-soluble organic matter) without the inorganic substances was obtained. The concentration of WS was measured with amylase by HPLC(Lin et al. 2020). Hydrothermal reaction of WS with PMo 12 WS was easily oxidized and degraded by PMo 12 under hydrothermal conditions. The hydrothermal treatment was conducted in a 250 mL three-port § ask reactor equipped with a stirring device and a condenser. A WS solution was prepared and transferred to the reactor, to which a certain amount of PMo 12 was added. Then, the mixture was heated in an oil bath to the desired temperature with a stirring speed of 300 r/min. The effect of hydrothermal temperature was investigated at 140 °C, 145 °C, 150 °C, 155 °C and 160 °C, and the PMo 12 dosage based on WS was investigated from 20 to 80 wt% for 60, 120, 180, and 240 min. Under direct heating, the solution gradually changed from yellow to dark blue. Then, 5 mL of sample was taken every 60 min for standby sampling and diluted to 1 mmol/L, and the reduction degree of PMo 12 was determined simultaneously by absorbance spectrophotometry at 700 nm. To this aim, the standard curve of the reduction degree versus absorbance of PMo 12 was obtained by titrating a PMo 12 solution with a KMnO 4 standard solution (0.01 mol/L).
Determination of total reducing sugar content after the hydrothermal reaction
The PMo 12 present in the mixture after the hydrothermal reaction was removed. After addition of a slight excess CsCl, a precipitate was immediately generated. The upper solution was cleared by centrifugation. Then, the supernatants were ¦ ltered through a ¦ lter with a pore size of 0.22 μ m, and the ¦ ltered liquid contained the total reducing sugar (TRS). Then, a mixture containing 2 mL of DNS reagent and 1 mL of ¦ ltrate was heated in a boiling water bath for 2 min, and 12 mL of DI water was added after the mixture was cooled down to room temperature. The color intensity of the mixture was measured in a UV757CRT Model spectrophotometer at a wavelength of 540 nm. The concentration of TRS was calculated according to a standard curve obtained from the concentration of glucose using Equation 1.
where c denotes the concentration of TRS, v is the volume of TRS, and m represents the WS mass.
The concentrations of glycolic acid, 5-hydroxymethylfurfural (5-HMF), levulinic acid, and formic acid were quantitatively determined by an external standard method; then, their quantities in the liquid products of hydrolysis were calculated according to the liquid yields. The concentration of the liquid products in the
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